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Design & Maintenance Guide 27
A Guide to Airfield
Pavement Design and
DEFENCE ESTATES
Design and Maintenance Guide 27
A Guide to Airfield Pavement Design
and Evaluation
3RD EDITION – FEBRUARY 2011
CONSTRUCTION SUPPORT TEAM
© Crown Copyright 2009.
All Crown Copyrights are reserved. Individuals are authorised to
download this text to file or printer for their own individual use. Any
other proposed reproduction requires the assent of Defence Estates,
Kingston Road, Sutton Coldfield, West Midlands, B75 7RL. Further
information is available from www.intellectual-property.gov.uk
This guide has been prepared under the patronage of the Construction Support Team, Defence
Estates, Ministry of Defence to provide guidance on the structural design and evaluation of airfield
pavements. It supersedes the previous edition published in 1989.
The design and evaluation methods presented in this guide are developments of previous methods,
incorporating the benefits of additional experience and research.
The aircraft/pavement classification system incorporated in this guide is the ICAO ACN-PCN
method. Methods for approximating the relationship with the previously used LCN/LCG system
are included.
Further technical assistance regarding the contents of this document can be obtained from Defence
Estates. Enquiries regarding this guide should be made to the airfield pavement technical
Authority:
Head of Airfield Pavements
B75 7RL
Tel: 0121 311 2119 or Sutton Coldfield MI 2119
This guide has been devised for the use of the Crown and of its Contractors in the execution of
contacts for the Crown and, subject to the Unfair Contracts Terms Act 1977, the Crown will not be
liable in any way whatever (including but without limitation negligence on the part of the Crown,
its servants or agents) where the guide is used for other purposes.
The guidance in this document has been prepared by TRL Limited and WSP Group in
conjunction with and under commission to the Construction Support Team, Defence Estates,
Page No
FIGURES ....................................................................................................................................... VIII
CHARTS ........................................................................................................................................... X
TABLES .......................................................................................................................................... XI
GLOSSARY .................................................................................................................................... XII
1 INTRODUCTION TO AIRFIELD PAVEMENT DESIGN IN THE UNITED KINGDOM .............1
2 CLASSIFICATION OF AIRCRAFT AND AIRFIELD PAVEMENTS ........................................5
3 THE SUBGRADE ..................................................................................................................11
4 DESIGN CONSIDERATIONS ................................................................................................29
5 RIGID PAVEMENT DESIGN .................................................................................................41
6 FLEXIBLE PAVEMENT DESIGN ..........................................................................................62
7 PAVEMENT EVALUATION AND STRENGTHENING ..........................................................70
8 OVERLOAD AND HIGH TYRE PRESSURE OPERATIONS ..............................................111
9 STOPWAYS, SHOULDERS AND BLAST PADS ...............................................................113
REFERENCES ..............................................................................................................................117
APPENDIX A EXTENDED CASAGRANDE SOIL CLASSIFICATION AND CBR/K
RELATIONSHIP ............................................................................................................................121
APPENDIX B ACNS FOR SEVERAL AIRCRAFT TYPES ...........................................................125
APPENDIX C DEFENCE ESTATES' SPECIFICATION FOR AIRFIELD PAVEMENT
WORKS .........................................................................................................................................154
APPENDIX D AIRCRAFT MAIN WHEEL GEAR ARRANGEMENTS ..........................................160
APPENDIX E PASS-TO-COVERAGE RATIO ..............................................................................161
APPENDIX F THE PAVEMENT DESIGN MODELS .....................................................................165
APPENDIX G CONVERSION OF LCN/LCGS TO PCNS .............................................................175
APPENDIX H EVALUATION BASED ON EXPERIENCE OF USER AIRCRAFT ........................179
APPENDIX I STRUCTURAL INVESTIGATIONS OF AIRFIELD PAVEMENTS...........................181
Figure 1 ACN Rigid pavement model ...................................................................................... 6
Figure 2 ACN Flexible pavement model .................................................................................. 7
Figure 3 Relative compaction requirements for subgrades under flexible pavements -
Single and dual main wheel gears - Cohesive soils ............................................. 18
Dual-tandem and tridem main wheel gears - Cohesive soils ............................... 19
Figure 5 Relative compaction requirements for subgrades under flexible pavements –
Single and dual main wheel gears - Non-cohesive soils ...................................... 20
Dual-tandem and tridem main wheel gears - Non-cohesive soils ........................ 21
Figure 7 Equivalency factors for the estimation of a design CBR on a layered
subgrade ............................................................................................................... 24
Figure 8 Estimation of a design CBR on a layered subgrade – Single and dual main
wheel gears .......................................................................................................... 25
Figure 9 Estimation of a design CBR on a layered subgrade - Dual-tandem and tridem
main wheel gears ................................................................................................. 26
Figure 10 Effect of granular sub-base on the modulus of subgrade reaction (k) for
rigid pavements.................................................................................................... 27
Figure 11 Reductions in runway thickness requirement ......................................................... 32
Figure 12 Mixed traffic analysis – rigid pavements ............................................................... 35
Figure 13 Mixed traffic analysis – flexible pavements ............................................................ 36
Figure 14 Zones of annual temperature variations applicable to rigid pavements .................. 42
Figure 15 Regions where high temperature warping stresses are likely to occur in
Figure 16 Concrete flexural strengths..................................................................................... 45
Figure 17 Dowelled expansion joint ....................................................................................... 49
Figure 18 Undowelled expansion joint with hot or cold poured sealant ................................. 49
Figure 19 Sawn contraction groove (not to be used for flint gravel aggregates) .................... 50
Figure 20 Formed contraction groove .................................................................................... 50
Figure 21 Dowelled contraction joint with formed groove ..................................................... 51
Figure 22 Undowelled construction joint ............................................................................... 51
Figure 23 Undowelled sealed construction joint .................................................................... 51
Figure 24 Dowelled sealed construction joint ........................................................................ 52
Figure 25 Cement-bound sub-bases for rigid construction ..................................................... 55
Figure 26 Typical longitudinal section through jointed reinforced concrete pavement................... 58
Figure 27 Flow charts for the evaluation of airfield pavements ............................................. 76
Figure 28 Reverse design for rigid pavements ....................................................................... 77
Figure 29 Reverse design for flexible pavements ................................................................... 78
Figure 30 Pavement design and thickness requirements for Chart 8 ....................................... 84
Figure 31 CBR/k Relationship.............................................................................................. 124
Figure 32 Main wheel gear types.......................................................................................... 160
Figure 34 Distribution curves for a dual main wheel gear .................................................... 161
Figure 35 Example main wheel gear for pass-coverage ratio ............................................... 163
Figure 36 Graph of γ against the distance from the wheel centre-line .................................. 164
Figure 37 Corner cracking .................................................................................................... 166
Figure 38 Halving cracking .................................................................................................. 166
Figure 39 Quartering and Delta cracking ............................................................................. 166
Figure 40 Flow diagram for computation of rigid pavement thickness ................................ 168
Figure 41 Flow diagram for the computation of allowable wheel load stress ...................... 171
Figure 42 PCN/LCN Rigid conversion ................................................................................. 177
Figure 43 PCN/LCN Flexible conversion ............................................................................ 178
Figure 44 PCN by user aircraft evaluation ........................................................................... 180
Figure 45 Identification of projects. ...................................................................................... 182
Figure 46 Information required from structural investigations. ............................................. 183
Figure 47 Homogenous Sections. .......................................................................................... 184
Figure 48 Test locations and frequency. ................................................................................ 185
Figure 49 Falling Weight Deflectometer ............................................................................... 188
Figure 50 Typical Surface Modulus Plots. ............................................................................ 191
Figure 51 Dynamic Cone Penetrometer................................................................................. 200
Figure 52 Typical Dynamic Cone Penetrometer test result. .................................................. 201
Figure 53 Standard techniques for structural investigations and interpretation and
integration of test results. ................................................................................... 208
Figure 54 Example Deflection Profile ................................................................................... 215
Figure 55 Cusum Chart (from Figure 54 profile) .................................................................. 215
Chart 1 – Design and Evaluation of Rigid Airfield Pavements (for Single Wheel
Gear)
Chart 2 – Design and Evaluation of Rigid Airfield Pavements (for Dual Wheel
Chart 3 – Design and Evaluation of Rigid Airfield Pavements (for Dual-Tandem
Wheel Gear)
Chart 4 – Design and Evaluation of Rigid Airfield Pavements (for Tridem
Chart 5 – Design and Evaluation of Rigid Airfield Pavements (Bituminous
Surfacing on High Strength Bound Base Material)
Chart 6 – Design and Evaluation of Rigid Airfield Pavements (Rigid Slab
directly on the subgrade or granular sub-base)
Surfacing on Bound Base Material)
Chart 8 – Evaluation of Conventional Flexible Airfield Pavements (using total
thickness – X axis and combined thickness of surfacing and granular
base – Y axis)
Table 1 PCN Subgrade Categories ............................................................................................ 8
Table 2 Relative Compaction Requirements for Subgrades .................................................... 17
Table 3 Frequency of Trial Pits/Boreholes .............................................................................. 23
Table 4 Depth of Trial Pits/Boreholes (mm) ........................................................................... 23
Table 5 Design Frequency of Trafficking................................................................................ 31
Table 6 Pass to Coverage Ratios ............................................................................................. 33
Table 7 Pass-to-Coverage Ratios for Aircraft with Single Main Wheel Gears ........................ 33
Table 8 Rigid Mixed Traffic Analysis Example ...................................................................... 38
Table 9 Flexible Mixed Traffic Analysis Example.................................................................. 40
Table 10 Maximum Joint Spacing for Unreinforced PQC....................................................... 48
Table 11 Design Thicknesses for Dowelled Constructions ..................................................... 57
Table 12 Dowel Size Requirements ........................................................................................ 57
Table 13 Suitability of Surfacing Materials (Temperate Climates) ......................................... 63
Table 14 Reverse design and overlay design procedures ........................................................ 75
Table 15 Minimum Top Slab Thickness for a Multiple Slab Construction ............................. 81
Table 16 Dowelled PQC Pavements on the Subgrade or on a Granular Sub-Base.................. 82
Table 17 Equivalency Factors for Base and Sub-base Materials ............................................. 84
Table 18 Condition Factors for Concrete Slabs ....................................................................... 91
Table 19 Stopway Constructions ........................................................................................... 115
Table 20 Shoulder Construction ............................................................................................ 116
Table 21 The Extended Casagrande Soil Classification ........................................................ 122
Table 22 Drylean Concrete aggregate grading requirements. ............................................... 155
Table 23 Marshall Asphalt – Test Requirements ................................................................... 157
Table 24 Minimum stability requirements for Marshall Asphalt........................................... 157
Table 25 Porous Friction Course ........................................................................................... 158
Table 26 Unbound Granular Materials .................................................................................. 158
Table 27 % Load Transfer at transverse joints in undowelled PQC in accordance with
the Specification. ............................................................................................... 167
Table 28 Poisson's ratios for use in back-analysis. ................................................................ 193
Table 29 Guide values for goodness of fit. ............................................................................ 194
Table 30 Typical values of penetration and resolution for various types of GPR ................. 197
Table 31 Recommended FWD geophone positions for stiffness evaluation testing (7
Sensors) ............................................................................................................. 211
Term Abbreviation Definition
Aircraft Classification ACN A number expressing the relative effect of an
Number aircraft on a pavement for a specified standard
subgrade strength. A component of the ICAO
ACN-PCN method.
All-up Mass/Weight A term meaning the total mass/weight of the aircraft
under defined conditions, or at a specific time
during flight. (Not to be confused with MTOW).
Blast Pad A length of pavement adjoining the runway end,
designed to resist jet blast from aircraft standing on
the runway before take off. Generally part of a
stopway.
Bound Base Material BBM Any material equivalent to a granular sub-base or
better, which uses a cement or bituminous binder.
British Standard BS A publication of the British Standards Institution.
California Bearing Ratio CBR An indication of the bearing capacity of a soil. It is
determined by comparing the penetration load of a
soil to that of a standard material.
Cement-Stabilised Soil A relatively low quality cement-bound material
produced by the addition of the cement to a natural
soil. Mixing can take place in situ or in a mixing
Cohesive Soil A soil which contains clay; forms a coherent mass.
For determining relative compaction requirement,
cohesive soils are taken as those with a Plasticity
Index greater than or equal to 6%.
Composite Pavements Pavements consisting of mixed rigid and flexible
layers.
Coverage The application of a maximum stress on a point in
the pavement surface.
Design Aircraft The aircraft which imposes the most severe loading
on the pavement.
Drylean Concrete DLC A low-strength Portland cement concrete generally
used as a sub-base and/or base course under PQC or
bituminous surfacing (see Rolled Drylean Concrete)
or as a working course. Water content and strength
requirements are specified.
Equilibrium Moisture The moisture content at any point in a soil after
Content moisture movements have ceased.
Equivalent Coverages The number of Coverages by one aircraft which has
the same damaging effect on the pavement as a
given number of Coverages by another aircraft.
Flexible Pavement A pavement which distributes the load primarily
through the sheer strength of the materials.
Formation The surface of the subgrade in its final shape
after completion of the earthworks.
Frequency of The level of Coverages for which the pavement is
Trafficking designed. There are three categories. High,
Medium and Low.
International Civil ICAO
Aviation Organisation
Load Classification LCG A range of LCN values.
Load Classification LCN A number expressing the relative effect of an
Number aircraft on a pavement or the bearing strength of
a pavement. The original LCN classification
system was developed in the UK in the late
1940s but in 1971 the method of calculating
LCNs was altered and the LCN/LCG system
introduced. LCN values from the two systems
are not compatible.
Main Wheel Gear The undercarriage leg used in ACN calculation.
Marshall Asphalt MA An asphalt designed by the Marshall method to
meet strict specification requirements in order to
provide a durable, high stability flexible
surfacing material.
Maximum All-Up MAUW The higher of MTOW and MRW.
Maximum Ramp MRW Maximum Take Off Weight plus any taxi/runup
Weight fuel load.
Maximum Take Off MTOW The maximum aircraft weight allowable at take
Weight off.
Mixed Traffic A mixture of aircraft types using a pavement, all
of which produce a calculable effect on the
fatigue life of a pavement.
Mixed Traffic Factor RMTF or A figure used in converting Coverages by an
FMTF aircraft with one ACN to equivalent Coverages
by an aircraft with a different ACN. There are
different MTF systems for Rigid (RMTF) and
Flexible (FMTF) pavements.
Modulus of Subgrade k A measurement of the bearing strength of a soil
Reaction obtained from a loading test with a 762mm (30
inch) diameter plate.
Movement Area Pavements intended for use by aircraft, including
runways, taxiways, aprons and other areas
provided for the operation or maintenance of
aircraft.
Multiple Slab Pavements consisting of two or more concrete
Pavements layers, with or without separating layers.
Non-cohesive soil A granular soil; does not form a coherent mass.
For determining relative compaction
requirements, non-cohesive soils are taken as
those with a Plasticity Index less than 6%.
Overlay An additional layer or layers of structural
pavements materials on an existing pavement.
Overload Use of a pavement by aircraft with a
classification (ACN) greater than the pavement
classification (PCN).
Overslab A concrete overlay.
Pass An aircraft movement over a particular section of
the pavement. Under certain conditions a pass
may be taken as a movement by departing
aircraft only.
Pass-to-Coverage Ratio The number of passes of an aircraft on a
pavement which produces one Coverage at a
point in the pavement.
Pavement A structure consisting of a layer or superimposed
layers of selected materials, whose primary
purpose is to distribute the applied loads to the
subgrade.
Pavement PCN A number expressing the bearing strength of a
Classification pavement for unrestricted operations by an
aircraft with a classification (ACN) of the same
number. A component of the ICAO ACN-PCN
Pavement Quality PQC A Portland cement concrete designed within
Concrete strict limits to give a durable material in
pavement applications.
Reflective Crack A crack in a pavement layer induced by a crack
in the underlying layer.
Relative Compaction The percentage ratio of the dry density of the soil
to the maximum dry density of that soil as
determined in a compaction test.
Rigid Pavement A pavement which distributes the load by means
of its high flexural stiffness.
Rolled Drylean A drylean concrete which is compacted by
Concrete rolling to give a dense material.
Shoulder A strip adjacent to the edge of a movement area
prepared to provide a transition in strength and ,
if necessary, in grade between the movement area
and the adjacent ground, to provide for use by
aircraft in an accident or emergency.
Stopway A defined rectangular area at the end of runway,
designated and prepared as a suitable area in
which an aircraft can be stopped if the take off is
aborted.
Subgrade The natural or made-up ground supporting the
Temperature Warping Stresses due to a temperature gradient through
Stresses the depth of the concrete slab.
Transport Road TRRL
Research Laboratory
Twin Slab Pavement A multiple slab pavement consisting of two slabs
laid at the same time to obtain a thicker
equivalent single slab thickness without
compaction problems.
Wander The width over which movements of an aircraft
centre-line are distributed 75% of the time.
Westergaard's k See Modulus of Subgrade Reaction.
Unrestricted A term meaning that the operator does not have
Operations to apply any limitations on use by an aircraft at a
particular ACN.
DMG 27 1 Introduction to Airfield Pavement Design in the United Kingdom
A Guide to Airfield Pavement
Design and Evaluation
1 Introduction to Airfield Pavement
Design in the United Kingdom
1.1.1. The design of an airfield pavement requires realistic methods of assessing the loading
characteristics of aircraft and the structural response of the pavement. It has long been
recognised that the severity of load-induced stresses in a pavement and subgrade depends on
the gross weights of the aircraft using the pavement and the configuration, spacing and tyre
pressures of their undercarriage wheels. The response of the pavement in resisting these
stresses depends on its thickness, composition, the properties of materials used in its
construction and the strength of the subgrade on which the pavement is built.
1.1.2. Through the years, these basic concepts have been developed and extended to include
the effects of fatigue, environmental factors, mixed traffic, overload operations etc. Major
developments in aircraft designs have required a continuing review of existing pavement
designs and the trend up to now has been that new generations of aircraft demand pavements
designs well ahead of any practical experience of previous aircraft use.
1.1.3. The design methods for airfield pavements have largely grown out of the experience
of pavement performance. For rigid pavements, which rely on the flexural stiffness of
concrete to distribute the loads from aircraft wheels to the subgrade, use has been made since
the early 1940s of theoretical approaches developed by Westergaard and others. Because of
difficulties encountered in developing a realistic mathematical model for flexible pavements,
which depend on the mechanical strength of compacted aggregates, empirical design methods
(e.g. the CBR method) are still commonly used.
1.2 DEVELOPMENT OF PAVEMENT DESIGN IN THE UNITED KINGDOM
1.2.1. In the UK, the history of airfield pavement design really began in 1937 when the first
paved runways were constructed, using road experience as a guide. Flexible pavements were
comprised of layers of brick or stone topped with two courses of tarmacadam and a sealing
coat of mastic asphalt. Concrete pavements were either 150mm or 200mm thick slabs
generally laid directly on to the ground after the removal of the topsoil. These early
pavements soon failed under the increasing weight of new aircraft and were overlaid with
65mm thickness of tarmacadam and a 20mm thick sealing course of rolled asphalt. The
overlays were remarkably successful on concrete and were the first composite pavements.
The flexible pavements on the other hand, kept failing and were either replaced by concrete
pavements or strengthened with further overlays of tarmacadam.
1.2.2. The Air Ministry Works Directorate, which was responsible for design, construction
and maintenance of all airfields for the UK Government, constructed some 450 airfields
between 1937 and 1945 without having the benefit of proven design methods. Nevertheless,
extensive data on pavement performance, construction details and subgrade characteristics
was collected and during the last stages of World War Two attention was being given to
developing proper methods of design for airfield pavements.
1.2.3. The first design method1, published by the Department* in 1945, used Westergaard's
equations for calculating the stresses induced in a concrete pavement by aircraft loads and
Bradbury's equations for calculating warping stresses induced by thermal effects. The
cracking of the slaps was controlled by limiting the allowable flexural stresses in concrete.
1.2.4. As aircraft increased in all-up weight and a wider range of tyre pressures was
introduced it became obvious that a system of classifying aircraft, according to the severity of
stresses produced in the pavement, was necessary. A series of plate-bearing tests was put in
hand to investigate the relationship between the load necessary to produce the failure of a
pavement and the contact area over which the load was applied. The results of these tests led
to the development of an empirical relationship expressed in the following form:
W1  A1  0.44
= (1)
W2  A2 
W1 and W2 are the failure loads and A1 and A2 are the contact areas for two combinations
with the same damaging effect.
1.2.5. In 1948, the Department published a load classification system2 which assigned a
Load Classification Number (LCN) to aircraft whose loads and contact areas (derived from
tyre pressures) were linked by Equation 1. The LCN represented the relative damaging effect
of wheel loads and tyre pressures of aircraft within a practical numerical scale ranging from 1
to 100. The LCN system is still used in some countries and at many military airfields.
1.2.6. During the early 1950s, a method of using plate-bearing tests was developed for
evaluation of airfields. Publication TP104/513, issued in 1952, included a formal description
of the LCN system which had by then been extended to cater for multiple wheel
undercarriages, evaluation techniques using plate bearing tests and advice on overload
operations. A year later, the Department published a paper4 describing its latest thinking on
the design concepts. Since good compaction of slabs thicker than 300mm was difficult to
achieve with techniques available at that time, a twin slab with the corners of the upper slab
supported at the centre of the lower slab was used to provide an equivalent construction. To
deal with the corner case more accurately the Teller and Sutherland modification to the
Westergaard corner case was incorporated into the design procedure. For the design of
flexible pavements two methods were introduced – a method based on the CBR equation and
the 'Search Plate' method which was abandoned later.
1.2.7. During the construction of flexible pavements including unbound granular materials,
problems had sometimes been experienced in uniformly compacting the high quality materials
to the levels required. These pavements produced poor performance in the short and long
term. Experiments were therefore carried out using full-depth bound constructions by placing
weak cement-bound layers beneath bituminous layers. These were very successful and full-
depth bound constructions have been adopted as standard construction by the Department
1.2.8. The cumulative developments in design methods and the associated construction
practices were brought together in the Department's publication entitled 'Airfield Design and
Evaluation'5 produced in 1959. It included design charts for rigid and flexible pavements
which used LCNs as the parameter for aircraft loading. For rigid pavements a procedure for
allowing two levels of trafficking – channelised and non-channelised – was introduced. The
possibility of using reverse design for evaluating the strength of airfield pavements was
mentioned. As the compaction of concrete thicknesses greater than 300mm had become
possible, the use of twin slabs was discontinued. Charts for the design of overlays on existing
pavements were included.
Throughout the guide 'the Department' refers to Defence Estates and its predecessors in the Directorate of
Civil Engineering Services (Airfields Branch), the Ministry of Public Building and Works and Air Ministry
Works Directorate.
1.2.9. Experience during the 1960s showed that the plate-bearing tests developed for
flexible pavements gave over-optimistic results when such pavements had cement-bound
bases. An alternative heavy rolling test was therefore introduced. It was also discovered that
the strength of twin slabs and overlays on rigid construction was being overestimated. A new
design technique, assuming a high subgrade strength on the surface of the underlying slab,
was therefore developed.
1.2.10. At a symposium organised by the Institution of Civil Engineers in London on 12
November 1970, the Department summarised its state-of-the-art on design, evaluation and
strengthening of airfield pavements. Three papers6,7,8 presented at the symposium discussed
the effects of multiple wheel undercarriages, limiting criteria for failure of rigid and flexible
pavements, types of pavement which had proven to be most satisfactory and design of
strengthening.
1.2.11. The Department's last guide, entitled 'The Design and Evaluation of Aircraft
Pavements 19719, introduced the concept of Load Classification Groups (LCG) which
categorised aircraft LCN valued into seven groups. Aircraft imposing similar stress levels on
particular pavement thicknesses normally used in construction were placed in one group.
This simplified the design and evaluation of pavements and was thought to be sufficiently
1.2.12. The Load/Contact area relationship used to develop the original LCN scale of relative
loading severity was also modified as follows:
W1  A1 
This relationship was considered to be more appropriate for the aircraft which were in service
at that time. The new LCN values derived from Equation 2 were different and unrelated to
those derived in the original LCN system†.
1.2.13. Although the LCG system was included in the 1977 edition of the ICAO* Aerodrome
Design Manual10 as one of the recommended methods for reporting pavement strength, it did
not become popular outside the Department. The LCGs embraced too wide a range for
practical use and the new LCNs were often confused with the previous LCN values.
1.2.14. Probably the most radical change in the 1971 publication was the formal recognition
of the Department's construction practices, which had for some years discarded the
conventional rigid and flexible pavement constructions by adopting cement- and bitumen-
bound bases for flexible pavements and lean concrete bases for rigid ones. The design
methodology for new pavement construction was modified to reflect these practices.
1.2.15. The 1971 design guide was substantially revised and updated and a new guide
published in 198911. The 1989 guide continued to build upon the development of previous
concepts with the emphasis fixed firmly on the use of proven design techniques developed
from past experience of pavement performance. Evaluation of concrete pavements nearing the
end of their design life in the late 1970s indicated that the more frequent failure criterion was
longitudinal halving cracking and this led to a more comprehensive fatigue model for
calculating the allowable stress in rigid pavements.
1.2.16. The analysis was still based on Westergaard's theories but the design model was
refined to include factors such as fatigue, growth in concrete strength with age and
temperature warping stresses. The structural contribution of lean concrete bases was also re-
appraised.
* ICAO – International Civil Aviation Organisation.
All subsequent references to LCN are in terms of the 1971 LCN/LCG system.
1.2.17. The 1989 guide incorporated improvements to the CBR method based on full scale
testing by the US Army Corps of Engineers12 and introduced Equivalency factors for cement
and bitumen bound base courses. Methods of equating multi-layer mixed constructions to
model rigid or flexible constructions based on pavement performance experience were
included.
1.2.18. The major change in the 1989 guide was the move from the previous LCN/LCG
classification system to the ICAO Aircraft Classification Number - Pavement Classification
Number (ACN-PCN) method linked to the design and evaluation methods.
1.3 CURRENT DESIGN PRACTICE
1.3.1. In 2006 a 2nd Edition of the guide was published, incorporating a number of
developments in aircraft and airfield pavement construction and site investigations that had
taken place since 1989. The 1989 guide was updated to cover:
(i) More damaging aircraft.
(ii) Tridem (6 wheel) main wheel gears.
(iii) Higher concrete strengths.
(iv) Increases in strength for Drylean Concrete in flexible pavements.
(v) Site investigation practice.
1.3.2. The basic design models were the same as those used for the 1989 guide. Detailed
consideration was given to the adoption of a design methodology based on Multi-Layer
Elastic Analysis. However, it was decided to maintain the traditional design methodologies
because of the problems of dealing with joints in rigid pavements, material behaviour that
changes significantly with trafficking, such as cement-bound bases in flexible pavements, and
major aspects of pavement evaluation including multiple slab pavements and Type 2 and 3
composite pavements.
1.3.3. The use of high strength Drylean Concrete in flexible pavements was dealt with by
the use of modified Equivalency Factors developed from full-scale testing of Drylean
Concrete undertaken by Defence Estates and analysis by multi-layer elastic theory.
1.3.4. For tridem main wheel gear an additional rigid pavement design chart and new main
wheel gear lines on the flexible pavement design charts were necessary because of the
differences in the variation of the damaging effect with subgrade strength and coverages when
compared to other main wheel gears. The use of the ACN for tridems on flexible pavements
was complicated by the fact that at the time of writing ICAO had not formalised the
calculation method. The flexible pavement design charts were based on ACNs for tridem
main wheel gears calculated using the "interim" Alpha Factor promulgated by ICAO.
1.3.5. The key change in the 3rd Edition is the modification of flexible pavement ACNs
following revisions to the ACN-PCN method promulgated by ICAO in September 2007. The
revised ACNs have necessitated major changes to Charts 5, 6 and 8, which cannot be used
with flexible pavement ACNs calculated using the original ACN-PCN method. In addition
changes have been made to recommendations for longitudinal joint design and minimum top
slab thicknesses for multiple slab pavements.
1.4 THE GUIDE
1.4.1. This guide supersedes all airfield pavements design and evaluation documents
previously published by the Department. It is a development of the previous guide
incorporating the latest pavement performance considerations, latest design thinking and
advances in construction materials and aviation technology.
1.4.2. Many of the charts, figures and tables have been revised to accommodate recent
developments such as the use of high strength drylean concrete, the emergence of larger,
heavy aircraft with more complex main wheel gears, and changes to the ICAO ACN-PCN
method. An additional Appendix on pavement structural investigation techniques has been
DMG 27 2 The Classiification of Aircraft and Airfield Pavement
2 Classification of Aircraft and Airfield
2.1.1. Several methods of classifying the load ratings of aircraft and bearing strengths of
airfield pavements have been in use for many years. The 1977 edition of the Aerodrome
Design Manual, Part 3 published by the ICAO described four different methods which
included the LCN and LCN/LCG systems originally developed in the UK. However, for safe
and efficient use of airfield pavements, the ICAO has been striving to formulate a single
universally accepted method of classification which would:
(i) enable aircraft operators to determine the permissible operating weights for their
aircraft;
(ii) assist aircraft manufacturers to ensure compatibility between airfield pavements and
the aircraft under development;
(iii) permit airport authorities to report on the aircraft they can accept and allow them to
use any evaluation procedure of their choice to ascertain the loading the pavements
can accept.
2.1.2. On 26 November 1981, the ICAO promulgated an internationally accepted reporting
method known as the Aircraft Classification Number – Pavement Classification Number
(ACN-PCN) method. Like the LCN and LCN/LCG systems the emphasis is on the evaluation
of the load rating of aircraft, for which a standard procedure is specified, rather than
evaluation of the pavement. The strength of the pavement is reported in terms of the load
rating of aircraft which the pavement can accept on an unrestricted basis.
2.1.3. Following Defence Estates' tradition of using the aircraft classification as the load
parameter for pavement design and evaluation, the ACN has been directly linked to the design
and evaluation methods described in this guide. For pavements previously designed or
classified in accordance with the LCN/LCG system, a procedure for conversion to PCNs is
included in Appendix G. Since there is no precise relationship between LCN/LCG and PCN
classifications, the conversions are only approximate.
2.2 DESCRIPTION OF THE ACN-PCN METHOD
2.2.1. A detailed description of the ACN-PCN method is given in the 1983 edition of the
Aerodrome Design Manual, Part 3 published by the ICAO11. However, a brief description of
the method and its application is given here.
2.3 AIRCRAFT CLASSIFICATION NUMBER (ACN)
2.3.1. The ACN of an aircraft expresses its relative loading severity on a pavement
supported by a specified subgrade. ACNs are calculated using two mathematical models, one
for rigid and the other for flexible pavements. The ACN of an aircraft is numerically defined
as twice the single wheel load (expressed in thousands of kilograms) at a standard tyre
pressure of 1.25MPa, which requires the same pavement thickness as the actual main wheel
gear of the aircraft for a given limiting stress or number of load repetitions. The pavement
thickness is known as the reference thickness.
2.3.2. The ACNs are reported separately for rigid and flexible pavements, four standard
categories of subgrade (representing ranges of subgrade strength and characterised by a
standard value at the middle of the range) and at Maximum Ramp Weight and a representative
operating empty weight.
2.3.3. The method of calculating ACNs for aircraft on rigid pavements is set out below with
reference to Figure 1:
(i) Calculate the reference thickness (tc), the thickness of concrete slab which when
loaded at the centre by one main wheel gear of the actual aircraft gives a maximum
flexural stress of 2.75 N/mm2 (fct)* on a subgrade whose Modulus of Subgrade
Reaction (k) is one of the standard values (see (iv)). The mathematical model for the
stress calculation is the Westergaard solution for an elastic slab on a dense liquid
subgrade (Winkler Foundation). The modulus of elasticity for concrete is taken as
27.6 x 103 MN/m2 and Poisson's ratio as 0.15.
(ii) Calculate the single wheel load (WR) which at a tyre pressure of 1.25MPa induces a
flexural stress of 2.75N/mm2, in slab of thickness tc.
WR WR
(iii) The ACN=2x 1000 = 500 where WR is in kgs.
(iv) Calculate ACNs for each aircraft for the following four categories of subgrade
characterised in terms of a standard k.
Subgrade Category k
High 150 MN/m2/m
Medium 80 MN/m2/m
Low 40 MN/m2/m
Ultra Low 20 MN/m2/m
Figure 1 ACN Rigid pavement model
* (fct) - the flexural stress of 2.75 N/mm2 for centre-case loading was selected by the ICAO to provide a
realistic assessment of the relative loading severity of different aircraft in relation to thicknesses of rigid
pavement construction on which they are likely to be operating. This may not necessarily be the allowable
wheel load stress used in this guide, which varies depending on the flexural strength of the concrete and the
load repetitions.
2.3.4. The method of calculating ACNs for aircraft on flexible pavements is set out below
with reference to Figure 2:
(i) Calculate the reference thickness (tf), the thickness of conventional flexible pavement
which allows 10,000* load repetitions by one main wheel gear of the actual aircraft
on a subgrade whose CBR is one of the standard values (see (iv)). The method of
calculation is based on the CBR Equation and Boussinesq deflection factors.
(ii) Calculate the single wheel load (WF) which at a tyre pressure of 1.25 MPa allows the
same 10,000 load repetitions on a flexible pavement of total thickness tf. The
calculation is carried out using the following formula:
WF WF
characterised in terms of a standard CBR.
Subgrade Category CBR
High 15%
Medium 10%
Low 6%
Ultra Low 3%
Figure 2 ACN Flexible pavement model
2.3.5. The ICAO has published ACNs for most civil aircraft13. For other aircraft, ACNs may
be obtained from the manufacturers. A list of aircraft ACNs with main wheel gear types is
given in Appendix B to this guide.
2.4 PAVEMENT CLASSIFICATION NUMBER (PCN)
2.4.1. By the definition of the ACN-PCN method, the PCN is the ACN of the aircraft which
imposes a severity of loading equal to the maximum permitted on the pavement of
unrestricted use.
2.4.2. PCNs are reported as a five part code as follows:
Part i The PCN Number: The highest permitted ACN at the appropriate subgrade
Part ii The type of pavement: R=rigid, F=flexible. If the actual pavement is of
mixed construction the engineer will need to decide whether the behaviour
and mode of failure of the pavement are likely to be those of a rigid or
flexible one, then classify accordingly. For guidance on the classification of
such pavements, see Chapter 7.
Part iii The pavement subgrade category:
A = High
B = Medium
C = Low
D = Ultra Low
The ranges of subgrade strength covered by these categories are shown in
Table 1. Note that these strength ranges are not equivalent for rigid and
flexible pavements.
Table 1 PCN Subgrade Categories
Subgrade Pavement Characteristic Range of Subgrade Strengths
Category Type Subgrade
A – High Rigid 150 MN/m2/m All k values above 120 MN/m2/m
Flexible CBR 15% All CBR values above 13%
B – Medium Rigid 80 MN/m2/m 60 - 120 MN/m2/m CBR 8% to CBR 13%
Flexible CBR 10%
C – Low Rigid 40 MN/m2/m 25 to 60 MN/m2/m
Flexible CBR 6% CBR 4% to CBR 8%
D – Ultra Rigid 20 MN/m2/m All k value below
Low Flexible CBR 3% 25 MN/m2/m
All CBR values below 4%
Part iv The maximum tyre pressure authorised for the pavement:
W = High, no limit.
X = Medium, limited to 1.5 MPa (217 psi)
Y = Low, limited to 1.0 MPa (145 psi)
Z = Very low, limited to 0.5 MPa (73 psi)
Refer to Chapter 8 for guidance on high tyre pressure operations.
Part v Pavement design/evaluation method:
T = Technical design or evaluation (see Chapters 5, 6 and 7 for detailed
guidance).
U = By experience of aircraft actually using the pavement (see Appendix H for
2.5 PAVEMENT CLASSIFICATION FOR LIGHT AIRCRAFT
2.5.1. The ACN-PCN method is not intended for reporting the strength of pavements meant
for light aircraft, i.e. those with a weight less than 5700kg.
2.5.2. The bearing strength of a pavement intended for use by light aircraft should be
classified in terms of the following data:
(i) Maximum allowable aircraft weight.
(ii) Maximum allowable tyre pressure.
2.6 THE DESIGN ACN
2.6.1. The design ACN, as used in this guide, is based on the Design Aircraft; which is
normally the aircraft with the highest ACN on the actual subgrade.
2.6.2. The actual weight of aircraft when using the pavement must be considered in
determining the design ACN. The Maximum All-Up Weight figure will normally be used,
but lighter weights are appropriate (see also Section 4.8) where:
(i) the runway length imposes restrictions on the operating weights,
(ii) the pavement is only used by landing aircraft (e.g. fast turn offs) and
(iii) the pavement is only used by unladen aircraft (e.g. the accesses to maintenance
hangars).
To compute an ACN at a weight between the published values it is assumed that ACNs vary
linearly with weight.
2.6.3. The design ACN should also relate the actual value of the subgrade under a
pavement. The ACNs listed in B are for four standard subgrade categories. If the value of
actual subgrade is not the same as that of a standard subgrade, the design ACNs are to be
calculated by linear interpolation or extrapolation of ACNs for the standard subgrades. The
procedure is illustrated in Examples 2.1, 2.2 and 2.3.
2.6.4. The high category subgrade for flexible pavements is for CBR 15%. When designing
pavements for subgrades with CBR greater than 15% the following rules may be applied:
(i) Single and Dual Main Wheel Gears
Take the ACN for CBR >15% to be the same as the ACN for CBR 15%.
(ii) Dual-Tandem Main Wheel Gears
Take the ACN for CBR ≥20% as equal to 0.95 x the ACN for CBR 15%.
Values for CBRs between 15% and 20% can be obtained by linear interpolation e.g.
ACN for CBR 17% = 0.98 x the ACN for CBR 15%.
(iii) Tridem Main Wheel Gears
Values for CBRs between 15% and 20% can be obtained by linear interpolation.
2.6.5. For rigid pavements, the effect of the higher subgrade values is less significant and it
is therefore acceptable to assume that:
ACN for k>150 MN/m2/m = ACN for k of 150 MN/m2/m.
2.6.6. For pavements which would subsequently be difficult to strengthen, it may be
appropriate to design for a higher ACN e.g. for aprons adjacent to hangars and terminal
buildings. Hangar floors designed in accordance with Chapter 5 will have an inbuilt element
of over-design (see also para. 7.11).
2.6.7. Where a design ACN of less than 10 is being considered a check should be made to
ensure that the pavement is strong enough for the expected use by aircraft servicing vehicles.
2.7 OVERLOAD OPERATIONS
2.7.1. Provided the PCN for a pavement is equal to or greater than the ACN of the aircraft
and the operating tyre pressure does not exceed the PCN limitation, unrestricted use of the
pavement by that aircraft (or those with lower ACNs) is permitted. The term 'unrestricted
use' of a pavement is not specifically defined. However, it is a pavement design parameter
which should reflect current and forecast use over an appropriate design life before major
maintenance is required. See Chapter 4 for further guidance on pavement use and design life.
2.7.2. Unless a pavement is subject to extreme overloading it is unlikely to fail suddenly or
catastrophically. Nevertheless regular overload operations can substantially reduce the design
life of the pavement. The Aerodrome Authority may wish to carry out an assessment of the
financial implications of increase maintenance or premature failure. Each aerodrome
authority in the UK is free to decide on its own criteria for permitting overload operations as
long as pavements remain safe for use by aircraft. See Chapter 8 for more detailed guidance
on overload operations.
EXAMPLES ILLUSTRATING THE ASSESSMENT OF ACNs AND THE REPORTING OF PCNs
Given a rigid pavement on a subgrade of k = 30 MN/m2/m. The Design Aircraft for the
pavement has been identified as the Boeing 747-400.
Determine the design ACN and the PCN for the pavement.
From Appendix B:
Subgrade Category
Low Ultra Low
(k 40 MN/m2/m) (k 20 MN/m2/m)
B747-400 ACN 74.4 ACN 84.1
(30 − 20)
84.1 − (84.1 − 74.4) × = 79.75 (1)
(i) By interpolation, the design ACN for k = 30 MN/m2/m is:
(ii) Having designed or evaluated the pavement for ACN 70 at k = 30 the PCN is reported
From Table 1 the subgrade category is Low (i.e. k is between 25 and 60 MN/m2/m) for which
the code is 'C'. The PCN is reported as the ACN of the aircraft on the standard subgrade
category, therefore assuming there is no tyre pressure limit for the concrete pavement, the
PCN is 75/R/C/W/T.
Given a flexible pavement on a subgrade whose actual CBR is 5%. The Design Aircraft for
the pavement has been identified as the Boeing 747-400.
(CBR 6%) (CBR 3%)
(i) By interpolation, the design ACN at CBR 5% is:
(5 − 3)
94.1 − (94.1 − 72.5) × = 79.7 (2)
(ii) Having designed or evaluated the pavement for ACN 79 at CBR 5%, the PCN is
reported as follows:
From Table 1 the subgrade category is Low (i.e. CBR is between 4% and 8%) for which the
code is C. Assuming there is no tyre pressure limit the PCN is 73/F/C/W/T.
Given an existing flexible pavement on subgrade which is known to be in the 'Low' category.
Experience of aircraft use shows that B737-200s have regularly used the pavement without
causing any apparent damage to it.
Determine the classification of that pavement.
From Appendix B, B737-200 on a Flexible Pavement Low subgrade has an ACN of 30.9.
If tyre pressure limit is 1.5MPa then the PCN is 31/F/C/X/U.
NB See Appendix H for advice on the reliability of classifications based on aircraft use.
DMG 27 3 The Subgrade
3 The Subgrade
3.1.1. The subgrade is the natural soil or made-up ground which supports the pavement and
the wheel loads imposed on it. The pavement spreads and thus reduces the high pressures
immediately under the loaded areas to pressures which the subgrade can tolerate without
unacceptable deformation. Thorough evaluation of the subgrade is very important, especially
for flexible pavements where the required thickness depends greatly on the sheer strength of
the soil. This evaluation of the subgrade includes the determination of subgrade strength and
the assessment of factors which can affect the uniformity of the subgrade with time: e.g.
shrinkage and swelling, frost action and mud pumping. It is also important to ascertain the
vertical profile of the soil types, densities and moisture contents.
3.2 SOIL CLASSIFICATION AND EVALUATION OF SUBGRADE STRENGTH
3.2.1. Several soil classification systems have been developed in order to relate solid
description to engineering properties. The most common is the extended Casagrande Soil
Classification shown in Appendix A. The group symbols used for coarse-grained soils are
derived from particle size distribution, and those for fine-grained soils are mainly derived
from the plasticity index and liquid limit. The tests to assess these parameters are fully
described in BS 1377-2: 1990,15 while the Casagrande system is described in Reference 17.
The Casagrande system enables the soil to be assessed for its likely behaviour as a subgrade,
including its sheer strength, shrinkage, drainage properties and susceptibility to frost heave.
Although an experienced engineer can often estimate the sheer strength and load/deflection
values for a subgrade from the classification tests, it is often necessary to carry out further
tests specifically to measure these characteristics.
3.2.2. The subgrade strength characteristics required for pavement design are the Modulus
of Subgrade Reaction (k) and the California Bearing Ratio (CBR) for rigid and flexible
pavements respectively. The design values chosen must be representative of the soil under
the pavement after construction. Therefore, they should be based upon a relevant moisture
content and density.
3.2.3. In selecting a design moisture content, consideration must be given to seasonal
variations and the likelihood of the post-construction moisture content being higher than the
pre-construction in situ value. There are some useful guidelines for certain conditions:
(i) A method of ascertaining the post-construction moisture content is to examine the
subgrade under an existing adjacent pavement. The accuracy of the assessment will
depend upon the similarity of pavement widths, subsoil drainage and permeability of
the surface layers.
(ii) In very dry climates, if no water is present, the in situ value of the natural subgrade is
likely to be representative.
(iii) In cohesive soils which are homogeneous with depth the moisture content at 1m down
may be representative17.
(iv) In the absence of any other information the moisture content of cohesive UK soils,
except those containing a high proportion of montmorillonite, seldom exceeds the
plastic limit plus 3%.
3.2.4. Selection of a representative density will depend on the in situ density, and the degree
of compaction likely during construction (see Section 3.6).
3.2.5. The test for k is a large scale in situ test, which measures the behaviour of the
subgrade as a whole and therefore tends to compensate for variations of density and moisture
content with depth. The CBR test only measures the properties of a very small volume of the
subgrade and it is more difficult to find a representative design value. However, in practice
the Modulus of Subgrade Reaction test is difficult to carry out and in some situations it may
be sufficient to assess k from the CBR value. Appendix A includes an approximate
relationship between CBR and k. Use this with caution, particularly when considering soils
uncommon in the UK (e.g. Laterites, corals and volcanic clinker/ash).
3.3 THE MODULUS OF SUBGRADE REACTION (K)
3.3.1. The Modulus of Subgrade Reaction k is determined from loading tests carried out on
the subgrade using a standard 762mm (30in.) diameter plate. The plate is loaded to give
increments of deflection of 0.25mm (0.01 in.). The pressure on the plate is plotted against
settlement and the k value is taken as the slope of the line passing though the origin and the
point on the curve corresponding to 1.27mm (0.05 in.) deflection. See Reference 17 for a full
description of the test method.
3.3.2. As the 762 mm plate test is an in situ test it is difficult to ensure that the density and
moisture content of the soils are appropriate to the post-construction conditions. It is best to
do this test on a section prepared to the appropriate density (e.g. during compaction trails).
An adjustment for moisture content is described in Reference 17.
3.4 THE CALIFORNIA BEARING RATIO (CBR)
3.4.1. The strength of the subgrade for flexible pavement s is measured in terms of the
California Bearing Ratio (CBR) of the soil. The CBR test compares the force required to
drive a plunger into the test material to a set penetration at a given rate, with the force
required to cause the same penetration in a standard crushed limestone. A full description of
the test is given in BS 1377-4: 1990. It is also possible to do field (in situ) CBR tests (BS
1377-9: 1990).
3.4.2. The laboratory CBR test should be carried out at a range of densities and, for each
density, at a range of moisture contents. This gives a series of curves of CBR against
moisture content from which a value applicable to the required condition can be obtained.
3.4.3. In conditions where it is difficult to choose a design moisture content, the test can be
done on 4-day soaked samples in order to give a reasonably conservative value18,19 These
conditions could include:
(i) Subgrades where there is a considerable variation of moisture content with depth, in
an otherwise homogeneous soil. This is likely when the water table lies near to or
within the depth of soil being considered (i.e. the recommended depth of boreholes as
shown in Table 4).
(ii) Areas where there is a large annual variation in moisture content due to a fluctuating
water table, or possibly a spring thaw.
(iii) Tropical monsoon climates.
3.4.4. A surcharge should be applied in the CBR test to allow for the weight of the
overlying soils and pavement construction. Defence Estates has adopted 6 kg as a standard
surcharge weight.
3.4.5. When carrying out in situ CBR tests care should be taken to ensure that the density
and moisture content are appropriate, as with the 762mm plate test. In situ tests are most
useful for testing soils under existing pavements, but two points should still be considered:
(i) Stones close to the plunger area may produce unrealistically high results;
(ii) Because the test only affects a limited volume of the subgrade it will not include the
presence of weaker underlying layers. It is therefore essential to know the soil profile
at depth.
3.4.6. Laboratory tests on granular materials can give unrealistically high results because of
the confining effect of the test mould. In situ tests may give lower figures but are often
inappropriate because of the difficulty in testing at the relevant density and moisture content.
The Casagrande Soil Classification can be used as a guide to selecting a design CBR value. It
is recommended that the maximum design CBR values for flexible pavements are 20% for
full-depth bound construction and 30% for unbound constructions (see para. 6.4.5).
3.4.7. Selecting a representative design CBR value can be difficult if the CBR varies
considerably with depth. There is no problem if the CBR increases with depth as the critical
value is the lowest one, i.e. at the formation. If the CBR decreases with depth (e.g. a layer of
sand or gravel overlying a clay), designing on a high CBR value representative of the top
layer could overstress the weaker underlying layer, but designing for the CBR of the lower
layer will lead to an uneconomic pavement. In this situation Figure 7, Figure 8 and Figure 9
can be used to obtain an equivalent CBR for the two layer system. (See para. 3.8.3 and
Example 3.2).
3.5 SUBSOIL DRAINAGE
3.5.1. Providing subsoil drainage may be desirable for several reasons:
(i) To increase subgrade strength by reducing the moisture content of the soils.
(ii) To reduce the chances of the moisture content increasing above that assumed in the
selection of a design subgrade strength.
(iii) To drain the formation and pavement layers during construction.
(iv) To drain any unpaved shoulders after construction.
(v) To drain granular layers in an unbound pavement structure after construction. In this
case the drainage is more likely to be essential rather than desirable as explained in
para. 3.5.5.
3.5.2. There are a number of reasons for changes in the moisture content of subgrades,
(i) seepage flow from higher ground adjacent to the pavement.
(ii) changes in the water table level.
(iii) transfer of moisture to and from soil adjacent to the pavement.
(iv) percolation of moisture through the pavement.
3.5.3. Maximum benefit can be obtained from subsoil drainage if it is designed to reduce the
moisture content of the soils prior to and during construction (e.g. by stopping seepage flow
or lowering the water table). After construction the drainage should work to maintain the
moisture content at or below that achieved during construction (e.g. by continuing to stop
seepage flow, by preventing a rise in the water table or by removing water entering through
the pavement or from the adjacent soil.)
3.5.4. It is possible to drain the formation and pavement layers during construction by
shaping and by protecting the formation and installing subsoil drains before construction
starts.
3.5.5. The large width of runways and other airfield pavements often makes it uneconomic
to lower or control the water table because the shape of the draw-down curve would require
drains to be installed at impracticable depths. In this case the pavement should be designed
for a higher water table. However, it is important that the water table is kept at least 300mm
below granular pavement layers to prevent them becoming saturated and to minimise the
pumping of fines into the layers by repetitive aircraft loading. A geotextile fabric can also be
used as a separator to control the latter problem. Ideally the same control of the water table
level should be applied to other pavements to prevent undue deterioration of their materials.
If necessary the formation should be elevated to raise the pavement far enough above the
highest likely water table.
3.5.6. In assessing whether to install subsoil drainage, careful consideration should be given
to the economic gains from potential benefits as compared to the cost of the system. Factors
to be considered include the actual effectiveness of the system (which will partly depend on
the permeability of the soil), the availability of a convenient outfall and the problems of
installing drainage before the min construction starts.
3.6 COMPACTION OF THE SUBGRADE
3.6.1. With the exception of those soils listed in 3.6.4 (i) and (ii) the subgrade should be
compacted to increase its density and sheer strength, and to prevent excessive settlement
under traffic.
3.6.2. Control of settlement due to repetitive loading by traffic is achieved by obtaining
specific relative compaction levels in the subgrade. (See Table 2 and Figure 3, Figure 4,
Figure 5 and Figure 6). As the subgrade under a rigid pavement is less highly stressed than
under a flexible one the relative compaction requirements are less stringent under rigid
pavements. Figure 3, Figure 4, Figure 5 and Figure 6 were developed from various
compaction trials.20,21,22
3.6.3. If the relative compaction requirement cannot be met, the subgrade should be
removed and replaced with fill or overlaid with an additional layer of fill, sub-base or base
material. The aim is that the uncompacted subgrade should be at a depth beneath the
formation where the in situ relative compaction is equal to or greater than that required. This
additional material can be taken as enhancing the subgrade, as long as the relative
compactions still comply with those required at the new subgrade strength.
3.6.4. The amount of compaction possible in a soil will largely depend on the natural
density and moisture content, but certain soils raise particular problems. These are:
(i) High and medium plasticity clays;
(ii) silts and very fine sands with a moisture content at or approaching saturation level;
(iii) uniformly graded non-cohesive materials.
3.6.5. High plasticity and some medium plasticity clays (see the Casagrande Soil
Classification) are liable to show a serious decrease in strength when compacted at high
moisture contents, especially when over consolidated. In the UK the natural moisture content
of these soils is normally well above the optimum for heavy compaction so their undisturbed
densities and strengths can rarely be improved by further compaction. In their undisturbed
state, these soils give relative compactions ranging from 85-92% and CBRs ranging from 2-
5% at typical moisture contents. From Table 2 and Figure 3, Figure 4, Figure 5 and Figure 6,
these relative compactions are similar to or slightly lower than those required immediately
under the pavement. However, experience in the UK has shown that rigid pavement with lean
concrete bases constructed on medium and high plasticity clays provide good long-term
performance without excessive settlement. It is therefore Defence Estates' practice to cause
the least possible disturbance when constructing on these soils. Once exposed, the subgrade
is usually covered as soon as possible to protect it from the weather and to provide a working
area for further construction operations.
3.6.6. In tropical monsoon climates the compaction of high and some medium plasticity
soils can present different problems (see also Section 3.10). In the dry season these soils will
generally have a natural moisture content well below the optimum for heavy compaction, and
thus if too highly compacted they are likely to swell in a later wet season. But if compacted at
too high a moisture content, a low dry density will be achieved and the soil is likely to shrink
during a dry period. Special care is therefore needed to achieve a moisture content and degree
of compaction which reduces subsequent swelling or shrinkage to acceptable levels. In
general the appropriate moisture content for compaction will be just above the optimum
moisture content.
3.6.7. Silts and very fine sands with moisture contents at or approaching saturation level
cannot be compacted. If it is not practical to drain these areas or remove and backfill them,
the pavement design should be based on a very poor subgrade strength which reflects a
saturated condition. With the pavement designs being based on a low CBR the density
requirement is unlikely to be critical. To reduce the effect of poor and variable subgrade
support however, a flexible or a rigid pavement design should incorporate a lean concrete
base (See Chapters 5 and 6).
3.6.8. It is difficult to achieve compaction of uniformly-graded non-cohesive materials.
One method of overcoming this is to compact through a thin layer (75-100mm) of a well-
graded material. This layer will have no significant effect on the subgrade strength (CBR or
k), which should be taken as that of the compacted underlying material.
3.6.9. To determine relative compaction requirements under flexible pavements using
Figure 3, Figure 4, Figure 5 and Figure 6.
(i) select the relevant Figure for the soil type;
(ii) select the relevant main wheel gear type;
(iii) enter the design subgrade CBR on the left hand vertical axis;
(iv) make a horizontal projection to meet the relative compaction line;
(v) make a vertical projection to meet the design ACN;
(vi) make a horizontal projection to the right hand vertical axis and read off the depth
See Example 3.1 for an application of this procedure.
3.7 VERY WEAK SUBGRADES (EXCEPT PEAT)
3.7.1. Subgrades with CBRs less than 3% of k less than 20 MN/m2/m include saturated or
nearly saturated high plasticity clays and silts. The support to the pavement provided by these
soils is non-uniform. In the long-term the performance of the pavements will therefore be
unpredictable and likely to be subject to premature localised failure.
3.7.2. Wherever practical these soils should be removed and backfilled with suitable fill
material. As a lesser alternative Section 3.8 sets out a procedure for improving subgrade
support by overlaying with suitable fill material. A thick layer of fill will provide a more
uniform support to the pavement, although high plasticity clays may suffer long-term
consolidation and loss of pavement shape.
3.8 SUBGRADE IMPROVEMENT
3.8.1. On poor subgrades an economic option may be to use suitable fill material which is
available locally to improve the effective subgrade support to the pavement and thereby
reduce the thickness of pavement required.
3.8.2. For flexible pavement design Figure 7, Figure 8 and Figure 9 set out a method of
assessing the subgrade improvement provided by suitable fill material. Figure 8 and Figure 9
relate ACNs, existing subgrade CBRs, and thickness of fill material to an enhanced CBR
design value at the top of the fill. The fill material must have a CBR value of not less than
15% at its anticipated equilibrium moisture content and must be compacted to the
requirements of Table 2.
3.8.3. To determine the design CBR for a two layer subgrade where the CBR of the upper
layer is greater than the CBR of the lower one:
(i) Select the relevant main wheel gear type.
(ii) On Figure 7 enter the CBR of the lower layer on the horizontal axis, make a vertical
projection to meet the curve for the CBR of the upper layer ad then a horizontal
projection to the vertical axis. Read off an Equivalency Factor from the vertical axis.
This represents the load-spreading ability of the soil in the upper layer compared with
that of a granular sub-base material.
(iii) Divide the thickness of the upper layer by the Equivalency Factor to obtain 't'.
Calculate t2/ACN on the vertical axis where ACN represents the loading severity of
the Design Aircraft on the CBR of the lower layer.
(iv) On Figure 8 and Figure 9 enter the CBR of the lower layer on the horizontal axis and
the value of t2/ACN on the vertical axis. Make horizontal and vertical projections
until they intersect. The design CBR on the subgrade is shown by the curve closest to
the intersection.
3.8.4. For rigid pavement design, Figure 10 sets out a method for assessing subgrade
improvement provided by a granular sub-base.
3.8.5. The pavement on the improved subgrade should then be designed for the ACN of the
Design Aircraft corresponding to the uprated CBR or k value.
3.9 CONSTRUCTION PRACTICE
3.9.1. Experience has shown that if the moisture content of the subgrade is allowed to
increase during construction the final equilibrium strength will be lower than if it had not. It
is therefore important that the specification requirements for protecting the formation are
compiled with, or the design CBR value should be reduced accordingly.
3.9.2. Construction traffic can damage or reduce the natural strength of the subgrade. The
use of the formation in areas of cut should be restricted to the minimum plant and equipment
essential for the overlying construction. For subgrades particularly prone to damage (e.g.
high plasticity clays and silts) a working course of drylean concrete or granular sub-
base/capping layer should be placed on the subgrade before construction continues. In fill
areas construction traffic should be restricted to prevent damage to compacted layers and the
subgrade. To allow reshaping and recompaction, rut depths in granular layers should not
exceed about 40mm.23
DMG XX
Table 2 Relative Compaction Requirements for Subgrades
PAVEMENT TYPE FILL/EMBANKMENT AREAS CUT AREAS
COHESIVE NON-COHESIVE COHESIVE NON-COHESIVE
Rigid incorporating a strong cement- 90% 95% The top 150mm The top 600mm
bound base If k ≥ 40 – 90% If k ≥ 50 – 95%
k < 40 – 85% If k < 50 – 90%
Rigid without strong cement-bound base 90% The top 150mm – 98% The top 150mm The top 150mm
The remainder – 95% If k ≥ 40 – 85% If k ≥ 50 – 98%
If k < 40 – 80% If k < 50 – 95%
Between 150mm and 600mm
If k ≥ 50 – 95%
If k < 50 – 90%
Flexible The top 225mm – 95% The top 225mm – 98% Refer to Figure 3 and Figure 4 Refer to Figure 5 and Figure 6
The remainder – 90% The remainder – 95%
Notes to Table 2 and Figure 3, Figure 4, Figure 5 and Figure 6
(i) For the purpose of determining relative compaction requirements non-cohesive soils are those for which the fraction passing the 425 micron sieve size has a plasticity index
(P1) of less than 6.
(ii) The density requirements are expressed as a percentage of the maximum dry density given by BS 1377-4: 1990, Section 3.5 or 3.6.
(iii) The compaction requirements in Figure 3, Figure 4, Figure 5 and Figure 6 apply to natural subgrades below flexible pavements. The relative compaction required at a
particular depth in the subgrade is a function of the vertical stress induced at that depth by the aircraft wheel loads and the number of load repetitions over the life of the
(iv) See Section 3.7 for subgrades less than CBR 2%.
(v) Subgrades which cannot realistically be compacted to the requirements in Table 2 and Figure 3, Figure 4, Figure 5 and Figure 6 should be removed and replaced with fill or
overlaid with an additional depth of fill, sub-base or base material. This additional depth of construction should be sufficient to ensure that the requirements for relative
compaction with depth beneath the pavement are achieved.
Figure 3 Relative compaction requirements for subgrades under flexible pavements - Single and dual main wheel gears - Cohesive soils
Figure 4 Relative compaction requirements for subgrades under flexible pavements - Dual-tandem and tridem main wheel gears - Cohesive soils
Figure 5 Relative compaction requirements for subgrades under flexible pavements – Single and dual main wheel gears - Non-cohesive soils
Figure 6 Relative compaction requirements for subgrades under flexible pavements – Dual-tandem and tridem main wheel gears - Non-cohesive soils
3.10 EXPANSIVE SOILS
3.10.1. Some soils can show large volume changes when the moisture content changes. This
can lead to loss of uniform support to the pavement, a reduction of bearing capacity of the
soil, and bumps, hollows and cracks in the pavement. Generally the problem is only severe in
climates where a long hot dry period is followed by a rainy season; the subgrade dries and
shrinks during the hot season, but then expands rapidly as the rainy season increases the
moisture content. As an appropriate the Plasticity index gives a good indication of the
expansive nature of a soil; values less than 20 are non-expansive; between 20 and 40 are
moderately expansive; and above 40 can be highly expansive. For more accurate assessment
a technique related to the shrinkage limit and expected range of moisture content is described
in Reference 18. Problems can also occur if an expansive soil is compacted in too dry a
condition or allowed to dry out during construction.
3.10.2. The effect of expansive soils can be much reduced by careful control of moisture
content during construction and the degree of compaction achieved (see para 3.6.6). If future
expansion is still likely to be excessive, soil swell can be limited by, for example, providing
sufficient fill/overburden.
3.11 FROST ACTION
3.11.1. For the UK and similar climates, material within 450mm of the pavement surface
should not be susceptible to frost. Where the subgrade is frost susceptible the thickness of the
base/sub-base must be increased if the proposed total thickness of construction is less than
3.11.2. Tests for frost susceptibility has been carried out by TRL on a variety of materials
used as subgrades, sub-bases and bases both in research and during routine testing for
motorway and trunk road projects. Test results and other aspects of frost susceptibility are
contained in TRL Report No LR90.24 The Frost Test method described in LR90 was latter
updated by TRL26. The current test method is given in BS 812-124:1989.
3.12 PEAT
3.12.1. Subgrades of peat are highly compressible and have very little bearing capacity.
Pavements constructed on them can suffer from serious differential settlement, so peat should
usually be removed and replaced with a suitable fill. A possible option is to surcharge the
peat with fill for a long time to reduce the short term consolidation substantially. But this
makes a long and phased construction necessary and in the long term the performance of the
pavement will be unpredictable; there will probably be localised failures and general loss of
shape. This alternative should not be used for pavements whose longitudinal and transverse
profiles are critical; e.g. runways and major taxiways. Consider it, however, for stopways.
3.13 SPRING THAW AND PERMAFROST
3.13.1. In certain parts of the world where frost conditions are severe, pavements must be
designed for the effects of spring thaw and permafrost. Both the spring thaw and intermittent
or partial melting of a permafrost layer can considerably reduce the load-carrying capacity of
the pavement.
3.14 GROUND INVESTIGATION
3.14.1. It is essential that an adequate ground investigation is carried out to obtain the
necessary soils information. Recommendations for the spacing and depth of trial pits or
boreholes are given in Table 3 and Table 4 Groundwater movements should be monitored
over a suitable period, preferably at least one year.
Table 3 Frequency of Trial Pits/Boreholes
Location Frequency
Runways/Taxiways 1 every 50m staggered across
centre line
Aprons and other areas To be positioned on a 30m
square grid.
Table 4 Depth of Trial Pits/Boreholes (mm)
(Below proposed formation in areas of cut and existing ground level in areas of fill)
ACN of the Subgrade Category (as ACN-PCN method)
Design Aircraft
on a Flexible
Ultra Low Ultra Low Medium High
Subgrade Low
20 600 800 1000 1000
40 800 1200 1400 1400
NB: If it is certain that the construction will be a rigid pavement then
the depth can be reduced to 50% of these figures, subject to a
minimum of 600mm.
Figure 7 Equivalency factors for the estimation of a design CBR on a layered subgrade
Figure 8 Estimation of a design CBR on a layered subgrade – Single and dual main wheel gears
Figure 9 Estimation of a design CBR on a layered subgrade - Dual-tandem and tridem main wheel gears
Figure 10 Effect of granular sub-base on the modulus of subgrade reaction (k) for rigid pavements
SUBGRADE EXAMPLES
Example 3.1
A flexible pavement is to be constructed on a sand subgrade with a design CBR of 10%. The
Design Aircraft has an ACN of 60, and a dual-tandem main wheel gear. Assess the Relative
Compaction requirements.
Using Figure 6
Relative Depth below
Compaction formation (mm)
100% 0-100
95% 100-800
90% 800-1450 (see Example Lines on
Figure 6)
85% 1450-2000
A subgrade consists of 500mm sand, CBR 10%, overlying a CBR 3% clay. Using Figure 7
and Figure 9 find a design CBR for a flexible pavement for an aircraft with an ACN of 60 on
CBR 3% and a dual-tandem main wheel gear. (See para 3.8.3 for a description of the
method).
(i) Equivalency Factor = 1.8
(ii) t = 500/1.8 =278
(iii) t2/ACN = 2782/60 = 1286
(iv) Design CBR for the subgrade is 4%.
DMG 27 4 Design Considerations
4 Design Considerations
4.1 DESIGN PARAMETERS
4.1.1. The design of a new pavement requires information on the following parameters:
(i) Pavement type – rigid or flexible
(ii) Quality of the pavement materials including the flexural strength of concrete
(iii) Subgrade strength
(iv) Design ACN
(v) Frequency of Trafficking. This is derived from a number of factors including
a. The Design Life
b. The pattern of trafficking and assessment of passes.
c. Coverages and Pass-to-Coverage ratio.
d. Mixed Traffic Analysis if there is more than one significant aircraft.
4.2 TYPES OF PAVEMENT
4.2.1. The design and classification method presented in this document requires a distinction
between rigid and flexible pavements as described below.
4.2.2. A rigid pavement comprises either wholly or partly concrete construction which can
be plain, reinforced or prestressed and which distributes the aircraft loading to the subgrade
by means of its high flexural stiffness. Chapter 5 gives a design method for the preferred new
rigid pavement constructions.
4.2.3. A flexible pavement is composed of bound or unbound granular materials. It
distributes the aircraft load primarily through the shear strength of the paving material.
Cement-bound granular bases beneath bituminous surfacings make pavements quite rigid in
their early years. However, for reasons discussed in para 6.3.7 this type of construction is
treated as a flexible pavement for design and evaluation purposes. Chapter 6 gives a design
method for the preferred new flexible pavement constructions.
4.2.4. Chapter 7 includes procedures for the design or evaluation of the following pavement
constructions:
(i) Traditional flexible constructions incorporating unbound granular bases and sub-
(ii) Traditional concrete pavements laid directly on the subgrade or on a granular sub-
(iii) Composite pavements – these comprise flexible-on-rigid construction and are
generally the result of various strengthening and maintenance overlays.
(iv) Multiple concrete slab construction – like composite pavements they have generally
evolved through strengthening overlays.
(v) Overlays and overslabs required for strengthening existing pavements.
4.2.5. The choice of pavement type depends on performance requirements and cost.
4.2.5.1 Performance requirements: In general, concrete is preferred where there is likely to
be venting of fuel, spillage of lubricating oils and hydraulic fluids, jet efflux gases from slow
moving high performance jet engines, or areas subject to locked wheel turns. Concrete should
therefore be used for the following pavement areas:
(i) Runway ends (typically for a distance of at least 150m).
(ii) Sections of taxiways adjacent to runway ends.
(iii) Holding areas.
(iv) Aprons and hard standings.
(v) Hangar floors.
(vi) Engine run-up platforms.
(vii) Compass calibration bases.
4.2.5.2 Cost: For many pavements this will be the main consideration and will depend on
such diverse factors as the availability of materials in the locality and the bearing capacity of
the natural subgrade on which the pavements are to be constructed. For rigid pavements there
is a minimum thickness of concrete below which its use is impractical, and a maximum
subgrade strength beyond which further increases in strength result in little saving of
construction depth. On soils of good bearing value, flexible construction is likely to be more
economical. The opposite is true for weak subgrades.
4.2.5.3 Other considerations:
(i) The absence of joints in flexible pavements gives them better riding qualities for high
speed operations than most types of rigid pavement.
(ii) If the only realistic option is to construct a pavement on an unpredictable subgrade
which is liable to long-term shrinkage or heave, a flexible pavement will generally be
the best option. This is because a flexible pavement can cope with greater movement
and remain serviceable; it can also be more cheaply and expediently overlaid to
rectify the loss of shape.
4.3 MATERIAL SPECIFICATION
4.3.1. The use of the semi-empirical design methods demands that the quality of material in
a pavement is at least as good as those in the pavements upon which the design methods are
based. This applies to the material specification and the level of quality control.
4.3.2. Relevant details of the necessary material specification are given in Chapter 5, 6 and
7, and Appendix C.
4.4 SUBGRADE STRENGTH
4.4.1. The determination of subgrade strength, and the other subgrade properties to which
consideration should be given during design, is described in Chapter 3. Some specific
considerations with respect to rigid and flexible pavements are discussed in Sections 5.5 and
6.4 respectively.
4.5.1. The method of determining the design ACN for a pavement is given in Chapter 2.
4.6 FREQUENCY OF TRAFFICKING
4.6.1. While the magnitude and configuration of the wheel loads are the dominant factors in
the design of airfield pavements, the effect of fatigue caused by load repetition is an important
secondary consideration for both rigid and flexible pavements. Laboratory and full-scale tests
clearly show that pavements subject to high frequencies of trafficking need to be significantly
thicker than those subject to low frequencies.12,27
4.6.2. The design methods given in this guide cater for 3 frequencies of trafficking: Low,
Medium and High, as shown in Table 5.
Table 5 Design Frequency of Trafficking
Nominal Number of Coverages*
Frequency of Trafficking over Design Life of Pavement
Low 10,000
Medium 100,000
High 250,000
*The definition of 'Coverages' is given in Section 4.9
4.6.3. To determine the appropriate frequency of the trafficking, the total number of
Coverages during the design life is calculated. This involves consideration of the design life,
pattern of trafficking and mixed traffic use.
4.7 DESIGN LIFE
4.7.1. The design method and the frequencies of trafficking in Table 5 assume the aircraft
movements are spread fairly evenly over the life of the pavement
4.7.2. In normal circumstances pavement deterioration is gradual, becoming noticeable over
a period of a few years. This deterioration can be due to surface weathering or structural
fatigue or both. In deciding on an appropriate structural design life, the following
considerations should be kept in mind:
(i) The need to keep major maintenance work on airfield pavements to a long term cycle.
(ii) The likelihood of a change in aircraft use after a number of years.
(iii) Durability of pavement construction. Concrete pavements are more durable than
blacktop pavements assuming both are constructed in accordance with Defence
Estates' Specification. The surface serviceability of concrete should, with the aid of
minor maintenance work, be adequate for 25-35 years. On the other hand bituminous
surfacings, as a result of surface weathering, generally require maintenance work in
the form of slurry sealing, the first coat being required after 7-10 years, and more
substantial restoration work after 20-25 years. In the case of friction case resurfacing
may be required after approximately 15 years.
(iv) The cost of rehabilitation. Concrete pavements generally cost more to rehabilitate
than flexible pavements.
4.7.3. With these factors in mind it is recommended that the structural design life be 20-30
years. The upper end of this range being for concrete pavements and the lower end for
4.7.4. The design method assumes an increasing degree of minor maintenance (e.g. crack
sealing) in the last few years of a pavement's life. Where such maintenance cannot be
tolerated, the engineer may wish to project a structural design life beyond the expected life of
the surfacing.
4.8 PATTERN OF TRAFFICKING AND ASSESSMENT OF PASSES
4.8.1. An aircraft movement over a particular section of the pavement normally constitutes a
pass. The total number of passes should be taken as the total number of movements and
Mixed Traffic Analysis used to consider the effect of aircraft operations at different weights.
It is conservative to consider all movements at Maximum Ramp Weight. If it is certain that
actual operations (e.g. landings) will always be at weights lower than this figure a more
accurate weight can be used (e.g. for fast turn offs, accesses to maintenance areas and where
runway length restricts Maximum Take Off Weight).
4.8.2. Runways and main taxiways leading to runway ends are the most heavily loaded
pavements as they carry the aircraft at their heaviest, when fuelled for take off. For these
pavements the number of passes can be taken as the number of departure movements only;
landing movements being accounted for by assuming that all passes are at Maximum Ramp
4.8.3. Aircraft parking aprons in rigid pavement construction should be designed for the
same design ACN and frequency of trafficking as the main taxitrack exit from the apron. This
is because it is difficult to predict movement patterns and to construct areas of concrete in
varying thicknesses.
4.8.4. The outer portions of runways can be designed to a reduced loading regime as shown
in Figure 11. However, where an airfield does not have a parallel or perimeter taxiway, the
assessment of the loading regime should include the additional use of the runway for taxiing
operations. 'Backtracking' (taxiing) down the runway by departing aircraft will approximately
double the Coverages (as defined in Section 4.9) on the runway. In addition, the length of
runway used by backtracking aircraft should be provided with the same full depth
construction across the width of the pavement to allow for taxiing being offset from the
centreline.
Figure 11 Reductions in runway thickness requirement
4.8.5. Reduction in construction thickness on the outer strips of runways, is particularly
beneficial when strengthening existing runways which have an inadequate camber. The
reduced thickness at the edge will allow improved transverse gradients and surface water
drainage.
4.8.6. On helicopter pads and Harrier VTOL pads the dynamic effects of landing aircraft
increase the loading factor. For these pavements the passes should be taken as the number
of take offs plus the number of landings at the ACN appropriate to the maximum weight;
the Pass-to-Coverage Ratio listed in Table 6 and Table 7 should also be adopted.
4.9 COVERAGES AND PASS-TO-COVERAGE RATIO
4.9.1. Coverages represent the number of times a particular point on the pavement is
expected to receive a maximum stress as a result of a given number of aircraft passes. The
relationship between passes and Coverages depends on several factors, including the number
and spacing of wheels on the aircraft's main wheel gear, the width of the tyre contact area and
the lateral distribution of aircraft wheel paths relative to the pavement centre-line or guideline
markings. The number of Coverages is calculated using the Pass-to-Coverage Ratio:
Coverages = Passes
Pass-to Coverage Ratio
4.9.2. Table 6 and Table 7 give Pass-to-Coverage Ratios for various main wheel gear
arrangements on runways, taxiways and stands. These ratios assume channelised trafficking
consistent with the initial stage of a take off run on runways, very channelised trafficking
about a centreline on taxiways, and operations on stands with designated stand centrelines,
especially when controlled by docking guidance systems. For aprons without stand
centrelines where the actual parking position varies the Pass-to-Coverage Ratios for taxiways
should be used.
4.9.3. For the background to the derivation of the Pass-to-Coverage Ratios see Appendix E
which also sets out a procedure for calculating Pass-to-Coverage Ratios for non-standard
wheel gear arrangements.
Table 6 Pass to Coverage Ratios
Main Wheel Gear Type* Pass-to-Coverage Ratio
Runway Taxiway Stand
Single See Table 7
Dual 3.2 2.1 1
Dual-Tandem 1.8 1.31 0.5
Tridem 1.44 1 0.33
* Refer to Appendix D for definition of landing gear arrangements.
Table 7 Pass-to-Coverage Ratios for Aircraft with Single Main Wheel Gears
ACN of Aircraft
Up to 10 11-20 21-40 Over 40 All
Runway Taxiway Runway Taxiway Runway Taxiway Runway Taxiway Stands
Up to 1.0 8 4 6 3 5 2.5 4 2 1
1.0 to 1.5 10 5 8 4 6 3 5 2.5 1
>1.5 12 6 10 5 7 3.5 6 3 1
4.10 MIXED TRAFFIC USE
4.10.1. At a military airfield the pavements are often designed for operations by a specific
type of aircraft, so the calculation of the loading regime is relatively straightforward.
However, where traffic forecasts indicate operations by a variety of aircraft, the loading
criteria will not be so readily assessed. In allowing for a variety of aircraft types it is
necessary to be able to relate the loading severity of each type of aircraft to that of the Design
Aircraft and thereby to calculate the number of Equivalent Coverages by the Design Aircraft.
4.10.2. The calculation of the loading regime for pavements subject to mixed traffic is
explained with reference to Examples 4.1 and 4.2:
(i) Decide on the required design life of the pavement (see Section 4.7).
(ii) Establish the aircraft types likely to use the pavement.
(iii) Establish the ACNs for each aircraft at the actual subgrade value and the appropriate
(iv) Use Appendix B to identify the main wheel gear type for each aircraft and establish
their Pass-to-Coverage Ratios from Table 6 and Table 7 (see Section 4.9).
(v) Establish the number of passes by each aircraft.
(vi) Establish the Design Aircraft.
(vii) For setting out the information refer to Table 8 and Table 9 of Examples 4.1 and 4.2
respectively. The tables show the aircraft (col 1), their ACNs (col 2), Pass to
Coverage Ratios (col 3) and annual passes (col 4).
(viii) Calculate the number of Coverages by each aircraft during the design life of the
pavement (col 5).
(ix) Calculate the ratio of the ACN of each aircraft to that of the Design Aircraft (col 6).
(x) For rigid pavements, use Figure 12 to obtain rigid mixed traffic factors (RMTF) from
the ACN ratios found in step (ix). For each aircraft, select the ACN of the Design
Aircraft on the left-hand ordinate and make a horizontal projection until it intersects
the curve with the appropriate ACN ratio. Make a vertical projection down the graph
and read off the RMTF. See Table 8 col 7. Having established the RMTF for each
aircraft the number of Equivalent Coverages by the Design Aircraft is equal to the
number of Coverages made by each aircraft divided by its respective RMTF (Table 8
col 8) Hence:
Equivalent Coverages Coverages by aircraft at
by Design Aircraft = less than the design ACN*
RMTF
(xi) For flexible pavements, use Figure 13 to obtain flexible mixed traffic factors (FMTF)
from the coverages found in step (viii). For each aircraft, select its respective number
of Coverages (Table 9 col 5) on the abscissa of Figure 13. Then make a vertical
projection until it intersects the curve. Make a horizontal projection and read off the
FMTF from the left-hand ordinate. See Table 9 col 7. Modify the FMTF for each
aircraft by multiplying it by the respective ACN ratio (Table 9 col 8). Select the
Modified FMTF on the left-hand ordinate of Figure 13. Using the graph in reverse,
read off the number of Equivalent Coverages by the Design Aircraft (Table 9 col 9).
(xii) Column 8 in Table 8 and column 9 in Table 9 give the mixed traffic loading in terms
of Equivalent Coverages by the Design Aircraft. Calculate the total Coverages at the
design ACN from:
Coverages by the Design
Total Coverages at = Aircraft at the design ACN
the design ACN plus the Equivalent Coverages
(xiii) From Table 5, select a frequency of trafficking to use as an input to the design charts.
* This could include the Design Aircraft at weights other than the maximum considered.
Figure 12 Mixed traffic analysis – rigid pavements
Flexible Mixed Traffic Factor (FMTF)
Figure 13 Mixed traffic analysis – flexible pavements
TRAFFIC ANALYSIS EXAMPLES
Design a new rigid pavement for the main taxiway at an international airport used by a wide
range of aircraft.
1. Design Life 30 years (see Section 4.7).
2. Expected Departures:
Aircraft Departures/Year
A321-200 28600
A340-500 2000
B737-800 5000
B777-300ER 1600
3. Aircraft Data:
RIGID PAVEMENT SUBGRADES - MN//m2/m
Pass-to-
Main Wheel Coverage
All Up
High Medium Low Ultra Low Gear Type for Ratio
Aircraft type Mass
150 80 40 20 Pavement
(kg) (Table
A321-200 89,400 56.5 59.4 62.1 64.3 Dual 2.1
A340-500 369,200 72.8 84.7 100 115.3 Dual Tandem 1.3
A330-200 233,900 53.7 62.4 74.3 86.9 Dual Tandem 1.3
B737-800 79,243 49.3 51.8 54.2 56.1 Dual 2.1
B747-400 397,800 52.4 62.7 74.4 85.1 Dual Tandem 1.3
B777-300ER 352,441 65.8 85.3 109.3 131.5 Tridem 1
4. Soil Survey: k = 50 MN/m2/m.
5. Aircraft ACNs at the requisite subgrade value (k = 50) interpolated from Step 3:
Aircraft ACN @
k = 50 MN/m /m
A321-200 61.4
B737-800 53.6
B777-300ER 103.3
The Design Aircraft is the B777-300ER with ACN 103.3.
6. Design Aircraft: Boeing 777-300ER.
7. Total Coverages (see Table 8): 76,775.
Table 8 Rigid Mixed Traffic Analysis Example
Aircraft ACN ACN Ratio Passes Pass-to- Coverages Rigid Mixed Equivalent
(Departures / Coverage (Col 4 / Col Traffic Factor Coverages
Year x Ratio 5) (From Figure (Col 6 / Col
Design Life) 12) 7)
A321-200 61.4 0.59 858000 2.1 408571 148.39 2753
A340-500 96.2 0.93 60000 1.3 45802 1.89 24290
A330-200 71.3 0.69 36000 1.3 27481 33.17 828
B737-800 53.6 0.52 150000 2.1 71429 682.42 105
B747-400 71.5 0.69 30000 1.3 22901 32.45 706
B767-300 51.9 0.50 114000 1.3 87023 939.6 93
B777-300ER 103.3 1 48000 1 48000 1 48000
TOTAL COVERAGES 76775
8. Design for Medium Frequency Trafficking (i.e. 100,000 Coverages) by Boeing 777-
300ER.
Design a new flexible pavement for the runway at an international airport used by a wide
1. Design Life 20 years,
FLEXIBLE PAVEMENT SUBGRADES - CBR
Main Wheel Pass-to-
All Up Coverage
High Medium Low Ultra Low Gear Type for
Aircraft type Mass Ratio
15 10 6 3 Pavement
Design (Table 6)
A321-200 89,400 49.4 52 57.6 63.2 Dual 3.2
A340-500 369,200 75.3 82.2 97.8 129.8 Dual Tandem 1.8
B747-400 397,800 53 59 72.5 94.1 Dual Tandem 1.8
B777-300ER 352,441 63.6 71.1 89.1 120.1 Tridem 1.4
4. Soil survey shows CBR 10%.
5. The actual subgrade value is one of the standard values. Therefore ACNs can be taken
directly from Step 3 above. The Design Aircraft is the A340-500 with ACN 82.2.
6. Design Aircraft: Airbus A340-500.
Table 9 Flexible Mixed Traffic Analysis Example
Aircraft ACN ACN Ratio Passes Pass-to- Coverages Flexible Modified Equivalent
(Departures Coverage (Col 4 / Col Mixed Mixed Coverages
/ Year x Ratio 5) Traffic Traffic (from
Design Life) Factor Factor Figure 13)
(from (Col 3 x Col
Figure 13) 7)
A321-200 52 0.63 572000 3.2 178750 1.46 0.92 5933
A340-500 82.2 1 20000 1.8 11111 1 0 11111
A330-200 63.5 0.77 55000 1.8 30556 1.17 0.9 5261
B737-800 45.4 0.55 100000 3.2 31250 1.17 0.65 849
B747-400 59 0.72 70000 1.8 38889 1.21 0.87 4082
B767-300 43.3 0.53 76000 1.8 42222 1.22 0.64 818
B777-
300ER 71.1 0.86 20000 1.4 13889 1.05 0.91 5368
8. Design for Medium Frequency Trafficking (i.e. 100,000 Coverages) by Airbus A340-500.
DMG 27 5 Rigid Pavement Design
5 Rigid Pavement Design
5.1.1. For over 50 years Defence Estates' preferred choice of new rigid construction has
comprised unreinforced pavement quality concrete (PQC) without dowels, tie bars or keys, on
a rolled drylean concrete (DLC) base. Defence Estates does not consider necessary the use of
traditional mechanical load transfer devices; experience has shown that with good base
support provided by drylean concrete together with aggregate interlock at the transverse
(contraction) joints of the PQC and the omission of regular expansion joints (see para 5.3.1),
an adequate level of load transfer is maintained for a considerable number of load repetitions.
Furthermore standard unreinforced, undowelled rigid pavement design simplifies construction
and gives reliable performance.
5.1.2. Longitudinal joints are simple butt joints without load transfer. In general the
absence of load transfer has not caused problems. However, where aircraft regularly traffic
across longitudinal joints, e.g. on some aircraft stands where the concrete is laid normal to the
stand centre-line, early failures have occurred. In these situations the load transfer should be
provided at the longitudinal joint, by dowels or a profiled joint, or the concrete slab thickness
should be increased as described in 5.6.3.
5.1.3. Apart from the preferred choice the following types of rigid construction are also
considered in this Chapter:
(i) Fully dowelled (unreinforced) – see Section 5.7
(ii) Jointed reinforced (with dowelled expansion, construction and contraction joints) –
see Section 5.8.
(iii) Continuously reinforced concrete – see Section 5.9.
5.1.4. In climates with a high seasonal temperature variation, omitting the dowels and
regular expansion joints must be considered with caution. Pavements constructed without
expansion joints at low temperatures in these climates may be subject to 'blow-ups'.
Conversely, those constructed without dowels at the high end of the temperature range would
have poor load transfer properties at open transverse joints in winter. Figure 14 gives three
zones of annual temperature variation, moderate, high and extreme. The preferred
constructions with undowelled PQC slabs without regular expansion joints (see para 5.3.1) on
drylean concrete bases apply without restrictions in the zone of moderate temperature
variation. For the zone of high annual temperature variation PQC slabs constructed within the
centre 80% band of annual temperature do not need dowels or regular expansion joints.
However, PQC slabs constructed outside this temperature range and those constructed in the
zones of extreme temperature variation, should be fully dowelled; when constructed at low
temperatures, regular expansion joints should also be provided.
5.1.5. Also to be considered are the excessive temperature differentials which can develop
between the top and bottom of a slab, causing high warping stresses. Figure 15 shows the
regions where temperature warping stresses are likely to be significantly greater than allowed
for in the rigid pavement design model (see Appendix F). In these regions the PQC thickness
requirements obtained from the design charts should be increased by 10% to allow for
excessive warping stresses.
Figure 14 Zones of annual temperature variations applicable to rigid pavements
Figure 15 Regions where high temperature warping stresses are likely to occur in rigid pavements
5.2 PAVEMENT QUALITY CONCRETE SLAB
5.2.1. Pavement Quality Concrete (PQC) must be strong enough to provide an economical
pavement thickness. The PQC must also provide a durable, hard wearing, weather resistant
surface so that the expense and disruption of major maintenance is seldom required. Air
entrainment should be used to provide resistance to frost and the action of de-icing chemicals.
5.2.2. The design flexural strength referred to on Charts 1, 2, 3 and 4 is the in situ mean
flexural strength of the PQC at 28 days (28 days is assumed to be the minimum time before
the pavement is brought into use). The in situ mean flexural strength parameter relates
directly to the failure criteria assumed in the design method i.e. 50% of the bays in the
trafficked area have developed cracks; as halving cracks initially develop at the underside of
the slabs this does not necessarily mean that 50% of bays will be exhibiting surface cracks at
failure (see Appendix F). The design flexural strength relates to the concrete in the pavement.
Quality control during construction should be based on strength tests on samples with the
same degree of compaction and cured in the same regime as the in situ concrete e.g. by using
cores from the slab, provided the flexural to compressive strength ratio is known.
5.2.3. Figure 16 provides guidance on relationships between 28 day in situ and laboratory
mean flexural strengths, 7 day laboratory mean and characteristic compressive strengths from
cubes and 28 day in situ characteristic compressive strengths from cores. The characteristic
strength is based on a 5% defective rate. The relationship between 7 day cube strengths and
28 day core strengths is based on the Defence Estate's experience, modified for the method
of determining compressive strength described in BS EN 13877-2:2004. It takes into account
the differences in both the curing regime and degree of compaction for cast cubes and cores
extracted from the pavement. The estimated laboratory mean flexural strength is based on a
20% gain in strength from 7 to 28 days. If evidence suggests that the actual gain in strength is
different, the strengths on the axis should be adjusted by the ratio of the actual gain in strength
to the assumed 20%.
Figure 16 Concrete flexural strengths
5.3 JOINTS IN PAVEMENT QUALITY CONCRETE SLABS
5.3.1. Recommendations on the frequency of expansion joints and the maximum spacing of
contraction and construction joints are given in Table 10 for unreinforced PQC. Joint
spacings are fundamental to the pavement design so these recommendations should not be
exceeded, except as modified in para 5.3.2.
5.3.2. Expansion joints. For PQC slabs 250mm or more in thickness regular expansion
joints are not usually required. However, for some situations it is advisable to provide them
to limit movement of the pavement, at maximum intervals such that expansion will not cause
unacceptable extrusion of the sealant from the joint. For pavements constructed at low
temperatures in climates with high or extreme annual variations in temperature as defined by
Figure 14 it is wise to provide regular expansion joints for all PQC slab thicknesses. The
spacing of expansion joints in these circumstances for slabs 250mm or more in thickness
should be similar to that required for 225mm thick slabs (Table 10). The actual spacing will
depend on the type of coarse aggregate, the annual range of temperature and the movement
accommodation factor of the joint filler and the sealing compounds. For slabs less than
250mm thick the spacing of expansion joints depends on the slab thickness and the type of
coarse aggregate used in the PQC. For all PQC thickness expansion joints should be formed
between new and existing pavements, at junctions, at tangent points of bends, around box
gutters and around other obstructions to the continuity of the slabs. Figure 17 and Figure 18
show details of dowelled and undowelled expansion joints. When constructing undowelled
unreinforced PQC, Defence Estates does not normally include any specific design provisions
(i.e. dowels or thickened slab edges) for expansion joints. This practice has not led to
premature maintenance problems primarily because of the stiff DLC base and the dearth of
expansion joints. If dowels are to be provided at expansion joints in an otherwise undowelled
pavement, care should be taken in the joint detailing to ensure that the overall movement of
the slabs as a result of moisture and temperature changes is not locally impeded in the
direction transverse to the dowelled joints.
5.3.3. Contraction grooves. Contraction grooves initially control the development of cracks
caused by drying shrinkage or a drop in temperature shortly after laying. Initial cracking due
to these factors rarely occurs at every contraction joint. In the long-term the spacing of these
grooves together with the construction joints is fundamental to the pavement design as it
controls the warping stresses caused by temperature differences between the top and bottom
of the slab. An undowelled contraction groove, which is standard Defence Estates practice,
relies on aggregate interlock to provide load transfer. Load transfer from aggregate interlock
decreases as the crack width increases. For thin slabs with regular expansion joints the effect
of joint opening is built into the design, but for thick slabs the assumption is that there are no
expansion joints. In situations where there induced cracks in slabs greater then 250 mm thick
may open more than expected, e.g. where it is necessary to provide expansion joints,
consideration should be given to dowelling the contraction groove. Figure 19, Figure 20 and
Figure 21 show typical details for unsealed contraction grooves, sealed contraction grooves,
and dowelled and sealed contraction grooves. The grooves can be sawn or wet formed, but
Defence Estates prefers the former in conjunction with the use of limestone coarse aggregate
which can be easily sawn. This is because wet forming grooves can cause the adjacent
concrete to become overworked leading to poor durability and long-term maintenance
problems. In arid regions where wind blown sand and dust are prevalent, the engineer may
prefer to provide wider contraction grooves which can be sealed to prevent abrasion and
'jamming' of joints (see Figure 20 and Figure 21). For reinforced concrete pavements the
spacing of joints can be increased as explained in Section 5.8.
5.3.4 Day work joints. Day work joints are usually simple butt joints. In most
circumstances they are infrequent and have little effect on the failure rate of the pavement. In
some situations frequent daywork joints become necessary, e.g. laying in winter with short
days. In these situations consideration should be given to dowelling the day work joints.
5.3.4. Construction joints. The spacing of these joints in unreinforced PQC should be the
same as the contraction groove spacing. This is because experience has shown that the effects
of wheel load and warping stresses are much reduced in square bays. Figure 22 shows the
standard construction joint detail used by Defence Estates. Figure 23 details a sealed joint
which may be considered more appropriate in certain circumstances for the same reason as
described in para 5.3.3. Where load transfer is required at construction joints (para 5.1.2) it
should be provided by dowels (para. 5.3.5) or a profiled joint (para 5.3.6). Both dowels and
profiled joints give greater quality problems than butt joints.
5.3.5. Dowelled Construction joints. The use of dowels, including the diameter and spacing
and the potential problems associated with them, is described in Section 5.7. Figure 24 details
a dowelled and sealed construction joint.
5.3.6. Profiled Construction joints. An alternative to dowelling construction joints to
provide load transfer is a keyed joint. In addition to traditional keys various curved profiles
for the faces of construction joints have also been tried, not necessarily for load transfer. The
dimensions of keys have been based on the slab thickness rather than a design for the specific
loading. Historically keyed joints have not performed well, often suffering premature failure.
due to factors such inadequate shear capacity in the key and very high stresses at sharp angles.
In addition the geometry of some key designs means that if the joint opens due to shrinkage or
movement caused by temperature changes, the faces do not come into contact when loaded
and the load transfer is lost.
5.3.7. Figure 25 shows a profiled joint specifically designed to provide load transfer to
meet the design requirement while avoiding high stresses at angles and being practicable to
construct with a shaped form. A profiled joint may be considered when:
(i) The slab thickness is greater than 250 mm.
(ii) It can be demonstrated that the capacity of the joint, based on its dimensions and the
concrete strength, is adequate for the proposed loading.
(iii) The detail can be offered as an option to dowels so that the contractor can confirm
that fixed forms and side-forms for slip-form pavers can be formed to the correct
dimensions, that the concrete mix allows formation of the profile and will have a
standard deviation less than or equal to the design assumption, and that any increase
in slab thickness is more economic than dowelling the standard design thickness.
5.3.8. To design a profiled joint:
(i) Design the pavement in accordance with Section 5.6.
(ii) Check the capacity of a profiled joint for the Design Aircraft, using the equation
(iii) If an increase the slab thickness is considered a viable option increase the thickness
until the Joint Capacity Factor is 1.
(iv) Check the Joint Capacity Factor for any aircraft in the design mix that may have an
individual wheel load greater than that of the design aircraft. (NB. Unlike overload of
a concrete slab which is unlikely to result in failure under a single load, overload of a
profiled joint may result in an immediate failure in shear. If significant overload is
foreseeable the Joint Capacity Factor should be checked for likely aircraft.)
(v) Detail the joint in accordance with Figure 25.
The capacity of a profiled joint within a given slab thickness is given by:
11.199 P
(i) JCF =
h fcm(1 − 2.33CV )
where: JCF = Joint Capacity Factor (must be ≥ 1).
h = slab depth (mm)
P = wheel load (kg)
p = tyre pressure (MPa)
fcm = mean 28 day in situ compressive strength of concrete (N/mm2)
CV = Coefficient of Variation of concrete (%)
The profiled joint is adequate for the load if the Joint Capacity Factor is greater than or equal
to one.
5.3.9. Profiled joints can be formed by manufacturing shaped side forms for fixed-form or
slip-form paving. It is vital that the profiled joint is formed without sharp angles or steps. For
instance forming the joint by welding a shaped plate to an existing steel shutter, leaving a step
along the top of the profile, is guaranteed to cause deep spalling along the joint. When slip-
form paving the concrete miz, and the mixing, delivery and laying processes, are critical to
accurately forming the profile.
5.3.10. Joint Rotation. When construction joints are loaded by wheels trafficking directly
along joint the deflection causes the bay edge to rotate towards the adjacent bay. Contact
between the two faces can cause deep spalling. Providing load transfer does not mitigate the
problem as transferring the load to the adjacent bay results in the same total rotation. Figure
25 shows a solution to the problem, eliminating the contact between the faces. This solution
may be applied to any of the other construction joint details. It may also be applied to
daywork joints where the same problem can occur.
Table 10 Maximum Joint Spacing for Unreinforced PQC
Nominal thickness of slab (mm)
Type of joint or groove Coarse aggregate 275 or over
Expansion Limestone 36 m 48 m 54.0 m None None
Other rocks and gravels 18 m 22.5 m 31.5 m None None
Contraction or Construction Limestone 4m 4m 6.75 m 6.75 m 7.5 m
Other rocks and gravels 3m 3m 5.25 m 5.26 m 6m
Depth of sealant based on
manufacturer's width to depth
recommendations. Hot or cold poured
Joint to be filled to 5 mm below joint sealing
surface level compound
Joint finished with bullnose not Seperator
exceeding 5 mm radius Membrane
mm Compressible
Dowel bar coated with
polymeric corrosion
resistant coating
Manufactured cap
joint filler board
Figure 17 Dowelled expansion joint
Figure 18 Undowelled expansion joint with hot or cold poured sealant
Figure 19 Sawn contraction groove (not to be used for flint gravel aggregates)
Depth of sealant based on Hot or cold poured
manufacturer's width to depth joint sealing
recommendations. compound
13 mm width
Joint to be filled to 5 mm below
surface level
Depth 40 mm Membrane
or one fifth
depth of slab Manufactured
which ever is joint filler board
Induced crack
Figure 20 Formed contraction groove
Figure 21 Dowelled contraction joint with formed groove
Figure 22 Undowelled construction joint
recommendations.
Hot or cold poured
Coat of bitumen emulsion
Figure 23 Undowelled sealed construction joint
Figure 24 Dowelled sealed construction joint
3 mm closed-cell foam
stuck to face of pilot rip
(NB. this detail may be
applied to any construction
joint type)
15 mm x1 = 14.71 mm
y1 = 12.06 mm
x1 line1h line1h = 2.18 mm
line1v = 10.88 mm
15 mm line2h = h/3-2x1-2line1h
line2h 35 mm
line1v y1
Figure 25 Profiled construction joint, with former to mitigate joint rotation
5.4 BASE
5.4.1. The standard base material used by Defence Estates is drylean concrete (see
Appendix C).
5.4.2. The purpose of the DLC base is:
(i) to provide a uniform and substantially improved support to the PQC, particularly at
slab joints,
(ii) to reduce the deflection at slab joints, caused by wheel loading, and thereby help to
preserve aggregate interlock at transverse joints so that a high level of load transfer is
maintained,
(iii) to act as a protective layer to moisture sensitive soils while PQC works is in hand and
also to form a level and firm working course on which to lay the PQC,
(iv) to prevent mud pumping,
(v) to reduce the rate of deterioration if cracking of the PQC occurs,
(vi) in the case of PQC pavements for high ACN values on poor subgrades, to reduce the
required PQC thickness.
5.4.3. Chart 3 gives an increased thickness of DLC for pavements on poor subgrades
subjected to heavy multiple wheel loads. This is to allow for the additional wheel load
interaction at depth and to prevent over-stressing or poor subgrades.
5.4.4. The DLC thickness shown on Charts 1-3 is the minimum thickness required under the
PQC slab. On poor subgrades it may not be possible to achieve adequate compaction and the
necessary finished level tolerances if this thickness is laid on one layer directly on the natural
formation. In this case the DLC should be place in two layers, the first one forming a
working course on which the second can be compacted. The working course should be laid
by hand with only very light compaction. As a guide, the working course thickness should be
100mm for k = 20-30MN/m2/m and 75mm for k = 30-40 MN/m2/m. As the top layer should
not be less than 75 mm thick the minimum total thickness of DLC which can be practically
laid directly on a poor natural formation will be 175mm for k = 20-30 MN/m2/m and 150mm
for k = 30-40 MN/m2/m. Alternatively the subgrade may be improved by using a granular
sub-base (see para 3.8.4).
5.4.5. The use of a cement-stabilised material may be considered instead of drylean
concrete. Figure 26 gives equivalency factors for cement-stabilised material in relation to
drylean concrete which depends on the 7-day characteristic cube strength (5% defective).
Note that Figure 26 gives reduced equivalency factors for cement-stabilised fine-grained
materials. This is because of the greater reduction in stability that occurs in fine-grained
materials after cracks eventually propagate in the cement-bound layer; there being
substantially less aggregate interlock. Cement-stabilised material may be produced by plant
mixing or in situ stabilisation as long as the required strengths are met. In other respects, such
as surface tolerances and densities, the specification for cement-stabilised material should be
the same as that for drylean concrete.
5.4.6. Rigid pavement designs for PQC slabs on granular sub-bases, or for PQC slabs laid
directly on the subgrade are not included in this Chapter. Chapter 7 includes a procedure
using Chart 5 for evaluating these types of construction. This procedure can also be used for
assessing strengthening requirements (see Chapter 7).
5.5 SUBGRADE
5.5.1. For details of subgrade characteristics, the test method for determining k, subsoil
drainage and sub-grade compaction requirements, see Chapter 3.
5.5.2. In assessing k, the presence of work underlying layers in the soil must be carefully
considered. Heavy multiple wheel loads induce large deflection basins in a rigid pavement
giving rise to significant stress levels in the subsoil at depth. Therefore pavements for aircraft
with heavy multiple wheel gears should be designed for a conservative k which reflects the
strength of the underlying weak soils.
5.5.3. Figure 10 sets out a method of assessing subgrade improvement provided by a
granular sub-base (see para 3.8.4).
Figure 26 Cement-bound sub-bases for rigid construction
5.6 DESIGN OF UNDOWELLED AND UNREINFORCED CONCRETE PAVEMENTS
5.6.1. Separate design charts have been prepared for single, dual dual-tandem and tridem
main wheel gears, Charts 1, 2, 3 and 4 respectively; see Appendix D for the definition of these
gear types. The use of the charts requires four design parameters:
(i) Flexural strength of the concrete (see section 5.2).
(ii) The Modulus of Subgrade Reaction k. See Section 5.5 and Chapter 3 for details of
subgrade characteristics. If subgrade improvement is to be carried out as detailed in
Section 3.8 the increased k value will be appropriate for design.
(iii) The design ACN (see Section 2.6).
(iv) The frequency of trafficking – either Low, Medium or High. Chapter 4 defines these
traffic levels in terms of Coverages by the Design Aircraft. For calculating the
number of Coverages for different areas of pavement and equating the loading effects
of different aircraft see Chapter 4.
5.6.2. Having established the above parameters, Charts 1-3 are used as follows:
(i) Select the frequency of trafficking (i.e. Low, Medium, High); for High Frequency
Trafficking see Section 5.10.
(ii) Make a horizontal projection until it intersects with the appropriate design flexural
strength.
(iii) Make a vertical projection from the intersection point to the design ACN.
(iv) From this intersection point make a horizontal projection to the k = 20 line. Trace a
line parallel to the curves until it intersects the vertical projection of the appropriate k.
At this stage read off the DLC base thickness required.
(v) From the last intersection point make a horizontal projection to the right hand
ordinate. Read off the PQC thickness and round it to the nearest practical
construction increment (Defence Estates works in 25mm increments). The minimum
PQC slab thickness is 150mm. This is because thinner slabs constructed to the
minimum practical by size in the Specification would crack prematurely due to
warping effects.
See Examples 5.1 and 5.2.
5.6.3. Where aircraft regularly traffic across the longitudinal joints and load transfer is not
provided the slab should be thickened (see paragraph 5.1.2). To provide the equivalent of
good load transfer the thickness of the slab should be increased by 25%.
5.7 FULLY DOWELLED CONCRETE PAVEMENTS
5.7.1. Generally Defence Estates does not specify fully dowelled concrete pavements (i.e.
with all joints dowelled). If dowelled concrete designs are being considered the following
points should be borne in mind.
(i) Dowels induce high localised stresses in the concrete. This can lead to crushing
and/or cracking of the concrete around the dowel, particularly if the concrete has not
been properly compacted in this area.
(ii) Long-term effectiveness of dowels depends largely on their accurate alignment which
reduces their tendency to seize up. If the movement at the joint is impeded this can
lead to 'blow-ups' in hot weather or the development of cracks parallel to the joints in
unreinforced concrete in cold weather. In addition excessive differential shrinkage
between newly constructed adjacent lanes of concrete can cause the dowels across the
construction joints to become jammed, so that any subsequent contraction of the bays
would induce tensile stresses in the concrete.
(iii) Experience has shown that the load transfer effectiveness of dowels lessens with load
repetition so in many cases it is considered that to incorporate dowels does not reduce
the thickness given by the standard undowelled/unreinforced designs (see para 5.7.2).
In climates with high annual variations in temperature it may be necessary to provide
a dowelled PQC slab to maintain load transfer effectiveness (see para 5.1.4).
5.7.2. With the following exception, the design procedure for fully dowelled concrete
pavements is the same as that set out in Section 5.6 for undowelled/unreinforced concrete
using Charts 1,2 and 3. Table 11 gives reductions in PQC slab thickness (determined from
Chart 1, 2 and 3) for dowelled concrete pavements less than 250mm thick.
5.7.3. For a fully dowelled concrete pavement dowels should be provided at all
construction, contraction and expansion joints to the requirements set out in Table 12. They
should be installed at mid-depth of the slab. See Section 5.3 for joint layout requirements and
5.8 JOINTED REINFORCED CONCRETE PAVEMENTS WITH DOWELS
5.8.1. Generally Defence Estates does not specify this type of construction which provides
little or no gain in structural performance and is more likely to present long term maintenance
problems than the standard undowelled unreinforced rigid pavement designs.
5.8.2. Jointed reinforced concrete pavement is usually constructed in long bays giving fewer
transverse joints. The long bays will tend to develop one or more transverse cracks due to
shrinkage and differential temperature stresses. The function of the reinforcement is to hold
the cracks tight and minimise deterioration. Failure of jointed reinforced concrete pavement is
generally by spalling of the transverse cracks. Monitoring and maintenance of the cracks can
be problematic, including increased disruption to aircraft operations. The risk of premature
failure is greater in this type of pavement than in jointed unreinforced concrete constructed in
square bays.
Table 11 Design Thicknesses for Dowelled Constructions
PQC slab thickness requirement Allowable reduction in PQC slab
from Charts 1, 2 and 3 (mm) thickness for dowelled
construction (mm)
equal to or greater than 250 0
Table 12 Dowel Size Requirements
PQC Slab Dowel Total Dowel Spacing (mm)
Thickness Diameter (mm) Length (mm)
175-200 25 450 300
225-275 30/32 450 300
5.8.3. Incorporating light reinforcement into a concrete slab to control shrinkage and
warping cracks does allow a considerable increase in the spacing of transverse contraction
joints. The quantity of steel required varies from 0.05% to 0.3% of the cross sectional area of
the slab and should be placed in the upper part of the slab with at least 50mm cover. This
does not improve the flexural strength of the slab and therefore the design thickness
requirements are the same as those for dowelled PQC slabs (see para 5.7.2).
5.8.4. The areas of reinforcement required in both the longitudinal and transverse directions
should be calculated from the following formula.
LC f Wh
As =
2 Fs
Where As = area of steel in mm2/m width of slab
L = distance between contraction joints (longitudinal direction) or construction joints
(transverse direction) in metres. The spacing of contraction joints should not exceed
23 metres.
W = weight of concrete in kN/m3
h = slab thickness in mm
Fs = working stress in reinforcement in N/mm2 (Fs = 0.75 yield stress)
Cf = coefficient of subgrade resistance to slab movement. This is dependent on the base
material and the slab dimension. For construction with a DLC base and a polythene
separation layer a value of 1.5 can be taken.
'As' should not be less than 0.05% of the cross sectional area of the concrete. Reinforcement is
usually in the form of mesh. Longitudinal laps should be at least 30 times the diameter of the
wire. Transverse laps should be not less than 150 mm or 20 times the transverse wire diameter,
whichever is the greater.
5.8.5. Dowels should be provided at construction, contraction and expansion joints in
accordance with the requirements set out in Table 11. Figure 27 shows a typical longitudinal
section through a jointed reinforced concrete pavement. See Section 5.3 for joint details.
Figure 27 Typical longitudinal section through jointed reinforced concrete pavement
5.9 CONTINUOUSLY REINFORCED CONCRETE PAVEMENTS
5.9.1. In Defence Estates' experience the use of continuously reinforced slabs has not
resulted in more cost-effective pavements. Continuously reinforced slabs of reduced
thickness generally suffer early spalling at shrinkage/warping cracks. The spalling is caused
by a combination of frost damage, excessive working of cracks from repetitive wheel loading,
jet blast and high tyre pressure.
5.9.2. Defence Estates does not have an established procedure for this type of construction.
However, where it is being considered the following points need to be remembered:
(i) To achieve a consistent and controlled development of cracks the amount of
longitudinal reinforcement required is likely to be between 0.5-0.7% of the cross-
sectional area of the concrete.
(ii) To avoid excessive deflections and consequent working of cracks due to trafficking
by heavy wheel loads, only a modest saving in PQC thickness should be considered. A
reduction in excess of 15% on the dowelled concrete designs may be unwise.
(iii) Special attention needs to be given to compaction around and under the reinforcement
particularly with the thicker constructions and their correspondingly higher steel
contents.
(iv) The advantage of improved ridability is perhaps not significant in relation to
unreinforced PQC with sawn contraction grooves.
5.10 DESIGN FOR HIGH FREQUENCY OF TRAFFICKING
5.10.1. The High Frequency design level is nominally 250,000 Coverages by the Design
Aircraft (see para 4.6.2). As Defence Estates lacks both experience and research data on
pavement performance at this level of use, the construction thickness requirements have been
extrapolated beyond proven designs. On this basis the PQC slab thickness for the High
Frequency design is 10% greater than that required for the Medium Frequency design.
RIGID DESIGN EXAMPLES
Example 5.1 A rigid pavement is required for nre stands at a small municipal airport, used principally for charter
traffic. The majority of departures are Boeing 737-800s.
Guide Reference
1. SUBGRADE: Soil Survey shows k = 30 MN/m /m
Appendix B a) ACN
B737-800 79,243 49.3 51.8 54.2 56.1 Dual 1
Appendix B b) Main Wheel Gear: Dual
Section 4.9, Table 6 c) Pass–to-Coverage Ratio: 3.2
3. AIRCRAFT USE: Expected departures are 3 Boeing 737-200s per day.
Para 4.7.3 4. DESIGN LIFE: 30 years.
5. FREQUENCY OF TRAFFICKING
(30 × 365 × 3)
Section 4.9 No of Coverages = = 32850
6. DESIGN CRITERIA
a) ACN: from 2a above, using linear interpolation between subgrade values and rounding to
the nearest integer, ACN = 55
Para 4.6.2, Table 5 b) Frequency of Trafficking: Low
Para 5.2.3 7. CONCRETE STRENGTH: 4.5 N/mm mean flexural at 28 days
Chart 2 8. REQUIRED CONSTRUCTION: 345 mm Pavement Quality Concrete
150 mm Rolled Drylean Concrete
Para. 5.6.3 Note: if the stands are constructed so that the butt longitudinal joints are normal to the
stand centreline, the joints should be provided with load transfer (e.g. dowels) or the slab
thickness increased by 25%. i.e.
430 mm Pavement Quality Concrete
Para 2.4.2 9. CLASSIFICATION:
a) Subgrade Category: Low (C)
b) PCN:ACN of the Boeing 737-800 on a Rigid Low Subgrade =54.2
c) Pavement Type: Rigid (R)
d) Tyre Pressure Limitations: No limitations on a concrete surface (W)
e) PCN 55/R/C/W/T
Example 5.2 Design a rigid pavement for a new parallel taxiway at an international airport used by a wide range
of aircraft.
Appendix B a) ACNs, Main Wheel Gears and Pass-to-Coverage Ratios.
Section 4.9 Table 6
3. AIRCRAFT USE: Proposed aircraft use shown in Table 8 (Page 38).
5. DESIGN CRITERIA:
a) ACNs of user aircraft calculated at k = 50 are shown in Table 8.
Section 2.6 The Design Aircraft is the Boeing 777-300ER. Design ACN = 103.
Para 4.6.2 Table 5 b) The Mixed Traffic Analysis is shown in Table 8. The total coverage is
76,775 and therefore Medium Frequency Trafficking is used.
6. CONCRETE STRENGTH: 5 N/mm mean flexural strength at 28 days.
b) PCN is the ACN of the Boeing 777-300ER on a Rigid Low Subgrade = 109.3
d) Tyre Pressure Limitations: No limitations on a concrete surface (W).
e) PCN 110/R/C/W/T.
DMG 27 6 Flexible Pavement Design
6 Flexible Pavement Design
6.1.1. For over 50 years Defence Estates' policy has been to construct 'flexible' pavements
with either cement or bitumen-bound bases. While an unbound base or sub-base can provide
the desired performance, the strict grading requirement together with the need for a high a
consistent level of compaction throughout can result in construction problems and unreliable
performance. This is particularly true on wet sub-grades, common in the UK. These
disadvantages are worsened in the case of pavements subject to regular trafficking by heavy
and high tyre pressure aircraft. On the other hand pavements with bound bases permit the use
of less stringent specification and give structural benefits over conventional flexible
pavements, allowing a saving in thickness over the granular base and sub-base requirements.
The bound base designs provide an economic and practical solution and most significantly
give reliable performance.
6.1.2. Sometimes, the availability of good quality materials, with or without self-cementing
properties, can make convention granular base and sub-base construction a practical and
economic choice. However, this type of construction is only recommended on good dry
subgrades, where it is possible to achieve the necessary high level of compaction. As the
aircraft weight increases there is an increasing possibility that failure to achieve uniform
compaction over the whole pavement area will lead to premature rutting due to consolidation
of the granular materials. Thus for heavy aircraft (ACN > 50) the granular materials should
have a well proven record of performance an the designer may consider proof rolling the base
course before laying the bituminous surfacing. The required subgrade conditions are more
likely to be found in certain overseas locations than in the UK. For these reasons flexible
constructions with unbound bases/sub-bases are not included on the design chart for this
Chapter (Chart 4). Chapter 7 includes a design and evaluation chart for these pavements.
6.2 SURFACING
6.2.1. The standard flexible pavement designs to Chart 5 require a minimum surfacing
thickness of 100mm. This will usually be made up to a 40mm surface course on a 60mm
binder course. A 20mm thick open macadam friction course is not considered to be a
structural layer and therefore should not be counted as part of the 100mm surfacing.
6.2.2. The principal bituminous surfacing materials used by Defence Estates are Marshall
asphalt surface and base course or hot rolled asphalt surface course on macadam binder
course. These materials should meet the specialist performance required of airfield
pavements:
(i) High stability to withstand the shear stresses induced by heavy wheel loads and high
tyre pressures.
(ii) Good ridability.
(iii) A durable hard-wearing weatherproof surface free from loose material and sharp
edges which might endanger aircraft.
6.2.3. To provide good wet weather braking characteristics on a runway an additional
surface treatment is usually specified. The standard surface treatments used by Defence
Estates are friction course, coarse slurry seal and grooving; the choice of treatment depends
on the availability of materials, site geography and performance requirement. A surfacing of
porous macadam friction course with cross-falls of 1.5%, gives runways an excellent all-
weather friction characteristic. The friction course must have an underlay of at least 100mm
of high quality asphalt. A friction course is not recommended in areas where the pores are
liable to silt up (e.g. by wind blown sand) or to freeze in extreme winter conditions (e.g.
where temperatures of less than -10ºC can be expected to last for periods o greater than 24
hours). Surface dressing should not be use for jet aircraft operations because of its tendency
to loose stones which present a FOD (foreign object damage) hazard. High speed taxiways
may also need treatment, either with a course slurry seal or by grooving.
6.2.4. For temperate climates, Table 13 gives guidance on the suitability of various
surfacing materials. The stability of hot rolled asphalt on a macadam base course is adequate
for the frequencies of trafficking and tyre pressures given in Table 13. The frequencies of
trafficking assumed in the table apply to a single user aircraft. Where mixed traffic use is
envisaged the aircraft with the highest category tyre pressure, not necessarily the Design
Aircraft, should be considered at the frequency of trafficking appropriate to that tyre pressure
category. For pavements in hot climates the Marshall asphalt specification should be used.
For guidance on high tyre pressure aircraft operations on blacktop surfacings see Chapter 8.
Table 13 Suitability of Surfacing Materials (Temperate Climates)
Frequency of Trafficking
W (> 1.5 Mpa) MA MA MA
X (up to 1.5 MPa) HRA/MB MA MA
Y (up to 1.0 MPa) HRA1/MB HRA/MB MA
Z (up to 0.5 MPa) HRA1/MB HRA/MB HRA/MB
MA – Marshall asphalt or alternatively Marshall Dense Tar Surfacing surface course on Marshall asphalt or
Marshall DTS binder course.
HRA/MB – Hot rolled asphalt on Macadam binder course.
Note 1 – Dense Tar Surfacing is acceptable as an alternative.
6.2.5. Marshall asphalt is a more highly controlled and consistent material than hot rolled
asphalt and is to be preferred wherever a contract is large enough to cover the enhanced level
of supervision and testing effort involved. To aid proper control and make sure that the
performance criteria will be met, Marshall asphalt should, wherever physically and
economically possible, be mixed on site.
6.2.6. If there is a requirement for a fuel resistant pavement surface such as for a runway
end or apron a tar-based slurry seal can be used to provide some resistance, although it should
be noted that they are susceptible to mechanical damage, especially early in their life, and are
not resistant to hydraulic fluid spillage.
6.2.7. Other surfacing materials which have been use to a limited extent by Defence Estates
include grouted macadam, concrete blocks and Stone Mastic Asphalt. Grouted macadam and
concrete blocks are fuel resistant and can be used on aprons, although fuel can penetrate the
joints between blocks to reach lower layers. In many cases grouted macadam has not given
good long-term performance, and the performance of Concrete Block Surfacing has been
variable. Concrete Block Surfacing should not be used on runways. Research shows that
Concrete Block Surfacing has little structural capacity until the blocks rotate sufficiently to
come into contact and develop interlock. Because of the surface tolerances required for
airfield pavements enough movement to create interlock is unlikely, and a Concrete Block
Surfacing will add little to the strength of a pavement. Concrete Block Surfacing should be
treated as being equivalent to 50mm of Marshall Asphalt or less. Stone Mastic Asphalt has
shown considerable promise, although long-term durability has not been fully proven.
Structurally it should be treated as equivalent to Marshall Asphalt.
6.3.1. The standard designs in Chart 5 require a bound base construction from the underside
of the surfacing down to the subgrade or improved subgrade (see Section 3.8). Therefore a
conventional unbound sub-base is not needed.
6.3.2. The bound base materials normally specified by Defence Estates are high strength
drylean concrete (Type FH DLC), and Marshall Asphalt. The thickness requirements on the
design chart can be made up of any one or a combination of these materials. However having
regard to stringent compaction and laying requirements for Marshall asphalt, DLC should
normally be used as the first layer of bound base material on the formation / subgrade.
6.3.3. The design model for Chart 5 is based on that used in Reference 11 for the design of
standard flexible pavements comprising bituminous surfacing materials on bound bases.
However, Chart 5 incorporates higher equivalency factors than those used in Reference 11 to
take account of experience gained since that time and more recent full scale testing of high
strength drylean concrete bases (Type FH DLC). Further details on materials and design
rationale are provided in Appendices C and F.
6.3.4. When laid on low strength subgrades (CBR less than 6%) it may be difficult if the
initial layers of Type FH DLC are laid directly on the subgrade to compact the layers
sufficiently to obtain the required minimum strength and density. In this situation a working
course should be provided before laying the initial layer, either an unbound capping or a
sacrificial working course of drylean concrete.
6.3.5. Chapter 7 deals with the evaluation and strengthening of existing pavements
incorporating DLC bases laid to the Defence Estates' specification prior to 1989 and now
designated Type F DLC, and also of pavements incorporating asphalts, including Hot Rolled
Asphalt and Macadam Base Course. Details of these materials are provided at Appendix C.
Chart 5 does not apply to these materials.
6.3.6. Chapter 7 also deals with pavements incorporating unbound granular base and sub-
base layers; Chart 5 does not apply to them.
6.3.7. The use of DLC as the principal base material results in a pavement with a rigid mode
of behaviour which gradually changes to a flexible mode as the stiffness of the pavement
reduces after cracks form in the cement-bound layer. Experience shows that DLC cracks into
irregular shaped bays, and the cracks eventually reflect through the surfacing. Where the
DLC is thick the resultant irregular bays tend to be large giving rise to wide cracks subject to
considerable movement. A requisite thickness of blacktop overlay should be provided; it will
substantially delay reflective cracking in the surfacing and postpone the need for widespread
maintenance with consequent loss in rideability, drainage and friction characteristics. For
minimisation of reflection cracking the thickness of bituminous material over the DLC should
be in accordance with Defence Estates Design & Maintenance Guide 3352.
6.3.8. The use of cement stabilised material instead of DLC may be considered as follows:
(i) Cement-stabilised fine or medium-grained material is unlikely to provide long-term
load-spreading characteristics comparable to DLC. Following the eventual
propagation of cracks in a cement-stabilised layer a fine-grained material will provide
minimal aggregate interlock with a consequent loss of stability and load distribution
properties. Therefore, cement-stabilised fine or medium-grained material of which
more than 60% passes the 5mm sieve should only be considered as a sub-base. See
Chapter 7, para 7.4.2.3 for cement-stabilised sub-bases.
(ii) To give performance comparable to a DLC base, strength characteristics of the
cement-stabilised material should comply with the minimum requirements for either
Type FH DLC or Type F DLC , as described in Appendix C, for use with Chart 5 or
Chart 7 respectively. Cement-stabilised materials are more likely to meet Type F
DLC requirements, in which case para 6.3.6 should be referred to. The strength
requirements for DLC are unlikely to be achieved with any degree of consistency
with an in situ stabilised soil.
6.4.1. For details of subgrade characteristics, the CBR test method, subsoil drainage and
subgrade compaction requirements, see Chapter 3.
6.4.2. Flexible pavements are more sensitive to sub-grade characteristics than rigid
pavements, making the assessment of a representative design CBR more critical than that of a
design k. On most sites the soil types at the formation levels are likely to vary. Where the
variation occurs in distinct and large areas of the site it may be feasible to consider separate
flexible pavement designs. However, if such variation occurs randomly, then a single design
based on the limiting soil type (i.e. lowest CBR) may be the only realistic solution. If the
change in subgrade support characteristics is considerable, the possibility of differential
settlement and densification, particularly in the transitional areas, may need to be considered.
6.4.3. The presence of weak layers in the subsoil must be carefully considered in assessing
the design CBR. This is particularly important when the pavement is to be designed for heavy
aircraft with multiple wheel main gears which induce significant stress levels at considerable
depths below the pavement surface, as reflected in the ACNs for poor subgrades. Para 3.4.7
sets out a procedure for assessing the design CBR when there is a weak underlying layer in
the subsoil.
6.4.4. For assessing the subgrade improvement provided by a granular fill see Section 3.8.
6.4.5. The maximum CBR value on Chart 7 is 20%. This is intended to limit the stresses
and strains in the bound base materials by imposing a minimum pavement thickness for a
given aircraft loading, i.e. the pavement thickness required for CBR 20%. The same subgrade
scale is shown on Chart 8, but in this case it is also possible to design for CBR 30% by using
the Y-axis only.
6.5 DESIGN OF FLEXIBLE PAVEMENTS WITH BOUND BASES
6.5.1. Chart 5 has been prepared for single, dual, dual-tandem and tridem main wheel gears;
see Appendix D for the definition of these gear types. The use of the Chart requires three
design parameters:
(iii) The CBR of the subgrade – see Section 6.4 and Chapter 3 for details of subgrade
characteristics. If subgrade improvement is to be carried out as detailed in Section
3.8 the increased CBR value will be the appropriate design value.
(iv) The design ACN (see Section 2.6).
(v) The frequency of trafficking – either Low, Medium or High. Chapter 4 defines these
of different aircraft, see Chapter 4.
6.5.2. Having established the above parameters the Chart is used as follows:
(i) Select the frequency of trafficking (Low, Medium or High); for High Frequency
Trafficking see Section 6.6.
(ii) Select the ACN scale appropriate to the Design Aircraft's main wheel gear type.
Enter the Chart with the design ACN and make a horizontal projection until it
intersects the vertical projection of the appropriate CBR.
(iii) From the intersection, trace a line parallel to the curves until it intersects the right
hand ordinate. Read off the thickness of bound base material required. The
minimum surfacing thickness required on top of the base is 100mm. See Section 6.2
for details of surfacing and Section 6.3 for details of bound base construction.
6.6 HIGH FREQUENCY OF TRAFFICKING
6.6.1. The High Frequency design level is nominally 250,000 Coverages by the Design
pavement performance at this level of use, the construction thickness requirements have
extrapolated beyond proven designs. On this basis the required thickness of bound base
material for the High Frequency design is increased to provide a total pavement thickness
which is 8% greater than that required for the Medium Frequency design.
FLEXIBLE DESIGN EXAMPLES
Example 6.1 Design a flexible pavement using a Type FH Bound Base Material for a taxiway at small municipal
airport used principally for charter traffic. The majority of departures are Boeing 737-800s.
1. SUBGRADE: Soil Survey shows CBR 5%.
Section 4.9 Table 6 c) Pass-to-Coverage Ratio: 2.1
Section 4.9 5. FREQUENCY OF TRAFFICKING
No of Coverages = = 10429
a) Design ACN: from 2a above, using linear interpolation between subgrade values and
rounding to the nearest integer ACN = 52
Para 4.6.2,Table 5 b) Frequency of Trafficking: Low
Chart 5 7. REQUIRED CONSTRUCTION: 100mm Surfacing
375mm High Strength Bound Base Material
Para 6.3.7 If most of the Bound Base Material is to be Type FH Drylean Concrete, a minimum thickness of
bitumonous material should be provided to restrict reflective cracking. Defence Estates Design &
Maintenance Guide 33 suggests that 220mm is required for a long-life pavement.
e.g. 40mm Marshall Asphalt Surface Course
60mm Marshall Asphalt Binder Course
60mm Marshall Asphalt Base Course
255mm TypeFH Drylean Concrete.
a) Subgrade Category: Low (C).
b) PCN is the ACN of the Boeing 737-200 on a Flexible Low Subgrade =51.
c) Pavement Type: Flexible (F)
Table 13 d) Tyre Pressure Limitations: No limitations for Marshall Asphalt
surfacings (W).
e) PCN 51/F/C/W/T.
Example 6.2 Design a flexible pavement using a Type F Bound Base Material for a runway at an international
airport used by a wide range of aircraft.
1. SUBGRADE: Soil Survey shows CBR 10%
Appendix B, Section 4.9 2. AIRCRAFT DATA:
a) ACNs, Main Wheel Gears and Pass-to-Coverage Ratios
Para 4.7.3. 4. DESIGN LIFE: 20 years.
5. DESIGN CRITERIA.
a) ACNs of the user aircraft calculated at CBR 10% are shown in Table 9.
The Design Aircraft is the Airbus 340-500. Design ACN = 82
Para 4.6.2 Table 5 b) The Mixed Traffic Analysis is shown in Table 9. The total coverages are 33,442,
therefore Medium Frequency Trafficking is used.
Para 6.3.7 If most of the Bound Base Material is to be Type F Drylean Concrete, a minimum thickness of
Maintenance Guide 33 suggests that 220 mm is required for a long-life pavement, if the
Surface Course is grooved.
Para 2.4.2 7. CLASSIFICATION
a) Subgrade Category: Medium (B)
b) PCN is the ACN of the Airbus A340-500 on a Flexible Medium Subgrade = 82
Table 13 d) Tyre Pressure Limitations: None (W)
e) PCN 82/F/B/W/T
DMG 27 7 Pavement Evaluation and Strengthening
7 Pavement Evaluation and
7.1 METHODS OF EVALUATION
7.1.1. For various reasons it may be necessary or desirable to reappraise the bearing
capacity of a pavement. This would apply in any of the following circumstances:
(i) A mid/end of life reassessment of the pavement to plan future maintenance work
and/or rehabilitation.
(ii) The pavement has been disused for some time and is to be rehabilitated.
(iii) The pavement is to be strengthened for regular use by heavier aircraft.
(iv) After several years service it has become apparent that the pavement's strength has
been reduced and it is showing signs of premature fatigue.
(v) There has been a change in the classification system.
7.1.2. Evaluation is carried out by 'reverse design', with pavement inputs determined by a
site investigation. There are no in situ test methods that directly measure pavement strength.
7.1.3. Reverse design works best when used with a pavement management system which
includes periodic maintenance inspections and records of construction, subgrade
characteristics and aircraft movements. Defence Estates has used reverse design extensively
for over 35 years. The method, as presented in this guide, requires the existing pavement to
be structurally equated to one of the standard constructions included in the design and
evaluation Charts 1-8. Using the Charts in reverse, the strength of the pavement can be
determined.
7.1.4. Any evaluation must be weighted by consideration of factors such as the pavement
condition, records of its operational use, future operational requirements and an estimate of
the pavement's residual fatigue life which must necessarily be subject to engineering
judgement.
7.1.5. If strengthening is required an overlay thickness can be calculated using the
procedures set out in Sections 7.5 to 7.10
7.2 INVESTIGATIONS FOR EVALUATION AND STRENGTHENING
7.2.1. Reverse design requires details of pavement construction and condition, and possibly
a record of its use. Section 7.3 provides an overview on assessment of pavement condition
and residual fatigue life. An evaluation is then made using the methods described in Section
7.2.2. Construction records are not always reliable and rarely give any indication of material
condition. A site investigation to determine the pavement inputs for reverse design is
therefore usually necessary. The investigation will:
(i) ascertain the existing construction,
(ii) ascertain the condition of the pavement,
(iii) ascertain material condition.
7.2.3. A detailed description of site investigation and interpretation methods for airfield
pavements in given in Appendix I.
7.3 PAVEMENT CONDITION AND RESIDUAL LIFE
7.3.1. Accounting for the Pavement Condition
7.3.1.1 Structural deterioration of pavement layers will reduce their load bearing capacity and
suitable allowances may have to be made in evaluation or overlay design. The formulae for
design and evaluation of composite and multiple slab pavements (Sections 7.9 and 7.10)
include specific Condition Factors which take account of cracking of underlying concrete
slabs. In some other cases the effective pavement thickness can be reduced by an analysis of
fatigue consumption (para 7.3.2.2). If the pavement is showing signs of serious structural
distress materials may be downgraded to ones of a lower structural value (paras 7.3.1.5 and
7.3.1.6). The following paragraphs describe how to determine Condition Factors and when
they should be used; advice on assessing Condition Factors by in situ testing is given in
Appendix I.
7.3.1.2 Normal deterioration caused by live loading or climatic effects is built into the
methods presented in this document. An airfield operator expects a pavement with a given
strength when new, and that the pavement will have the same strength until the end of its life
before major maintenance. The operator does not expect to change aircraft use in the middle
of the pavement life because normal deterioration has occurred. Unless deterioration is
excessive the pavement classification should not be altered during the design life, and future
performance must be defined by considering together the initial strength, the residual fatigue
life and the expected life of the pavement materials. If a mid/end of life evaluation is being
made to obtain a classification or to estimate a pavement's residual fatigue life, condition
factors should not be applied to the most recent layers of construction. The condition factors
for other layers should not generally be changed from those used in the design of the last
strengthening, unless it has been agreed with the operator that excessive deterioration is best
expressed by a decrease in PCN rather than by a decreased pavement life.
7.3.1.3 If a pavement is being strengthened, the overlay thickness requirement should be
calculated on the basis that it provides a renewed design life (see Section 4.6). The evaluation
of the existing pavement should therefore reflect its current condition. This will generally
necessitate making due allowance for the deterioration of all layers in the existing
construction.
7.3.1.4 If deterioration is excessive the likely causes are overloading, fatigue, poor quality
construction or reduction in subgrade strength due to a change in moisture content. The
reason for the failure should be established and an evaluation of the various layers of
construction made. The pavement should be overlaid or reconstructed to restore serviceability
at the current classification. Where adjacent level constraints are critical reconstruction may
be the only choice.
7.3.1.5 Structural failure of concrete pavements is indicated by cracking of bays in the wheel
path (see Appendix F). The following points should be considered:
(i) Condition Factors related to cracking of concrete are given in Table 18 and should be
applied to slabs that are part of a composite construction, or to concrete slabs that are
to be overlaid, including all layers of existing multiple slab constructions (see
Sections 7.9 and 7.10). The Condition Factors should not be applied to the top layer
of an existing multiple slab construction for a mid/end of life evaluation.
(ii) If the state of the pavement is significantly worse than the failure condition described
in Appendix F the concrete is down-graded to an equal thickness of drylean concrete.
Alternatively localised areas of severe failure could be reconstructed and the
pavement as a whole assigned an appropriate condition factor.
7.3.1.6 Structural failure of a flexible pavement is indicated by rutting with associated
heaving and/or cracking in the wheel paths (see Appendix F). (Note: Rutting is sometimes
due to compaction of the pavement layers or subgrade by aircraft operations which while
giving rise to a serviceability problem, does not cause a loss of bearing capacity). Provided a
pavement is not showing signs of a severe failure it should still possess enough residual
strength to form an integral part of a strengthened construction. In developing a design
concept for rehabilitation of a failed flexible pavement the following points need to be
considered:
(i) If the failure has occurred suddenly or unexpectedly or there is uncertainty in
assessing the condition of the various layers of construction the engineer may
consider it wise to do deflection tests to reassess the behaviour of the pavement.
Tests in failed areas and in areas adjacent to the wheel tracks may indicate whether
pavement strength has been significantly reduced.
(ii) Severe shear failure of a pavement (i.e. rutting and heave in excess of twice that given
as the failure criterion in Appendix F) is likely to result in loosening of the pavement
construction and/or subgrade. In these circumstances either reconstruction, or
recompaction and reappraisal of material strengths should be done.
(iii) If small areas of severe rutting have occurred, the pavement may be reconstructed
locally and assigned the same residual strength as the sections which are deteriorating
normally.
(iv) A surface showing appreciable cracking but with little or no ravelling of the cracks
should not be considered as being any better than a granular base (see para 7.8.3).
Again, if the crack pattern along the wheel tracks is extensive and well defined with
ravelling along the cracks, the surfacing should not be considered as being any better
than a granular sub base (see para 7.8.4), or it should be removed before overlaying.
(v) Structural failure resulting from shear failure within unbound base and sub-base
layers or within the subgrade can lead to substantial decreases in the in situ CBR
values of the layers. Ideally a failed base should be regraded and recompacted but
this will be difficult if there is a thick layer of bituminous materials overlying it.
Recompaction of sub-bases and the subgrade will only be possible if the pavement is
completely reconstructed. The alternative to recompaction is to do in situ CBR tests
in the failed and unfailed areas and then use the lowest results for overlay design.
This would normally lead to the downgrading of granular bases and sub-base to sub-
base and capping layer respectively and to a reduced CBR value being taken for the
(vi) If subgrade shear failure is due to the reduced load bearing characteristics of a
degraded bound base the structural value of the bound base will also need to be
reconsidered. Depending on its density and grading, an aged and embrittled
bituminous bound base will be reassessed as a granular base course or granular sub-
base. An extensively cracked drylean concrete base will be worth little more then a
granular sub-base.
(vii) When an underlying concrete slab has undergone extensive multiple cracking with
subsequent shear failure of the subgrade the broken slabs should be equated to an
equivalent thickness of lean concrete base. (See Table 18). As in (vi) above, the
subgrade should also be reassessed.
7.3.2. Estimating Residual Fatigue Life
7.3.2.1 Records of aircraft use are essential for residual fatigue life calculations, and are very
useful when designing pavement overlays where adjustments can be made to existing
pavements thicknesses to allow for past fatigue. They are also helpful in assessing the
classification of a pavement, since in conjunction with the pavement condition they provide a
good indication of the integrity and strength of the pavement. The greater the previous use
the more significant this factor becomes in the evaluation. Care should be taken to ensure that
too great a reliance is not put on the evidence of a few movements.
7.3.2.2 If a pavement has been regularly used for several years by aircraft at or approaching
its PCN it must be considered that a portion of the structural fatigue life has been used up. A
mid/end of life appraisal of a pavement will indicate its remaining structural life. The first
step is to evaluate the pavement. As explained in paragraph 7.3.1.2 condition factors for the
most recent layers of construction need not be incorporated into a reverse design and
generally condition factors for other layers should not be altered from those used for the
previous strengthening design. The next step is to use the records of aircraft movements to
assess the past fatigue, in terms of an aircraft with an ACN equal to the PCN of the pavement.
The procedures for mixed traffic analysis detailed in Chapter 4 should be used for this
purpose. The residual fatigue life is the difference between the design Coverage level for the
evaluation and the equivalent number of Coverages by the aircraft to date (see Example 7.9).
7.4 EVALUATION BY REVERSE DESIGN
7.4.1. Procedure
7.4.1.1 Where an existing pavement can be equated to one of the standard constructions in
Chapters 5 and 6 reverse design is carried out using the design Charts 1-8. For existing
flexible pavements equating to the standard construction described in Chapter 6 but with Type
F DLC Chart 7 can be used in the same way as Chart 5 (para 7.4.2.8). In other cases use
Charts 5, 7 and 8 as described in Section 7.6 and 7.7. The subgrade strength, pavement
thickness, material properties and the relevant frequency of trafficking are determined and
then entered on the charts to give the ACN of the Design Aircraft. The ACN is then modified
to allow for the difference between the actual subgrade strength and one of the standard
subgrade categories in the ACN-PCN method.
7.4.1.2 If the pavement construction is not similar to one of those shown on the relevant
Charts then equivalency factors are used to convert materials in the actual pavement to an
equivalent thickness of one of the standard materials assumed in the Charts. The methods of
equating various types and combinations of construction to an appropriate type covered by the
Charts are set out in and, Figure 29 and Figure 30. The procedures for converting composite
and multiple slab constructions to equivalent standard ones are set out in Sections 7.9 and
7.4.1.3 Pavement condition is dealt with either by using residual life calculations to allow for
past fatigue and give an equivalent pavement thickness (see Example 7.9) or, in the case of
concrete slabs, by using 'Condition Factors' as set out in Table 18. An explanation of
determining Condition Factors and when Condition Factors should be used is given in section
7.4.2. Pavement Constructions
7.4.2.1 Many existing pavements may not directly correspond with the construction assumed
for the design charts. Techniques for dealing with them are described below.
7.4.2.2 Subgrade improvement: Capping layers under flexible pavements and granular sub-
bases under rigid pavements are allowed for by calculating an effective subgrade strength
using the techniques described in Chapter 3. For capping layers, evaluation will be an
iterative process as it is first necessary to estimate the ACN of the Design Aircraft from which
an effective CBR at the formation is calculated. The effective subgrade strength is the value
entered on the charts.
7.4.2.3 Cement-stabilised soils: Cement-stabilised soils under rigid pavements should be
converted to an equivalent thickness of drylean concrete (see para 5.4.5 and Figure 26). In
flexible pavements cement-stabilised gravels and crushed rock can be treated as a bound base
material (see para 6.3.4), otherwise cement-stabilised soils should be converted to an
equivalent thickness of granular sub-base, using the equivalency factors given in Table 17,
and Chart 8 for the evaluation.
7.4.2.4 Excess or deficiency of Type R DLC under rigid pavements: Reverse design using
Charts 1-4 implies that a certain thickness of Type R DLC base exists for a given concrete
slab thickness and subgrade strength. If this thickness does not exist in practice the drylean
concrete thickness can be altered by converting PQC thickness to drylean concrete thickness
or vice versa, using an equivalency factor for PQC to drylean concrete of 3. The maximum
deficiency of drylean concrete is 100mm (i.e. not more than 33mm of PQC should be
converted to drylean concrete) and the maximum excess is 50mm (i.e. not more than 50mm
drylean concrete should be converted to PQC). Any additional thickness of drylean concrete
should be ignored. If the PQC-drylean concrete conversion is insufficient to give the required
thickness of drylean concrete the existing drylean concrete should be ignored.
Table 14 Reverse design and overlay design procedures
Construction Pavement Type Procedure Chart
Reverse Design Overlay Design
Rigid If base thickness greater or less than See Composite or Multiple Chart 1, 2, 3, 4
(PQC on Type R DLC) the design chart requirement para Slab (7.1)
Rigid Para 7.4.2.2 See Composite or Multiple Chart 6
(PQC on the subgrade or on an unbound sub- Section 7.6 Slab (7.2)
base) Effective k on a granular sub-base para
Rigid Para 7.4.2.5. Para 7.5.4 Chart 1, 2, 3, 4
(PQC on a bituminous base)
Flexible Para 7.4.1.1 Para 7.5.2 Chart 5
(100 mm Marshall Asphalt surfacing on Type Capping layer Para 7.4.2.2 (7.4, 7.9)
FH DLC or Marshall Asphalt Bound Base Effective CBR on a capping layer para
Material, on an optional capping layer) 3.8.2
(100 mm Marshall Asphalt surfacing on Type F Section 7.7 Section 7.7
DLC or Bituminous Bound Base Material, on an Capping layer Para 7.4.2.2
optional capping layer) Effective CBR on a capping layer para
Flexible Para 7.4.2.9, Para 7.5.2 Chart 8
(100 mm Marshall Asphalt surfacing on Para 7.4.2.10 Section 7.8
unbound granular base and sub-base, or a Section 7.8
combination of Bound Base Material and Capping layer Para 7.4.2.2
unbound materials, on an optional capping Effective CBR on a capping layer para
layer) 3.8.2
Case Composite Para 7.4.2.11 Para 7.5.3 Type 1
β ≤ 0.5 Type 1 Section 7.9 Section 7.9 Case 1, 3, 4 Chart 6
β≥1 Type 2 Case 2 Chart 1, 2, 3, 4
0.5 < β < 1 Type 3 (7.3)
Case 1, 2, 4 Chart 5 or 7
Case 3 Chart 8
Case 1, 4 Chart 6, and 5
Case 2 Chart 1, 2, 3, 4,
and 5 or 7
Case 3 Chart 6 and 8
β = t / he (7.8)
Composite Para 7.4.2.12 Para 7.5.3.3 Chart 5
(Crack and Seat)
Rigid Para 7.4.2.13 Para 7.5.5 Chart 1, 2, 3, 4, 6
(Multiple Slab) Section 7.10 Section 7.10 (7.10)
(Multiple Slab with cement-bound or bitumen- Section 7.10 Section 7.10 (7.2)
bound inter-layer)
Pavement Quality Concrete Crack and Seat Type R or FH DLC or Type F DLC or equivalent
equivalent cement-bound base cement-bound base
Marshall Asphalt Bituminous Bound Materials Granular base and / or sub- Granular fill beneath
other then Marshall Asphalt base flexible pavements
Figure 28 Flow charts for the evaluation of airfield pavements
Figure 29 Reverse design for rigid pavements
Flexible Capping Layer?
Estimate the CBR on the
Capping Layer (para 7.4.2.2)
Concrete Layers?
Crack & Seat?
Yes Ye
Equivalent to Type F DLC? Cement-stabilised soil?
Assume equivalent to Type FH
(para 7.4.2.12)
Convert to granular sub-base Mixed bound and unbound Convert to Type F DLC
(para 7.4.2.3) layer? (para 7.9.1(ii))
Convert bound material to Are BBMs all Type FH DLC
All unbound?
granular base (para 7.8.3.3) or Marshall Asphalt?
Convert excess granular base Convert Type FH DLC and
Calculate total equivalent
to granular sub-base (para Marshall Asphalt layers other
unbound thickness
7.8.3.3) than 100 mm Surfacing to
Type F DLC (para 7.7.2.1)
Determine ACN from Chart 8,
Estimate the thickness of X-Line
Surfacing plus Base required Calculate total
and convert excess to Sub- BBM thickness
Determine the Surfacing plus
Base requirement from Chart
Yes 8 Y-Line
Determine ACN Determine ACN
Is the actual (or estimated)
Is the actual thickness of from Chart 5 from Chart 7
thickness of Surfacing plus
No Surfacing plus Base less than
Base greater than the
the requirement
requirement (para 7.8.3.3)
Y-Line
ACN is determined from Chart
8 X-Line
Calculate t2/ACN and
Capping Layer Yes determine the CBR on the
capping layer (para 3.8.3)
Apply any necessary
corrections to convert to the Is the estimated CBR on the
subgrade reporting category capping layer correct?
(para 7.4.3)
Figure 30 Reverse design for flexible pavements
7.4.2.5 Rigid pavements with bituminous base layers: The bituminous base layers should be
treated as Type R DLC.
7.4.2.6 Rigid pavements without a bound base: These pavements are evaluated using Chart
6, as described in Section 7.6.
7.4.2.7 Dowelled and/or reinforced concrete: For dowelled concrete pavements the slab
should be treated as described in Sections 5.7 or 7.6 for rigid pavements with and without
bound bases respectively. Jointed reinforced concrete pavements (see Section 5.8) should be
taken as plain concrete. Continuously reinforced concrete pavements (see Section 5.9) are
outside the scope of this guide and should be evaluated using the method by which they were
originally designed.
7.4.2.8 Flexible pavements incorporating Type F DLC, as described in Appendix C: These
should be evaluated using Chart 7 as described in Section 7.7.
7.4.2.9 Flexible pavements incorporating unbound granular bases and sub-bases: These
pavements are evaluated using Chart 8, as described in Section 7.8.
7.4.2.10 Mixed bound and unbound flexible constructions: The procedure for dealing with
these constructions is set out in Section 7.8. Bound layers should be converted to unbound
layers so that the strength at the top of the unbound layers is properly checked in the
evaluation. Converting an unbound layer to a bound layer risks missing the possibility that the
surface of the unbound layer is the critical point for the pavement strength.
7.4.2.11 Bituminous layers on concrete (including bituminous layers on thin drylean concrete
regulating courses on concrete): These pavements are defined as composite and the
techniques for evaluating them are described in Section 7.9. Thin drylean concrete regulating
courses will crack and behave as a flexible material; they should therefore be included as part
of the bituminous material thickness.
7.4.2.12 When an existing concrete slab has been or is to be treated by Crack and Seat
techniques before overlaying the cracked concrete should be treated as Type FH DLC as
described in Appendix C, and the pavement designed or evaluated using Chart 5.
7.4.2.13 Multiple slabs: Concrete slab on concrete slab, concrete on bituminous layers on
concrete and concrete on drylean concrete on concrete are evaluated using the techniques
described in Section 7.10.
7.4.2.14 Concrete Block Surfacing: The structural contribution of Concrete Block Surfacing is
discussed in Section 6.2. Concrete Block Surfacing provides very little structural strength, in
particular on strong bases, it should be considered as equivalent to no more than 50mm
bituminous surfacing.
7.4.3. Determination of PCN
The allowable ACN obtained from the Charts corresponds to the actual subgrade value. To
establish a PCN several aircraft should be selected with ACNs at or near this value so that the
ACN corresponding to the reported subgrade category can be interpolated. This is the reverse
procedure to that described in Section 2.6 and Examples 2.1 and 2.2; it is shown in Example
7.3. If the allowable ACN is greater than that for any existing aircraft the ACN adjustment (to
correspond with the reported subgrade category) will have to be based on a percentage
increase or decrease determined from the aircraft with the closest ACN. If the subgrade
strength is greater than that represented by CBR 15% for flexible pavements or k =
150MN/m2/m for rigid pavements then the allowable ACN should be modified as described in
Section 2.6 to give the PCN. The choice of the pavement type, i.e. rigid (R) of flexible (F),
should be based on the chart used for the evaluation. If Charts 1 to 3 or 5 are used the
pavement is classified as rigid; if Charts 4 or 6 are used the pavement is classified as flexible.
Composite Pavements Type 3 (see para 0) should be categorised as flexible (F).
7.5 PAVEMENT STRENGTHENING (DESIGN OF OVERLAYS)
7.5.1.1 The procedure for establishing the strengthening requirements is as follows:
(i) The existing pavement is evaluated by reverse design (see Section 7.4).
(ii) The likely composition and mode of behaviour (i.e. rigid or flexible pavement
incorporating a bound or unbound base) of the strengthened pavement is addressed
and the appropriate design/evaluation chart is used to establish the full depth
pavement required for the Design Aircraft; Table 14 can be used as a guide for
selecting the appropriate design method and chart together with the procedures set out
in paras 7.5.2 to 7.5.5.
(iii) The existing pavement evaluated in (i) above is assigned an equivalent structural
value in terms of the new construction calculated in (ii) above. For composite
pavements this necessitates using the semi-empirical equivalency factors given in
Section 7.9. When an existing flexible pavement is to be overslabbed it can only be
structurally equated to a rigid pavement base (see para 7.5.4).
(iv) If a flexible overlay is being provided the required thickness is given by the
difference between (ii) and (iii) above. For a concrete overslab the thickness can be
established using the method set out In para 7.5.4, or in the case of multiple slab
construction from the empirical design method set out in para 7.5.5 and Section 7.10.
7.5.2. Flexible Overlays on Existing Flexible Pavements
7.5.2.1 The existing pavement is evaluated using the procedures described in Section 7.4.
7.5.2.2 If a renewed design life (see Section 4.5) is to be provided by the strengthening
overlay an additional thickness of overlay is needed to allow for the reduced effective value of
the existing pavement (see Example 7.9).
7.5.2.3 Where the existing construction is showing signs of impending failure or deterioration
of any layer, its structural value is appropriately reduced below its original design value and a
strengthening overlay provided to give a renewed design life. See para 7.3.1.6 for assessment
of flexible pavement constructions which are showing signs of fatigue.
7.5.2.4 If the pavement is to be used by a Design Aircraft with an ACN greater than the PCN
of the pavement the required construction is obtained from Chart 5 (see Chapter 6), Chart 7 or
Chart 8 (see Sections 7.7 and 7.8), whichever is appropriate. The overlay requirement is then
the shortfall in construction between the new requirement and the existing.
7.5.3. Flexible Overlays on Existing Rigid Pavements
7.5.3.1 Unless it is necessary to retain a surface with a high resistance to fuel spillage and jet
blast strengthening of a rigid pavement will generally be more expediently and economically
achieved by a flexible overlay, in spite of the problems of cracking of bituminous layers laid
over unreinforced concrete. The thickness of the bituminous surfacing over existing
unreinforced concrete should not in any case be less than 100mm. If the existing concrete is
jointed early reflective cracking will occur at the transverse joints. For minimisation of
reflection cracking the thickness of bituminous material over the DLC should be in
accordance with Defence Estates Design & Maintenance Guide 3352. In apron areas, either
concrete blocks or grouted macadam will give adequate resistance to fuel spillage and jet
blast. However, as stated in Section 6.2 these surfacing materials are not yet fully proven.
7.5.3.2 Flexible overlays on existing rigid pavements are defined as composite pavement and
are designed using the methods described in Section 7.9.
7.5.3.3 Numerous methods have been tried in attempts to control reflective cracking of
existing joints and cracks in concrete pavements through flexible overlays52. The most
effective technique has been found to be crack and seat, where the existing concrete slab is
cracked at regular intervals to minimise movement and cracks and joints. Overlay design
should be based on the recommendations of para 7.4.2.12. The specification and construction
of crack and seat overlays is described in Reference 53. The discussion of design in the
reference is superceded by this document.
7.5.4. Overslabbing Existing Flexible Pavements
7.5.4.1 The overslab design method considers the existing flexible pavement either as a
bound base or simply an improved subgrade. When the existing pavement includes a good
quality blacktop surfacing and bound base of adequate thickness and is in sound condition, it
may be considered equivalent to a drylean concrete base; Chart 1, 2, 3 or 4 (see Chapter 5)
may be used to design the overslab. Otherwise, the existing pavement should be equated to a
7.5.4.2 If there is a difference in thickness between the actual bitumen/cement-bound flexible
construction and the base requirements of Charts 1, 2, 3 or 4, the overslab thickness may be
modified as described in para 7.4.2.3.
7.5.5. Overslabbing Existing Rigid Pavements
7.5.5.1 The basis of the design method is the multiple slab empirical design formula
developed by the US Army Corps of Engineers35 and described in Section 7.10.
7.5.5.2 The joint layout of the overslab should as far as possible correspond with that of
existing slab unless the overslab is at least 1.25 x the thickness of the existing slab, or the
existing slab is showing multiple cracking.
7.5.5.3 To allow for differential temperature effects the minimum top slab thickness for an
unreinforced and undowelled slab should not be less than that given in Table 15.
Table 15 Minimum Top Slab Thickness for a Multiple Slab Construction
Minimum top slab thickness
ACN for k = 150 Low Frequency Trafficking Medium Frequency Trafficking
MN/m2/m
>50 275 300
7.6 CONCRETE SLABS LAID ON THE SUBGRADE OR ON A GRANULAR SUB-BASE
(DESIGN, REVERSE DESIGN AND OVERLAY DESIGN)
7.6.1.1 Chart 6 has been developed for the design or evaluation of PQC slabs founded on
either a granular sub-base or directly onto the subgrade.
7.6.1.2 The same design model as that described in Appendix F for new reinforced, rigid
pavement designs was used to produce Chart 6, except that the structural contribution of the
lean concrete base was not included (i.e. no enhancement of subgrade support taken) and a
reduced value of load transfer at transverse joints is adopted for slabs less than 300mm thick.
The pavement designs have been linked directly to ACNs as described in Appendix F. The
omission of a cement-bound base layer allows the three standard main wheel gear types (i.e.
single, dual and dual-tandem (see Appendix D) to be included on one chart.
7.6.1.3 The improvement in the subgrade support provided by a granular sub-base can be
assessed using Figure 10 (see Section 3.8).
7.6.2. Use of Chart 6
7.6.2.1 The use of design/evaluation Chart 6 requires the following parameters:
(i) Flexural strength of the concrete. This can either be established from construction
records (28 day core strengths) or by tests on samples taken from the pavement (see
para I8.5).
(ii) If the Chart is being used for evaluation purposes, the thickness of the concrete slab
and granular sub-base (if any).
(iii) If Chart 6 is being used for design purposes the design ACN (see Section 2.6).
(iv) The Modulus of Subgrade Reaction k. Chapter 3 and Section 5.5 give details of
subgrade characteristics. If subgrade improvement in accordance with Section 3.8 is
to be allowed for, the increased k will be appropriate for design.
(v) The frequency of trafficking; either Low, Medium or High. Chapter 4 defines these
traffic levels in terms of Coverages by the Design Aircraft. For equating the loading
effects of different aircraft, see Chapter 4.
7.6.2.2 In addition to the above parameters the bay layout and the load transfer effectiveness
of transverse joints also have a significant bearing on the future performance of the pavement.
If it is suspected that load transfer is substantially below that assumed in the rigid pavement
design model (see Appendix F) the evaluation will need to be done conservatively. For
details of joints and spacing requirements see Section 5.3.
7.6.2.3 For slabs less than 300mm thick a fully dowelled pavement should provide a
significantly greater load transfer than that assumed in the design model for Chart 6. Table 17
gives allowable reductions in the PQC thickness requirements of Chart 6 for fully dowelled
slabs (i.e. dowelled expansion, construction and contraction joints).
Table 16 Dowelled PQC Pavements on the Subgrade or on a Granular Sub-Base
Chart 6 Allowable reduction in PQC
Design thickness of thickness for fully dowelled
PQC (mm) slabs (mm)
The minimum slab thickness is 150mm
7.6.2.2 Having established the design parameters, Chart 6 is used in the same way as Charts 1
to 4 for design. The procedure for evaluation is as follows:
(i) Select the appropriate PQC thickness on the right hand ordinate.
(ii) Make a horizontal projection until it intersects the vertical projection of the
appropriate k. From this intersection point trace a line parallel to the curves until it
intersects the left-hand ordinate which is also the k = 20 line.
(iii) At the k = 20 line make a horizontal projection; this projection must be maintained.
(iv) Select the design frequency of trafficking (i.e. Low, Medium, High); for High
Frequency of Trafficking see Section 5.10. Make a horizontal projection until it
intersects the appropriate flexural strength.
(v) Make a vertical projection until it intersect the horizontal projection maintained from
(iii) above. Read off the design ACN.
7.7 FLEXIBLE PAVEMENTS INCORPORATING TYPE F DLC (DESIGN, REVERSE DESIGN
AND OVERLAY DESIGN)
7.7.1.1 Chart 7 deals with the Department's pre 1989 specification for drylean concrete now
designated Type F DLC as well as Hot rolled asphalt and Macadam bases and equivalent
materials. Further details on material types is provided at Appendix C. Higher quality
materials can be converted to Type F DLC using equivalency factors given in Table 17: this is
for the purposes of evaluation of composite pavements (see Section 7.9 ) and multi-layer
pavements incorporating combinations bituminous and/or Type F and HF DLC.
7.7.1.2 When used for design, the thickness of bituminous material (including the 100mm
surfacing) over the DLC should not be less than one third of the total thickness of the bound
pavement materials, as described in para 6.3.7, unless special measures are taken to control
reflective cracking, e.g. the use of geotextiles or crack and seat techniques, as described in
Defence Estates Design & Maintenance Guide 3352.
7.7.2.1 Chart 7 has been prepared for single, dual, dual-tandem and tridem main wheel gears;
(i) If Chart 7 is being used for evaluation purposes – the thickness of the surfacing, and
BBM. The thickness of BBM entered into the chart should be the total thickness of
surfacing plus BBM minus a standard 100 mm allowance for surfacing.
(ii) If Chart 7 is being used for design purposes, the design ACN (see Section 2.6).
(iii) The CBR of the subgrade. See Chapter 3 and Section 6.4 for details of subgrade
characteristics. If subgrade improvement in accordance with Section 3.8 is to be
allowed for, the increased CBR value will be the appropriate design value.
effects of different aircraft see Chapter 4.
7.7.2.2 Having established the above parameters, the following sets out the procedure for use
of Chart 7 for pavement evaluation:
(i) Select the appropriate BBM thickness and trace a line parallel to the curves until it
intersects the vertical projection of the appropriate subgrade CBR.
(ii) From the intersection point make a horizontal projection. Read off the design ACN at
the relevant trafficking level.
When designing pavements use Chart 7 in the same way as Chart 5
7.8 FLEXIBLE PAVEMENTS INCORPORATING A GRANULAR BASE AND/OR SUB-BASE
DESIGN, REVERSE DESIGN AND OVERLAY DESIGN)
7.8.1.1 Chart 8 has been developed for the design or evaluation of flexible pavements
incorporating granular bases and/or sub-bases.
7.8.1.2 The same design model as that described in Appendix F for new flexible pavements
was used to produce Chart 8, except that the Equivalency Factors for bound base materials
were omitted. Figure 31 sets out the pavement construction for use with Chart 8. To enable
various other combinations of construction to be considered in the design evaluation, Table 17
sets out Equivalency Factors relating the structural value of a granular base and sub-base as
given by Chart 8 to that of cement and bitumen-bound materials; see para 7.8.3.3 for the
application of Equivalency Factors. The Equivalency Factors for materials are related to a
number of parameters, including the quality of materials, the subgrade strength, the thickness
of construction and the magnitude of the loading. Consequently they vary particularly with
regard to the sub-base. They have largely developed from studies of full scale tests and are
set out in Table 18 in relation to subgrade CBR. For practical design purposes the
Equivalency Factors can be linearly interpolated for intermediate subgrade CBRs.
7.8.2. Bituminous Surfacing
7.8.2.1 A minimum thickness of 100mm is necessary to comply with the requirements set out
in Section 6.2. Pavements designed for regular use by aircraft with an ACN greater than 50
should have a minimum surfacing thickness of 125mm if constructed on a granular base. This
is to prevent early fatigue cracking being developed in the surfacing by high wheel load
deflections on a granular base. The additional 25mm thickness can, in these circumstances,
be subtracted from the granular base requirement.
Table 17 Equivalency Factors for Base and Sub-base Materials
Material Structural Equivalency Factor to apply to Chart 8 requirements
Base Sub-Base
Subgrade CBR
3% 10% 20%
Granular Sub-base - 1.0 1.0 1.0
Granular Base 1.0 2.0 1.5 1.0
Marshall Asphalt Surface and Base Course 1.5 3.0 2.3 1.5
Other Bituminous Materials 1.15 2.3 1.75 1.15
Type FH DLC (see Appendix C) 1.5 3.0 2.3 1.5
Type F DLC (see Appendix C) 1.15 2.3 1.75 1.15
Cement stabilised fine grained material with - 1.0 1.0 1.0
a minimum compressive cube strength of 4
N/mm2 at 7 days
Figure 31 Pavement design and thickness requirements for Chart 8
7.8.3. Granular Base
7.8.3.1 The base thickness requirement on Chart 8 relates to the granular base material shown
in Figure 31. For medium/low severity loading (i.e. nominally less than ACN 30) the
granular base requirements can be reduced to CBR 80%.
7.8.3.2 In circumstances where long-term use has shown good performance of a different
type of base material to those listed in Table 17, the engineer may assess it as being
structurally equivalent to the standard granular base. Some base materials in overseas
locations have good self-cementing properties including limestone, coral and certain laterites
and may well give adequate performance particularly in respect of medium/low severity
loading (i.e. nominally less than ACN 30). Field performance of cement-stabilised fine-
grained material may also indicate structural equivalency as a pavement base at this level of
loading. However, as explained in Section 6.3 it is unlikely that this type of construction will
provide long-term load-spreading characteristics equivalent to the base construction listed in
7.8.3.3 If, in addition to granular base and/or sub-base, the pavement contains a bound base
material (see Sections 6.3 and 7.7) the material should be converted to an equivalent thickness
of granular base using the equivalency factors given in Table 17. If when the first stage of the
evaluation is complete there is found to be an excess of granular base, some of the excess can
be converted to granular sub-base, using the equivalency factors given in Table 17; this will
give a new total thickness and surfacing plus base course thickness. This process is repeated
until the surfacing plus base thickness is equal to that required for the critical ACN obtained
from the total thickness (see Example 7.7).
7.8.4. Granular Sub-base
7.8.4.1 The sub-base thickness requirement on Chart 8 relates to the granular sub-base
material shown in Figure 31. For granular sub-bases with CBRs between 20% and 30% see
para 7.8.5.2. vi. Materials less than CBR 20% should not be considered as a sub-base.
7.8.5.1 Chart 8 has been prepared for the three standard main wheel gear types i.e. single,
dual and dual-tandem (see Appendix D) and the constructions are linked directly to ACNs as
described in Appendix F. The use of Chart 8 requires the following parameters:
(i) If Chart 8 is being used for evaluation purposes – the thickness of the surfacing, base
and sub-base.
(i) Select the appropriate total pavement thickness on the X ordinate and trace a line
parallel to the unbroken line curves until it intersects the vertical projection of the
appropriate subgrade CBR.
(iii) Check the base thickness required for this classification. Retrace the horizontal
projection to the ACN in (ii) until it again intersects the vertical projection of the
(iv) From the intersection point trace a line parallel to the broken line curves until it
intersects the Y ordinate. The minimum combined thickness of surfacing and base
required can then be read off.
(v) If the required thickness of base an surfacing is greater than the actual thickness in the
pavement then the PCN will be limited by the thickness of the base. If the actual
thickness of the base and surfacing is greater than the required thickness, the excess
thickness can be converted to granular sub-base using the equivalency factors given
in Table 17. This will give an equivalent total pavement thickness which can be re-
entered on the X-Axis.
(vi) If the sub-base is between CBR 20% and 30% the base thickness requirement is
greater than that determined in (iv) and can be derived from the Chart by the
following method. From (ii) retrace the horizontal projection to the ACN and project
it across until it intersects the X ordinate. The X ordinate then represents the
combined thickness of surfacing and base required above the sub-base.
7.8.5.3 When designing pavements use Chart 8 in the same way as Chart 5; except that,
having drawn a horizontal line through the design ACN to meet the subgrade CBR line,
follow both the continuous and dotted curves to obtain the total thickness and surfacing plus
base thickness from the X and Y axes respectively. Obtain the sub-base thickness by
subtracting one from the other.
7.9 COMPOSITE PAVEMENTS – REVERSE DESIGN AND STRENGTHENING
7.9.1.1 Pavements comprised of flexible overlays on concrete slabs are termed composite.
The methods for designing and evaluating these pavements depend on how they behave, and
in particular the form of their failure. When viewed in this way composite pavements, other
than those using crack and seat techniques to minimise reflective cracking (para 7.5.3.3), can
be divided into three types:
(i) Type 1: Composite pavements with relatively thin flexible overlays.
In this case reflective cracking of structural cracks in the underlying concrete will
lead to the failure mechanism described in Appendix F. From long-term performance
of pavements, Equivalency Factors have been obtained to convert the thickness of
flexible overlay to an equivalent concrete thickness. The pavement is then treated as
rigid.
(ii) Type 2: Composite pavements with relatively thick flexible overlays.
In this case reflective cracking is delayed sufficiently for a considerable amount of
structural cracking to occur in the concrete slab, leading to a transfer of the load to the
subgrade and eventual failure by subgrade shear, as described in Appendix F. From
long-term performance of pavements, Equivalency Factors have been obtained to
convert the thickness of the concrete slab to an equivalent thickness of bound base
material which is added to the thickness of the overlying flexible overlay. The
pavement is then treated as flexible and Charts 7 or 8 should be used for reverse
design and strengthening.
(iii) Type 3: Composite pavement with overlays which fall between Types 1 and 2 above.
In this situation the pavement cannot be defined as rigid or flexible and there is no
clear cut failure criterion. The evaluated strength of these pavements is found by
interpolating between rigid and flexible strengths calculated for nominal constructions
conforming to Types 1 and 2 above.
7.9.1.2 Type 1 and 3 composite pavements are likely to suffer from reflective cracking from
the joints in the concrete slab before structural cracking of the slab occurs. Techniques for
controlling reflective cracking when designing flexible overlays of concrete slabs are
described in Defence Estates Design & Maintenance Guide 3352
7.9.1.3 Reverse design and overlay design for composite pavements using crack and seat
techniques to minimise reflective cracking is described in para 7.5.3.3.
7.9.2. Reverse Design of Composite Pavements
The following formulae should be used for evaluating composite pavements:
(i) Type 1 If β ≤ 0.5
hc = C t h e +
(ii) Type 2 If β ≥ 1
h f = t + 1.8C t h e + be
(iii) Type 3 If 0.5 <β<1
PCN = PCNR + ( PCNF − PCNR)(2β − 1)
where β = t/he
he = thickness of existing concrete slab
hc = equivalent concrete thickness
hf = equivalent thickness of flexible pavement (surfacing plus bound base
material)
t = thickness of bituminous surfacing
be = thickness of existing bound base, if any
PCNR = PCN of nominal Type 1 Composite pavement with β = 0.5.
PCNF = PCN of a nominal Type 2 Composite Pavement with β = 1.0
Ct = Condition Factor (see Table 18)
NB: If Ct<0.85 and β<1 then reliable performance of the pavement cannot be guaranteed and
measures should be taken to increase the overlay thickness.
7.9.3. Overlay Designs for Composite Pavements
The following formulae should be used for designing flexible overlays to concrete pavements,
producing a composite pavement.
(i) If the overlay thickness is less than or equal to half the concrete thickness
t = 1.8 hc − C t he )
(ii) If the overlay thickness is greater than or equal to the concrete thickness
t = h f − 1.8 C t he − be
(iii) If the overlay thickness is greater than half and less than one times the concrete
 −  1
β =  PCN P PCN R + 1 ×
 PCN F − PCN R  2
and t = he β
where t = bituminous overlay thickness required
hc = concrete slab thickness required for the design
hf =total flexible pavement thickness (surfacing plus bound base material)
required for the design
PCNR = PCN of a nominal Type 1 composite pavement with an overlay thickness
equal to half the concrete thickness
PCNF = PCN of a nominal Type 2 composite pavement with an overlay thickness
equal to the concrete thickness
PCNP = Design ACN for the strengthened pavement
NB: If the value of Ct is less than 0.85 then the concrete should be converted to an equal
thickness of drylean concrete, and an overlay at least equal to the concrete thickness applied
to ensure reliable performance.
7.10 MULTIPLE SLAB PAVEMENTS – REVERSE DESIGN AND STRENGTHENING
7.10.1. Pavements comprised of two or more successive concrete slabs are termed multiple
slab pavements. These pavements are designed or evaluated using the empirical formula
developed from full scale testing by the US Army Corps of Engineers28. The formula can be
expressed in the form
h r = C1h1 + C2 h2
n n n
where hr is an equivalent single slab thickness
h1,h2 are the component slab thicknesses
n is a factor depending upon the bond between the layers
C1,C2 are condition factors
7.10.2. Three conditions of bond are used:
(i) Fully bonded: by very careful preparation of the existing surface the two concrete
layers are bonded together and behave as a monolithic slab. This form of
construction should only be used if the existing surface is in good condition.
(ii) Partially bonded: the two slabs are place on top of each other with no attempt at
producing a bond between layers, although some shear transfer is achieved at the
interface through friction and mechanical interlock. The Defence Estates normally
places a membrane between the layers; the value of n given below for partially
bonded slabs is based on analysis of the long-term performance of this type of
(iii) Unbonded: in some situations it may be necessary to use a layer of regulating
material between the two slabs. When separated in this way the slabs will act more
independently of each other than in the partially bonded case. Regulating courses of
bituminous material or bituminous materials on thin cement-bound or drylean
concrete layers less than 150mm thick can be accounted for the evaluation by
converting them to an equivalent concrete thickness. This thickness should be added
to the thickness of the underlying slab using the composite pavement equation shown
in para 7.9.2 (I). The maximum amount of material assessed in this way should be
150mm; anything in excess of this is to be ignored. Techniques for dealing with thick
drylean concrete regulating courses are given in para 7.10.3. As the thickness of a
bituminous regulating course increases, the behaviour of the top slab will be
increasingly governed by elastic deflections of the bituminous materials, reducing the
effect of the lower slab. Therefore as much as possible of a thick regulating course
should be in cement-bound materials.
7.10.3. Reverse Design of Multiple Slab Pavements
The following formulae should be used for evaluating multiple slab pavements.
(i) h r = n C1h1 + C 2 h n2
(ii) for multiple slab construction containing a drylean concrete regulating course greater
than 150mm thick.
 hd 
 1 .8 
h r = 1.6 C1h1 + + C 2 h12.6
 
where hr = equivalent slab thickness
h1, h2 = top and bottom slab thickness
C1, C2 are the condition factors (see Table 18)
NB C1 will only be required if the evaluation is being carried out as part of an overlay design
(see para 7.3.1.3)
n = 1.0 for a fully bonded pavement
n = 1.6 for a partially bonded pavement
n = 2.0 for an unbonded pavement
hd = thickness of drylean concrete.
7.10.4. Overlay Design for Multiple Slab Pavements
The following formulae should be used for the design of an overslab to an existing concrete
(i) h o = n hcn − C 2 hen
(ii) for a multiple slab construction containing a drylean concrete regulating course
greater than 150mm thick
ho = 1.6 h1c.6 −  1.8  −C 2 h1e.6
where h0 = thickness of overslab required
hc = thickness of a single concrete slab required for the design
he = thickness of the existing concrete slab
hd = thickness of a drylean concrete regulating course
C2 = condition factor (see Table 18)
n = 2.0 for an unbonded pavement.
7.10.5. If the top slab is thicker than the underlying one, the formulae given above may be
conservative and evaluations or overlay designs should be checked by assuming k =
150MN/m2/m on the surface of the bottom slab and then using Chart 8.
Table 18 Condition Factors for Concrete Slabs
Conditions of concrete bays in the wheel Condition Factors
track area
Ct Ci
a) No more than a few cracks 1.0 1.0
b) 30%-50% contain halving, quartering 0.85 0.75
or delta cracks
c) Virtually all cracked with 30%-50% DLC N/A DLC N/A
containing corner cracks or having
cracked into 5 or more pieces
d) Many with multiple cracking and Gbc N/A Gbc N/A
some deformation
Abbreviations: DLC = equate to drylean concrete
Gbc = equate to granular base course
N/A = formulae in Section 7.9 are not
NB: i = 1, 2 etc see para 7.10.1
7.11 EVALUATION OF HANGAR FLOORS
7.11.1.1 The rigid design model and the Charts include a factor for the effects of temperature-
induced stresses. However, in hangar floors, the full effects of the temperature range are not
often experienced, and thus evaluation based on Charts 1, 2, 3, 4 or 6 will underestimate the
bearing strength of the floors. For existing hangar floors where the normal daily temperature
range is 50% (or less) of that for the external pavements, the PCN obtained from the charts
may be factored by:
(i) 1.5 for Low Frequency Trafficking
(ii) 1.2 for Medium Frequency Trafficking
if the aircraft use suggests that the evaluation without these factors is over-conservative.
EVALUATION EXAMPLES
Example 7.1 CONCRETE SLAB ON A ROLLED DRYLEAN CONCRETE BASE
1. CONSTRUCTION:
275mm Pavement Quality Concrete
150mm Rolled Drylean Concrete.
2. USE: The runway end on a busy military airfield.
MAIN USER AIRCRAFT: ACN – 16
Main Wheel Gear – Single
Tyre Pressure – 1.3 MPa
Para 4.9.2, Table 7 Pass-to-Coverage Ratio – 8
MOVEMENTS: 75 Departures a day for about 350 days of the year.
Para 4.7.3 DESIGN LIFE: 30 years.
75 x30 x350
FREQUENCY OF TRAFFICKING: Coverages = = 98438
Para 4.6.2, Table 5 Take Medium Frequency Trafficking
3. SUBGRADE: A clay with k = 30MN/m /m, obtained from in situ testing.
Para 5.2.3 4. MATERIAL QUALITY: Concrete produced to Defence Estates' Specification. Assume the
concrete flexural strength = 4.5N/mm at 28 days
5. PAVEMENT TYPE: Rigid.
Chart 1 6. EVALUATION: PCN 33.
k = 30 MN/m /m is not a standard subgrade reporting value and it may therefore be necessary
to correct the PCN to an appropriate value for k.
Para 2.4.2, Table 1 In this case the reporting category is Low (k = 40MN/m /m) but the ACN for a single main
wheel gear does not change with the subgrade support. (NB. Examination of published ACN
data may show a variation of ACN with subgrade support for single wheels, but this is due to
the difference between the actual tyre pressure and the standard ACN-PCN tyre pressure).
b) PCN: 33.
c) Pavement Type: Rigid (R).
e) PCN 33/R/C/W/T.
Example 7.2 MULTIPLE CONCRETE SLAB CONSTRUCTION
Guide Reference 1. CONSTRUCTION:
200mm Pavement Quality Concrete} Twin slab construction
200mm Pavement Quality Concrete} with a building paper separating membrane.
150mm average Rolled Drylean Concrete
150mm Concrete
300mm Granular Fill.
2. USE: A parallel taxiway on a busy military airfield with an expected use of 50,000 Coverages
by dual and dual-tandem aircraft during a 30 year life i.e. Medium Frequency Trafficking.
3. SUBGRADE: Clay. k = 30 MN/m /m obtained from in situ testing.
4. MATERIAL QUALITY: The construction records for the twin slab show that the concrete
strength was low. The 150mm concrete slab is wartime construction. Cores show that the
mean flexural strength of the top slab is 4.7 N/mm . There are no visable cracks in the
pavement surface.
6. EVALUATION:
Para 7.4.2.13 (i) Convert the twin slab construction to an equivalent single slab.
Para 7.10.4 h r = n C1h1 + C 2 h n2
n = 1.6 for a twin slab with a separating membrane.
C1 = 1.0 as the surface layer is considered as new.
C2 = 1.0 for a slab in good condition.
1.0 × 2001.6 + 1.0 × 2001.6 = 308mm
(ii) Convert the equivalent single top slab, DLC regulating course and bottom slab to an
equivalent single thickness pavement.
From Para 7.10.4 h r = 1.6 C1h1 + + C 2 h12.6
It is conservative to assume that the wartime concrete slab has suffered some degree of
cracking; take C2 = 0.75.
Table 18 1.6 1.0 × 3081.6 + ( )
+ 0.75 × 1501.6 = 375mm
Para 7.4.2.2 (iii) Calculate an effective k on the granular sub-base.
Para 3.8.4, Figure 10 300mm granular sub-base on k = 30MN/m /m gives 48MN/m /m.
(iv) Equivalent construction 375mm PQC on k = 48MN/m /m.
Chart 6 (v) PCN 41.
Para 2.4.2 Table 2.1 The standard subgrade category is Low (k = 40MN/m /m). Examination of a
Para 7.4.3 number of aircraft with an ACN close to 40 on a Rigid Low Subgrade (see below) shows that
the change in ACN between k = 48MN/m /m and k = 40MN/m /m is less than 1.
RIGID PAVEMENT SUBGRADES - MN/m2/m
Aircraft High Medium Low Ultra Low
type 150 80 40 20
A318 68,400 36 38.4 40.6 42.5
B737-500 60,800 36.4 38.4 40.2 41.7
B757-200 116,100 30.7 36.8 43.4 49.3
C130H 79,379 35.8 38.6 41.6 44.5
Embraer 47,790 41.8 42.2 42.6 42.9
Nimrod 83,461 31.9 36.2 40.4 44.1
MR Mk 2
Example 7.3 THIN FLEXIBLE OVERLAY ON A CONCRETE SLAB ON A THIN ROLLED DRYLEAN
40mm Hot Rolled Asphalt Surface Course
60mm Macadam Binder Course
2. USE: A taxiway for dual-tandem aircraft. Expected use is less than 10,000 Coverages in a 20-
year life, i.e. Low Frequency Trafficking.
3. SUBGRADE: Sand. k = 60MN/m /m from in situ testing.
4. MATERIAL QUALITY: Cores in the Pavement Quality Concrete at 20 years give a mean
compressive strength of 56N/mm , and show a crushed rock aggregate. A conservative
estimate of the flexural strength is 56/10 = 5.6N/mm . Take a 4.5N/mm mean flexural
strength at 28 days to allow for gain in strength with age. Concrete bay joints have reflected
through the surfacing, but there is no sign of structural cracking.
Para 7.4.2.10 5. PAVEMENT TYPE: Composite pavement where the ratio of the flexible overlay
Para 7.9.2 to the concrete thickness is less than 0.5 i.e Type 1.
Para 7.9.2 (i) hc = C t he +
hc = 1 × 300 + = 355
Chart 3 (ii) 355mm PQC on subgrade k = 60 MN/m /m requires 150mm DLC.
Para 7.4.2.4 (iii) Convert some of the PQC slab to DLC to make up the deficiency.
= 17mm
Modified PQC thickness: 355-17 = 338mm.
(iv) Equivalent construction is 340mm PQC
150mm DLC.
Chart 3 PCN 70.
Para 2.4.2, Table 1 (v) Subgrade Category – Medium (k = 80 MN/m /m). For aircraft with dual-tandem main
wheel gears the ACN varied considerably with the sub-grade strength. Consider a range of
aircraft with an ACN close to 70 on k = 60.
Aircraft k80 k60 k40 (1)
B747-200 56 61 67 0.92
Concorde 71 76 82 0.93
DC10-30 53 58 64 0.91
L1011-500 59 65 72 0.91
The ACN on k = 80 is approximately 0.92 times the ACN on k = 60
70 x 0.92 = 64
Table 2.1 a) Subgrade Category: Medium (B).
Para 6.2.4, Table 13 d) Tyre Pressure Limitations: X.
e) PCN 64/R/B/X/T.
Example 7.4 ASPHALT ON TYPE F DRYLEAN CONCRETE
40mm Marshall Asphalt Surface Course
63mm Macadam Base Course
450mm Drylean Concrete.
2. USE: The main runway on a busy military airfleld, with the majority of
movements by aircraft with single main wheel gears. 200,000 Coverages expected in a 20-year
life, i.e. High Frequency Trafficking.
3. SUBGRADE: Clay. CBR 3%.
4. MATERIAL QUALITY: All materials to Defence Estates' Specification or one of its
predecessors. Cores show the Drylean Concrete has a compressive strength of 11 N/mm2;
therefore take as Type F.
5. PAVEMENT TYPE: FLEXIBLE.
(i) Correct for High Frequency Trafficking (and use the Medium Frequency Trafficking line
on the Chart).
TotalThickness 650
Para 6.6.1 Equivalent Thickness = = = 600
(ii) Equivalent Construction 100mm Surfacing
500mm Bound Base Material.
CBR 3% is a standard subgrade category, so no correction is required to the PCN.
Para 2.4.2, Table 1 7. CLASSIFICATION:
a) Subgrade Category: Ultra Low (D).
b) PCN 30.
c) Pavement Type: Flexible (F).
Para 6.2.4, Table 13 d) Tyre Pressure Limitations: No limitations for Marshall Asphalt (W).
e) PCN 30/F/D/W/T.
Example 7.5 FLEXIBLE OVERLAY ON A THIN CONCRETE SLAB
25mm Asphalt Surface Course
65mm Tarmacadam
20mm Sand Asphalt.
2. USE: Main taxiway of a provincial airport with an expected use of 10,000 Coverages by dual-
tandem aircraft in a 20-year life, i.e. Low Frequency Trafficking.
3. SUBGRADE: A dense silty sand. CBR 10%.
4. MATERIAL QUALITY: Cores show that all the materials are in good condition.
Inspection Reports produced before the first Hot Rolled Asphalt/Macadam Base Course
overlay was placed indicate that considerable reflection cracking was present showing
longitudinal, quartering and corner cracking in the underlying concrete.
Para 7.4.2.11 5. TYPE: Composite pavement where the ratio of the flexible overlay to the
Para 7.9.2 concrete thickness is greater than 1 i.e. Type 2.
6. EVALUATION
Para 7.9.2 (i) h f = t + 1.8C t h e + be
Table 18 Take Ct = 0.85 because of evidence of structural cracking in the concrete.
hf = 310 + 1.8 × 0.85 × 150 + 0 = 540mm
(ii) Equivalent Construction: 100mm Surfacing
CBR 10% is a standard subgrade category.
Table 1 a) Subgrade Category: Medium (B).
Para 6.2.4 Table 13 d) Tyre Pressure Limitations: X.
e) PCN 68/F/B/X/T.
Example 7.6 ASPHALT SURFACING ON A GRANULAR BASE AND SUB-BASE
250mm Granular Base
650mm Granular Sub-base.
2. USE: Taxiway on a provincial airport. Expected use 10,000 Coverages by dual aircraft in a
20-year life, i.e. Low Frequency Trafficking.
4. MATERIAL QUALITY: Site investigation confirms that the granular materials have a grading
and in situ density compatible with Defence Estates' Specifications for base and sub-base
materials. There are no signs of rutting.
5. TYPE: FLEXIBLE.
Section 7.7, Chart 8 6. EVALUATION:
(i) Evaluate PCN from the total pavement thickness.
Total Pavement Thickness = 1000mm.
From X-Axis PCN = 48.
(ii) Check thickness of base plus surfacing.
From Y-Axis, PCN48 on CBR 4% requires 275mm of base plus surfacing, which compares
with 350mm in the actual pavement.
(iii)Convert excess granular base to granular sub-base.
Estimate base + surfacing requirement as 275mm.
Excess Granular Base = ( 250 − ( 275 − 100) = 75mm .
Para 7.8.5.2v Equivalency Factor (by interpolation between the published values)
( 2.0 − 1.5)
= 2 .0 − × 1 = 1.93
Equivalent Thickness of Granular Sub-Base = 75 × 1.93 = 145mm .
Equivalent Construction 100mm Surfacing
800mm Granular Sub-Base
Chart 8 (iv) If we round up to the nearest 100mm, i.e. total thickness of 1100mm, Chart 6 indicates a PCN
greater than 50 and a base plus surfacing requirement of 275mm. However for a PCN greater
than 50, 125mm of surfacing is required, therefore
Para 7.8.2.1 the PCN of the actual pavement is restricted to 50.
Para 2.4.2, Table 1 (v) The Standard Subgrade Category is Ultra Low (CBR 3%).
Consider a range of aircraft:
Aircraft CBR 6% CBR 4% CBR 3%
(1) (2) (3) ( 2)
A321-200 46.8 50.7 52.6 1.038
B727-100 49.2 52.6 54.3 1.032
MD90-30 49.5 51.6 52.6 1.020
The PCN on CBR 3% is approximately 1.04 times the PCN on CBR 4%.
50 x 1.03 = 51.
Para 6.2.4, Table 13 d) Tyre Pressure Limitations: W.
Example 7.7 MIXED BOUND AND UNBOUND FLEXIBLE CONSTRUCTION (INCLUDING
CAPPING LAYER)
20mm Friction Course
650mm Type F Rolled Drylean Concrete
550mm Capping Layer.
2. AIRCRAFT USE: Main runway for an international airport. Expected use is equivalent to
100,000 Coverages by Boeing 767-200 in 20 years i.e. Medium Frequency Trafficking.
3. SUBGRADE: Silty Clay CBR 2%.
4. MATERIAL QUALITY: Asphalt, Rolled Drylean Concrete and Granular Sub-base are
compatible with Defence Estates' Specification. The capping layer is a granular material with
a minimum CBR of 15%.
5. PAVEMENT TYPE: FLEXIBLE
Para 6.2.1 (i) The Friction Course is ignored.
Figure 7 Dual-Tandems Determine Equivalency Factor for the Capping Layer. 1.3
Para 7.4.2.2 (ii)
1stEstimate 2nd
Determine Design ACN
a) CBR on Estimate 3% 3.5 3 3.5
Capping
Para b) Calculate 1.15
7.4.2.9 Equivalency
Para Factor of BBM
7.8.1.2, to Granular
Table 17 Base.
Calculate 2 ((2- 2 1.96
Equivalency 1.5)/(
Factor of 3-
Granular Base 10))*(
Course to 3.5-
Granular Sub- 3)+2=
base 1.96
Para d) Surfacing 100
7.8.2.1 Requirement.
Para e) Convert BBM 650x1.15=75
7.8.3.3 to Granular 0
Convert Excess 0 (750- (750- (750-
Granular Base 275)* 375)*2 325)*
Course to 1.96= =750 1.96=
Granular Sub- 930 830
f) Calculate Total 100+750+0+ 100+2 100+3 100+
Thickness (X). 300=1150 75+93 75+75 325+
0+300 0+300 830+
=1605 =1525 300=
Chart 8 g) Determine 32 71 55 67
X-line Design ACN
Chart 8 h) Determine 275 375 325
Y-line required
Surfacing and
Base Thickness
Check CBR on Capping Layer
Calculate 550/1.3=423
Thickness of
Capping Layer
as Granular
Sub-base
Calculate 4232/32=559 4232/ 4232/5
t /ACN for the 1 71=25 5=325
Capping Layer 20 3
Determine the 3.5 3 3.5
CBR on the
i) Estimate 350
Surfacing plus
Base required
j) Convert Excess 1000
Base to Sub-
Para 7.4.2.9 b) Calculate Equivalency Factor of BBM to Granular 1.15
Para 7.8.1.2, Table 17 Base.
Para 7.8.1.2, Table 17 c) Calculate Equivalency Factor of Granular Base to 2
Sub-base.
Para 7.8.2.1 d) Surfacing Requirement. 100
Para 7.8.3.3 e) Convert BBM to Granular Base. 750
1st 2nd
Estimate Estimate
f) Calculate Total Thickness. 1150 1650
Chart 8 X-line g) Determine PCN. 37 70
Chart 8 Y-line h) Determine Surfacing and Base Thickness required. 275 350
i) Estimate thickness of Surfacing plus Base required.350
j) Convert Excess Base to Sub-base. 1000
k) Return to step (f).
(iii)Check the CBR on the Capping Layer.
Calculate Equivalent Thickness of Capping Layer as Granular Sub-base. 423
Figure 9 Calculate t /ACN for the Capping Layer. 2557
Determine the CBR on the Capping Layer. 3%
e) PCN 70/F/D/W/T
(This pavement is likely to suffer from early and extensive reflection cracking form shrinkage
cracks in the DLC base, and is not recommended for a new pavement. Further details are given in
Defence Estates Design & Maintenance Guide 3352)
Example 7.8 FLEXIBLE OVERLAY ON CONCRETE WITH 0.5<β<1
40mm Hot Rolled Asphalt
225mm Pavement Quality Concrete.
2. USE: A Taxiway. The use is expected to be less than 10,000 Coverages by dual-tandem
aircraft in a 20-year life, i.e. Low Frequency Trafficking.
Figure 32 3. SUBGRADE: A silty sand. CBR 10%/k = 50 MN/m /m.
4. MATERIAL QUALITY: All materials complied with Defence Estates' Specification.
Concrete strength is 5.3 N/mm .
Para 7.4.2.10 5. PAVEMENT TYPE: Composite pavement where the ratio of the flexible
Para 7.9.2 overlay to concrete thickness lies between 0.5 and 1 i.e. Type 3.
Para 7.9.2 6. EVALUATION:
(i) Evaluate an imaginary pavement with β =1
Construction is 225mm Asphalt
225mm Concrete.
Para 7.9.2(ii) hf = t + 1.8 Cthe + be (be = 0)
= 225 + 1.8 x 225 = 630
Chart 7 PCNF = 95
(ii) Evaluate an imaginary pavement with β = 0.5
Para 7.9.2(i) hc = Cthe + 1t.8
= 1.0 x 225 +112/1.8 = 287
Equivalent Construction = 285mm PQC
Chart 6 PCN(R) = 30
(iii)Evaluate the final PCN of the actual construction.
Para 7.9.2 β= = 0.89
Para 7.9.2(iii) PCN = PCN(R) + (PCNf-PCN(R)) (2β-1)
= 30 + (95-30) x 0.78
= 81.
a) Subgrade Category: Medium (B).
Para 2.4.2 c) Pavement Type: Flexible (F).
e) PCN 81 /F/B/X/T
(In determining the final overlay requirement the critical factor for the overlay requirement may be
reflection cracking rather than structural strength. Further details are given in Defence Estates
Design & Maintenance Guide 3352)
OVERLAY DESIGN EXAMPLES
Example 7.9 FLEXIBLE OVERLAY ON AN EXISTING PAVEMENT
Guide Reference 1. REQUIREMENT: A taxiway currently carrying Boeing 727-200 and lighter aircraft is to be
strengthened to take Boeing 747-100.
Appendix B a) ACN, Undercarriage Type and Pass-to Coverage Ratio.
Para 4.9.2, Table 6
FLEXIBLE PAVEMENT SUBGRADES
Main Wheel
Aircraft Type Gear Pass-to-
Ultra Low Coverage Ratio
High Medium Low
747-100 44 48 58 78 D-T 1.6
727-200 40 42 48 53 Dual 3.2
3. AIRCRAFT USE: The existing pavement is 15 years old and had had an average use of 1200
departures per year by Boeing 727-200.
Para 4.9.1
15 x1200
Use to date = = 5625 coverages.
The expected future use is 500 departures a year by Boeing 747-100.
5. FREQUENCY OF TRAFFICKING:
Para 4.6.2, Table 5 a) For the evaluation of the existing pavement use Low Frequency Trafficking based on the use to
Para 4.7.3 b) For future use the optional coverages in a 20 year life are
Para 4.6.2, Table 5 i.e. Low Frequency Trafficking
6. EXISTING CONSTRUCTION:
425mm Type F Rolled Drylean Concrete.
7. SUBGRADE: CBR 6%.
8. MATERIAL QUALITY: The pavement materials are compatible with Defence Estates'
Specification. Some rutting is present in small areas.
9. EVALUATION OF THE EXISTING PAVEMENT:
100mm Surfacing
Para 7.3.2.2 The aircraft use to date equates to 5625 coverages by Boeing 727-200, which have an ACN of
48 on CBR 6%. The Equivalent Coverages on the actual pavement can be calculated as
ACN/ ACN FMTF Modified Coverages
PCN Ratio FMTF
Pavement 50
Boeing 727-200 48 0.96 0.92 0.88 4400
Since the evaluation is for Low Frequency Trafficking – nominally 10,000 coverages – the
remaining life is 10,000-4,400 = 5,600 Coverages. if the pavement is to be overlaid to give a
further 20 year life for a heavier aircraft the existing construction should be re-evaluated to find
a lighter load which will allow a new 10,000 Coverage life. Therefore find a PCN which will
allow a further 10,000 Coverages with an equivalent damaging effect to 5,600 Coverages at
ACN 50.
Mixed Traffic Factor 10,000 Coverages = 1.0
Mixed Traffic Factor 5,600 Coverages = 0.91.
ACN Ratio = = 0.91
PCN = 50 x 0.91 = 45
Chart 7 Equivalent Construction is 100mm Surfacing
400mm Bound Base Material
10. DESIGN REQUIREMENT FOR NEW LOADING
The design requirement is for Low Frequency Trafficking by Boeing 747-100.
(see also Example 4.2) The ACN of the aircraft on a Flexible Low Subgrade is 58.
Chart 7 100mm Surfacing
11. OVERLAY DESIGN
(i) The existing pavement is equivalent to a total of 500mm.
(ii) The new design requirement is for a total of 600mm.
Overlay requirement = 600-500 = 100mm.
Overlay with 100mm asphalt or equivalent materials.
Example 7.10 RIGID OVERSLAB OF AN EXISTING RIGID PAVEMENT
Guide Reference 1. REQUIREMENT: An existing hardstanding is to be uprated to take dual wheel gear
short/medium range transport aircraft, of which McDonnell-Douglas DC9-51 will be the most
severe loading case.
Aircraft Type RIGID PAVEMENT SUBGRADES
High Medium Low Ultra Low
DC 9-51 35 37 39 40
Para 4.9.2, Table 6 c) Pass-to-Coverage Ratio: 3.2.
3. AIRCRAFT USE: The expected use is less than 10,000 Coverages by DC9-51 in 30 years, i.e.
Low Frequency Trafficking.
6. SUBGRADE: k = 40 MN/m /m.
7. MATERIAL QUALITY: The concrete is of good quality and from the available information it
is thought appropriate to use the 5.3 N/mm line on Chart 6. A few of the existing bays have
one or more corner cracks and about 30% of the bays have halved. There are no signs of
differential settlement or mud-pumping.
8. EVALUATION: Only an estimate of the effect of the granular sub-base is needed.
Para 3.8.4. Figure 10 300mm of granular sub-base on k = 40 MN/m /m gives an effective K of 60MN/m /m.
9. DESIGN REQUIREMENT FOR NEW LOADING:
The design requirement is for Low Frequency Trafficking by DC9-51. Design ACN = 38 (on k
= 60MN/m /m).
Chart 6 325mm Pavement Quality Concrete
h0 = n h c − C 2 h e
Para 7.10.4
The overslab will be laid on a polythene separating membrane, directly on the underlying slab;
therefore n = 1.6.
Table 18 From the existing degree of cracking C2 = 0.75.
h0 =
3251.6 − 0.75x1751.6
= 265mm PQC
Overlay with 275mm PQC.
Para 7.10.4 NB.Chart 6 shows that if the existing construction is considered as a sub-base giving an effective k
of 150 MN/m /m, the overlay requirement is also 275mm. If the existing slab was thinner, the
method used above to calculate the overslab thickness becomes pessimistic and it is more
economic to consider the existing construction as a good sub-base.
8 Overload and High Tyre Pressure
8.1.1. Individual aerodrome authorities are generally free to decide on their own criteria for
permitting overload operations as long as pavements remain safe for use by aircraft. Unless
severely overloaded, (e.g. an aircraft with an ACN four times greater than the PCN) it is most
unlikely that a pavement will suddenly or catastrophically fail. Nevertheless regular overload
can substantially reduce the design life of a pavement, resulting in high rehabilitation costs
and the inconvenience of a main runway or taxiway out of action. The limiting criteria for
overload must be somewhat arbitrary, representing a reasonable balance between operational
flexibility and the need to avoid undue damage to pavements. On that basis the following
guidance has been developed:
(i) A 10% difference in ACN over PCN involves an increase in pavement working
stresses which is generally considered acceptable provided the following conditions
are satisfied.
a. The pavement is more than 12 months old.
b. The pavement is not already showing signs of structural distress.
c. Overload operations do not exceed 5% of the annual departures and are
spread throughout the year.
The 5% must be calculated from the number of departures of aircraft with
ACNs at or near the PCN of the pavement (i.e. 5% of the 'design traffic').
Otherwise if there is a high frequency of use by light aircraft which are
well below the PCN, 5% of the total movements could represent a
substantial proportion of the actual coverage level for the pavement (see
para 4.6.2) and lead to an unacceptable rate of deterioration.
The effect of maintaining overload operations at this level and frequency cannot be
accurately predicted owing to the number of variables; for example, the type of
construction (i.e. rigid and flexible), its condition, the type of aircraft (e.g. pass-to-
coverage ratios) and the environmental factors at the time. As an approximate guide
the standard rigid and flexible pavement design models were used to establish
average results; these gave a 5-15% reduction in the remaining design life.
(ii) Overload operations representing a difference in ACN over PCN from 10% to 25%
justify regular inspections of the pavements by a competent person in addition to
satisfying the criteria for 10% overload. Overload operations should stop as soon as
distress becomes evident; the higher loading should not be reimposed until
appropriate pavement strengthening work has been completed.
As for the 10% overload case the standard rigid and flexible design models were used
to assess the implications of maintaining 25% overload operations at a frequency of
5% of the 'design traffic' The results varied from 25-75% reduction in design life
depending on the pavement type and aircraft type. Therefore overload operations at
this level and frequency should only be short-term.
DMG 27 8 Overload and High Tyre Pressure Operatiions
(iii) Overload operations representing a difference in ACN over PCN from 25% to 50%
should only be permitted very occasionally. They call for scrutiny of available
pavement construction records and test data and a thorough pavement inspection by a
pavement engineer before and on completion of the movement to assess any signs of
pavement distress.
(iv) Overload operations representing a difference in ACN over PCN of more than 50%
should only be undertaken in an emergency.
8.2 HIGH TYRE PRESSURE OPERATIONS
8.2.1. For practical design purposes the tyre pressure produces the intensity of the load on
the pavement. The primary consideration for excess tyre pressure operations is the risk of
undue damage to the surfacing. The consequences of pavement damage as a result of
overstressing of the surfacing layers are likely to be less serious than a deep seated structural
failure. Nevertheless an engineer must carefully weight the problems of carrying out
maintenance work in the event of damage before allowing occasional excess tyre pressure
operations for the sake of maintaining operational flexibility. The following notes are for
guidance:
(i) Occasional movements by aircraft with tyre pressures over the maximum authorised
for unrestricted use (see para Error! Reference source not found.) of the pavements
are unlikely to have significant effect on the performance of the pavement except in
circumstances described in (iii). The factors which affect surface stability make it
inappropriate to lay down rules.
(ii) Concrete pavements are not subject to surface indentation by high tyre pressure
(iii) Bituminous surfacing of other than high stablility Marshall asphalt or with less than
100mm of Marshall asphalt are liable to indentation by high pressure tyres. The
amount of indentation depends on the following factors:
a. The stability of blacktop surfacings is temperature dependant and therefore
they are more liable to identation by high pressure tyres on hot days. This
is particularly the case for tar-bound surfacings (e.g. dense tar or tar
macadam).
b. Although ready for use within hours of laying, bituminous surfacings
continue to harden for some months. This depends on the type of mix and
the climatic conditions. The full stability of surfacing is not realised for
several months after laying.
c. Due to creep, indentation is more likely to occur on a bituminous surface
when aircraft are parked on it. Metal plates can be used to spread the load
beneath the tyres of parked aircraft, they will protect a low stability black-
top surfacing.
d. Shallow pavements comprising less than 100mm of bituminous surfacing
on low-grade granular bases (i.e. CBR <80%) are liable to structural
damage by high tyre pressure aircraft, particularly where the aircraft are
parked. In practice this situation will effectively represent a combination
of overload and excess tyre pressure and will therefore need to be carefully
considered.
(iv) Use of pavements by aircraft with tyre pressures three categories above the
designated PCN should only be considered in an emergency.
DMG 27 9 Stopways, Shoulders and Blast Pads
9 Stopways, Shoulders and Blast Pads
9.1.1. Whether stopways and shoulders should be provided is explained in the ICAO
publications Annex 1413 and the Aerodrome Design Manual Parts 1 and 211. Stopways and
shoulders should be strong enough to support any aircraft which the runway is designed for,
without introducing structural damage to the aircraft. They should also be able to support
rescue and fire fighting vehicles. The definitions of strength and serviceability are open to
some interpretation; the following sets out the design concept and method Defence Estates
uses for establishing Stopways and shoulder construction
9.2 STOPWAYS
9.2.1. A stopway provides a safe 'run out' for an aircraft if take off is aborted. It can be
included as part of the Accelerate Stop Distance Available (ASDA) which is one of the four
declared runway distances in Annex 14. Note: this distance is referred to as the Emergency
Distance Available in the AIP).14
9.2.2. A stopway surface can be unpaved or paved. A low-cost unpaved stopway could be
designed in accordance with the procedure set out in Section 9.3 for shoulder construction.
However, such a stopway would probably require some regrading and reconstruction after
each pass of an aircraft. It would also result in a surface with a variable ridability and braking
characteristics. The degree of variability will depend on the prevailing moisture contents of
the pavements and the subgrade.
9.2.3. The paved stopway designs are intended to provide support to the Design Aircraft for
0.1% of the design frequency of trafficking for the runway, before major maintenance is
required. This is achieved by designing for a reduced ACN at the design level of trafficking.
9.2.4. Using Charts 1-6 a paved stopway design can be established by the following
(i) The design ACN is the ACN of the Design Aircraft divided by 3 for flexible
pavements or 2 for rigid pavements. To allow for use by emergency vehicles the
design ACN should not be less than 5 with a minimum concrete thickness of 150mm.
(ii) The frequency of trafficking used for the runway design should be selected for the
Charts.
(iii) The pavement thickness is obtained form the relevant chart. The actual make-up of
the construction should be in accordance with Table 19.
9.2.5. With the exception of blast pads a flexible pavement is preferable to a rigid one to
provide easy future rehabilitation. A failed flexible pavement with a granular base and sub-
base can be recompacted, regraded and surfaced. A flexible pavement with a cement-bound
base can be provided with a thin bituminous overlay, or the existing surfacing can be planed
off and replaced. However, a failed rigid pavement requires a thick bituminous overlay or
complete replacement.
9.2.6. As the use of a stopway is unpredictable, and it is designed and constructed to a lower
standard than the movement areas, some maintenance work should be expected if the stopway
is to have the same life as the runway.
9.3 SHOULDERS
9.3.1. The shoulders should be able to support an aircraft running off a runway or taxiway.
The surface of the shoulders should not be susceptible to erosion and the blowing up of
debris. On a runway, grassed surface shoulders will generally suffice provided the climate
and topsoil are capable of sustaining them. However, taxiways used by large jets with
outboard engines extending beyond the edge of the pavement may need shoulders with a
paved surface to prevent erosion and foreign object damage to the aircraft.
9.3.2. Using Chart 6 or Figure 7 and Figure 8 a shoulder construction can be established in
accordance with the following procedure:
(i) The ACN design parameter is the ACN of the Design Aircraft divided by 3. To allow
for use by rescue vehicles the design ACN should not be less than 5.
(ii) For paved shoulders the frequency of trafficking used for the runway design should
be selected. For grassed shoulders the Low frequency of trafficking should be used.
(iii) The pavement construction should accord with Table 20.
9.3.3. On grassed shoulders regrading and some reconstruction would most likely be
required after each pass of an aircraft. Wheel penetration is unlikely to exceed 150mm on
prepared grassed shoulders and it would be substantially less on paved shoulders. The design
concept is based on References 28 and 29. As failure criteria and design methods are not
precise, the designs cannot be expected to be accurate and may therefore be a little
conservative.
9.3.4. Paved surfaces can give rise to a lack of visual contrast between the runway and the
shoulders. This can be overcome either by providing a good visual contrast between the
surfacings of the runway and shoulders or by applying a distinctive marking at the edge of the
runway.
9.4 BLAST PROTECTION
9.4.1. Areas adjacent to movement areas, especially those immediately off the end of
runways may be subject to blast from jet engines. In these situations paved shoulders and
blast pads should be provided. They should be large enough to prevent surface erosion and
migration of foreign materials onto the movement areas. The width of the paved shoulders
will depend on the taxiway width and the position of the outboard jet engines of the user
9.4.2. Shoulders and blast pads forming part of a stopway should be designed to the
recommendations given in Sections 9.2 and 9.3. For aircraft with high velocity turbojet
engines (e.g. fighters) a concrete surface is preferable for the blast pad, otherwise the
minimum thickness of asphalt surfacing should be 75mm.
Table 19 Stopway Constructions
PAVEMENT TYPE
SURFACING BASE/SUB-BASE DESIGN CHART
Rigid Pavement Quality Concrete (PQC) (Section 5.2) DLC - Section 1.1 Chart 1, 2, 3 or 5 as appropriate.
Cement-stabilised base - Section 1.1
Granular base. - Section 3.8
Flexible Marshall asphalt or DLC - Section 6.3 Chart 4 or 6 as appropriate.
Hot rolled asphalt or Cement-stabilised base - Section 6.3
Dense bituminous macadam Cement–stabilised base - Section 6.3
The thickness of surfacing can be reduced to 50mm with these Granular base and sub-base - Section 7.7
materials but the total thickness of construction (including surfacing + If the base is either wholly or partly cement-bound the minimum
base/sub-base) should be kept the same as that required by the Chart thickness of surfacing should be not less than 1/5th of the total
i.e. increase the base course thickness by 50mm. thickness of bound pavement construction.
Proprietary surfacing DLC - Section 6.3 Chart 4 or 6 as appropriate.
Cement-stabilised base - Section 6.3 The total thickness of construction (including
This should be of proven durability. Granular base and sub-base. - Section 7.7 surfacing + base/sub-base) should not be
If laid over a cement-bound base it should not be subject to premature less than that required by the Charts.
reflective cracking.
Possibilities are proprietary blacktop materials, concrete blocks and
grouted macadam.
Table 20 Shoulder Construction
DESIGN AIRCRAFT ACN AND TYRE PRESSURE CONSTRUCTION THICKNESS
SURFACING BASE/SUB-BASE CALCULATION
ACN … 30 Either topsoiled and grassed with a maximum topsoil If the subgrade is equal to or better than CBR 15% no Use Figure 7 and Figure 8 to calculate the thickness of
Tyre pressure … 1.5 MPa depth of 100mm or as for Table 19(Flexible). base/sub-base is required. The CBR 15% must still be granular fill required to provide a CBR 15% support level
valid in wet weather. or use Chart 6 if the depth of granular subbase required
If the subgrade is less than the design requirement this to give CBR 30% proves more economical.
can be improved with granular fill (Section 3.8).
ACN > 30 If the subgrade is equal to or better than CBR 30% no Use Chart 6 to calculate the thickness of granular sub-
and all the aircraft with tyre pressures base/sub-base is required. base required; the thickness is the X ordinate minus the
1.5 MPa The CBR 30% must still be valid in wet weather. If the Y ordinate. This will provide CBR 30%. The equivalent
subgrade is less than the design requirement this can be thickness of cement-bound base can be calculated.
improved with granular sub-base or, for paved shoulders (Section 1.1 and 6.3).
only, the equivalent thickness of cement-bound base
(Section 6.3).
NB. The ACN is the design Aircraft CAN before dividing
by 3.
DMG 27 References
1. Air Ministry Works Department. Design and Construction of Concrete Pavements.
Air Publication No. AP3129A. 1945.
2. Air Ministry Works Department. Load Classification of Runways an Aircraft
Technical Publication 102. 1948.
3. Air Ministry Works Department. Airfield Evaluation. Technical Publication 104.
4. Air Ministry Works Department. The Fundamentals of Airfield Pavement Design.
Technical Publication 107 1953.
5. Air Ministry Works Department. Airfield Design and Evaluation. Technical
Publication 109. 1959.
6. J L Dawson and R L Mills. Undercarriage Effects on (a) Rigid Pavements (b)
Flexible Pavements. ICE Proceedings of Symposium on Aircraft Pavement Design
7. F R Martin, A R Macrae. Current British Pavement Design ICE Proceedings of
Symposium on Aircraft Pavement Design 1970.
8. H Jennings, F L H Straw. Strengthening of Pavements. ICE Proceedings of
Symposium on Aircraft Pavement Design 1970
9. Department of the Environment. Design and Evaluation of Aircraft Pavements 1971.
10. International Civil Aviation Organisation, Aerodrome Design Manual Parts 1-3, First
Edition 1977, Second Edition 1983.
11. Property Services Agency. A Guide to Airfield Pavement Design and Evaluation.
HMSO. 1989.
12. US Army Engineer Waterways Experiment Station. Procedures for Development of
CBR Design Curves – Instruction Report S-77-1. 1977.
13. International Civil Aviation Organisation. Annex 14. Aerodromes – Internationa
Standards and Recommended Practices. Eight Edition 1983
14. Civil Aviation Authority. United Kingdom Aeronautical Information Publication.
London 1982.
15. BSI. Methods of test for soils for civil engineering purposes - Part 2: Classification
tests. BS 1377-2: 1990 incorporating Amendment No. 1 May 1996.
16. Road Research Laboratory. Soil Mechanics for Road Engineers. HMSO 1952.
17. D Croney. The Design and Performance of Road Pavements. HMSO London 1977.
18. US Army Engineer Waterways Experiment Station. Field Moisture Content
Investigation, October 1945 – November 1952 Phase. Report No. 2 1955.
Investigation, November 1952 – May 1956 Phase. Report No. 3. 1961.
20. W A Lewis. Full Scale Compaction Studies at the British Road Research Laboratory.
Highways Research Board Bulletin 254. Washington 1960.
21. US Army Engineer Waterways Experiment Station. Compaction Requirements for
Soil Components of Flexible Airfield Pavements. Technical Report No 3-529. 1959
22. W J Turnbull and Charles R Foster. Proof Rolling of Subgrades. Highway Research
Board Bulletin 254. Washington. 1960.
23. W D Powell, J F Potter, H C Mayhew and M E Nunn. The Structural Design of
Bituminous Roads. Report LR 1132. Transport and Road Research Laboratory,
Crowthorne, Berks. 1984.
24. D Croney and J C Jacobs. The Frost Susceptibility of Soils and Road Materials. RRL
Report LR 90. Transport and Road Research Laboratory, Crowthorne, Berks. 1967.
25. BSI. Testing aggregates - Method for determination of frost-heave. BS 812-124:1989.
26. P G Roe and D C Webster. Specification for the TRRL Frost Heave Test,
Supplementary Report 829. Transport and Road Research Laboratory, Crowthorne,
Berks. 1984.
27. R G Packard. Fatigue Concepts for Concrete Aircraft Pavement Design.
Transportation Engineering Journal. 1974.
28. R L Hutchinson. Basis for Rigid Pavement Designs for Military Airfields.
Miscellaneous Paper No 5-7. US Army Corps of Engineers, Ohio River Institute.
Washington DC 1958.
29. US Army Corps of Engineers Waterways Experiment Station. Validation of Soil
Strength Criteria for Aircraft Operations on Unprepared Landing Strips. Technical
Report No.3-554. July 1960.
30. Military Engineering Experimental Establishment Sinkage of a Dual Aircraft Wheel
Assembly. Report No 925. Christchurch. October 1965.
31. D N Brown and O O Thompson. Lateral Distribution of Aircraft Traffic.
Miscellaneous Paper S-73-56 July 1973. US Army Engineer Waterways
Experimental Station, Vicksburg. 1973.
32. H M Westergaard. Stresses in Concrete Pavements Computed by Theoretical
Analysis Public Roads. Vol 7 No 2. 1926.
33. G Pickett, M E Raville, W C Jones, F J McCormick. Deflections, Movements and
Reactive Pressures for Concrete pavements. Kansas State College Bulletin 65.
34. R G Packard. Computer Programme for Airport Pavement Design, Portland Cement
Association. Chicago, Illinois. 1967
35. G Pickett. Concrete Pavement Design, Appendix III: A Study of Stresses in the
Corner Region of Concrete Pavement Slabs under Large Corner Loads. Portland
Cement Association, Skorkie.1946.
36. US Army Corps of Engineers. Final Report on the Dynamic Loading of Concrete Test
Slabs – Wright Field Slab Tests. Ohio River Division Laboratories, Mariemont, Ohio
37. R H Ledbetter. Pavement response to Aircraft Dynamic Loads. FAA Report No.
FAA-RD-74-39-III. June 1973.
38. Highway Research Board. Joint Spacing in Concrete Pavements: 10 year Reports on
Six Experimental Projects. Research Report 17B. 1956.
39. B E Colley and H A Humphrey. Aggregate Interlock in Joints in Concrete Pavements.
Highway Research Record Number 189. 1967.
40. L D Childs. Effect of Granular and Soil-Cement Sub-bases on Load Capacity of
Concrete Slabs. Journal of the PCA Research and Development Laboratories, Vol 2,
No. 2. 1960.
41. L D Childs and J W Kaperick. Tests of Concrete Pavement Slabs on Gravel Sub-
bases. Proceedings ASCE, Vol 84 (HW3). October 1958.
42. L D Childs. Tests of Concrete Pavement Slabs on Cement Treated Sub-bases.
Highway Research Record 60, Highway Research Board. 1964.
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Record 189, Highway Research Board 1967.
44. Air Ministry Works Department. Investigation into the Value of Lean Concrete as a
Base in Rigid Pavements. Air Ministry Tech Memo No.4. 1955.
45. R D Bradbury. Reingforced Concrete Pavements. Wire Reinforcement Institute.
Washington DC. 1938.
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Public Roads Vol 16, No 8, 9 and 10, 1935; Vol 17 No 7 an 8, 1936; and Vol 23, No.
8 1943.
47. J Thomlinson. Temperature Variations and Consequent Stresses Produced by Daily
and Seasonal Temperature Cycles in Concrete Slabs. Road Research Laboratory.
June 1940.
48. US Army Corps of Engineers. Lockbourne No 1 Test Track. Final Report. Ohio
River Division Laboratories, Mariemont, Ohio. March 1946.
49. Charles R Foster and R G Ahlvin. Notes on the Corps of Engineers CBR Design
Procedures. Highways Research Board Bulletin 210, Washington, 1959.
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MEXE Report No 592.
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Guide 33. Defence Estates. Sutton Coldfield, UK, 2005.
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Functional Standard 13. Ministry of Defence. 1996.
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