Why Does FHWA Have A Chemistry Lab?

A state-of-the-art renovation in 2011 increased the laboratory's utility and effectiveness. Find out how the
laboratory can work for you.

These FHWA researchers are at work in the agency's new state-of-the-art chemistry laboratory in McLean, VA.

On
the face of it, paving a highway with hot-mix asphalt is a straightforward
process. In reality, it is very complex. Many decisions, based on a number of
variables, need to be made at every stage of the process -- from selecting the
raw materials, proper blending of the aggregate sizes, and laying the pavement
itself to create a sound structure. At any stage, something can go wrong that
may not be manifested until long after the pavement is in service, possibly
resulting in a drastically shortened service life. Extensive repairs or
replacement of pavements can be extremely costly.

The focus of the Pavement Materials Team in the Federal Highway
Administration's (FHWA) Office of Infrastructure Research & Development is
to evaluate pavement and materials to optimize their use, extend pavement life,
and reduce costs. The chemistry laboratory at FHWA's Turner-Fairbank Highway
Research Center (TFHRC) in McLean, VA, is part of that effort. One of the lab's
key objectives is to provide Federal researchers, State agencies, and industry
partners with a forensic toolbox to assist them with controlling the quality of
materials and investigating premature failures. In addition, the TFHRC research
facility sometimes evaluates commercial materials that are widely used in the
paving industry as part of a research study to determine their potential
efficacy and longevity in pavement structures.

A chemistry laboratory has existed within the Federal highway
system for more than 100 years. Prevost Hubbard was chief of the Physical and
Chemical Labs, Bureau of Public Roads, U.S. Department of Agriculture from
1905-1919. The laboratory moved to its present location in 1950. In 2011 the
lab underwent a major renovation to make it a state-of-the-art facility for
research. Today, it is housed in three rooms: One consists of a wet chemistry
laboratory, the second contains various chemical and spectroscopic instruments,
and the third houses a scanning electron microscope.

"Fundamental challenges we need to address with regard to
engineering problems are related to the chemistry of the component materials
and their interactions," says Jorge E. Pagán-Ortiz, director of FHWA's Office
of Infrastructure Research and Development. "To enable us to resolve these
complex challenges we need a sophisticated and well-equipped chemistry
laboratory."

Chemistry:
The Molecular Perspective

Traditionally,
some people think of chemistry labs as dark, smelly places containing lots of
glassware. Strange liquids bubbling away, producing ominous odors. This is
traditional "wet chemistry" where many analyses and chemical reactions are
carried out with liquids in glass flasks. Typically, these analyses and
reactions involved processes like distillation, titration, filtration, and
chemical reactions. These types of chemistry laboratories still exist,
including one at FHWA where the staff does sample preparation.

This instrument is measuring the Raman spectrum of a sample of quartz.

The world has changed. Chemistry has changed with it. Analyses
that used to take hours or days now can be completed in minutes with the aid of
new electronic analysis equipment. The main instrumentation lab at FHWA clearly
shows the difference. This room houses most of the electronic analysis
equipment. Another room houses a powerful scanning electron microscope that
enables the staff to visually examine samples of aggregate, concrete, and other
materials in great detail.

Paving and bridge engineers concern themselves mostly with the
bulk physical properties of materials. Yet, steel and concrete, like everything
else, are composed of atoms and molecules. It is the bonding between these
atoms and molecules that give strength to structures like bridges. If engineers
know more of what is happening at the atomic and molecular levels, they are in
a better position to judge the condition of a bridge or pavement.

Atoms and molecules are extremely small. For example, a piece 0.66
inch by 0.66 in. (17 millimeters, mm, by 17 mm) of 1-in. (25.4-mm)-thick steel
gusset plate would contain 6.0221415 x1023 atoms of iron (Avogadro's
number). This is such a huge number that it is difficult to conceive just how
large. Bill Bryson provides an interesting way to comprehend this number in his
book, A Short History of Nearly Everything: If the atoms
in that small sample of gusset plate were the size of popcorn kernels, they
would cover the entire United States to a depth of 9 miles (15 kilometers).

While atoms and molecules cannot be seen with the naked eye, they
can be excited in various ways using heat, light, or x-rays, for example, and
their responses to those stimuli can be measured. From this, researchers can
deduce information about the chemical structures. The response generally is
plotted against a variable like wavelength of light or x-ray energy. These
plots are known as spectra, which are produced using machines called
spectrometers. To make a spectrum easier to read, the usual practice is to show
the wavelength as a wavenumber, the reciprocal of the wavelength.

FHWA researchers at the TFHRC chemistry lab have an array of rapid
spectroscopic, optical, and analytical tools at their disposal that enable them
to study pavement materials at the atomic and molecular levels. These
techniques, along with more traditional wet chemistry methods, offer a powerful
combination that FHWA researchers can use to investigate paving phenomena and
assist other researchers in examining pavement structures.

By studying molecules and atoms, chemists look at things with a
different perspective than engineers do. For instance, pavement contractors
have used phosphoric acid for many years as a low-cost way to stiffen asphalt
and help it resist rutting caused by traffic. A premature pavement failure in
Nebraska, at first blamed on the use of phosphoric acid, led to unsubstantiated
fears in the industry concerning phosphoric acid. The FHWA chemistry laboratory
carried out a research program to investigate these concerns. Mostly, the fears
had no technical merit and were unfounded.

The chemistry lab researchers learned: Yes,
you can safely use phosphoric acid as an additive. No, it won't cause the
asphalt to age more rapidly. Yes, you can use it with limestone aggregates.
Yes, you can use it with some antistrip additives. Yes, it might lower the
moisture resistance of the pavement. Although the exact cause of the pavement
failure in Nebraska is still under investigation, the lab researchers
determined that the appropriate use of phosphoric acid can be suitable for
modifying asphalt mixtures to improve rutting resistance and pavement life.
These findings reassured some State departments of transportation (DOTs),
although some still do not allow use of phosphoric acid.

During this 6-year research program, the team developed a quick
and simple test method requiring no specialized knowledge or equipment that
State DOTs could use to detect the presence of phosphoric acid in asphalt
binders. The American Association of State Highway and Transportation Officials
(AASHTO) has adopted this test: Detecting the Presence of Phosphorous in
Asphalt Binder AASHTO Designation TP 78-09.

Putting
Chemistry To Work

Hot-mix asphalt pavements contain approximately 95
percent aggregate and 5 percent asphalt binder, the black sticky residue left at the end of the refining process
after all the fuels and oils have been removed. Its chemical composition and
properties are dependent on the source of the crude oil from which it came.
Many materials can be added either to the asphalt or the aggregate to improve
performance and extend the life of the pavement.

When water penetrates asphalt pavements, some asphalts can lose
their adhesion to the aggregate, resulting in the demise of the pavement via
stripping (separation of the asphalt film
from the aggregate in the presence of water). Pavement engineers can use
additives to prevent this adverse condition from occurring. One approach is to
treat moisture-sensitive aggregate with lime (calcium hydroxide). Since
no direct test method existed for determining the presence of lime, it was
impossible for DOTs to determine if an aggregate had been treated with lime.

To address this gap, the FHWA researchers at TFHRC developed a
test method that is now an AASHTO provisional method. Part of the method, which
answers the question of whether lime is present in the pavement, uses a
technique called Fourier Transform Infrared Spectro-scopy (FTIR).

How is FTIR applied to determine the presence of lime in asphalt?
Well, molecules are always in motion, vibrating and flexing in different ways.
When they vibrate, they absorb energy at different wavelengths of the
electromagnetic spectrum. Sunglasses, for instance, absorb in the visible
region. Carbon dioxide, the greenhouse gas, absorbs in the infrared region. By
measuring the absorption at different wavelengths, chemists can tell a lot
about the molecular structure of the material. The plot of absorption against
wavelength (more commonly, wavenumber) is an FTIR spectrum, which contains a
number of peaks. The positions of the peaks provide information about the kinds
of chemical groups in the sample, while the area under the peaks is indicative
of the amount present.

Application of the technique is simple. A small sample is placed
on the diamond window of an accessory called an Attenuated Total Reflectance
bridge, and the spectrum collected. The test typically takes less than a
minute. Asphalt produces a very characteristic spectrum, whereas the FTIR
spectrum of lime is completely different. Lime has a very sharp peak at 3,600
wavenumbers that can be used as a marker. If the asphalt contains lime, the
distinctive marker is clearly visible.

The FTIR spectrum will show whether lime is present. Determining
exactly how much lime is contained in the sample is more complicated, but this
part of the test can be completed in a few hours. The FHWA test method is now
an AASHTO provisional test method (AASHTO TP 72-08 [2010]) that the industry
can use to ensure that lime has been added to the mix.

A similar approach can be applied to asphalt binders modified with
polymers. To improve the damage resistance (that is, resistance to rutting and
cracking) of asphalt pavements, common practice is to add polymers to the
asphalt binder. These materials are
expensive compared to the other components in the mix. The most common polymer
used in the United States is SBS, a rubbery polymer made from styrene and
butadiene. This polymer confers some elastic properties on the binder. Many
State DOTs use time-consuming methods to measure the elastic properties
of the binder to ensure that they are obtaining the materials for which they
are paying. Determining the presence of these polymers can be achieved rapidly
and simply by taking an FTIR spectrum of the material in question. Some DOTs
use this method, and the FHWA researchers at TFHRC have the capability of
running the test in the chemistry lab. Two peaks in the spectrum indicate the
presence of the styrene and butadiene, and the size of the peaks can be used to
calculate the quantities present.

FTIR Spectra: Asphalt and Lime

This FTIR spectra clearly demonstrates the presence of lime.

A third application of FTIR spectroscopy is to monitor the aging
of an asphalt binder. Asphalt is a black sticky solid that reacts slowly with
oxygen in the air and oxidizes. With time, it becomes brittle and may begin to
crack and fall apart. This aging is a major factor limiting the life of an
asphalt pavement. Researchers study asphalt aging in order to monitor the
degradation of a pavement with a view to predicting and extending pavement
life. By using FTIR to measure the amount of carbonyl and sulfoxide, two of the
oxidation products that contribute to the pavement's embrittlement, researchers
can study the rate of aging to determine its extent or to find ways of slowing
it down.

Over the years, a number of materials have
been marketed as additives for asphalt binders with the promise of extending
pavement life. Some of these additives can be detected using x-rays. While FTIR
spectroscopy provides information about molecular environments, x-ray
fluorescence spectroscopy (XRF) can provide information about the elements
themselves. With XRF, samples are irradiated with x-rays, and a complete
analysis of all the elements from sodium to uranium in the Periodic Table is
provided in just a few minutes. FHWA initially purchased the XRF unit for the
chemistry lab to analyze cement and concrete, but the team has found it useful
for investigating asphalt binders as well.

Examination of the trace metals in asphalt binders recovered from
pavements can provide insights into what materials have been added. These binders
can be helpful in forensic investigations.

Sample AAK-1 is a reference asphalt used in the Strategic
Highway Research Program. The crude oil source from which the sample was
derived came from Venezuela. All crude oil contains vanadium, but this particular
one from Venezuela contains an unusually high level of this element.

Sample B6286 had been modified by a special process in which
rubber ground from used tires was digested into the asphalt. The high level of
zinc in this asphalt came from the tire rubber.

Sample B6269 also contained high levels of zinc as well as iron
and copper. This too had been blended with ground tire rubber by a simple
thermal shearing process used in Arizona.

Sample XRF1-75-2 was from a forensic study in
cooperation with the Maine Department of Transportation. The sample had been
modified with phosphoric acid.

Sample AS1-134-2, the last sample, had been modified
with a residue obtained from waste engine oil. Calcium and zinc found in the
sample came from additives used in the manufacture of the engine oil; iron and
copper were metals worn from the engines.

FTIR Spectrum of Asphalt Binder Containing SBS Polymer

The styrene and butadiene bands in this FTIR spectrum show that the asphalt
binder contains SBS polymer.

Another application is illustrated by a recent forensic study in
which the FHWA researchers at TFHRC were asked to help identify the root cause
of a premature pavement failure in Nevada. The distress mechanism was described as
top-down stripping (moisture
damage) and fatigue cracking. Federal Lands personnel who submitted the request
suspected that lime had been omitted from the mix and that the asphalt binder
had been contaminated with heating oil or diesel fuel. By using the two
techniques together (FTIR and XRF), FHWA was able to show that lime had indeed
been added to the mix and that the asphalt binder had been modified with waste
engine oil residues. Characterization of the cores turned up nothing out of the
ordinary except that the effective asphalt film thickness was found to be below
specification and was the most likely cause of failure.

Current
Research

The
FHWA chemistry lab also is involved with research into liquid antistrip
additives sometimes used in asphalt binders. Liquid antistrip additives are
used to improve the moisture resistance of asphalt pavements and function in a
way similar to the addition of lime. The major differences are that the liquid
additives containing amines or phosphate esters are added to the asphalt binder
and not to the aggregate as is the case for lime usage. Since many liquid
antistrips are available and customers have their own preferences, asphalt
producers usually meter them into the truck while it is being loaded at the
asphalt terminal. No method exists, however, to determine accurately whether
the correct quantity of the specified material was added and ended up in the
asphalt mixture. If the pavement shows signs of moisture damage early in its
life, this sometimes leads to disputes between DOTs and contractors. The FHWA
chemistry lab is developing a method to identify and accurately determine the
quantity of liquid antistrip in asphalt binders.

XRF Elemental Analysis of Asphalt Samples

Concentration in Parts Per Million

Sample Reference

AAK-1

B6286

B6269

XRF1-75-2

AS1-134-2

Phosphorous

950

870

570

8,140

3,060

Sulfur

52,140

31,270

37,890

41,600

15,650

Calcium

2,256

Iron

13

15

164

19

284

Copper

89

447

Zinc

298

2,540

1,202

Molybdenum

16

96

Lead

Vanadium

1,483

270

887

273

146

Nickel

154

59

115

68

81

Another part of the lab's current research is
to find a rapid method to identify the presence of alkali-silica reaction (ASR)
gels in concrete. The presence of ASR gels causes a destructive expansion that
takes place in some concrete structures. No reliable field test to detect the
presence of ASR gels exists. The chemistry
laboratory is using a technique called Raman spectroscopy to detect the presence of ASR gels. Raman
spectro-scopy is a technique widely used by the FBI to investigate forgeries
and by the art world to examine paintings. The sample is irradiated with a
powerful laser light, which polarizes the electrons around the molecule and
results in a Raman spectrum, similar to the FTIR spectrum described earlier.
The reason for this research is not only to develop a field test, but also to
come up with a rapid test to determine the potential of an aggregate to form
ASR gels when the aggregate is used in concrete. This test would replace the
mortar bar test ASTM 1260, which takes 16 days, and ASTM1293, which takes 1-2
years.

The analytical techniques discussed so far deal with atoms and
molecules. The lab also can look at larger entities, namely crystals that have
a uniform chemical packing. The atoms in crystals arrange themselves in a very
precise way, giving the crystal a characteristic shape that depends on the
material. Table salt, as an example, under a microscope appears as tiny cubes.
Many materials exhibit characteristic crystalline structures and have the
ability to diffract x‑rays in characteristic patterns.

FTIR Spectrum of an Aged Asphalt Binder

The presence of the carbonyl and sulfoxide peaks reveals that this is an aged
asphalt binder.

These diffraction patterns are obtained using an x-ray
diffractometer. The sample is exposed to a narrow x-ray beam. The detector
measures the response from the sample, and the signal strength then is plotted
against the angle of the x-ray beam to the sample. The characteristics of these
patterns facilitate the identification of the various crystals present in a
sample, and the lab's researchers use them to study cement hydration and fly
ash being used as a substitute for cement in concrete.

The manufacture of cement produces carbon dioxide, which is
considered a greenhouse gas. The industry can reduce the amount of cement
manufactured and the greenhouse gas emissions produced by substituting fly ash
for the cement used in concrete. Fly ash is the dust collected from the stacks
of coal-burning power plants. In the United States, coal plants produce more
than 100 million tons (90.7 million metric tons) of fly ash per year, of which
30 million (27.2 million metric tons) are utilized, some of it to replace
cement, and 70 million (63.5 million metric tons) are landfilled. The problem
is that concrete made with high amounts of fly ash takes longer to set than
regular concrete, although its ultimate strength might be higher. A concrete
pavement usually can be opened to traffic a week or so after construction. High
levels of fly ash probably could double this time.

A little crane lowers the cups containing samples into the x-ray chamber of the XRF spectrometer.

Cement contains materials with delightful names like alite,
belite, aluminate, and ferrite. When concrete is made by mixing cement with
water, sand, and aggregate, chemical reactions take place and these materials
change. They have a characteristic x-ray diffraction pattern that changes as new
substances are formed. These changes can be measured using an x-ray
diffractometer. By placing a small sample of wet cement in the machine,
researchers can accurately measure the rate at which these materials form,
indicating how rapidly the concrete will set. When fly ash is introduced, the
reaction products and the rate at which they are formed change. Researchers can
use these changes to explore ways of increasing reaction rate so that the
concrete sets more rapidly and can accommodate highway traffic sooner.

Chandni
Balachandran,
with the FHWA
chemistry lab
at TFHRC, is
using the Raman
spectrometer
to examine a
specimen of
an ASR gel.

Summary
of Major Points

FHWA has a new state-of-the-art
chemistry facility that is available to assist State DOTs and industry partners
in evaluating pavement materials and premature pavement failures.

Major problems solved and
accomplishments include technical information to support limited use of
phosphoric acid, detection of lime, and evaluation of the characteristics of
degradation through aging.

Current research at the lab is
tackling difficult challenges at the forefront of highway research, such as
detecting ASR gels and reducing the highway industry's carbon footprint.

What's
Next?

This
snapshot of some of the chemistry lab's activities perhaps can suggest to
readers how their agencies might make use of its capabilities. Though the lab's
researchers use complex equipment to conduct much of their work, they have an
eye toward developing simpler devices for use in the field.

Forensic investigations are far from trivial exercises. For
readers who find themselves scratching their heads over tough problems, the
lab's team urges them to call -- and adds, "Even if you don't have a problem,
come and see the lab. Visitors are always welcome!"

Raman Spectrum of an ASR Gel

The FHWA lab at TFHRC is believed to be the first to successfully identify an ASR gel using Raman spectroscopy.

Terry Arnold manages the chemistry research complex at TFHRC. A
native of England, he has
a bachelor's degree in chemistry from the Royal Institute of Chemistry and is a
fellow of the Royal Society of Chemistry.

Gretchen Stoeltje works in the Texas Department of Transportation's Office of Strategic Policy and
Performance Management. She researches, writes, and makes films about
transportation and its connection to other areas of public policy. She earned a
bachelor's degree in film theory and a graduate certificate in film production from the University of California Santa Cruz. She earned a law degree from Santa
Clara University.