Masonry is frequently used as an "expensive" exterior wall decoration, with complete disregard for its structural properties. In general, masonry, like concrete, is very strong in compression but very weak in tension. One method to overcome this structural deficiency is to reinforce masonry with steel bars, similar to reinforced concrete. Another approach, which looks more attractive in the case of masonry, is to use prestressing. For example, Curtin et al (1982) show that for a 20% increase in cross sectional area, a diaphragm wall can have 15 times the bending resistance of a cavity wall. Post-tensioning will provide a further increase in resistance, perhaps up to 150 times that of the original cavity wall.
It is believed that the simplest, yet most effective and cheapest technique of prestressing masonry is post-tensioning with unbonded steel tendons. However, serious problems have been reported regarding the performance of unbonded steel tendons used for post-tensioned masonry and, in general, concrete structures. One of the most common problems is associated with steel corrosion, even when the "right" protection technique is used. Significant loss of prestressing may occur as a result of the tension corrosion and may lead to catastrophic failure (e.g. Elliott and Morrison, 1995). Thus, what starts as a dream of having an economic and aesthetic structural element may turn into a continuous nightmare of rehabilitation.
As an alternative for steel tendons, new advanced corrosion-free materials have been introduced. These promising new products are Fibre-Reinforced-Plastic (FRP) materials.
In order to use FRP tendons in masonry, an anchorage system must be designed which allows for the development of the full strength of the prestressing cable, but which has minimal creep and loss of loads at transfer. The traditional anchorages for FRP tendons involve either epoxy resins or soft metals between tendon and anchorage. Loss of load due to displacements in these systems is likely to make them inadequate for the "short" spans of masonry. Hence, the first stage for post tensioning masonry with FRP tendons is the development of an appropriate anchorage system.
The concept of making fibre reinforced composite materials for improved performance is very old: in ancient Egyptian civilization straw was used to reinforce clay bricks. Masonry reinforced with iron rods was used in the nineteenth century, leading to the development of reinforced concrete. During the early twentieth century, Phenolic resins reinforced with asbestos fibres were introduced (Daniel and Ishai, 1994).
In the early 1940s, the first fibreglass boat was made, followed by filament winding which was introduced in 1946 and incorporated into missile applications in the 1950s. The first high strength carbon fibres were introduced in the early 1960s and were used in aircraft industry by 1968. KEVLAR.TM. (or aramid) fibres were later developed in 1973. By the late 1970s advanced composites were utilized widely in the aircraft, automotive, sporting goods and biomedical industries, as well as many other fields. The 1980s marked a significant increase in high-modulus fibre utilization (Daniel and Ishai, 1994).
As a result of their high durability and corrosion resistance, FRP's have been pioneered in recent years (late 1980s and 1990s) as an alternative to prestressing steel tendons, especially in bridges. Most FRP's used today are reinforced with glass (GFRP), aramid (AFRP), and/or carbon (CFRP). Both CFRP and AFRP have been recently used for both pre-and post-tensioned applications. The world's first highway bridge, with a span of 47 m, prestressed with CFRP was build in Germany 1986 (Ballinger, 1991). In 1994, two masonry footbridges were lowered into place in the UK. One of them incorporated PARAFIL.TM. rope prestressing tendons and the other was prestressed by steel tendons (Shaw and Baldwin, 1995 and Shaw et. al. 1995). In Canada, another bridge prestressed with CFRP tendons was built in Calgary (Grant et. al. 1995).
The typical stress-strain relationships of FRP tendons show that none of them exhibit the inelastic response typical of steel tendons, and thus no ductility is observed in the failure of this kind of material. This shortcoming must be addressed in the design codes before there will be any widespread practical usage of FRP in prestressing applications.
GFRP offers the cheapest alternative to steel tendons where its price is very comparable to steel. However, its mechanical properties are disappointing compared with the other two types of FRP. GFRP has the lowest tensile strength (Holte et. al., 1993a), and is very sensitive to fatigue damage (Multi et. al., 1991). Furthermore, GFRP suffers creep rupture more than the other two types where midterm failure is observed at 33% of the ultimate load compared to 50% and 80% for AFRP and CFRP respectively (Slattery, 1994). GFRP's are also very sensitive to alkaline media and lose much of their strength when exposed to moisture and/or increased temperature (Hercules aerospace, 1995).
Although CFRP is more expensive, it has the more appropriate structural properties. Of the FRP considered, CFRP exhibits the highest tensile strength (Hercules aerospace, 1995), excellent fatigue strength compared with steel tendons (Rostasy et. al., 1993) and very low relaxation (Rao, 1992 and Santoh et. al., 1993). The biggest advantage of CFRP is the high durability and corrosion resistance compared with steel tendons.
In the post-tensioning method, the material is constructed about a tendon which is not bonded to the material. Once the material (either concrete of masonry) gains its strength, the tendon is anchored at one end and a jack is usually used at the other end to stretch the tendon. When the required level of prestressing force is reached, the tendon is anchored with a suitable anchorage system to transfer the prestress to the masonry and/or concrete. The jack is then released. As may be deduced from this sequence, the key-point in the post-tensioning technique is the anchorage system.
FRP tendons are much more sensitive to loads in the transverse direction compared to steel tendons. However, conventional anchorage systems cause stress concentration in the transverse direction around the wedge teeth which will generally lead to cable/rod failure (failure mode type 2). Thus, new anchorage concepts are required for FRP. The most common types of FRP anchorage used to date are (Holt, 1993a,b):
Split wedge. A metal wedge in a conic housing is used to grip the tendon. (Tokyo Rope, 1990 and Iyer et. al., 1991). The main anchorage concept is that the wedges compress the perimeter of the tendon and teeth in the wedges grip it. Wedge teeth lead to fracture of the tendons due to the biting action of the wedge. Enka (1986) used a plastic wedge system but the usage of this system is limited to pre-tensioning prestressing.
Plug in-cone. A bundle of tendons is placed in a conical housing socket. A solid cone (spike) is then driven into the bundle centre to splay out the tendons and gripping them individually between the spike itself and the socket. The system is reported to perform well under a static load (Burgoyne, 1990). The system has the advantage of not using resins around the tendons and thus suffers no creep deformation and is not sensitive to elevated temperature. The main disadvantage of this system with respect to FRP tendons is that the tendons are not straight at the front of the anchorage which may shatter the fibres apart.
Resin-sleeve. An epoxy resin is injected between a cylindrical steel shell (sleeve) and the tendon. The inside surface of the sleeve is usually deformed or threaded to improve the load transfer (Wolff and Miesser, 1989 and Tokyo Rope, 1990). In addition to suffering excessive creep deformation and being sensitive to moisture and thermal loads, rod bond failures were also reported for this anchorage system (Holte et. al., 1993a).
Resin-potted. The resin-sleeve anchorage system is modified to this geometry to achieve better anchorage. The resin-potted anchorage system is actually a combination of the split wedge system and the resin sleeve system where the compressive action of the split wedge is developed while the continuous bond of the resin releases the biting action of the teeth. However, creep deformation and sensitivity to thermal loading and moisture are still major problems for this type of anchorage (Dolan, 1991 and Iyer et. al. 1991).
Soft-metal overlay. This anchorage is used by Tokyo Rope (1990). The gripping pressure is transferred to the FRP rods through a soft metal tube (sleeve). With this configuration the metal sleeve is permanently bonded to the cable and gripping is achieved using a conventional strand chuck. Typically the soft metal is aluminum or an aluminum alloy. These materials corrode in concrete and are thus unsuitable for use in masonry as well.
Swaged anchor. In this type, the rod/cable is embedded in a resin and transverse stress is generated along a steel shell using bolts and nuts. Increased friction along the surface of the tendons is generated and provides the required gripping (Sippel, 1992).
From an understanding of the prior art as set out above, the inventors have identified requirements for post-tensioning prestressing anchorage system for masonry, as follows:
The anchorage must develop the maximum tensile capacity of the prestressing strands: that is the tendon should fail at its maximum capacity rather than slip out of the anchorage, fail prematurely or cause anchorage failure. The system should develop a minimum of 95% of the ultimate tensile strength of the tendon which is referred as the anchorage efficiency: this is a major requirement for the anchorage. The anchorage must also allow correlation between the prestressing force and the elongation of the tendons.
At the release of the jacking force, the anchorage must undergo a very small, predictable deflection. This is because a large deflection would reduce the load in the tendon substantially, particularly in the "short" lengths which may be expected in masonry walls.
The anchorage must perform at the same level throughout the lifetime of the structure. The stressing operation should only have to be performed once. Thus creep in the anchorage must be minimal.
In addition, the inventors have identified that the most common failure modes of FRP anchorage systems can be summarized as follows:
Rupture of the cable-rod within its free length. This mode is the one which indicates that the anchorage is working as planned. It demonstrates that the tensile capacity of the FRP cable/rod is totally developed.
Shear failure in the anchorage zone. The cable/rod may be pinched due to the large shear stress concentration that occurs with certain anchorage geometries. The shear stress causes premature failure of the tendon.
Bond failure between the epoxy and the cable/rod (for epoxy anchorage systems: eg. Type 3 or 4 anchorages defined above). Due to bond failure, no load transfer occurs between the cable/rod and the anchorage which causes this type of failure.
Excessive deflection and/or long-term creep. The low elasticity modulus epoxy resin (epoxy anchorage system) is very sensitive to high temperature and exhibits long term creep deformation as well. As a result, the undesired longitudinal deformation resulting from these two shortcomings may lead to significant loss of prestress force.
Slip failure between the cable/rod and the grip. This type of failure is catastrophic and leads to complete loss of prestressing force due to cable/rod pulling out from the anchorage.