The aim in prestressing concrete beams may be, according to Garden and Mays , either to increase the serviceability capacity of the structural system of which the beams form a part or to extend its ultimate limit state. According to El-Hacha , FRP are well suited to prestressing applications because of their high strength-to-weight ratio that provides high prestressing forces, without increases on the self-weight of the post-strengthened structure. The prestressing technique may improve the serviceability of a structural element and delay the onset of cracking.
When prestressed FRP are used, just a small part of the ultimate strain capacity of the material is used to prestress the FRP, the remaining strain capacity is available to support external loads and also to ensure safety against failure modes associated to peeling-off at the border of flexural cracks and at the ends of the post-strengthening. However, in some cases, it may be advantageous to bond FRP sheets or strips onto the structural element surface in a prestressed state.
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According to the Bulletin 14 of fib , prestressing the FRP prior to bonding has the following advantages:. On the other hand, prestressing FRP systems are more expensive than the non-prestressed ones, due to the greater number of operations and the equipment that is required to prestress the FRP.
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Prestressed FRP bonded to concrete structures are sujected to prestress losses, as it happens in any prestressing system. Such prestress losses may be instantaneous, due to immediate elastic deformation of concrete, or time dependent, due to creep and shrinkage of concrete and relaxation of the FRP. If prestress is applied by reacting against the structural member there will be no loss. It happens because if the prestressing device if fixed on the structural element that will be post-strengthened, a compensation occurs: as the PRF is being stressed, the concrete is being compressed.
However, FRP elements that have already been prestressed will experience a loss of prestress due to the shortening of the beam upon the prestressing of subsequent FRP elements. In such cases it is necessary to determine the average loss of prestress per FRP element. The document also informs that losses due to relaxation of fibers may be neglected when CFRP are used, since the relaxation of carbon fibers is very low. Results of a research program developed by Triantafillou et al.
Garden and Mays  consider that prestressed FRP also suffer prestress losses due to the shear transferred through the adhesive and into the concrete by the FRP tension. This shear action is sufficient to fracture the concrete even at low prestress levels so it is necessary to install anchorages at the ends of the FRP element to resist this action.
Figure  a , by Triantafillou et al. Horizontal shear cracks propagated from both ends of the CFRP strip through the concrete layer and stopped at a certain length. Figure  b shows that this failure mode may be prevented if anchorage systems are used at the ends of the strips.
Failures in Concrete Structures: Case Studies in Reinforced and Prestressed Concrete
Thus, the addition of anchors at the end of the prestressed FRP sheets or strips reduces the shear deformation that occurs within the resin or adhesive layer upon releasing the prestress force and reducing the shear stresses transferred to the base of the concrete section.
Thereby, anchorage systems minimize the possibility of premature failures El-Hacha . According to El-Hacha et al. It is important to have in mind that, when post-strengthening is prestressed the modulus of elasticity of the FRP is of great significance, once the FRP element needs to be stiffer to hold up a significant loading that, before the post-strengtnening, was made only by the steel reinforcement El-Hacha, .
In this method, the FRP strip is first prestressed then bonded at the beam that will receive the post-strengthening. Since it is very complicated to grab and prestress the FRP strip, due to its anisotropic behavior, a prestressing device was designed, as one can see in Figure . The prestressing device consists of two wheels which are connected to a beam of the required length, as shown Figure  a. The FRP strip 1 is wrapped around the wheels 2 and clamped at its ends 3 as shown in Figure  b.
The strip can be prestressed by rotating one or both wheels 4a or displace the wheels 4b.
As one can see in Figura  a , the prestressing device with the prestressed FRP strip is temporarily mounted to the structure and can be pressed against the structure with a constant pressure by means of an air-cushion 5 between the FRP strip and the beam. Stoecklin e Meier . In a new version of the prestressing device developed by Stoecklin and Meier , two separate prestressing units at each end of the strip are directly mounted to the structure, what means that the FRP strip is prestressed against the structure, as shown in Figure . To overcome anchorage problems at the ends of the FRP strips, the prestressing force can be reduced gradually from the mid-span to both ends of the FRP strips.
As described by Meier et al. A system of electric heating may be used to speed up curing of the adhesive in the bonded section within the pot life of the adhesive. After curing the central part of the FRP strip at mid-span, the prestressing force is slightly reduced and another section is bonded at each side of the strip also using the electric heating system to speed up curing the adhesive. This process is repeated in several stages until the entire length of the strip is bonded and the prestressed level at the ends of the strips has been reduced to a low level, as one can see in Figure .
In this way, anchorages are not required at the end of the prestressed strip. In the prestressing method developed by Stoecklin and Meier  the strip is prestressed before the application at the beam.
In such case, prestressing is applied by reacting against the structural member, since the prestressing device is mounted to the structure. When the first FRP strip is prestressed, using the device developed by Stoecklin and Meier , imediate losses of prestress due to elastic deformation of the concrete, that happen when the prestress force is released, can be neglected, since prestressing is applied by reacting against the structural member.
However, strips that have already been prestressed will experience a loss of prestress due to the shortening of the beam upon the prestressing of subsequent FRP strips. According to Hollaway , the anisotropic behavior of the composite materials leads to a complex rupture mechanism that may be characterized by extensive damages on the composite material when submitted to static and cyclic loading. The level of damage, however, depends on the properties of the composite material and on the applied loading.
Failure modes of reinforced concrete structures post-strengthened with FRP include crushing of concrete, yielding of steel reinforcement or tensile failure at the FRP. Teng et al. Thus premature failures, in general, are associated to:. Concrete structures post-strengthened with prestressed FRP also show premature failures as described by Teng et al.
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