Keywords: CFRP, rehabilitation, composite materials, new technology.

The strengthening of reinforced concrete structural elements by using externally bonded CFRP seems to be an effective solution and also the most economic over a long time period. Sometimes the compressed zone of concrete fails since it is not capable of sustaining the new stress state produced by the increase in resistance ability of the structural element. In this work we give a new scientific contribution in terms of both the experimental and the analytical study of the use of carbon fibers applied to civil engineering structures. With this aim, the investigation was carried out on a damaged and then repaired reinforced concrete frame. The behavior of the tested structure was firstly examined both in absence and in the presence of the CFRP. The innovative technology, proposed in the paper, consists in applying the CFRP by means of the interposition of a thin element of polyurethane glued at the internal face of the frame mesh as shown in Figure 249.1. The polyurethane is deprived of any structural feature but it has the scope of centrifuging the CFRP layer so as to increase the flexural resistant required for the retrofitting. This technology was already tested by the same authors on a single reinforced concrete beam, and it has given excellent results with an enhancement of the ultimate resistance area (not only in terms of the flexural capacity), up to two or three times higher with respect to the original section. This is a result of increasing the internal couple of the cross section; but also in terms of the compression strength as a result of the domain rotation towards the right side of the shown diagram [1]. This solution seems to be very useful in the case of the change of the use of a civil engineering structure and at any time an increase in the resistant section is required.

The frame structure now strengthened by the proposed methodology, will provide both ductility, capable of resisting the horizontal loads, and a greater resistance to the vertical loads, by avoiding any dangerous increment of the gravitational weight of the traditional strengthening techniques which consists of inserting either of concrete or steel plates. The analytical investigation of the strengthened frame is based on a finite element analysis carried out by a specific numerical code (ANSYS) while the transverse section verification is based on the ultimate limit state theory, according to european and international codes. Some new limit fields were defined for the transverse section of the structural element in order to consider the CFRP contribution.

The experimental test is carried out on a reinforced concrete frame at a 1:2 scale. The frame has a constant vertical load applied on each column and also a variable and a cyclic horizontal load. The behaviour in terms of load displacements, of both the investigated systems is reported in Figure 249.2. The hysteretical loops of the reinforced frame, derived from the tests, seems to be sensible. Besides polyurethane is an extremely deformable material, and does not suffer from either plastic deformation or fragile breaks. So it appears to be a very suitable material for the prefixed scopes to enhance the bearing capacity of the structural members of a frame while avoiding dangerous increment in the structural mass.

1

Anania L., Badalá A., Failla G. (2003). "Increasing of the Flexural Performance of r.c. Beams Strengthened with C-FRP Materials", Construction And Building Materials.19 (2005) 55-61. doi:10.1016/j.conbuildmat.2004.04.011

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