Strengthening of reinforced concrete two-way slabs using mechanically fastened FRP systems
A new technique for strengthening reinforced concrete (RC) slabs using FRP composites is investigated. It is referred to as the mechanically fastened (MF) technique, which is based on fixing FRP materials to concrete using closely spaced powder-actuated fasteners.A special FRP material, known by the trade name SAFSTRIP ª , is used in this method. It is characterized by a high bearing strength as well as a high tensile strength. This new technique is very rapid compared to the conventional bonded FRP techniques. Furthermore, it allows for immediate use of the structure after completion of the installation process. In addition, unlike the bonded system, the use of the MF system results in a desirable ductile behaviour of the strengthened structure. The commonly used fasteners in this technique are embedded into the concrete using a powder actuated fastening"gun". The shot of this fastener is very strong and may sometimes cause initial cracking or concrete spalling or may locally damage the FRP if the fastener is overdriven. To overcome these problems we have proposed a new fastener based on screwing rather than shooting the fastener into the concrete. This screwed fastener has a better anchorage into the concrete, and displays a much better performance. This study attempts to investigate the behaviour of RC slabs strengthened in flexure with the mechanically fastened FRP system. Two series of large-scale reinforced concrete slabs are tested. The first series is comprised of five slabs without a cut-out, and measuring 2600 x 2600 x 120 mm; the second series consists of four slabs of the same dimensions with a central cut-out measuring 800 x 800 mm. In each series, one slab is left unstrengthened to serve as a reference while FRP strips are used to strengthen the remaining slabs. Different strengthening patterns are used with different spacings between fasteners.Also, the conventionally bonded strengthening technique is used for the sake of comparison. The mechanically fastened system is found to be an interesting alternative to the bonded system resulting in a rapid, economic, and effective system. The gained increases in ultimate capacities of the MF FRP-strengthened slabs range between 30 to 70% over those of the unstrengthened specimens. Moreover, the use of the MF system shows a desirable pseudo-ductile mode of failure.As a complementary study, experimental results from 27 direct shear tests on FRP strips mechanically fastened to concrete blocks are reported. Based on the results from these tests, new analytical models that describe the interfacial behaviour between the MF FRP and concrete substrate are developed. These models are referred to as"bearing-slip" models. Finite element models are then introduced to address the interfacial behaviour between the FRP strips and the concrete substrate for the MF FRP/concrete direct shear specimens. Results are presented in terms of ultimate load capacities and load-slip relationships. The numerical predictions are verified against the experimental data, and very good agreement is obtained. Based on this verification, the proposed bearing-slip models are subsequently used in finite element analyses to simulate the cases of RC beams and slabs strengthened with the MF system.Also, the theoretical part of this study is extended to include finite element modelling of bonded FRP-strengthened slabs. The interfacial behaviour between the FRPs and the concrete substrate is accounted for by using appropriate interfacial models. It is shown that the numerical models can be applied to arbitrary FRP configurations, and can also accommodate both passive as well as prestressed FRP strengthening schemes. Results are presented in terms of load-deflection relationships, ultimate load capacities, failure modes, and interfacial slip and stress distributions. When compared to test results reported in the literature, the analysis is shown to lead to very good predictions.
- Génie – Thèses