ANCHORAGE OF COMPOSITE LAMINATES IN RC FLEXURAL BEAMS

The flexural capacity of reinforced concrete (RC) beams can be increased by applying carbon fibre reinforced polymers (CFRP) in the form of fabric or laminates. Concrete elements strengthened with bonded FRP systems are often showing a brittle failure mode with premature delamination of the FRP ply at the concrete-FRP interface. This can be quite catastrophic and is undesirable since it does not allow exploiting the FRP mechanical properties to be fully utilized. The failure is often brittle. It has been was demonstrated that the use of anchoring systems (i.e., U-wrap or anchor spikes) are effective in order to raise the ultimate load; failure of the beam remains essentially brittle with a substantial drop of the load after the peak. This paper basically focuses on flexural strengthening with pre-cured FRP laminates applied on RC beams with different anchoring devices chemically and mechanically applied. One beam was maintained as un-strengthened control sample. Two beams were strengthened using CFRP pre-cured laminates: for one of them, glass fibre anchor spikes were applied, the other was kept without anchoring. Two other beams were strengthened using hybrid pre-cured laminates having high longitudinal bearing strength: the laminate was bonded on one beam (steel anchors were also used to prevent/delay peeling) while was mechanically fastened (MF system) to the other one. The results showed that the ultimate load of the strengthened beams was 35-80% higher than the control beam, depending of the type of strengthening system applied. The lower value corresponds to the beam strengthened using CFRP without anchor spikes, whose use increased the ultimate load of about 14%. The best performance of the strengthening was obtained for the beams strengthened with the hybrid laminates: steel anchors reduced the load drop after the peak while, in the case of hybrid system and mechanically fastened CFRP, the use of the MF system resulted in a pseudo-ductile mode of failure. The experimental results were compared with analytical results obtained from the application of recently developed guidelines, that were published by the Italian Research Council (CNR). It is shown that design equations, even if material safety coefficients are disregarded, resulted conservative in both cases of unanchored and anchored FRP plate.