Effects of adhesive properties on strengthening of concrete beams with composite strips

This paper reports a recent study regarding the influence of adhesive properties on mechanical/structural behaviour of reinforced concrete beams strengthened with fibre reinforced plastic (FRP) strips. The FRP strips are bonded to the concrete beams using epoxy adhesives containing rubber modifier of different contents. The results from flexural tests are compared with those bonded with a conventional neat epoxy resin and those without strengthening FRP strips. The flexural tests results confirm the general findings of previous reports. While the FRP reinforcements improve significantly the stiffness and strength, they show detrimental effect of much reduced ductility. Of note is that the rubber modifier increases both the maximum bending force and the corresponding displacement, compared to those bonded with a neat epoxy resin, for all beams tested with different number of layers of FRP strip. The improvement of flexural ductility is particularly remarkable as the rubber modifier can mitigate the harmful effect of reduced ductility. Improvement of FRP delamination resistance and stability is mainly responsible for the observed results, as confirmed by the interlaminar fracture toughness measurements based on the cantilever beam tests. Apart from the apparent improvement of delamination stability, the rubber also has a beneficial synergy of strengthening due to the penetration of epoxy resin into the concrete surface layer. Practical implications of the results are discussed.