Parametric study of bond failure in concrete beams externally strengthened with fibre reinforced polymer plates

The use of adhesively bonded fibre reinforced polymer (FRP) plates to enhance in-service concrete beams and slabs in flexure is rapidly expanding within the structural rehabilitation industry. Failure of FRP-plated concrete members commonly occurs by brittle rupture of the plate-to-concrete bond. For plated reinforced concrete beams the most commonly reported mode of such bond failure initiates at the ends of the plates and propagates inwards along the beam. In this article, it is shown that two other bond failure modes, which initiate either at midspan with propagation out to the ends of the plates or within the inner end of the shear span with propagation to both the ends and middle of the plates, can also be critical for FRP-plated concrete beams. It is also shown that both failure modes propagated mainly through the concrete near the bondline. One use of the FRP plating in this context is to replace corroded internal steel in the deficient concrete structure. However, the local beam section changes due to the presence of corroded steel rebar generate high local plate-to-beam bond stresses, which trigger bond failure. Therefore in the study reported here, varying extents of steel corrosion were simulated in different plated beams, which were tested to bond failure. The thickness of the adhesive layer was introduced as another test parameter. The effects of these variables on the bond failure loads and modes of the plated beams are reported in this paper. Commercial plates and adhesive were used in the tests. No rupture of the plates was observed. Key video images of the final sudden debonding failure, showing the crack propagations through, and explosive rupture of the cover concrete adjacent to, the plates are presented.