Effect of shear-span/depth ratio on debonding failures of FRP-strengthened RC beams

Debonding of fiber-reinforced polymers (FRP) from the concrete substrate is a major concern that compromises the effectiveness of FRP-strengthening. The intermediate-crack (IC) debonding and concrete cover separation (CCS) are the two main types of debonding failure of FRP-strengthened beams. Multiple factors may affect and lead to any of these two debonding failure modes. In this paper, the influence of shear-span/depth ratio (a(v)/d(s)) on the ultimate capacity and failure mode of FRP-strengthened beams is explored. A total of 14 beams comprised of 10 strengthened in flexure with carbon FRP (CFRP) sheets along with 4 control beams were investigated covering varying a(v)/d(s) ratios in the range of 1.5 to 7. For all the strengthened beams, the CFRP sheets were externally bonded to the soffit of the beams and extended close to the supports. The beams were simply supported and were tested in four-point bending. The results of this study showed clear effect of a(v)/d(s) ratio on the type of debonding failure and strain at failure. For the beams tested in this study, a threshold value of a(v)/d(s) ratio of 3.0 was identified at and below which the failure mode shifted from IC debonding to CCS. The results also indicated that all beams with CCS failed at the same shear load irrespective of a(v)/d(s) ratio. Comparison of experimental results with provisions of various design codes and guidelines to guard against debonding failures clearly shows the need for improved prediction of CCS failure.