Effects of coarse aggregate volume on CFRP-concrete bond strength and behavior

A major challenge in using fiber reinforced polymer (FRP) composites for repair and strengthening of concrete structures has been debonding of FRP from concrete substrate. It is, therefore, essential to identify the parameters involved in FRP-concrete bond behavior to predict the loading capacity of the bond and, thereby, possibly postpone the debonding of the strengthening sheet. One such important parameter is the coarse aggregate at the interface between the substrate concrete and the FRP sheet which becomes exposed after surface preparation of the concrete for adhering FRP sheets; the coarse aggregate might affect FRP-concrete bond behavior and strength. The present study was designed and implemented to investigate the effects of coarse aggregate volume on FRP-concrete bond strength and behavior. To achieve this objective, 52 concrete specimens were cast and strengthened with carbon FRP (CFRP) sheets 200 mm long and 50 mm wide using the externally-bonded reinforcement (EBR) technique. To better understand the effect of coarse aggregate, a parameter called fine fraction was defined as the ratio of fine aggregate to total concrete aggregate, with values in the range of zero to 1.0; zero for concrete mixes containing only coarse aggregate and 1.0 for those with only fine aggregate. The specimens thus prepared were then subjected to direct single lap-shear test. Particle image velocimetry (PIV) technique was also utilized to determine the displacement fields. The single lap-shear test results revealed that variations in fine fraction led to changes in bond strength such that fine fraction values of 0.3-0.6 led to reduced bond strength while those from 0.6 to 1.0 increased it. Comparison of experimental results obtained from the current study and those predicted by codes indicated the underestimation of bond strength by the codes in the absence of such surface parameters as fine fraction. (C) 2018 Elsevier Ltd. All rights reserved.