Bond performance between substrate concrete and repair mortar: Effect of carbon fibre and expansive agent

This study aims to investigate the impact of an expansive agent and carbon fibre in repair mortar (overlay) on the bond strength at different water-to-binder ratios (w/b). Measurements of the flexural and compressive strengths, drying shrinkage, and mercury intrusion porosimetry (MIP) were used to evaluate the properties of the overlays, and the compressive strength, drying shrinkage, and splitting tensile strength were adopted to evaluate the properties of substrate. The exposure surface after splitting a cube into two halves was used as repair surface of substrate. The bond strength was evaluated by a splitting tensile test, and backscattered electron images were acquired to observe micro-cracks of the repair bond. The experimental results indicated that the drying shrinkage of the repair mortar was reduced by the carbon fibre and expansive agent. The total porosity of the repair mortar was increased and the compressive strength was decreased by the expansive agent. Interface failure accompanied by partial substrate failure and repair mortar failure accounted for 85.7% of bond failures. The incorporation of the expansive agent and carbon fibre in the repair mortar resulted in growth rates of 25.18% and 33.56%, respectively, for the splitting tensile bond strength at w/b of 0.30. This enhancement decreased as the w/b decreased. The repair effectiveness improved as the compressive strength ratio (repair mortar/substrate concrete) increased with a certain range (no more than 1.45). Therefore, the expansive agent and carbon fibre have the potential to improve the bond strength when the strengths of the substrate and of the repair material are equivalent and the repair bond under splitting tensile stress.