Effects of synthetic fibres on the fracture behaviours of recycled coarse aggregate concrete

Economic and environmentally friendly polypropylene (PP) fibre and polyvinyl alcohol (PVA) fibre were found to be beneficial to the strength of recycled coarse aggregate concrete (RCAC), but the effects in the fracture properties of the RCAC has been overlooked. This study added PP fibre and PVA fibre to enhance the ability of the RCAC to resist crack instability expansion. The three-point bending tests were performed to compare the fracture properties of the RCAC with fibres and without fibre. The reinforcement mechanism of fibres was analyzed by the single-fibre pullout tests and scanning electron microscopy tests. The experimental results indicated that fibres could not effectively increase the compressive strength of RCAC but the splitting tensile strength of RCAC was significantly improved with the increase of the volume fraction of fibres and the optimal volume fraction of fibre was 1.5% PP fibre. The PP fibre and PVA fibre both effectively improved the fracture properties of RCAC but acted in the post-cracking stage and the linear loading stage, respectively. As a concrete manifestation of the PP fibre was more effective in increasing the peak load, unstable toughness, and fracture energy of RCAC than the PVA fibre while the PVA fibre was more effective in increasing the initial cracking load and the initial toughness. The 1.0% PP fibre had the best effect on the fracture properties, which increased peak load, unstable toughness, and fracture energy by 47.2%, 42.3%, and 5500%, respectively. The 0.2% PVA has the best effect on the fracture properties of RCAC at the stage before the cracks developed which increased initial cracking load and initial toughness by 25.2% and 43.7%, respectively. In addition, the development of cracks of RCAC with PP fibre was more tortuous than that of RCAC with PVA fibre. A reinforcement mechanism analysis showed that there were obvious diamond-shaped depressions on the surface of PP fibre which provided mechanical interlock to enhance the RCAC. The hydrogen bonds of PVA fibre also enhanced the RCAC through the hydration products attached to the surface of the PVA but it is weaker than diamond-shaped depressions. These results provided theoretical and experimental support for the research gap of PP fibre and PVA fibre in RCAC. •Roles of PP fibre and PVA fibre in the fracture properties of RCAC were explored.•The crack propagation of RCAC with PP and PVA fibre was observed based on the DIC technique.•The fracture mechanis