Effect of coarse recycled aggregate with embedded fibres on the mechanical properties and microstructure of polypropylene fibre-reinforced concrete

Previous studies have shown the feasibility of using recycled aggregates in new concrete production. However, the recycling of fibre-reinforced concrete (FRC) introduces a novel challenge: the emergence of a new aggregate type that can be identified as recycled aggregate with embedded fibres. Therefore, in this study, in order to investigate the differences, the mechanical properties and microstructures of polypropylene fibre-reinforced concrete (PPFRC) made of natural coarse aggregate, coarse recycled aggregate and coarse recycled aggregate with embedded fibres were compared. Polypropylene fibre contents of 3 and 9 kg/m³ (0.33% and 1.0% by volume, respectively) were chosen for all concretes. The mechanical properties, including stress–strain behaviour in compression and flexural behavior, were tested. Scanning electron microscopy (SEM) was used to examine the microstructure and elucidate the effects of different aggregates on PPFRC properties. The results indicated that recycled aggregate with embedded fibres did not enhance compressive strength and elastic modulus compared to recycled aggregate without fibres. However, when the fibre content is low, its contribution to flexural behavior is significant, even surpassing that of PPFRC made with natural aggregate.