Investigation on the flexural toughness evaluation method and toughening mechanism of polypropylene fiber reinforced coral aggregate concrete

Coral aggregate concrete (CAC) is critical for marine infrastructure in remote islands, yet its inherent brittleness and limited research on early-age flexural behavior hinder its application. Polypropylene fiber reinforcement (PPF) offers a potential solution to enhance ductility, but systematic studies on its efficacy in CAC under early curing conditions are scarce. The influence of polypropylene fiber content (0 kg/m3, 0.455 kg/m3, 0.91 kg/m3, 1.365 kg/m3, 1.82 kg/m3) and curing age (7d, 14d, 21d, 28d) on flexural properties of coral concrete was investigated through four-point bending tests. Test results were analyzed following ASTM C1609 and JG/T 472–2015, coupled with failure mode analysis via scanning electron microscopy (SEM). A novel performance index was proposed to overcome limitations in existing standards. Failure and toughening mechanisms in polypropylene fiber-reinforced coral concrete (PPFCAC) were characterized via scanning electron microscopy (SEM). Experimental results indicated that adding PPF enhanced the flexural strength of CAC compared to the plain control group. The PPF content that maximized flexural strength was 0.91 kg/m³ at 7, 14, and 28 days, the flexural strengths are 3.61 MPa, 4.39 MPa, and 5.14 MPa respectively; whereas 1.365 kg/m³ was optimal at 21 days, the flexural strength is 4.73 MPa. Moreover, it can be seen by SEM that because coral aggregate has complex pore characteristics, cement matrix and PPF can enter its pores, effectively improving the brittleness of coral concrete. These research results provide a reference for the application of early-age PPFCAC. • Few studies have examined the flexural performance of early-age polypropylene fiber-reinforced coral concrete. • The response surface methodology was used to optimize the concrete mixture proportions and predict the flexural strength. • A novel index is proposed for evaluating the flexural toughness of polypropylene fiber-reinforced coral concrete. • The toughening and fracture mechanisms are analyzed based on Scanning Electron Microscope images.