Multi-scale Mechanical Characteristics and Fiber Reinforcement Mechanisms in Fiber-Reinforced Coral Shotcrete

To investigate the multiscale mechanical properties and fiber effects of Fiber-Reinforced Coral Shotcrete (FRCS), C30 coral aggregate shotcrete specimens were prepared using the wet-mix shotcreting method with either steel fibers or polypropylene fibers incorporated. Nanoindentation and uniaxial compression tests were subsequently conducted, supplemented by acoustic emission (AE) monitoring and scanning electron microscopy (SEM) to analyze crack evolution and failure mechanisms. The results indicate that the fracture toughness of coral aggregate ranges from 0.04 to 1.445 MPa·m^0.5, which is significantly lower than that of the interfacial transition zone and cement matrix, establishing it as the weakest phase in the FRCS system and the dominant factor governing macroscopic fracture behavior. This mechanism differs from the conventional understanding where the interfacial transition zone typically controls failure in traditional concrete. With increasing curing age, the compressive strengths of polypropylene fiber-reinforced concrete (PFRC) and steel fiber-reinforced concrete (SFRC) developed to 43 MPa and 46 MPa, respectively, while their toughness decreased as the hardening stage in the stress-strain curves contracted. During crack evolution, PFRC was primarily characterized by tensile cracks with an RA value less than 0.5 ms/V, accompanied by relatively weak AE activity. In contrast, SFRC was dominated by shear cracks with an RA value greater than 0.5 ms/V, showing significantly enhanced AE signals. Microscopic failure analysis reveals that polypropylene fibers mainly inhibit crack propagation through a bridging mechanism, whereas steel fibers enhance strength and stiffness via strong interfacial bonding and frictional energy dissipation during debonding and pull-out processes. This study clarifies, from a multiscale perspective, the dominant role of coral aggregate in the fracture of fiber-reinforced systems and elucidates the distinct reinforcement mechanisms of different fibers, providing a theoretical basis for the design and performance regulation of such materials in island and reef engineering.