Experimental and numerical investigation of multiscale mechanical properties of coral aggregate seawater shotcrete

The study of the dynamic compression characteristics of coral aggregate seawater shotcrete (CASS) is crucial for evaluating the performance of island projects under potential dynamic military strikes. Nanoindentation tests were conducted to determine the micro-mechanical properties of the concrete. Subsequently, the effect of aggregate type on the mechanical parameters of the concrete was investigated using a Split Hopkinson Pressure Bar (SHPB) system. Dynamic compression tests were carried out at four different impact pressures (0.10, 0.20, 0.30, and 0.40 MPa) on CASS and ordinary aggregate seawater shotcrete (OASS), both of which had identical mix proportions but differed in the type of aggregate used. The analysis then focused on the fractal dimension and energy characteristics of CASS. Additionally, a coupled numerical model combining the finite difference method (FDM) and the discrete element method (DEM) was developed for simulations. This model was calibrated using the nanoindentation test results. The results revealed a positive correlation between the elasticity modulus and fracture toughness across different phases of the concrete. Coral aggregate significantly influenced the macroscopic fracture properties. In contrast to OASS, where internal cracks propagated along the interfacial transition zone (ITZ), cracks in CASS propagated directly through the coral aggregate. The primary failure mechanism for both concrete specimens in the dynamic compression tests was dominated by tensile failure. Under the same loading strain rate, specimens with higher strain rate sensitivity exhibited larger fractal dimensions in the fragments after failure. As the strain rate increased, the specimens reached damage saturation due to the limitations in energy absorption capacity. As strain rates continued to increase, the proportion of reflected energy rose while the proportion of absorbed energy decreased, leading to a higher incidence of shear failure and more severe damage. The numerical model calibrated by the nanoindentation test results accurately replicated the heterogeneous concrete structures.