Static and Dynamic Mechanical Responses and Synergistic Mechanisms of Concrete Modified with Nano-CaCO₃ and PP Fibers

To investigate the synergistic effects and underlying mechanisms of nano-CaCO₃ (NCC) and polypropylene fibers (PPF) on the static and dynamic mechanical properties of concrete, this study systematically examines hybrid-modified concrete with varying additive contents. Compressive strength, splitting tensile strength, and split Hopkinson pressure bar (SHPB) tests were conducted to evaluate the mechanical responses under quasi-static and high-strain-rate impact loadings. Additionally, scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR) were employed to elucidate the multi-scale synergistic enhancement mechanisms. The results indicate that hybrid modification significantly improves both static and dynamic performance. Specifically, the optimal hybrid proportion was identified as 1.5% NCC and 1.5 kg/m³ PPF. Under static conditions, this combined addition effectively increases compressive strength by 51.56% and enhances splitting tensile strength while also substantially improving material toughness. Under dynamic impact conditions, dynamic compressive strength is notably elevated by 62.47%, demonstrating a pronounced strain-rate strengthening effect. Microstructural analyses confirm the presence of a nano-densification and macro-crack bridging mechanism, wherein NCC chemically accelerates hydration and optimizes the cementitious matrix, thereby strengthening the fiber-matrix interfacial transition zone (ITZ). This robust ITZ maximizes the physical crack-bridging and energy dissipation capacities of the PPF network. Ultimately, this study provides critical experimental evidence and theoretical guidance for designing high-performance, impact-resistant concrete composites.