Investigation on the mechanical properties and failure mechanism of high-strength recycled concrete enhanced with basalt fiber and nano-calcium carbonate under impact loading

With the advancement of the resource utilization concept of construction waste, recycled aggregate (RA) is increasingly used in concrete. However, RA weakens mechanical properties—especially in high-strength concrete—due to interfacial defects and brittleness, limiting its performance under impact loads. Previous studies have shown that fibers and nanomaterials are typically used separately to enhance recycled concrete; however, their combined effect on strengthening HSRC under impact loading has not been systematically investigated. To improve the dynamic performance of high-strength recycled concrete (HSRC) under impact loading, this study adopts a synergistic modification using basalt fiber (BF) and nano-calcium carbonate (NC), investigating its mechanical behavior and multiscale failure mechanisms. Dynamic compression tests using a Split Hopkinson Pressure Bar (SHPB) examined the effects of strain rate, BF/NC content, and recycled aggregate (RA) ratio on failure modes, stress–strain response, dynamic strength, and dynamic increase factor (DIF). A recalibrated DIF prediction model was developed based on material-related variables, and the post-impact microstructure and failure mechanisms of the modified high-strength recycled concrete (MHSRC) were thoroughly analyzed. Results show that RA reduces impact resistance, while optimal additions of BF and NC synergistically enhance dynamic properties, with 2.5 % NC and 0.5 % BF yielding the best strength–toughness balance. NC strengthens interfaces by pore filling and interfacial transition zone (ITZ) refinement, while BF improves crack resistance via bridging and pull-out. The proposed DIF model provides improved prediction accuracy the DIF values across different mix proportions. Failure mechanism analysis reveals that NC contributes to interfacial strengthening by filling pores and refining the ITZ, while BF enhances crack resistance through bridging and pull-out mechanisms. Under impact loading, a multiscale synergistic reinforcement effect is established through interfacial enhancement and crack inhibition. This study provides theoretical insights and technical guidance for optimizing the performance and structural applications of MHSRC under dynamic loading conditions.