Microcrack evolution and mechanical performance of fiber-reinforced high water-resistant concrete backfill: A quantitative multi-scale study

Fiber-reinforced high water-resistant concrete backfill (FCB) is increasingly recognized as a promising material to maintain underground structural stability in green mining applications. A quantitative understanding of how fiber content governs microcrack evolution and its subsequent impact on macroscopic mechanical performance remains lacking. In this study, the influence of fiber content on the mechanical behavior and crack propagation characteristics of FCB was systematically investigated through mechanical tests combined with multi-scale microstructural characterization. To reveal the micro-damage evolution mechanism, three microcrack indices (crack ratio P f , average crack length L f , and crack connectivity P c ) were proposed to establish a quantitative linkage between microcrack development and macroscopic failure behavior. The results demonstrate that fiber content plays a decisive role in regulating both strength development and crack propagation characteristics. An optimal dosage of 0.3% fibers enhances unconfined compressive strength (up to 26%) and residual strength by promoting pore refinement and effective crack-bridging, thereby suppressing crack initiation and coalescence. In contrast, excessive fiber content (0.5%) leads to agglomeration and weak interfacial zones, which accelerate localized crack growth and deteriorate mechanical performance. Multi-scale observations further demonstrate that the incorporation of 0.3% fibers transforms the failure mode from brittle splitting to ductile shear failure characterized by distributed microcracking, which is confirmed by microcrack analysis. The study provides new insights into the micro-to-macro damage evolution mechanism of FCB materials and offers a scientific basis for the optimized design and engineering application of FCB in underground mining systems. • A quantitative multiscale framework links microcrack evolution to macroscopic mechanical behavior is proposed. • Fiber content governs mechanical performance and crack propagation in high water-resistant concrete backfill. • Optimal 0.3% fibers enhance multiscale crack resistance through fiber bridging and crack-bridging mechanisms.