Toughening Mechanism and Nonlinear Water-to-Binder Ratio Effects in Multi-Scale Steel Fiber-Calcium Carbonate Whisker Reinforced Concrete.

The toughening efficiency of multiscale fibers is strongly coupled with the matrix state, yet how the water-to-binder (w/b) ratio shifts the governing reinforcement mechanism remains unclear. Two w/b regimes representing distinct matrix conditions were therefore incorporated into a three-factor, four-level orthogonal design, with water, steel-fiber, and calcium carbonate whisker dosages as variables. Compressive, flexural, and direct tensile strengths, together with failure characteristics, were evaluated to track the transition in mechanical response across the two regimes. In the low w/b ratio regime, the w/b ratio controlled matrix compactness and interfacial quality, significantly affecting compressive and flexural strengths (p = 0.0005 and p = 0.0016, respectively). As the matrix became more susceptible to cracking, the contribution of w/b ratio diminished, whereas steel-fiber bridging became dominant and significantly influenced flexural strength (p = 0.0027). This regime transition demonstrates that the governing mechanism evolves nonlinearly from matrix-controlled strength to fiber-controlled crack resistance. Calcium carbonate whiskers delayed damage initiation through microfilling, crack-tip blunting, and microcrack bridging, but excessive dosages promoted agglomeration and local defects. Steel fibers subsequently restrained macrocrack growth through interfacial bonding, bridging, slip, and pull-out. Together, these actions established a hierarchical toughening mechanism spanning microdefect regulation and macrocrack bridging, transforming abrupt brittle fracture into progressive cracking while sustaining structural integrity. These findings clarify the matrix-dependent nature of multiscale reinforcement and provide a mechanistic basis for regime-specific cementitious composite design through coordinated optimization of matrix quality, microscale crack control, and macroscale bridging.