Multi-scale macro-micro properties and interfacial bonding mechanisms of basalt fiber-reinforced UHPC

Ultra-high performance concrete (UHPC) is an advanced cement-based composite renowned for its ultra-high strength and excellent durability, offering broad application prospects in modern engineering. However, to fully exploit the reinforcement efficiency of basalt fibers (BF) in UHPC, a precisely tailored matrix and a deep understanding of its specific interfacial mechanisms are essential. This study systematically investigates the multi-scale macro-micro properties of basalt fiber-reinforced UHPC (BF-UHPC). Macroscopic mechanical evaluations, fiber pull-out tests, cohesive zone model (CZM) finite element simulations, and surface thermodynamic analyses were conducted. Experimental results revealed that tailoring the matrix with a water-to-binder (W/B) ratio of 0.17 and a BF dosage of 1.0 vol% successfully balanced workability and mechanical capacities. In comparative assessments, the BF-reinforced system significantly outperformed the conventional ST-reinforced system in macroscopic load-bearing capacity, achieving a higher ultimate tensile strength (9.5 MPa), while the apparent interfacial bond strength of BF (21.4 MPa) was 1.3–1.5 times that of steel fibers (ST). This exceptional adhesion shifted the interfacial failure mode from typical frictional pull-out (ST) to abrupt fiber rupture (BF). Numerical simulations reproduced the pull-out behavior and elucidated that this in-situ rupture mechanism is dictated by severe localized stress concentrations at the matrix exit. Furthermore, microstructural and surface thermodynamic analyses demonstrated that BF possesses a substantially higher work of adhesion ( W a = 88.92 mJ/m²) and surface polarity than the ST ( W a = 75.48 mJ/m²), suggesting a highly favorable surface thermodynamic potential for interfacial affinity with calcium silicate hydrate (C-S-H) gels. The 1.0 vol% BF addition effectively refined the interfacial transition zone (ITZ) pore network, achieving a peak thermodynamic fractal dimension ( D f = 2.909) that directly correlated with macroscopic strength enhancement. Ultimately, this research reveals that the superior interfacial anchorage of BF in the tailored UHPC matrix is governed by the synergistic combination of mechanical interlocking from its bundled geometry and favorable interfacial affinity.