Synergistic Self-Healing and Seepage Resistance of Basalt Fiber-Reinforced Microbial Concrete for Pressurized Diversion Tunnel Linings

Concrete linings in pressurized diversion tunnels are susceptible to cracking and seepage under sustained hydraulic pressure and saturated service conditions, threatening their long-term durability, water-conveyance reliability, and operational safety. To improve the autonomous repair and seepage-control capacity of cracked linings, basalt fiber-reinforced microbial self-healing concrete was developed using modified basalt fibers as both reinforcing components and microbial carriers. Four fiber volume fractions of 0%, 0.1%, 0.2%, and 0.3% were investigated. Mechanical properties, impermeability, crack closure, internal structural recovery, and healing products were evaluated through mechanical tests, water-permeability tests, ultrasonic pulse velocity measurements, crack morphology observations, and microstructural analyses. A normalized integrated self-healing index was further established by combining the crack area healing ratio, ultrasonic pulse velocity recovery ratio, and reduction ratio of water penetration height. The results showed that an appropriate basalt fiber content enhanced the mechanical performance, crack-healing capacity, and seepage resistance of the concrete. The specimen containing 0.2% basalt fiber exhibited the best overall performance, with a 57.6% reduction in permeability coefficient relative to the reference group, a crack area healing ratio of 84%, successful healing of cracks up to 0.65 mm in width, and an integrated self-healing index (SEI) of 1.000 under the adopted internal normalization. Microstructural observations revealed dense CaCO3-rich deposits within the cracks, with preferential mineral accumulation on and around the basalt-fiber surfaces. These features, together with the mineralogical evidence, were consistent with an MICP-related contribution to crack filling. However, the 0.3% group exhibited markedly poorer impermeability recovery despite relatively high crack closure, indicating that apparent surface healing did not ensure complete functional sealing. These findings demonstrate that apparent crack closure alone is insufficient to characterize self-healing effectiveness and that impermeability recovery provides a more sensitive indicator of internal healing quality. Among the investigated mixtures, the 0.2% basalt fiber formulation exhibited the most favorable overall performance under the continuously saturated laboratory conditions adopted in this study, providing a basis for subsequent validation under representative pressurized seepage conditions.