Effects of Steel Fibers, a CaO-MgO Composite Expansive Agent, and Fly Ash–Slag Replacement on Early-Age Cracking and Water Penetration Resistance of Tunnel Lining Concrete

Tunnel lining concrete is prone to early-age cracking and leakage during service, which provides pathways for water and harmful ions and consequently threatens the safety and durability of tunnel structures. To improve the crack resistance and water penetration resistance of lining concrete, this study investigates the effects of steel fibers, a CaO-MgO composite expansive agent, and fly ash–slag replacement on workability, mechanical properties, ultrasonic pulse velocity, early-age cracking, and water penetration resistance. The results show that steel fibers had the most pronounced effect on crack-width control; at 1.0% steel fiber content, the total cracking area and water penetration height decreased by 71.9% and 68.2%, respectively. The composite expansive agent showed an optimum dosage of 10%, at which cracking resistance and water penetration resistance were both improved. The largest reduction in water penetration height was observed when slag was fully replaced by fly ash, with an 87.5% decrease. These findings provide a material-design reference for improving the crack resistance and durability of tunnel lining concrete under restrained early-age conditions.