Development of Cross-Scale Structured Hybrid Fiber-Reinforced Shotcrete.

With the increasing demand for tunnel construction under extreme geological conditions such as high geo-stress, rock bursts, and fault zones, the performance requirements for shotcrete in initial support systems have become more stringent. This study develops a cross-scale structured hybrid fiber-reinforced shotcrete by incorporating alkali-resistant glass fibers including HP and HD types with different lengths and carbon nanotubes (CNTs) into a conventional shotcrete matrix. An orthogonal experimental design at four factors and four levels was adopted to investigate the effects of fiber and CNT contents on the mechanical properties and microstructure of shotcrete. Uniaxial compressive strength, splitting tensile strength, slumping, rebound rate, and microscopic characteristics such as SEM were evaluated at 3, 7, and 28 days. Results show that the optimal mix proportion is 4% HP fiber (24 mm), 2% HD fiber (18 mm), 2% HD fiber (6 mm), and 0.2% CNT. Under this formulation, the 28-day compressive and splitting tensile strengths reached 43.53 MPa and 4.85 MPa, respectively, with a rebound rate as low as 3.85%. The enhanced performance is attributed to the multi-scale reinforcement mechanism. Long fibers suppress macroscopic cracks, short fibers bridge micro-cracks, and CNTs densify the interfacial transition zone. This study provides a parametric reference for the development of high-performance shotcrete and its engineering application in complex underground excavations.