Orientation evolution of basalt fibers and its regulation mechanism on the microstructure and macroscopic properties of shotcrete

This study systematically investigates the performance enhancement of wet-mix shotcrete using basalt fibers, addressing critical field issues such as high rebound, early-age cracking, and microstructural non-uniformity. The mechanisms through which fibers regulate flow behavior, crack resistance, and matrix densification were elucidated. Transparent-concrete analog experiments and rheological tests were employed to characterize the evolution of fiber orientation and spatial distribution during mixing and pumping, as well as aggregate particle migration. Results demonstrate that 12-mm fibers at a 0.15 % volume fraction achieve the most stable preferential alignment, increasing the splitting tensile strength by 11.48–15.46 % across all curing ages and extending the time to cracking under restrained shrinkage from 2.65 d for the plain mixture to approximately 4.5 d, while optimized mixtures reduce the peak hoop tensile stress by up to 63.41 %. With 12 mm basalt fiber, the rebound rate is reduced by up to 35.9 % and the deposited thickness increased by more than 25 % compared with the no fiber mixture. Computed tomography (CT) and scanning electron microscopy (SEM) analyses further reveal that the combined action of fibers and reactive mineral admixtures reduces the near-surface porosity from 0.24 % in the control to 0.05 % in optimized mixtures, refines the pore-size distribution, and enhances the interfacial microstructure, thereby suppressing the initiation and propagation of microcracks. ● A transparent concrete analog and cracking tests elucidated the fiber kinematics and fresh-state behavior of shotcrete. ● Optimal fiber length and content enhanced tensile strength and delayed restrained shrinkage cracking in shotcrete. ● Combined use of basalt fibers and SCMs refined microstructure and improved matrix densification and crack resistance.