The microstructure and mechanical properties of cementless ultra-high-performance alkali activated concrete considering geometrical properties of steel fiber

This study investigates the impact of steel fiber geometrical properties on the interfacial bond and tensile behavior of ultra-high-performance alkali activated concrete (UHP-AAC). Two types of steel fibers, straight and hooked-end, with aspect ratios of 65, 97.5, and 125 were examined. Results showed that the hooked-end steel fiber exhibited the highest average and equivalent bond strengths in the UHP-AAC matrix, measuring 15.13 and 11.46 MPa, respectively, which were approximately 66% and 94% higher than those of straight steel fibers in the same matrix and bonding area. The best tensile performance of UHP-AAC was achieved using straight steel fibers with an aspect ratio of 97.5, demonstrating the highest tensile strength and strain energy density of 12.28 MPa and 55.24 kJ/m³, respectively, and the second highest strain capacity of 0.533%. Straight steel fibers were more effective in increasing the tensile strength of UHP-AAC than hooked-end steel fibers with the same aspect ratio. Higher fiber aspect ratios resulted in larger microcrack widths, with the straightened end-hooks acting on the crack plane to transmit tensile force more effectively, contributing to increased microcrack width and deformability.