Experimental study on flexural performance of ultra-high-performance concrete with recycled aggregate co-modified by nano-silica and steel fiber

Abstract

This study investigated the feasibility of applying defect-containing recycled aggregates (RA) in ultra-high-performance concrete (UHPC) and studied the effect of nanosilica (NS) and steel fiber (SF)) together on its modification. The effects of RA substitution rate, SF content and NS content on the flexural behavior and microstructure of RA-UHPC were analyzed by four-point bending test. The experimental results showed that the specimens without added SF exhibited a linear elastic growth on the load-deflection curve, with a sharp decrease in load after the peak without a significant falling segment. In contrast, the specimens with added SF showed a typical three-phase characteristic with a smoother and flatter descending phase. The cross-distributed SF improves the ductility of the specimens after the peak by bridging effect and changes the damage behavior from quasi-brittle to ductile. Increasing the SF content significantly improved the flexural strength and toughness of the specimens, while increasing the RA substitution rate and NS content slightly decreased the flexural strength. The trends of the residual strengths calculated by the two methods were basically the same, while the specimens with brittle failure had almost no residual strengths. In addition, crack locations could be predicted prior to peak loading using digital image correlation techniques, while SEM analysis revealed the pore space between the cement matrix, interfacial transition zones, and fibers, as well as the ameliorating effects of SF and NS on micro- and macro-scales. These findings are valuable for understanding the flexural mechanism of fiber-nanocomposite concrete.