Analysis of bond performance of steel bar in steel-polypropylene hybrid fiber reinforced concrete with partially recycled coarse aggregates

To promote structural sustainability and resource recycling, the bond performance between steel bars and steel-polypropylene hybrid fiber reinforced recycled aggregate concrete (HFRAC) confined with/without stirrups was investigated through the pullout test on 72 specimens. The specimens with varying stirrup spacing, polypropylene fiber volume fraction, and steel fiber volume fraction were designed, and the replacement rate of recycled coarse aggregates was kept at 50% for all specimens. The strain gauges were bonded in the slotted steel bar of specimens for detecting the strain of steel bar during the loading process. According to the measured strain of steel bar, the distribution law of bond stress and relative slip for the specimens with/without stirrups were analyzed. Results showed that the lateral confinement of stirrups dramatically improved the bond stiffness and rendered the distribution of bond stress more uniform than that of specimens without stirrups. The hybridization of steel fiber and polypropylene fiber improved the bond strength of recycled aggregate concrete specimen without/with stirrups, in which the stirrup spacing were taken as 80 mm and 40 mm up to 12%, 8.97% and 5.69%, respectively. Smaller stirrup spacing leaded to a slight reduction in bond strength when 1.5% steel fibers were added into the specimens. Based on the thick-walled cylinder model and fictitious crack model reflecting the softening behavior of HFRAC under tension, the solutions of bond strength of steel bars in HFRAC with/without stirrups were deduced, respectively. Finally, the bond-slip model of steel bars in unconfined/well-confined HFRAC was proposed, which considered the effect of anchorage position.