Bond stress-slip behavior of ultrahigh-strength steel bars with spiral grooves embedded in plain and steel fiber high-strength recycled aggregate concrete

This study investigated the bond behavior of ultrahigh-strength steel bars with spiral grooves (UHSSBs) embedded in high-strength recycled aggregate concrete (RAC). Forty-five pullout specimens under monotonic loadings were designed to evaluate the effect of several parameters on the bond strength. The variables included concrete strength (50 MPa-60 MPa); recycled concrete aggregate (RCA) replacement ratio (0 %, 50 %, and 100 %); embedment length (5db, 10db, and 20db); concrete cover (25 mm, 45 mm, and 68.7 mm); surface treatment; volume fraction of steel fibers in RAC and stirrup ratio. The failure modes of these tests mainly concentrated on two types, pullout-type bond failure and splitting-type bond failure. According to the typical bond stress-slip curves and trendlines of plotted values, the bond strength increased with increasing of concrete compressive strength. For specimens with splitting-type bond failure, the higher normalized bond strength was observed in higher RCA replacement ratio and concrete cover thickness. The factors of embedment length and surface treatment were also proven to be effective for bond strength. For specimens with pullout-type bond failure, the presence of steel fibers and stirrups substantially restricted the splitting crack growth and improved the bond failure modes. Combined with previous experimental results, simple empirical equations for the average bond strength and bond stress-slip relationship between UHSSBs and RAC under different types of bond failure were developed by regression analysis, which can capture the experimental data with high accuracy.