Study on the bond properties of NPR steels and high-strength steels with ultra-high performance concrete

Negative Poisson's Ratio (NPR) steel bar is a new type of steel with high strength and ductility. Combining UHPC with high-strength steels could solve strain incompatibility in the reinforced structures. The study investigated the bonding properties of the UHPC reinforced with NPR, HRB635, and HRB400 through pull-out test in combination with acoustic emission (AE) observation. Parameters as fiber volume, cover thickness, rebar type, and rib shape were examined. The failure mode, bond-slip curve, maximum bond stress, and peak slip are measured and compared. It showed that the failure modes of NPR and HRB are slightly different. The bond strength of NPR was higher than HRB400 but lower than HRB635 despite the similar yield strength and rib shape. Peak slips of NPR were much higher than those of HRB635 and HRB400 steels. Bond properties of NPR and HRB635 exhibited different parameter sensitivity to anchorage length and fiber volume. An increase in rib spacing decreased the bond stiffness but had no noticeable impact on maximum bond strength. Decreasing the rib's height reduced the bond strength and the corresponding slip. AE count, frequencies, energy and AF-RA analysis were used to discriminate damage evolution of specimens. The degree of matrix damage and fiber-matrix interaction under different conditions was identified. An expression was proposed to calculate the maximum bond stress in UHPC specimens reinforced by HRB635 and NPR bars.