Quantifying the rebar arrangement effects on bond strength in non-contact lap ultra-high-performance concrete joints

Designers tend to ignore the rebar arrangement effects on bond strength that actually exist in real non-contact lap ultra-high-performance concrete (UHPC) joints, which increases structure risks and limits the use of UHPC. In this paper, UHPC joint’s bond failure processes under tension and bending loads are studied by tests and numerical simulation. The rebar arrangement effects on bond strength are then quantified based on the parameters of interest, such as the spacing of same-side longitudinal rebars, the spacing and overlap length of opposite-side longitudinal rebars, and the existence of transverse rebars, to establish formulas to instruct efficient and safe joint design. Results show that bond failure of joint happens after wavy transverse cracks appear. Same-side longitudinal rebars cause tensile stress areas to overlap and same-direction tensile stress to superimpose, which makes UHPC crack earlier and weakens bond strength. The smaller the spacing of the same-side longitudinal rebars, the stronger the weakening effect. Opposite-side longitudinal rebars produce compressive stress areas in UHPC between rebar anchorage sides to constrain cracking and enhance bond strength. The smaller the spacing and the bigger the overlap length of the opposite-side longitudinal rebars, the stronger the enhancing effect. Transverse rebars could improve joint ductility by limiting wavy transverse cracks development. A recommended design parameter table considering rebar arrangement effects is finally proposed. Designers could check the table and directly use the parameter groups to facilitate the design process of non-contact lap UHPC joints, thus improving design efficiency and safety. • Same-side longitudinal rebars weaken bond strength of each rebar. • Opposite-side longitudinal rebars restrain UHPC cracking and enhance bond strength. • Transverse rebars limit transverse cracks’ development and improve joint ductility. • A design parameter guidance table for fast joint design without verifying the safety.