Experimental investigation on UHPC-NSC composite beams

Abstract

A hybrid beam composed of ultra-high performance concrete (UHPC) and normal strength concrete (NSC), is experimentally evaluated. The layout optimizes cost and performance by placing the UHPC full-depth section at mid-span of the hybrid beam. The length of the UHPC section compared to the span length (Lu/L) as well as the angle between the UHPC and NSC interface is varied experimentally, and the performance evaluated against fully NSC and fully UHPC control beams. The fully UHPC beam (BU) had an ultimate load 63.5% higher compared to the NSC beam (B). All three hybrid UHPC-NSC beams B0.5, B0.6, and B0.5I, having Lu/L ≥ 0.5, achieved ultimate loads of 92.3%, 95.8% and 94.2% of the fully UHPC beam ultimate load respectively. The failure of the hybrid beam with the inclined joint (B0.5I) was more gradual when compared with the beam with the straight joint (B0.5) having the same volume of UHPC. The hot joint between UHPC and NSC in the hybrid beams showed no joint separation. The developed 3D FE model was able to predict the load-deflection behaviour as well as crack locations and crack localization. In hybrid UHPC-NSC beams, steel yielding can initiate near the UHPC-NSC joint location, where unlike within the UHPC portion, the steel rebars are required to carry the entire tensile force. An analytical method is proposed to provide conservative design loads considering the different failures expected from the hybrid UHPC-NSC beam.