Experimental, analytical, and numerical investigation on flexural behavior of hybrid beams consisting of ultra-high performance and normal-strength concrete

• A bending test was performed on five hybrid beam specimens consisting of ultra-high performance and normal-strength concrete. • UHPC substrate greatly enhanced the load-carrying capacity but impaired the ductility performance. • An analytical load–deflection curve of hybrid beam specimens is derived, including the post-peak branch. • Numerical modeling is performed, employing the nonlocal phase-field model for cementitious materials. In this study, the flexural behavior of hybrid beams consisting of an ultra-high-performance concrete (UHPC) substrate and normal-strength concrete (NSC) overlay is investigated. From the experimental perspective, five UHPC-NSC hybrid beams were loaded to failure, and their failure patterns, load–deflection curves, ductility, strain distributions, and crack evolutions were compared. The UHPC substrate was found to significantly enhance the load-carrying capacity and suppress massive cracking of the hybrid beams. Interestingly, the increasing thickness of the UHPC substrate was observed to impair the beams’ overall ductility, leading to the apparent descending post-peak branch of the load–deflection curve. In other words, the agreed ductility of UHPC at the material level did not always benefit the deformation capacity of the beams at the structural level. Furthermore, an analytical derivation is proposed to capture the load–deflection curve of the hybrid beams. Specifically, sectional analysis in OpenSees was adopted to obtain the moment–curvature relation. This numerical strategy significantly alleviates the complicated derivation of the moment–curvature relation from conventional analytical formulas, owing to the significant tensile contribution of the UHPC material. Finally, finite element (FE) modeling of the hybrid beam specimens was performed. A nonlocal phase-field approach was implemented for UHPC and NSC materials. The numerical simulations are in good agreement with the experimental observations in terms of stiffness, load–deflection relation, and crack evolution.