Prediction of Moment–Curvature Response and Maximum Bending Resistance for Hybrid NSC-UHPC Elements

Exceptional mechanical properties of ultra-high performance concretes (UHPC) offer strong strengthening capacities in bending and shear when used as overlay on normal strength concrete (NSC) structures. Nonetheless, lack of simple and intuitive design models for hybrid elements in design guidelines refrain designers from using UHPC overlays for structural applications. Thereby, a simplified sectional analysis model for NSC-UHPC hybrid elements was developed based on the philosophy of the Canadian Bridge Design Code CSA-S6. By using a new average stress distribution for NSC in hybrid elements that considers the strain at the extreme compressed fiber, equilibrium of forces can be solved by a second-degree equation with direct computation. The simplified model provides the complete moment–curvature behavior of hybrid elements for design purposes, thus allowing verifications in service and ultimate state conditions. An empiric equation is also proposed to evaluate the maximum bending capacity of hybrid elements for predesign. It only uses an approximation of a lever arm between forces in the hybrid cross section and thus offers a quick and easy way to evaluate the bending capacity. Both tools were validated on a detailed and iterative sectional analysis program and with results of four international experimental campaigns. The simplified sectional analysis model and empirical equation showed very good accuracy at reproducing the behavior of a wide range of NSC-UHPC hybrid elements configurations.