A piece-wise linear model for modelling the tension-stiffening effect in steel-reinforced UHPC composites

This study examines the tensile behaviour of Ultra-High Performance Concrete (UHPC) specimens reinforced with varying steel fibre content and rebar diameters. An experimental campaign involving 18 reinforced UHPC specimens was conducted, following individual tensile response characterizations of UHPC and reinforcing steel. These specimens were designed and fabricated with threaded welded steel cages to transfer loads to the central measurement area. Force-displacement curves were measured to derive stress-strain curves for the UHPC component, evaluating the impact of reinforcement rebar and steel fibre content on UHPC's mechanical response. The analysis revealed that steel fibre volume content significantly influences the tensile response of UHPC reinforcement more than the steel bar diameter and yield strength. Comparing the strain in steel bars to the average strain in reinforced UHPC specimens indicated that post-cracking deformation concentrates at the crack. At the same time, the bond between reinforcement and UHPC remains intact in uncracked sections, allowing effective load transfer. UHPC specimens reinforced with 14 mm diameter steel bars displayed a significant horizontal segment in the descending phase of the stress-strain curve. In contrast, specimens with 18 mm or 22 mm diameter bars exhibit a progressive stress reduction at a nearly constant rate. Stress-strain curves of UHPC were fitted with a five-fold piecewise empirical model applicable to UHPC with a reinforcement ratio higher than 0.09%. The resulting model can be used to model the axial behaviour in UHPC, considering the effects of steel reinforcement.