Effect of strain gradient on the stress-strain relationship of FRP-confined ultra-high performance concrete

In the case where strain gradient prevails, whether the stress-strain curves of confined ultra-high performance concrete (UHPC) obtained from concentric compression tests can offer sufficiently high accuracy is still open to question. This study aims to understand the effect of strain gradient on the stress-strain relationship of UHPC with fiber-reinforced polymer (FRP) confinement. The response of FRP-confined UHPC cylinders subjected to different levels of strain gradient was experimentally and numerically investigated. The stress-strain curves were then calculated by performing a numerical regression method to study the strain gradient effect. The results show that the ultimate axial strain and the slope of the second branch are the two parameters most affected by the strain gradient effect, with a maximum increase of 20.8% and a decrease of 24.2%, respectively, under the applied highest level of the strain gradient. The findings in this paper reveal that the effect of strain gradient on the first branch of the stress-strain curve of FRP-confined UHPC is negligible, but the effect on the second ascending branch depends generally on FRP confinement levels.