Quantitative characterization on the restrained expansion of core concrete in SSCFST based on ultrasonic pulse velocity – A novel approach

Abstract

The expansive agent (EA) is generally added in the concrete-filled steel tube (CFST) to produce the micro-expansion for core concrete to ensure the synergistic load bearing between the steel tube and core concrete, which guarantees its high strength. The quantitative analysis of the expansion induced by the expansive agent in practical engineering becomes difficult, due to the small magnitude of the micro-expansion. To address this issue, this study proposed a new method for characterizing the time-varying expansion of the self-stressed concrete-filled steel tube (SSCFST) based on the ultrasonic pulse velocity (UPV). The surface strain of the steel tube, the UPV of core concrete, and the porosity of core concrete under conditions of uniform casting and the simulated segregation were investigated. The results showed that the prismatic specimen expanded continuously in its free state, accompanied by a rapid increase in the associated UPV during the initial stage, followed by a subsequent plateau phase. However, in SSCFST, the expansion of core concrete initially increased but subsequently dropped, due to the restriction of the steel tube. As a result, the associated UPV increased. Further, the relationship between the difference in UPV and the expansion consumption was established through an intermediate variable, i.e. the pore-filling increment. And, a computational model for evaluating the circumferential expansion of SSCFST based on UPV was then proposed. The comparison between predicted results and actual measurements confirmed the applicability of the proposed model. This study is believed to offer a new technical solution for non-destructive testing and the quantitative characterization of the time-variant expansion of SSCFST.