Compressive behavior and stress–strain relationships of confined and unconfined UHPC

Abstract

Superior physical and mechanical behavior of ultra-high performance concrete (UHPC) has attracted lots of interest for its use in joints and connections of prefabricated elements. However, there is still a great potential for application of UHPC for larger applications and full structural elements, such as bridge columns. Some fundamental knowledge gaps at the material level still need to be filled to promote future structural designs. One of these gaps is understanding the compressive behavior of confined UHPC as compared to unconfined UHPC. To date, no extensive studies examined the confining effects of spirals representing transverse reinforcement in typical columns. This study focused on UHPC cylinders to establish a better understanding of the compressive behavior and stress–strain relationships of confined UHPC materials. More than 130 unconfined UHPC and UHPC cylinders confined with steel spirals with varying volumetric ratios of steel fibers (0%, 2%, 4%) and transverse reinforcement ratios (0%, 2%, 4%, and 8%) were tested under uniaxial compression. Two cylinder sizes were considered (2 × 4 in and 3 × 6 in) and effects of different specimen geometry aspects (e.g. end planeness) as well as test setup were investigated. The behavior of confined UHPC cylinders was obtained and compared with reference unconfined UHPC cylinders. Overall, the study shows that both steel fibers and steel spirals have contributory roles in confining effects but the effect of steel spirals is more compelling. Moreover, full stress–strain relationships of confined and unconfined UHPC are provided for future use.