Experimental and analytical investigation of UHPC confined concrete behavior

Abstract

Ultra-High Performance Concrete (UHPC) is a self-levelling cementitious material with enhanced mechanical and durability properties when compared to normal strength concrete. Accelerated construction of concrete columns can be realized using a precast hollow UHPC shell as permanent formwork. Also, as a retrofit solution, UHPC can be placed around existing concrete columns with deficient confinement details. The continuous confinement provided by UHPC shell to both the concrete core and longitudinal reinforcement could be used to improve the seismic behavior of columns or reduce the amount of confining hoop reinforcement required in traditional column designs. An experimental and analytical study is performed with a main objective of quantifying the confinement effect provided by plain UHPC shell. The experimental investigation compressed of axial testing of 228 mm (9 in.) and 280 mm (11 in.) dia circular columns, 203 mm (8 in.) and 305 mm (12 in.) square columns confined with UHPC shell. Experimental investigation included a total of two normal strength concrete column specimens as control specimens and 11 column specimens confined with 1 in. and 2 in. thick UHPC shell. All the specimens were tested to failure under axial compression and were extensively instrumented. The confinement provided by the unreinforced UHPC shell improved the peak compressive stress and the crushing strain values of the core concrete by 15%–30% and 26%–46% respectively. The effectiveness of plain UHPC shell to provide confinement was found to considerable in circular columns compared square columns. A detailed 3D FEA models were developed in ATENA software and were validated using the experimental data. The FEA models captured observed responses accurately and provided insight into the observed experimental behavior. Based on the experimental and analytical study, it is recommended to include some transverse reinforcement in UHPC shells, especially for non-circular cross-section for improved confinement effect.