Stress-strain behaviour of high-strength concrete with lateral pre-tensioning confinement

Abstract

High-strength concrete is currently being used in columns of multi-storey building all over the world. Although, it offers superior properties, it is relatively a brittle material. This study intends to improve the ductility of the concrete using a pre-tensioning lateral confinement technique. It investigates the effects of the technique on strength and ductility of the concrete and develops model equations to predict the stress-strain behaviour of confined concrete. The basis of the technique is to apply a pre-tensioning force to relatively low cost steel straps wrapped around the cylindrical concrete specimens. The experimental work was carried out using cylindrical specimens with a dimension of 100mm in diameter and 200mm in height and concrete compressive strength were fixed at 50, 60 and 80 MPa. The parameters studied including different properties of steel straps, various spacing between straps, number of layers of straps and different levels of pre-tensioning stress in the steel straps. The confined specimens were tested in compression until failure under monotonic and cyclic loading conditions. Data collected from the test included mode of failure, loads at peak and ultimate condition, and longitudinal and lateral strains in both concrete and straps. The data were analysed based on volumetric ratio of confinement which is a function of strength of steel straps, compressive strength of concrete, spacing and number of layers of straps. The experimental results show that the pre-tensioning technique using steel straps enhanced the ductility as well as strength of the concrete as the volumetric ratio increases. The results also depicted the ability of the technique to improve the ductility and strength of the concrete, especially for concrete with higher compressive strength by effectively utilising the confinement material. Moreover, the layers of straps had also delayed the onset of volumetric expansion and, undoubtedly, the concrete failure. Based on the analysis, new equations of strength and strain enhancement with confinement coefficients of 2.62 and 11.6 respectively, using pre-tensioning technique had been developed to predict the stress-strain behaviour of confined high-strength concrete.