Lateral dilation and limited value of volumetric ratio of stirrups for ultra-high strength concrete confined with spiral stirrups

Ultra-high strength concrete (UHSC) has high compressive strength and tends to fail in brittle under axial compression. The lateral confinements of spiral stirrups can be significantly enhanced the ductility of UHSC. However, it was a huge challenge to evaluate the lateral dilation and determine the proper ranges of the volumetric ratio of stirrups in confined UHSC. Seventy-five UHSC circular columns confined with high-strength spiral stirrups were tested under axial compression to reveal strain development of stirrups and determine the proper ranges of the volumetric ratio of stirrups. The parameters investigated include the concrete strength in the range of 99.86–176.42 MPa, the tensile strength of stirrups was 873–1215 MPa and the volumetric ratios of stirrups in the range of 0.9–2.0%. The experimental results showed that both the axial stress–lateral strain curves and the axial strain–lateral strain curves of confined UHSC were consisted of three parts, especially, the axial strain–lateral strain curves were consisted of two linear parts connected with a transition part. The prediction models for the lateral strain–axial strain curves of confined UHSC were established. The strain of stirrups at the peak stress of confined UHSC had close relationships with the compressive strength of concrete and the volumetric ratio of stirrups. The prediction models for the strain of stirrups at the peak stress were proposed. Furthermore, the appropriate volumetric ratio of stirrups in confined UHSC was determined, which the confined UHSC can satisfy the requirements of both ductility and efficient lateral confinements.