Evaluation of moment-curvature response and curvature ductility of reinforced UHPC cross-sections

Abstract

This study deals with the evaluation of moment-curvature response and curvature ductility of reinforced ultra high performance concrete (R-UHPC) cross-sections. Curvature ductility is calculated based on six definitions. The impact of these definitions is demonstrated through the calculation of strength reduction factors and comparisons of factored flexural strength of R-UHPC and reinforced normal strength concrete (R-NSC) cross-sections. Strategies for improving the curvature ductility of R-UHPC cross-sections are presented. A versatile framework to predict the complete moment curvature response of R-UHPC cross-sections is proposed. The proposed procedure is used to study the effect of tension reinforcement ratio, UHPC strain at crack localization, UHPC tension model, UHPC ultimate tensile strain, and compression reinforcement ratio on curvature ductility and complete moment curvature response. It is determined that for a given cross-section, changing the material from NSC to UHPC causes notable reductions in curvature ductility. This reduction applies to: 1) tension and compression reinforcement ratios that range from 0.69–2.78 % and 0 – 1.37 %, respectively; 2) UHPC tension models that feature elasto-plastic and linear strain hardening responses with and without residual branches; and 3) UHPC ultimate tensile strains that range from 0.005–0.015. Such reductions in curvature ductility render the considered R-UHPC cross-sections as members in the transition zone – a classification currently prohibited for R-NSC sections according to ACI 318–19 (22) - while their R-NSC counterparts qualify as tension-controlled. This results in lower strength reduction factors for R-UHPC cross-sections and limits the benefits of UHPC when flexural strength controls design. It is concluded that R-UHPC cross-sections that feature either the highest or the lowest reinforcement ratios provide the highest added benefit from the point of view of enhancing factored flexural strength compared to R-NSC sections. The most effective method to elevate curvature ductility is an increase in the UHPC strain at crack localization. A minimum UHPC strain at crack localization of 0.006 is required to change the classification of cross-sections from the transition zone to tension-controlled. The use of compression reinforcement - an established technique for R-NSC members to induce a change in the flexural failure mode by elevating curvature ductility - was determined to have almost no influence on the curvature ductility of R-UHPC cross-sections.