Experimental study on bond performance of UHPC-to-NC interfaces: Constitutive model and size effect

Abstract

This study investigates the constitutive model of the adhesive bond between normal concrete (NC) and ultra-high-performance concrete (UHPC), focusing on the impact of varying bond dimensions. Two distinct series of UHPC-NC bonding specimens were prepared and tested, comprising eight direct-shear and fifty-six pull-off specimens. The experimental results were presented and thoroughly discussed, including failure modes, bond stress, load-displacement curves, and strain histories of the UHPC-NC interfaces. The findings indicate that an average surface roughness of 4 mm on NC substrates guarantees adequate shearing and tensile bonding strength with UHPC, adhering to the standards outlined in ACI 546.3R-14. An increment in the bond length, ranging from 100 mm to 440 mm, reduced interfacial shear stiffness by 50.8 % while enhancing the shear strength by 14.4 %. Conversely, an enlargement in the core diameter, from 50 mm to 100 mm, decreased the tensile bond strength at the interface by 18.1 %. Furthermore, theoretical models were formulated to predict the bonding properties across the UHPC-NC interface, utilizing the size effect law and Weibull theory principles. The accuracy of these models was confirmed through a validation process using both current experimental data and external sources. The comprehensive findings of this study provide a valuable reference for optimizing the application of UHPC in concrete bridge restoration projects.