Strength and stiffness characterization of ultra-high performance concrete (UHPC) cement paste phases through in-situ micro-mechanical testing

Abstract

Compressive strength and stiffness of concrete materials involve the complex interaction of aggregates and hardened cement, which can involve even more complex phases formed through hydration processes. Typically, the strength and stiffness of concrete materials are determined through material-scale testing, focusing on the bulk behavior while ignoring individual constitutive material interactions. This study aims to identify individual cement phase contributions to bulk concrete properties through the characterization of individual hardened cement-paste phases using micro-scale compressive testing. In this study, in-situ micro-compression measurements are used to generate statistical distributions of compressive strength and stiffness for five hardened cement phases and one phase composite. Results from the in-situ micro-compression testing indicate that AFt/AFm and CH/C–S–H micropillars exhibit higher stiffness and strength when compared to hardened C–S–H and C-A-S-H phases. Comparing measured micropillar strength and stiffness values with traditional nano-indentation methods indicates that nano-indentation approaches may provide a higher estimation of individual phase stiffness due to material confinement boundary effects.