Mechanical Behavior of Cement Paste and Alterations of Hydrates under High-Pressure Triaxial Testing

High confining pressure works on concrete under various conditions such as concrete structures deep underground/sea, the lower floors of an extremely tall building and on the foundation concrete piles of an enormous structure such as dam. Understanding the performance of concrete and the deformation mechanism under high confining pressure is important for avoiding unexpected risks and for rationalization of design. A high-pressure triaxial test was conducted on cement paste to understand the mechanism of mechanical performance of concrete under a high confining pressure. The deviatoric stress-axial strain relationship of cement paste was independent of confining pressure during ductile deformation under confining pressures greater than 30 MPa. Ion-milled cross-sections of specimens were examined by scanning electron microscopy, and the obtained backscattered electron images were darker after the test. This darkening may indicate the alteration of hydrates at the molecular level caused by deformation involving crystal plasticity. Furthermore, the pore volume of the sample tested at 400 MPa was drastically reduced.