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

Abstract

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.

Deformation Mechanism of Hardened Cement Paste and Effect of Water

“…Moisture prevents strain hardening by accelerating the dislocation climb and pressure solution to continue plastic deformation. As mentioned in the introduction, the triaxial test results and SEM observation on HCP in our previous study (Sakai et al 2016) support the crystal plasticity being caused by dislocation and twinning. These results correspond to the results obtained in this study.…”

“…The SS curves in our previous triaxial test on HCP did not depend on P c (Sakai et al 2016). Elastic deformation generally took place first in the triaxial test on rocks.…”

“…Understanding such changes in the cement paste is important for estimating the long-term performance and mass transfer resistance of concrete under confining pressure. In a previous study, the authors (Sakai et al 2016) conducted a triaxial test on hardened cement paste (HCP).…”

Deformation Mechanism of Cement Paste and Hydrates under High Stress

“…Concrete can deform extensively under high confining pressure without stress drop, though such a situation causes macroscopic damage (Gabet et al 2008;Malecot et al 2010;Poinard et al 2010;Sfer et al 2002). On the other hand, hardened cement paste (HCP) can undergo large deformation without damage under confining pressure (Sakai et al 2016;Sakai and Kishi 2017). In this case, HCP does not fracture; therefore, the mechanism of the large deformation clearly involves more than just the initiation and development of cracks.…”

“…Considering that rocks are generally subjected to high pressure, Sakai et al (2016) applied a method used in rock mechanics to understand the deformation mechanism of HCP. In this method, the deformation mechanism is classified by the slope of the differential stress-strain rate relationship.…”