Experimental study on seepage evolution and microscopic characteristics of initially damaged concrete under variable confining pressure

Abstract

This study aims to examine the impact of service loading and dry-wet-freeze–thaw environments on the initial damage caused to concrete, and conducts initial damage concrete permeability testing and microscopic characterization under varying confining pressure. The evolution pattern and mechanism of specimen permeability are analyzed using the confining pressure sensitivity coefficient, permeability recovery rate, and causeless radial strain. The results indicate that permeability exhibits nonlinear characteristics during the process of confining pressure loading and unloading. The initial permeability increases with increasing load level and the number of dry-wet-freeze–thaw cycles. The loading process can be divided into a sensitive area, a transition area, an insensitive area, and a steady area, according to the confining pressure sensitivity coefficient. The permeability in the sensitive area decreases the most with the increase of confining pressure, while the permeability in the steady area hardly changes. A smaller permeability recovery rate after confining pressure unloading leads to a larger dimensionless radial strain and lower permeability value. Utilizing nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM), it was observed that the initial damage caused the expansion of microcracks and micropores in the specimen, leading to a subsequent amplification of the seepage channels, ultimately resulting in a substantial increase in the specimen's permeability. When confining pressure is loaded, the internal pore structure of the specimen undergoes elastic–plastic deformation. Upon confining pressure unloading, the proportion of macropores in the specimen decreases by 78.79%, resulting in incomplete restoration of the permeability value.