Investigation on degradation of cement mortars subjected to cyclic freezing and thawing by micro X-ray computed tomography

For marine concrete structures in cold climates, frost damage is one of the most critical durability issues. However, the degradation process, i.e., development of internal cracks before measurable degradation occurs, is still not fully understood. In this study, micro X-ray computed tomography (X-ray µCT) is employed to investigate the degradation process of mortar exposed to cyclic freezing and thawing. The results illustrate that both internal cracking and surface scaling can be clearly traced and visualized in the light of µCT. The cracks initiate from the interfacial transition zone (ITZ), traverse the paste and the ITZ, develop in a layered pattern along the altitude (upward direction perpendicular to the water-contacting bottom surface), and then connect with each other. The maximum width and connectivity of the cracks increase following the freeze-thaw cycles. The sequential development of cracks leads to dilation and surface scaling. In addition, the evolution of void volume and solid phases indicates that the damage induced by freeze-thaw cycling is cumulative and self-accelerative. The volume dilation ratio determined by µCT image analysis can be used to evaluate the degree of degradation.