In this paper, we present the corrosion behavior of F30S30 concrete used for a bridge cap which was investigated in simulated seawater with 2.0% NaCl, 0.35% MgCl2, and 0.25% Na2SO4under wetdry cycles. This behavior was compared with those in 3.5% NaCl, 5.0% Na2SO4, and 3.5% NaCl + 5% Na2SO4solutions. Results show that the change in pore structure obtained through X-ray computed tomography could reflect the amount of corrosion products and the degree of destruction. The variety and quantity of corrosion products varied with the corrosive environment. The damage mechanisms of concrete also changed according to the corrosive environment. The effects of Mg2+and SO42-were superimposed on each other for the simulated seawater environment, thereby posing a serious threat to the structure, primarily through damage and coarsening of the pore structure caused by Mg2+.

The X ray computed tomography (X-CT) was applied to test the cracking resistance of cement paste, and the hydration process was monitored to study the effect of fly ash on the early age cracking performance. The results showed that the hydration heat reduced with the increase of fly ash under the same water-cement ratio. Within 24h, the porosity increased with time. The addition of fly ash increased the proportion of large holes and then changed the internal stress state. Using X-CT test method and by comparing the number of cracks, the sample with 20% FA was found to have the most serious cracks, whereas the sample with 30% FA had the best crack resistance.

Three-dimensional reconstruction and quantitative calculation were performed by X-ray computerized tomography to propagate cracks in paste, mortar, and concrete specimens. Cracking could be observed, and the number and width of cracks increased with increasing strain and load. In mortar and concrete specimens, the number and width of cracks remained unchanged with increasing strain and load; however, a sudden increase in the number and width of cracks nearly destroyed the specimens. Cracks developed along the interface of the aggregate and the mortar in the mortar specimen. In the concrete specimen, cracks continued to grow through the coarse aggregate when cracking occurred along the stress direction. However, cracks developed along the interface of the aggregate and the coarse aggregate when cracking was independent of the stress direction.