Influence of microstructural change under stress on the strength-related properties of hardened cement mortar and paste

Microstructural change of hardened cement paste and mortar under various stresses was studied to obtain the basic data for judging the safety of concrete structure repeatedly receiving the stress. The crack revealed generally at 60% of the fracture stress, and it grew rapidly, exceeding at 80% of the fracture stress. The growth of the cracks in the hardened body was more conspicuous in paste than in mortar, in fly ash cement than in normal portland cement and blastfurnace slag cement, at high W/C than at low W/C, and in repeated loading than in single loading. Results were discussed in connection with the changes of pore structure and backscattered electron images of hardened bodies after loading the stress.