Temperature sensitivity of MgO expansive agent and its application in temperature crack mitigation in shiplock mass concrete

The compensation of thermal shrinkage with the delayed expansion of MgO expansive agent has been used not only in mass concrete, but also in some underground sidewall in subway stations in China. This study investigated the expansion behaviors and crack resistance of three kinds of MgO with different reactivity values (90 s, 150 s and 250 s) in concrete based on a comprehensive test system including the length test using the differential strain gauge, a dual-ring test and the environment simulation test chamber. The results showed that the expansion behavior of MgO expansive agent was closely related to its reactivity value and the temperature history of concrete. MgO with lower reactivity value (90 s, 150 s) showed larger expansion in concrete than MgO with higher reactivity value (250 s) under the temperature history of low temperature rise and high cooling rate, while MgO with higher reactivity value showed larger expansion at temperature fall stage and better crack resistance under the temperature history of high temperature rise, low cooling rate and long cooling period. The results indicate that the reactivity value of MgO expansive agent should be properly selected according to the characteristics of temperature histories of different concrete structures to give full play of its shrinkage compensating efficiency.