Influence of reactivity and dosage of MgO expansive agent on shrinkage and crack resistance of face slab concrete

Abstract

Enhancing cracking resistance of face slab concrete is essential for the structural integrity and normal operation of concrete-faced rockfill dams (CFRDs). In this study, the effects of MgO with three reactivities and three dosages on the shrinkage and crack resistance of face slab concrete were systematically investigated by slab test, restrained drying shrinkage test and temperature stress test machine (TSTM). The results indicate that: (1) 7 %–25% of the total drying shrinkage and all (or most of) the autogenous shrinkage can be compensated by adding 5 %–10% MgO. The reactive MgO (M60) starts to compensate the drying and autogenous shrinkage at 1 day and compensates more shrinkage at early age than the moderate reactive MgO (M150) and the weak reactive MgO (M300), while M300 begins to compensate the shrinkage at about 50 days and produces larger compensation than M60 and M150 at late age. (2) M60 performs better than M150 in improving the crack resistance of face slab concrete to constraint and thermal stress. The increase in dosage of M60 and M150 from 0 to 10% prolongs the initial cracking time of face slab concrete by 10.0–27.5 h, increases the cracking strain by 9.2 %–25.7%, enhances the cracking tensile stress (σ) by 4.7 %–18.8% and lowers the cracking temperature (Tc) by 2.6–7.4 °C. Conversely, M300 weakens the cracking resistance. (3) The reactive MgO with relatively high dosage is suggested to eliminate the early shrinkage and to improve the cracking resistance of face slab concrete.