Research on the effect of steam curing temperature and duration on the strength of manufactured sand concrete and strength estimation model considering thermal damage

The steam curing temperature and duration of manufactured sand concrete result from the comprehensive consideration of early strength growth rate and late strength. The effects of steam curing temperature and duration on the compressive strength during the constant temperature stage were studied experimentally to identify an economical and practical steam curing regime. Moreover, the heat of hydration, temperature change and microstructure were analyzed under different steam curing temperatures. After steam curing at 40 °C for 6 h, the hydration products covered only a small area, leaving unhydrated cement and leading to low early-stage strength. When the high-temperature steam curing duration exceeds 48 h, the hydration products are not uniformly distributed, and the C-H-S crystals tend to grow into a reticular structure with large voids, which results in an additional decrease in strength at the late stage. The results show that the duration of the constant temperature stage of steam curing at 40 °C, 50 °C and 60 °C should be controlled at 6.2–31 h, 4–19 h and 2.7–13 h, respectively. The classical maturity theory overestimates the strength of concrete at high temperatures of the steam curing stage. This paper shows how the maturity function can be modified by considering the thermal damage of the high-temperature steam curing, resulting in a more accurate strength estimation.