Effects of Curing Temperature and Relative Humidity on the Early-Age Compressive Strength and Water Impermeability of C20 Concrete

Concrete is the most widely used building material in civil engineering, and its long-term performance and durability are directly related to structural safety and service life. To investigate the effects of early-age curing temperature and relative humidity (RH) on the performance of C20 concrete, this study designed two independent single-factor experiments: five curing temperature levels (20, 30, 40, 50, 60 °C) were set at a constant relative humidity of 55%; and five relative humidity levels (55%, 65%, 75%, 85%, 95%) were set at a constant temperature of 20 °C. Compressive strength and impermeability tests were conducted on C20 concrete specimens cured for 7 days, and comparative analysis was performed with specimens under standard curing conditions at different ages. The results show that under 55% RH, compressive strength first increases and then slightly decreases with rising temperature, with a more pronounced increase in the 40–50 °C range and a decline at 60 °C; the penetration height (the water stain height measured vertically upward from the compressed bottom surface of the specimen) first decreases and then increases with rising temperature, from 11.09 cm at 20 °C to 8.35 cm in the 20–40 °C range, and then rises to 12.21 cm in the 40–60 °C range, indicating that a moderate temperature increase is beneficial to the performance development of C20 concrete, while excessively high temperature leads to performance degradation. At 20 °C, as relative humidity increases, the compressive strength of C20 concrete continues to rise and the penetration height decreases significantly, indicating that higher humidity is conducive to strength formation and impermeability improvement. Compared with 7-day curing, C20 concrete under 28-day standard curing exhibits higher strength and better impermeability. These results indicate that, within the material system and working condition range of this experiment, suitable early-age curing temperature and relatively high humidity are favorable for the early-age strength and water impermeability development of C20 concrete.