Study on the process of isothermal continuous drying and its effect on the strength of concrete of different strength grades

Dry-based concrete specimens must be obtained to investigate a moisture effect, but the drying law and strength change vary for concrete of different strength grades under different drying temperatures. In this work, concrete specimens with different strength grades are dried in a thermostatic electric drier at constant temperatures of 60, 80, 105, 120 and 150 °C, and the changes in mass, ultrasonic data and strength of the specimens before and after drying are analysed. The test results demonstrate that the drying stages of concrete with different strength grades at different drying temperatures are generally the same even with variations in drying rate, time and the total water mass loss. Considering the drying temperature and strength grade, a model for concrete drying is obtained based on the modified Page model II. After drying, changes in the concrete compressive and splitting tensile strengths of different strength grade materials are highly similar and are slightly higher than those before drying. The relative strength initially decreases and subsequently increases with increasing drying temperature. The drying temperature 100-125 °C is recommended as the optimal drying temperature for different strength grades of concrete according to the minimal strength change. Further analysis shows that the change in concrete strength is the result of mutual competition between the damage caused by water evaporation and the density increase caused by drying.