Degradation Law and Prediction Model of Elastic Modulus of High-Performance Concrete under Thermal Cycling

In this study, a thermal cycling method was proposed in the temperature range of 25 degrees C-65 degrees C to investigate the effect of thermal cycling on the elastic modulus of high-performance concretes (HPC40 and HPC60). In addition, the applicabilities of infrared thermography and ultrasound techniques were compared to detect defects in concrete under the effect of thermal cycling. Furthermore, the influence of the microstructure on elastic modulus was studied using scanning electron microscopy and backscattered electron analysis. Finally, based on the four-sphere model, a prediction model of elastic modulus evolution was established considering the influence of the microstructure. The elastic modulus of the HPC decreased with the increasing number of thermal cycling tests, and as the strength grade of concrete increased, the decrease of elastic modulus became more evident. With the increasing number of thermal cycling tests, the maximum value of the average surface temperature increased, while the ultrasonic velocity decreased. The experiment illustrated the infrared thermography method was appropriate for characterizing concrete with severe defects, while the ultrasonic technique was more suitable for characterizing concrete with less damage. Under the effect of thermal cycling, microcracks clearly appeared in the matrix and interface transition zone, and the structure of the hydration products changed from dense to loose, resulting in the degradation of the elastic modulus. The predicted elastic modulus values from an evolution model, developed to describe the effect of microstructure on elastic modulus, matched the experimental values well.