A Multiscale Assessment of the Mechanical Behaviour of Concrete at Elevated Temperatures for Nuclear Structures

The evolution of load induced thermal strain (LITS) plays a key role in the pre-tension loss of concrete in containment structures of a nuclear power plant under accidental conditions. The imposition of load during the first heating of concrete activates inelastic LITS and creates a specific damage mechanism. LITS and related damage mechanism can severely impair the structural integrity of pre-stressed concrete structures. Therefore, defining the mechanisms governing LITS is essential in improving the reliability of current engineering models. The mechanism underlying the evolution of LITS is still not fully understood and the role of concrete heterogeneity, particularly the interaction between aggregates and the cement matrix has not been previously examined. To address this, a multi-scale experimental programme was conducted. Specimens with varying coarse aggregate size, and volume fraction and sand content were tested under sustained load at temperatures up to 600 °C. A bespoke experimental setup was designed and built to perform the transient thermal tests, employing a non-contact measuring technique based on 2D point-tracking. Scanning electron microscopy was also used to characterise the microstructure of specimens subjected to heating only, loading only, and combined heating and loading. The results show that the evolution of LITS in concrete with temperature is highly sensitive to aggregate gradation, with smaller aggregate sizes producing higher LITS values. LITS does not always decrease with increasing coarse aggregate content; instead, a threshold is observed beyond which LITS increases. The absence of coarse aggregate in mortar specimens leads to development of significant magnitude of LITS and revealed LITS is restrained by the presence of coarse aggregates. In addition, the microstructural observations using backscattered electron imaging confirms that cracks always developed along planes parallel to the loading direction, supporting the hypothesis that crack closure is a key mechanism governing LITS.