Strain of heated concrete during two thermal cycles. Part 2: strain during first cooling and subsequent thermal cycle

The strains of three nuclear reactor type concretes during first cooling from constant test temperatures of 110°C to 600°C and during a second heat cycle have been analysed in detail. Drying shrinkage and load-induced thermal strain (LITS) are largely absent from the strains measured during the subsequent three thermal transients because the ‘delayed’ components were largely (but at lower temperatures not totally) dissipated in the five-day period at constant temperature. An important conclusion derived from the strain measurements during the three last thermal transients is that they provide a sensitive indication of the thermal stability of the concretes as deduced from thermal cycling tests with and without compressive loading. It is from these results that the concept of the ‘concrete-specific critical temperature’ emerges, and it is being used for the first time in the current paper. Up to this ‘critical’ temperature, a second thermal cycle would not cause significant additional damage. At higher temperatures, the influence of thermal cycling, particularly for unloaded concrete, would be to cause marked progressive cracking. The onset, and degree, of damage depend primarily upon the type of aggregate and to a lesser extent upon the type of cement paste. Orders of influence of environmental and material factors are presented for the different strains analysed in the current paper.