Strain of concrete during first heating to 600°C under load

Abstract

Synopsis This is the second of three papers presenting the results of an investigation into the transient thermal strain behaviour of concrete during the first heat cycle to 600°C under load. The results during first heating to 600°C under uniaxial compressive load of eight different unsealed concrete andcement paste mixes are presented. The thermal strains were shown to consistof ‘free’ and ‘load-induced’ components which possessed different and distinct properties. The free thermal strains of the unloaded concretes were dominated by the thermal expansion of the constituent aggregate, whilst the load-induced thermal strains (LITS) were identical for temperatures up to about 450°C, irrespective of type of aggregate used, provided certain criteria were met. A ‘master’ LITS curve therefore existed which represented LITS of Portland-cement-based concretes in general. Also LITS was not significantly influenced by the age of the concrete (1 and 9 years) or the initial moisture condition (moist and air-dry). These findings could considerably simplify the analysis of heated concrete structures, particularly since only two strain components are required. Replicate tests were performed to provide a statistical basis for analysis of the results. The effect of aggregate restraint was determined by comparing results from concrete and cement paste specimens. In addition to ‘material’ factors, the effects upon free and load-induced thermal strain of the following ‘environmental’ parameters were examined: temperature level, initial moisture conditioning, preheating, rate of heating and stress level.