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Effects of different cooling methods on residual mechanical properties of normal-strength concretes produced with slag-modified cements were not reported in previous literature. Therefore, a (2 x 2 x 4) factorial experiment was carried out in the present study to investigate the compressive strength and elastic modulus of slag-modified cement concretes with different strength levels (characteristic compressive strength of 20 and 30 MPa) subjected to different maximum temperatures (200, 400, 600, or 800ºC) and cooling procedures (slow or fast cooling). According to analyses of variance (ANOVA), air-cooled specimens showed higher residual mechanical properties. Higher residual elastic modulus was observed in concretes with higher initial strength, whereas residual strength was only affected by initial strength for higher temperatures. Effects of different cooling methods were more pronounced in slag-modified concretes than in concretes produced with ordinary Portland cement, especially for temperature up to 400ºC. Since slag-modified concretes have lower calcium hydroxide content, volume expansion and cracking propagation due to lime rehydration during slow cooling were mitigated, leading to higher post-fire mechanical properties. In contrast, these types of concrete exhibited significant temperature difference along their cross-section when fast cooling was used, so that a substantial thermal shock caused limited post-fire mechanical properties.
Teixeira et al. (Wed,) studied this question.