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Concrete strength deterioration is a major concern in evaluating the reusability of fire-exposed reinforced concrete (RC) structures and the residual service life of constantly heated RC structures. The maximum temperature reached during heating and the exposure duration are two key factors in weakening concrete. Numerous research has experimentally investigated the relationship between concrete residual strength and exposure duration at constant heating temperature. The temperature in a real fire or in a high-temperature service environment, however, is not constant. Within this context, an algorithm based on the concept of damage accumulation is developed to evaluate concrete mechanical decay in a variable-temperature environment. An experimental campaign was performed and its results closely match the predictions of the proposed algorithm.

This paper demonstrates how artificial neural networks (ANN) can be used to predict the residual strength of heated concrete. The authors report on ongoing research that is trying to account for the effect of exposure time (to high temperatures) on residual strength of concrete. Results reveal that loss of strength becomes significant as exposure times are prolonged at exposure temperatures of 400 deg C. and higher. Analyses of residual pulse velocity and residual strength shows that a certain linear relationship exists for exposure times between 30 and 120 min. Due to the absence of a theoretical relationship, neural network analysis is applied to identify a possible general relationship between residual strength and variables including exposure temperature, exposure time, water-cement ratio (w/c), and residual pulse velocity. The authors note that ANN bears better tolerance than regression analysis to an imprecise data set. The authors conclude that good linear relationships are found when comparing the residual strength predicted by networks with correspondent target values. This type of testing may prove valuable as part of the assessment for fire-damaged concrete prior to the repair of structures.