Analyzing mix parameters in ASR concrete using response surface methodology

The purpose of this experimental study was to investigate the effect of some critical concrete mix parameters on the ASR expansion and engineering properties by employing response surface methodology. Box–Behnken experimental design was utilized. Three independent variables (i.e., alkali content, water–cementitious ratio, ground clay brick percentage) were evaluated in three levels—minimum, maximum and midpoint of the experimental domain. The chosen ranges for alkali content, water–cementitious ratio and ground clay brick content were 2–8 kg/m3, 0.40–0.55 and 5–25%, respectively. 15 concrete mixes were prepared. Four responses (i.e., ASR expansion, compressive strength, flexural strength and modulus of elasticity) were measured in duplicates. The regression analysis showed the fitted model explains over 85% of the variations for all four responses. The plotted response surfaces showed that (a) ASR expansion decreases with ground clay brick quantity and increases with alkali content, and increases with increasing water–cementitious ratio at high alkali levels; (b) ground clay brick has positive effect on compressive strength at high alkali levels; (c) flexural strength increases with decreasing water–cementitious ratio and with increasing ground clay brick content; (d) modulus of elasticity increases with increasing ground clay brick and decreases with increasing alkali content, moreover, modulus of elasticity increases with decreasing water–cementitious ratio when brick content is high or alkali level is low, in other words, when ASR is low.