Effect of Synthetic Fibers and a Rheology Modifier on Settlement Cracking of Concrete

Abstract

Three series of concrete mixtures, consisting of 133 batches, were tested. The first series consisted of 27 control mixtures with a 24.3 percent cement paste content by volume and a water-to-cement ratio of 0.5. The second series consisted of 18 control mixtures with a 27 percent cement paste content by volume and a water-to-cement ratio of 0.45. The third series consisted of 88 mixtures with a 27 percent cement paste content and a water-to-cement ratio of 0.45. Fourteen batches in the third series served as controls; the remaining 74 mixtures were used to evaluate the effectiveness of crack reduction technologies on settlement cracking performance; fifty-seven mixtures contained from 1.5 lb/yd3 (0.89 kg/m3) to 7.5 lb/yd3 (4.45 kg/m3) of one of four different synthetic fibers, and 17 mixtures contained a rheology modifier in the form of a dry viscosity modifying admixture (dosed at 0.05% of mixture material dry weight). The results showed that settlement cracking increased as the slump of the mixture increased. The addition of fibers or the viscosity modifying admixture reduced settlement cracking compared to the mixtures without the additions. For a given slump, mixtures containing fibers exhibited less settlement cracking than the mixtures containing the viscosity modifying admixture. For a given slump, concrete containing the viscosity modifying admixture was more workable when shear force is applied compared to concrete containing fibers. With one exception, differences in settlement cracking as a function of fiber type were not statistically significant.