Effect of mineral admixtures and viscosity modifying admixtures on plastic shrinkage cracking of cementitious composites

Abstract

The effects of mineral admixtures (MAs) and cellulose-based viscosity modifying admixtures (VMA) on bleeding and plastic shrinkage cracking of cementitious mortar exposed to a windy environment were examined for this article. Cementitious composites containing 25% of three types of MAs, i.e. fly ash and two finenesses of slag were incorporated with two molecular weights of VMA with various mixing ratios in the mortar specimens. Plastic shrinkage cracking tests were conducted on the restrained plate subject to a constant evaporation rate of 1000 g/(m² h) using a speed adjustable fan in an environment chamber. Bleeding and total crack areas (TCA) and crack widths and crack width distribution were evaluated. In addition, the viscosity of cementitious composite and surface tension of pore solution were also investigated. Without VMA, with the addition of slag, viscosity increased and bleeding reduced, and thus TCA increased. The opposite was found when fly ash was added. With the addition of VMA, bleeding and surface tension reduced significantly. Cementitious mortars with specified VMA dosages appeared to be significantly effective in reducing TCAs, average crack width, and large cracks. The crack reductions are fairly well correlated with surface tension, VMA dosage, and polymer film formation.

Keywords:

• viscosity
• admixture
• plastic shrinkage
• cracking

Acknowledgments

We would like to gratefully acknowledge the National Science Council for the financial support (NSC-96-2622-E019-005-CC3).