Improvement of viscosity-modifying agents on air-void system of vibrated concrete.

• Stabilization effect of VMA on entrained air bubbles in fresh concrete was clarified. • Improvement of VMA on air voids system of hardened vibrated concrete was validated. • The relation of air-void system parameters to rheological properties was presented. The influence of viscosity-modifying agent (VMA) on air-void system of self-compacting concrete has attracted considerable attention in recent years. But for conventional concrete with a sharp loss of air content due to vibration operation, the role of VMA on air-void system remains unclear. This paper aims to validate and clarify the positive effects of VMA on air bubbles stability and air voids homogeneity in vibrated concrete. Three typical VMAs with variable dosages were incorporated to prepare twenty air-entrained fresh mixtures. Rheological properties of mortar matrix portion of fresh concrete, surface tension of aqueous solution and time-dependent air content were evaluated for every mix. The air-void system in hardened concrete was inspected at three casting depths of prism specimen. Results showed that the surface tension of VMA solution played a key role in the stabilization effect on air bubbles, which strongly varied with VMA molecules. A higher plastic viscosity induced by the higher VMA dosage was favorable for maintaining the homogeneous distribution of entrained small air bubbles and reducing the formation of large voids in vibrated concrete. Therefore, a proper incorporation of VMA can improve the freeze-thaw resistance of vibrated concrete with lower spacing factor and smaller chord length. [ABSTRACT FROM AUTHOR]