What is the primary reason for using viscosity-modifying admixtures (VMAs) in self-consolidating concrete (SCC)?

The primary reason for using viscosity-modifying admixtures (VMAs) in self-consolidating concrete (SCC) is to enhance its stability and prevent segregation and bleeding. Self-consolidating concrete is designed to flow easily and fill formwork under its own weight without the need for vibration. This high flowability can make SCC prone to segregation, where the heavier aggregate particles separate from the cement paste, and bleeding, where excess water rises to the surface. VMAs increase the viscosity of the mix water, which in turn increases the cohesiveness of the entire concrete mixture. This increased cohesiveness helps to suspend the aggregate particles and prevent them from settling out of the mix, thereby reducing segregation. VMAs also help to retain water within the mix, minimizing bleeding by holding the water in suspension and reducing its tendency to migrate to the surface. By preventing segregation and bleeding, VMAs ensure that the SCC remains homogeneous and maintains its desired properties throughout the placement process. For example, in complex formwork or heavily reinforced sections where vibration is difficult or impossible, VMAs are crucial for ensuring that the SCC fills the form completely and achieves uniform density and strength.