New-generation high-range water-reducing admixtures have the ability to challenge even the most tried-and-true industry standards, and yet we still do not have a standard name for the concrete they produce. One of the hottest and most controversial subjects in the concrete industry is what's known as self-leveling (SLC), self-placing (SPC), self-consolidating, or self-compacting concrete (SCC). Several admixtures are designed to produce concrete that fits any of these four often-interchangeable descriptions. Most of these concretes actually have less water than standard mix designs.
Because of the fluid nature of the concrete, a more continuous grading of aggregates is required. Also, many producers have pursued cost-effective SCC solutions through the use of high-volume fly ash replacement and still achieved optimum performance from the mix design. But the real magic in producing this type of concrete comes from the chemical admixtures.
SCC, SLC, and SPC are produced through a combination of superplasticizers and what are called viscosity-modifying admixtures (VMAs). According to Tony Schlagbaum, group marketing manager at Master Builders, VMAs offer two major advantages: 1. extreme rheology control and 2. improved engineering properties vs. a high fines mix-design approach. The use of a VMA creates a certain amount of "fluidity forgiveness" in the mix if excess water content is present. Under ordinary circumstances, this would produce a mix that would tend to segregate. But with the use of a VMA, the mix remains cohesive and maintains its viscosity.
The article includes definitions of SCC, SLC, and SPC; a list of commercially available SCC admixtures; and a list of SCC workability test methods.