Self-consolidating concrete (SCC), a high-slump technology imported from Japan, has the ability to revolutionize concrete placement, resulting in faster
JOAN BLECHA, NPCA
Self-consolidating concrete (SCC), a high-slump technology imported from Japan, has the ability to revolutionize concrete placement, resulting in faster pours requiring potentially fewer workers. For precasters, it provides better flowability within forms, reducing vibration and increasing form life, as well as superior consolidation to produce a higher quality product that needs no extra finishing after form removal. Cast-in-place practitioners find SCC advantageous due to its lower viscosity as compared to conventional concrete and, thus, greater flow rate when pumped. Lower pumping pressure reduces wear and tear on pumps and the need for cranes to deliver concrete in buckets at the job site.
To achieve a high flow rate and avoid obstruction by closely spaced reinforcing, SCC is designed with limits on the nominal maximum size (NMS) of the aggregate as well as its quantity and grading. However, when flow rate is high, the potential for segregation and loss of entrained air voids increases. Another challenge associated with SCC technology is shrinkage, which may result in more cracks in restrained concrete elements, accelerating the deterioration of both concrete and reinforcement.
Such problems can be alleviated by designing a mix that incorporates a high fine-to-coarse aggregate ratio, a low water-to-cementitious material ratio (w/cm), good aggregate grading, and a high-range water-reducing admixture (HRWRA). Viscosity modifying admixtures (VMA) are also used to reduce the tendency for segregation and enhance the stability of the air-void system.
We can’t afford to be complacent, says National Precast Concrete Association president Ty Gable. If the industry doesn’t embrace new technology, then we will be replaced. But, SCC is a technology that we are adopting enthusiastically.
NPCA engineers have been looking closely at SCC technology, exploring how it can best be used in precast plants and gauging its use by industry producers. Engineers’ observations are posted for the benefit of members and specifiers at the association’s Web site, www.precast.org
Self-consolidating concrete’s ability to flow three-dimensionally Û requiring very little, if any, vibration Û greatly reduces surface defects typically associated with conventional concrete, notes NPCA Technical Services Engineer Adam Neuwald. In addition, a typical SCC mix design will develop higher early strengths than conventional concrete used in precast applications.
Many within the industry believe that SCC will revolutionize the manufactured concrete products industry, while others feel that SCC will find a niche market similar to zero-slump concrete or high-strength concrete, Neuwald adds. Only time will tell how SCC will ultimately affect the manufactured concrete products industry, but one thing is certain: Self-consolidating concrete is here Û and, it’s here to stay.