The ABCs of SCC: as a cast-in-place concrete contractor, here's what you should know

Concrete Construction, March, 2003 by Joe Nasvik

Self-consolidating concrete can be placed into a form at a single location and flow 100 feet or more without segregating. SCC is so fluid that its consistency is measured by "slump flow" rather than by slump. When a standard slump cone is filled and lifted, the size of the puddle it makes, rather than how far it slumped, is measured--slump flow can be as much as 34 inches.

SCC isn't a, new idea, although the current technology is different from what it was in the past. Marketing of "flowing concrete" started in the early 1980s in the United States, not long after the introduction of superplasticizers, originally formulated in Japan. SCC evolved from technologies used in underwater concrete placement in 1988 by Professor Okamura at the University of Tokyo. In 1989, Master Builders, Cleveland, developed and marketed high-strength concrete with a slump flow of 23 inches to 26 inches in a high-rise application. This concrete, however, still required some minimal vibration for consolidation.

Today many believe that SCC is the way of the future, backed up by tens of millions of dollars of research being conducted throughout the world. This investment is providing the information needed to ensure that SCC performs well for the many uses planned. This year will bring significant marketing efforts to promote it for cast-in-place concrete.

Mark Bury, senior product manager for Master Builders, estimates that 3 years ago only 10% of the precast concrete industry had experimented with and used SCC. Today, an estimated 90% of precast producers have tried SCC, and about 40% are using it on a daily basis. Congested steel reinforcement and good control of batching and placing concrete make this an ideal concrete for precasters. For ready-mix producers, however, embracing this technology is more problematic because there are many more variables to control, adding to the educational and marketing challenges.

Gaining acceptance of SCC in the cast-in-place market is more a matter of education than promotion. Contractors must look past the added upfront cost of SCC and focus on the "in-place cost" where significant savings may occur. Furthermore, contractors using SCC may be able to allocate labor differently although that will require better levels of communication and planning with ready-mix producers. Richard Szecsy, vice president of new product development for Lattimore Materials, McKinney, Texas, uses the word "partnership" to describe the necessary focus on communication. But the greatest challenge for acceptance of SCC will be in educating jobsite personnel, who often make the decisions about which concrete mix design and even which ready-mix producer to use--especially in the residential construction industry.

What is SCC?

SCC is a highly flowable yet stable concrete that can flow readily into place and fill the formwork without undergoing any consolidation and without significant segregation. It's a tightly controlled mix design with few options for adjustment on the jobsite. Szecsy refers to SCC as "hypersensitive" concrete. Small changes to the mix, especially the amount of water, can have very large repercussions. That's why SCC is normally sold as a value-added performance product. And as with any new product, there's a learning curve, and all parties have to gain some experience to work out the details.

The three primary components of SCC include a properly proportioned concrete mixture designed for the application, a synthetic high-range water reducer (superplasticizer), and sometimes a viscosity modifying admixture (VMA)--more about these later. The quantities of each component can be varied to achieve a wide range of results.

Increasing the flowability

Superplasticizers provide the high flowability of SCC. Although high-range water reducers (superplasticizers) have been on the market for 30 years, a new generation based on polycarboxylate chemistry is emerging as the leading superplasticizer. Bury states that synthetic high-range water reducers are engineered at the molecular level to provide desirable properties while minimizing the undesirable ones. Polycarboxylate-based superplasticizers are powerful cement dispersants that require less mix water while significantly increasing flow characteristics. Previous generations of superplasticizers retarded initial set, but some polycarboxylate superplasticizers are engineered for a more rapid initial set. They also provide better, more complete hydration of cement particles, yielding increased "high-early" setting characteristics. Bury adds that polycarboxylates are now available from 50% of U.S. ready-mix producers.

Mix design considerations

Aggregate gradations play an important role in mix proportioning for SCC. Well-graded aggregates (including well-graded fine aggregates) make the best SCC because they require less cementitious material and less mix water and therefore cause fewer shrinkage, curling, and efflorescence problems. The cementitious content for a good well-graded mix, according to Szecsy, can be as low as 470 pounds per cubic yard (five bags)--20% to 25% of which can be fly ash. He stresses that fly ash should be included because it increases flowability.