Self-Consolidating Concrete
Bryant Mather was a legend in the concrete industry. I'll never forget the long conversations I had with him about what most normal people would find completely irrelevant. He once took an hour to explain to me in magnificent detail why concrete flatwork bleeds (it is basically settling of the particles) and how entrained air can control it. Then there was self-consolidating concrete, or SCC. When this was first showing up in the U.S. many people, me included, called it self-compacting concrete. The difference between compacting and consolidating seemed insignificant, but when I foolishly used that terminology in a public meeting where Bryant was sitting in the front row guarding the purity of the concrete industry, he clarified the issue. He explained in a way that even a moron (like me) could understand that concrete is not compacted it is consolidated. Compaction implies squeezing something together to increase its density, like soil, while consolidation is getting the entrapped air out. Most people would say compaction, consolidation, who cares? But Bryant cared and he was right. I never used the term self-compacting again and eventually the entire business went along with Bryant. The meaning of those words is important because it is the self consolidating nature of SCC that make it so valuable in construction—we're not trying to increase the concrete's density, just get it completely consolidated in the forms—get all the air out. SCC can flow into very intricate forms or forms that have a lot of reinforcing bars (rebar congestion) and still leave no voids. Normal concrete would have to be heavily vibrated in those applications to try to work out all the entrapped air bubbles next to the forms and reinforcing and get the concrete to move in. Today, self-consolidating concrete (SCC) is recognized as one of the greatest advances in the concrete industry over the past 20 years. Using new admixtures and some mix modifications, we can now produce concrete that flows easily without segregating (where the coarse aggregate separates from the cement paste). Concrete that segregates loses strength and results in honeycombed areas next to the formwork. Some would ask, so why don't we use SCC everyplace? "It's cultural!" said Caroline Talbot, admixture marketing manager with Euclid Chemical. "Americans don't like change. It's very frustrating. In Canada we have been using SCC since the early 1990s and even earlier in Europe but it didn't start appearing in the US until the late 1990s. Today, finally, the precast industry seems to be catching on but not so much for ready mix." The perception is that it's the cost that prevents wider use of SCC—these mixes use fines and admixtures that can increase the price of a yard of concrete by $10 to $15, although some producers may add a surcharge as high as $30. The best approach to controlling the cost is to work with your ready mix producer to get the optimum mix for the intended use. Some ready mix providers may increase the cost simply because, especially when they were really busy, they didn't want to deal with the extra QC necessary, but in larger markets and with more sophisticated producers that is no longer an issue. "In New York City the difference is about $10 a yard," said Euclid Chemical's Bill Phelan. "But the cost of concrete construction reached a peak on high rise buildings of $1100 to $1200 per cubic yard in place—so what's ten bucks! So maybe the ready mix guy says it takes some extra attention so that might add some cost but it's not $30 per yard like some contractors think. In the boom years in California, one of Webcor's producers said they didn't want to have anything to do with high-performance concrete or self-consolidating concrete—but they were busy with the easy stuff and just didn't want to deal with it. With the conditions now, though, all of a sudden they can do HPC and SCC just fine." The other big problem has been controlling air entrainment and slump retention when using polycarboxylate water reducer (superplasticizer). The first time I saw SCC on a project, the concrete from the first truckload flowed perfectly, but the day turned out to be warmer than expected and the second truckload would barely flow down the chute and jammed the pump hose and had to be re-dosed with superplasticizer. So that's a potential problem, but one that can be easily dealt with, as we'll see. Use the links above to see where SCC works best, the materials and methods that make SCC work, and how to test it to make sure it measures up to its capabilities. Find a Concrete Contractor 24 Services in 221 Metros -- U.S. and Canada 
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