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A reasonable tolerance on SCC spread is plus or minus 2.5 inches.
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Slump flow, a test that is often performed with an upside down cone, measures SCC's flowability.

But another term is gaining importance when referring to SCC's quality. Robustness is a mix's ability to maintain its properties in the face of minor changes, such as water content (aggregate moisture content), admixture dosage, cement type, aggregate gradation, or mixing and handling.

A lengthy article on robustness is available at www.selfconsolidatingconcrete.org. The bottom line is that SCC mixes tend to be less robust than ordinary concrete because the mix is more complex, and minor variations can lead to serious problems on the jobsite in terms of flow and segregation.

For many SCC mixes, the solution to this lack of robustness is to use high percentages of powder, high dosages of VMAs, or both. But then the problem is that with all the added fines and admixture, the cost of the materials begins to outpace the cost reduction from labor savings on the jobsite.

Ask any contractor if he is willing to pay a higher price for a less predictable concrete mix to slightly reduce his labor costs, and he will show you the door in a hurry. Added risk is something to avoid in construction.

SCC is one of the greatest advancements in concrete technology in the past 20 years. But moving this technology from lab to the field will only happen if a robust mix can be provided at a reasonable price.

Using new technology and reasonable tolerances, producers have proven our industry can accomplish this. The remaining challenge is convincing contractors and owners.

For more information, also see Achieving Robust SCC Economically

William D. Palmer Jr. is with Complete Construction Consultants. E-mail wpalmer@cee3.com.