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

Concrete Construction, March, 2003 by Joe Nasvik

Kaufman prefers to have well-graded aggregates in his SCC mixtures. His in-place cost saves him money, and he installs more slabs with the same number of workers.

SCC for walls

For all the above reasons, SCC is ideally suited for concrete wall construction. It flows long distances, including moving around corners in the formwork, and it provides good contact with reinforcement. The caution however, is that formwork must be designed to handle a full liquid head, and there must not be openings where concrete can leak out during placement.

Bob McCracken, vice president for research and development for EFCO, Des Moines, Iowa, is a member of ACI Committee 347 "Formwork for Concrete." This committee is currently working to define lateral pressures produced by SCC. Recently, Committee 347 revised the lateral pressure formulas to accommodate today's common mixes (Ref. 1), introducing a chemistry coefficient into the formula for calculating lateral pressure that varies as follows:

* 1.0 for normal concrete

* 1.2 for concrete containing fly ash, meaning that formwork must have 20% more strength.

* 1.4 when fly ash and superplasticizers are used together, meaning formwork must be 40% stronger.

Last year, EFCO poured SCC walls and columns at their facility in Des Moines. McCracken reports that the SCC performed well and the pressures observed would conform to pressures predicted using the 1.4 coefficient. However he urges caution during the placement of SCC. When possible, forms should be designed for full liquid head (unit weight of concrete x depth of fluid concrete in the form). This allows unrestricted pour rates. If liquid head forms are not practical, forms may be designed using the 1.4 chemistry coefficient as long as the pressure in the form is monitored during the pour to ensure that the pressure rating of the form is not exceeded. A practical method for doing this is to push a long rebar into the concrete to determine the depth at which a change in stiffness is found. The distance from that change in stiffness to the top of the pour would be the liquid pressure.

McCracken further states that the pour rate for concrete walls and columns depends on the strength of the form, the temperature of the concrete, and the combination of cement and additives used in the concrete mix. (See the graphs on page 44 for pour rates using forms of different strengths.) "There can be significant risks, along with all the benefits, of using SCC in vertical formwork applications," he adds.

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Marketing and education

Szechy contends that SCC should be called "SCT"--T for technology. It's not so much a new product as it is a new technology. The current marketing effort is intended to teach the technology to ready-mix producers and contractors. He advocates the need for better communication and planning with his contractors. "Trusting partnerships between producers and contractors is a vital component for cast-in-place SCC," he adds.

Ready-mix producers like Lattimore and Lafarge (which sells an SCC product called Agilia) are developing the SCC market. They prefer selling "performance" rather than just concrete. Specifiers, they contend, should tell them what kind of performance they require, and they will provide the concrete to achieve that result. It may well turn out that once contractors know the ingredients and how they perform (without worrying about batch weights and mix proportions), they will make better partners. For structural engineers, however, who carry the ultimate responsibility for the concrete in a structure, more knowledge may be required.