Only a few years ago there was very little cast-in-place self-consolidating concrete (SCC) being used in the United States; this year, according to the National Ready Mixed Concrete Association (NRMCA), 2.8 million cubic yards will be placed, and 6 million cubic yards will be placed in 2006. Although SCC mixes are more expensive, contractors are learning that it's possible to save money on installation, and the results for architectural concrete are much better than when more standard mixes are used.

But constructing formwork for walls using SCC is challenging for contractors because lateral form pressures can be much higher than with stiffer mixes. Bob McCracken, vice president for research and development at EFCO Forms, Des Moines, Iowa, says that at his company's research facility they have measured lateral pressures on forms from SCC of 3000 pounds per square foot. ACI committee 347, Formwork for Concrete, is currently discussing what the maximum pressures for SCC might be. Their current recommendation is:

... When working with mixtures
using newly introduced admixtures
that increase set time or slump characteristics,
such as SCC, "full liquid
head" should be used until the effect
on formwork is understood by
measurement.

About SCC

A definition for SCC currently being considered by ACI committee 237, Self-Consolidating Concrete, is:

Self-Consolidating Concrete (SCC)
is highly flowable, non-segregating
concrete that can spread into place,
fill the formwork, and encapsulate
the reinforcement without any mechanical
consolidation.

This definition provides considerable latitude for mix ingredients and performance requirements, so the design of a mix for a particular job application becomes very important. For this reason, a representative from the ready-mix producer should be included in planning sessions on the formwork systems and any discussion of lateral form pressures, to help work out SCC mix design criteria. Concrete temperature at the time of placement must be a part of this discussion, too, since temperature determines how quickly the mix loses slump.

An admixture commonly found in SCC is a polycarboxylate superplasticizer. This new generation of superplasticizers combines high water reduction, cohesiveness improvement, and no set retardation at normal dosages--three advantages that benefit SCC. But depending on dosage and the product used, polycarboxylate admixtures can have an accelerating, a retarding, or a "set-neutral" effect. So it's important for you to talk with your supplier about the setting characteristics of the product you are using. Even though SCC is very fluid, stiffening can start 20 to 40 minutes after batching at concrete temperatures of 80[degrees] F. Richard Szecsy, vice president of new product development for Lattimore Materials, McKinney, Texas, notes also that polycarboxylate superplasticizers are cement sensitive, meaning that different portland cements affect them in different ways.

Viscosity modifying admixtures (VMA) are also used for some SCC mixes to control segregation, such as when there is a gap-graded aggregate mix. VMAs are inert admixtures, not influencing setting characteristics or reacting with other admixtures. Tony Schlagbaum, product line manager for Degussa Admixtures, Cleveland, adds that they may slightly reduce form pressures.

There are good reasons to use SCC for wall construction:

* For placements with congested steel reinforcement, SCC can ensure consolidation around the bars and good bond.

* SCC can flow as far as 130 feet from the point of placement, making it easy to place concrete in areas where access is difficult.

* When SCC is placed properly, almost no bug holes appear on form surfaces, making it ideal for architectural applications.

* SCC is less permeable, develops high early strength, and provides higher durability than regular concrete.

Determining form pressures by full liquid head

Should your formwork be designed to withstand full liquid head pressures? If you design for full liquid head, the full height of concrete in the forms is assumed to be in a liquid state. To calculate the lateral pressure, multiply the full height of the concrete by its density; 150 pcf is a typical assumption. So a 12-foot-high SCC placement would produce 1800 psf of lateral pressure at the bottom of the form. To be safe, this is the best way to calculate form pressure.

Designing and building forms assuming full liquid head also makes it possible for the contractor to place concrete by the fastest means possible. Since there is no longer any concern about whether the forms can handle the full load, there is no restriction on rate of placement. This also allows walls and columns to be pumped from the bottom. McCracken says that when you pump concrete into the form from the bottom, you must calculate full liquid head plus an additional 25% to account for pump pressures. The greatest advantage of this method is almost total elimination of bug holes.