Tilt-up concrete construction: tips on choosing materials for tilt-up panels

Concrete Construction,  Sept, 2003  by Joe Nasvik,  Bill Palmer

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How much influence does the contractor have on the materials incorporated into a tilt-up panel? "On the concrete mix, we usually stick with the spec," says Mike Berguin, Tilt-Up Construction, Mesa, Ariz., "but I have a huge voice in the bondbreaker." Whether you stick with the specification for materials or suggest alternatives, here are some tips on the decisions you might have to make.

As for the mix itself, there is no magic. According to Greg Frigerio, vice president for the tilt-up group at Clayco, St. Louis, you need between 2500 and 3000 psi compressive strength and between 450 and 500 psi flexural strength in a tilt-up panel in order to lift it. His company typically lifts panels 4 or 5 days after they are cast and can easily achieve these requirements with most mixes. Some contractors will "change to a high-early-strength mix for the last couple of pouring days in order to compress the curing time," notes Tom Klemens in a recent article (CC, July 2003, pp. 53-58).

The only controversy we've found with mix design is the use of fly ash. While some recommend a maximum of "15% fly ash in the floor/casting slab and none in the wall panels," in order to get low porosity concrete in the slab and get a good separation when lifting (CC, March 2003, p. 90) others insist that "fly ash is an acceptable part of every type of concrete" (CC, June 2003, p. 16; also see Reader Response, p. 16 in this issue). Our feeling is that when proportioned properly, fly ash concrete is fine for either the slab or the panels, but all ramifications of this decision should be considered in advance.

Aggregate size can also be an issue-make sure it's not too big for the concrete cover between the bars and the panel face. "If the bars are 3/4 inch from the face," notes Beguin, "you need smaller aggregate so that it's not pushed down, leaving nothing but paste over the bars, which can crack or leave lines."

Admixtures

The use of admixtures doesn't play a large part in concrete mixes for tilt-up construction. The only admixture Clayco regularly uses is superplasticizer. It typically orders concrete with a 4inch slump and adds enough plasticizer at the jobsite to increase the slump to between 6 and 7.5 inches. Slump checks are performed at the point of discharge before and after the addition of the admixture. When temperatures fall, Clayco will use a nonchloride accelerator.

Woodland Construction, Jupiter, Fla., constructs tilt-up buildings from Miami to Vero Beach, Fla.--mostly along the coast. Temperatures usually reach 90[degrees]F during the day, with 100% relative humidity. And rainstorms are possible in the afternoons. Clay Fischer, president of Woodland, says that his company uses only a water-reducing admixture in its concrete. In their hot-weather climate, this gives additional time for placement due to the retarding effect of water reducers. He adds that Woodland typically places floors at night, when temperatures are lower, the chance of rain is diminished, and concrete is more readily available.

Bondbreakers

The selection and proper application of bondbreakers is critical for tilt-up construction. They are placed on floor slabs prior to forming operations for tilt-up panels--as much as 6 weeks prior to the concrete being placed. The bondbreaker's purpose is to ensure that a clean separation occurs between the wall panel and the floor when the panel is lifted. It's also important that bondbreakers allow a good finish on the lower part of the wall panel.

Only a few companies are making bondbreakers for use in tilt-up construction, according to Jim Baty, technical director for the Tilt-Up Concrete Association, Mount Vernon, Iowa. Burke by Edoco, Cresset, Conspec, Dayton Richmond, Meadow-Burke, and Nox-Crete are the principal manufacturers. According to Kirk Plumb, national sales manager for Nox-Crete Products, Omaha, these products are usually marketed directly to contractors. "You don't sell a job, you sell a contractor," he says. In fact, many project specifications call out bond breakers as "contractor's choice."

Beguin's guidance on bondbreakers is to use products from the same company that manufactures the floor curing/sealing compound. "That way you're sure there won't be compatibility problems. I've had some disclaimers from cure-and-seal manufacturers that indicate a specific temperature and a specific time of day--we can't always do that in the real world."

There are three types of bondbreakers on the market, those that form physical barriers, those that are reactive with the concrete, and combinations of the two. Physical barrier products are made with wax or resins. Reactive bondbreakers have ingredients that react with the free lime in port-land cement to produce metallic gels.

When choosing a bondbreaker, consider the following:

* Some states, California for instance, have more stringent VOC requirements than federal regulations.