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Building with Tilt-Up | Frequent Questions

FAQ: Engineering

The TCA Technical Committee provides answers to common questions about Tilt-Up construction as well as some tips and techniques.

 

What is the recommended floor thickness?
A minimum of 5 inches is recommended to provide adequate brace anchorage and permit the crane to operate on it without damage. The use of the crane on the slab, however, does require attention to the load transfer through the outriggers and the pads they rest on.
What sub-base is recommended under the floor?
This depends somewhat on the soils report, but normally about 6" of crushed rock over 95 percent compacted soil.
Is a moisture barrier required under the floor?
Not under warehouse-type uses, unless specified by the soils report. However, it might be considered in floor areas where floor coverings are used and the water table is high.
What strength concrete, slump and type of mix is recommended in the floors?
ACI 302 – “Guide for Concrete Floor and Slab Construction” has recently been changed to ACI 302.1R-04. This revised guide specifies 3,000 psi compressive strength for Class 1, 2, and 3 floors and 3,500 psi for Class 4, 5, and 6 floors with a maximum slump of 5 inches. Because minimizing shrinkage and resulting slab curl is of prime importance, ACI 302 suggests paying special attention to selecting the best possible concrete mixture proportions. In general, the committee recommends a well graded aggregate mix design with a coarseness factor of 50% - 75% and a workability factor of 33% - 43%. The bottom line is the concrete mix design must have low shrinkage, good finish ability, good place ability and the required strength.
What type of joints and how frequently should they occur in the floors?

The typical floor will have joints both ways at no greater than 20 feet on center. They will be steel keyed cold joints in the long direction and control joints should be performed in the perpendicular direction using "Soft-Cut® International" or similar. The joints should be detailed on the panel layout plans.

Read Joints in Slabs as published in the February 2005 edition of Concrete Construction for additional references to key ideas about joints in slabs.

What equipment is needed to cast the concrete floors?
The floor needs to be carefully built to act as the casting bed, the crane platform and the working surface of the building. It must be strong at an early age for the ready-mix trucks and crane, and smoothly finished since it will mirror the panel finish. For these reasons, it needs to be vibrated well and struck off in large widths. The Texas Vibratory Screed does this very well and also serves as a straight-edge for inserting the zip strip joint material. However, any long screed apparatus can be used in conjunction with good vibration and finishing techniques.
How should the concrete floor be cured?
ACI 302 recommends the use of wet burlap, polyethylene film, combination burlap/polyethylene film or liquid membrane-forming curing compounds. Wet burlap, polyethylene film or combinations of both can result in the formation of salt deposits on the floor slab surface which appear as blotchy stains. If a liquid membrane forming curing compound is selected it must be compatible with the intended Tilt-Up bondbreaker or it will be necessary to remove it prior to application of the bondbreaker. Some tilt-up bondbreakers also serve the dual function of both a cure and bondbreaker. A combination cure and bondbreaker has the added advantage of minimizing the number of different products on the job site and the potential for a mistake.
What can you fill the necessary saw cut joints in a slab with prior to pouring panel on top of it that will not read or "telegraph" on to the face of the panel?

To our knowledge nothing exists that will consistently prevent saw cut joints from telegraphing onto the reflective panel surfaces. The problem boils down to whether you prefer grinding and patching or just patching panel surfaces. Some companies manufacture a vinyl rubber based temporary joint filler, which when properly placed in a saw cut joint, forms a uniform depression on the reflective panel surface which is easily patched using a tilt wall patching compound. Most Tilt-Up contractors would agree that it is far easier to apply a patching compound to a straight and uniform void than to carry a grinder 20-30 feet in the air in order to grind a concrete fin.

For painted faces, filling with silicone has been the most successful material. The ridge created when the bead cures is simply removed with a pass over from a grinder. Patching or sacking products are supplied by a variety of TCA Global Suppliers and can be used as needed to repair the grinding marks and fill any low areas. A durable concrete patching compound should be used that can be sanded with ease.

It is imperative that when preparing slab surfaces to produce an exposed aggregate finish, whether sand-blasted, acid- or power-washed or by other means, the joints in the slab must be concave to produce an “outy” on the surface of the panel. This “fin” that exists on the panel is much easier to grind and blend to the exposed aggregate. It is nearly impossible to patch an exposed aggregate panel requiring more extensive repair work.

Large construction joints are best filled with melted wax. One note on the finishing of panels, however, is that while previously a textured finish was required to hide blemishes, today the advanced technology of concrete placing and patching products allows an extremely smooth finish. A high quality patching product and paint is still recommended.

One contractor put it like this "you can't afford to cut corners because you're always remembered for your last project."

Will the crane damage the floor?
Not if it has been properly constructed with the correct sub-grade and compaction. The crane should extend outriggers when lifting heavy loads, and use rolling outriggers when driving with heavy loads. The crane should not be allowed to get near the edge of the slab, especially when deep fills are used.
How should electrical, mechanical and structural floor penetrations be blocked outs?
Wherever possible, electrical, mechanical, and structural objects should be stopped below the floor surface and the opening filled with sand. Place a thin layer of plaster over the sand and trowel smooth. Coat with bond-breaker to prevent adhesion. Joints in the floor that will have panels cast over them should also be filled with plaster and coated with bond-breaker.
Are isolated or continuous foundations better for Tilt-Up?
This varies with soil conditions and the slope of the terrain. Both can work well, but if there is much panel extension below the floor, the continuous footing will be needed to permit the back-filled soil pressure. If there are a number of openings in the panel, the continuous footing allows better support for the panel. The continuous foundation also permits getting closer to property lines. The typical design today usually utilizes continuous foundations under the panels.

About the TCA

Founded in 1986, the Tilt-Up Concrete Association (TCA) strives to improve the quality and acceptance of site-cast Tilt-Up construction, a method in which concrete wall panels are cast on-site and tilted into place. Tilt-Up construction is one of the fastest growing industries in the United States, combining the advantages of reasonable cost with low maintenance, durability, speed of construction and minimal capital investment. At least 10,000 buildings, enclosing more than 650 million square feet, are constructed each year using this construction method.

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