One word describes successful concrete work: consistency. Exercise the same level of care at every stage of the process.
Preparing the Base
If the gravel subgrade settles after you've poured, cracks can occur. The two prime offenders of subgrade settlement are soils with different load-bearing capacity across the slab area and insufficient gravel compaction.
Make sure the soil under the slab is free of topsoil and organic matter. If there's any doubt about the soil's bearing capacity, check with the local building department, with area builders, or test the soil. The few hundred dollars a soil analysis and compaction test costs is nothing compared to fixing a failure.
Compact the subgrade soil uniformly. Use a plate-tamper or jumping-jack compactor until it leaves very little impression in successive passes. Compact soil in 6- to 10-inch layers to remove pockets of soft soil.
When adding water to dry soils during compaction, don't add too much. If you see water-saturated soil near the compaction area, correct it. You have two options: wait for the soil to dry or remove and replace it with dry material.
Subgrade surfaces should be as flat as possible. Grade soil to the same level across the full slab width. Subgrade should be level to within 1/2 inch. Where the subgrade elevation is inconsistent, the slab thickness varies, causing uneven curing and stresses.
Slab shapes affect cracking. An inside corner, for example, is a pressure point. Try to divide slabs into symmetrical squares using contraction joints (or control joints) rather than creating irregular shapes. Contraction joints can be hand-tooled right after the bleed-water leaves the surface, cut with a saw right after finishing, or formed in wet concrete using a proprietary plastic or metal joint material. A contraction joint should be one-fourth as deep as the slab thickness. And, squares are better than rectangles. On a typical 4-inch-thick residential slab, 8- to 12-foot joint spacing is good.
If the slab is contained by foundation walls–or there are structural columns inside the slab area–you must provide expansion (or isolation) joints. This allows slab movement independent of the wall or columns. Residential expansion joints are often 1/2-inch-thick fiberboard strips. The slab must be isolated from the footing, too. Use sand or 15-pound felt between the slab and footing as a bond breaker.
Proper placement of steel reinforcement is critical. At a minimum, reinforcing mesh, pipes, and cables should be suspended in–not lying on the bottom of–the slab. Wire mesh should be in the middle of the slab to work properly. Pipe or conduit laid directly on the ground dramatically weakens slabs. Use steel or plastic rebar chairs to locate it correctly. Don't use brick; it pulls the moisture from the surrounding concrete too quickly, creating a stress point.
Measure the exact volume of concrete required and add 10 percent. If you come up shy and call for a short load you may get cracks where the late-arriving concrete meets the earlier mix. Make sure your supplier has enough trucks to supply your project without waiting between loads. Excessive delay can result in a "hot load." Send hot loads back. Proper water content is vital, but adding water for easier flow is bad. A slab mix's "slump" should be no greater than 3 to 4 inches. Instead, add super plasticizer for smoother flow.
Concrete is actually a delicate material until fully cured. How you treat it during placement is crucial. To avoid segregating the aggregate from the cement paste, never drop it more than 6 feet out of a chute or pump hose, use the proper concrete tools, and don't drag or vibrate it into place. If the soil is dry or you're working in hot, windy weather, wet the subgrade, but don't create mud or standing water. In cold weather, do not place concrete on frozen ground.
Have sufficient manpower to maintain continuity in placing the concrete. If placing a slab takes too long, the first concrete placed starts setting before the last concrete is placed, making it difficult to finish the slab in one continuous process.
Screed and bull-float the concrete as it's placed. Don't allow finishing to begin until the bleed-water has disappeared. Bleed-water worked into concrete while finishing weakens the surface. Don't add water to ease finishing.
A well-organized, well-timed, uninterrupted work flow is essential. Hand-troweling the perimeter along walls and around obstacles as concrete is placed is a good start. Have the power-trowel ready to go before you need it, too.
Take care during curing. Initially, concrete's temperature rises quickly, peaks, then drops. This can cause failures. Wet-curing keeps the slab from drying out too fast. Cover a fresh slab with burlap immediately after finishing, and keep the burlap wet with a lawn sprinkler for at least three days (ideally seven days). Or cover it with plastic. Wet curing ensures an even and controlled cure. In cold weather, make sure fresh concrete is protected from freezing.
–Gabe Martel is a project manager and construction superintendent in Pembroke, Ontario. This article was reprinted with permission from The Journal of Light Construction. For subscription information, call 800-375-5981 or visit www.jlconline.com.