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Tips for Crack-Free Concrete Slabs - Continued

When It's Time for Concrete

Measure the exact volume of concrete required, then add 10% as a buffer. If you underestimate and have to call for a short load, not only do you waste time, but you may get cracks. Make sure the concrete supplier will have the trucks needed to supply your project without having to wait for an hour between loads. Excessive delay can result in a hot load. If the mix comes out of the chute hot, then send it back. Slump. Proper water content is one of the most important elements of good quality concrete; adding water to concrete to make it flow has a measurable negative result. A slab mix should not have a slump greater than 3 to 4 inches. If smoother flow is required, add superplasticizer, which gives the effect of a 6- to 8-inch slump without weakening the mix (Figure 6).

Figure 6. Although few residential contractors use them, slump tests ensure that the concrete coming out of the truck is not too wet (top). If better flow is needed, add a super-plasticizer (above).

Placing and Screeding

Concrete is actually a delicate material until it has fully cured. How you treat it during placement is crucial to your success. To avoid segregating the aggregate from the cement paste, never drop concrete more than 6 feet, use the proper concrete tools (not steel rakes), and don't drag or vibrate the concrete into place. If the soil is particularly dry or you're working in hot, windy weather, you should wet the subgrade, but not enough to create mud or leave standing water. In cold weather, do not place concrete on frozen ground.

Make sure you have sufficient manpower on site to maintain continuity in placing the concrete. If placing a slab takes too long, the first concrete placed will begin to set up well before the last concrete placed. This makes it difficult to finish the slab in one continuous process.

Finishing

Make sure concrete is screeded and bull-floated as it's placed, but don't allow finishing to begin until the bleed water has disappeared (Figure 7). If bleed water is worked into the concrete while finishing, the surface will be weakened. For the same reason, don't allow finishers to add water in an attempt to ease finishing.

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Concrete should be bull-floated as soon as it has been screeded (top). Bleed water will begin to appear soon after (above), but make sure it's gone before finishing begins.