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What makes a concrete driveway or slab strong? Here's a sampling of tips and opinions from professional concrete masons. Avoid cracking and spalling problems and learn how to make a concrete slab that will last as long as you do.
Because concrete slabs “float” on the soil, soft ground or voids underneath may cause unsupported areas to crack under heavy weight like vehicles. Pack about 4 in. of sand or gravel over clay and other poorly draining soils to provide even support.
Don't rely on steel mesh or plastic fibers. They help prevent cracks but do little to improve strength.
Adding water to the concrete makes it flow down the chute and fill your forms more easily, but it also weakens the final slab. A drier mix is more difficult to pack into your form, especially around the edges, but makes for a stronger, more crack-resistant slab.
If you trowel concrete too soon, too much cement (paste) will rise to the surface, weakening the surface layer. But once the concrete begins to firm up and the water disappears, be prepared to move quickly with your troweling. Concrete can harden fast.
Control joints should be cut about one fourth the thickness of the slab during the finish-troweling process. Concrete shrinks slightly as it cures and will crack at regular intervals. You want the cracks to occur at the control joints. Space the joints about every 4 ft. on walks and at least every 10 ft. on larger slabs. If the concrete hardens before you can cut the grooves, cut them the next day with a circular saw and diamond blade.
Concrete will cure harder and stronger if it has a chance to dry slowly. In cool and moderate conditions, cover it with plastic. Under hot conditions, above 80 degrees F, frequently water it down with a garden hose to keep it moist. If kept wet, concrete will reach about 90 percent of its maximum strength after about a week.
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