Better Garage Floors

Six-mil poly will satisfy the vapor-barrier requirement of most building codes, but it can be easily damaged during a concrete pour. Vapor barriers specifically designed for underslab use are thicker—between 10 mils and 20 mils—and much more puncture-resistant than ordinary poly. They’re also less likely to decay under the slab.

This surface spalling could have been prevented if the concrete mix had included air-entrainment.

Rebar is more effective than wire mesh at reinforcing a slab, but both must be propped up on concrete block or chairs so that they’re positioned in the middle of the slab after the concrete has been poured.

Isolation joints, like the one around this post, allow the slab to move independently from penetrations and perimeter walls. The joints are typically made with asphalt-impregnated fiberboard or other 1/2-inch-thick material.

Sawed contraction joints produce more effective crack control because joint depths are deeper and more consistent. Joints can be cut with an early-entry saw within one to four hours after troweling.

Hand-tooled joints (bottom) often aren’t deep enough or installed soon enough to be effective.

For the best surface, the slab should be kept wet during its initial seven-day setting period. This can be done either by spraying or by trapping its own moisture with a curing compound, a poly covering, or a curing blanket, as shown here.

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