Garage Slab Specifications

Slab Support

• The longevity and performance of the slab depend on the reliability and integrity of the ground soil. If the sub-grade fails, so does the concrete slab. Most specifications call for the use of a thin layer of stones for the fine grading materials. Compacting the material helps control the thickness of the concrete slab and reduce resistance between the support and the slab.

Concrete Mixture

• Generally, the mixed concrete for slabs must meet the ASTM C-94. The compressive strength must be 4,000 pounds per square inch (psi) after 28 days of curing. The blend should have air content of 5 to 7 percent for most climates. The proper air content enhances the workability of the concrete mixture and improves the long-term durability of the slab. Most garage slabs use concrete with a 4-inch "slump"--a measurement of the amount of water in the concrete--but not exceeding 5 inches. Many specifications call for coarse aggregates of ¾ to 1 inch. Some guidelines may advise the contractor to consult with the concrete producers to get their recommendation for the proper concrete mixture for a particular region and climate.

Moisture Control

• The ultimate dependability of a garage slab on ground installation entails proper moisture control. Contractors typically install vapor barriers or vapor retarders to reduce the transmission of moisture from the ground up through the slab. The material used for this purpose comes in direct contact with the sub-base and requires a vapor transmission rate or "permeance" of 0.3 perm. Many contractors use 6-millimeter (mil) polyethylene for vapor retarders. However, the correct vapor barrier should meet the standards of the American Standards for Testing and Materials (ASTM) E 1745 and have a minimum thickness of 10 mils.

Slab Reinforcement

• The Wire Reinforcement Institute and American Concrete Institute recommend the use of bar mats or welded wire reinforcement sheets to add strength to the slab. Usually, this requires the placement of the material one-third of the way from the top of the slab's surface. Some specifications may call for the elimination of reinforcement at construction or contraction joints. This allows freedom for movement and load transfer.

Curing

• One of the most essential specifications for a garage slab entails the curing process, which runs from three to seven days. Curing helps retain the slab's moisture to make the concrete stronger. It also slows down shrinkage until the slab gains strength, which prevents moisture evaporation. Curing involves covering the surface with sand, straw, burlap or canvas--kept damp throughout the curing duration. Some contractors may use a chemical application.