How to Build Cement Slab

May 24, 2011


How to Build Cement Slab

Homeowners can save thousands of dollars by learning how to build a cement slab and doing the work themselves rather than hiring a concrete contractor. Even though we think of the cement slab as the foundation of the garage or shed it actually needs a proper base prepared before pouring the concrete to prevent foundation failure.

If the cement slab support, or base, is not properly prepared and level there will be stress where the concrete will try to bend. Concrete is very strong with compression, but not so much with tension. Tension stress is when the cement is pulled apart such as when concrete is bent (one side of the bend is being compressed while the other side is pulled apart) and this can occur if the base is not level or properly compacted.

The most important aspect of a proper slab base is that it is uniform, or level, to provide consistent support throughout the entire area. This is even more important than the strength of the base so special attention should be used to accurately level the base.

To clarify the terminology the American Concrete Institute’s definitions follow:

Subgrade – native soil (or improved soil), which is usually compacted.

Subbase – layer of gravel on top of the subgrade.

Base – layer of material on the subbase (directly under the cement slab)

For garage and shed cement slabs there is typically only one size gravel for a base. This depends on the soil conditions and any engineered design requirements. In cold climates a 24′ x 24′ garage slab on solid ground that is quite level may only require leveling with a Bobcat or mini-excavator and then adding one dump truck load of 3/4” gravel.

Compaction is achieved with a jumping jack tamper or plate tamper. Several passes over the entire base is required to reach maximum compaction to reduce the chances of cement slab cracks cause by future settling. This equipment can be rented from any local tool rental location.

In warm climates it may be acceptable to place the cement slab on top of the natural ground if it is stump-free and compactable. One complication of this method is efficient drainage. Gravel is porous and therefore excellent for drainage. Special attention to grading away from the cement slab is important to prevent water from entering the garage or shed and also to prevent slab floating. Floating occurs when the water level under the slab pushed up on the slab due to poor drainage or a high water table. Poor drainage will also complicate construction with a constantly web jobsite, base compaction reduction, and base leveling.

ACT 302 recommends subbase material be compactible, easy to trim, granular fill that will remain stable and support construction traffic. 3/4” gravel is often used for subbase and it is cost effective and usually readily available. The minimum thickness is 4” and is often much thicker.

Build the concrete forms with lumber such as 2×2, 2×6, 2×8, or 2×10 and double up or stack as necessary for cement slabs with thickened edges. Place supports every four to six feet on the outside of the forms to prevent the concrete from blowing out the planks. This can be achieved by using 2×4 stakes pounded into the ground flush with the form, backfilled with gravel or earth, or supported by nearby structures such as trees. Use a level or transit to level the forms at the desired elevation.

Install foundation foam if appropriate. This is typically 2.5” styrofoam in 2′ x 8′ size.

Install vapor barrier over the foundation foam.

Install rebar as required. This is often installed with two rows within the thickened area of the slab and held in place by tying to shorter rebar that are used as stakes.

Install #10 wire mesh if appropriate. The wire mesh comes in 6′ x 6′ sections and should overlap by at least 6”. The wire mesh can sit on special “chairs” that are available from the local build materials supplier for a nominal fee.

When the concrete truck arrive pour the back of the slab first and work forward starting at the perimeter. Pour a wheelbarrow’s worth of concrete every eight feet from the edges of the slab. Level this patch of concrete using the transit or other level. This is now the reference and an 8′ length of 2×4 can be used to level the concrete between the patch and the edges of the slab. It is easier to attach two handles (strapping or other small wood) on the edge of the 2×4 in the center about two feet apart. Use the handles to pull the 2×4 to you while scapping off excess concrete to get the level correct. Repeat as necessary to complete the rest of the cement slab.

Use a bull float or a fresno trowel to level and smooth the concrete.

Use a concrete trowel to level and smooth the edges of the cement slab.

Use a concrete machine trowel to machine polish the slab once the concrete is hard enough to support the weight.

Allow the concrete a day to cure and then strip the form and backfill as necessary.

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