Home Improvement: How to Pour a Concrete Slab

Patios, driveways, and bases for various outdoor items require a concrete slab to be poured. Projects like these make good do-it-yourself jobs. Pouring and finishing a concrete slab is a simple task if you do not need some type of special finish that requires expertise. The idea is simple, but the labor can be intense because concrete is extremely heavy.

Stake out the site.

Determine the size and location for the slab. Use wooden or metal stakes to mark the corners of the slab. Tie a string to the first stake and use the string to make a perimeter around the site. This will guide you when you start to prepare the site for the slab.

Decide how thick the slab should be.

If the slab does not need to bear much weight, it can be thinner than a slab that will be used to support a car. Usually a patio or sidewalk will be 4 or 6 inches thick. A driveway may need to be 8 or 10 inches thick. Pouring the slab too thin for its use can cause it to decay rapidly. Extra thick slabs are not always more durable and may cost you more than you need to spend for concrete.

Dig away enough soil to allow the slab to rise above the ground to the desired height.

Think about how you want your finished slab to look. Will it be flush with the ground or stick up somewhat? It is a simple matter to decide. This is almost always a matter of personal preference unless you need to prevent excess water from flowing across it. Remove soil until the depth of the depression where the slab will rest is about 2 inches deeper than the amount of the slab that will be below ground level.

Spread gravel or sand to build a base for the slab.

Rake the gravel until it is level and at a uniform depth of about 2 inches. This will let water beneath the slab drain away without eroding the soil. Without a good base, the concrete will settle rapidly into the ground during wet seasons instead of staying where you intend it to be.

Use plastic sheathing for a moisture barrier if needed.

For slabs that will be a foundation for a shed or other building, a moisture barrier is a good idea. Spread the plastic over the entire surface of the gravel and let the edges protrude. These will bump your forms and eventually be buried when the soil is replaced around the edge of your slab.

Put some type of metal over the gravel to give the concrete strength.

For a small slab, some type of grid work like metal fencing will work well. If you do not have this, you can use rebar. These metal rods can be bought at most home centers. Use wire to lash the rods together in a loose framework. When you pour the concrete, pull up on the grid or fencing to get it an inch or so off of the bottom of the slab. This will give good reinforcement to the concrete slab.

Build forms to hold the concrete in place while it sets up.

Follow the edge of the area that you dug out for the slab as you build the forms. Use wood for the border and drive stakes in every foot or so to hold it in place. The weight of the concrete can cause your forms to bow or give if they are not reinforced with stakes. Set the forms down to the top of the gravel unless the sides of the opening are precise enough to function as forms. Use extra bracing at the corners.

For large slabs, you may need to establish crack joints.

This is especially true if the slab will be a driveway or sidewalk. These joints can be made with a finishing tool by creating a slot across the width of the concrete about one inch deep 4 to 6 feet apart.

Another way to create crack joints is to put permanent spacers at each joint.

This essentially means that you are making several smaller slabs out of one big one. If you do this, drill holes through the board or object that you use for this purpose and put some of the rebar through them so that each smaller slab is connected to the one next to it. This will help prevent one of the slabs from rising up and creating a hazard for people walking along the slab.

Strike off the slab.

You can use a professional tool for this or find a sturdy straight 2×4 that is long enough to reach across the width of the slab. For small slabs, you can do this by yourself. If the slab is more than three feet across, it is a good idea to have a person on each end of the board or tool.

Use the 2×4 with the 4 inch side being vertical.

Hold the board or tool tightly against the forms on each side of the slab. Use a sawing motion and work your way from one end of the slab to the other. This will remove any excess concrete and prepare the surface for finishing. If there is still excess concrete rising above the forms, you may have to do this again.

Use a float to make the finish.

For small slabs, a flat hand trowel can be used as a float. Hold the leading edge of the trowel up so that the last 2/3 or so of the trowel drags over the concrete. This will force any rocks that may be protruding to go down and allow more of the liquid to rise to the surface. Do not apply too much pressure because you want to create a smooth level surface.

For large surfaces, use a bull float.

This is a larger version of the trowel except it is made so that you do not have to worry about raising the edge up as you work. These come with various length handles so that you can reach the entire surface of a large slab.

Edge the slab.

Use an edging tool to complete the finishing around the perimeter of the slab. The edging tool will make a more rounded corner. If the edging tool leaves a line, use a hand trowel to repeat the float process around the perimeter to clean this up.

For sidewalks and other surfaces that will be walked on a lot, texture the surface.

Use a push broom for this process. Standing on one edge of the slab, reach across the slab with the broom and let the bristles rest on the surface. Apply a slight downward pressure and drag the broom back toward you until it reaches the edge where you are standing. Carefully line up the broom with the edge lines that you just created and repeat the process. Do this over and over until the entire surface has the desired texture. Applying more downward pressure will cause a more pronounced texture.

Let the concrete harden.

If rain is in the forecast, you may want to use plastic and build a tent over the slab until it hardens. It will need about 24 to 48 hours to cure enough for light use.

You can remove the forms at this point.

Give it an extra day or two before driving on or over it with a car unless you had an additive like calcium chloride put into the concrete to cause it to harden faster. The problem is that the additives can reduce the strength of the concrete because they can react with the iron or steel reinforcing rods.