Crack-Free Concrete Slabs

Tips from a commercial concrete contractor you can use to avoid cracks in concrete slabs.

Gabe Martel
Spring 2003

There is a saying among concrete contractors, “Concrete does two things – it gets hard, and it cracks.” But I disagree with that claim. In fact, as a commercial construction superintendent, I have poured 15,000-square-foot slabs containing 260 cubic yards of concrete without a single crack. And I only do what thousands of other commercial contractors do everyday. Residential concrete doesn’t usually need to meet the same high quality requirements as most commercial concrete work, but you can eliminate uncontrolled cracking even in residential construction using the techniques and principles used on big projects.

One word describes the secret to successful concrete work: Consistency. For the best results you must exercise the same level of care at every stage of the process – designing details, preparing the subgrade (the gravel base area under the slab), placing the concrete, and finishing and curing it.

Preparing the Base

If the gravel subgrade settles after you’ve poured a slab, you increase the likelihood of a crack. It may seem obvious, but this is the cause of far too many problems. Several things can contribute to subgrade settlement, but the two prime offenders are soils with different load-bearing capacity across the area of the slab, and insufficient compaction of the gravel.

Always make sure the soil used under a slab is free of topsoil and organic matter. If you have any doubt about the bearing capacity of the soil you’re building on, check with the local building department or other area builders. If you’re still not sure, get the soil tested. The few hundred dollars you’ll pay for soil analysis and a compaction test is nothing compared to going back and fixing a failed foundation.

Make sure that the soil is uniformly and properly compacted. Run a plate-tamper or jumping-jack compactor over the subgrade until it leaves very little impression with each successive pass. Only compact soil in layers no thicker than six to ten inches. If you attempt to compact greater depths, the surface will look good but the soil underneath will remain uncompacted and will eventually settle. Also, check for uniform bearing capacity of the subgrade so the slab will rest evenly on its base.

Optimal moisture content happens in a narrowly defined range, so while you may need to add water to dry soils during compaction, take care not to add too much. If you see evidence of water saturated soil near the area you are compacting you must stop and correct this condition. Your choices are limited at this point: Either wait for the soil to dry out or remove the soild and replace it with dry material.

Subgrade surfaces should be flat. It’s helpful to picture your slab upside down. It should look almost as smooth on the bottom as you’ll want it to look on top. Soil must be graded to the same level across the full width of the slab. Your subgrade should be level to within 1/2 inch. Where the subgrade elevation is inconsistent, then the slab will vary in thickness, which will cause it to cure unevenly and stress the concrete, increasing the likelihood of a crack.

Laying Out Contraction Joints

The shape of the slab can greatly affect its cracking. An inside corner, for example, is a pressure point and, as far as the concrete is concerned, a great place to crack. Try to divide your slab into symmetrical squares using contraction joints (also called control joints) rather than creating irregular shapes with lots of re-entrant corners (places where cracks can start). Contraction joints can be hand-tooled right after the bleed-water leaves the surface, cut with a saw right after finishing, or formed in the wet concrete using one of the proprietary plastic or metal joint materials.

No matter how you make them, all contraction joints work the same way. By creating an intentionally weakened point in the slab, stress from the inevitable shrinkage (as a result of drying or temperature changes) is relieved in a predictable, controlled manner.
A contraction joint should be one-fourth the depth of the slab, or 1 inch deep in a 4-inch slab. Any less, and it may not function as designed; any more, and you will unnecessarily weaken the slab, risking vertical displacement of the concrete if the subgrade should move.

When laying out joints in a slab, keep in mind that squares are better than rectangles. When laying out contraction joints on a typical 4-inch thick residential slab, 8- to 12-foot spacing between joints is reasonable. Under ideal conditions, spacing up to about 15 feet between joints may be possible.

Expansion Joints

If the slab is contained by foundation walls, you must provide expansion joints (also known as isolation joints) around the inside perimeter of the foundation to allow the slab to move independently of the wall. The same thing holds true for structural columns – if they are inside the area of the slab, they should be surrounded with expansion joints.
Residential expansion joints are often made with 1/2-inch-thick fiberboard strips. Remember that the slab must be isolated from the footing, too. Either sand or 15# felt between the slab and footing works well as a bond breaker.

Placing the Steel

Proper placement of steel reinforcement is critical. At the very least, reinforcing mesh, pipes, and cables should be suspended so that they don’t lie on the bottom of the poured slab. Wire mesh should be in the middle of the slab if it is to do its job properly. Pipe or conduit laid directly on the ground will dramatically weaken the slab; it has the same effect as scoring a ceramic tile before snapping it.

Make use of steel or plastic rebar chairs to get the rebar in the right place. Do not use brick, because it will pull moisture out of the surrounding concrete too quickly and create a stress point.

When It’s Time for Concrete

Measure the exact volume of concrete required, then add 10% as a buffer. If you underestimate and have to call for a short load, not only do you waste time, but you may get cracks where the late-arriving concrete meets the earlier mix. Make sure your concrete supplier will have the number of trucks needed to supply your project without having to wait for an hour between loads. Excessive delay can result in a what’s called a “hot load”. If the mix comes out of the chute “hot”, then send it back.

Proper water content is one of the most important elements of good quality concrete; adding water to concrete to make it flow easier has a negative result. A slab mix should not have a “slump” greater than 3 to 4 inches. If smoother flow is required, add superplasticizer, which gives the effect of a 6- to 8-inch slump without weakening the mix.

Placing and Screeding

Concrete is actually a delicate material until it has fully cured. How you treat it during placement is crucial to your success. To avoid segregating the aggregate from the cement paste, never drop concrete more than 6 feet out of a shoot or pump hose, use the proper concrete tools (not steel rakes), and don’t drag or vibrate the concrete into place.
If the soil is particularly dry or you’re working in hot, windy weather, you should wet the subgrade, but not enough to create mud or leave standing water. In cold weather, do not place concrete on frozen ground.

Make sure you have sufficient manpower on site to maintain continuity in placing the concrete. If placing a slab takes too long, the first concrete placed will begin to set up well before the last concrete placed. This makes it difficult to finish the slab in one continuous process.

Finishing

Make sure concrete is screeded and bull-floated as it’s placed, but don’t allow finishing to begin until the bleed-water (water that rises to the surface as you bull-float it during placement) has disappeared. If bleed-water is worked into the concrete while finishing, the surface will be weakened. For the same reason, don’t allow finishers to add water in an attempt to ease finishing.

A well-organized, well-timed, uninterrupted work flow is essential. Hand-trowelling the perimeter along walls and around obstacles as concrete is being placed is a good way to start. Have the power-trowel fueled-up and ready to go before you need it.

Curing

Don’t forget to take care during the curing stage. Initially, the temperature will rise quickly, peak, and then start to drop. This process is stressful on a new slab and can cause failures. Wet-curing keeps the slab from drying out too fast during this process. Cover a fresh slab with a layer of burlap immediately after finishing, and keep it wet with a lawn sprinkler for at least three days (ideally seven days). Or, cover the slab with a layer of plastic. Wet curing will ensure an even and controlled cure. In cold weather, make sure that fresh concrete is protected from freezing.

Gabe Martel is a project manager and construction superintendent in Pembroke, Ont., Canada, with over 35 years of experience.