Why Concrete Cracks and Heaves

Friday, June 21, 2019

Concrete Joints

So you’ve had a beautiful concrete porch or driveway poured. It’s so pristine you barely want to walk across it. Then one day, you see a crack going right through the center! Why did this happen!

There are many reasons why concrete heaves and cracks with the number one being contraction and expansion, or an abundance or loss of water, in the concrete itself or in the ground.

In new concrete when it hardens, or cures, shrinkage always occurs. Because concrete is not an elastic material, cracks are inevitable.
To solve this type of cracking, a joint will be cut into the concrete in an effort to control where the cracks will occur. Typically, the crack will run down the line of the joint and not disturb the look of the concrete. These are called contraction joints, because concrete tends to contract when it is curing.

The type of soil the concrete is poured on may cause cracking as well. As the surrounding soil next to and below the concrete absorbs moisture and then dries out, it will cause the soil to contract and expand. Depending on how quickly this occurs, cracking will almost always occur.

Assessing the soil and putting in place the proper drainage to move any excess water away from the concrete may help to reduce this type of cracking.

Heaving may be caused in much of the same way as cracking but mostly it is due to an abundance of water in the soil. This could be from rainwater, broken sewer pipes or surface water not draining away from the concrete structure. In our area, frost in the ground is another main reason of heaving. As the ground freezes it will expand and push against the concrete, resulting in movement or cracking and heaving. Assuming the ground, and therefore the concrete, has not moved horizontally, the heaving should subside and the concrete lay back down. But nonetheless, there will still be an unavoidable crack.

Tree or shrub roots are the other main reason for concrete to move, crack and heave. The roots will push up on the concrete causing that unsightly heave and crack. Short of removing the root and possibly the tree or shrub, there is little that can be done to prevent this.
Although cracks and heaving may be unsightly, the integrity of the concrete slab will always remain in effect. The only time for concern will be when the crack is larger than an eighth of an inch.

For more information on concrete cracks and heaving contact the American Concrete Institute.