Why Concrete Cracks

Concrete, used extensively in the construction of foundations, basements, slabs, bridges and other products, requires a high degree of structural integrity. Unfortunately, poured concrete is prone to cracking in many instances. Very few basements will survive for one year without a few hairline cracks. Due to the movement of the soil, some cracks are inevitable but you can minimize cracking by using some professional concrete techniques.

History

A precursor to today's concrete is found in archaeological digs in the Middle East and in Europe where our early forebears combined mud and sand with water to make a paste to cover their huts. When the mixture dried, it formed a protective barrier from the sun and blowing wind. In the mid 1800's, Portland cement was developed from crushed limestone and as a result, it provided a much stronger finished concrete product. Today's mixtures use aggregate and air-entrainment materials to strengthen concrete slabs and walls.

Geography

The soil where you live is a determining factor in the level of cracking in your finished concrete project. Clay soils tend to shift and slip, cracking even the strongest foundation while soils high in sand content, are more stable and offer a greater compaction ratio. When pouring concrete in an area known for soil movement, use sand beneath your concrete footings and flatwork.

Features

You can reduce the likelihood of cracks by reinforcing the interior of your concrete. Accomplish this by placing rebar and steel mesh in the forms before pouring the wet concrete. After the concrete cures and you remove the forms, the steel reinforcement, deeply embedded in the concrete, provides increased structural integrity. Before pouring basement walls, contact your local building code enforcer to determine the amount of steel reinforcement required in your area.

Effects

Freezing temperatures are second only to soil movement as a leading cause of concrete cracks. The denser the concrete, the more likely it is to crack when the temperature drops below freezing. Contractors reduce the likelihood of cracks by using air-entrainment additives, injected into the wet concrete at the concrete plant. Known as "light concrete," these additives encourage the formation of tiny bubbles in the semi-fluid mixture, reducing the weight of the concrete and its density. In addition, air-entrained concrete resists surface scaling when cured.

Warning

The less moisture the wet concrete has, the better it will resist cracking after it cures. However, pouring dry concrete or air-entrained concrete is more labor intensive because the mixture may begin to cure within minutes of pouring. If you want to pour this type of concrete, have extra workers ready and all your concrete tools within easy reach. Appoint one skilled concrete specialist to oversee the project. Once the wet concrete is in the forms, you have a very short time in which to work it.

Resources

• Calculate your concrete needs online.