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Material World: Concrete

Concrete is versatile material found throughout the home: slab foundations, footers, basement walls, concrete masonry units, basement floors, crawlspace walls, driveways and other places. Despite its popularity, it is one building material that offers a lifetime guarantee: it’s guaranteed to crack sometime in its lifetime. Even the best laid concrete will crack, but there are ways to limit and control the cracking.

Some people refer to concrete as “cement”, but cement is actually one of the ingredients in concrete. The others are: sand, water, aggregate (gravel) and admixtures (such as a quick set admixture for faster setting of concrete). The ultimate strength of the concrete relies on precise mixtures of the ingredients. Adding more water to a mixture of concrete after it starts to set can greatly reduce its strength and later lead to excess cracking. One way to limit excess cracking is to make sure that a quality mixture is used and that it isn’t tampered with.
 

Another way to strengthen concrete is to add metal reinforcement. Wire mesh is often used in concrete driveways and slabs. Footers often have reinforcing steel bars (rebar) put in place to help add strength to the concrete. Additionally, metal reinforcement will keep the concrete together if (and when) cracking does occur.

Since cracks are inevitable, installers will use control joints and isolation joints to limit where those cracks occur. An example of an isolation joint is often found on concrete driveways: you’ll see a joint between two slabs of concrete that is filled with wood or a fiber board to allow the slabs to have some give. Control joints are most often cut into concrete after it has a chance to cure. A big saw blade mounted on a machine will cut narrow, straight lines. If you look down in control joints on older concrete jobs you may see a hairline fracture on the bottom – this controlled where the crack would occur.

One more way to limit cracking is to provide a good base for the concrete and to provide drainage for the base. A common base is gravel that has been tamped down. Proper drainage will ensure that water does not wash out that base. In colder climates, if the drainage isn’t effective standing water can freeze and expand causing the concrete to crack. Additionally, concrete footers need a good base to reduce issues of settling.

Installing concrete can be a messy business, but if done correctly it will provide a solid foundation for most of the homes today – cracks and all.
 

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