December 18th, 2013

Defects in Concrete Slabs

Home Owners Network

Questions about concrete slab defects are a frequent subject at our “Ask The Experts” service. Let’s discuss some of the issues related to concrete slab defects so we can understand whether and how to deal with them. We’re going to discuss defects in horizontal concrete slabs such as those found in basements, garages, and in homes built on concrete slab foundations. Most of what we’ll discuss also applies to outdoor concrete slabs such as driveways, walkways, and patios.

It’s said that there are two kinds of concrete, concrete that already has defects, and concrete that’s going to have defects. In theory, if the ground under the concrete is properly prepared, and if the concrete is properly mixed, and if it’s properly placed and finished, and if it’s properly cured (dried), then concrete should not have defects. That’s a lot of ifs. In practice, concrete is seldom installed “by the book” so it often has defects.

The most common concrete defect is the crack. Cracks are when the concrete separates. Cracks can be horizontal, vertical, and both horizontal and vertical. Crack horizontal width, vertical height, and on-going activity matter a lot when determining whether and how to deal with the crack. Crack length is usually a lesser concern compared to width, height, and activity.

The first question when dealing with a crack is whether the crack is still active. Most cracks occur and grow within the first year or two after the concrete is laid. After that, they usually stop. Cracks that are inactive are often not a concern. You should consider having an engineer evaluate cracks that are active, especially after the first two years. There are crack measuring and monitoring devices available to help you determine if the crack is active; or you can just use a ruler and take pictures.

Horizontal cracks that are less than about 3/16 inch wide are almost always non-structural. These cracks are usually caused when the concrete shrinks during initial curing. Shrinkage usually occurs because too much water is added to the concrete and because the concrete dries too quickly. Repairs for these cracks are often ineffective because the cracks are not large enough to be filled with enough crack repair product. The crack repair product often dries, shrinks, and comes out of the crack.

Horizontal cracks that are wider than about 3/16 inch are also usually non-structural; however, cracks wider than about ¼ inch and that run through the footings could indicate a structural problem. You may want to have an engineer evaluate these cracks. If this size crack is non-structural and inactive, you may fill it with one of the crack repair products available at home centers.

Vertical cracks cause more concern because they are more likely to indicate structural problems. Vertical cracks are most often caused because the soil under the concrete either settles or rises.

Settlement is the more likely cause and usually occurs because the ground under the concrete is not properly compacted. Concrete needs well-compacted soil underneath to provide support. Without support, the concrete cracks and starts falling. One example of settlement is in an attached garage next to a basement. There can be several feet of fill soil in these garages and this soil is seldom properly compacted. The soil can, over time, settle several inches. Repair of cracks caused by soil settlement often involves injecting material under the concrete to fill the gap between the concrete and the soil. This procedure can be very expensive.

Much of the country has areas with some unstable clay soil. This soil expands when wet and shrinks when dry. If the concrete was poured on dry unstable clay soil, the soil will expand if it becomes wet and push up the concrete slab causing cracks. Water management around the foundation is the best way to prevent problems caused by unstable soil. Gutters and proper grading around the foundation (six inches of fall in the first ten feet) helps prevent soil instability and the resulting concrete cracks. Gutters and proper grading are good practice for all foundations, not just those with unstable clay soil.

Other common concrete defects include crazing, spalling, and dusting. Crazing is when the concrete surface shows many very small cracks in a random pattern. Crazing looks like a dry lake bed. Spalling is when small areas of the finish surface pop off leaving a small depression. Dusting is when the surface produces a fine powder that behaves like dust. Crazing, spalling, and dusting are non-structural cosmetic defects. These defects are caused by improper mixing, placing, or finishing of the concrete. Cosmetic repairs are available for these defects, but these repairs do not treat the root causes of the defects. Sometimes, the repair looks worse than the defect. Most people just learn to live with these defects.

Efflorescence is sometimes mistaken for a concrete defect. Efflorescence is what remains of the minerals in the concrete when water passes through the concrete and evaporates. It appears as a white powder on the surface of concrete slabs and concrete and concrete block foundation walls. Efflorescence indicates a water problem under or behind the concrete. It is not a concrete defect. Repair of efflorescence involves finding and correcting the water source.

Most concrete defects are cosmetic and are not a structural concern. If you are unsure about a defect, an experienced home inspector or contractor may be able to help you evaluate the defect and recommend what, if anything, to do about it.

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