Being Wise About Cracks
You just can't ignore cracks in a foundation or concrete slab. They are not only unsightly, but can often lead to more extensive damage where water is involved. Filling in these cracks and stopping them from spreading is essential to preventing serious structural issues in your foundation.
Foundations or structural slabs with an excessive number of smaller cracks or with cracks that are larger than 1/4 inch in width should be examined by a structural or civil engineer to assess the extent of the damage and to determine the source. If the damage is severe, a geotechnical or soils engineer should be consulted to suggest solutions to correct the problem once and for all.
Even in the best of conditions, concrete moves a fraction of an inch here or there, not always resulting in a crack. And, believe it or not, concrete expands and contracts in conjunction with the temperature. Concrete expands on hot days and shrinks when the weather is cold.
Therefore, when patching cracks in concrete, we suggest that you use a product that gives a little here or there. The more elastic the product, the less likely a crack will reappear. One of our favorite patching products for cracks that are wider than 1/8 inch is a vinyl concrete patch, which usually comes packaged dry in a box.
We like to mix the dry patch powder with latex instead of water to give the product some added elasticity and adhesion. Latex additives, like patching compounds, can be found at most hardware stores or home centers.
Just as with painting, the key to successful crack repair lies in the preparation. To begin, the crack should be clean and free of any loose chips. For cracks wider than 1/8 inch, use a small sledgehammer and a cold chisel to chip away loose material.
Always mist a crack with water before installing a patching product. The water prevents the moisture in the patch material from being drawn out by dry concrete, which causes the patch material to crack and not form a solid bond.
Mix the concrete patch to the consistency of a thin paste and trowel it into the crack with a trowel. Scrape away any excess paste to create a smooth and uniform finish.
Mix only a small amount of paste at a time because most products tend to dry pretty quickly. Tools and buckets should also be cleaned up immediately to avoid drying.
If you can't find vinyl concrete patch material, mix 1 part Portland cement to 3 parts sand, then add enough concrete bonding agent to make a stiff mixture about the consistency of mashed potatoes. Take a small part of the mixture and add more bonding agent so that it becomes a soupy consistency. Brush the soupy mixture into the crack using an old paintbrush and then pack the stiff mixture into the crack with a metal trowel. Use the trowel to create a smooth finish.
If the existing concrete surrounding the patch is rough, you can match the finish by sweeping it with a broom.
Large gaps of 1/4 inch or greater in concrete (such as those between a concrete slab and foundation wall) require a slightly different patching method. Here, a latex caulk that contains silicone or a polyurethane caulk should be used. These products are pumped into the joint with a caulking gun and remain pliable to allow for ongoing expansion and contraction. Because most of these caulking products are self-leveling, no troweling is required.
Be sure to lightly spray the patch with water twice a day for a week to help it cure and prevent cracking.
The cost of a small box of concrete patch and a tube of concrete caulk along with a small investment of your time will do wonders to improve the overall appearance and longevity of your home's foundation.