Frost heaves in concrete
Last year, we had frost-heave problems with several patios and driveways that we built. When the temperature dropped to near 0°F for several days straight, some of the patios rose up more than 2 in. When the ground thawed, the patios sank to below their original level. We tried removing the soft soil next to the foundation and filling the void with crushed stone but still experienced frost heaves. Is there a technique for putting in concrete patios and driveways that minimizes the possibility of frost heaves? Does doweling into an adjacent foundation help?
Jim Goodrum, Asheville, NC
Rocky Geans, a concrete contractor in Mishawaka, Indiana, replies: There are two ways of putting in exterior concrete slabs that minimize frost heaves. The first is to build a structural slab that spans the patio area and that sits on top of its own foundation or pilings that extend below the frost line. However, the prohibitive cost of this option makes it a poor choice for residential construction. A better option is making sure that you use the right soils below the slab, that the soils are properly prepared and that there is adequate drainage. This second, more-reasonable option works to prevent moisture from collecting beneath the patio or driveway that, when frozen, can cause frost heaves.
If the virgin or native soils in your area are sand and gravel, and if there is enough slope away from the slab and the house to provide ample drainage, then compacting these soils is all that is required before you proceed with your slab. However, it sounds as if the soils in your area may be expansive soils, which are fine, compact soils, such as clay. These soils not only hold moisture but also draw moisture up to the underside of the slab. If you do have expansive soils where you live, you need to install a capillary break.
A capillary break is a layer of sand and aggregate that is installed between the expansive soils and the slab. Voids in the sand and aggregate prevent capillary action from drawing moisture up underneath the slab. The capillary break should be at least 6 in. thick and should provide drainage to a lower area away from the patio or driveway, which may require extending the sand and aggregate horizontally to the point where the lawn or the surrounding land naturally slopes down. This layer of sand and aggregate can be covered with topsoil and grass where it extends beyond the concrete slab. If any type of natural drainage is impossible, then installation of a water-collection system might be necessary.
Dowels should be installed in an adjacent foundation just below the concrete slab. Placing the dowels here allows the patio to move up during any frost heave but prevents the slab from settling any lower than dowels. For this application, 1/2-in. rebar works well, and it should be spaced about 2 ft. apart.