Replacing Cracked Concrete
All you need to know when the porch, sidewalk, or patio go
Four years ago I had a concrete porch, sidewalk, and patio poured around my new house. The concrete is now a maze of cracks that resembles a broken pane of glass. What can I do before the concrete starts coming up in chunks?
— Justin, Lewisburg, TN
Tom Silva replies: Cracks like this can be caused by several factors, but I suspect it has to do either with a drainage problem beneath the concrete or a lack of expansion and control joints in the slabs. Poor drainage holds water in the soil, which expands when it freezes, causing the concrete to heave and crack.
Expansion joints and control joints can't help with frost heaving, but they are necessary to mitigate other kinds of cracking. Expansion joints, which go all the way through the slab, allow minor movement of the concrete — something you can't prevent. These joints essentially break up a larger slab into smaller, less vulnerable pieces. Control joints, or relief joints, run only an inch deep and provide a channel for the harmless cracks that happen as the concrete cures and shrinks.
On sidewalks, both these joints run across the width of the walkway. Expansion joints shouldn't be more than 24 feet apart. Control joints should be spaced the same distance as the width of the sidewalk. On patio slabs, the joints are often divided into a grid, with the control joints about 8 feet apart and expansion joints no more than 24 feet apart. There should also be expansion joints wherever the patio butts the foundation or forms an inside corner.
Now let's talk about a fix. It's possible for a masonry contractor with a special saw to make relief cuts in an existing slab, but it sounds as if your concrete is well past saving and will probably have to be removed. Still, have a couple of masonry contractors come out to make a firsthand diagnosis.
If the old concrete does have to go, make sure the new work is done right. Your contractor should remove any soil that contains organic matter, compact the subsoil with a power compactor, and add at least a 6-inch-deep layer of fast-draining gravel — also well-tamped — to serve as the foundation for the concrete. The concrete should be poured at least 4 inches thick over a 10-gauge welded wire mesh that's positioned in the lower third of the slab. The mesh helps prevent cracking.
Also check that the concrete your contractor uses is strong enough. According to the International Residential Code, which guides most local codes in this country, patio concrete in a moderate climate should have a strength in compression of at least 3,000 psi after 30 days. If your slab will be exposed to severe weathering, you need concrete that's at least 3,500 psi. Most of Tennessee is subject to severe weathering, but check with your local building department to see what they require.