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Q: We are getting bids for an addition to our 2007 home. I was asking for anything but concrete on the patio as the current patio is awful. Cracks everywhere started 18 months after we moved in. They fill with weeds and we trip on the uneven surface. My contractor assures me that even though the current patio is bad, his will be better, because he will add more expansion joints. (The current patio is 20 feet by 36 feet, and it has just one of these joints.) He also said he would compact the base, add gravel, and wait until spring to compact again, then pour the concrete. Will this work or should we switch to something else, like flagstones?
A: There are numerous reasons concrete cracks. They stem from two properties of concrete: It needs water in the initial mix, and it isn’t very strong when used as a bridge.
Concrete is basically a mixture of sand, gravel and Portland cement, plus water to make the mixture fluid enough to form and to trigger the chemical reaction that hardens the cement. Relatively little water is needed for the chemical reaction, but with only that much water, the mix is stiff and hard to smooth into a level pad. So mixed concrete usually contains more. The more water, the easier it is to form the wet concrete.
But it’s also true that the more water, the more likely it is the concrete will crack. The excess water goes to the surface as the concrete is troweled, creating tiny tunnels that remain as the mix hardens. The tunnels make the concrete more porous, which can lead to shallow surface cracks and later, frost damage. Also, as the concrete cures, the water in it evaporates, reducing the volume of the concrete. The more water, the more the concrete shrinks, resulting in deeper cracks.
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To minimize cracks, concrete suppliers often add ingredients that make concrete fluid without as much water. And builders add what are called expansion joints, although they probably should be called shrinkage joints. The idea is to divide a concrete slab into small enough sections so that each one can shrink independently while staying intact. A large slab has too much weight to slide inward in one piece as it shrinks, so it cracks.
Multiply the slab’s thickness in inches by 2.5 to get the maximum distance in feet that a slab can go without needing an expansion/shrinkage joint. For concrete four inches thick — the minimum recommended for patios — there should be a break every 10 feet. A patio 20 feet wide by 36 feet long would need one joint dividing the width in half and three perpendicular joints dividing the length into four sections, each nine feet long. So eight sections in all. The joints can be created by forms or cut into the hardening concrete. Concrete stiffens within hours — about three hours on a 70-degree day — but it isn’t fully hard for about a month.
It’s reassuring that your contractor says he will wait until spring to install the concrete, because freezing weather that arrives too soon after concrete is poured can also lead to cracks. Ice crystals occupy about 9 percent more space than liquid water does, so if an early freeze hits before the concrete cures and hardens, it can crack severely.
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Beyond all the water-related cracks, there are also cracks that stem from the physical characteristics of concrete. Concrete is strong in terms of being able to hold up heavy loads without crumbling, but it needs reinforcing to act as a bridge. If sections of the base material erode or sink, the concrete can’t bend to accommodate the new shape underneath. Instead, it cracks.
From the pictures you sent, it appears at least some of the cracks are related to poor preparation of the base under the patio. Builders often disturb soil near a house during construction, so unless they take care to backfill and compact correctly, a patio adjoining a house can wind up with an unstable base — and lots of deep cracks.
It’s sometimes possible to pour concrete directly over soil, provided the soil is compacted, free of organic material and not dominated by clay that expands when damp and shrinks when dry. Often, it works better to excavate and add a layer of gravel at least four inches deep. Good contractors know what works and what doesn’t with local soil conditions.
As to whether it makes sense to go with concrete or switch to a different kind of paving, such as flagstones, part of the issue is aesthetics. Concrete, provided it doesn’t crack extensively, is very easy to care for, and the level surface works well with tables and chairs. A flagstone patio can be gorgeous and look more inviting, but if the base is uneven, some of the stones are likely to lift. Weeds are sure to grow in the joints, too, even if you plant thyme or other suitable ground cover or fill the joints with acrylic-modified sand. Don’t switch to flagstones just because of the weed and tripping issues. For a patio the size you have now, concrete probably costs less, too.
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