If I had a dollar for every phone call and email I have received about cracks that appear in new concrete sidewalks, driveways, patios, basement floors, slabs on grade, and foundation walls, I would throw a party for all of us that could last for several days. I am constantly shocked to discover that some builders, remodelers and even concrete masons do not tell homeowners the truth as to why cracks occur.
I have yet to figure out whether they don't really know the answer, are unwilling to initiate a repair, or are just making excuses. I may never discover the reason, but have decided to try to help by telling all I have learned, so that you know exactly what to expect from the new concrete used at your home.
Years ago, when I was still building, I discovered that concrete shrinks as it dries and cures. On average, a concrete slab shrinks 1/16th inch for every ten linear feet. This may not seem much to you, but what this shrinkage does is produce significant internal stress within the slab. This stress or force is considered a tension-type force as the concrete is trying to pull itself apart much as you pull on two ends of a piece of newspaper. Pull hard enough and the paper tears.
Because I knew there was a chance for concrete to crack, I actually had a special section of my contract that told people I guaranteed their concrete would crack. I realize this sounds nuts, but it afforded me with enormous protection. I further stated in my contract that I would do everything in my power to minimize the cracks and encourage the concrete to crack at predetermined locations. But even with all of this, the concrete could develop a random crack all on its own.
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Surely, you have had to tear a piece of paper in half before and not had a scissors handy. If so, you probably creased the paper with your fingernail several times and then tore the paper neatly along this crease. The creasing action creates a pre-weakened zone in the paper by bending and breaking some of the fibers in the sheet of paper.
Concrete masons can do the same thing by creating a line in concrete slabs as they are finished or immediately after they are poured. A saw cut or tooled line that creates a groove in a slab actually reduces the thickness of the slab at that location and makes it easier for the slab to crack. In the trade these lines are called control joints as we are trying to control where the crack will occur.
These control joints need to be a minimum depth to be effective. Note I say minimum; there is nothing stopping you or your contractor from exceeding this minimum so you increase your chances of success. The Portland Cement Association (www.cement.org) and the American Concrete Institute (www.concrete.org) seem to agree that the minimum depth of a control joint should be 1/4th the thickness of a slab. This means the grooved lines you see in a typical sidewalk should be one inch deep, as many sidewalks are poured four inches thick. Measure your grooves and guess what? I'll wager they are only 5/8 inch deep or perhaps 3/4 inch if you are lucky. A concrete saw can be placed in these grooves to increase the depth of the groove.
Placing reinforcing steel, wire mesh and even synthetic fibers in with the concrete will do wonders to help hold the concrete together in the event it does crack. I am a huge fan of one-half inch steel bars placed at two-foot-on-center intervals in slabs poured on grade. This steel works well if it is in the center of the slab or just slightly below the center point. The steel has a far greater tensile strength than the concrete and holds the artificial rock together much the same way as the strings you find in common brown packaging tape.
Be sure you discuss what measures your builder intends to take to ensure your concrete cracks where it is supposed to crack. Keep in mind that your builder can't give you an absolute guarantee that the concrete will do what he or she says. If this person makes this lofty promise, then you might want to consider talking with another builder.