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Concrete Cracks

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.


17 Responses to Concrete Cracks

  1. I had an 8" roof poured on a 20'x20' shelter with an 8" center wall. The contractor used metal decking material with 1/2" rebar 2" from from decking at 12" centers. All concrete is 6 bag with fiber. My question is, if the decking bows when the forms are removed, can I put screws in it without weakening the concrete ?

    Thank You.

  2. They poured my house foundation 6 wks ago. Today i notice a crack the whole lenght of the living rm. The homebuillder says its a stress crack and filled it in says I have no worries. In middle of crack it has raise a little they say this is caused by temperture being so hot. My question is do I have anything to worry about.

  3. Is there any thing I can do to keep a cement crack from getting bigger or deeper. The concrete is only about 3 months old.

  4. we have a crack along sidethe front of house on the floor it goes along side of the basement floor it doesnt go all the way down but it is long what can we do to fix this cant afford a professional thanks

    • Sharilyn, your question requires lots of typing, plus I have some questions for you so I can give you the correct answer(s). I only do pithy answers here in the comment section. If you want to protect the investment you have in your house and not waste time or money *hoping* you make the right decision, you should talk to me on the phone for just 15 minutes. It'll be the best investment you've ever made in your home!

  5. Hello,
    We had our driveway poured on a Wed. By Friday we had a crack all the way across one section of the driveway. I do realize that concrete cracks. The weird part about our crack is that it's about 3" away from a control joint? I believe the water line is directly over this area of the driveway also. Could the water line have been put in incorrectly?
    Thank you !

  6. I just had a slab poured and shop build on it. The slab is 30x40 and has a very large crack 30 ft across. I think my control joints are maybe 1/4 inch if that and total of six. Three running 40 ft side and three running the 30 ft crossing over each other. They look a little jagged also, past cement the cuts where much deeper and smooth. What should I do, I don't want this to get worse. My ground seems to move a lot so I am really concerned.

  7. One of the control joints in our new construction (6 months old) basement has increased from a hairline crack to approximately 1/8 inch wide (I can lose a quarter in the gap). The same thing has happened around the sump pump. Should I be concerned?

  8. Concrete contractor poured a small rectangular concrete pad last week of Oct 2013. I noticed a crack across the first control seem of slab in late April 2014. It can only be seen when wet. Contractor used his saw to saw the first line down to the dirt. It will stop that crack but temps will probably push the two parts up and down separately. What is a proper fix?

    • Please go up the page and use the search engine tool of mine. Type this into it:

      concrete control joints

      READ every column I have about the topic and your answer and MORE is there!!! Woo Hoo!

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