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Concrete Cracks

Concrete shrinkage cracks look just like this. The darker gray concrete slab touched the lighter gray foundation the day it was poured. Over a period of months, even up to a year or more, the concrete slab shrinks in size and creates a gap. Copyright 2018 Tim Carter

"On average, a concrete slab shrinks 1/16th inch for every ten linear feet."

Concrete Shrinkage Cracks Checklist

  • Concrete shrinks over time as it hardens and subsequently cures
  • Shrinkage creates tension forces that can rip the concrete apart
  • Control joints in the concrete help to prevent ugly random cracking
  • Shrinkage is about 1/16th inch for every ten linear feet

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Concrete gets smaller as the water in the concrete ever so slowly evaporates from the artificial stone.

I receive calls and emails from homeowners all the time about cracks that appear in new concrete sidewalks, driveways, patios, basement floors, slabs on grade, and foundation walls. Most homeowners are quite upset by the cracks.

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Do Contractors Know Concrete Shrinks?

I don't think that most contractors know the true cause of cracks in concrete. They'll tell you that cracks are normal and shrug their shoulders.

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.

What Happens When Concrete Shrinks?

As concrete shrinks, stress is created 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 that sentence in the contract 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.

Can You Control Where Concrete Cracks?

You can encourage concrete to crack where you want, but I wouldn't always guarantee this. The best way is to create a crease or straight line in the fresh concrete. Large slabs, driveways, sidewalks, etc. will require many of these lines. Contractors call them control joints because you try to control where the concrete will crack.

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.

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How are Concrete Control Joints Created?

Concrete masons create control joints 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.

How Deep Should Control Joints Be?

The Portland Cement Association and the American Concrete Institute 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're lucky. A concrete saw can be placed in these grooves to increase the depth of the groove.

Will Steel or Mesh Help Hold Concrete Together?

Placing reinforcing steel, wire mesh, and even synthetic fiberglass fibers in with the concrete will help hold the concrete together in the event it does crack. I'm a huge fan of one-half inch steel bars placed at two-foot-on-center intervals in slabs poured on grade.

rebar before pouring concrete

Reinforcing steel is the best thing you can include when pouring concrete. When concrete cracks, the steel holds it together. (C) Copyright 2018 Tim Carter

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.

How Do You Hold the Steel Bars Up During the Pour?

For years I've placed small pieces of solid brick under rebar to hold them up so they are suspended in the air and concrete can flow under and around the steel bars. If you're pouring a 4-inch-thick slab, you want about 1.5 inches of concrete under the bars. If the brick is thicker than 1.5 inches, then you need to dig a small recess for the brick to rest in so it puts the steel at the correct height.

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.

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Concrete Shrinkage Cracks Are Ugly But Normal
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Concrete Shrinkage Cracks Are Ugly But Normal
Concrete shrinkage cracks are normal. Concrete shrinks as it hardens and cures. It can take many months for the shrinkage to stop.
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40 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!

  9. We had concrete pool deck and patio poured approx 1 1/2 months ago. 75-80% of joints have crack and there are 4 cracks across squares away from joints that lead to edge of pool. Contractor says this is normal and there is nothing we can do about it. Your thoughts?

    • Kim,

      I think you should go read my column again. The answer is right there in the column if it's NORMAL for concrete to crack. You're asking the wrong question in my opinion.

      You should be asking did the contractor do something wrong so that the cracks didn't happen in the CONTROL JOINTS?

      The answer is probably. Go read ALL of my past columns about Concrete Cracks and the depth of Control Joints.

  10. I had a new home built last year with a concrete porch 30' wide and 7' length about 4" thick. It crack all the way through from to back 3 weeks after being poured. Still awaiting for the builder to repair it. The crack has separated more about 1/8 of inch. He say the concrete hasn't failed. I say it wasn't done correctly due to they only hand tamp the fill. Normal concrete no rebar and one solid pour. The builder said stress crack at first and now he says shrinkage. He wants to fix it with an epoxy and overlay I want to porch ripped out and redone correctly. The porch also drains towards the house and other crack have have developed.

  11. hello. new home construction and builder recently poured driveway... less than a week has crack in it.. builder says it stress crack and normal.. also driveway poured uneven having areas where it collects water and doesn't run off.. its was horribly broomed since some surface smooth and other rigid.. this normal??

  12. Hello, I'm performing a new construction home and have several questions that I believe the contractor is doing wrong but he is telling me otherwise on some cases. Would it be possible if I can bother you for a lil bit and pick your brain if you wouldn't mind that is. Thank you I greatly appreciate your time.

  13. New construction
    Concrete poured
    Started out as a thin crack but now
    It goes all the way through to the dirt underneath the concrete
    This is the foundation of our house
    It's started out small and now is getting bigger I say at least a 16th of an inch
    We are in Texas new home new state
    They are trying to blow it off
    But my question is if it's foundation and the studs are up and the concrete is cracking almost straight across the middle of the house am I in trouble
    Their current contractor was fired our plumbing for the refrigerator is in the wrong place also am I in trouble ?
    Where did the good builders go to I'm so frustrated
    K Maxwell

  14. If a contractor did not use an isolation joint or or expansion joint against my building before pouring cement right up against it, can I have a major problem or is this just not an IDEAL situation? thank you Tim!

  15. I'm a business owner and I lease a building that was built in the 1950s mostly out of cinder block.
    We've been experiencing flooding during our now rainy season. Turns out I have large cracks along the main back wall and water is coming through the cracks. It has done substantial damage to three rooms. Insurance companies have already been called but offer very little help. My building owner is 93 yrs old and has left it up to me to find someone to "fix" the problem. I have no idea who I should be calling.I'm assuming a contractor? I apologize if I sound a but dull ... just want to know the right steps to take.

    • No problem Cindy. This situation you're in is complex and I've got questions. I've offered for years a special concierge phone call service to help people like you to make the correct decisions. CLICK HERE and I can be on the phone with you in just hours.

  16. If you use inch and half fibers in the pour of a 25x40 slab should you saw cut or not? Kind of makes sense both ways but not sure.

  17. I'm creating a 22" x50" x4" cement foundation for a 200+ lb air conditioning unit. Do I need to add rebar to prevent cracking from the weight and vibration of the ac unit?

  18. We had a new home built and the builder did not put any relief lines (controlled crack lines) in my driveway or sidewalk. I recently asked him about this and he said that they are not required and not a big deal since all concrete cracks. Is he telling me the truth or should I push for him to put relief lines even though I'm guessing this would mean it would all have to be jack hammered and reported? Thanks!

  19. The guy that did my garage pad says this ;

    I have looked at pics and that is a normal thing. Concert is going to crack. We always hope it will be in the joints we cut but sometimes it does not. Thanks for reaching out to me and if I can answer any more questions please fell free to call me


  20. Hi mate,
    My driveway got poured 16 days ago and there are multiple visible long cracks right throughout, but the most concerning one is bigger than a hairline, goes right over the side edge of the driveway and right down to the earth! I don't know jack about driveways but I know that can't be good so soon!

  21. Tim, I have a raised concrete patio poured over metal sheets. It was acid stained when poured. Now I have spider vein cracks over a big parts of it. No one has any ideas as to what to do. I have a stone border around the outside edge the same level as the pad. Any suggestions on this problem. Thanks Steve

  22. I have a 20x24 detached garage that has 20x24 long hairline cracks in both directions! I want to cover them up and make the garage look nice. What products specifically do you suggest to fill the cracks (if necessary) and then cover the floor with a product that might prevent future cracks from appearing?

  23. We are looking at a 1983 slab home duplex to purchase. The entryway has a crack running from the wall that is raised. In walking through the property we can feel other cracks in two other rooms. The duplex is 696sf so this seems extreme to us. Do we walk away and look for a different property or is there any repairing for this situation? We like the local, neighborhood and the overall feel of the duplex otherwise. Thank you. Karen

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