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Our home was new construction and the basement floor was poured around 1.5 years ago. The builder installed control joints in several locations in the basement slab. Several of these joints have widened. The widest crack is now around 1/8th inch wide with certain sections as wide as 3/16th inch wide. When i shine a flashlight down this wide section it looks as if i see gravel below although i'm not sure if that is just rough concrete since the floors are supposed to be 4 inches thick.

Is this widening normal? Should i seal these cracks to prevent moisture and soil gasses from getting into the house? control crack in floorpic2pic3

  • I would say almost for sure yes that this is a problem. Hairline cracks are not uncommon, but that is less of a crack and more a separation. I would reference THIS question and response. – Derek D Jun 26 '18 at 0:10
  • Are the cracks all straight with the exception of where it is close to the wall or possibly over the footing? The line looks two straight to be a shrinkage crack to me, being an escivated area the soil should not have required compaction but any rock brought in should have been packed. I would want to wait to seal things until the movement stops. – Ed Beal Jun 26 '18 at 0:26
  • The house is still under warranty. Should I be asking the builder to fix something? If so, what ? Isn’t it a given that control joints will expand as the concrete dries ? – bradforj287 Jun 26 '18 at 1:25
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+100

All NEW buildings move and settle. However, we design them for “uniform” settlement. That is to say, we want the entire building to move together at a specific rate. We don’t want DIFFERENTIAL settlement.

Differential settlement can occur from one portion of a building moving up, down or sideways differently than another portion.

It’s always a result of bad planning. I live where the ground is mush, but we plan for it and provide larger footings, piers or piling and thus have no problems. However, when we build over a “soft spot” that we didn’t account for, one portion of the building will settle more than another and we have problems. When this happens, slabs or footings will always “step” apart.

Your pictures show the slab “level”, so you do not appear to have any of these conditions.

However, EXCESSIVE movement caused by excessive amount of water in the concrete mix is more common and appears to be your problem.

There is an exact amount of water to be used (mixed) with Portland Cement to give it it’s ideal working consistency and strength. We call this it’s water/cement ratio.

However, it’s tempting for workmen to add additional water to the recommended mix design so that it is easier to “work” the concrete before it “sets”.

However, when this is done, then more water causes greater shrinkage when the concrete dries (cures). Your concrete must have been very “wet” because there is an extremely large “shrinkage” crack now. In fact, now the tile has cracked where the slab has shrunk and cracked.

When we get involved in fixing these types of cracks we monitor the crack to make sure it has stopped moving before we fix the problem.

I’d recommend that you tape a plastic ruler to one side of the crack, install a box over the ruler so it can’t be moved, and then create a photo journal taking pictures each month for 1 year and writing down observations: like width of crack, outside temperature, room temperature and humidity. Usually after 1 year you’ll know if it’s stopped moving. (That is to say, you’ll know if the slab has stopped shrinking.)

Then, to repair the problem, I’d add a tile “expansion joint” directly over the crack.

Because the concrete crack is irregular, the two slabs will probably not move up or down causing differential settlement. Just make sure the slab has stopped shrinking. It may take up to a year.

  • What do you mean when you say the tile has cracked? – bradforj287 Jun 26 '18 at 1:12
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    @bradforj287 I saw the joint with crooked edges as tile. It’s concrete too, isn’t it. So, the good news is that the crooked edge crack was planned, they just did a terrible job of installing it. It’s a “control joint” for expansion and contraction. Fortunately, the joint was installed there to control the cracking. Unfortunately they did not extend it to the top of the slab. So the top cracked leaving a crooked edge. – Lee Sam Jun 26 '18 at 15:02
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    @bradforj287 With regard to gasses entering your home: I’d contact your local Department of Environmental Quality or your local Building Department to see if you live where there are RADON GAS. Either way, I’d seal the joint with silicon caulk. (Make sure you get one for floor installations. You won’t find it at your local Building yard.) – Lee Sam Jun 26 '18 at 15:08
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    @bradforj287 No, you do not have a claim. There is nothing failing. It’s of reasonable size (I’ve seen them 3/8” wide) and relatively straight. (Failures appear random and at an angle to the home.) – Lee Sam Jun 26 '18 at 15:12
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    @bradforj287 Also, when you go to caulk the joint, don’t try to fill the joint up. That is to say, you only need to fill the top 1/8” or so. If you fill it up then it won’t expand and contract and thus fail by tearing away from one edge of the joint. (That looks like what happened in picture 2.) They make backer rods which you can push into the joint to keep a uniform depth for caulking. – Lee Sam Jun 27 '18 at 16:33

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