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Crack on a Brand New Foundation

We have a new town home under construction. We did a pre-sheet rock inspection of this town home. We see a crack running all the way from the front to back of the foundation from the garage door entrance into the home to the backyard.
This crack is bigger than hairline crack. I would say about a dental floss size crack. The builder says it is a hydraulic crack nothing to worry.
Can some one shed more light on this kind of crack on foundation?
We have requested the builder to do a engineer review of this crack.


Re: Crack on a Brand New Foundation

Was there rebar in the foundation?

Are there similar cracks in everyone else's foundation?

I'd get another opinion than the builder, who is out to cover his ....

Re: Crack on a Brand New Foundation

Do what Houston said.

Your slab is doing what concrete does - it cracks, which is normal for concrete, even with rebars (which is code for foundations and slabs). What causes cracks? sometimes it's the concrete quality, dryness shrinkage and the prep work, other times it's external factors, like earth movements, settling, temperature when poured, etc.

If you get nowhere with your builder, at least fill the crack with a flex concrete filler (there are different kinds available: epoxy, acrylic, patch, paste, liquid, etc), to make sure your slab is sealed.

Re: Crack on a Brand New Foundation

I think I would start with a call to the local building code enforcement. Maybe take a picture of it before the builder has a chance to cover it up.

A crack like this so soon can mean that the gravel underneath wasn't properly tamped down and/or the concrete was a substandard grade.

Re: Crack on a Brand New Foundation

I began my construction career as a Form Carpenter where I learned a lot about concrete in general, and a lot about slab construction. Let me begin by saying that I have never seen a residential builder who truly understood what it takes to do a slab correctly. Some do OK, most do worse, and a few do worthless but get by with it through dumb luck. It is not that they intend to do poorly, it's that they see concrete as being simple when it's not! Just covering the basics of proper slab construction would fill a couple dozen pages, mostly simple things but all of which make a difference in the end, and some of them very much so. To cover basically concrete as a whole would take ten times that or more, and to cover it all in depth requires a textbook. And the local building inspector probably isn't a lot better with understanding concrete unless that background came from industrial concrete work or from schooling as a civil engineer. There's a whole lot more to it than re-bar, cement, sand, and aggregate poured then finished!

Hairline cracks (including what you describe as 'dental floss' size) may mean nothing and are common. The usual cause is too wet of a mix in pouring, too rapid dry-out on the surface after finishing, or both. It's easier to smoothly pour and finish a wetter mix, which is the usual with residential slabs. My peers here can look up "slump test" as a beginning point to understand why you never wet a mix without a good reason to do it. Luckily concrete is fairly forgiving in this as most applications are over-engineered.

The biggest issue with residential slabs is incorrect control jointing (like the grooves you see in sidewalks). They are called that because they control where the slab is going to crack, thus keeping it from cracking where you don't want it to. (some call these "expansion joints" but those are something totally different). Control joints may not be more than 12' apart across any span- actually 8'-10' is a lot better. They should be at least 1/4 of the thickness of a slab (which in residential work never happens; the usual is 3/8" to1/2" deep and formed with a tool, or worse made with "zip strip", a "T" shaped part pushed into the pour and finished flush with the upper part peeled off after initial curing has happened). Their layout also matters. They should never bisect a corner (creating a triangle which could break off) although they may run to a corner point. They should not run lengthwise near or under a load-bearing wall but may run perpendicular to and underneath it. Sometimes a lesser depth joint will be adequate to control the cracking so don't let their depth alone guide you here so long as they aren't excessively shallow. Now to the answers you need to know:

If the control joints are correct, and if the shallow 'hairline cracks' do not appear to be getting wider after the initial cure (a few days to a week based on temperature) then you're probably OK. If you see any more significant width cracks other than in the control joints you've got problems. If control joint cracks open to more than half the width of a pencil lead this early in the game you've got problems; especially if they are growing wider. Once the initial cure occurs, widening cracks indicate a poor base and will require redoing at least part (but preferably all) of the slab with better attention paid to the base the next time.

Most importantly, if any builder tries to convince you that any of the cracking I called a problem is normal, they do not know what they are talking about and you may need to get a certified engineer to asses things before continuing. Definitely do this with growing cracks as that may indicate serious structural failure of the slab. With a good contract there should be an arbitration clause which will cover this expense should your fears prove to be well-founded, otherwise you will have to pay for this on your own. It's money well spent either way for as the slab goes so does the rest of the house!


Re: Crack on a Brand New Foundation

You guys are great. Thanks everyone for the detailed information, very useful.
Take Care!

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