New house concrete cracks, what can we do?

Hi,
Our new house is just complete and we are about to close. Today during the final checkup we found the 2-car garage slab has a hairline crack in the middle, running from the front to back for 3/4 of the depth, ending at a point about 6' from the inside wall. We put a 6' straight edge (steel ruler) across the crack and found the two halves of the slab are at an angle. When one end of the ruler sits on the slab at one half, the other end is at least 1/4" about the floor. When the ruler is entirely on either half, it sits straight and square on the slab. So it is apparent that one side of the slab has settled more. It appears that the front corner that is on the far outside (away from the other part of the house) has sunk. When looking for more evidence, we checked the foundation wall around the garage. The wall is about 1' tall around the slab. We found at three places has 2" long hairline crack at the top edge. They do not run all the way to the slab, though. The locations of these small cracks confirmed it is that corner that has settled more. Subsequently we carefully inspected all foundation walls in the crawlspace. The other part (around the heated space) look fine.
We know many houses will eventually have some fine cracks due to differential settlement. But this appeared before we move in. It happened so soon. The concrete was poured in October, only four months old. When the concrete was poured, we asked GC if he had compacted soil in the ditch he said no need because that's undisturbed soil. We have clay soil here and it is as hard as rock at the back of the house but at the front it appears not as hard. But we trusted GC.
We just found it today and will speak to the GC tomorrow but we'd like some opinion from poeple here. Is it a concern big enough to stop the closing process? What the GC can do to remedy it? The city inspectors OK'ed the house last Friday. Don't know if they noticed the slab crack. We like the house and want it but if the cracks grow larger and larger in the coming years it is scary.
TIA
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Someone told us concrete shrinks after curing, resulting fine cracks. It bothers us that the two sides are not even, not like the results of shrinking.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi again,
Some source (not an engineer) told us that the garage slab may not be structural. In the south they use slab foundation thus the slab is what the house sits on. We are in the northwest. The foundation is foundation wall and the studs sit on the foundation concrete wall not the slab. Slab is just to cover the garage floor. The slab is somehow not well connected to the foundation walls. Our foundation walls were poured early October but the slab was poured one month later after the framing and roof were built up.
With this info we went to check again in the evening. The crack IS higher and the steel ruler sits across it bit like a seesaw. With 3' (half of the 6' ruler) sitting on one side of the crack, the other end is about 1/4" about the floor. We extrapolate this 1/4" from 3' to 11' (half of the garage width), we get about 0.9". That would be the amount the far end slab has settled. We checked where the foundation wall and slab meet all around the garage. There is NO sign that there is shift between the slab and the wall. It looks more like there has been some clay soil heaving near the front middle part of the slab so the crack open up and is a bit higher than either side - the soil under the gravel is clay, and the house has gone through freezing temperatures of Dec and Jan without garage door in place. The three 2" long cracks on top edge of foundation wall are probably not uncommon. If the walls had settled down together with the slab at one corner for 1" or so, we'd have larger cracks in wall.
We know the builder put rebar in the slab. He told us he put rebar everywhere: driveway and patio.
So it doesn't sound as bad as it did at the beginning.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Your concrete possibly was not originally finished flatter than 1/4 inch in 6 feet so I would not asume the cracks are due to settling. Hairline shrinkage cracks are very common in concrete and if stable should cause no concern. Don Young

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I would immediately get opinion of a commericial or residential structural engineer, and not one selected by the GC. Hairline cracks are not uncommon, but differential shifting could be serious on a four month job, as you suspect, and may be due any one of many causes, including incorrect soil treatment, insufficient thickness of compacted ballast gravel pad under the slab, concrete's too thin, improperly mixed, or undersized or missing re-bars, or incorrectly linked rebars. Onsite hands-on inspection by a consulting specialist, paid by the visit or hour, is the way to go, as the most you'll get from a newsgroup is armchair speculation as to root cause, including this opinion..
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Roger T. wrote:

suggestions are right on the mark.
My 2-car garage has a crack in it as does the drive way, but they developed over time. And it is clear now that that the fault was in the initial prep. My foundation walls also have a couple of cracks (crawl space) which shouldn't be there but have caused no problems. But cracks in four months? Contrary to others, I've never seen cracks, small or even fine hairline cracks, in well placed concrete. I have never seen it any any of my concrete whether wheelbarrow, small mixing machine, or big truck stuff. And, I have never seen it in old industrial type stuff. Don't know about new industrial stuff, but I'll bet hairline cracks are due to poor mix or poor placement.
I've watched foundations poured around here and have been appalled at the way new concrete structures are treated. Speed is of essence for contractors and sometimes wall forms are removed in less than 24 hours and usually removed in 36-48 hours. The fresh concrete is usually neither covered and nor kept damp, and never kept warm in freezing temperatures, so the concrete strength is very poor compared to what it could be. Neither is there any steel reinforcement, and initial ground prep often looks rather shoddy.
We also have lots of clay which expands and contracts with water content. From October until now, the moisture content of the clay base could have radically changed. So, you can expect some movement. That means that good ground preparation is needed to minimize the movement. If one corner of the garage has sunk, then ground preparation must have been poor in regard to compaction and base prep or drainage.
I would have to really like the house to accept it and would demand that the problems found by the consultant (hopefully not the brotherinlaw of the GC) be fixed to my satisfaction and guaranteed for a 10 year period.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
opinions from people in this newsgroup would be the blind leading the blind, visual inspection and other procedures are necessary to answer your questions
hire an independent structural engineer to determine if the slab is a problem, otherwise you'll be worrying about it until you get an expert opinion, whether it is a problem or not

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The only way your going to know for sure is to hire an engineer and inspect the area. The way that homes are being done in my area the garage is part of the structure. God help you if the builder is KB.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Kathy wrote:
While cracks are not at all unusual and most are not cause of alarm or an indication of a problem or fault, I would be worried about the one you are describing. It will take an engineer to be sure, I would hold off until I was sure on this one.
I doubt if it is going to turn out a big problem, but if it is a big problem it can be a really big problem.

--
Joseph Meehan

26 + 6 = 1 It's Irish Math
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.