Got a question...trying to figure out how concerned to be about this issue.

Wife and I are building a new home in development on north side near Conroe. Here is the situation:

- Slab poured around 6/14 or 6/15 while it was still hot and dry;
- As of today slab still has not been tensioned despite typical timeline of 7-10 days;
- Of course it rained quite a bit in between slab being poured and today prior to slab being tensioned;
- On Saturday inspector and I found several surface cracks throughout the interior of the foundation;
- At least 3 cracks extend out to the exterior pier and run from top of slab down to soil;
- Construction manager wants to have slab tensioned, then see how it looks...I'd like to have engineer assess before I make a decision to invest in something that is compromised.

For those who have experience, how likely is it to expect any cracking at the exterior of the slab on a new foundation? Is this something to be really concerned about, or just have it checked and it'll probably be fine?

Do you have the mix design for the concrete? Very slim chance I know, but you didn't happen to make any cylinders for testing? Finally, any pictures?? Not all cracks are created equally. I've done elevated slabs for commercial work and had hairline cracks that you could see on the top and bottom, or in other words went straight through, and the engineer said "eh, that's not an issue. just float over it if the architect doesn't like how it looks". Now, don't take that to mean that just b/c your cracks might be hairline that everything is A-ok...again, not all cracks are created equally.

All this is to say, if it is a concern to you, enlist an engineer. It won't be cheap, but if it gives you peace of mind for your already substantial investment, it's worth it. I highly highly doubt an engineer is going to stamp/sign anything for you where they woudl assume any liability as they 1) did not design it 2) never got to see the re-steel or excavation before or during the pour 3) you most likely don't have the mix design so they don't know what the concrete SHOULD do and 4) you have no cyclinders to break so they don't know what it DID/WILL do. You didn't do anything wrong by not having this, but it's just that without it, it would be too risky for an engineer to officially say one way or the other. They will certainly write a field report (with a MASSIVE disclaimer) saying that their findings during their VISUAL inspection were otherwise not indicative of structural failure and then probably another 10 paragraphs of disclaimers.

[This message has been edited by DwightSchrute (edited 6/28/2011 9:20p).]

http://www.flickr.com/photos/30009715@N04

If the link works right, there are photos of 3 external cracks and two of where they run from the inside of the house.

I would be scared to death over I #1 and I to E #2

Consult an engineer, but here is my opinion fwiw.

What you are seeing is most likely "shrinkage cracks." Typically they are non structural, but they can be if severe enough. Basically these cracks are caused when the surface of the concrete dries out faster then the rest. The evaporation of water from below the surface (also called the bleed out) will cause the surface to crack. This usually occurs within a couple of hours of curing or within a few days. Relative humidity, temperature, and wind all have an effect on how likely shrinkage cracks are to occur. Low humidity, high temps and wind will speed up the surface drying time and increase the probability of the cracks.

Concrete finishers adding too much water to a mix design will increase the probability of this happening as well. The concrete cures slower below the surface then it would have without the extra water, and thus this extra water will need to escape through evaporation. If curing compounds, wet burlap, or even a sprinkler were used to slow the surface drying, then these cracks most likely wouldn't have occured.

A great/horrible example of why to pour a slab at 4am instead of 10 and also know when/how to use curing compounds.

This is my theory after seeing pictures (keeping in mind this is just a theory):

granted they are unnerving b/c of the size, those do look like shrinkage cracks. If you've ever noticed the control joints cut across sidewalks every 4' to 5', look at them closely and you'll see a crack running all the way across the width of the sidewalk in nearly every one. This is b/c when concrete cracks, and it will crack, it does so running to the weakest point, even if it's only weaker b/c of a 1/4" joint cut into it. Now, expand that to the area of your house and put one giant weak spot in it due to a slightly thinner slab thickness b/c the underlying base was a mere 1/2" thinner at 5.5" thick instead of 6" thick...you can see where i'm going.

the larger the surface area of the slab in relation to the thickness of the concrete, the more cracks you can expect. Your slab is substantially large surface to thickenss. As your slab is probably intended to be covered by flooring of some kind, there were no control joints cut in b/c any cracks will be covered. It just happens that instead of a lot of smaller joints, you got one or two hellacious joints.

I too would consult with an engineer. Those appear to be more than shrinkage cracks. Shrinkage cracks are generally superficial and generally do not extend through the exterior beam. A couple of major wet/ dry cycles may really open those up. Probably want to nip this in the bud now instead of 2 years down the road.

My 1965 house has a crack like that. As a homeowner, I would not expect a new house to have it!!!

Appreciate the feedback...we are working to get advice from engineer and determine right plan forward.

One more question regarding the post-tensioning process...are there further potential issues now that it is at least 15 days out from being poured and the entire frame is up without being tensioned? No brick, just rough out and roof.

stinky, i'm unfamiliar with the schedule ofconstruction on post tensioned slab on grade foundations but must say I would not have thought it would start going vertical before tensioning, although, it's not like the slab is going to "squeeze together" a measureable amount. I think this would be a good question for the engineer.

[This message has been edited by DwightSchrute (edited 6/29/2011 11:58a).]