If this was my slab... Id have them rip it out and pour it again...

something went wrong... Items for concern curing process type of mix did it match the structural drawings, rebar location and size does it match the structural drawings. and soils recommendation for base..? MAJOR ISSUE

Is this slab post and tension ?

Take a lot of pictures. I am not a concrete expert, but this could be a problem and could be a bigger problem later. Make note of the extent of the crack and record it. Talk to your builder and talk to an experienced concrete person that is not involved in your project or is not a friend or enemy with your builder.

Am I correct to assume this is a builder's house which you will purchase when complete and not a custom build for you? If so, then it's the builder's house until you take possession at the end of construction. Check you contract wording to see what rights you may have during construction (or not).

Have you met onsite with the builder and discussed this, as well as remedies?

If it was my house I would have a structural engineer inspect and give a written report of findings and recommendations.

I'm not sure what dimensions we're looking at here, but I find it odd that there isn't a single control joint to be found.

Thank you all, for the comments and your thoughts on this. (MSDG, I am not sure if this is a post and tension slab or not. It was poured in stages, with the steps put in first and then the top slab poured, over a gravel layer. ) We have talked with the builder about it, and they put some polyurethane sealant on the crack but it seems like it did not close it all the way. They don't seem too worried about it, and the agent keeps saying it is no big deal and very common, but it just seems like it is more than a hairline crack and I worry it will just get bigger.

Settling cracks are common place, I'd be very curious as to why they poured the slab in stages. Your cracks could well be attributed to that, That appears to be a fairly significant crack and more than I would tend to call a settling crack. I would want to know how far across the foundation it goes and putting a "sealant" on the crack is like putting lipstick on a pig, it's not going to have any structural bearing. It may "look" better but that's about all that's for. You now have what is called a "COLD JOINT".

My understanding of concrete is that in the curing process the concrete
is forming crystalline "needles" that knit it together. If there is a
cold joint this cannot occur between the two sections of the pour, so
you are relying on a mechanical connection between the two pours... the
roughened surface, keys, or reinforcing steel. Bonding agents are simply
glues trying to enhance that mechanical bond. I am going out on a limb here and assuming that the pours were both reinforced with rebar to create your mechanical joint.

FWIW, I would prefer a single stage pour to allow to slab to cure as a whole vs the 2 pours which will NOT form a chemical bond since you've got a cold joint.

"Is this slab post and tension ?"

It's Post Tensioned. Post as in after -- the tendon (steel reinforcing) is placed in the slab and once the concrete has reached a specified strength, the tendons are pulled, placing them in tension.

"I'm not sure what dimensions we're looking at here, but I find it odd that there isn't a single control joint to be found."

Control joints are cut or formed in a slab to encourage the slab to crack in a specific location. It's mostly for looks, so you don't see random cracking.

Residential slabs are typically covered with a finish floor material, so random cracking is not a problem.

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As for your problem, It seems obvious that the floor outlet may have something to do with this. Just guessing that it might have been buried in the slab, and someone used a jackhammer to remove the concrete to locate it and extend the conduit. This could have caused the crack.

You did not say where the ends of the crack are.

The important thing is that it does not open up. The slab should be reinforced, and that reinforcement is designed to keep a cracked slab together.

You don't want to seal now, because it needs to be monitored, but have them seal it before it gets covered.

Thanks for all of the useful comments and explanations. Yes; I wondered about the floor outlet. I looked and found a "before" photo before patching and this is what it looked like. What sort of reinforcement might be possible? The crack runs all the way across the living room, wall to wall. I am not sure if it goes any farther than that.

T

The crack is clearly there prior to the work on the outlet

It looks like the crack runs to an exterior wall -- on the left in your 3rd picture. Is the slab and crack visible from the outside?

The reinforcement in the slab should keep it together. No real way to reinforce after the fact.

Ask your builder what sort of reinforcement is in the slab. Is it the commonly used 6 X 6 welded wire mesh, which doesn't have much reinforcing strength? WWM is commonly used in driveways. Or is it No. 4 reinforcing bars each way, which do have sufficient reinforcing strength...

Do you remember seeing the reinforcing before the concrete was poured?

The size and width of the crack is substantial, and it will be interesting for you to follow it in both directions, especially to the outside perimeter stem wall foundation, to see if it occurs there as well, or is limited to slab only.

For piece of mind you need an investigation and report from a structural engineer. Call one and get them out to your site.

@PineBaron

"Concrete expands 1/4" for every 10'. "

Could you expand on this a bit? What causes it to expand, and when does it expand? Also, how does this relate to the crack issue?

PPF: All materials have a coefficient of linear thermal expansion. Coefficient of linear thermal expansion for concrete is roughly the same as for iron which is why it (rebar) works well with concrete. Expansion/contraction is significant for long linear surfaces such as the large concrete slab with the crack OPs thread. Though there is a much more to material science, especially concrete and types of concrete, hopefully the following excerpt provides an adequate explanation.

THERMAL EXPANSION AND CONTRACTION
Materials expand or contract when subjected to changes in temperature. Most materials expand when they are heated, and contract when they are cooled. When free to deform, concrete will expand or contract due to fluctuations in temperature. The size of the concrete structure whether it is a bridge, a highway, or a building does not make it immune to the effects of temperature. The expansion and contraction with changes in temperature occur regardless 0of the structure’s cross-sectional area.
Concrete expands slightly as temperature rises and contracts as temperature falls. Temperature changes may be caused by environmental conditions or by cement hydration (the exothermic chemical process in which the cement reacts with the water in a mixture of concrete to create the calcium silicate hydrate binder and other compounds). An average value for the coefficient of thermal expansion of concrete is about 10 millionths per degree Celsius (10x10-6/C), although values ranging from 7 to 12 millionths per degree Celsius have been observed. This amounts to a length change of 1.7 centimeters for every 30.5 meters of concrete subjected to a rise or fall of 38 degrees Celsius.
Thermal expansion and contraction of concrete varies primarily with aggregate type (shale, limestone, siliceous gravel, granite), cementitious material content, water cement ratio, temperature range, concrete age, and ambient relative humidity. Of these factors, aggregate type has the greatest influence on the expansion and contraction of concrete.
Severe problems develop in massive structures where heat cannot be dissipated. Thermal contraction on the concrete’s surface without a corresponding change in its interior temperature will cause a thermal differential and potentially lead to cracking. Temperature changes that result in shortening will crack concrete members that are held in place or restrained by another part of the structure, internal reinforcement or by the ground. Forexample, a long restrained concrete section is allowed to drop in temperature. As the temperature drops, the concrete tends to shorten, but cannot as it is restrained along its base length. This causes the concrete to be stressed, and eventually crack.
Joints are the most effective way to control cracking. If a sizable section of concrete is not provided with properly spaced joints to accommodate temperature movement, the concrete will crack in a regular pattern related to the temperature and restraint directory. Control joints are grooved, formed, or sawed into sidewalks, driveways, pavements, floors, and walls so that cracking will occur in these joints rather than in a random manner. Contraction joints provide for movement in the plane of a slab or wall, and induce cracking caused by thermal shrinkage at preselected locations. One of the most economical methods for making a contraction joint is by simply sawing a continuous cut in the top of the slab with a masonry saw.

Thanks for all of the very useful comments and ideas on this; much appreciated. I do live in an area that has a fairly high incidence of radon, so that is a concern. There is some sort of passive radon vent option, to activate in case needed, but not sure if that will be effective if slab is cracked. We are planning to test for radon. The initial plan is to close on the house in two and a half weeks; they say it will be ready by then. I do think an inspection is a good idea, though, before that, to make sure this is addressed first in ways that it can be. The crack appears to be level, one side not higher than other. I don’t know if this makes a difference or not, but the slab is about 5-6 months old; footers /steps poured/put in Dec/Jan I think, slab poured in February, with a few days around 30 degrees. There is a gravel layer in between, with concrete blocks edging it and separating areas. I think I remember seeing some rebar in the footers/steps; I don’t know about what was used with the slab.
I am attaching a picture of what it looked like fairly soon after they poured on the slab. (The garage area is toward the front of photo, living room/kitchen to the back of photo). It looks like they slanted the garage part of the slab a bit. You can see the outline of the area for the floor outlet towards the back right of the photo.

I am not sure how you'd ventilate a slab or do this AFTER the pour? We installed a radon BARRIER (not retarder) under our slab - specifically tested as a barrier and 15 mil plastic.

I am not 100% sure how it works, but I think it is a pipe they put through the slab, and if needed, they further connect it to a pipe running to the roof to divert the radon outside. "passive system relies on natural pressure differentials and air currents instead of a fan (used with active radon mitigation systems) to draw radon up from below the home."http://radondiva.com/blog/passive-vs-active-mitigation-systems/  But, it seems like, if the slab is cracked, there would be more than one way for the radon to go up rather than just through the pipe.

Concern is really only warranted regarding the finished floor. If you're installing some sort of floating floor, I wouldn't worry about this another minute. If you're polishing the concrete, it's a tear-out. Don't even think about ceramic tile or stone without Dirta, please.

This is a problem. I've been building for 37 years and concrete issues are complicated and get worse overtime. Give the builder a 24 hr notice to correct or inform him that you will be contacting the lender, his insurance and bond agents. Stop all payments until this is resolved. It would be in your interest to find a local concrete consultant and have them intervene. Trying to pass off such a flagrant defect is cause to have the entire house inspected by a respected third party consultant.

Best of Luck.