Well this is a pretty big deal because we don't know the cause.
First let's go over common reasons we get cracks on new floors.
- Soil wasn't properly compacted. Soil should be compacted with a rock bed on top.
- Bad mixing at site. Especially in the summer contractors pump too much water in the mix. The water makes the concrete weaker and it does crack easier.
- Bad mix at plant. Just a poor mix of materials coming in.
- Bad environment for mix. Too hot, rain, too long to site, whatever.
- They didn't cut the concrete or didn't cut it quick enough. The concrete needs to be cut (I suggest every 10x10 foot area) to allow for expansion. It really needs to be cut the first day.
- Didn't use rebar. This is a common cost cutter and I have seen rebar pulled before pour.
- House was built on unstable land. Could have been a landfill, could have springs underneath, whatever.
- Soil type wasn't conducive to their install method. Really too many things to list here but basically different types of soil expand and contract more due to moisture levels. Also it is possible that it was pushed even further by either a lot of rain or a drought.
Those are what I got off the top of my head. I am positive there are more reasons for cracks in your basement. If I were to totally guess I would say they didn't prep right in that corner - It seems like the corner is dropping on you already and maybe it wasn't floor wasn't compacted correctly. The crack starts on the outward "L corner" which is normal but heads right across the corner - most insignificant cracks in concrete tend to be straight. This crack has its shape for a reason.
What do I think you should do? First ask questions. See if the contractor knows why it cracked. "It happens" is not an answer. My reply would be "Me not paying you happens too." If the contractor/builder doesn't want to take blame (it is their fault) then you need to get a psi reading on the floor, measure its thickness, and also prove that the floor is rebarred/reinforced correctly. You will almost certainly need an engineer. Even if the problem is obvious your builder is less likely to move until it is proved from a creditable source.
You cannot sit on this. If the mix was bad - well did they use the same mix for the walls too? If it was bad prep, what happens when you have issues in a couple years? I have seen basements that have had to be repoured and it is a mess. Also will set you back 10K in the states.
Again, no matter what anyone tells you - cracks in a fresh floor are not acceptable. This is a sizable crack. Even if an engineer checks everything out I would demand a longer warranty on the basement floor and to have them seal it. Either that or they start over.
Note (based on additional pictures: I am not there so it is a bit of conjecture. But it is not a good sign when an area is relatively dry and you have a corner that is not only wet but it seems dropped too. It is a guessing game for me too at this point, not seeing what you have going on. My top 2 guess (and they are guesses) -
#1 - They dug and compacted. While digging they left over dug out that corner and then filled it. Then they never compacted it correctly. After it rained a few times, the corner started dropping. Also when I have a slab compacted the rock is part of the equation. I don't see any rock in your pictures.
#2 - You have some sort of natural underground current or spring. I have a stream that is 3 feet under, about a foot from my house - and I can't imagine if it where a foot over and 2 more feet down.
(Do you have any pictures right before they poured?)
You can pour a new slab up to the level of the foundation, not higher. In this case, you could basically fabricate a new concrete patio that's below the level of the previous one and build some steps leading down from the door to the patio.
You could also forget about making a concrete patio and build a wooden (or composite decking) patio at the current level of the door.
In general, I would recommend against pouring concrete right up against the foundation, because it provides hidden termite entry points and makes it very difficult for you or future owners to insulate the exterior of the foundation later (if this is an appropriate foundation insulation and hasn't already been done). Also, you have to get the slope away from the house perfect or else you're redirecting water into your foundation.
Best Answer
You mention a column as the footer, so I'm assuming this is a post framed type of structure with the posts sitting on concrete piers. If so, the posts must be supported directly on the piers, not on a slab poured over the pier. You can do this by either extending the pier to the top of the slab and using a support like this (Simpson EPB44T) with the slab poured around the pier:
Or you can bring your pier to the bottom of the slab and use this style of support (Simpson PPBZ Porch Base), which you anchor to the pier, and then pour the slab over/around it:
This is NOT engineering advice - I suggest you talk to one to properly spec out this component for your application!
(just realized this question is 5 months old and probably OBE, not sure how it ended up on my feed - hopefully you got it right)