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?)
Yes, you're being paranoid.
If it was installed correctly, there really isn't much of anything that will go wrong with it from sitting - most septic problems are from using the system and not maintaining it, leading to material that should have been pumped out getting into the drain field and clogging it. If it's not used, that won't happen. The materials do not degrade in the ground. (Orangburg pipe does degrade, but a system built circa 2004 is a good 30+ years removed from the end of Orangeburg - nasty, hateful stuff - basically tar-paper pipes.)
If you keep hunting you can probably find someone who will take your money to run an inspection camera into the pipes. If you like, you could dig up (or open the access manhole, if your area was moving into modern ideas 10 years ago and did not allow the access to be buried below-ground) the septic tank access - but DO NOT stick your head into even an unused septic tank - they can still collect/generate poisonous gases and should aways be treated as if they were, in fact going to kill you if you did that - use a flashlight & mirror on a stick, or hire a pro. If the system is accurately mapped, the distribution box could also be dug down to for a look. If it passed inspection 10 years ago, there's not likely to be much of anything to see in either place, though the tank at least should be looked into (Title V inspection may, in fact, be required before sale, even though it's unused as yet) just to make sure it hasn't shifted or something, and to be sure you know where the access is if it's not above-ground access (consider adding a riser and manhole cover if you have to dig to find the access.)
Don't be surprised if you find it full of water - rainwater seeps in and won't flow out until the liquid inside reaches the outlet level. 10 years allows for quite a bit of rain to fall.
If it was not installed with a particulate filter on the outlet, you might want to have one retrofitted before starting to use it. It may have been - 10 years ago those were starting to become standard in many locales.
Best Answer
No matter what type of steel ,fiberglass,pre stress cables are used there are 2 types of concrete, #1 the concrete that is cracked and # 2 the concrete that will crack. Since it is summer and it has been hot I would hope the ground was wet when poured and after finished it was kept wet or damp for several days but other than that see the 2 types above.