looks a little large to be shrinkage cracks. if its a slab on grade, i would bet on some sort of soil condition that has heaved the slab to open a gorge like that.
Looks like a shrinkage crack to me. They can be large, with stress risers like reentrant corners, also little or no reinforcement, high w/c ratio concrete. In other words, fairly typical of poor construction.
Yes, this is a shrinkage crack. In the last photo you can see that it likely started from a couple of discontinuous plastic shrinkage cracks. You can also see evidence of laitance at the surface, indicating a lot of bleed water. As hokie66 noted you have a re-entrant corner condition by the sliding glass door, also near the original plastic shrinkage cracks. Also as hokie66 noted....typical of poor construction.
Don't forget one of the main tenets of contracting.... "There are two kinds of concrete. Concrete that has cracked and concrete that will crack"
(Similar beliefs about Skylights and leakage)
It's a typical residential slab built in Florida. probably no more than 4" thick with no reinforcement. I was thinking this was due to shrinkage but the size was what I was a bit concerned about. It was built back in the 70's and they probably didn't compact, added a bunch of water, poured it in a rain storm in 100 degree weather.
I'm thinking no reinforcement too. If there were any reinforcement 1 1/2" to 2" down, then wouldn't a crack that wide mean steel had yielded or de-bonded?
Heave? This is typical in clayey soils expanding from moisture right? This area isn't known for clayey soils; its close to the beach and more than likely clean sands. Unless the builder imported a bunch of cheap fill with clay...
This may fit into the situation where it is a typical "D" shaped crack. Experts have discussed that years ago, but this OG can't recall the reason. Has to do with mix and curing I think.
OG...not D-cracking...that's very specialized. This one is simpler...just shrinkage. As kmart30 notes, likely just relatively clean sands and poor construction!
I recently removed the floor covering from a house I bought in 1994 (in Florida). The family room had a similar crack in the floor. Clean fine sands below slab. I didn't bother with "why"...that much was obvious from my 40 years of experience with concrete construction in Florida. Filled crack with epoxy and put "engineered hardwood" flooring over it. No issues.
