Randall,
I certainly did not intend for this to get into a discussion about the principles and characteristics of concrete, but here goes. Concrete in itself is a very stable platform for many construction projects and has been for many many years. We all assume that it is a static material and once hard, will remain so. The fact is that when concrete is poured, it can move as much as 1" in 100ft, and so on, 1/2 " 50ft. That is why we, in the business, cut relief joints in it to control this, it's what we call a Control joint. The majority of the time, when concrete cracks, it is an upward crack, not downward. When steel is introduced, this strengthens it yes, but does not stop it from cracking. This does not mean there is any structural integrity loss. We use the phrase, if you want concrete not to crack, then don't pour it. Many other factors come into play, temps, moisture, compaction of base, mix, and the time between mixing and installing, generally, 1 hour is max, after that it becomes HOT and can cause several other things.
So, in summation of this, and adding all you have done, you're good to go. High Strength mix, + re-bar and the depth you poured, you should have no issues. as far as cracking, no guaranty that it won't.
The vapor barrier thing is a separate issue. this is governed by the local building code. Some demand it , some expect it, and some give a chit. Being that this is a garage, it should not be an issue but since you dug down much further than the slab, you have created "the lowest spot" in the garage, sort of a cistern effect since you filled it with gravel. Is it going to be a problem, I highly doubt it. If you are going to drain some water into a pit of sorts, I would give it someway to ventilate so as to prohibit mold spores from forming. A simple PVC pipe open to the atmosphere is sufficient, no more than 3/4 in pipe.
I don't know your soil conditions and rainfall but you mentioned sand and rock, both of which holds moisture. I see looking at the drain setup, you have accomplished the Venting issue.
once again I did not intend on being an alarmist here, but it is my nature and what I earn my living doing.
Just want the best for my CVO bros.
And, this is not a $5mil project, but a small garage slab rework. This is going to hold your baby.
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