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I had a pad poured last year and almost soon after I noticed some hairline cracks coming from under the tub and out the sides.... My pad is 4" thick and was the same PSI as the drive way.. I think 3500 or 4500.. no rebar or anything in it and no fiberglass was used.
The FOOLS that put in my driveway at my new house apparently didn't understand this principle which is why they now refer to me as "plaintiff."
I had a 15'x20' slab poured. The only prep done was that I removed all of the sod then the guy poured the concrete over top the dirt. There was one crack that appeared about 1 month after the pour but it really doesn't matter because we are going to put slate tiles over top of the concrete whenever the weather gets better so that I will have a week or more of guaranteed dry, warm weather. I'll post photo's when done.