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Regarding the cracks, you shouldn't have much to worry about. It is a slab on grade and not structural. Concrete cracks and 1/16 is acceptable. The norm for slab on grade with a properly compacted soil is 4". 6" and thicker with #5 rebar 16" o.c. w/3000 psi or higher concrete becomes structural, and does not depend on the underlying soil, although it can crack too. Although you say the slab was poured on virgin soil, that does not mean that it is of proper compaction of 90% or better. You need to have performed borings and soils tests to determine that, unless it was obviously bedrock. If the virgin soil does not meet proper compaction, it needs to be excavated to a layer that is, and backfilled in lifts until you meet your subgrade elevation. I am guessing that if your contractor poured an 8" slab, no soils testing was performed and your contractor was not confident of the soils conditions. Was this ever performed? Typically a city will not allow a building permit unless a final soils report was submitted. Take a look at it and read the analysis and design recommendations. Did the contractor tie a mesh of rebar across the entire subfloor prior to the pour (not wire mesh)? Was the mesh lifted off the ground with little concrete blocks over a properly prepped subgrade -typically 6mil visqueen under 2" of sand? If your underlying soil is clay, it will expand and shrink dramatically as the moisture content changes, causing cracks even in 8" concrete. You can only mitigate and control the cracks visually by having saw cut expansion grooves in the slab the day after the pour about 5' o.c. In any event, I would be more concerned about proper waterproofing of your basement exterior walls than visual cracks on the floor. That is not a simple or inexpensive process to do properly.
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