Pouring concrete directly onto prepared soil is a common construction practice, but the underlying ground must be properly graded, leveled, and compacted to prevent future shifting, settling, and cracking. Before pouring, the soil must be free of organic matter, roots, and debris that could decompose over time and leave voids beneath the slab. In most residential applications, contractors lay down a compacted sub-base consisting of crushed gravel or stone aggregate, followed by a vapor barrier sheet to block ground moisture from seeping upward through the concrete. Pouring unreinforced concrete directly onto loose, wet, or uncompacted clay soil without a proper gravel sub-base is strongly discouraged because seasonal shifting and moisture expansion will quickly cause the slab to fracture and break apart.
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