Omitting a compacted gravel or crushed stone sub-base beneath a concrete slab can lead to severe structural issues, premature cracking, and uneven settling over time. Without a gravel layer to act as a stable foundation, the concrete rests directly on native soil, which is frequently prone to shifting, expanding when wet, and contracting when dry. Furthermore, a proper gravel sub-base breaks the capillary action that pulls ground moisture upward through the slab, preventing efflorescence and dampness. If poured directly onto uncompacted mud or organic soil without drainage stone, seasonal freeze-thaw cycles and ground movement will quickly cause the concrete slab to fracture, sink, and break apart.
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