replying to mister_friendly, clint wood wrote:
I'm surprised that not a single person here actually knows what aggregate
beneath a concrete slab is actually used for. It has nothing whatsoever to do
with moisture drainage. I think we all know that concrete is not water
permeable. Any water that makes its way beneath concrete has wicked through
the dirt from adjacent areas and will continue to wick its way out from
beneath the concrete. Aggregate beneath a concrete slab is used entirely to
minimize settling and to more evenly distribute the load the concrete imparts
onto the soil beneath. Soil compacts easily, and the weight of a vehicle will
cause your driveway to eventually compress the soil beneath and ultimately
lead to concrete failure. Aggregate spreads a concentrated load over a larger
area, thereby minimizing soil compaction. Generally speaking aggregate is not
needed beneath a sidewalk because a sidewalk is typically exposed to loads
that are insufficient to produce soil compaction.

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