The goal is to place concrete parking areas on properly prepared subgrades that are uniform in soil composition and compaction. A properly prepared subgrade will provide uniform support to the concrete slab and minimize, if not eliminate, subgrade related cracking.
The subgrade under all pavement should be brought to the required lines and grades and compacted to a uniform density. Any pockets of soft material that cannot be compacted should be removed and replaced with suitable material. Ready mixed concrete trucks and other equipment can operate on the prepared subgrade providing the subgrade is kept smooth and compacted prior to placing concrete. At the time of concrete placement, the subgrade should be in a moist, but not muddy condition. Adding granular sub-base material is not required.
Subgrade is defined as the natural ground, graded and compacted, on which the concrete parking area is constructed.
Because of the rigidity of concrete slabs, the pressures on the subgrade are very low. Thus, concrete slabs do not necessarily require strong support from the subgrade. It is important, however, that subgrade support be reasonably uniform without abrupt horizontal changes from hard to soft, and that the upper portion of the subgrade be of uniform composition and density.
No special sub-base material is required with concrete parking areas provided that you have properly prepared the subgrade. Sub-base is defined as a thin layer of sand or gravel placed on top of a prepared subgrade.
The major causes of non-uniform support from the subgrade are hard and soft spots, back-filling and frost susceptible soils.
See the Parking Area Subgrade Checklist to determine if potential subgrade problems exist.
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