Subbases and Subgrades
A reasonably uniform subgrade or subbase, with no abrupt changes in support, is ideal for any concrete pavement. Most native soils are not too uniform and thus require some improvement or additional layers to compensate.
A subbase is a thin layer of material placed on top of the prepared subgrade. Subbases provide uniform support to the pavement and a stable platform for construction equipment. Subbases also help prevent movement of subgrade soils at transverse pavement joints in roads subject to a large volume of truck traffic. Subbases may be gravel, stone, cement-modified soil, asphalt, or econocrete (low-strength concrete).
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Permeable subbases (sometimes open-graded subbases) are used to drain water away from beneath the pavement. Permeable subbases have become popular over the last decade. These subbases either may be untreated or stabilized with portland cement or asphalt. To drain water freely, permeable subbases do not have as much fine or small particles as a typical dense graded granular subbase material.
Water easily passes around the aggregate as it flows through the permeable subbase layer. A collector pipe and outlet system discharges water away from the pavement. However, to be effective, an important balance must be met between the degree of drainage and the stability of the subbase layer. Subbase stability should not be sacrificed for the sake of drainage A target permeability of 200-300 ft/day will produce a stable, draining layer that will last. |
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