What Is the Proper Base for a Concrete House Slab?

Soil Support for a Concrete Slab

• Concrete slab-on-grade structures are the most simple form of construction for foundations. Slabs are commonly used in regions where soils don't freeze and the soil is suitable for supporting the slab. The supporting soil has to be uniform for the concrete to maintain its integrity. Concrete can flex to some degree without damage, but if the slab has to cross over gaps in soft soil, it may bend to the point that it cracks all the way through.

Subgrade and Subbase for Slabs

• The subgrade is native or improved compacted soil; it may be sufficient for placing the slab if it drains well and has adequate bearing capacity. If there are problems with the soil, such as the conditions are muddy, the soil doesn't drain well, or it doesn't compact well, a subbase of granular material is added and compacted. Thick subbases let the slab bear heavier loads. The subbase can serve as a capillary break, keeping groundwater from moving upward to the slab.

Base Layer for Slabs

• The granular material for subbases is a coarse gravel containing few fines, or smaller particles. This material is compacted to within plus or minus 1 1/2 inches of the desired elevations for the slab. A base layer of fine gravel is laid on top of the subbase as a leveling course, to achieve the correct elevations. The base material should be compactible, granular material containing 10 to 30 percent fines -- specified to a certain size -- and no clay, silt or organic materials.

Effect of Base on Slab

• The fine material used for the base layer is fine-tuned to meet the elevations for the slab, within a low range of plus or minus tolerances. The result is a flat surface that provides a uniform support for the slab. This saves concrete costs; slabs have to be made thicker to compensate for lack of soil support. The slab can also slide easily as it shrinks, reducing cracking from contraction as the concrete sets.