Proper Aggregate for Under Concrete

Small Aggregate

• The smallest aggregate is also referred to as "screenings" and measures under a 1/8 inch. The stones pieces often breakdown to form a course sand-like consistency. Due to its small size, this aggregate tamps down to form a dense, nearly solid surface. Unless you live in a dry area and have excellent drainage, small aggregate is not a good fit for a concrete foundation. It doesn't absorb tension or allow underground water to escape, which can cause large cracks in the surface.

Medium Aggregate

• Medium aggregate averages about 3/4 inch and provides a strong, sturdy foundation for concrete. Stones this size wedge together when compacted so they prevent the concrete from sinking. They offer enough flexibility to absorb both ground tension and stress from heavy weight loads. The cracks between the stones are large enough for moisture to seep through but small enough to ensure stability. If the soil has a clay consistency, lay down geotextile fabric on the ground first to reinforce the aggregate.

Large Aggregate

• Resembling the size of a golf ball, large aggregate is about 1 1/4 inch, and works better as ground cover than a concrete foundation. While large aggregate enables water to steep through, the gaps between compacted stones are larger and less stable than medium gravel. If you're pouring a driveway in an area with clay soil and poor drainage, a 4-inch thick layer of large aggregate beneath an 8-inch layer of medium aggregate is appropriate. The large aggregate forms a barrier between the clay soil and medium gravel, preventing the concrete from sinking.

Recycled Aggregate

• Using recycled aggregate, or crushed concrete, saves money and conserves natural resources. Recycled concrete is separated from metals and grass and broken down into irregular shapes of different sizes. In addition to containing the small stones originally mixed into the concrete, recycled aggregate has bits of hydrated cement paste. These porous particles make a foundation more absorbent than a natural aggregate foundation.