How Much Gravel With Concrete Slab?

Types of Gravel

• In addition to the proper depth, it's important to use the right type of gravel for a concrete base. Avoid using pea gravel or any type with smooth, rounded surfaces because it won't stabilize. Instead, use processed gravel with a wide gradation of rocks from fine sand powder to one-inch pieces. The gravel must be clean of organic material that will shift and squeeze when exposed to moisture.

Depth

• The ensure strength and stability, the depth of a gravel base is typically twice the thickness of the slab. A concrete walkway or patio should be at least four inches thick, while a driveway should be six to eight inches thick. For a walkway or patio over a site with good drainage, two inches of gravel is enough to support a four-inch slab.

Installation

• Installing a gravel base involves more than digging out the foundation and dumping in the gravel. You should first stabilize the subsoil by forcing out moisture and air pockets with a compactor machine. Add the gravel in two- to three-inch layers and compact each layer down before adding more. The sharp, irregularly-shaped pieces pack snug together to form a solid base strong enough to support concrete, but flexible enough to allow moisture to seep away. Use a bubble level over each section to ensure the base is level.

Tips

• There are a few optional steps that improve the strength and durability of your slab. Once the gravel is compacted, lay down wire mesh over the rocks as reinforcement. If you're pouring a driveway or patio, consider laying down rebar near the outsides and through the center. Set the rebar on rebar chairs to lift them so they will run through the center of the concrete.