A well-laid concrete slab provides a sturdy, long-lasting surface. Used for patios, drives, paths and building foundations, concrete is designed to support heavy weight loads. Slabs require a solid foundation to support them, absorb tension, facilitate drainage and serve a variety of other purposes. Gravel is widely used because it is as durable as concrete and serves each of these purposes well.
Types of gravel
The best type of gravel to use for a concrete base is 1.8 cm (3/4 inch) medium gravel. This size comes in irregular shapes with sharper edges that wedge together when they are tamped. Medium gravel comes with smaller, sand-like particles that reinforce the rocks between crevices. Smaller gravel can be used as a base if your soil has excellent drainage because the small rocks don't leave enough room for underground moisture to escape. While larger gravel doesn't stabilise enough to be used alone, you can lay down an extra 10 cm (4 inch) layer beneath the medium gravel in an area with dense, clay soil for extra support.
Advantages of gravel
Gravel is used over other types of landscaping material because it offers both strength and flexibility. Irregular shapes allow them to form a nearly solid foundation strong enough to prevent the slab from sinking, but flexible to expand and contract with changing temperatures. The air pockets between rocks allow water to seep through so it doesn't freeze beneath the slab and crack it. Gravel is also affordable and easy to work with.
Installing a gravel base is tedious, but not complicated. Concrete slabs call for a 10 to 25 cm (4 to 10 inch) deep foundation depending on if it is meant for foot or vehicular traffic. The depth of the excavation must be deep enough for the foundation plus at least 10 cm (4 inches) for the slab. Compacting the subsoil allows you to stabilise the ground before pouring in the gravel. Add the gravel in 5 to 7.5 cm (2 to 3 inch) layers at a time and compact the rocks between each layer to make the base as solid as possible.
While gravel is widely used, it is not the only option for a concrete base. Recycled crushed concrete is another affordable, durable option and it conserves natural resources. It is composed of concrete removed from buildings and other structures, and processed so glass, metal and other contaminants are extracted. Another alternative to pouring concrete on gravel is to lay it directly on soil. If soil is compactable with good drainage, you can pour a concrete walkway or patio. However, a slab with only a dirt subbase will not be able to support heavy loads and is more vulnerable to frost heave and drainage problems.