· Michael Blair
Thickness is the number people compare because it is easy to compare. On Middle Tennessee clay, what goes under the slab decides more about its life than an extra inch on top.
What the base does
A compacted gravel base spreads load and, more importantly, drains. Water held against the underside of a slab in clay is what produces heave and settlement.
Without it, the slab sits directly on soil that expands when wet and shrinks when dry, and it moves with the seasons regardless of how thick it is.
Reasonable specifications
Residential driveways are commonly four inches of concrete, increased to five or six where trucks or an RV are involved. Base is typically four to six inches of compacted crushed stone.
Subgrade should be compacted and free of organic material and soft spots. Topsoil left under a slab decomposes and leaves a void.
- Compacted subgrade, organics removed
- Four to six inches of compacted crushed stone
- Four inches of concrete residential, more for heavy vehicles
- Reinforcement appropriate to the use
- Control joints cut early, at a quarter of slab depth
Reinforcement
Wire mesh or rebar does not prevent cracking. What it does is hold a crack together so the two sides stay level, which is the difference between a line and a trip hazard.
It only works if it sits in the middle of the slab. Mesh lying on the ground and poured over does nothing at all, and it is a common shortcut.
Drainage around it
A driveway that collects water at its edges is undermining itself. Grading should carry runoff away, and any downspout discharging beside it should be piped past.
This is the single most common reason a Middle Tennessee driveway settles at one corner within a few years.