Q: This is my second year as a contractor doing residential and some commercial work, primarily walks, driveways and parking areas. In looking for recommendations on what the standards ought to be I've run into a lot of different opinions from people I thought would know the answers.

I'd like to know:

  • Can I really get by with 4 inches of concrete for a parking area?
  • Should I use wire mesh?
  • Should there be a layer of gravel under the pavement?

I'd like to know I'm doing the job right, but I don't want to waste money on things that aren't necessary.

The different opinions may reflect some changes in standards that are taking place. For many years asphalt has dominated the parking lot market, for no better reasons than its being cheap and easy to apply. Concrete was almost always overdesigned for the purpose and therefore more expensive. In recent years the concrete industry has made a concerted effort to capture a larger share of that market. This has resulted in the application of more realistic minimum standards that are involved in all three of your questions.

A 4-inch thickness is more than is needed for the typical parking lot traffic of passenger cars and light pickup trucks. However, it is a practical minimum for construction. If the traffic pattern will bring heavier trucks near the free edge of the slab (at the slab perimeter or at an expansion joint) thickening the slab edge gradually from 4 inches to 6 inches over a distance of 36 inches from the edge would be a good precaution against overloading.

You will not usually require wire reinforcing mesh. It doesn't increase the load carrying capacity of the slab; its primary purpose is to prevent the opening of shrinkage cracks that occur between joints. In a nonreinforced 4-inch slab, control joints should be spaced at a maximum of 8 to 10 feet and cut to a depth of 1 inch. If that is done, the shrinkage is provided for, and there is no need for wire mesh. If it is desired to space joints farther apart than 10 feet, mesh would be helpful.

A granular subbase is used to prevent pumping at joints in concrete highways subjected to heavy loads moving at high speeds. Since these conditions do not exist for driveways or parking lots, a granular subbase is not required. The recommendation is merely that the subgrade be uniformly compacted. Apart from concrete pavement design, however, there are conditions under which some granular material may be helpful in construction. Heavy clay soils can be difficult to grade accurately. It may be economical to bring in a couple of inches of granular material to allow more accurate fine grading, especially when working with a minimum pavement thickness. Some granular material may also provide a better working platform in case of a wet subgrade.

Don't forget some other important standards about which you didn't ask: 3500 to 4000 psi concrete, air entrainment in severe climates and a maximum slump of about 4 to 5 inches.