Q: Is a granular base needed beneath city streets or parking lots that will be paved with concrete? Some specifications require a subbase and some don't. If one is needed, how thick should it be?

A.: There's an excellent article on this subject in the August 1989 Ohio Paver. Concrete pavements built on properly compacted and shaped subgrades, with no subbase, provide satisfactory performance for many years with little maintenance. Using a subbase adds significantly to the cost of a concrete pavement but doesn't appreciably increase its load-carrying capacity. Thus, a subbase may not be cost-effective and shouldn't be specified and used unless warranted for other reasons. There are four reasons for using a subbase under concrete pavements:

  • To prevent mud pumping (ejection of fine material from under the pavement) at joints and cracks
  • To help control volume changes in highly expansive subgrade soils
  • To help reduce excessive differential frost heave
  • To provide a working platform for pavement construction

The primary purpose of a subbase is preventing mud pumping. Mud pumping occurs at joints when the subgrade soil goes into suspension, when there's free water between the pavement and subgrade, and when there's heavy truck traffic. The Ohio Paver article defines heavy truck traffic as 100 to 200 trucks per day, excluding panel, pickup, and four-tire single unit trucks. If any of the three conditions that lead to mud pumping is absent, pumping won't occur and a subbase isn't warranted.

The other three reasons for subbases are secondary. Volume changes and frost heave can usually be controlled by proper earthwork and compaction methods. These methods also provide the needed working platform for paving. Regardless of whether or not a subbase is constructed, shaping and compacting the subgrade is important. Porous subbases permit water to enter between the pavement and subgrade. The water must flow out of the pavement structure, transversely or longitudinally. Low spots will collect water that weakens the subgrade.

The article recommends specifying subbases under pavements only when they're needed to assure satisfactory performance. Adequately designed concrete pavements without subbases are suitable for most city streets, county roads, low-traffic-volume highways, and general aviation airport runways. Subbases aren't warranted for most concrete parking areas, sidewalks, bike paths, golf cart paths, driveways, and patios.