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Concrete Parking Lot Design

The basic idea in design is that parking lots are subjected to higher loads than floors but those loads are mostly static (or low speed) so flatness and smoothness are not as critical as for streets and highways. With a parking lot, a good subbase, the proper thickness, drainage, and traction are more important.

Parking lots are usually designed in accordance with ACI 330R, Guide for Design & Construction of Concrete Parking Lots, although the design method shown in that document is actually based on the Portland Cement Association's thickness design method. The nomograph shown here will yield a design thickness knowing the subgrade reaction (k), the expected loads (how heavy the vehicles will be), and the number of load repetitions expected over the 20-year pavement life.

Here are a few things to understand about design of concrete parking lots:

  • An important consideration is that there's more to the parking lot than just pavement. Parking lots include slabs, joints, curbs, light poles, and drainage facilities—and all of these need to work together.
  • For cars and light trucks, a 4-inch pavement is generally OK. For bigger delivery trucks, the pavement will need to be 5 or 6 inches thick. This is dependent on the subgrade, the total number of load repetitions, and the weight of the vehicles.
  • Parking lots are typically placed directly on the existing soil rather than on a compressed subgrade. Find more information on subbases and subgrades. The important thing is to get the compaction of the subgrade uniform so that some areas don't sink and crack the pavement.
  • Parking lots generally drain simply to the edge of the pavement or into gutters. Sometimes drains are located within the paving area. In either case, pavements should slope a minimum of 1% (1/8 inch/foot); 2% (1/4 inch/foot) is better; 6% is the maximum slope in areas where cars park. Slope of entrances to the parking lot should not exceed 8% to prevent cars from dragging.
  • Jointing in a concrete parking lot is no different than for any slab on grade. See a complete description of joints. Parking lots should be isolated from any buildings, drains, or light post foundations by installing isolation joints. Contraction joints should be continuous (not staggered), in square panels, and spaced at the intervals shown in the table below.
Spacing of contraction joints according to ACI 330
Pavement thickness, in. Maximum distance between joints, ft.
4 10
5 12.5
6 15

  • Reinforcement is generally not necessary in a concrete parking lot. If, for any reason, the contraction joints are spaced farther than recommended in the table, reinforcement might be specified to hold cracks together. Similarly, dowels at joints to transfer the vertical loads between panels are unnecessary unless very heavy loads are anticipated—as for heavy truck traffic.
  • The exception to the rule about no dowels or reinforcement is tie bars. To prevent the first section of slab from moving away from the parking lot, the first joint in from the edge of the pavement should be tied to the rest of the pavement with tie bars. Tie bars should be ½ inch in diameter, 24 inches long, and spaced at 30 inches on center.
  • The edges of parking lots should be thickened if cars will park close to the edge. Edges can be thickened by using an integral curb or by thickening the bottom (see diagram).
  • Parking lots can be given just about any exterior decorative treatment, although large lots would be expensive to stamp or texture. Integral color is a good option and may keep the lot looking a little cleaner.
  • Check ACI 330, Appendix D for parking space dimensions and the overall geometry of lots. Also don't forget to include handicap accessible spots according to ADA requirements (more info on ADA requirements)

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