Parking lots are paved areas intended for vehicle parking and can vary widely
in size, function and design. This page gives some general guidance when
designing at-grade parking lot pavements.
Figure 1: Large Parking Lot
Figure 2: Smaller Parking Lot
Assumed Traffic
Low speed automobiles and trucks. Typically, automobiles may travel
anywhere in the parking lot while heavy trucks travel to and from delivery and
pick-up points only. Thus, recommended parking lot structural design is divided
into two categories:
Light loads. Parking lot areas expected to experience little
or no truck traffic can be designed for car and light truck loads only.
Typically light load parking lot areas encompass the parking stalls and
parking stall access lanes. Truck traffic in these areas is limited to a few medium trucks and the occasional heavy truck
that may stray into the parking stall area after hours.
Heavy loads. Parking areas expected to experience significant
truck traffic should include a thicker pavement structure to accommodate these
heavier loads. Typically, heavy load parking lot areas encompass the
entrance and exit lanes from the parking lot, the area directly in front of a
store, truck loading/unloading areas and access routes to these areas. The recommended
minimum pavement thicknesses shown here are for typical parking lot heavy load
areas. If it is likely a particular parking lot will experience
excessive heavy loads, it should be designed using an
approved procedure.
Design Considerations In general, most owners like parking lots to be smooth, pedestrian
friendly and easily maintainable for a minimal cost. Specific
considerations are:
Heavy truck traffic. Most parking lots will experience at least some
heavy truck traffic
related to goods delivery and pick-up. If possible, heavy truck travel
should be restricted to designated areas and then only these areas should be
designed to accommodate heavy truck loading. Thus, pavements are made
thicker where heavy trucks are likely to travel (e.g., to and from the loading
dock) and are made thinner where only passenger cars and light trucks are
expected (e.g., in the individual parking stalls).
Design parking lot geometry to minimize handwork and maximize long straight
paver pulls. Handwork, the placement of HMA by hand shoveling and
raking, is more expensive and results in a rougher surface texture than pavement placed by a paver.
Although these areas should last just as long as the rest of the parking lot,
they often do not look as good and their roughness may be unpleasant to walk
on or roll a shopping cart across. In short, the less handwork the better.
Additionally, the highest quality pavement is generally placed by a continuously
moving paver that does not have to repeatedly stop and start. Specific
guidance follows:
Design parking plans should allow for long, uninterrupted, straight paver pulls.
Unusual, non-square geometry and odd-shaped islands make for short paver pulls
and create portions of the parking lot inaccessible to the paver that have to be
paved by hand.
Minimize the use of planter islands in the middle of the parking lot. Lots
of planter islands result in shorter paver pulls and more paver-inaccessible
areas requiring handwork. As an alternative, consider placing planters
around the parking lot perimeter or paving the parking lot then sawcutting out
areas to be built into planters.
Drains should follow straight lines so the paver can parallel these lines
during laydown. Since
only a fixed contour can be laid in one pass of an asphalt paver, designs
calling for a meandering gutter or valley require hand placing.
Drain lines that parallel the long
dimension of the parking lot allow for longer paver pulls.
Drainage.
Well-drained parking lots last significantly longer than poorly drained parking
lots. To ensure adequate drainage:
Design parking lots with a minimum slope of 2 percent (0.25 inches per foot).
Slopes less than this are difficult to construct and may not prevent pooling of
water during wet weather. It is extremely difficult to prevent pooling in
parking lots sloped less than 1 percent.
Where parking lot geometry necessitates hand placed
pavement, an increase in minimum slope to 4 percent (0.5 inches per foot)
should be considered. Fine grade control is more difficult with hand
placement than with machine placement.
Parking lots placed in cut areas are more susceptible to moisture problems.
When placing a parking lot in a large cut area cannot be avoided, take extra
care in developing a good drainage plan.
Consider using slightly more asphalt
binder than standard
WSDOT mixes. More asphalt binder (on the order of 0.5 percent more
than a standard WSDOT mix) will ease placement and help create a smooth,
pedestrian friendly surface. Typically, increased asphalt binder content
makes a pavement more susceptible to
rutting, however in
a parking lot area where few, if any, trucks are expected rutting is not a great
concern.
Construction Considerations
It is important to maintain adequate drainage during construction and keep
water away from the subgrade soil to be paved over. A saturated subgrade
soil will be weak and may not adequately support the overlying pavement
leading to depressions and
fatigue cracking.
The subgrade should be graded to the final desired parking lot elevation
minus the planned pavement thickness. It is difficult to make up grade
differences with pavement thickness alone. The subgrade should have a
maximum variance of 0.75 inches per 10 feet.
Where there is the possibility of recurrent vegetation growth, a quality
commercial grade herbicide should be used.
Generally, a
prime coat
underneath a parking lot is not warranted.
Maintenance Considerations
Once completed, parking lot pavements can last a long time. Generally,
loads are low so the primary modes of deterioration are subgrade failures and
aging effects such as
block cracking and
raveling.
Cracks should be
sealed
in a timely manner to prevent water infiltration into the subgrade through the
cracks.
A periodic application of a
fog seal or
slurry seal can prolong
parking lot pavement life and improve its appearance.
In Eastern Washington and areas where it frequently snows, planter islands
are difficult to locate underneath a layer of snow and are often damaged by
snowplows. Consider marking these islands or only building them on the
parking lot perimeter.
In Eastern Washington and areas where it frequently snows, consider
plowing snow completely off the pavement. Large snow piles from plowing
operations that sit on pavement surfaces will melt and may result in
water infiltration into the
subgrade.
Recommended References
Asphalt Institute. Full-Depth Asphalt Pavement for Parking Lots,
Service Stations and Driveways, 5th Edition. IS-091. Asphalt
Institute. Lexington, KY. (www.asphaltinstitute.org)
