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Structural Design

For parking lots where most of the traffic will be passenger vehicles, the structural requirements are not significant. The material thicknesses are determined based on (1) water storage needs for the base aggregates and (2) minimum thickness requirements for the porous asphalt.


While the vast majority of projects constructed to date were designed to carry light automobile traffic only, there is substantial anecdotal experience supporting the use of porous asphalt for applications with trucks. In Oregon, open-graded cold mixes have been designed and built for farm-to-market applications for over 30 years. These pavements are generally 3 to 5 inches of open-graded cold mixes over dense aggregate, typically with a chip seal surface. They have performed very well (inch for inch, better than a dense mix would in the same application), even though based on modulus testing an engineer would probably assign a layer coefficient much lower than dense mix.


The Arizona DOT constructed a porous roadway in Chandler, Arizona, in 1986 and evaluated its structure. This pavement consists of 6 inches of an open-graded asphalt friction course (3/8-inch maximum aggregate size) over 6 inches of asphalt-treated permeable base over 8 inches of open-graded subbase (stone recharge bed). The pavement has performed well for more than 20 years.


There also is recent experience with the Pringle Creek subdivision in Salem, Oregon, in which the asphalt-treated permeable base handled heavy construction truck traffic with no distress.


Clearly the porous aggregate bases used with porous pavement structures perform very well both because they tend to be thick and because they are just as good wet as dry, unlike conventional dense aggregate bases. It is probable that the layer coefficient of the open-graded base rock is substantially better than dense aggregate when seasonal effects are considered.


NAPA's Structural Design Guidelines for Porous Asphalt Pavements (IS-140) provides guidelines for the use of AASTHO 93 Design Guide methodology when determining structural thicknesses for full-depth porous asphalt pavements. The book includes guidance on determining structural design input values, calculating minimum layer thicknesses, and limits on effective structural thickness. Design examples are included for full-depth porous asphalt pavement systems with and without an asphalt-treated permeable base, as well as a catalogue of required layer thicknesses for different traffic levels. To order, click here.