Let it Rain……It’s Porous

With the unusual amount of rain the St. Louis has had this summer we thought we would take the opportunity to talk a bit about porous asphalt.Developed in the mid 1970’s porous asphalt pavement is slowly gaining popularity in the St. Louis area and nearby urban sprawls. Porous pavement offers owners, developers and planners another option for managing storm water. 

These pavements, used mostly for parking lots, allow water to drain through the pavement surface into the stone base and infiltrate into the soils below the pavement. The system is similar to a conventional asphalt parking lot however there are a few key changes.

watch this video to see porous pavement in action!

watch this video on porous pavement installation.

THE DESIGN: Porous vs. Conventional Asphalt Pavement

Porous Asphalt in itself is an open-graded mix design (minimal dust and fines in the mix). A “rock on rock” mix design allowing water to flow through the asphalt pores. Underneath this asphalt is a clean stone rock base. Clean stone consists of very little fines. It is in fact “clean”. As the water drains through the porous asphalt and into the stone bed, it slowly infiltrates into the soil. The stone bed is usually 18 to 36 inches while the porous asphalt is usually 3 to 4 inches in depth. See NAPA’s website for additional information on design.

Conventional asphalt pavement mixes have more fines in the designs making a “tighter” mix not allowing water to infiltrate into the system like a porous system. The rock base below also consists of a minus stone which has many fines in it. A conventional asphalt parking lot usually consists of 3 to 5 inches of hot mix asphalt and 6 to 10 inches of rock base.

THE CONSTRUCTION: Porous vs. Conventional Asphalt Pavement

Porous parking lots are quite simple in their construction. Porous pavements eliminate the need for sewers and drainage tie in’s, concrete curb barrier diverting water to the sewer system, as well as grading the parking lot to ensure proper drainage!  If the parking lot is flat- no problem. It will drain……into the earth below as it is designed to do! Porous asphalt also is a much more natural and environmentally friendly asphalt pavement system  by using infiltration directly into surrounding soils in lieu of draining into a sewer system.

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Conventional parking lots on the other hand must be built ensuring proper drainage (beginning in the grading and earth moving phase), a sewer tie in system (piping all water adequately to the local sewer utility), and a curb system on the perimeter of the parking lot help divert water to the sewer system.

THE COST: Porous vs. Conventional Asphalt Pavement

While the initial cost of materials (asphalt and rock) will be higher than conventional pavements (due to the additional thickness of rock and cost of materials versus conventional pavement materials) porous pavements can provide cost savings. Not only in the construction process of excavation/grading, sewer installation, and curb installation but also individuals should consider the amount paid to their local utility company in water shed fees over a 20 year time span for non permeable pavements. This truly adds up!

This is also a GREEN Pavement Technology promoting natural infiltration and improved local water quality! Now let it Rain!

If you would like more information on porous pavements please feel free to contact us or check out National Asphalt Pavement Association!

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