Pervious concrete has been around for a while. You’re likely familiar with the environmental benefits and have heard its catch phrase “when it rains it drains”. Yet with all the information available about pervious concrete, it’s a “brand new” product the Virginia Department of Transportation (VDOT) is just beginning to explore as an alternative to traditional asphalt.

What triggered this shift? Overcrowding at the Park & Ride at Exit 140 in Salem. The existing lot contained less that 100 spaces on 2.5 acres of land, but there was no room for expansion given the need for storm water management that traditional asphalt would require. Enter pervious concrete. On paper, the product seemed to be a great fit for what needed to be accomplished. It would allow for additional parking spaces within the existing footprint without necessitating additional acreage for storm water management.

The first thing the team set out to do was establish criteria for testing acceptance and evaluation of cores for strength, hardened density and voids. The Pervious Concrete Test Slab Acceptance Criteria they outlined are as follows:

• Fresh density and void content (ASTM C 1688) = Density of 125 pcf to 140 pcf, voids of 15% to 25%
• Infiltration Rate (ASTM C 1701) = Greater than 100 inches per hour
• Hardened density and void content (ASTM C 1754) = Density of +/- 5 pcf of approved mix design, voids +/- 4% of approved mix design
• Core length (ASTM C 174) = -3/8 in. to +1.5 in. of design thickness
• Average compressive strength at 28 days (ASTM C 39) = Greater than 2,000 psi

One true test slab was created on No. 57 stone to assess the pervious concrete against the established criteria and determine which method of cross-rolling worked best. The team also utilized a “pizza cutter” for joints. While the test slab yielded some positive results, it didn’t meet the guidelines for hardened density and void content, or 7-day average compressive strength.

As the team moved toward placing production slabs, additional adjustments were made along the way based on test results. These included:

• Replacing the cross-rollers with a “heavy roller”
• Variations on the strips used on the motorized screed
• A slightly altered mix design to allow for more sand and less voids (new target of 19.5%)
• Exploring a pan float for compaction purposes

While not every acceptance criteria was met for the various production test slabs, in the end the team felt they landed on a mix design and placement plan that achieved their goal: optimum balance of durability and permeability. Given that the Park & Ride is relatively close to the district office in that region, the VDOT team is keeping a close eye on the performance of the pervious concrete and has implemented a vigorous maintenance program to keep the lot clean.

One additional fact that can’t be ignored? This project was completed shortly after another Park & Ride was finished in Christiansburg. Both lots contain 250 spaces. However, the lot in Christiansburg used traditional asphalt with a detention pond and took four acres of land to construct, versus the 2.5 acres in Salem.

If you’re interested in learning more about this project, we encourage you to register for the seminar VRMCA and its Southwest Virginia Concrete Advisory Council is holding on Thursday, June 7th. More information – including how to register – can be found here.