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Stormwater Rule Spurs Porous Pavement Research

Private research and development of new asphalt pavement products is being driven by stringent stormwater control requirements spelled out in ...

October 13, 2008

Private research and development of new asphalt pavement products is being driven by stringent stormwater control requirements spelled out in New Hampshire's Comprehensive Shoreland Protection Act (CSPA), which became effective July 1, 2008.

Issued by the state's Department of Environmental Services (DES), the new regulation limits the area of impervious surface to 20 to 30 percent of the total area of a building project within the DES-defined protected shoreland of New Hampshire's myriad rivers, streams and lakes. The protected shoreland extends up to 250 feet from a reference line at the edge of the water.

Impervious surfaces include paved driveways and walkways, and even the building itself, whether it's a private residence or a commercial project and its parking lot. This means fully 70 to 80 percent of a project's total area cannot produce stormwater runoff.

Porous Pavement Solution

"They (DES) want the water to go straight down — no more runoff, storm drains or catch basins," said Mary Wescott, quality control engineer for Pike Industries, a 125-year-old asphalt contractor/mix producer headquartered in Belmont, NH. Wescott is part of a team spearheading the company's research and development efforts to create new porous pavement products to meet CSPA's impervious surface area requirements. Much of the work is taking place in the company's lab at Belmont and at its asphalt and aggregate production facility in Hooksett, NH.

Pike has modeled its products on specifications outlined in a white paper on porous asphalt pavement and infiltration beds produced by the University of New Hampshire (UNH) Stormwater Center in Durham. In line with this, the producer apprises officials at the New Hampshire Department of Transportation of its progress with field tests of the specified product.

How It Works

Porous pavement (also referred to as pervious pavement) is an open-graded mix with high voids content achieved by using a smaller percentage of fines than dense-graded mix, and primarily just two sizes of coarse aggregate — in this case 1/2-inch and 3/8-inch fractured granite ledge. This formula yields a pavement mix with approximately 18-percent voids as opposed to about 4-percent voids in a conventional dense-graded mix, allowing water to percolate through the pavement and into underlying stone infiltration, or recharge, beds.

Stone beds provide underground storage and allow water to slowly infiltrate into soil below. This has the effect of recharging groundwater, improving water quality, and eliminating the need for detention basins that take up valuable surface real estate.

UNH's specs call for porous pavement to be installed in a single 4-inch lift over the stone beds, an engineered system consisting of several variable-thickness courses of different stone. Among these, from top to bottom, are a 4-inch-thick choker course of 3/4-inch crushed stone; a minimum 12-inch-thick filter course of bank run gravel; a 3-inch-thick filter blanket of 3/8-inch pea gravel; and a reservoir course comprised of a minimum 4-inch-thick layer of 3/4-quarter-inch crushed stone plus 4- to 6-inch-diameter perforated subdrains.

The Binder Is Key

The success of a porous pavement relies on the ability of asphalt binder to hold aggregate together and ensure structural integrity over wide variations in temperatures, while maintaining sufficient voids to permit the passage of water. To achieve this, the specs require the use of 76-22 performance-graded asphalt binder. This designation means asphalt must perform satisfactorily under anticipated traffic throughout a range of plus 76 to minus 22 degrees Centigrade (plus 169 degrees and minus 8 degrees Fahrenheit).

It also indicates the binder is modified with an additive. According to a rule of thumb used by engineers and asphalt producers, when the difference between high and low temperatures exceeds 90 degrees, an additive is called for. Often, it is a polymer.

Polymer modifiers expand the effective temperature range of the binder, help prevent stripping of asphalt from aggregate, and improve resistance to rutting caused by high temperatures and thermal cracking generated by low temperatures.

In-Plant Modifying Cuts Costs

One of the challenges of using PG 76-22 binder in porous pavement mix is its cost, according to Wescott. She pointed out that PG 64-28 is the binder usually required by the NHDOT and thus is readily available.

"We truck it in to our plants every day when we are in production," she said. However, PG 76-22 has to be trucked in from far away — and in quantities of 9,000 gallons at a time.

"We don't have the tanks to store it, and it has to be agitated and kept at a high temperature." All this can cost a fortune, she added.

Consequently, as they planned their experiments with porous pavement last winter, the research team decided to see if they could produce PG 76-22 binder by adding polymer to PG 64-28 at the plant. The economic benefits of being able to produce various binders in small quantities as needed were enormous.

First they needed to determine how much polymer to put in and how to add it to the pugmill of their H&B plant to make porous pavement mix. And they needed to find out how the mix handled during laydown.

Injecting The Latex

Pike's laboratory team worked all through the winter of 2007/2008, blending various quantities of a polymer, SBR (styrene butadiene rubber) latex, with PG 64-28 asphalt binder until they created a product that met the PG 76-22 specifications during testing. Next they needed a mechanism to introduce the latex into the asphalt at the plant.

A pump, metering and injection system for this was provided by Fred Mello, a representative of Rub-R-Road, whose company supplies SBR latex manufactured by BASF Corporation to asphalt mix producers. The relatively compact unit fits on a pallet and is easily transported.

The final step of this experiment was to actually produce porous pavement mix in the H&B plant and lay the material down in a test area. A location at the Hooksett facility subjected to low-volume traffic had been selected for the trial, which took place several weeks before the new stormwater rules went into effect. Mix was produced at their plant, with the latex injection system working without incident as designed.

Laying The Mix

Pike's heavy and highway construction crews had previously excavated the site and installed stone beds. Employing a Blaw Knox 4410 track paver, the paving crew laid the porous mix in a single, 4-inch lift. It was not an easy task, said Wescott, pointing out that the mix was fairly sticky, so handling it was a challenge. But the crew installed the mix successfully, and compacted it with a 530 Hyster roller, a BOMAG double-drum vibratory roller and a CAT 434D finish roller.

Since that time Wescott has routinely monitored the pavement. She said it is wearing well and continues to allow water to pass through in spite of an accumulation of silt on the surface in one section. She is sharing knowledge gained by this experiment with NHDOT personnel, in hopes of seeing the agency develop performance specifications for porous pavement.

Why Performance Specs?

Performance specs are important, since one of the drawbacks to injecting latex directly into the asphalt during plant production is that it's impossible to prove that PG 76-22 was actually created in the process.

Typical pavement specifications require an asphalt mix producer to submit samples of component materials to the appropriate transportation agency before being run through the pugmill. Later, cores of the installed pavement are taken, analyzed and compared to the design specs. Analysis involves using a solvent to remove asphalt and performing a sieve analysis to determine gradation — the quantities of different sizes of aggregate.

The amount of asphalt binder that was put into the mix can be determined, but its characteristics cannot. While it can be proven in the lab that X grams of SBR latex pre-mixed with Y grams of PG 64-28 binder will create PG 76-22 binder, in an operating plant there's no way to prove latex changed the binder from the PG 64-28 originally fed to the plant into PG 76-22 as required by laboratory mix design. Therefore, there is a critical need for performance specs for porous pavement.

But none are currently available, and won't be until there is sufficient evidence that a porous pavement mix created by injecting latex directly into asphalt binder during plant production produces the desired pavement performance over time.

"We are trying to build a history," said Wescott. "Results do count."

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