A New Era For Permeable Pavements

Road Science

A New Era For Permeable Pavements

Today’s next-generation permeable pavements are
safer, quieter, and more durable.

Today’s next-generation permeable, drainable pavements comprise an idea that holds water — because they don’t hold water.

These drainable asphalt pavements — better known as open-graded friction courses — offer state transportation departments a better-performing, driver-friendly pavement, but at a 30 to 40% cost premium over conventional asphalt mixes.

They feature an “open” aggregate structure (without fines) in which larger-sized aggregate is held in place by polymer-modified and fiber-modified Superpave performance-graded liquid asphalts. This open structure of 15% or more voids allows water to drain right through the driving or friction course to an impervious intermediate course below, and out into roadside ditches.

The result is the near-complete elimination of tire spray and hydroplaning, making a safer pavement.

Also, because noise generated at the tire/pavement interface is attenuated within the spaces between the aggregate, they are significantly quieter pavements. The noise reduction can be on the order of 3 to 5 decibels, representing at least a 30% decrease in volume. And the frequencies of tire whine are reduced as well, producing a less annoying sound.

As a result of these virtues, open-graded friction courses are being used and recommended.

In a quest for quieter pavements, the Arizona Department of Transportation is launching a three-year program to surface all continuously reinforced concrete expressways in the Phoenix area with asphalt-rubber open-graded asphalt pavement.

Supporting noise reduction in pavements are the Quiet Pavement Pilot Programs of the Federal Highway Administration. The Arizona and California DOTs are developing proposals for these programs, reported National Asphalt Pavement Association Vice President Dave Newcomb, P.E.

Although the FHWA programs do not focus on specific pavement types, Arizona currently uses asphalt- rubber open-graded pavement as a quiet pavement, and California will most likely use conventional OGFCs for the pilot program, NAPA said.

New research released from the Texas Department of Transportation in February supports the use of open-graded asphalt pavements in that state. There, a recent asphalt-rubber “porous friction course” overlay of a concrete interstate in San Antonio substantially reduced noise levels, improved its ride quality by 61%, and increased the skid resistance by 200%, according to the state.

On the downside, in addition to their cost premium, OGFC pavements over a period of years can clog with roadway fines, inhibiting their drainage abilities. In Europe, self-propelled machines have been developed that force water at high pressure into the pavement, forcing out fines, and vacuuming the fines and water in a single pass. These machines have yet to be evaluated in the United States.

Instead, experts recommend that OGFCs be used on high-volume, high-speed roadways such as interstate highways, where the suctioning action of the tires on the pavement tend to pull detritus from the porous lift. Open-graded pavements on lower-volume, slower-trafficked local roads have been less successful.

Classic OGFC

A conventional open-graded friction course is a layer of asphalt that incorporates a skeleton of uniform aggregate size with a minimum of fines. In the past, these pavements typically had a void content as low as 12% and as high as 15 or 16%.

Most open-graded friction courses are 0.75-inch thick, and never thicker than 2 inches. The OGFC should be elevated above the shoulder, as the water drains onto the shoulder and hence to a roadside ditch.

In dense-graded asphalt mixtures, a thin film of asphalt and compaction are required to keep the mix glued together. The final density of such dense-graded mixes directly reflects the strength and durability of the mix.

But the open graded mix uses a grading of mostly 0.375-inch stone, the idea being to build up a thick film of asphalt on the stone without the mixture draining or flushing. The asphalt film viscosity is usually four to six times that of a dense-graded mix.

An open-graded asphalt pavement should not be used over an existing surface that is uneven, reports the FHWA. “An uneven surface should be milled or leveled and all cracks sealed prior to OGFC application. It should also be rolled immediately following placement,” FHWA said.

New generation pavements

The new generation of open-graded friction-course pavements that are being built in the U.S. and Europe have considerably higher air void contents than before, up in the range of 17 to 22%. They are more open, with more voids.

Today’s OGFCs are polymer-modified and include spun mineral or cellulose fibers. Use of polymer modifier or fiber does not preclude the other; instead, each complements the other in the liquid asphalt.

The polymer modifier stiffens the asphalt binder, while adding flexibility, helping it resist raveling of the top layer of aggregate.

Spun mineral fibers such as those manufactured by Fibrox Technology Ltd. are 6 microns wide (almost microscopic), and about 0.25-inch long. They constitute 0.3 to 0.4% of the liquid asphalt by volume.

The fibers disperse evenly, and despite their tiny size, overlap and form a mat, which keeps the liquid asphalt from draining to the bottom of the layer before it cools, not unlike the action of gauze in keeping a wound from seeping.

On the other hand, Arizona’s asphalt rubber-modified open-graded mix differs from others in that it is dense below the initial surface, thus doesn’t provide all the drainage capability of full-depth OGFCs. However, its noise attenuation and durability are superb.

“Where other OGFCs without rubber have a very high air void content, the Arizona DOT spec fills a lot of those voids with binder,” said Doug Carlson, Rubber Pavements Association, Tempe, Arizona. “It is open in relation to gradation, but not in density of pavement. The surface has an open texture, but the lift does not have interconnected voids below.”

Uphill battle

Despite their benefits, open-graded friction course pavements have been fighting an uphill battle in the United States. That’s because they suffered conspicuous failures when first implemented in this country in the 1970s and 1980s.

These early attempts failed because the liquid asphalt was not stiff enough, creating drain-down of asphalt into dense “fat” spots, while encouraging raveling of the top layer of aggregate.

Also, in an attempt to fight drain-down in earlier OGFCs, mix temperatures may have been too low for successful paving. While combating drain-down, aggregates may not have been heated enough to drive off moisture, thus contributing to stripping of asphalt from aggregate.

Now, modified asphalt mixes are mixed at higher temperatures, thus more efficiently drying the aggregate in the drum and improving adhesion.

In addition to those problems, some northern states thought that open-graded pavements were more susceptible to icing, frost-heave damage, and clogging with deicing salt.

As a result, permeable pavements got a bad rap.

But that was then. The new technology is prompting another look, as reported by Ken Kandhal and Rajib Mallick of the National Center for Asphalt Technology at Auburn University, in their 1998 survey, Open Graded Asphalt Friction Course: State of the Practice.

“While many transportation agencies have reported good performance, many others have stopped using OGFC due to poor performance,” they said.

The 1998 survey showed improvements in open-graded pavements since their introduction in the 1950s. “These improvements have been achieved with the help of good design and construction practices,” they wrote. “Half of the agencies surveyed in this study indicated good experience with OGFCs.”

More than 70% of the agencies that use open-graded surface courses reported service life of eight or more years. About 80% of the agencies using such pavements have standard specifications for design and construction.

Their secret to longer-life success is polymer modification of the asphalt binder. “A vast majority of agencies report good experience using polymer-modified asphalt binders,” Kandhal and Mallick said.

The entire report may be downloaded from NCAT at http://www.eng.auburn.edu/center/ncat/reports/rep98-7.pdf.

Expanding interest

Today, as Kandhal and Mallick observed, interest is once again growing in open-graded friction courses.

“There’s more interest now, based on recent projects states have done,” said Don Watson, P.E., civil engineer, National Center for Asphalt Technology at Auburn University. “Now that we’ve used polymer-modified asphalts, and fiber stabilizers to reinforce the asphalt film to minimize drain-down, more and more states are taking interest in OGFCs.”

Now, Georgia, Florida, and Alabama require open-graded friction courses to be used on all interstate projects. And an ongoing, multi-year, pooled-fund study by 13 states will expand interest in the pavement across the country.

While typecast as only of interest to Sunbelt states — due to OGFCs’ perceived vulnerability to icing — interest now is growing in frost-belt states as well.

Northern-tier states are among those participating in the 13-state pooled-fund study. “In the past, northern states were reluctant to look at OGFCs because of the snow and ice removal issues and the amount of water draining through the pavement,” Watson said. “Now they’re taking another look, due to safety benefits and the improved performance of the new mixes.”

Georgia, Florida, South Carolina, Texas, Arizona, Colorado, Utah, Michigan, New Jersey, Rhode Island, and Vermont are among those joining lead state Alabama in the study.

In Oregon, use of open-graded pavements has declined slightly from a very active level, in an effort to identify where they are best applied. “We went through a period in which we put it everywhere,” said Jim Huddleston, executive director, Asphalt Pavement Association of Oregon. “It’s not perfect for every application. They’re homing in on where the best use will be.”

Adapting to Superpave tests

“With the modifications made to these mixes in the last few years, significant improvements in mix performance have already been noted,” NCAT’s Watson said. “Agencies which have used this mix in the past and experienced problems should reconsider the possibility of using this modified mix on high-volume traffic facilities.”

Much leadership on development of next-generation open-graded asphalt pavements suitable for use in the United States is coming from Watson and his colleagues at NCAT. In 2000, NCAT research led to a mix design procedure for this new type of OGFC.

That work evaluated different gradations and additives to recommend a mix design process for OGFCs. But it involved only one aggregate source.

“NCAT is in the process of refining this mix design procedure to make sure it’s applicable to other aggregate types used in surface mixes throughout the U.S.,” Watson said. “States have expressed a desire for an OGFC mix design that could be adapted to Superpave technology, and replace the modified Marshall method used in the past.” Now, NCAT is verifying the use of Superpave test procedures for OGFC mix design testing.

About Georgia’s mix

Georgia DOT’s 0.5-inch standard mix has been used statewide since 1993 . It is composed of aggregate, polymer-modified asphalt cement, stabilizing fibers, and hydrated lime.

The inclusion of stiffer polymer-modified asphalt cement gives the mix greater film thickness, which safeguards against the weathering problems experienced by asphalt cements in earlier mixes.

Mineral fiber has also been added, typically 0.4% of the total mix. Georgia adds hydrated lime as an anti-stripping agent.

In 1992, Georgia specifications were changed to require a coarser gradation. The coarser gradation enhances permeability and resistance to rutting.

Georgia DOT primarily uses two polymers — styrene butadiene and styrene butadiene styrene — to modify asphalt cements used in open-graded mixes. Since the implementation of Superpave binder grading, empirical tests have been dropped and Superpave PG 76-22 binder is now used. Other states are using SBR (latex modifier) and asphalt rubber.

In Georgia, the conventional OGFC was placed at very low temperatures (110 to 120 degrees C, or 230 to 248 degrees F) because of excessive asphalt cement drain-down during production and hauling. The modified open-graded asphalt can now be produced at much higher temperatures than conventional OGFC without drain-down problems because of the addition of polymers and fibers. Typical mix temperatures now range from 160 to 170 degrees C (320 to 338 degrees F).

While the standard open-grade friction course had a typical service life of eight years, the modified OGFC is expected to last 10 to 12 years, Georgia reported. Based on annualized costs, the modified pavement would become a cost-effective alternative if it lasted just 19 months longer than the conventional mix.

Georgia approves Fibrox for key pavements

The Georgia Department of Transportation has approved Fibrox 300 mineral fiber for use as an asphalt cement stabilizing additive to open-graded friction course and stone matrix asphalt mixes.

Like open-graded asphalt, SMA is a gap-graded mix containing a higher percentage of coarse aggregate than that of conventional asphalt. For both mixes, a key ingredient is mineral fiber, which assists in stabilizing the high asphalt

In this application, Fibrox 300 stabilizes and reduces “drain-down” of the high content asphalt mix. Additionally, Fibrox 300 will increase the film thickness and surface tension, which enhances the wear and load bearing capacity of the road.

More information is available from Fibrox Technology Ltd., 613-938-9921, or visit http://www.fibrox.com.

NAPA offers new technical guidance

In late 2002 the National Asphalt Pavement Association released a new technical document, Design, Construction, and Maintenance of Open-Graded Asphalt Friction Courses. This publication is a valuable reference for pavement designers and specifiers, plant foremen and operators, and paving crews.

Numerous states currently using open-graded surface pavements have experienced excellent performance in terms of safety and durability. New-generation OGFCs are more open with higher permeability than those previously used in the U.S. The mix design procedure in the publication addresses the concerns of northern-tier states with cold climates by including abrasion tests on new and aged mixtures, and subjecting the mix to freeze-thaw cycles.

The 22-page booklet can be purchased for $12 for government agency/non-profits, or $16 for all others, plus shipping. NAPA’s toll-free order line is 888-468-6499.

Cement-Treated Permeable Bases Prolong Pavement Life

Not to be confused with open-graded friction courses, cement-treated permeable bases for roadways are one way to prolong the life of portland cement concrete pavements.

These bases are formed from a type of concrete that is an agglomeration of coarse aggregates coated with a paste consisting of cement, water, and perhaps fly ash or other pozzolan.

These permeable bases can minimize pavement ills caused by pumping, faulting, and cracking. Use of a permeable base may provide up to a 70% increase in the service life of a concrete pavement.

This drainable, permeable concrete base should be constructed with good quality, well-graded aggregate mixed with up to 6% cement. It is roller compacted and often used in conjunction with drains imbedded along pavement edges (see Making Edge Drains Work, January 2003, pages 42-48). Its higher initial cost compared to a standard base is balanced by long-term life cycle cost benefits.

Creating quiet roads

Open-graded friction courses are getting closer attention from state transportation departments because of their ability to attenuate pavement noise. But an open-graded pavement is just one part of a successful noise reduction strategy along highways, roads, and streets.

The quietest roadways are obtained when these road surfaces are used in conjunction with noise barriers (sound walls), effective landscaping and buffers, and effective speed control. Future use of quieter tires now being developed also will help the problem.

Transportation departments are spending more on noise mitigation. On new projects, sound walls that were once considered an extravagance now are standard operating procedure.

Any contribution that the pavement can make to reduce noise can pay off for local governments and road agencies, because sound walls can be costly. Noise barriers can cost on the order of $1 to $1.5 million per linear mile.

Proof that OGFCs are quieter

In their 1998 paper Comparative Field Measurements of Tire Pavement Noise of Selected Texas Pavements, McNerney, Landsbeger, Turen, and Pandelides presented the results of tests that could be used in future quiet pavement technology research.

Using on-board and roadside microphones, sound measurements from a variety of typical Texas pavements were made. This “apples-to-apples” comparison involved sound from a single type of tire, at a consistent speed of 62 miles per hour, with consistent ambient conditions.

Although the intent of the study was not to reach conclusions on noise generated by pavement type, the data showed that open graded friction courses were the quietest surfaces tested. An aged, proprietary OGFC — Novachip, by Koch Pavement Solutions — produced the least noise. All of the top five quietest pavement types were HMA-based, as were seven of the top nine.

In the U.K., it was reported that porous asphalt roads have been measured to reduce noise by four decibels in dry conditions and up to eight in wet conditions. That’s equal to halving the volume of traffic or doubling the distance between the listener and the road.

A 1997 report to the California Department of Transportation on mitigation of noise along existing S.R. 85 in Santa Clara County said reducing of speed by 10 miles per hour would be the most cost-effective solution, reducing noise by about 3 decibels.

But an open-graded surface course overlay would work better, said consultants Acentech Inc. “The resurfacing of the roadway [with OGFC] would reduce the Route 85 traffic noise by over 4 decibels and benefit all areas where Route 85 traffic is audible.”

Reprinted from Better Roads Magazine
April 2003