Open-graded mixes: better the second time around

With asphalt modifiers providing stability, noise-suppressing open-graded mixes get the applause they deserve after an initial mixed response.

Open-graded asphalt friction courses (OGFCs) — also known as “popcorn” or porous asphalt — are getting a second look by pavement owners, designers and builders. Bolstered by new technologies (such as the use of the newest polymer modifiers) that lead to a more durable asphalt binder under the unique circumstances of the open-graded design, the open-graded friction courses of today are winning rave reviews from motorists.

Highway users benefit from fast drainage of water from the driving surface — as well as the virtual elimination of tire spray and hydroplaning — that the OGFC provides. As water is the all-time, long-standing enemy of pavements, the highway itself benefits from the quick drainage of water, right through the top layer of asphalt, that the OGFC allows.

And for long-suffering neighbors of urban expressways, OGFCs provide immediate reduction in pavement noise, because of the attenuating effect that the open-graded structure of the asphalt layer provides to the sound energy generated at the tire/pavement interface.

Research in the United States and overseas shows OGFCs provide instant reduction in annoying tire/pavement noise by as much as 5 decibels (dBA), although the effect may also diminish with time.

But these benefits can come at a higher price, with OGFCs costing up to 35 percent more per ton than same-sized conventional asphalt pavements.

However, since OGFCs are lighter in weight than conventional mixes, a ton of material is able to cover more square yards of pavement surface.

But even the cost disadvantage is far outweighed when long-term life-cycle costing is used, both in terms of reduced subsequent maintenance and in reduced delay costs to highway users during maintenance operations.

And despite the fact that some research indicates that OGFCs may complicate winter snow and ice control, with higher levels of salt required, Oregon — a leader in OGFC research and use — has found that use of OGFCs does not preclude use of sand as friction material for winter driving.

UNIFORM AGGREGATE, MINIMUM FINES

Just what are open-graded friction courses and so-called “porous” asphalts? Both terms describe layers of asphalt that incorporate a skeleton of uniform aggregate size with a minimum of fines.

“Typically, the open-graded friction courses that we’ve built in the past have a void content as low as 12 percent and as high as 15 percent to 16 percent,” says Dale Decker, vice president, research and technology, of the National Asphalt Pavement Association (NAPA).

“But the new generation of OGFCs and OGFC pavements that they’re building in Europe have considerably higher air void contents, up in the range of 17 percent to 22 percent. They are more open, with more voids,” he says.

And the voids of an OGFC — along with its stone-on-stone skeleton — are what give this type of mix its popular attributes.

The porous nature of the OGFC allows immediate drainage of water from the pavement surface.

Much like the stone matrix asphalt (SMA) mixes imported from Europe in recent years, the stone-on-stone structure can hold up better to heavy traffic, especially trucks, than a variety of other mixes.

The texture of the larger aggregate without fines provides better traction (early examples were the “popcorn” mixes of the 1970s).

As well, the voids capture sound energy as tires roll over the pavement with accompanying reduction in surface noise.

HIGHER INITIAL COSTS EQUAL LONGER LIFE

The higher costs for enhanced mixes, such as OGFCs and SMA, are balanced by long-term lower costs. “In Georgia, typically it will run 30 percent to 35 percent more in increased initial per-ton cost,” says Ronald Collins, state materials and research engineer for the Georgia Department of Transportation.

“We do 30-year life-cycle cost analyses, and we calculate what the annualized yearly costs will be for modified systems versus conventional systems. We’re finding that the extended life we get from SMA and porous mixes is well worthwhile,” he says. “It is much cheaper to go with the higher cost initially over a 30-year life cycle.

“With conventional mixes over 30 years, we’re looking at three rehabilitation cycles. On the `supermixes’ we’re talking about two rehabilitation cycles. And when you factor in user costs and traffic delays, it’s a much cheaper buy,” Collins says.

“We think we’re making a break-through,” he adds. “We’re looking at the whole life-cycle costs of a system. We feel we’re spending our money wisely in protecting our investment. We want to do it right the first time, even though we may have to pay a little more for it.”

And the drivers approve, to the point of actually contacting public officials to compliment them. “We probably get more calls in that regard than for any single thing we do,” Collins says.

These comments relate mostly to the condition of the pavement during rain. “They’ll say, `There was no standing water, no backspray, no backsplash, good visibility.’ Those are the kind of things that the public sees.”

OREGON’S TYPE F MIX

Oregon continues to be a leader in use of OGFC in the United States. The Oregon Department of Transportation used the popcorn mix in the 1970s, half-inch minus aggregate size placed one-and-a-half-inches thick or thinner, as a friction course.

As with other states, early disappointments were followed by a reconsideration of the mixes.

“In 1979, we began experimenting with a coarser, thicker product,” says Jim Huddleston, executive director of the Asphalt Pavement Association (APA) of Oregon, located in Salem.

Oregon’s Type F mix is similar to the European porous asphalts, which typically use three-quarter-inch minus aggregate placed in thicker lifts.

Oregon will place the Type F mix two inches thick, as opposed to the thinner popcorn mixes, using a three-quarter-inch minus aggregate, instead of the half-inch or three-eights minus and will use void ranges from 14 percent to 18 percent.

“We’ve found that mix performs much better than the old popcorn mix — it’s coarser, with larger stone, and thicker,” Huddleston says.

“I believe that because it’s thicker and larger, there is a certain degree of interlock that affords it more durability and stability and better drainage characteristics than thinner OGFCs.

“It can handle more rain, and still reduce splash and spray, while getting the same kind of frictional characteristics. We use it everywhere.”

For example, OGFCs are the specified surface course for interstates throughout Oregon. Moreover, DOT use of OGFCs throughout Oregon has trickled down to county jurisdictions.

“Several counties are specifying open-graded,” Huddleston says. “They like them a lot. The mixes tend to be more forgiving.”

NOISE REDUCTION A PLUS

In addition to OGFCs’ high drainage ability — combined with minimized spray, hydroplaning, less glare at night and improved skid resistance — the material helps to attenuate highway noise. This capability has been studied extensively in Europe, where the environmental impact of highways in high-density urban environments has been under extremely close scrutiny for years, as it has in the United States.

New research from the Nordic Road & Transport Research journal indicates immediate reduction of noise emissions by 3 decibels (dBA) to 5 dBA when an optimum drainage asphalt with air voids of 22 percent to 23 percent is employed.

“This is a considerable reduction in noise, and it is comparable to a traffic reduction of 50 percent, or a 100 percent increase in the protective distance from the road,” according to the researchers.

Oregon State University has conducted research into the sound-attenuating characteristics of OGFCs and found noise-reducing attributes consistent with other research. “They found up to 4 dBA reduction in roadside noise,” Huddleston says.

He adds that the research found the OGFC pavements were producing noise at different frequencies than those produced by conventional dense HMA mixes, and this caused bystanders to assume that noise levels had dropped, even when the instruments did not indicate a relative reduction in noise.

Conversely, instruments can measure a decline in noise, but humans may not perceive it. The subjectivity of human perception of noise and dBA levels is a continuing challenge to acoustic researchers.

A project under construction in 1996 in Bend, in central Oregon, will be a full-depth OGFC pavement in an urban environment, which the state asphalt industry feels will prove the benefits of OGFC noise-reducing qualities in a city.

“Research on noise-attenuating characteristics of full-depth indicates a reduction of as high as 8 dBA or 9 dBA,” Huddleston says.

In general, the clogging of the drainage structure of the porous asphalt will lead to a reduction in its drainage ability and in its ability to attenuate sound.

OGFCs can reduce some of the noise that traffic produces. A reduction in up to 3 dBA of noise can be expected from the tire/pavement interface, although the pavement surface itself cannot reduce noise that comes from engines.

The perception of the reduction of noise, and its measurement in dBA, is a subjective matter. “The decibel is a dimensionless unit that combines the magnitude question of how much energy is involved with the human response,” Cohn says. “We’re concerned more about noise, as opposed to pure sound energy, because of how noise affects us as humans.”

Each dBA represents a tenfold increase in energy from the unit below it. “That’s true for the energy, but for the human response, it’s different,” Cohn says. “A 10-dBA increase basically is a doubling of loudness in human response in which a listener can say that it was twice as loud as the preceding sound. Three dBA is the threshold of perception of change. You have to crank something up 3 dBA to really tell the difference.”

EUROPEAN DESIGNS GIVE BOOST

Even as Americans were wrestling with OGFCs, trying to refine mixes to retain their pluses while minimizing the minuses, European researchers were enthusiastically fine-tuning porous asphalt pavements to get the performance they desired.

Much of this research work was done not by government road agencies, but by contractors.

Unlike the situation in the United States, there is a close working relationship between contractor and government road agency with centralization of authority in a national road agency (instead of 50 different state DOTs, each with its own specs), and much fewer, but more influential, vertically organized road contractors (unlike the proliferation of smaller contractors in the United States).

Quite often, the European contractor develops his own proprietary techniques and mixes, which are placed and warranted by the contractor for the road agency, but without the oversight and inspection by the road agency that is common in the United States. This tradition — which could even be construed as illegal in the United States — nonetheless has been an incubator for new technologies that have been of benefit to the U.S. road users, such as stone matrix asphalt (SMA), proprietary modified asphalt cements and perfected porous asphalt pavements.

The European practice in the early ’90s was to use porous mixes as surface courses. “We found they were not too much different from the conventional open-graded mixes we were using in Georgia,” Collins says.

In fact, Georgia’s OGFC contains voids up to 20 percent, similar to the European porous pavement.

“The gradation was coarser, and they were using a larger top-size aggregate in it. Consequently, they had to place it in a thicker lift.

“The Europeans were using additives and modifiers in the asphalt in order to achieve thick film coatings,” Collins says.

“They were trying to get higher AC contents in the mix. When you do that, the AC has a tendency to drain off the aggregate, thereby creating the potential for flushing, or `fat’ spots in the pavement.

“When we looked at the European experience with fibers and polymers and put down some test sections like those, we really liked what we saw,” he says. “We did not have the fat spots. We were able to get a thicker film coating, and we knew that would increase durability.

“So we took our present gradation for open-graded mixes and coarsened them up a bit. We put the polymers and fibers in it, and it has become the mix that we now use on our high traffic volume roads,” he says.

Georgia now uses open-graded mixes on all its interstate hot-mix asphalt (HMA) pavements. Depth of placement depends on the distress of the existing pavement.

“If we’re overlaying a portland cement concrete (PCC) pavement, we’ll use two inches of coarse SMA mix, one-and-a-half inches of fine SMA and three-quarters of an inch in OGFC,” Collins says. “We have a substantial pavement cross-section. But it is less thick than what we would do with out conventional mix.

“We’d be using five inches to six inches of mix for a conventional overlay. The SMA is so much more stable and more resistant to rutting than conventional mixes that we don’t feel we have to use as thick an application,” he says.

If Georgia were overlaying an asphalt pavement, it might put down one-and-a-half inches of fine-graded SMA and three-quarters of an inch of OGFC.

“It has to be customized for each job,” Collins says. “All jobs are not alike.”

Open-graded mixes, European-style, basically have the same coarse aggregate skeleton as stone matrix asphalt, but without the fines.

SMA mix is an impermeable mix that is resistant to the intrusion of water. Mineral filler and fine aggregate tends to plug up voids in SMA, while in an open-graded or porous mix, those fines are taken out.

In addition, mineral or cellulose fiber may be used to prevent migration of liquid asphalt.

“In Georgia, we use the polymer modifiers [such as styrene-butadiene-styene] and the fibers in open-graded mixes, just like we do the SMA mixes, but we take all the fines out because we want at least 20 percent voids in the open-graded mixes to open channels so the water can percolate down through it,” Collins says.

After an initial period of testing in the 1970s — much of it driven by the Federal Highway Administration — disappointment with the performance of OGFCs led to a movement away from the mixes.

“The term open-graded friction course is used mostly in the United States,” Collins says. “The FHWA pushed open-graded mixes a lot to the states in the 1960s and ’70s, and there were some problems with those mixes.

“Many of the states stopped using them or put a moratorium on their use because of stripping problems in the dense-graded layers underneath.

“Open-graded mixes are not new to us in Georgia,” he says. “We started using open-graded mixes in 1970, so they’ve been around for some time. But we did not get the performance out of them that we wanted. However, the public likes open-graded mixes very much. In particular, drivers like the drainage characteristics.”

Specifically, the extremely hot summer of 1980 accentuated the pavement distresses that gave road agencies reason to reconsider use of OGFCs.

Collins reports that after that summer, a task force conducted a comprehensive pavement damage survey, in which significant cases of rutting, shoving, texture loss, blisters, slippage and stripping were found in most OGFC mixes, but most dramatically, with the mix immediately beneath the OGFC, where delamination was occurring.

“Raveling in OGFC was also a problem, since the presence of moisture and air accelerated the oxidation, or aging, process,” Collins says.

Early problems also included raveling in the top layers. “OGFC mixes had gotten a bad reputation,” says Gerald Huber, associate director of research for The Heritage Research Group, Indianapolis, “primarily because of a failure mode of very rapid loss of material. It can wear away from the surface, down.

“And when this happens, it will happen very rapidly. Within the course of one year, what looks like a pavement with very little distress may rapidly deteriorate,” Huber says.

In the summertime sun, the liquid asphalt cement will flow under gravity if it is exposed for a long period of time. The result is that without refinements, the liquid asphalt can migrate downward in the OGFC. “The upper layer becomes starved for asphalt, and all of a sudden, it starts to come apart. Becomes unglued, as it were,” he says.

While declaring its own moratorium on OGFCs in 1981, the state of Georgia did not abandon research into them. Investigation into moisture intrusion in the early 1980s led to changes not only in mix design, but in quality control and placement specifications, as well.

Changes included use of so-called “harder” (viscosity grade 30) asphalt cements at higher percentages, the discontinuance of asphalt emulsions as a tack coat and use of hydrated lime as an anti-strip additive.

“The research also spawned development of slightly coarser friction courses to improve water drainage properties,” Collins says.

Subsequent placements in 1985 performed well and still are yielding long-term data. Georgia reinstituted the use of OGFCs in 1986, as long as 1 percent hydrated lime was included.

When necessitated by incompatible aggregate and asphalt cement, other anti-strips are used.

LONG LIFE: THE KEY

“By using a modified asphalt binder, greater film thicknesses are achieved on the asphalt particles, thereby increasing the life of these pavements,” NAPA’s Dale Decker says.

“They pavement’s ability to resist trucks and carry the loads without undergoing permanent deformation is super because there is a true aggregate skeleton.

“In an ideal asphalt mixture, we would always like the load to be carried by the stone and the asphalt to be a glue that holds everything in place,” Decker explains.

“The porous pavements are ideal, and with the modified asphalts, the glue stays in place and retains its properties. We get a long life with durability and rut resistance.

“The pavements maintain their rut resistance throughout their life because of their stone-on-stone contact. Reduction in noise levels add to the list of advantages of open-graded friction courses,” he says.

A practical “how-to” guide for calculating and balancing the four main aspects of production — facility (manufacturing), transportation, paving and compaction — for a variety of conditions has just been released by the National Asphalt Pavement Association.

“Balancing Production Rates in Hot Mix Asphalt Operations” discusses the HMA facility production rate, its capabilities and the demands on those capabilities.

In addition, the publication provides a method for calculating production rates for each area and a section with handy fill-in-the-blank forms to assist in making those calculations. The guide is available at the list price of $16 per copy.

Recycling Hot Mix Asphalt Pavement. NAPA has also recently released “Recycling Hot Mix Asphalt Pavement,” a publication that provides both specifiers and processors with a current look at methods for reclaiming, sizing, storing and processing reclaimed asphalt pavement (RAP) materials.

This new publication covers types of RAP removal, sizing and stock-piling RAP, processing RAP and placement of recycled mixes.

A special section on processing RAP discusses the mixing process in drum and batch HMA mixing facilities, including the various techniques and devices currently in use, and how they affect the percentage of RAP that can successfully be processed.

Also discussed are indirect heat transfer methods and microwave heat transfer technology.

Calculations for determining the savings from using RAP are provided, as well as a bibliography for additional sources of information on many of the processes and techniques. The publication is available at the list price of $16 per copy.

Roller Operations for Quality. A revised and updated “Roller Operations for Quality,” NAPA’s most popular publication on the compaction of HMA, includes a helpful reference table on minimum laydown temperatures for various thicknesses and guidelines for selecting the proper amplitude on vibratory rollers for various pavement parameters.

Detailed instructions and diagrams covering proper procedures for compacting thin and thick lifts, curves and longitudinal and transverse joints are included. The publication is available at the list price of $12 per copy.

To order, contact the NAPA, 5100 Forbes Blvd., Lanham, MD 20706-4413; (301) 731-4748; fax: (301) 731-4621. Address orders to NAPA’s Internet homepage to: http://www.hotmix.org.

An Arizona Department of Transportation (ADOT) truck equipped with an Hex-Foam truck mounted attenuator (TMA) arrived just in time to save at least nine lives in an accident on 1-10 near the California border.

Maintenance crews had joined firefighters in the aftermath of a semi-truck fire on the highway just outside of Toponah, according to Department of Public Safety Officer Thomas Perkins.

The right-hand lane before and after the fire scene was in the process of being cordoned off from traffic, and the ADOT truck, equipped with the TMA, had just pulled into place upstream from the scene to protect the personnel against on-coming traffic.

The truck had barely been placed in position when it was struck by a pickup truck traveling approximately 75 miles per hour. A woman accompanied by her daughter and three-year-old granddaughter was driving the pickup.

“Had the shadow truck not been there, the pickup would have crashed into my squad car and probably killed the half-dozen fire-fighters and ADOT maintenance workers who were standing with me in front of it,” Perkins says. “Also, the two women and the child likely would have been killed — none were wearing seat belts.”

The driver, who said she was blinded by the early morning sun, stated that she saw the ADOT truck “at the last minute” and, although she tried to switch lanes, still hit the TMA.

First introduced by Energy Absorption Systems, Chicago, in 1981, the TMA is designed to protect a stationary maintenance vehicle from an impacting vehicle at speeds up to 45 miles per hour.

While this crash occurred at a much higher speed, enough of the impact was absorbed by the matrix of foam-filled hexagon-shaped honeycomb cells to prevent personal injury.

The driver and her passengers needed overnight treatment and observation at a local hospital and were released the next day.