Road Science

Asphalt Rubber Makes a Quiet Comeback

Left for dead in the early ’90s, rubberized asphalt is getting
star billing today as a thin-lift, noise-dampening friction course.

by , Contributing Editor

Asphalt rubber pavement vaulted into the road industry’s limelight when the sweeping Intermodal Surface Transportation Efficiency Act of 1991 mandated its widespread use. Industry experts were shocked. Asphalt rubber pavement was a promising concept that a few agencies were working with, but many others were not ready to implement it on a grand scale.

Though the industry was successful in postponing and eventually killing the asphalt rubber provisions of ISTEA, the pavement concept was dealt an enormous blow by the ensuing debate over its benefits and liabilities.

For many in the road industry, asphalt rubber pavement was a dead issue after that debacle, and it received little attention in most parts of North America for the rest of the decade.

How times have changed. In recent months, asphalt rubber pavements have become the new paradigm of the asphalt industry’s quiet pavements initiative. Ground zero of that marketing campaign is a three-year, $34-million Arizona noise-attenuation project to surface about 115 centerline miles of Phoenix-area freeways with rubberized asphalt open-graded friction course.

Since the ISTEA-mandate debacle, asphalt rubber pavements have evolved quietly in Arizona, California, and other places and now appear to be poised for widespread use in the United States.

Advocates for the technology range include the Federal Highway Administration and the National Asphalt Pavement Association. The FHWA features asphalt rubber pavements as a choice in its Quiet Pavements Pilot Program (QP3). NAPA works beside the Rubber Pavements Association to promote asphalt rubber pavements as both quiet and environmentally sound.

A number of important developments has helped propel asphalt rubber pavement to its current level of acceptance, including new tire-shredding technologies and the establishment of tire recycling facilities in every part of the United States. In addition, the Arizona DOT has demonstrated that crumb rubber modifier is compatible with Superpave-type performance-graded liquid asphalt binders, making asphalt rubber a viable candidate for high performance pavement designs.

In addition to their environmental benefits, well-constructed asphalt rubber open-graded friction courses also provide smoother rides for highway users, and reduce tire spray in wet weather due to their highly drainable nature. In Arizona, citizen satisfaction with those issues is reinforcing the state and local government’s decisions in going to asphalt rubber thin overlays on all major Phoenix expressways.

Of course, not everything about asphalt rubber is perfect. These mixes bring fumes and odors to the paving process, and there are still questions about the recyclability of reclaimed asphalt rubber pavements.

Asphalt rubber pavements are also more expensive than conventional asphalt pavements, and can be difficult for paving contractors not accustomed to working with the sticky product, which, like Superpave, must be placed and compacted within a relatively narrow temperature window.

What is asphalt rubber?

Asphalt rubber, or rubberized asphalt, is a chemically reacted mix of liquid asphalt binder with 15 to 22% crumb rubber. The rubber is obtained from reclaimed tires, and added to liquid asphalt. It’s reacted at elevated temperatures prior to being mixed with aggregate.

In this so-called “wet process,” the rubber crumbs react with the liquid asphalt; there is also a so-called “dry process” in which rubber crumbs are added with the stone as another type of aggregate.

Use of rubber in asphalt varies significantly from mix to mix. “There’s a spectrum of applications of rubber in asphalt,” said Doug Carlson, executive director, Rubber Pavements Association in Tempe.

“Rubberized asphalt is the overall term, and it means any kind of rubber placed in any kind of asphalt,” Carlson told Better Roads. “But Asphalt Rubber is very specific. It’s an ASTM material, typically 15% ground tire rubber combined with liquid asphalt and other additives.”

Two types of asphalt rubber (wet process) are used in California. Type 1 contains rubber only from ground tires, and Type 2 contains rubber from both ground tires and natural rubber. Caltrans favors Type 2 binders because they have better resistance to reflection cracking.

Asphalt rubber has been used both in hot-mix applications and in spray surface treatments like chip seals, stress-absorbing membranes (SAMs), and stress-absorbing membrane interlayers (SAMIs). The enhanced performance of asphalt rubber helps agencies deal with applications requiring thin lifts, such as when reduced overlay thickness is needed for controlling grades and next to curbs and medians.

Today, asphalt rubber’s revival stems from its use in gap- and open-graded mixes, but it continues to be used in those spray applications.

Asphalt rubber can’t be placed during weather extremes. In Arizona, the concrete pavement surface to be overlaid must be between 85 and 145 degrees F for the overlay to adhere and cure properly.

Asphalt rubber has drawn some environmental scrutiny, mainly when used to overlay concrete pavements. The objections center on covering an environmentally friendly concrete surface, with its higher thermal mass and excellent reflectivity of heat and light, with a black overlay that will cause the pavements to absorb more heat, adding to a city’s “heat island.”

Arizona disputes that criticism, claiming that because the rubberized asphalt friction course is porous and less dense, it has less mass to begin with and will radiate heat more quickly than concrete. The Urban Heat Island Group at Arizona State University is conducting research to address this issue.

Asphalt rubber stumbles

Through the 1980s, awareness and application of asphalt rubber pavements grew slowly but steadily. But in a textbook example of the Law of Unintended Consequences, asphalt rubber got a black eye when its use was mandated by the 1991 Intermodal Surface Transportation Efficiency Act.

This act mandated increasing use of reclaimed rubber in asphalt pavements, beginning with 5% in 1994, 10% in 1995, 15% in 1996, and 20% in 1997 and each year thereafter.

Reputed to be the brainstorm of an environmentalist staffer in the office of the late Senator John Chafee (R-R.I.), then chairman of the Senate Environment and Public Works Committee, the mandate required use of recycled rubber regardless of the type of asphalt cement used, the region of the country, the traffic loadings of the pavement, or the source of the recycled rubber.

Especially galling to the industry was the fact that the mandate totally ignored the purpose of the Strategic Highway Research Program (1987), a five-year, $150-million research program funded entirely by the states that was investigating the validity of performance specifications for asphalt pavements based on a road’s specific climate and projected traffic loadings.

SHRP’s research, of course, led to today’s Superpave system of pavement design. But ISTEA’s asphalt rubber mandate — which would have required reclaimed crumb rubber in every liquid asphalt, regardless of its chemistry or mix application — in effect trashed the entire asphalt research effort.

Even worse, the mandate would have wiped out an existing road-related industry, built by entrepreneurs, which utilized reclaimed tire rubber. The mandate was supposed to reduce the number of scrap tires going into landfills, but because the source of the rubber was not codified, in reality the source of reclaimed rubber would have been buffings of tire carcasses being prepped for recapping.

These buffings already were going into rubber products such as rail crossings, bases for temporary road signage, and traffic cones. As the mandate was phased in, the asphalt market would have consumed these buffings long before scrap tires would have come into play, and driven an industry out of business.

Intense public/private lobbying resulted in postponement of the mandate deadlines in yearly appropriation bills, until the entire section was scrapped in face of withering opposition. A major industry complaint was not rubber in asphalt, but mandated recycled materials in pavements.

Partnering again

But now, the hatchet is buried, and the public and private sectors are partnering to enhance asphalt rubber use. This year, a technical presentation on asphalt rubber pavements was part of NAPA’s 2004 annual meeting program in Phoenix in January, and in March 2004 the association published an article by the National Center for Asphalt Technology’s Ken Kandahl, P.E., on quiet pavements and the Arizona initiative.

“As an association, Rubber Pavements has been striving to improve its relationship with NAPA,” said RPA’s Carlson. “In the crumb-rubber mandate era, NAPA rightly picked sides. There were a number of reasons. Most technologies were patented at the time, and that would have severely hampered implementation of the mandate. It would have allocated a huge portion of the national market to a handful of contractors. It was an ill-conceived, ill-timed mandate.” Carlson added that many of RPA’s members belong to NAPA and have worked to change attitudes.

In 1992, the patent for rubberized asphalt expired, making the technology openly available in the public domain, resulting in more extensive consideration of the paving material.

“I think the light bulbs finally have gone off regarding market opportunities,” Carlson said. “Arizona and California have had tremendous success with very thin asphalt rubber porous friction courses [OGFCs] on top of concrete, and it’s opened up a new market for HMA, even for brand new concrete pavements, which normally would have been out of the running for an overlay for as long as 30 years.”

Even the complaint that heightened asphalt rubber use would drive other users of tire carcass buffings out of business has been quelled, thanks to improvements in tire recycling technology and the development of tire recycling plants in most states.

“At the time of the mandate, there were only two plants in the whole country that could process the entire tire,” Carlson said. “Today, it’s just the opposite. The increased demand for crumb rubber for all purposes has brought new players in, and there are as many as 100 facilities that can process complete tires into crumb rubber and use the remaining steel belts.” Carlson added the wire is of a very high quality of steel and often finds its way into reinforcing steel for concrete.

Indeed, asphalt rubber pavements have extended well beyond Arizona and California, with installations in Connecticut, Florida, Tennessee, Texas, New Mexico, Colorado, Nebraska, and Ontario.

And in Michigan, a technical conference on asphalt rubber, cosponsored by RPA, NAPA, the Rubber Manufacturers Association, the Asphalt Institute, and the National Center for Asphalt Technology at Auburn University will be held later this month. The 1st Rubber Modified Asphalt Conference is planned for Grand Rapids May 19-20. More information is available from www.rubberpavements.org.

”Win with Thin” in Arizona

With the slogan “Win with Thin”, as in thin overlays, the Valley of the Sun is currently in the second year of a three-year, $34-million project to surface about 115 miles of Phoenix-area freeways with rubberized asphalt.

Arizona’s Quiet Pavements initiative was spearheaded by the Arizona DOT in cooperation with the Maricopa Association of Governments (MAG), the county regional transportation planning body, and local municipalities.

It is being financed by $34 million from MAG regional transportation funds and projects, funded by a county-wide sales tax for highways. The program operates in parallel with a MAG Regional Freeway System scheduled for completion by 2007.

The program began as residents and visitors to housing areas near a freeway that had been paved in 2002 with an asphalt rubber thin-lift OGFC began to talk among themselves about the quieter pavement. Word spread, letters were written, and radio talk shows buzzed with conversation. Pressure began to mount on government officials.

“It was a grassroots undertaking,” said George B. Way, P.E., chief pavement design engineer, Arizona DOT. “As enthusiasm over the initial quiet pavement section mounted, the question came up, ‘Why aren’t we doing this across the entire freeway system?’ They asked, now that we have a quiet pavement design, why aren’t we using it everywhere?”

In December 2002, the Arizona DOT announced plans for the program, and after a ramping-up period, began to resurface the first 21 miles of freeways in September 2003.

The DOT used testing to determine where noise was most obnoxious, and let the OGFC overlays begin there. “We looked at where the noise is the worst, and targeted those segments first,” Way told Better Roads. “Then we will gradually fill in the others.”

The Arizona DOT is a pioneer in the use of rubberized asphalt in paving projects, with more than 4.2 million tons of rubberized asphalt paved on Arizona highways since 1988. The state has spent $225 million on rubber asphalt paving projects during that period. The state estimates that over 15 million tires have been recycled since 1988 by paving Arizona highways with rubberized asphalt.

In the 1970s and 1980s, the state studied asphalt rubber open-graded friction courses for use in snow country, then for their ability to resist reflective cracking (1980s-1990s). In the 1980s and 1990s the improved OGFCs were applied as overlays for portland cement concrete pavements, and were utilized for smoothness in the 1990s. Their use for noise attenuation continued from the 1990s into the 21st century.

“The cold weather in snow country did not have any detrimental effect,” Way said. “We use it throughout Arizona, in both desert and higher elevations, and it’s performed very well. In the Flagstaff area we have snow and ice, and at times chains are required on tires to get through mountain passes. We found that the asphalt rubber OGFC is a tough surface and we don’t get as much chain wear as we used to get in cold climates, and snow plows don’t chip away at the surface in a pattern the way they did.”

Asphalt rubber OGFCs for noise attenuation has a similarly nuanced development in Arizona. An industry study of their use for noise control appeared in the 1990s, but it was not until 1995 that Arizona DOT began its first formal research in that area.

The year 2002, the year of the public reaction, was a watershed for asphalt rubber OGFCs in Arizona. That’s when the Arizona DOT built its ISO Noise Trailer and the state began collaborative research with Caltrans. The two agencies launched a network-level evaluation of asphalt rubber friction courses, as well as tests on noise from different PCC pavement textures.

In 2003, the state partnered with the FHWA and its Quiet Pavement Pilot Program (QP3). This research effort is a 10-year program involving flexible (hot-mix asphalt) and rigid (portland cement concrete) pavements, and includes a study of the environmental effects of the thin surfacings.

Generally, the Arizona DOT’s asphalt rubber friction courses use minus 0.37-inch aggregate combined with 9 to 9.5% asphalt rubber binder. The thin lift is placed 0.50-inch deep when used on flexible HMA pavements, and 1-inch deep when used on rigid PCC pavements. A noise attenuation of up to 4 dBA is attained from these thin surfacings.

A closer look at the binder

Arizonans went on record at this year’s Transportation Research Board) meeting in January. There, grad students Aleksander Zborowski and Andres Sotil, and assistant professor Kamil E. Kaloush, Ph.D, P.E., Arizona State University, and the Arizona DOT’s Way, described ongoing research in asphalt rubber mixes in that state in the peer-reviewed paper, Material Characteristics of Asphalt Rubber Mixtures.

This study conducted experiments on asphalt rubber mixes to obtain their typical engineering properties and understand their field performance. Most of the laboratory program was based on tests recommended by the National Cooperative Highway Research Program, NCHRP 9-19 Project, which dealt with recommending Simple Performance Tests (SPT) for the evaluation of asphalt mixes.

The lab tests included consistency binder tests, triaxial shear strength, repeated load permanent deformation, dynamic modulus, flexural beam fatigue, and indirect tensile tests. The results obtained for the asphalt rubber mixtures were also compared, when possible, with results obtained for conventional mixtures. They found that lab results reflected the performance of the mixes in the field.

Five hot asphalt rubber mixes were obtained from the field during construction, and compared to a single PG-asphalt control mix. The mixes were reheated and compacted at the air voids level specified for each of the projects.

The six mixes were:

Arizona I-40 Asphalt Rubber Asphalt Concrete Gap Graded Mixture, which is referred to as the Arizona ARAC. This mix had an in-situ air voids level of 11% and an asphalt content of 6.8%. The stock binder used for this mix was a PG 58-22.

Arizona I-40 Asphalt Rubber Asphalt Concrete Friction Course Open Graded Mixture, which is referred to as the Arizona AR-ACFC. This mix was placed on top of the Arizona ARAC and had an in-situ air voids content of 18% and AC of 8.8%. Similar to the Arizona ARAC, the stock binder was a PG 58-22.

Arizona I-17 ARAC PG 58-22 Gap Graded Mix, which is referred to as the Arizona PG 58-22. This mix had in-situ air voids of 8% and AC content of 7.5%.

Arizona I-17 ARAC PG 64-16 Gap Graded Mix, which is referred to as the Arizona PG 64-16. This mix had in-situ air voids of 5.5% and AC content of 8.0%.

Alberta ARAC Pen 150-200, which had an in-situ air voids of 9.7% and AC content of 8%. The stock binder was a Pen 150-200. A control virgin dense graded Pen 150-200 mixture was also included within the testing program. This mix had in-situ air voids of 5.4% and AC content of 5.4%.

The control mix, from the MnRoad test site in Minnesota, was the 18 Dense Graded Mix, which is referred to as the MnRoad-18 PG 64-22. This mix had in-situ air voids of 5.6% and AC content of 5.83%. The stock binder used for this mix was an AC-20.

A comparison of the crumb rubber-modified binders with the virgin stock binders showed that at both high- and low-temperature conditions, all asphalt rubber binders had improved viscosity-temperature susceptibility, that is, characteristically moderate viscosity changes due to temperature.

The results of the Triaxial Shear Strength Test show that the conventional dense-graded mixture had much higher cohesion (the characteristic ability to hold together under stress) than the asphalt rubber open-graded mix, and a little higher cohesion than the asphalt rubber gap-graded mixes. At the same time, both of the Arizona AR mixes had higher angle of internal friction compared to the conventional dense-graded ADOT mixture.

The results of the Flow Number Test at 37.8 degrees C (100 degrees F) showed 2.5 to 13 times higher flow numbers for the asphalt rubber mixture compared to the conventional mixes. For tests conducted at 54.4 degrees C (130 degrees F) the asphalt rubber mix showed a flow number over 16 times higher than the conventional mix. The higher flow numbers indicate asphalt rubber’s characteristic enhanced liquidity, but nothing else, Way told Better Roads, especially considering the enhanced toughness of asphalt rubber in the field.

The results of Axial Strain at failure showed that the asphalt rubber mixtures have three to four times higher strain at failure compared to the conventional mixes. Higher strain at failure is an indicator of good mixture stability to the applied loads.

For the Flexural Beam Fatigue Tests the relationships were rational in that higher binder content mixes yielded higher fatigue life despite the air void content variations between the mixtures. It was generally observed that the asphalt rubber mixture resulted in higher fatigue life than the conventional mix.

The results from the Indirect Tensile Strength Test showed higher strength values for the conventional mix when compared to the asphalt rubber mixes at all three test temperatures. On the other hand, higher tensile strains were obtained for the asphalt rubber mixtures. Higher energy until failure was also observed at lower temperatures, and may be indicative of an advantage for the asphalt rubber mixes compared to the conventional mixes.

For the Indirect Tensile Creep Test there was a general observation that the values of the parameters as well as differences between mixtures decreased with decreasing temperature.

Above all, the study results indicated that much of the test data obtained in the laboratory successfully described the observed field performance of the asphalt rubber mixes.

Asphalt rubber challenges

Asphalt rubber is not a panacea. There are drawbacks to it, chiefly in terms of its workability, its recyclability, and the fumes it emits during the paving process.

One California contractor told Better Roads that he prefers not to work with the material. “The material is very sticky and must be compacted quickly,” he said. “And you can’t believe the odors that come off the paver and mat.”

Another issue has been the blue smoke that originates when milled asphalt rubber RAP is mixed with hot asphalt mix at the plant.

In late 1994 the city of Los Angeles repaved Olympic Boulevard with hot-mix asphalt that included 15% asphalt rubber millings from that same street. The resulting study (www.rubberizedasphalt.org/
airquality/index.htm) found the millings were recyclable and no hazards of particulate materials presented themselves to employees. However, this RAP contained only 1.5 to 3% rubber, so it fell well below the 18 to 22% levels used today. The analysis also included recycling in a microwave-heated asphalt plant, which is extremely uncommon.

How widely asphalt rubber pavement will be used in the future remains to be seen, but it is

certain to get a long look in many noise-sensitive metropolitan areas for many years to come as road managers weigh its disadvantages against the benefits of a quieter, smoother, more user-friendly surface.

Benefits of Asphalt Rubber Surface Pavements

California’s Rubberized Asphalt Concrete Technology Center, a cooperative effort by Los Angeles County, Sacramento County, and the California Integrated Waste Management Board, promotes use of crumb rubber from scrap tires in roadway rehab projects by providing education, training, and consultation services to local agencies within California.

The center is funded by the California Integrated Waste Management Board in an effort to reduce the state’s stockpile of scrap tires and conserve the state’s landfills. The state generates 30 million scrap tires annually. You may visit them at www.rubberizedasphalt.org.

The center lists a dozen benefits to asphalt rubber open-graded friction courses:

1. Improved resistance to surface-initiated cracking due to higher binder contents.

2. Improved aging and oxidation resistance due to higher binder contents.

3. Improved resistance to fatigue and reflection cracking due to higher binder contents.

4. Improved resistance to rutting due to higher viscosity and softening points.

5. Increased night-time visibility due to contrast between the pavement and the striping.

6. Reduced tire noise due to increased binder film thickness and open texture.

7. Reduced splash and spray during rain storms due to open texture.

8. Reduced construction times because less material is placed.

9. Lower pavement maintenance costs due to improved pavement performance.

10. Better chip retention for chip seals due to thick films of asphalt.

11. Lower life-cycle costs due to improved performance.

12. Savings in energy and natural resources by using waste products.

For More Information

An abundance of research on asphalt rubber pavements and asphalt rubber open-graded friction courses is available to road builders seeking more information. Consider beginning with some of the references below.

Rubber Pavements Association. The first stop for any road planner or pavement engineer should be the RPA Web site which maintains a current research library with many downloadable documents. You’ll find them at www.rubberpavements.org, or contact Doug Carlson, executive director, RPA, 1801 South Jentilly Lane, Suite A-2, Tempe, Arizona, 85281-5738, voice 480-517-9944, fax 480-517-9959.

Quiet Pavements Pilot Program. The Arizona DOT has established a Web site with more information about its Quiet Pavements Pilot Program (QP3), including a map of expressways to be paved with asphalt rubber OGFCs this year. Access it at www.quietroads.com.

Federal Highway Administration. The FHWA’s User Guidelines for Waste and Byproduct Materials in Highway Construction is available from its Turner-Fairbank Highway Research Center at www.tfhrc.gov/hnr20/recycle/waste/begin.htm.

Recycled Materials Research Center. The Center, based at the University of New Hampshire, operates a Web site at www.rmrc.unh.edu/ Partners/UserGuide/begin.htm. The site includes the FHWA material, and a lot of other content on waste tires and other materials in pavements.

Rubber Manufacturers Association. This group’s Web site has a section on scrap tires, with links to suppliers for purchase. Access it at www.rma.org/scrap_tires/.

European research. Recycled Materials in European Highway Environments: Uses, Technologies, and Policies (FHWA-PL-00-025) is a valuable tool for state and local road managers as they ponder the utilization of recycled materials in their pavements. Access a .pdf file of it at www.international.fhwa.dot.gov/Pdfs/recycolor.pdf.

Rubberized Asphalt Concrete Technology Center. While it is based on California experience, the RACTC of Los Angeles and Sacramento counties is a great resource for municipal and county governments considering use of asphalt rubber. Their documents include an online field inspection guide, asphalt rubber design and construction guidelines, a report on the status of rubberized asphalt traffic noise reduction in Sacramento County, and an asphalt rubber overlay noise study update. Obtain them at www.rubberizedasphalt.org/.

Better Roads. Last year, Better Roads published an in-depth look at OGFCs: A New Era for Permeable Pavements, April 2003, pp 28-32.

Reprinted from Better Roads Magazine
May 2004

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