Recycling of tires in asphalt pavement is an idea that has proven as resilient as rubber itself. Yet in the approximately 50 years since first being tried, the idea has failed to revolutionize the United States paving industry.
That could change.
New technology and old concerns about the environment seem to be re-energizing the rubber asphalt revolution.
The primary obstacles to more widespread reuse of tires in asphalt have been relatively high cost and uniform testing processes that tend to work against rubber asphalt while standards for highway projects are set.
These fundamental barriers to acceptance have been offset to some extent by repeated demonstrations in the United States and abroad that rubber asphalt holds up well under heavy traffic and absorbs the weight of passing vehicles in a way that reduces freeway noise. And the indisputable advantage of rubber asphalt is that it recycles tires that otherwise are piling up in landfills and ditches.
Yet in 2006, rubber asphalt remains something of a niche product. It is used extensively as a highway pavement in a handful of states, but represents a tiny segment of the U.S. asphalt paving market overall.
The rubberized product is more popular overseas in countries ranging from Australia to Belgium. This global presence is evident in an international rubber asphalt conference that is coming to Palm Springs, CA, Oct. 25 to 27, a first in the United States. The initial conference was in Portugal in 2000, the second in Brazil three years ago. The October program features technical papers and industry networking.
Each year in the United States, approximately 4 million tons (3.6 million t) of asphalt rubber hot mix is laid down, according to Doug Carlson, executive director of the Rubber Pavements Association. That compares to 525 million to 550 million tons (476 million to 499 million t) of conventional hot mix asphalt spread each year across the country.
Carlson noted that comparing the two materials is somewhat misleading because the properties of the mixes differ enough to warrant different applications. Their strength and flexibility characteristics vary enough that, for example, some jobs that recommend 4 in. (10 cm) of conventional asphalt will specify 2 in. (5 cm) of rubber asphalt, with corresponding differences in tonnage. A rule of thumb is that a 1-in. (2.5 cm) thick layer of conventional asphalt consumes 4 tons (3.6 t) of material per lane mile.
In Arizona — where the Rubber Pavements Association has its headquarters —rubber asphalt accounts annually for 85 percent of surface area paved in department of transportation projects, yet just 20 percent of asphalt tonnage used in all projects, Carlson said. The rubber asphalt often is spread thin — an inch or less — on top of conventional asphalt or concrete bases.
A Little History
Any primer on asphalt begins in ancient Mesopotamia where Babylonian builders used naturally occurring asphalt to build firm walkways and caulk water tanks. The Greeks employed it, as did other early Europeans. But not until the 1700s did a British engineer use it as a highway pavement and another 100 years passed before asphalt was introduced to American roadways.
At the turn of the 20th century, refined petroleum asphalt — instead of natural asphalt shipped to the United States from South American deposits — began to take over the industry. Production of the artificial compound encouraged its usage and now 94 percent of paved roads in the United States are asphalt covered, according to the National Asphalt Pavement Association (NAPA).
The paving material has improved over the decades. Materials, binders and technical standards all have evolved in response to problems of pavement cracking, rutting or aging prematurely. One of the major developments in quality control in the industry was the introduction 20 years ago of a quality-control testing system to establish uniform criteria for acceptable pavement.
Called Superpave, for Superior Performing Asphalt Pavements, the system devised by the Strategic Highway Research Program set performance standards for asphalt binders, mixes and materials. The idea was to wed each asphalt mix to a particular highway application and climate, thereby increasing the chance of a pavement performing as desired.
The testing system became the standard across the country with nearly every state’s department of transportation now requiring Superpave criteria in formulating asphalt pavement batches. While that generally is a plus, it has had the unforeseen consequence of penalizing rubber asphalt mixes.
Superpave testing uses various pieces of metering and compacting equipment to determine the suitability of an asphalt product. The problem is that the equipment cannot accommodate the size and variability of some of the crumb rubber used in rubber asphalt mixes. Because the rubber binder cannot be analyzed on the universal equipment, the subsequent mixes are deemed unsuitable … for simple lack of adequate testing.
Carlson said that the rubber asphalt industry has responded by coming up with a different device to measure its materials — with repeated good results. However, the industry has not been able to win Federal Highway Administration approval of the test.
“We are not a very big industry,” he said and having to win approval of its product state by state using its own equipment “is a tremendous challenge” for the rubber asphalt industry.
John D’Angelo, leader of the asphalt team in the Office of Pavement Technology of the Federal Highway Administration, said Carlson’s contention is “quasi-correct. Some of the processes that use rubber don’t quite fit into our equipment, not when there are big chunks of rubber floating around. But that’s only for processes where there are relatively large rubber pieces.”
However, D’Angelo said that state Department of Transportation (DOT) officials and contractors sometimes get around that by testing rubber asphalt mixes at the end of the batching process, rather than testing binders up front. Such testing, D’Angelo said, is acceptable but “a little bit more complicated.”
The testing hang-up has created a Catch-22 in another way. One piece of specialized blending equipment is needed when rubber is a component of asphalt, but many contractors are reluctant to purchase that piece of equipment in the absence of Superpave approval. Conversely, because state departments of transportation see little movement by contractors to equip themselves for rubber asphalt, DOT officials feel little pressure to tweak the Superpave testing process.
Turning tires into an asphalt ingredient dates to the 1960s when an engineer for Phoenix, AZ, developed a successful process.
Arizona remains the leader in the use of rubber asphalt in terms of percentage of total highway work. In three recent years, for example, Arizona transportation officials paved approximately 660 lane mi. using rubber asphalt as part of a “quiet pavement pilot program” to reduce tire noise. The rubberized treatment reduced decibels from each tire, which, among other things, allowed the Arizona Department of Transportation to lower the height of freeway sound-containment walls.
Measured another way, California has claimed leadership of rubber asphalt usage in terms of the percentage of total tons of all asphalt used — by approximately 35 percent last year.
Rubberizing Process
Tires only become useful for asphalt use after they are reduced to small pieces. Bags of tires shredded to chunks perhaps 6 in. square are delivered to rubber asphalt processors. The shredded rubber then is reduced to pieces ranging in size from less than a millimeter on up to approximately .5 in. This is accomplished either by grinding and shearing or by deep-freezing the material using liquid nitrogen and then fracturing it. Steel belting material is removed from the shredded tires.
Two methods subsequently are employed to introduce the crumb rubber into an asphalt mix — wet and dry. The wet process blends the crumb rubber with the asphalt at high temperature until the materials join and modify one another, whereas the dry process essentially uses crumb rubber as an aggregate. The binder created in the wet process is used in a greater variety of asphalt pavement applications, but both have been used successfully.
Blending the rubber and asphalt at high temperatures and binding them in a curing process requires special pieces of equipment. Several companies manufacture the reactor-blenders — including CEI Enterprises in Albuquerque, NM, and Valley Slurry Seal Co. of West Sacramento, CA — and some contractors fabricate their own equipment.
Approximately 35 blenders are operating around the world and 20 of them are in California, Arizona and Texas, Carlson said. That seems to illustrate the concentration in the Southwest United States of the rubber asphalt industrial community and shows the industry’s size compared to asphalt contracting in general.
One proponent of asphalt rubber is Mark Belshe of FNF Construction Inc. of Tempe, AZ. FNF is a $200 million-a-year company that concentrates on projects in the Southwest. But Belshe, FNF vice president, ventured to Nebraska a few years ago to take a 15,000-ton (13,600 t) rubber asphalt paving contract.
He credits Bob Rey of the Nebraska Department of Roads for introducing rubber asphalt to that state.
“He had the courage to say, ’I really like this stuff and I want to see it go in our state.’”
An experienced FNF crew executed the work on the initial contract and on a second job a year later. By the third year, a Nebraska contractor became interested and FNF returned to its Southwest roots.
Rey subsequently left the Nebraska department and is working with Dobson Brothers Construction of Lincoln, NE. A successor in the highways department, Mick Syslo, has overseen two rubber asphalt paving projects a year with “good results.” However, the price of the product has become less competitive because just one contractor has the needed specialized equipment to bid for the projects. As a result of the noncompetitive bidding, no rubber asphalt contracts were let by the state last year.
Syslo said the state is trying to persuade an asphalt supplier to blend and cure the rubber asphalt so that more than one Nebraska contractor can bid paving jobs. While blending the rubber asphalt off site holds some risks because of delay in application, Texas successfully employs such a system and Syslo said it can be adapted for Nebraska.
DOT officials in adjacent Kansas have tried rubber asphalt in pilot projects. Other states that have at least experimented with the process include Washington, Minnesota, New York, New Mexico and South Carolina. Florida has used it widely in open-graded friction courses.
Belshe said the asphalt industry is slow to embrace rubberized asphalt because it is “sort of a chicken and egg proposition.” That is, most states won’t let such contracts without at least a test strip being laid, but the reactor-blender equipment isn’t available in the Northeastern United States, for instance, to lay such strips. The equipment costs thousands of dollars to buy or to truck in from the West.
“That usually kills the deal,” Belshe said. “If you had a ready market, industry would respond.”
The FNF executive acknowledged that rubber asphalt generally is going to be more expensive at this stage of development — maybe 20 percent more expensive.
“But in the right application, it is extremely competitive. If you’re looking at a pavement’s life cycle, then it becomes very competitive. But so many people don’t look at the long term.”
D’Angelo, the Federal Highway Administration official, agreed the rubber asphalt industry “can produce an excellent product that can give excellent performance.” But he added that a major reason rubber asphalt has not become more popular is because “it takes a great deal of knowledge on the part of contractors and the construction industry to produce a good product. When you get outside the few states where it is used widely, that experience drops off dramatically.”
Belshe said that he has come to believe the innovative product needs a political champion and a technical champion in each state. He cited Tennessee where in the late 1990s the governor’s office pushed for rubber asphalt because of the recycling aspect. Some projects were undertaken, but when that chief executive’s office lost interest, momentum died.
Environmental Factors
Federal and state environmental agencies are on the side of anyone wanting to reuse tires in new pavement. The U.S. Environmental Protection Agency promotes the concept in the Resource Conservation Challenge section of its Web site. Noting that rubber asphalt is the single biggest user of crumb rubber (12 million tires a year), the agency lists the benefits of the rubberized asphalt — “longer lasting road surfaces, reduced road maintenance, cost effectiveness over the long term, lower road noise and shorter braking distances.”
In Nebraska, the Department of Environmental Quality helps pick up the cost differential on some rubber asphalt experimental strips to encourage its use. In several other states, environmental agencies support incentives that are being offered to contractors who buy specialized reactor-blender equipment, viewing the cost as an investment in resource conservation.
In California, a state assemblyman championed a measure that requires the state’s department of transportation to use more tires in asphalt paving work. Some 40 million used tires are believed to exist in the state in any given year with 1.5 million of them in landfills or back lots. Gov. Arnold Schwarzenegger signed the bill last year, declaring that “increasing the use of recycled tires in road construction is another way that the environment and economy can prosper together.”
Depending upon the percentage of rubber in asphalt — 18 percent is often the target — 500 to 2,000 tires are used in each lane mile of pavement. Because some applications of rubber asphalt are thinner than comparable conventional asphalt courses, less paving material is used. This conservation of building resources also appeals widely to environmentalists.
Mark Belshe, the FNF executive in Arizona, said he is more pessimistic today than he used to be about the growth of the rubber asphalt segment.
“The only way I see large-scale use [of rubber asphalt] is environmentalists pushing it.”
Far more optimistic is Brittany Blacklidge, executive vice president of Blacklidge Emulsions Inc. in Gulfport, MS. His company is a leading producer of asphalt binders and specialty asphalt compounds. Blacklidge and Coe Polymer LLC of Los Angeles, CA, have embarked on a joint commercial process that promises to increase the value of discarded tires as an ingredient in asphalt.
Polymer modified asphalt is the industry’s response to fatigue cracking, thermal cracking, rutting and other failure of pavement under stress. Synthetic elastic polymer — usually styrene-butadiene-rubber (SBR) and styrene-butadiene-styrene (SBS) — is introduced to a hot mix asphalt to increase its durability. An estimated 50 billion lbs. of synthetic polymer are produced each year, much of it in China.
What is relevant about this is that polymer chains are the primary structural components of rubber. What Blacklidge and Coe are attempting to do is reclaim polymer from synthetic and natural rubber in tires.
“If we can produce the same polymer out of tires, it is gonna’ be a big deal,” Blacklidge said.
William Coe was a Lockheed Corp. consultant in California a decade ago when he began to examine polymer reclamation. He eventually set his sights on what he called a P2 Ground Tire Rubber reactor. In 2004, Coe and fellow engineers and scientists working in a Lockheed spinoff company called Ecostar Systems made an electromagnetic breakthrough, figuring out how to reduce crumb rubber to tiny particles. This nanotechnology allows retrieval of polymer from tires.
Blacklidge likened the process to a cake. Once rubber has been vulcanized into a tire product, reclaiming the individual polymer strands heretofore was not possible. Using P2GTR technology, Blacklidge said vulcanized rubber now can be reduced to its elements just like reclaiming pristine salt and sugar from a baked cake.
“That’s the future,” Blacklidge said of permutated ground crumb rubber. “Take all those tires from the landfills, run them through this process and get back the polymer. This is going to shatter the cost of importing virgin polymer.”
If it develops anywhere along the lines Blacklidge envisions, the process could kick-start a rubberized asphalt revolution that will make tires the source for polymer modified asphalt hot mixes. Blacklidge Emulsions — which is a $70 million a year company — is fabricating a portable multi-million-dollar reactor that will “crank up” in California this fall.
Blacklidge gives Coe credit for keeping his eye on the environmental benefits of the tire reclamation.
“We have a mess on this planet and we need to clean it up.”
However, a full measure of success for the P2GTR project might empty landfills of tires faster than the asphalt paving industry wants. The reclaimed polymer can be used in applications other than pavement.
“Ever since Mr. Coe has come up with this process, a lot of brains are starting to turn,” Blacklidge said, counting among the thinkers the sellers of virgin polymer whose market might be threatened. But he added that auto industry executives also are taking note.
“Tire companies are going to want it [the polymer] back in tires. If you can recycle a tire back into a new tire, the tire industry is going to be all over that.” CEG
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