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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Public Roads - Spring 1997

CMCRA: Where The Tire Meets The Road

by Dr. Brian Chollar and Dr. Mohammed Memon

Rising amidst the fashionable homes and horse stables of Smithfield, R.I., stands an immense mountain of discarded automobile tires that residents fear is a latter-day Vesuvius capable at any time of ruining their idyllic village. A testament to the nation's love of the automobile and its ecological consequences, the site and its 10 million tires recently made headlines when the Environmental Protection Agency announced an agreement to spend $32 million to clean up the toxic waste beneath the tires. But residents of the area also want something done about the tires which, in the event the pile ever caught fire, would create a serious health hazard for the local population.

Unfortunately, this Rhode Island "tire farm" represents only a fraction of the more than 850 million used tires scattered across car-crazy America. Worse still, more than 250 million old tires are added to these piles each year, making the problem a festering environmental nightmare with few solutions. Fires are rare, but when they do occur, the gallon of oil and ready-made oxygen supply contained in each tire can create smoldering infernos that last for more than a year and belch toxic gasses into the atmosphere.

To help alleviate the problem, Congress in 1991 ordered states to begin incorporating crumb rubber (chopped tires) into their asphalt mixes for pavement construction and repair under the auspices of the Intermodal Surface Transportation Efficiency Act (ISTEA). While in theory a noble and ecologically sound idea, in practice many states found the process inconvenient, costly, and ineffective. Offered little technological assistance from the federal government, most states' efforts resulted in little success as they struggled to meet the new legislative requirements.

Here Comes the Cavalry: FHWA Adopts the Crumb Rubber Cause

Many states have experienced a vast number of problems in preparing pavements using the crumb rubber modified asphalt binder. Some of the more significant problems include the settling of rubber particles during heated binder storage and raveling of pavements that include the crumb rubber modified asphalt (CRMA).

Too often the states found that the crumb rubber and asphalt were separating into two or more phases, meaning that the binder was deteriorating to the point of separation. What the states needed was a "chemical bridge" that would facilitate molecular bonding between the asphalt and crumb rubber to radically reduce the separation and create a homogenous product less susceptible to deterioration.

Fortunately for the residents of Smithfield and others living beneath the shadow of a tire farm, the Federal Highway Administration (FHWA) adopted the crumb rubber cause and tasked the authors of this article to develop a modified crumb rubber mix that would help the states meet federal guidelines. Working in our laboratories at FHWA's Turner-Fairbank Highway Research Center, we developed a chemically modified crumb rubber asphalt (CMCRA) that not only eliminates the problem of rubber particles settling while in storage but also expands the useful temperature range of the binder. The latter is particularly important because producers generally improve the low-temperature performance of a binder by using a petroleum distillate that is not "cut" so deeply to produce asphalt during the distillation process. This makes the asphalt softer, but it also requires that needed heating or motor oil distillates be left in the asphalt fraction, something that is both economically and ecologically undesirable. By using chemically modified crumb rubber to improve the low-temperature rheological performance of CMCRA made from regular asphalts, refiners potentially can save millions of dollars; savings that inevitably make CMCRA a more attractive product to users.

In conducting our research, we sought to achieve three objectives: (1) to improve both the low- and high-temperature rheological properties of CMCRA; (2) to achieve a proper dispersion of CMCRA, resulting in improved homogenity in the CRMA matrix; and(3) to produce a CMCRA that states would use by choice rather than statute. We wanted the states to voluntarily use the material because an effective, cost-efficient product beats a congressional mandate any day of the week.

Expanding the Low-Temperature Rheological Envelope

The first hurdle was eliminating the propensity of crumb rubber modified asphalt to separate and settle during heated storage. Normal or "neat" asphalt shows a separation (the difference in stiffness between the top and bottom third of the sample blend after a predetermined period of heated storage) of between 2 percent and 4 percent. Conventional crumb rubber modified asphalt resulted in a non-homogenous blend with up to 25 percent separation. But the chemically modified crumb rubber asphalt had a separation range of 5 percent to 7 percent. The result is a homogenous product that significantly increases the life of a pavement.

This achievement is due to a process in which we are chemically bonding the asphalt and crumb rubber instead of physically mixing the blend. The markedly reduced separation rate gives the chemically modified crumb rubber asphalt a competitive edge over more conventional crumb rubber products.

But in addition to passing the settling test, any asphalt incorporating a crumb rubber modifier must also have a large useful temperature range and be stable at storage temperatures. Although previous tests already had determined that a crumb rubber modified asphalt offers improved rheological (deformation and flow) properties over conventional asphalt, the asphalt modified with chemically modified crumb rubber showed improved rheological properties for both low and high temperatures.

This is particularly important because while a number of polymers improve the high-temperature properties of asphalt, they show little or no changes in low-temperature rheological properties of the asphalt. The improvement in low-temperature utility makes CMCRA more competitive from both a practical and economic standpoint. Producers currently improve the low-temperature performance of the asphalt binder by distilling the crude oil in the manner previously noted or by blending a portion of the high-boiling distillate back in with the asphalt. This makes the asphalt more expensive because the distillate generally commands premium prices as compared to the asphalt.

Although production of CMCRA is approximately 60 percent more expensive than conventional crumb rubber asphalt, the additional costs are offset by longer pavement life. More testing is scheduled at the lab, but word of the initial CMCRA successes is generating interest across the country.

Indeed, there may be hope yet for the folks in Smithfield and other Americans yearning to be rid of their tire farms.

Dr. Brian Chollar is a research chemist in the Office of Engineering Research and Development at the Turner-Fairbank Highway Research Center in McLean, Va. Dr. Chollar has worked for FHWA for 23 years. His areas of expertise are snow and ice chemicals for winter maintenance and asphalt chemistry. He received his bachelor's degree in chemistry from Purdue University and his doctorate in organic chemistry from the University of Minnesota.

Dr. Mohammed Memon is also a research chemist in the Office of Engineering Research and Development at TFHRC. He has worked for FHWA for five years. His areas of expertise are asphalt and polymer chemistry. He received his doctorate from the University of Sindh in Pakistan.