By Dwight Walker, P.E.
At one time, the push to use tires in asphalt seemed to primarily be just a means of getting rid of piles of scrap tires. For many agencies, their first experience with crumb rubber in asphalt resulted from the mandate included in the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1993. But now, the emphasis of using tires in asphalt binder is to improve pavement performance.
Asphalt binders modified with ground tire rubber (GTR) have several positive qualities. Adding ground tire rubber to asphalt can contribute to improved rutting resistance, skid resistance, ride quality, pavement life and reduced pavement noise levels. Adding rubber to the asphalt liquid retards aging and oxidation of the resulting binder, which increases pavement life by lessening brittleness and cracking. Rubber-modified asphalt binders can be used in open-graded asphalt mixtures, which have reduced hydroplaning, vehicle spray and reduced pavement noise.
Some basic descriptions
Defining some of the terms commonly used regarding tire rubber in asphalt may be helpful. The descriptions included here are intended to give a general understanding of meaning rather than to constitute a specification or standard. Various organizations and agencies have established definitions and standards, which should be used where applicable.
Crumb rubber modifier (CRM) or ground tire rubber (GTR) is recycled tire rubber which has been ground into very small particles to use as an asphalt modifier.
Asphalt rubber (AR) is defined by American Society for Testing and Materials (ASTM) as a blend of hot paving grade asphalt cement, reclaimed tire rubber and additives in which the rubber content is at least 15 percent by weight of the liquid asphalt binder and has reacted to cause swelling of the rubber particles.
Rubberized asphalt is a term applied to rubber modified asphalt with less than 15 percent by total weight of the asphalt cement.
Dry process refers to mixing crumb rubber into the mixture as a small part of the aggregate or filler rather than blending the rubber with the liquid asphalt. This process is normally only applicable to hot mix applications.
Wet process refers to blending or reacting the crumb rubber with the liquid asphalt at elevated temperatures before incorporating the resulting binder for use in an asphalt paving or surfacing application.
In the terminal blending of rubberized asphalt, tire rubber is blended into the asphalt binder at the asphalt terminal or refinery and shipped to the asphalt production plant as a finished binder, with no additional handling or processing required. The tire rubber is incorporated into the asphalt to provide styrene, butadiene, carbon black and aromatic oils yielding a stable, homogenous material.
How tires are processed
It is important to recognize that today CRM is typically a highly controlled material. The process is no longer just grinding up a stockpile of old tires and adding the rubber to hot asphalt. The handling and shredding process is carefully planned and monitored to produce a clean and highly consistent rubber material. The crumb rubber is produced through a process of grinding rubber tires into very small particles.
During the process, the tire’s reinforcing wire and fiber is removed. The steel is removed by magnets and the fiber is removed by aspiration. The rubber particles are sieved and separated into different size fractions, as specified by the customer. The resulting rubber particles are consistently sized and very clean. Automated bagging systems help ensure proper bag weights and eliminate cross contamination.
Fully PG-compliant asphalt rubber binders
Wright Asphalt Products in Houston is an example of today’s rubber modified asphalt producer. They began producing tire rubber modified asphalt with a single product for high volume chip seal applications and have expanded the product line to include a full range of modified asphalts and emulsions. Through 2009, Wright has more than 1.5 million tons of rubber modified liquid asphalt in service.
Wright’s process completely digests and incorporates the ground tire rubber into the asphalt cement. Consequently, their terminal blended asphalt is fully compliant with the PG binder specifications, including the solubility requirement. In their operation, the rubber particles are reduced to micron size, rather than swollen within the asphalt, by a combination of temperature and pressure. And since the rubber is completely incorporated into the asphalt at the terminal, the hot mix produced with their binders is fully recyclable.
Wright produces a concentrate containing 20 to 25 percent rubber, which can be blended with polymer to produce a polymer/rubber hybrid binder. Wright contends that the rubber additive enhances rather than replaces polymer. The rubber is not a substitute or replacement for polymer but functions as a durability enhancement. The rubber does not provide elastomeric improvements but lessens premature aging and cracking by lessening UV degradation. A weatherometer, commonly used for testing asphalt roofing material, is used to test their paving asphalts.
According to Wright, on average, adding rubber to asphalt increases the binder cost by 5 to 8 percent and adds about $2 per ton to the price of the hot mix. They estimate that their rubber modified hot mix can last an additional 5 to 8 years over unmodified mixes and their chip seals can last an additional 4 to 6 years.
AR in OGFC and warm mix
I-78 in Somerset County, New Jersey, was originally constructed in 1965 and rehabbed in 1999. In 2009 longitudinal joint raveling and minor rutting indicated the need for preservation work on the eastbound lanes from mileposts 30.9 to 42.7. At that time, the average International Roughness Index (IRI) number was 70 inches per mile. The three 12-feet wide lanes carried 77,000 vehicles per day, with 30 percent trucks.
The paving work was performed by Della Pello Paving Inc., with mix produced by Stavola Companies. The overlay mix was a 9.5 mm open graded friction course (OGFC) with 100 percent crushed trap rock aggregate and 8.5 percent asphalt rubber binder.
The asphalt rubber binder was blended by All States Materials Group (ASMG) using a portable blender/mixer and reaction tank. The asphalt contained 15 to 20 percent crumb rubber. The blending and reaction occurred at 290 to 350° F and was allowed to mix for one hour to allow the rubber particles to swell and suspend within the asphalt.
The blender was connected to the mixing plant and the binder addition was tied to the mix plant’s automation. The AR mix was placed using conventional equipment. Hand-working of the mix was minimized.
The completed overlay was smooth; the IRI following paving was 35 inches per mile—half the roughness before the resurfacing. The paving work received the maximum bonus for a single lift overlay.
In addition to the conventional OGFC placement, a small warm mix asphalt (WMA) test section was placed as part of the work. The mixing temperature was reduced by 40 to 100° F (with an approximate 50° average reduction) using Evotherm. Again, the AR-modified WMA was produced and placed with conventional equipment.
ASMG also furnished AR binder to another WMA project. This one was located on Route 3 in Plymouth, MA. The mix was a ½-inch nominal maximum size, gap-graded mix containing 7.5 percent AR binder. The WMA modifier was ECOBIT, a blend of liquid asphalt cement and Sonneborn Inc.’s Ad-Rap, produced and distributed by ASMG. 20,000 tons of mix was placed in a 1 1/4-inch compacted layer during night-time paving.
AR in Illinois SMAs
In October 2008, the Illinois State Toll Highway Authority released a report evaluating field trials of HMA pavements on the Jane Addams Memorial Tollway (I-90) in the Rockford, IL, area. The primary research focus of the project was to evaluate the use of high contents of fractionated recycled asphalt pavement (FRAP). During the study, 15 percent more RAP was allowed than under then current Illinois DOT or Tollway specifications.
The high FRAP contents were incorporated into nine different mixtures, which are regularly used in Tollway pavements. Three of the mixes were SMAs using a PG 76-22 with ground tire rubber, with different coarse aggregates.
Seneca Petroleum supplied the terminally blended GTR-modified liquid asphalt. It complied with the PG 76-22 grade with the exception of the solubility test. The use of the GTR-modified binder eliminated the use of fibers normally added to prevent drain-down of the asphalt mastic during storage and hauling of SMA mixes.
According to the tollway report, the material properties of the high FRAP mixes were evaluated on how they related to pavement design—specifically, the fatigue performance curve and the HMA modulus or stiffness. The dynamic modulus determines how much the pavement flexes as a truck passes over it, resulting in strain in the asphalt pavement. This strain is evaluated against the fatigue curve, which relates the strain to the loads the pavement can carry.
The three GTR-modified SMA mixes showed excellent fatigue performance using the flexural beam fatigue test. Also, the GTR-modified mixes easily lasted the 8,000 cycles of the rubber hose variation and the 20,000 cycles of the submerged steel-wheel configuration of the Asphalt Pavement Analyzer test (AASHTO TP 63-03.)
The report concluded that the asphalt rubber modified SMAs had materials properties similar to those of other SMA mixes. It is expected that these high RAP, GTR-modified SMA mixes will perform similarly to other SMA materials produced with virgin aggregates and SBS-polymer modified asphalt.
Conclusion
Today, only a small percentage of waste tires are being land-filled. The recycled rubber is being used in new tires, in tire-derived fuel, in civil engineering applications and products, in molded rubber products, in agricultural uses, recreational and sports applications and in rubber modified asphalt applications.
The benefits of using rubber modified asphalts are being more widely experienced and recognized, and the incorporation of tires into asphalt is likely to increase.
Dwight Walker, P.E., is a contributing editor for Asphalt magazine and a consulting engineer specializing in asphalt materials and construction.