ADOT'S USE OF CRUMB RUBBER IN ASPHALT PAVEMENTS

George Way has been in materials and research with ADOT since 1969. He is currently head of the Materials Pavement Services. George's department handles both Pavement Design and Management and is responsible for over 100 projects per year, representing about 300 million dollars of construction. George conducts an annual survey of all state highways in terms of ride smoothness, cracking, rutting and skid resistance. From this information he establishes a recommended preservation policy involving overlay and seal coats representing about 70 million dollars of projects. George is a graduate of Arizona State University, receiving his B.S. E. in 1969. He is a Registered Professional C.E. in Arizona and is the recipient of many prestigious awards such as the 1979 WASHTO award for outstanding contributions to transportation development in the west, the 1982 Management Science Achievement Award and the Arizona Society of Professional Engineers 1983 Young Engineer of the Year He is a member of the NSPE, the AzSPE, the ASCE, AAPT and the American Institute of Chemical Engineers. He served as Chairman of a major TRB Committee and was selected by, AASHTO in 1985 for the Long Tenn Pavement Performance committee, part of the National Strategic Highway Research Program, He has authored and co-authored numerous papers on pavement design and performance and has made many presentations at engineering conferences and universities. George has worked with Asphalt-Rubber since 1969 and is one of the leading experts in the field.

A b s t r a c t

The Arizona Department of Transportation (ADOT) has used crumb rubber from ground tires. since the late 1960's to primarily reduce reflective cracking. In 1988, ADOT started to use crumb rubber mixed with hot asphalt, commonly referred to as asphalt rubber as a binder in hot-mix asphalt. Typically, these mixes are either open-graded or gap-graded and from 12.5mm to 25min or 25mm to 50mm in thickness, respectively. Open-graded mixes generally contain 9 to 10 percent asphalt rubber binder, whereas the gapgraded contains generally 75 to 8.5 percent. To date, field performance has been very good with very little reflective cracking, rutting. As an extra benefit, the ground tire rubber from over four million tires in Arizona has been recycled since 1988, in the making of hot-mix asphalt with asphalt rubber.

ADOT'S USE OF CRUMB RUBBER IN ASPHALT PAVEMENTS

History

Charles MacDonald, as the City of Phoenix Materials Engineer, began to experiment with mixing crumb rubber from ground tires with asphalt in the early 1960's. He, along with other associates, patented what is currently described as the MacDonald Process or Wet Process for making Asphalt-Rubber.

The Arizona Department of Transportation (ADOT) monitored the development of asphalt rubber and placed a band aid type maintenance application in 1964. In 1968. technology had advanced after trial and error and the burning of a couple of distributor boot trucks, to a degree where these processing problems were satisfactorily solved and asphalt rubber could be safely and consistently placed with a distributor truck by using a diluent (kerosene), From 1968 - 1972 ADOT placed six projects with asphalt-rubber in-seal coat type application using boot truck distributor. In these early applications the ground tire crumb rubber was introduced into the top of the boot truck and mixed by rocking the truck forward and backward. Even with this rather primitive early technology it was possible to construct the first ADOT field experiment in 1972 using asphalt rubber as a seal coat or Stress Absorbing Membrane (SAM), as well as an interlayer under a hot-mix asphalt surfacing. The interlayer application is typically referred to as a Stress Absorbing Membrane Interlayer (SAMI). Both the SAM and SAMI applications showed great promise in reducing reflective cracking (1). From 1974 until 1989 approximately 1100 kilometers of state highways were built using a SAM or SAMI application of asphalt rubber. in addition to this, ADOT and the Federal Highway Administration sponsored numerous. research studies which resulted in 42 research reports being published, thus greatly increasing the state-of-the- knowledge concerning asphalt rubber.

In addition to reducing reflective cracking, it was noted early on that asphalt rubber could be used as a waterproofing membrane. Several projects were built to control subgrade moisture in order to control expansive (swelling) clays or to reduce structural pavement sections. This application proved to be very successful (2).

Current Practice

In 1989, Larry Scofield documented in a research report the history, development and performance of asphalt rubber at ADOT (3). in that report the following conclusion is stated , "asphalt rubber has successfully been used as an encapsulating membrane to control pavement distortion due to expansive soils and to reduce reflection cracking in overlays on both rigid and flexible pavements. During the twenty years of asphalt rubber use, ADOT has evolved from using slurry applied asphalt rubber chip seals to utilizing reacted asphalt rubber as a binder in open and dense-graded asphalt concrete. " He noted that asphalt rubber could be used as a binder for hot-mix asphalt. Concurrent with this conclusion, it became evident that it could be used as a binder to provide a hot-mix asphalt suitable for addressing cracked pavements.

By 1988 boot truck processing of asphalt rubber had advanced enough such that it was now possible to pump the material into a hot plant in such a manner as to make a suitable hot-mix. In 1988, a 25mm layer of an open-graded asphalt rubber asphalt concrete friction course (AR-ACFC) was placed on several miles of Interstate 19, south of Tucson. This AR-ACFC mix, containing 10.0 percent asphalt rubber by weight of the mix as the binder (note: diluent is no longer used) was placed on top of a plain jointed concrete pavement since 1988, no cracks reflected through until 1996, when only a few transverse cracks appeared over the concrete joints. Following that project, many more projects 'have been built with asphalt rubber as the binder. To a great extent this has been made possible as a result of significant improvements in the mixing and reacting of the asphalt rubber at the project. Pumping the material from a boot truck is no longer necessary as it is now more economical and practical to produce the reacted asphalt rubber at the construction project by using the appropriate equipment. The asphalt-rubber contains 20 percent ground tire crumb rubber by weight of the asphalt content. These projects were built with the expressed purpose of controlling reflective cracks with a very thin layer of very elastic material. To date, all projects have performed as expected. As a further extention of this work, a structural overlay was designed and built in 1990 on Interstate 40 near Flagstaff, using asphalt rubber as the binder for a gap-graded hot-mix asphalt and the ACFC binder (4). This project also contained numerous Strategic Highway Research Program SHRP) test sections as well as (ADOT) test sections. The purpose of the project was to overlay a severely cracked and failed concrete pavement. As of the most recent review of this project in July, 1997, the asphalt rubber sections built as the top portion (overlay top 50min AR-AC, 12.5mm ARACFC) have the least percentage of reflective cracks. Cost comparisons would indicate that asphalt rubber binder can be from two to four times as expensive as asphalt. After incorporating the asphalt rubber into the hot-mix asphalt the finished product is generally from 25 to 75 percent more expensive for the gapgrade mix than the typical densegraded hot-mix and 80 to 160 percent more expensive than the typical open-graded friction course. These higher costs need to be examined in light of actual usage which is to place less thickness of asphalt rubber mixes than typical dense-graded mixes. On the 1-19 project, only a 18.75mm AR-ACFC was placed for the same cost as a 37.5mm dense-graded hot-mix asphalt overlay. Such a thin densegraded hot-mix surely would have cracked after the first winter, whereas after seven winters, the ARACTC did not crack. Similarly, on the 1-40 Flagstaff project a 100mm dense-graded hot mix asphalt overlay showed the highest percentage of reflective cracking, whereas the section with an overlay combination of a 50mm densegraded hot-mix asphalt covered with a 50mm gap-graded asphalt rubber hot-mix has show the least percentage of reflective cracking after seven winters. in terms of field performance asphalt rubber appears to be a cost effective material when used as a relatively thin overlay (50mm or less) or ACFC.

Old PCCP Condition I-40 Arizona

With regard to the price of asphalt-rubber, Table I shows usage and bid prices since 1985. in 1985, one Arizona company became the owner of all asphalt rubber patent rights, which had previously been owned by two Arizona companies. As can be seen, since 1985 asphalt rubber prices have been going down. ADOT monitors the price of all the products it buys and has used asphalt rubber only when its usage appeared to he well suited to the problem and cost effective. In 1992, the patents on asphalt rubber began to expire. Since then, the asphalt rubber price has continued to drop with increased competition. Table 2 shows the cost of asphalt rubber mixes compared to dense-graded mixes made with neat asphalt binders.

Reasons for Use

In general ADOT is using asphalt rubber as a binder in mixes to reduce reflection cracking, improve durability of surface courses and in urban areas to reduce noise. By using asphalt rubber as a binder the film thickness is increased to a value of 19 - 36 micrometer compared to the typical dense- graded hot-mix asphalt film thickness of about 9 micrometer. The grade of asphalt binder used as a base to make asphalt rubber is an AC-IO, in contrast to typically stiffer grades of AC-20 and AC-30 used in mountains and AC-30 or AC-40 used in the deserts in dense-graded mixes. The 20 percent ground tire crumb rubber particals change the asphalt rubber temperature susceptibility such, that at high temperatures, it is much more viscous than the neat asphalt. However, at cold temperatures, the asphalt rubber acts more like an AC-10 asphalt. SHRP asphalt binder tests indicate that it can be graded from a PG 70-22 to a PG 82-28, which is indicative of a low temperature susceptible asphalt binder. Typically, the asphalt rubber mixes are 12.5mm to 25mm thick when open-graded and 25mm to 50mm thick when gap-graded. For Arizona's climate and materials, asphalt rubber appears to provide an excellent durable wearing course. As an extra benefit, the ground tire crumb rubber from nearly four million tires has been recycled since 1988 in the making of hot mixes with asphalt rubber.