Summary of report from the A-R 2000 Conference These comparisons were generally either made on the binder or mix properties. Extensive data has been presented and it would be far too exhaustive to detail the contributions of each papers. Two papers presented original research results with Accelerated Pavement Testing (APT). In most instances, laboratory results were in line with the field observations presented above; however, occasionally laboratory results and the conclusions these tests led to did not correlate with field observation. The reader is invited to go in depth into each of those papers. In many of these cases, comparisons were not made with materials identical to those used by State or other governmental agencies. Dr. Visser and Van Kirk independently reported on a joint effort between the California Department of Transportation and the South African Council for Scientific and Industrial Research, using a Heavy Vehicle Simulator APT device to accelerate the development of pavement distresses. The resistance to reflective cracking of 75mm (3") of conventional AC was com pared to 37 mm (1 1/2" ") and 25mm (1"), respectively, of a gap-graded Asphalt- Rubber mixture. The results proved to be more resistant to reflective cracking than 75mm of conventional AC, used as an overlay above fatigue cracked asphalt pavement. It was also apparent during the Conference that the term "Asphalt-Rubber" has been rather loosely used. The term as originally coined referred to the process currently implemented in Arizona and California, taking into consideration the wet process and 18-22% crumb rubber in the binder, satisfying the properties of ASTMD-6114. It is interesting to see that some of the papers presented in the conference referred to "Asphalt-Rubber" with processes that are quite different, as shown below.
 Larry Smith, PE, Florida DOT, retired, clearly demonstrated the influence of the amount of crumb rubber and the process type on trial sections in the State of Florida. Here it can be seen that the dry process under performs even conventional mixes. However with the wet process, performance increases (i.e., less cracking and patching) as the rubber content in the binder increases up to about 15% using dense graded aggregate mixes. It is clear that not all methods of processing crumb rubber will yield identical results. The reader must be aware of these differences when making comparisons or drawing conclusions about the merits of each "Asphalt-Rubber" product. A few papers also addressed the added benefits to the functional characteristics of pavements where Asphalt-Rubber has been incorporated, together with the proper aggregate gradation, on surface courses. Theron Roschen, PE, Sacramento County Public Works: The conclusions of the 6 year-study indicated that the use of rubberized asphalt on Alta Arden Expressway resulted in an average of a four decibel reduction in traffic noise levels compared to conventional asphalt concrete overlay. Way also reported in one of his presentations that tire road noise was reduced by three to six decibels when Asphalt-Rubber overlays were used. Mark Belshe, PE, FNF Construction, Arizona: After completion of each project, the different paving sections were measured for good rideability and numerous contractors participated in the bidding process and project award through completion. Data gathered indicated one thing -the more structural sections that contained rubber, the better the overall rideability. Way presented the results of a comparison of the evolution of IRI over several sections traveled by the same traffic at the same location. Based on all the information presented and discussed at the Conference, we feel that it is appropriate to state that no evidence was presented that indicated the Arizona or California method of producing Asphalt Rubber (wet method: 18-22% crumb rubber in the binder) cannot be implemented and used in most parts of the world. Specifically, it appears that the high binder content and high percentage of crumb rubber in the binder approach is best suited to inhibit, or even prevent, reflective cracking through overlays due to traffic loads. It can be seen as corroborating
information supporting this conclusion, the fact that George
Way reported on Arizona DOT's plans to construct several new
freeways with very high capacity, totally more then 400 million
dollars in construction costs, in which Asphalt- Rubber is specified
1-110 the upper surface course. One of these facilities will
have seven lanes each direction over a distance of 12 miles (19
km). 2: Asphalt-Rubber pavements may not be cost effective 3: Asphalt-Rubber may not be recyclable or environmentally friendly Based on the presentations made at the Conference, case after case, we are convinced that sufficient information has been presented so that those myths have been dispelled. It was clearly shown that the "wet" method of manufacturing Asphalt-Rubber has proved to be very successful, in almost any climatic region, as long as proper mix design, construction techniques and quality control during construction are carried out. The places with the longest history of using the wet method for routine A-R production are in Arizona, California and South Africa. In many cases, pavements laid more than 20 years ago are still performing flawlessly in all three of these areas. Some of the papers have tried to bring to light the reason for this successful performance and some are of the opinion that it is both related to slower aging mechanisms and the existence of unreacted rubber and reacted rubber within the Asphalt-Rubber hot mix. Although there were other methods of construction good quality Asphalt-Rubber pavements presented at the Conference, each of which either shows great promise or has been used successfully in specific applications, the methods used in Arizona, California and South Africa appear ideally suited to inhibiting reflective cracking through A-R overlays. These A-R mixes and surface treatments contain both high binder contents and 18 to 22 percent rubber in the binder, while still retaining their stability and resilience. However, it cannot be assumed that there is no room for improvement in Asphalt-Rubber. Although experience has driven the utilization and acceptance of A-R products, further research and a better understanding of its full potential are still needed. The following list resents further research and efforts needed, as identified by the Technical Advisory Board of the Rubber Pavements Association. This maybe a starting point for those wishing to pursue research in these areas, so that new efforts will expand our envelope of knowledge on the subject of Asphalt-Rubber. Comparison of Technical Merits
of Asphalt Rubber and Terminal Blend Products  In his closing statement, Dr. Jorge Sousa, conference chairman, stated "Based on data presented, as a taxpayer, I would I like to save on both construction and life-cycle costs by utilizing Asphalt-Rubber in our highways and streets. There is no reason to continue to use higher cost and lower benefit solutions, just because these solutions may have worked satisfactorily in the past. Where Asphalt- Rubber is an appropriate and advantageous engineering solution to a pavement problem. I would like to see it being used, to make better use of my tax money and to help save our environment." In other words, if Asphalt Rubber is cheaper, longer lasting, quieter, smoother, and makes sensible and environmentally safe use of our scrap tire piles, it should be used-in every country and throughout every jurisdiction. The Organization Committee would like to extend a note of gratitude to the sponsors and exhibitors for their monetary contributions and their help in making Asphalt-Rubber 2000 a huge success. The tremendous efforts of the Technical Committee of Asphalt Rubber 2000 are also hereby recognized and appreciated. It really looks as if this Conference may have contributed to make Asphalt-Rubber the Pavement Material of the 21st Century. Editors note: Copies of the AR 2000 proceedings are available. Please contact Richard Stubstad of Consul pav International at (805) 649-1211 or by e-mail: stubstad@aol.com |