Summary of report from the A-R 2000 Conference

From the 14th through the 17th of November 2000, this Conference, devoted solely to Asphalt-Rubber, was held in Vilamoura, Portugal In terms of both attendance and technical content, the conference was a great success, with more then 200 attendees from over 30 countries, many of whom had excellent papers to present. All accepted papers are now published in a Book of Proceedings, which was handed out to all registered attendees and is now available on a widespread basis. All published papers were reviewed and refereed to an independent technical committee of highly qualified experts.

Judging from the topics covered in the Proceedings and a field trip to a nearby crumb rubber facto and adjacent A-R construction site, the following three specific Asphalt-Rubber topic areas were addressed: Field Performance, Cost Effectiveness, and Environmental Effects. Remaining areas are grouped into Other Topics of general interest.

Field Performance

One of the areas where most data has been collected refers to the field performance of Asphalt-Rubber. In most cases, detailed follow-up studies were conducted after the application of A-R pavements. For in-service pavements, more than 30 years of information is already available. During the Conference, several authors summarized specific information covering this experience, as highlighted here.

George Way, PE, Arizona DOT.- "In Arizona, open-graded mixes generally contain 9 to 10 percent of Asphalt-Rubber binder whereas gap-graded contains generally 7.5 to 8.5 percent A-R binder. More than 2,500 miles of A-R pavements have been built on the state highway system since 1988 and, to date, field performance has been very good. As an added benefit, the crumb rubber from over 8,500,000 tires has been recycled into these good-performing A-R HMA projects."

Way presented details of the findings regarding the relative comparison of the ability of A-R to resist reflective cracking. The overlays with Asphalt-Rubber-Asphalt Concrete Friction Course (AR-ACFC) are often placed in Arizona, with thicknesses that range from a third to half of those normally use with conventional overlays or inlays. Even with far less maintenance costs Asphalt-Rubber overlays have markedly fewer cracks. Way also reported on a project where 50mm (2") of Asphalt-Rubber overlay 'is outperforming 100-200 (4-8")0 conventional mix.' Exactly the same traffic and environmental conditions exist on these two comparable pavement sections after more than eight years of heavy traffic loads located in a wet-freeze zone on an Interstate Highway in Northern Arizona. Other researchers and engineers also reported similar successes, a few of which are highlighted here.

Mark Watkins, Maintenance Engineer, City of Ventura, California: "The City of Ventura has been very pleased with the performance of its A-R pavements, all of which have been constructed within the past 10 years. Current plans include the use of spray-applied A-R chip seals as well. "

Dr. Shakir Shatnawi, California DOT. "Generally, the fatigue performance of thin Gap-Graded A-R overlays has been demonstrated to be superior to conventional dense-graded conventional AC overlays with unmodified binders."

"Please note that the term "gap-graded" as used in California and Arizona refers to a specific grading of the aggregate, which is not the same as is used elsewhere world when a gap-graded mix is referred to. In this case, there is not a true "gap" in the California and/or Arizona aggregate gradation but rather somewhat less material is contained within certain fractions compared to "dense" or continuously graded asphalt concrete mixes."

Shatnawi also reported on a review of A-R projects executed in California. "A review was conducted jointly by a team from Caltrans and industry. Among the 113 projects observed, 101 projects were rated good, six were rated fair and six were rated poor. This shows that 90% of the projects were rated good. The other 10% (the fair and the poor) need to be investigated."

"These projects exhibited stress but some of it is clearly not a material problem but due to other problems such as base failure, etc. It should be mentioned that all of these projects, which were overlays, were designed at reduced thicknesses (i.e. 50% of the required DGAC overlay). Also, I would like to add that the criteria used by the Caltrans-industry team were defined as follows:

Good: exhibited little to no distress and are expected to achieve their design life.

Fair: exhibited moderate distress but are expected to achieve their design life.

Poor: exhibited moderate to severe distress and are: not expected to achieve their design life."