Tires Make The Road - The Latest In Asphalt Rubber Pavements

By Les Jorgenson

I recently participated in an Asphalt-Rubber Design and Construction Workshop sponsored by the Northern California Rubberized Asphalt Concrete Technology Center and the Rubber Pavements Association, where the latest information on the use of asphalt rubber was presented.

Asphalt rubber is defined in ASTM D8-88 as "a blend of asphalt cement, reclaimed tire rubber, and certain additives in which the rubber component is at least 15% by weight of the total blend and has reacted in the hot asphalt cement sufficiently to cause swelling of the rubber particles." Although it is also referred to as asphalt rubber concrete, rubberized asphalt and asphalt rubber hot mix, in this article I will use the term asphalt rubber to describe the mixture of crumb rubber, ground from tires, that is blended with a paving grade asphalt for use primarily in pavement layers and chip seals.

Over the past twenty years, the design of asphalt rubber mixes has taken many paths. The states of Arizona, California, Florida and Texas have led the way in evaluating asphalt rubber pavements. In the late 1980's, Arizona began using gap-graded mixes. A gapgraded aggregate specification allows space for the crumb rubber particles. California followed with the creation of an Asphalt Rubber Hot Mix Gap Graded Specification. Today, this Caltrans specification is the most popular mix used by agencies in the United States.

Generally, the rubber component is 15% by weight of the total asphalt blend, although some states use as much as 20%, while other states use as little as 10%, with good results. The higher rubber content will enhance the performance of the asphalt rubber pavement, but a rubber component rate above 20% by weight will make the binder more viscous, which in practice has resulted in problems at the batch plant and with the paving machine. This is one case where more is not better.

Research studies over the past fifteen years have confirmed the success of the reduced thickness design approach for Asphalt Rubber Hot Mix - Gap Graded Mixes. The Caltrans design method allows for a 50% reduction in pavement thickness when this mix is used in lieu of the standard dense graded asphalt concrete mix. The gap graded mix allows for a higher binder content, and when combined with the crumb rubber, results in a pavement with much greater flexibility and durability. To date, Caltrans has constructed over 750 successful reduced thickness projects.

Except for the addition of the crumb rubber, the process for making asphalt rubber is the same as that for making conventional asphalt concrete. The "wet process" consists of adding crumb rubber to the paving asphalt prior to delivery to the pug mill mixer, and has proven to be the most desirable method. In this process, the crumb rubber is added to the paving asphalt in a blending tank. After blending is achieved the mixture is pumped to a reaction tank where it is kept at 375 F for 35 to 45 minutes before being sent to the pug mill mixer. The use of asphalt rubber mixes will normally tie up the hot plant for the full days production, so that the hot plant will not be available to produce other mixes during this time. Aggregate and asphalt mixing temperatures must be higher than normal to deliver an asphalt rubber mix at 300 to 350 F to the project site. Due to the high mix temperature, blue smoke may be a problem, as well as noxious vapors. The vapors are not considered toxic but odor masks should be provided to the paving crews and inspectors.

The placement and compaction of the asphalt rubber can be performed with conventional equipment. Due to the stiffness of the asphalt rubber, the breakdown rolling must be carried out at 300 to 325 F to achieve the desired compaction. The finish rolling must be completed above 175 F to avoid pickup. The asphalt rubber is usually tacky until the paving mat cools. It may be necessary to apply a dust-coat or lime-water so that traffic can be returned sooner to the pavement. The best paving results are achieved when the surface temperature is greater than 85F. The pavement thickness should also be 0.15 feet or greater. Special attention is required during the paving operation to control the speed of the paving machine, so that the breakdown rollers stay close to it and so that the material stays in the truck until ready to be spread. This is best accomplished by requiring the contractor to submit a Paving Operations plan that details the entire process.

The slower production of asphalt rubber mixes could result in less tonnage per hour delivered to the project -- thus increasing costs. Move-in costs for special equipment, higher electrical power costs and the 80% to 90% reduced production capacity required for the asphalt rubber blending and reaction process also increases costs. Small projects, such as those under 10,00 tons, will also cost more. Recently the cost of asphalt rubber has been 60% to 90% more than the cost of standard dense graded asphalt concrete mix.

Since it is no longer considered an experimental product, the use of asphalt rubber is on the rise. The documented benefits of using asphalt rubber include reduced reflective cracking, traffic noise, design thickness and life cycle costs and increased fatigue life and resistance to rutting. Since asphalt rubber concrete consumes about three tires per ton mix, another primary benefit of it's use if the recycling of discarded tires, a big plus for the environment.