Rubber Modified Asphalt

Asphalt Rubber Pavements Rubber additives to asphalt paving materials have been in limited use in portions of the south and southwest since the early 1960s, and as SAMIs since the early 1970s, but have not caught on in widespread usage due to a variety of factors, further described below. The recycled content of the rubber makes it an option to consider if seeking to reduce the impact of construction on the environment, and to reduce used tire waste. The benefits of this type of paving will likely fall outside the scope of typical architecture-related paving tasks, since the primary benefits are best realized in large scale applications in higher volume traffic situations.

There are two basic versions of asphalt paving material, one is known as Rubber-Modified Asphalt (RMA), and the other is referred to as Asphalt Rubber (AR). Both contain granulated rubber (called Crumb Rubber Modifier, or CRM) added to an asphalt paving material. RMA is a wet process production method, and AR is a dry process production method. In simple terms, wet process adds the rubber into the asphalt cement binder to allow a chemical reaction of the two, while the dry process includes rubber in the aggregate of the paving material during mixing.

Asphalt rubber paving contains 15% to 20% rubber, either Type I (crumb rubber) or Type II (crumb rubber and natural rubber). Rubber raw materials are generated from waste rubber products, primarily ground up tires, and sometimes ground up tennis ball manufacturing waste. Since about 270,000,000 waste tires are created every year, there are significant storage and disposal concerns. A typical tire will provide roughly 15 pounds of rubber, ground into particles 1mm in diameter. Approximately 1-2 million tires are used each year by the paving industry.

Benefits of asphalt rubber include:
Reduced traffic noise
Improved cracking and crack telegraphing resistance
Ability to extend road life (used as SAMI - Stress Absorbing Membrane Interlayer) under traditional asphalt to improve durability and reduce crack telegraphing
Better freeze thaw characteristics
Reduced road-spray
Improved surface friction in certain formulations (typically wet process)
Improved surface drainage reduces hydroplaning risk
Thinner pavement lifts required
Downcycle use of waste tires and manufacturing waste
Potential longer maintenance intervals

Limitations of rubber added to asphalt include:
Higher cost and reduced asphalt plant output
Lack of contractor experience
More finicky installation - temperatures, timing, rolling, vicosity
Lack of standard specifications
Lower surface friction in certain formulations
Less stone retention in certain formulations
Worse odor than typical asphalt
Questions about recyclability of old AR pavement

In terms of environmental inpacts, the use of recycled tires in asphalt roadways is compelling. In addition to reducing waste tire quantities, there is also a limited reduction in the quantity of other raw materials that need to be supplied for every ton of asphalt, offset by the equivalent amount of rubber. But the realities of the applications make the benefits less clear-cut if you look beyond traffic safety and road maintenance. In a mature industry like paving, changes are often difficult, especially with a product that may have additional challenges to implementation that discourage investment in the time and plant modifications to produce the product. For most building-related applications (parking lots, driveways, low volume roadways, etc) the added cost, lack of material availability, and contractor inexperience in many parts of the country prohibit its use even for those looking to incorporate more environmentally friendly materials into their work.

First image at left is Copyright © 2000 International Surfacing Systems

AGB 5/2005