Scrap Tire News recently sat down with Redmond Clark PhD, Asphalt Plus LLC, to talk about the use of crumb rubber in asphalt pavement in the U.S.
STN: Why do we modify asphalt?
RC: The vast majority of paved roads in the U.S. are asphalt construction. In about 20 percent of those asphalt roads, use conditions are so challenging (heavy traffic, many trucks, difficult climates) that the asphalt pave-ment requires further strengthening. Chemically modified. asphalts – usually modified with various types of poly-mers added to liquid asphalt – have been used success-fully in those applications. Those polymers are usually added into the liquid asphalt oil and then “cooked” before use in asphalt mixes.
STN: Crumb or ground tire rubber is one of the materials used to modify asphalt today. Does crumb rubber make a difference in modified asphalt?
RC: When crumb rubber was first introduced into asphalt pavements many decades ago, the challenge was to first find out if rubber made a difference in pavement performance, and the second challenge was finding out whether any improvements were cost-effective. After more than fifty years of trial and error, there is no question that the proper use of crumb rubber will produce modified pavements that are the equal of polymer-modified asphalts. In addition, the best data on rubber modification of asphalt pavement is pretty straightforward: crumb rubber serves as a flexible aggregate, not as a chemical modifier of the liquid asphalt binder. The questions on cost are still open.
STN: What is the status of crumb rubber use in asphalt in the U.S?
RC: At the moment, best guesses about the amounts of crumb rubber going into asphalt pavement top out at about 200 million pounds per year, making this a moderately important application for recycled tires (<3% of the weight of recycled U.S. tires). Most of that rubber is being used to produce modified asphalt in three states, but the crumb rubber /modified asphalt pavement market potential is enormous. In the U.S., the modified asphalt market could consume more than half of the recycled tire rubber annually.
STN: What is holding rubber asphalt back?
RC: The answer is complicated, but it boils down to three primary reasons: state DOT (Department of Transportation) pavement specifications, where /how the rubber is added to asphalt and the costs of rubber use in asphalt.
- Every DOT has different use environments and different materials, so every state has developed their own ways for building and maintaining roads. The introduction of new materials (like crumb rubber) into one state’s specification often becomes a very focused effort that does not move easily_ to another state. It takes time and money to change a specification.
- Rubber can be added to the liquid binder at an oil blending terminal (terminal blend, wet process) or rubber can be added at the asphalt mix producer site (dry process or Plant Mix). The termi-nals already blend polymers. At present, most rub-ber is also added to the binder at terminals, and as a result, rubber modified asphalt binder costs as much or more than polymer modified asphalt.
- If costs and performance for rubber modified asphalts are equivalent to polymer modification, states have little incentive to add rubber to their specifications and to their pavements.
STN: Can you tell us about new technologies gaining ground in the modified asphalt market today?
RC: A number of states have now specified or are considering specifying the addition of rubber to asphalt during the mix production process. The “dry process” or “Plant Mix Process” allows the metered introduction of engineered crumb rubber directly into the mill where heated aggregate and binder are mixed together. Field performance testing shows no material difference between polymers, terminally blended rubber and rubber added at the asphalt producer site. By eliminating terminal blending and associated fees, asphalt producers can dramatically lower their costs to modify asphalt, often by as much as 40 percent. Unlike wet process rubber additions, Plant Mix asphalts do not require special handling equipment during asphalt laydown and compaction. Lower costs are expected to spur more rubber use in pavements.
STN: What does the future look like for rubberized asphalt?
RC: Well, the Georgia DOT was an early adopter of this approach and has incorporated the process in their specifications. More than half of the modified asphalt pavement in the state now includes crumb rubber, and that percentage is expected to grow. Other states are following. Ten states now have dry mix or Plant Mix test projects on the ground with up to ten years experience in pavement performance, and all report results similar to those in Georgia.
Asphalt pavements represent one of the great untapped markets for recycled rubber, and with the growing number of Plant Mix successes, there is real hope that this rubber asphalt will make great strides in the next five to ten years.