What is Asphalt-Rubber?

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”. However common practice in Arizona, California, and Texas use at least 18% rubber.

When was A-R invented?

Engineers and chemists have been trying to incorporate rubber into asphalt since the 1920’s. However, most attempts were without success. In the 1960s, Charles McDonald, a former Federal Bureau of Highways (now FHWA) employee and later the Engineering Supervisor Materials Testing Section for the city of Phoenix, Arizona, developed the first successful time/temperature formula for incorporating scrap tire rubber into an asphalt paving material. This process is often referred to as the McDonald process, the “Arizona” process, or the “wet” process.

Why isn’t A-R more widely used?
There are several obstacles the industry needs to overcome:
1. Formerly a patented process prevented industry development. Last of patents expired in 1992.
2. Long held in “experimental” status.
3. “If it ain’t broke, don’t fix it” mentality. Many state highway departments prefer to use old materials and methods.
4. ISTEA mandate for use caused political fallout. RPA does not support mandates. A-R should be used by free choice based on its superior qualities.
5. “Impostors” (processes with non-reacted rubber additives) failures associated with A-R. F. Some specialized equipment is required.
6. A-R lowers life cycle costs but initial costs can be an obstacle.

How many tires are recycled with A-R?
It depends on the mix design specified. A two-inch thick overlay of A-R Hot Mix will use about 2,000 tires per lane mile. In a spray applied method (seal coats), about 500 tires will be used each lane mile. About ten million tires are recycled each year in paving applications.

Depending on the application selected, between 500 and 2,000 scrap tires can be used in each lane mile of pavement. This means that for a one-mile section of a four-lane highway, anywhere between 2,000 and 8,000 tires can be used in creating a safer, quieter, longer-lasting road!

The passenger tires are composed of in total about 70% rubber. The rubber is composed of synthetic rubber (27%), natural rubber (15%) and carbon black (28%). Other components include 15% steel and 16% fabric.

So, here is the formula how we calculate that about 1,000 tires are used in one lane mile one inch thick:  383 tons of GG mix are needed to pave one lane mile one inch thick.  If the binder is 8% by weight of mix, then 30.64 tons of binders are needed per lane mile inch.  If rubber content is 20% of the binder
then it is 6.12 tons of rubber.  The 6.12 tons of rubber is equal to 12,256 pounds, If 12 pounds of rubber are recovered from each passenger tire, then divide 12,256 by 12 = 1021 tires.

For more details, an excel file is provided with the calculation of pounds of rubber per lane mile in various paving materials and applications, please click here.


How much does it cost?
High Oil Prices have driven up the cost of asphalt over 250% since the mid 1990’s. According to some state DOT figures, asphalt was going for $140 a ton in the 1990’s and is now over $500 per ton of liquid. Asphalt has gone from 7 cents to 30 cents a pound. Recycled tire rubber has held steady between 12 to 17 cents a pound ($240 to $340 a ton) in the same period. Adding 20% rubber in the asphalt binder can lower raw material costs. The Cost Savings also comes from the long life cycle, decreased maintenance, and use of less material. In most cases due to the flexibility and strength of A-R, only half of the material is required when compared to regular asphalt concrete.
What are the benefits of using A-R?
There are many diverse benefits associated with asphalt rubber. It can be applied in reduced thickness. In the age of conservation of natural resources and recycling, the reduced thickness of A-R can save on the amount of aggregate required to resurface a road. That means less material will be mined from diminishing reserves and transported to the job site. Another benefit is the reduction of traffic noise. International studies have shown that A-R pavements can reduce traffic noise up to 85% in some cases. Generally, A-R will provide a 50% reduction in noise. (Also see Noise Study) The noise reduction can also reduce costs regarding the construction of sound walls, which run between $200.00 to $400.00 per linear foot. The most notable side benefit of using A-R rubber is that it consumes scrap tires. If 5% of the nation’s roads were surfaced with A-R annually nearly all the nation’s discarded tires could be completely removed from the waste stream. Waste tires cause problems. If buried whole in a landfill, they can “float” to the surface, break the “cap” or cover on the landfill and expose once buried waste to the environment. In whole tire stockpiles, they are ideal breeding ground for a variety of pests, such as mosquitoes. The EPA estimates that over $5,500,000 is spent each year to combat mosquito borne diseases such as encephalitis and yellow fever. In shredded tire stockpiles there is danger of fires which create environmental disasters.  

Another benefit which is seldom mentioned is the “thermal blanket” effect it has on helping to preserve the underlying concrete pavement.  Concrete pavements, which represent a substantial taxpayer investment, are subjected daily to stresses caused by the top and bottom of the slab not being the same temperature.  
These thermal stresses can be just as damaging to the structure as traffic loads and any method to mitigate these stresses will lead to a longer life of the investment.  A thin, easily-renewed layer of asphalt rubber open graded friction course mitigates the temperature differentials and it has been estimated this can reduce these stresses by as much as 20 percent.  Not bad for a construction product that also recycles as many as 1,000 scrap tires per lane mile!

How much does it save?
When A-R rehabilitation strategies are used on deteriorated Portland Concrete Cement Pavements, the savings have been substantial, often in the millions. When used as an alternative to reconstruction, the A-R can often cost one-fourth the reconstruction costs. Keep watch for the Life Cycle Cost Analysis due out in late 1998. (See also the report by George Way)

What kind of equipment is needed to make A-R?
Conventional paving equipment is used to apply A-R, the only specialized equipment required is the “rubber plant” which blends rubber with asphalt at the asphalt plant. These units are portable and are set up and operating on site in as little as one to three days. In spray applications, the tips of the spray bars must be enlarged to accommodate the viscous material.

Is A-R proven and reliable?
The RPA recommends only those A-R processes that have been subjected to extensive laboratory research and analysis, construction evaluated research, successful field performance and are routinely used by two or more states. The Asphalt-Rubber Wet process is the only one that currently meets this criteria. Many processes have been developed that use rubber, not all are successful. Unfortunately, they are often confused with A-R but do not have the same proven credentials. Asphalt-Rubber has been studied extensively by the FHWA, many Universities, States, and the industry since the late 1960’s. It is the most extensively researched modifier for asphalt pavements.

How long does it last?
There are many projects in Arizona where A-R was developed that have been in service for 20 years. A comprehensive life cycle cost analysis, comparing A-R to other paving materials has been done The study was done by Dr. Gary Hicks, Oregon State University with assistance from Dr. J.R. Lundy, OSU and Dr. Jon Epps, University of Nevada Reno. The final report is available through our library.

What makes it last longer?
The chemicals contained in the rubber retard the aging and oxidation of the asphalt preventing it from becoming brittle and cracking. Because of the flexibility of rubber in asphalt, A-R resists and reduces cracking. Cracks allow water and pressure to act on sub-surfaces which creates pot-holes and deterioration of the road surface.  22,937,600 rubber particles per ton of mix help fight cracking.  A-R allows substantially higher binder contents in hotmix and greater application rates in spray applications.  These higher binder quantities dramatically increase the life of the pavement structure.

Is it quieter to drive on?
Traffic noise studies have been conducted on all types of pavement surfaces. A-R has been proven to reduce the decibel level substantially as much as 85% when compared to concrete. A significant factor in traffic noise is the “grooves” cut in concrete to add traction and the joints and seems between concrete slabs. Tires make less noise on A-R. (See also noise study report)

Does the use of rubber cause dangerous emissions?
The federal government has funded comprehensive emissions testing program of asphalt and asphalt rubber fumes at seven sites through the U.S.  Initially reports indicate the emissions caused by using tire rubber in asphalt are no greater the conventional asphalt. The study was conducted by NIOSH. All asphalt  plants are held to stringent air quality standards. Responsible contractors have no unusual problem with emissions related to A-R.  The only noticeable difference is the aroma of heated rubber.
Can A-R be recycled?
Many agencies engage in the recycling of paving materials. A-R has been successfully recycled on many occasions. One of the reports is by the City of Los Angeles. LA., recycled a 12 year old A-R pavement on Olympic Blvd. They also performed an air quality impact of grinding, transporting and processing the asphalt rubber. The results of the testing showed that the recycled A-R reclaimed from Olympic Blvd. Met specifications and passed all test and is recyclable using either microwave technology or conventional mix design technology. The Air Quality testing report employee exposure to air contaminants well below the CAL/OSHA permissible exposure limits (PEL) and in most cases, below the detection limit.

Is A-R safe to use?
In the reconstruction of concrete roadways, less time is required to get the road back in a safe condition. Which means less time of congestion as a result of construction and less time that maintenance crews are working near moving traffic. The emissions of an A-R plant are nearly the same as a regular asphalt plant.

Can A-R be used in cold climates?
A-R has been tested in many cold regions. The State of Arizona routinely uses a combination of gap graded A-R Hot Mix and an open graded A-R friction course in the Flagstaff area.  The elevation is over 7,000 ft in that area of the state and the winters are extremely harsh with temperatures dipping to -20°F. The State of California has also successfully used A-R in the high Sierra Mountains. It has performed remarkably well. In Sweden, which we think anyone would agree is a challenging, cold climate, engineers have developed a special gradation using asphalt rubber which has proven to be very resistant to wear from tire chains and snow plows.  Asphalt rubber is also currently being used in such states as Alaska, Massachusetts and New Jersey as well as the Provinces of Ontario and Saskatchewan in Canada.  While we don’t recommend placing asphalt rubber in the cold, it performs very well in cold climates.  It is just a matter of proper mix design and following good construction practices.