Recycled rubber hits the road

by Texas Department of Transportation

Out in West Texas, where there are miles and miles of highway, the rubber really hits the road. And West Texas motorists are glad of it.

When the Texas Department of Transportation determined an aging and cracking surface on US 385 from the Crane/ Ector county line to the south city limits of Crane needed to undergo a major rehabilitation to forestall a severe maintenance problem, a TxDOT engineer seized the opportunity to use an innovative design he is convinced adds years to highway life and provides external customers (the motorists who regularly used the highway) with an extraordinary ride.

The design uses crumb rubber from more than 200,000 recycled tires as an additive in the surface hot mix. Dan Dalager, who oversees TxDOT construction activity for the counties of Andrews, Crane, and Ector as the area engineer in Odessa, uses the recycled rubber in a crumb rubber modified hot mix asphaltic concrete technique for the surface treatment - with a hot rubber underseal.

The design used by Dalager called for 17 percent rubber in the asphalt binder, which retards the aging and oxidation of the asphalt, preventing it from becoming brittle and cracking. And the flexibility of the rubber in the asphalt resists and reduces cracking.

"Taken together, those capacities reduce the need for costly maintenance," said Dalager. "Studies have confirmed that the proven advantages of crumb rubber modified hot mix asphaltic concrete include increased pavement life; resistance to rutting, aging, and reflective cracking; improved skid resistance, and a significant reduction in road noise."

Although regular hot mix normally costs the Odessa District $30 per ton and the crumb rubber modified hot mix asphaltic concrete is $35 per ton, Dalager said that the increased life cycle and decreased maintenance costs more than offset the $5 difference, especially in light of the fact that the process requires no unconventional and costly equipment for placement.

As of today, two years after the completion of the rehabilitation project, US 385 from the Crane/Ector county line to the City of Crane is a picture-perfect West Texas highway. There is no noticeable wear, no surface cracking, no evidence of rutting, ample skid resistance, and the driving lanes exhibit an exemplary ride - both in terms of noise and smoothness.

At the beginning of the project, representatives of TxDOT, Jones Bros. and its major subcontractor, Cox Paving, discussed how to achieve consistent production rates of the crumb rubber modified hot mix asphaltic concrete to be used on a 14-mile stretch.

Cox Paving set up its mobile crumb rubber blending plant adjacent to the hot mix plant at the Jones Bros. project yard on US 385 and, after an initial period of experimentation, the combined plant began producing a consistent rate mobile ended asphalt for the laydown operation. The crumb rubber was provided from PolyTek, an Arizona firm which specializes in recycled rubber material. Production rates of between 3,500 and 4,200 tons per day were quickly established.

Daily tests were performed to insure that the materials and laydown process met everyone's expectations regarding quality and production. Dalager had used the design once previously, in an Interstate 20 project in the district, and Cox Paving had been involved in several of the state's dozen-or-so prior crumb rubber modified hot mix asphaltic concrete projects.

Jones Bros., a longtime highway contractor for TxDOT in West Texas, and its subcontractors brought in veteran crews experienced in every phase of conventional highway construction from plant setup and operation to initial traffic control and cleanup. The experience of the laydown crew was critical because one of the few disadvantages to the mix is its adhesive quality, which makes the material difficult to "rework."

The team commitment to problem solving proved its worth. In addition to dealing with the design of the plant, all parties participated in daily testing processes to insure material and job quality.

The ride achieved earned the contractor a $4,500 bonus, based on profilograph results, and earned accolades from the motoring public. There also was a noticeable unmeasured decrease in the amount of road noise.

According to tests, crumb rubber modified hot mix asphaltic concrete can lower noise by as much as 5.7 decibels. Since human hearing can distinguish noise level differences of 3 decibels or more, the average driver was immediately aware of the difference.

A motorist who uses the highway frequently added her own evaluation of the quality of the project when contacted regarding another project nearby. "Will this highway be like the one at Crane," she asked? When questioned she said, "That's the smoothest, quietest highway I have ever been on."

The project was completed under budget, with a savings of $116,333, and on schedule. Due to the combined efforts of Dalager, Jones Bros., Cox Paving, and other subcontractors, and the patience of the motoring public and local business owners, it produced win-win results for everyone.

Florida leads

Editorial by D. J. Carlson

Despite a strong run from ADOT, which used over 2.4 million tires in pavement in the 1998-99 construction season, Florida DOT still leads the nation by using 3 million tires in its pavements annually.

The Florida "process" is slightly different from that used in Arizona, California, and Texas in that it uses a lower percentage of rubber. None-the-less, Florida is successful in meeting the expected design life.

When Florida, a recycling state, was put on alert to begin using tires in highways its DOT did all the right things. It studied emissions, recyclability, skid resistance in the rainy climates of their state, and the draining capabilities of their mixes in the lab and in the field until it was satisfied it could build safe, cost effective roads using crumb rubber modified asphalt.

It is rumored that Florida went to a lower percentage of crumb rubber to avoid running head on into the patents that were then in place but have since expired. With lower percentages, many of their mixes do not meet the ASTM definition of Asphalt-Rubber, but that doesn't mean the product is not good. To the contrary, Florida reports that it is getting good performance from its rubber pavements.

Some have been critical of RPA for focusing only on the ASTM defined A-R, but there is only one criteria adopted by the RPA board that separates Florida from the A-R used in the west. That is a requirement that any RPA supported process be "routinely used by two or more states."

RPA decided it needed to set the bar high enough to exclude some of the "johnny come lately" products that flooded the market in anticipation of a federal mandate in the 1990s. Most of those products failed miserably and left states with bad memories about using crumb rubber modified asphalt.

The Florida process meets the following RPA criteria: It has had extensive laboratory research. It has had extensive field testing. It has performed successfully.

It just isn't routinely used by two or more states. Maybe that will change or perhaps Florida will decide that more rubber is better. The important thing to remember is that Florida DOT did the right engineering evaluation and came up with a good public process.

Remember that the next time the proprietary folks come calling. Meanwhile, a salute to Florida for its leadership.