WASHINGTON—When choosing to use crumb-rubber-modified asphalt for road projects, states have two processes available, but a third method is gaining popularity. To date, the predominant technology in the asphalt rubber field is the so-called ``wet'' process, which describes any method in which crumb rubber is added to asphalt cement prior to incorporating the binder. In this way it differs from the ``dry'' process, in which crumb rubber is added directly to hot mix asphalt in the mixing process.
Arizona and Florida use only the wet process. So does California, with the exception of an experiment with the ``PlusRide'' dry process for hot mix asphalt in the 1970s, according to Bob Doty of the Pavement Branch of CalTrans Headquarters Laboratory in Sacramento, Calif.
The El Paso district of the Texas Department of Transportation laid some dry process asphalt rubber in 1993, said David Head, director of construction for that district. But the El Paso project didn't turn out well, according to Mr. Head.
``At the time, the course matrix high binder was new to the state of Texas,'' he said. ``There were problems with the mix not sealing totally.''
While there are several different varieties of wet-process asphalt rubber, there are two basic technologies—one invented by Charles McDonald, an engineer with the City of Phoenix, and the other developed by Arizona Refining Co.
There is very little difference between the two technologies, according to Donna Carlson, Rubber Pavements Association executive director. The main difference is the Arizona Refining method allows the use of extender oils.
California uses mainly the Arizona Refining process, whereas the McDonald method holds sway in Arizona and Florida. Both processes are used in a formula the states have refined for their own use, Ms. Carlson said.
The RPA recommends using wet-process asphalt rubber containing at least 15 percent crumb rubber. It is only asphalt rubber of this ilk that meets the RPA criteria for laboratory results, field performance and continuous real-world use in more than one state, according to Ms. Carlson.
Advocates of dry-process asphalt rubber, however, point to successful projects—mostly in Southern California—as proof of the intrinsic value of that method.
``The only difference between the wet process and the dry process is the method of introducing the crumb rubber to the asphalt,'' said Barry Takallou, president of TAK Consulting Engineers in Laguna Hills, Calif.
Mr. Takallou developed the ``generic'' dry technology, which is based on PlusRide but allows greater variability of application.
Frank Lancaster, materials engineer for Los Angeles County, speaks highly of both the wet and generic dry process.
``We've laid over 1 million tons of asphalt rubber since 1992,'' Mr. Lancaster said. ``I think we've saved millions of dollars over that time, because we lay it only at half the thickness of regular asphalt. There have been no failures yet, and some of it's been down for more than 10 years.''
Up to now the county has used only wet-process asphalt rubber, but it is switching to the dry process, he said.
``We've been evaluating the dry method for the past several years, and we can't see any difference between it and the wet process,'' Mr. Lancaster said.
Meanwhile, another technology may be arising to challenge both the wet and dry process. Mr. Lancaster said he is impressed by the so-called ``terminal blend'' developed and patented by Wright Industries in Texas and marketed by Paramount Petroleum in California.
With terminal-blend asphalt rubber, the rubber is blended into the asphalt at the refinery. ``They ship the asphalt rubber binder to the asphalt plant, just like regular asphalt,'' Mr. Lancaster said. ``We just buy it pre-mixed.''
Los Angeles County has used the terminal blend in two projects so far, as well as conducting extensive tests on it.
``So far its performance has been equal to wet and dry,'' Mr. Lancaster said. ``The new terminal blend is turning out a little bit better.''
Ted Flanagan of Wright Industries, who developed terminal-blend asphalt rubber, could not be reached for comment.
Manufacturers of asphalt rubber remain adamant their products eventually will prove themselves beyond the southern rim of the U.S.
``Climate and topography are not a factor,'' Mr. Takallou said. ``It all depends on the contractors. If they're trained for it, either the wet or the dry system will work.''
Polyphalt Inc., a Toronto-based manufacturer of various blends of rubber-and polymer-modified asphalt, said it has proof of asphalt rubber's worth in northern climates through its own projects.
The company recently completed two projects for CalTrans involving asphalt rubber—one on Interstate 30 on the south shore of Lake Tahoe, the other on Highway 88 in the Amador district of the Sierra foothills.
Both areas are mountainous, with snow and extreme temperatures, said Polyphalt's Larry Firmin, but the asphalt rubber pavement performed extremely well in both places.