Q: Back in 2007 or 2008 you answered a question about rubberized asphalt for road paving and said that PennDOT was doing some test pavings to try out the technology. Can you follow up to see how the test paving projects went, and what PennDOT is planning for using the technology? How is rubberized asphalt faring elsewhere in the road-construction marketplace?
— Bruce Wilson, Upper Saucon Township
A: You're referring to a test project by PennDOT's Harrisburg-based District 8 using rubberized asphalt — macadam with "crumb rubber" or finely chopped pieces of used tire added to the mix — in a chip-seal paving job in York County in 2007, Bruce.
The results, according to state Deputy Secretary for Highway Administration Scott Christie, were quite good. But that was a fairly limited test in a chip-seal application — a very thin paving layer intended to squeeze a few more years of life out of the deteriorating road surface, delaying the need for a more extensive and costly milling and repaving.
In the meantime, PennDOT has been testing rubberized asphalt in a couple of full-scale repaving projects, and — to skip a lot of technical stuff involving provisional specifications and other details — officials have given the green light to statewide use of the product.
PennDOT spokesman Steve Chizmar was unaware of any tire-rubber road-paving work coming our way immediately in Allentown-based District 5, but project engineers now are free to use it. It's another materials option, Christie said, not unlike determining whether a concrete or steel-beam bridge works best in a specific application.
"It's another tool for them to choose, depending on circumstances," added District 8 spokesman Greg Penny.
There are two basic manufacturing processes for rubberized asphalt, according to Mark Belshe of the Rubber Pavements Association in Tempe, Ariz.
To oversimplify, in the "wet" process, crumb rubber is incorporated into the liquid asphalt before it is mixed with the aggregate (basically sand and gravel), Belshe said. You're modifying the liquid, which binds the aggregate, so this process is known as "binder modification." In the "dry" form, the rubber is added to the aggregate, replacing some of it; normally there's no modification of the liquid asphalt. The wet process was developed first, but it requires expensive specialized equipment, and promising new "dry" techniques have been developed, Belshe said.
PennDOT has approved the use of the dry form by adopting specifications for the product, and plans to begin pilot testing of wet rubberized asphalt, according to Christie.
Rubberized asphalt generally costs more than the plain stuff, though proponents contend that some or all of that money can be recouped by using less product and still getting longer life with lower maintenance costs. Some officials in California, where it's been used for more than 30 years, make exactly those claims and more.
Motorists in Arizona and Florida also have logged many miles on the stuff, which initially got me wondering whether it might wither under colder weather. However, it's also been used in Massachusetts, Nebraska and, to slam the brakes on my concerns, in Alaska. Studies have shown that rubberized asphalt reduces cold-weather cracking on Alaska's roadways. Belshe said the finished product performs well in cold climes, but warm conditions are required for application, "so your window for paving is smaller" and procedures must be followed closely.
Here in Pennsylvania, where the freeze-thaw cycle can be extra rough on roads, "We hope to gain [durability] on both ends of the temperature scale" with rubberized asphalt, Christie said.
And of course, there's a tractor-trailer's worth of environmental benefits. The tire industry claims an 80 percent recycling rate, which is great. But in our car-crazed culture, that still leaves some 60 million scrap tires with no good place to go every year.
According to Clemson University's Asphalt Rubber Technology Service, 500 to 2,000 discarded tires are put to good use in each lane-mile of rubberized asphalt, diverting them from landfills or fire-prone stockpiles, or preventing them from littering the roadside. The amount depends on factors including the process used and the thickness of the layer applied. (Asphalt, it turns out, is a surprisingly complex topic when you start cracking the books on it. For a primer on rubberized asphalt, turn to the 13-minute video "California Rubberized Asphalt 101" at rubberpavements.org.)
Clemson also reports that the product reduces "reflective cracking" in concrete overlays — a common problem that PennDOT officials have tried to address with other techniques, including on Route 378 in Bethlehem — and improves skid resistance while lowering the volume on tire noise.
Wayne Bowen of he Pennsylvania Recycling Markets Center, which helped jump-start PennDOT's approval of rubberized asphalt, said that essentially, the rubber content acts as a stabilizer preventing the liquid asphalt from draining down through the aggregate as quickly, holding things in suspension more effectively for even distribution and thus added strength and durability. Polymers, paper and other products sometimes are utilized for the same purpose, he said, but those materials don't provide a useful purpose for scrap tires.
Clemson reports that rubberized asphalt generally costs 20 percent to 30 percent more than the plain variety, but so does asphalt with polymer stabilizers added, and some or all of that money could be recouped through extended roadway life.
Does rubberized asphalt cost more than the plain variety? "Not necessarily in the end," Christie said.
According to Belshe's trade group, asphalt prices have risen far more sharply than the cost of crumb rubber in the past decade or so, tilting the road surface back in the direction of rubberized asphalt.
What's not to like about rubberized asphalt, so long as the costs can be managed? That's one of the issues PennDOT is researching in approving the dry type for regular use and turning the key on testing of the wet form.
I'll be shock-absorbed if motorists aren't riding on rubberized asphalt somewhere in the Lehigh Valley within the next few years, Bruce.
Road Warrior appears Mondays and Fridays, and the Warrior blogs at mcall.com. Email questions about roadways, traffic and transportation, with your name and the municipality where you live, to hartzell@mcall.com, or write to Road Warrior, Box 1260, Allentown, PA 18105-1260.
