On a steamy summer day, a crew spreads asphalt on a road that looks, to a casual observer, like a typical rural road in Alabama. It's really a test track run by the National Center for Asphalt Technology, or NCAT. The tests done on the 1.7-mile loop are an important step in the continuing journey to build better roads.
"We're working to determine the best combinations of materials and the ideal thicknesses of those materials in order to minimize the life-cycle costs and investments in pavements," said Raymond "Buzz" Powell, who manages the test track.
Critical to building better roads, is making better asphalt. It covers 97 percent of the roads in the United States, roads that by any account are in rough shape.
A recent report by the transportation research group TRIP found 18 percent of U.S. roads are in poor condition, and 40 percent are in mediocre to fair condition. TRIP estimates the damage done by potholes, and extra time stuck in traffic because roads are being fixed, costs the average driver an extra $516 dollars a year in gas and repairs. Meanwhile, if taxpayers want them fixed, they are looking at an annual bill of $20.5 billion through 2028, according to the Federal Highway Administration.
That is where the Auburn, Alabama-based research center comes in. A research cooperative funded by 17 states, NCAT tests various asphalt mixes the states submit, looking for ones that can stand up to heavy traffic and extreme weather, and use enough local and recycled materials to keep costs low.
"The holy grail of asphalt is to use as much recycled, reclaimed material as possible," said Powell. "But do it in a way that has no impact on pavement life, or has a positive impact on pavement life."
Currently the asphalt you see poured on roads around your town contains anywhere from 20 percent to 30 percent reclaimed asphalt, or asphalt taken from roads being repaired and replaced, and reclaimed roofing shingles. The rest of the mixture is made up of a petroleum-based binder that acts like a glue, and local rocks or aggregates.
"Georgia has different rocks than Alabama, Alabama has different rocks than Nebraska," said lab manager Jason Moore. "You want to use stuff that's locally available 'cause that helps keep the costs down."
Moore said he and his staff will mix up four or five different asphalt recipes a day. The recipes are submitted by the states, and they are mixed up by interns from Auburn University or other lab technicians. The mixtures are baked for two hours and the resulting asphalt samples are tested in the lab's various machines for rutting, cracking and their ability to repel water.
"Moisture is one of the biggest problems for asphalt because we want them to be impermeable," said Moore. "Up north, where you have a lot of freeze-thaw problems, the water gets in there and freezes and it takes the binder off the asphalt, you have unraveling and (eventually) potholes."
Recipes that are rejected make asphalt that cracks too easily, absorbs too much water, proves fragile in extreme temperature or easily forms ruts from repetitive motion. Recipes creating asphalt that meets certain standards required by a state may be adopted once the lab testing is complete, or sent to NCAT's test track for additional testing.
Right now the track is being resurfaced with 46 types of asphalt. Each section is 200 yards long. Some of the sections are very thick, while others are very thin. The thick sections are studied to see how the asphalt mixes withstand the wear and tear of heavy traffic, the thinner sections are studied to get a sense of how thick the layer of asphalt needs to be to keep a road smooth for a long period of time. Once the track is covered, two eight-hour shifts made up of five trucks carrying payloads that are twice the legal limit, will drive the track every day for two years.
Results from the tests done on the track have resulted in significant cost savings for states, according to Powell. He said tests done on the track led to a change in national standards for how thick the asphalt needs to be on a road. Modern materials and modern paving methods have cut the thickness required, said Powell, saving state departments of transportation hundreds of millions of dollars and saving the country billions.