Rough Roads Asphalt Still Rules Road Science Has Yet To Come Up With A Better Way To Build A Highway
It’s not your father’s Oldsmobile. But it is your father’s pavement.
In the 40 years since America started its road-building blitz, cars have gotten faster, trucks heavier, tires sturdier and streets more crowded. The number of licensed drivers has more than tripled.
But the science of street paving has changed little, even though roads are taking more abuse than ever.
There is no road-building equivalent to the jet engine, the transistor or the microchip. There has been no light bulb in a scientist’s mind to radically change America’s 2.4 million miles of pavement.
“The changes are evolutionary, not revolutionary,” said Joe Mahoney, a civil engineering professor at the University of Washington and a student of pavement for 30 years.
Some communities add rubber and others have tried adding glass, but most stick with the original recipe for pavement - 95 percent crushed rock and 5 percent asphalt, a petroleum byproduct. With minor variations, the formula has been used for thousands of years.
Applied under the right conditions and at the right thickness, and given proper maintenance, most Washington roads will hold up fairly well for 20 years.
Yet many European countries build roads to last a lifetime. And streets in Phoenix require less maintenance and are far quieter than those in Spokane.
Engineers say it’s no secret why those roads are better. Phoenix doesn’t have our harsh winters and spends half again as much as Spokane on pavement. Swedes pay $4 a gallon for gasoline, with a good share earmarked for roadwork.
“We can build roads (to last) for 50 years. They’re just going to cost two and a half times as much,” said Gary Nelson, state road designer.
When a street is paved in Spokane, the process is strictly by the book.
The soil is graded, then compacted to a depth of about two feet, or replaced with more suitable fill. That’s topped with a bed of crushed rock up to 6 inches deep.
Asphalt goes on top - three inches for residential streets, four to six inches for arterials. There may be two layers: a base made with large gravel for strength and a cap made with smaller rocks for a smooth, quiet ride.
The cost ranges from $15 a square yard if new asphalt is laid right over the old, to $43 if the road must be reconstructed.
In many Western European countries, the soil is compacted much deeper, and the layers of gravel and asphalt poured much thicker.
There are more important differences between American and European approaches, said John Harvey of the Pavement Research Center at the University of California at Berkeley.
In America, the government typically tells contractors how to build a road. As long as the work meets government specifications, the contractor has met his commitment. Anyone who exceeds the specs will be under-bid by a competitor.
“They’re being paid to meet the requirements and come in at the lowest cost,” said Harvey. “They’re not paid to optimize creativity.”
Often in Europe, companies are told how long the pavement must last, not how it should be built, Harvey said. The winning bidder is expected to maintain the pavement for five, 10, or maybe even 30 years.
“The initial cost is much higher because you’re passing the risk from the government to the contractor,” said Harvey.
The rubber meets the road
When a Phoenix street needs new blacktop, engineers turn to one of the most common discards of the mobile society.
Old tires are ground up, then mixed with asphalt oil at a ratio of one part tire for every four parts asphalt, said Phoenix street engineer Equbal Charania. The soup, called rubberized asphalt, is heated to the melting point before gravel is added.
“It’s more durable, it lasts longer, it’s more flexible,” Charania said.
Rubberized asphalt costs about 50 percent more than standard mixes, but requires less maintenance, he said.
“Over a 10-year period, I think you’re better off,” said Charania, adding that the rubber cuts road noise by 60 percent.
Some roads in Washington, including a few in Spokane, were built with old tires in the asphalt. Engineers say it seldom lasts longer than traditional mixes, and sometimes doesn’t hold up as well.
“Is it our somewhat colder climate? Is it our wetter climate? All I know is, I’ve seen a lot of this stuff tried, and it’s had real mixed results,” said Mahoney.
Ground tires also have been used as fill beneath roads in Washington, with disastrous results. In two cases, the rubber caught fire and crews had to dig up the roads and haul the chips to landfills, after first dousing them with water. It cost $3 million to put out the fire beneath one road in Western Washington.
Contractors bidding on state-financed projects can add rubber to the asphalt if they want, said Robyn Moore, state pavement engineer. But since the lowest bid wins, it typically isn’t used.
Rubber isn’t the only thing added to asphalt. Some East Coast communities tried glass, for streets that sparkled like Oz. But it didn’t last longer and cost more, Harvey said.
From time to time, entrepreneurs suggest mixing asphalt with everything from roofing shingles to broken porcelain sinks and toilets. The idea is rarely to make streets more enduring, Harvey said, but to get rid of something that’s a recycling headache.
The case for concrete
East of Sullivan Road in the Valley is a stretch of Sprague Avenue that’s a disgrace. Cracked and pockmarked, it gives one of the roughest rides of any pavement in the county.
Like other early Spokane roads, that patch of pavement is concrete, not asphalt.
Laid in the 1920s, the road’s long life illustrates what’s good about concrete. Its sorry state illustrates the bad.
Concrete streets cost about twice as much as asphalt. They hold up better under heavy loads and stop-and-go traffic, several engineers said.
So concrete was a natural for the bus zones around Spokane Transit Authority’s downtown station, and for a stretch of University Road outside the Valley bus station. It also was a good choice when the state resurfaced Interstate 90 up Sunset Hill, where heavy, slow-moving trucks caused rutting in the old asphalt.
“We’re trying it on some of the more heavily traveled intersections,” said Eldon Brown, city construction engineer.
But patching concrete is expensive and difficult. So, it’s a bad choice for urban streets that must occasionally be torn up for utility work. And it’s expensive to replace.
“There isn’t anything we can do with it … once it starts getting broken up,” said Chad Coles, Spokane County construction engineer. “Once it goes to heck, it goes in a big way.”
The future of road building
The mixed reviews for rubberized asphalt and the expense of concrete means Washington residents will continue to drive on traditional blacktop most of the time.
Careful installation is the key to extending its life, said Harvey.
Hot asphalt should be compacted until its air content is less than 8 percent, the maximum allowed for Spokane County roads, and the recommendation for city roads. Any more air, Harvey said, and it may last only half as long as expected.
Just as important is finding the right quality of asphalt and size of gravel for the conditions.
The most striking local example of a mismatch was Interstate 90 in the Valley, where asphalt that was supposed to last eight years deteriorated twice as fast before it was replaced in 1995. The porous asphalt, which was designed to control water spray, worked well in other states but fell apart under studded tires.
“That’s an example of a good effort gone bad,” said Mahoney at UW.
As part of a three-year federal study, pavement engineers have developed a computer program to help design the best roads possible.
“Superpave,” as the program is called, takes local weather and traffic patterns into account before recommending a mix. It was used for the first time in Spokane this year, to plan this summer’s repaving of a section of Francis Avenue.
The computer recommended a mix that is slightly different than state engineers would normally require, said Moore of the state transportation department.
There will be other, slight changes in road construction, though few that drivers will notice.
Mahoney said the state is starting to give contractors more latitude, and asking for performance guarantees. The transportation department is starting to build some roads to last 40 years, rather than 20.
Cities and counties probably are better off sticking with 20-year roads because guessing a neighborhood’s needs beyond two decades is difficult, said Harvey.
For instance, streets built for residential use must be strengthened if Costco moves into the neighborhood. An agricultural community needs wider roads if a couple of farmers subdivide their land.
“(Cars) may be twice as heavy,” requiring stronger roads, said Harvey. “Or we may be out of oil,” and people will be forced to change the way they commute.
, DataTimes ILLUSTRATION: Color photo
MEMO: This sidebar appeared with the story: DIFFERENT APPROACH In America, the government typically tells contractors how to build a road. Anyone who exceeds the specs will be under-bid by a competitor. Often in Europe, companies are told how long the pavement must last, not how it should be built, Harvey said. The winning bidder is expected to maintain the pavement for five, 10, or even 30 years.
This sidebar appeared with the story: DIFFERENT APPROACH In America, the government typically tells contractors how to build a road. Anyone who exceeds the specs will be under-bid by a competitor. Often in Europe, companies are told how long the pavement must last, not how it should be built, Harvey said. The winning bidder is expected to maintain the pavement for five, 10, or even 30 years.