Minnesota has found the secret to building quieter superhighways: AstroTurf.
Dragging the dense, wiry, plastic grass over fresh concrete gives roads a skid-resistant surface without the annoying high-pitched whine that turned Minneapolis neighborhoods against concrete in the 1990s, engineers say.
The whine is produced by tires passing over crosswise grooves put into the concrete to drain water and reduce skidding. To stop the noise, Minnesota has become the first state to eliminate the grooves in new highway construction in favor of surfaces roughed up with weighted AstroTurf or stiff brushes.
Miles of quieter concrete lanes now are opening in the Twin Cities metropolitan area, including the widened and rebuilt sections of Interstate Hwy. 694, and Hwy. 100 through the northwest suburbs. Drivers say they can hear the difference.
"It's a different hum, much better," said Bernice Swanberg of Golden Valley, who regularly drives the new stretch of Hwy. 100. "The difference is so marked," she said, that when she drove recently over old-fashioned grooved concrete on Interstate Hwy. 35, "I thought what in the world was wrong with my car."
Pete Livingston of Plymouth gives a similar report on I-694's new lanes. He drives the freeway daily and says: "The surface of 694 is much better -- the noise level is way down. It's a smooth ride."
The quest for quiet concrete began in the late 1970s after the Federal Highway Administration required grooves or "tines" to be raked across concrete pavement to drain water and improve traction.
Almost immediately, the noise of tires passing over the grooves became a public concern, said David Rettner, a former pavement research engineer for the Minnesota Department of Transportation who led a research effort to solve the noise problem.
By the 1990s, the whine had given concrete such a bad name that residents of the Bryn Mawr neighborhood west of downtown Minneapolis campaigned for a softer, quieter blacktop surface for the construction of Interstate Hwy. 394.
MnDOT chose concrete anyway, arguing that it provided a more durable surface for the high-traffic highway. But in 1997, just five years after the freeway opened, legislators ordered a costly application of blacktop over the new concrete to deaden noise.
With some legislators threatening to ban new concrete freeways in the metro area, MnDOT stepped up efforts to silence the whine. The department turned to Rettner, who was its pavement research engineer from 1994 to 1999. Two Wisconsin Transportation Department engineers also worked on the research.
"We had been building pavement with AstroTurf drag for lower-volume roads for a lot of years and it had always measured quiet, but it wasn't allowed by Federal Highway [on freeways] until we could demonstrate the skid resistance," Rettner said. "So we asked for permission to do a research project to see if we could make it quieter and make it skid-resistant."
In tests on different pavement textures for three years, the right texture evolved. One promising recipe offered good traction at first, but then wore down. On the next try, researchers made the surface so rough that it could withstand years of snow plowing without losing skid resistance, Rettner said.
"What we have out there now, if you were riding a bike and fell on it, it would rip all the meat off your bones. It is very skid resistant," said Rettner, who now is principal engineer for American Engineering Testing Inc. in St. Paul.
MnDOT won approval from the Federal Highway Administration to eliminate grooves on new concrete lanes in Minnesota. In new construction since 1999, Minnesota contractors have textured highways with AstroTurf or a similar technique employing stiff-bristled brushes.
The technique won't make older, grooved roads quieter. That requires refinishing the road surface. One refinishing process uses diamond grinding tools and costs $25,000 per mile for each lane, Rettner said. It typically is done as part of pavement repairs, not just for noise deadening, he said.
Other states are experimenting with AstroTurf texturing for new pavement, but they have not eliminated the whine-producing grooves, according to the Federal Highway Administration.
The new texture appears to be just as safe as the grooves, said Curt Turgeon, MnDOT's current pavement engineer. "We have looked at accident data and we haven't been able to see any difference," Turgeon said.
Steve Gerster, vice president of the concrete paving division of Progressive Contractors Inc. of St. Michael, said his company used the AstroTurf method on I-694 and on Hwy. 52 in Rochester.
"No question it's quieter," Gerster said. "They can't say a thing about noise with our slabs any more. And they stay that way forever."