Several major Baltimore-area highways have suffered an epidemic of shallow, gravel-filled potholes this winter, leading to numerous cracked windshields, chipped paint and rattled motorists.
State highway engineers, who are scrambling to repair more than a dozen stretches of road, blame the problem on an asphalt surface material that has begun to deteriorate at least a year before it was supposed to.
Greg Horton, manager of Smith's Auto Glass in Towson, said yesterday that the pitted pavement has been good for business. In the past week, three drivers had come to his shop complaining that their cars were damaged by loose stones on Interstate 83 in Baltimore County.
"They say there's a gravel road up there, not a highway," he said.
Charles R. Harrison, the State Highway Administration's district engineer for Baltimore and Harford counties, said that a three-quarter-inch layer of what is called "popcorn mix" surface asphalt began to erode -- or, as he put it, "unravel" -- late last year in isolated patches along state-maintained roads. The roadway leprosy spread much further, he said, after a rapid succession of freezes and thaws in January.
Popcorn mix, used widely in this country since the 1970s, is a porous material designed to drain water away from the road surface, reducing hydroplaning, skids, tire noise and puddles that get splashed on windshields. In Maryland, use of the material has been limited to interstate highways and certain other major roadways.
The mixture is generally expected to last a minimum of between six and seven years, Mr. Harrison said. But the formula used by the state between 1982 and 1987 apparently has begun decaying after as little as five years, he said.
The erosion, Mr. Harrison said, creates a rough, noisy ride and has triggered complaints about minor damage from spattering stone. But the pitted roads still provide plenty of traction and protection against skids, he said.
The problem appears worst along I-83 southbound between Shawan Road and Middletown Road, Mr. Harrison said. Other patches of loose popcorn are scattered along Interstate 795; the Baltimore Beltway through the I-795 interchange; U.S. 40 West in Baltimore County; the York Road overpass at the Beltway; Liberty Road near the city line; Route 140 near the Carroll County line; and along the Beltway near the Anne Arundel County line.
There is similar erosion along stretches of U.S. 15 in Frederick County, U.S. 40 West and Route 32 in Howard County and the Capital Beltway, he said.
P. Russell Ulrich, a spokesman for the Highway Administration, said most of the damage appears to be in the Baltimore region.
The crumbling, Mr. Harrison said, seems to occur where a "dry"popcorn mix, which used 1 percent less liquid asphalt than the current formula, was applied in the mid-1980s.
One theory state highways officials are investigating is that the stone used in the dry mixture may have trapped more of the liquid asphalt than it was supposed to, weakening the mixture and making it susceptible to freeze damage.
Different popcorn mixtures used in Maryland before 1982 and after
1987 have shown no unexpected deterioration, and some early popcorn mixtures have lasted up to 12 years.
Sweeper trucks have been cleaning up loose asphalt daily along the effected roadways in Baltimore County, Mr. Harrison said, and seven patching crews are temporarily covering popcorn potholes. On I-83, road crews using milling machines have ground off the eroded surface. The state plans to lay a new surface when the weather turns warm.