They look like ordinary potholes, and to motorists who skim over them at freeway speeds, they are only a minor nuisance.
But highway engineers say the small craters that have developed in the slow lane of the Ventura Freeway in Agoura and Westlake Village are a major headache.
The holes are evidence that the concrete used on the freeway is defective and probably will have to be resurfaced at great cost.
State Department of Transportation officials have diagnosed the Ventura Freeway's ills as a case of "reactive aggregate," a malady that four years ago surfaced on the Simi Valley-San Fernando Valley Freeway in Simi Valley.
On that freeway, breakdown of the concrete grew steadily worse until Caltrans undertook a $5.7- million resurfacing along a seven-mile stretch between Kuehner Drive and First Street.
The resurfacing, which is half finished, is scheduled for completion in a month.
Freeway Built 17 Years Ago
Resurfacing the affected part of the Ventura Freeway would cost $7 million to $9 million, Caltrans officials said.
The two stretches of freeway, both completed 17 years ago, are the only California freeways to be thus afflicted, according to Gene Berthelsen, Caltrans spokesman in Sacramento.
He said Caltrans engineers have traced the problem to unusually acidic rock, or aggregate, from P. W. Gillibrand Co.'s gravel pit in Simi Valley.
"With new concrete formulas we have developed, we now can neutralize the reaction," he said. "But of course, we had no way of knowing this would happen when those freeways were paved."
He said Caltrans officials were "somewhat alarmed" when the deterioration began.
"We have been aware of reactive aggregate since the 1930s, but thus far we have been free of this condition," Berthelsen said.
Caltrans officials said tests to determine which rocks will lead to defective concrete have only recently been perfected. However, it takes five years for the tests to provide an answer, said Walter Maloney, Caltrans Southern California spokesman.
The chemical process that destroys the concrete occurs when highly acidic rock reacts with cement, which is alkaline, the engineers said.
The reaction causes the rocks to expand and become brittle, said Bruce Dyar, Caltrans' Ventura region manager. Under pressure from traffic, the stones break loose, creating small, shallow holes.
Dyar said that, when the Simi, or 118, Freeway began to deteriorate in 1982, it "behaved exactly the way the Ventura Freeway is going now."
Between Chesebro Road and Westlake Boulevard on the Ventura Freeway, the lane farthest to the right, where most trucks are driven, has become pockmarked with holes 1 to 2 inches deep and 6 to 12 inches wide.
New Holes Keep Appearing
Caltrans crews have filled most of the holes with asphalt, but new ones keep appearing.
"On the 118, we filled holes for three years," Dyar said, "but the holes got more numerous and they spread from the slow lane to the fast lane."
Caltrans' solution was to put two three-inch layers of asphalt on the Simi Freeway.
Dyar said the asphalt should keep water, an essential part of the reactive process, away from the defective aggregate, thereby ending the deterioration.
"We figure that the surface should last another 20 years," he said.
Maloney said there is little Caltrans can do about the possibility that reactive aggregate will become a problem on other freeways.
"We can only sit and wait and hope these are isolated cases," he said.