Friday, March 2, 2012

ITD takes blame for micro-seal failure

Agency says materials may have been laid too late in the year

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By TERRY SMITH
Express Staff Writer

The Idaho Transportation Department is taking the blame for failure of a micro-seal coating on a 3.25-mile section of state Highway 75 just south of Ketchum, suggesting that the material may have been applied too late last year to successfully compact and harden.

"We're still not completely sure what caused the failure on that," Devin Rigby, district engineer for ITD District 4 in Shoshone, told local officials Thursday at a meeting of the Blaine County Regional Transportation Committee.

The micro-seal, applied in late September 2011 by Valley Slurry Seal, a subcontractor to Knife River Corp., began to visibly deteriorate in December. The micro-seal was laid as part of a 10-mile highway resurfacing project between Hailey and Ketchum, with Knife River as the prime contractor.

Rigby said ITD is responsible for the failure because the subcontractor "met the contract and they did what they were supposed to do and what we directed them to do."

The 3.25-mile section of highway, which stretches from Timber Way just north of East Fork Road to the bridge over the Big Wood River near St. Luke's Wood River, only received a 1-inch asphalt overlay, followed by the micro-seal, because that portion of highway is set for reconstruction and widening in 2013.

Much thicker asphalt was placed on the remainder of the 10-mile stretch of road and application of a micro-seal there was put off until this summer because of delays encountered in construction by Knife River.

Rigby acknowledged that ITD took "a risk" in having the micro-seal applied late in the season on the 3.25-mile section, but said a seal was thought necessary to protect the thin overlay from moisture.

Despite deterioration of much of the micro-seal, Rigby said, enough material bonded with the overlay to protect the asphalt.

Rigby said ITD is considering a hypothesis developed by Valley Slurry Seal that the micro-seal is failing because it was applied when temperatures were not warm enough for successful compaction.

"We did not get the warm weather to get the traffic to compact it down," Rigby said.

He said ITD is still researching the oil emulsion used in the micro-seal to see if there was proper "ionic bonding" with the gravel in the mix.

"Don't ask me what ionic bonding is supposed to be, but our lab checked it out and said the rock and oil were a proper fit," he said.

Rigby said ITD wants to make sure it knows what caused the problem before a micro-seal is applied to the remaining portion of the 10-mile stretch of highway.

He said ITD has worked with Valley Slurry Seal on numerous occasions in the past without any problems.

"This is the first micro-seal failure we've had," Rigby said.

Terry Smith: tsmith@mtexpress.com

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