Recycling with CRABSby Kevin Hoagland
Idaho looks at an alternative solution to pavement rehabilitation.
In 2007, the Idaho Transportation Department (ITD) plans to rehabilitate Cleveland Boulevard, a portion of the Interstate 84 Business Loop (I-84B) in Caldwell. Last year, ITD retained Lenexa, Kan.-based engineering firm Terracon Consultants Inc. to provide design recommendations.
The project is complicated by the presence of mid-1900s trolley tracks located near Cleveland Boulevard's centerline. When the current roadway was constructed, the tracks were paved over, resulting in an excessive crown and preventing rehabilitation with conventional overlay. This trolley line will need to be removed to vertically adjust the crown.
Analysis by ITD and Terracon indicated that cement recycled asphalt base stabilization (CRABS), an in-place recycling technique, would result in a 10% to 15% grade raise or “fluff” of the compacted mixture.
CRABS traditionally uses existing pavement pulverized to a maximum particle size of 3 inches and blended with underlying base materials to depths up to 12 inches. Then, cement is added to the blended materials at approximately 2% by dry weight of the blended mixture; water is added to the roadway as it is shaped and compacted to produce a recycled base layer in preparation for an asphaltic concrete surfacing.
The addition of cement binds the fines to help stabilize the base layer. The method requires removing pulverized materials to match existing grade at intersections, curbs, and gutters on urban projects.
Rehabilitating the Cleveland Boulevard segment containing the tracks will require excavating and removing the tracks and existing pavement section to a depth of 16 inches. Useable materials will be processed offsite for reuse in the excavated roadway. A drainable, nonwoven geotextile will be placed on the sub-grade, followed by a 13-inch-thick layer of pulverized material.
The CRABS rehabilitation option breaks up the aged and cracked existing flexible pavement layer, reducing reflective cracking potential while providing a suitable base layer for supporting the new plant mix pavement.
“Reusing or recycling the existing pavement section materials will likely reduce the cost of rehabilitating this portion of I-84B,” says Steve Weiss, ITD project designer. In addition to the new plant mix pavement, existing pavement section materials will be reused in the CRABS layer, reducing disposal and new material costs. Construction costs for the I-84B project are estimated at $950,400.
— Kevin Hoagland, P.E., is a geotechnical engineer in Terracon's Boise, Idaho, office.