Iowa City director of public works Rick Fosse heads up maintenance for some 280 roadway miles; he estimates 70% are concrete, and the remaining 30% are concrete overlaid with asphalt.
Iowa City's concrete pavement restoration (CPR) efforts consist of full- and partial-depth patching, diamond grinding, and sawing and sealing joints. Fosse uses diamond grinding to correct faulting, fix slab warping, improve the ride, and quiet tire-to-pavement noise. “Neighbors along our diamond-ground roads say they appreciate the lack of noise, and motorists like the improved ride,” he says, adding that the pavement where the city first performed CPR, eight years ago, is still holding up very well. “My advice to others is, ‘Be generous with your patching, both full- and partial-depth, and you'll be happier with the pavement in the long term,'” he says.
Like Fosse's team in Iowa City, road departments are increasingly turning to CPR to repair and prolong the life of their pavement. The technique's affordability, durability, and ease of use make it an appealing alternative.
CONCRETE: TRIED AND TRUE
Founded in 1823, McDonough, Ga., had aging concrete pavement around the city square and on access roads from the north or south. Some entrance routes had been widened with concrete in the 1940s.
In 2003, officials scrapped plans to replace with asphalt—deciding the change would detract from the city's aesthetic appeal—and opted to keep it concrete. Because the project was state-funded, it needed state approval.
“We expressed our desire to the state DOT not to overlay the project with asphalt,” says James Lee, city administrator. “It worked. We restored some of the concrete and replaced other sections. We were very pleased with the project.”
To minimize traffic disruption, Conyers, Ga., contractor Pittman Construction performed most of the work at night. The $6.2 million project covered 11 lane miles and featured:
Some construction on new alignment
- Grinding and resealing one section of concrete
- Completely removing and replacing some slabs
- Removingl an asphalt overlay and replacing the underlying concrete in another section.
In total, Pittman performed 25,000 square yards of full-depth slab replacements, 80,000 feet of joint sealing and resealing, and 68,000 square yards of diamond grinding. Use of a high-early-strength mix made it possible to open the area to traffic four to six hours after placement.
Douglas County, Colo., maintains 280 lane miles of concrete pavement, much of it on six-lane urban arterials with auxiliary lanes. In recent years the pavements—most more than 20 years old—have developed spalling and slab separation at the longitudinal joints.
Three years ago, the county experimented with Thermacrete, a patching material supplied by Deery American Corp., Grand Junction, Colo. In the past two years, the county has placed 33 tons of the polymerized material, which contains an aggregate and is heated inside a melter towed behind a truck. Hot liquid product flows from the melter; workers use shovels to scoop the material into place.
“On a 50° F day, we can open up to traffic in 20 minutes after we place the mastic product,” says Randy Teague, county road and bridge manager. “High-early-strength concrete would take several hours.” Typically a nine-man crew blocks off two lanes, jackhammers the spalls down to hard concrete, applies a conditioner, and places the Thermacrete.
By blocking off two lanes, Teague says, “In one day, we can apply one ton of Thermacrete in two longitudinal joints—one on each side of the slab for a half-mile. That's one mile of joint repairs in a day.”
The material costs $3.20 per pound, or $6400 per ton—enough to repair one linear joint mile. Teague figures the cost is just 3% of the cost to remove and replace a concrete panel.
— Brown is a freelance writer in Des Plaines, Ill.Do you know CPR?
Road departments find concrete pavement restoration to be a viable process.
“Transportation officials have transferred their emphasis over the past several years from system expansion to system preservation,” says John Roberts, executive director of the International Grooving and Grinding Association (IGGA). Road departments are increasingly turning to concrete pavement restoration (CPR) as a way to preserve their pavements.
According to the American Concrete Pavement Association (ACPA), the technique directly corrects the cause of pavement distress and adds seven to 12 years to pavement life. The association holds that some CPR projects have lasted more than 17 years before needing a second round of repairs.
Diamond grinding removes 0.1 to 0.25 inches of concrete from the surface. Grinding a pavement corrects faulting at joints and cracks, enhances surface texture, and reduces noise. The total square national yardage of diamond grinding jumped from 9.9 million in 2003 to more than 35 million in 2005, according to IGGA.
Common CPR terms:
Cross-stitching: adding reinforcing steel to hold longitudinal cracks or joints together tightly. This method is used on cracks in low-severity condition.
Diamond grinding: removing a thin surface layer using closely spaced diamond saw blades; creates a smooth, uniform pavement profile.
Dowel bar retrofits (DBRs): cutting slots across the joint or crack, cleaning transformation the slots, placing dowel bars, and backfilling with new concrete. DBRs link slabs at transverse cracks and joints so that the load is evenly distributed across the crack or joint.
Full-depth repairs (FDRs): removing at least a portion of the existing slab and replacing it with new concrete.
Joint and crack resealing: minimizes the infiltration of surface water and incompressible material into joints, thereby reducing subgrade softening, slowing pumping and erosion of the subgrade or sub-base fines, and limiting corrosion caused by deicing chemicals.
Partial-depth repairs (PDRs): removing deteriorated concrete, cleaning the patch area, placing new concrete, and reforming joints. PDRs correct surface distress and joint-crack deterioration in the upper third of the slab.
Slab stabilization: restores support by filling small voids that develop underneath the concrete slab at joints, cracks, or pavement edge.
For more information, contact the ACPA at 847-966-2272 or visit www.pavement.com.Recycling with CRABS
by 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.