Fighting fatigue

Asphalt Article February 12, 2008
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Brooks Davis expects alligator sightings in his native state of Florida. It’s the alligator cracking on roadways he won’t accept.

“These roads aren’t heavily traveled, yet they’re under all sorts of pressure,” said Davis, reclamation specialist for Walton County. “Eventually, they get fatigued. It’s our job to make the most of the money we have and fix them.”

It is an ongoing battle, to say the least. Elements affecting the roads include extreme temperature fluctuations and tropical storms. “There has been a lot of rainfall from the hurricanes over the last few years,” Davis said.

The conditions take a toll. Eventually the road base breaks down, and the surface quickly follows suit. The deteriorating roads historically have not been cheap to repair—at least not if the work is done right. But committed communities like Walton County are finding ways to get the work done properly and at an affordable price. Rebuilding a road that lasts typically required excavators, haul trucks, aggregate and blade work. “It was quite a process,” Davis explained. “It was expensive in terms of materials and hauling costs—not to mention aggregate.”

But Walton County now uses reclamation and stabilization almost exclusively. “This road we’re working on here could cost $45,000 the old way,” Davis said. “When we’re done with it, it will last longer and cost a fraction of that amount.”

 

Doing it themselves

Davis and his crew handle the county’s reclamation and stabilization work, a proc­ess many other governmental agencies outsource.

“We’re efficient; it costs the county substantially less than hiring an outside contractor to handle the work,” Davis said. “We also feel we get it done more quickly because Walton County roads are our one and only priority.”

The county employs well-trained operators. It also has engineers and other experts with advanced degrees with an emphasis on asphalt paving. In addition, Walton County recently invested in a Cat RM500 Rotary Mixer. “It’s a cut above anything else that’s out there,” said Davis. The rotary mixer is designed to work well in both full-depth reclamation and soil stabilization applications.

 

Reclaiming the road

The crew worked this spring on Woodland Bayou Road, which had an unstable base and an excessively cracked surface. The road is one of many leading to residences off state Highway 98. Woodland Bayou Road runs for 4,250 ft before dead-ending.

The crew stabilized and reclaimed the road with the Cat RM500. The process began with a truck placing between 88-100 lb of lime per cu yd of base material. Each truck held 25-27 tons of lime. It took the contents of three trucks to cover 1/2 mile.

The lime was placed on the existing asphalt surface, which is typically 1-2 in. but can run as deep as 6 in. The RM500 then made a pass at a width of 8 ft and depth of 8 in. The rotary mixer blended the lime with the material that resulted from the pulverization of the existing asphalt.

Leading the rotary mixer was a truck carrying 5,000 gal of liquid soil stabilizer. A hose connected that truck to the rotary mixer. The RM500 mixed the stabilizer with the lime and the aggregate. The process required 29 oz per cu yd, or 10,000 gal per 1/2 mile. The base seal is a silicon co-polymer. Details are proprietary, but Davis said the liquid is “completely environmentally sensitive” and requires no special handling.

The RM500 covered 8 ft during each pass and 24 ft overall. The new asphalt surface, placed a week or two after the reclamation, has a width of 20 ft. “That extra length is insurance,” Davis explained. “It means we only pave on the prime materials that are away from the edges.”

The RM500 moved at 35-40 in. per minute on this job. “There is solid clay material underneath here, and it can handle that pace while properly grinding the asphalt and mixing the materials,” Davis said.

 

Finishing the job

A 12-ton roller followed the RM500. The 8-ft-wide roller made a pass up the middle, then back; to one outside area, then back; to the other outside area, and back. It then went back up the middle and kept going, thereby repeating the process. The roller was a single-drum set on the highest vibratory setting. The crew typically reached its compaction goal of 98% without much trouble, Davis said.

 

Happy with the process

Walton County officials are pleased with the stabilization and reclamation process. “It has many advantages,” said Aaron Warren, engineer for the county. “It’s environmentally sound and cost-effective. Not having to buy or haul aggregate is a substantial saving.”

Warren is a graduate of Auburn University and had considerable training at the National Center for Asphalt Technology. While studying, Warren learned about the benefits of soil stabilization and reclamation. After graduation he went to work and saw Walton County apply those practices.

“This road will last 30 to 40 years,” Warren stated. “It is resistant to storms, drought and freezing.” In fact, an American Association of State Highway Officials road test found soil stabilization creates one of the strongest possible bases, he said.

“The lime and base-seal product work together,” Warren said. “Lime makes a very durable base. The base seal accelerates the compaction rate. It acts as a preservative, when dried, but it also retains flexibility. It almost acts like rubber.”

The base has PSI approaching 300, compared to portland cement, with a PSI of about 3,000, Warren said. “We don’t want a PSI of 3,000,” he said. “We want some flexibility because of the conditions. The worst thing we can do is make it too rigid. If we add too much lime, the base will take on more rigid characteristics and with time will form block cracking as seen in portland cement concrete pavements. The 300 psi compressive strength goal will allow the base to exhibit more flexible characteristics, which in turn complements the flexible pavement overlay. This will actually last longer than concrete.”

The proof was in the reclaimed road—completed quickly, and at a fraction of the cost, when compared with other methods of rebuilding.

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