Park Road is Ideal Warm Mix Project

When the Texas Department of Transportation solicited bids for paving work at Lake Mineral Wells State Park & Trailway, located west of Fort Worth, the original contract specifications called for a hot-mix asphalt design. The Lane Construction Corp.'s Roanoke, TX division proposed a warm mix design and was awarded the contract.

Along with the environmentally-friendly benefits warm mix provides, which was definitely a plus when considering the pristine beauty of the 3,282-acre park, the contractor had logistical issues that only warm mix could address.

According to John Rauer, plant manager for Lane's Bridgeport, TX facility, located approximately 60 miles from the project, hauling hot mix that distance proved challenging. 

Another challenge that had to be addressed in executing the project had to do with the tight working conditions in placing asphalt on some of the campsite areas included in the contract. Once mix arrived via 22-ton lowboy trailers, it had to be dumped at a staging area, loaded into tandem dump trucks that could then maneuver the tight paving sites.

And to add yet another challenge, the project was scheduled for construction between November 2008 and March 2009, when cooler temperatures would make it difficult to achieve density specifications with mix hauled a long distance and then transferred into smaller equipment before paving and compaction could actually begin.

Project details

Along with repaving numerous campsite and parking lot areas, the Lake Mineral Wells State Park project included reconstructing the main park roadway, which had to be milled, stabilized and then resurfaced with a 2-inch lift of warm mix.

"We had to mill 1 to 3 inches of old asphalt off the park roadway and then treat the flexible (aggregate) base with a cement additive (18 pounds of Portland cement per square yard)," explains Rick Schumann, project superintendent who's been with Lane for 24 years. "We couldn't just mix the old pavement into the base material (which is often done in a full-depth reclamation/stabilization preservation process), because there was a lot of vegetation growing into and out of the old surface."

After mixing the cement additive and water into the base, an 84-inch pad-foot roller was used to compact the material and a motor grader was used to smooth the surface and establish a one percent cross slope from the centerline to each edge.

"We used a sonic tracker to establish and achieve the correct slope before applying an AEP (16,314 gallons of anionic emulsion prime - cut 50/50 to minimize the environmental impact) prime coat over the new stabilized base," Schumann says.

On the campsite pads, Lane expanded the existing paved area by a foot on each side of the pads (replacing soil with Type B 1-in. minus aggregate) before covering the pads with a chip seal prior to placing the new warm mix overlay.

"We had to sealcoat everything (campsite pads, parking areas and roadway) before we could begin paving," notes Rauer, who's been with the company for 27 years. "We used 35,000 gallons of CRS1P (cationic rapid set polymer) emulsion and a Grade 5 (1/2-in. minus) aggregate for the chip sealcoat."

After completing all the preliminary work, the paving crew moved in with a Cat AP-655 8-foot track paver and a Cat AP-1055B 10-foot track paver to begin placing the 17,000 tons of TxDOT Type D warm mix required to overlay the 4.6 lane miles of park roadway, as well as the 87 campsites and parking lot areas. The PG 64-22 warm-mix design, with 3/8-in. minus aggregate and the liquid Evotherm DAT warm-mix additive, was produced at Lane's Bridgeport facility, which is an Astec Six Pack portable plant. The Bridgeport and Justin production facilities support two paving crews, as well as the needs of outside customers.

"We used the smaller paver on the tight campsite areas of the project and the larger paver with extensions on the screed to pave the 20-foot-wide park roadway in one pass," Schumann says. "We achieved our density requirements (92-96 percent) with a rolling pattern that included two Sakai SW900s and one Sakai SW328."

Mix left the plant at approximately 250 degrees F and dropped to approximately 245 degrees by time it arrived at the project, according to Scott Huber, Lane QC/QA manager.The mix was produced with Evotherm DAT, a chemical package at a dilution rate of 90/10, with the additive metered into the AC line at 5 percent per liquid ton. Huber says the 90/10 dilution rate provided better stability of the mix during placement, eliminating "tender" compaction characteristics.

"We recorded temperatures in the 220 range coming out of the screed and we were achieving density air voids of 5 1/2 to 6 1/2 percent with a rolling pattern of eight passes over the mat," says Huber, a 12-year veteran with the asphalt producer/contractor. "Each of our two large rollers made four passes over the mat, with the first roller making two vibratory and two static passes, and the second roller making one vibratory and three static passes. The smaller roller was used to make one final static pass to finish off the surface.

"With the rolling pattern right behind the paver, we've been able to achieve in the range of 93.5 to 94.5 percent density in a short time and we don't have to wait for the mix to cool down like you do with hot mix that has a tender zone," Huber adds. "We're following TxDOT's 341.020 warm mix spec and we've had no problems meeting the requirements when the temperature (ground and ambient) is cooler. I think that would have been difficult if we were working with hot mix on this project at this time of the year, especially with double handling the mix. And since we were able to achieve density a lot quicker with the warm mix, we were able to maintain the schedule and  reopen camping areas in time for weekend visitors."

Taking the lead

Lane's Texas division is taking the lead in proposing and constructing TxDOT warm mix projects. Since installing equipment to add Evotherm at the company's Bridgeport and Justin facilities in late 2007, the contractor has placed over 90,000 tons of warm mix.

Initially, crews experienced some difficulty working with a PG 76-22 warm mix design, but that was quickly resolved by changing the dilution rate of the warm-mix additive from 85/15 to 90/10 which allowed easier workability of both PG 76-22 and PG 64-22 binders.

According to Rauer and Huber, the simplicity of using the liquid Evotherm additive has made it possible to deliver a cost-effective mix design that meets TxDOT specifications, which follow the standard dense-graded hot-mix asphalt requirements, with specific requirements, including:

  • Warm Mix Asphalt (WMA) is defined as additives or processes that allow a reduction in the temperature at which asphalt mixtures are produced and placed. WMA is allowed for use at the contractor's option unless otherwise shown on the plans. The use of WMA is required when shown on plans. When WMA is required by the plans, produce an asphalt mixture within the temperature range of 215 degrees F and 275 degrees F.
  • Place (WMA) mixture when the roadway surface temperature is equal to or higher than 45 degrees and rising for night paving of PG 64 design, 50 degrees and rising for daytime operation; 55 and rising for night paving of PG 70 design, 60 degrees and rising for daytime operation; and 60 degrees and rising for night paving of PG 76 design, 60 degrees and rising for daytime operation.

John Cordary, P.E., area engineer for the Weatherford office of TxDOT's Fort Worth Division, says the park warm-mix project was the first for his office and he's pleased with the results.

"There have been several (warm mix) projects completed in the Fort Worth area, but this was the first opportunity we had to use warm mix on one of our projects," notes Cordary, who's been with TxDOT for 16 years. "I happy with the results and I anticipate using warm mix on future projects."

That seems to be the general sentiment TxDOT has expressed regarding the success of warm-mix projects if constructed over the past several years.

With the performance and environmental benefits WMA has to offer, TxDOT is receptive to using warm mix on any hot-mix project as long as the mix meets all the current hot-mix specifications and it doesn't add to the overall project costs.

As for the execution of the Lake Mineral Wells State Park WMA project, Lane Construction added yet another success story on the advantages of this emerging technology.

"On this job (Lake Mineral Wells State Park), the warm mix design allowed us to pave during the time of year when temperatures are cooler and working conditions required more handling of the mix, but at the same time we were able to successfully meet all TxDOT specifications and do so in a way that minimized disruption to park visitors," Schumann explains. "And in a setting like this, where wildlife roams freely and visitors hike throughout the park, the cooler temperature of the warm mix provided a much safer environment. It was really a great project to work on."

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