Lehman-Roberts Co. test drives warm mix asphalt

While the Terex warm mix asphalt system is new for asphalt plants, it’s not an untested technology. As a matter of fact, the “new” system is adapted from the manufacturer’s patented foamed asphalt system, developed for its reclaimer/stabilizers and milling machines. It has been used in recycling and reclamation applications for 10 years.

“When we set off to develop our warm mix asphalt product, we wanted to build a system that was simple to install and operate, could be easily adapted to a wide range of asphalt plant technologies and would not significantly impact the producer’s cost per ton to make asphalt,” says David Emerson, director of product management for Terex Roadbuilding. “It was a natural progression for us to adapt our foamed asphalt technology for asphalt producers.”

To operate the system, a producer needs only water, a power source and a signal from the asphalt plant controls package. As a field retrofit, the warm mix asphalt system is equipped with a programmable logic control (PLC) to accurately meter up to four percent of water by mass weight of the liquid AC.

“We can vary the percentage of water, but we are finding that two percent mass of water is working well,” explains Joe Musil, senior engineering fellow for Terex Roadbuilding. With new plant purchases, operation of the system will be integrated into the plant controls package, eliminating the need for a separate PLC control.

The heart of this new Terex warm mix asphalt system lies in an expansion chamber that provides single-point mixing of water and heated liquid AC just prior to the entering the drum. A three-way valve installed to the liquid AC supply line allows producers to choose between running traditional hot mix and diverting binder flow to the expansion chamber for making foamed asphalt.

A high pressure, five horsepower, piston-type pump operating in a closed-loop system accurately injects water into the asphalt. “It’s really a simple system. Other than the water pump, there are no moving parts,” Nelson mentions.

Once the water and hot AC are mixed in the expansion chamber, the foamed asphalt is immediately piped into the drum to evenly coat the virgin aggregate and RAP. The pipe is designed to fit the producer’s specific drum mixer and is equipped with up to 24 spray nozzles, in which at all times all nozzles are used to coat the aggregate and RAP regardless of whether the plant is running at 200 or 500 tons per hour.

“The system can produce up to 500 tons per hour of warm mix asphalt,” Emerson explains. “Our system will work with either volumetric or mass-flow metering systems,” Musil adds.

Positive field results

Originally, Lehman-Roberts planned to produce the warm mix asphalt for a paving project Desoto County, MS. However, due to the inclement weather during the evaluation and since this was the first time the producer worked with the new mixing process, company officials decided to move the evaluation to a more controllable application.

The crews paved the Lehman-Roberts’ office parking lot and the land surrounding the asphalt plant.

In an effort to obtain quantifiable test results, a Tennessee Type E surface mix with virgin aggregate and a PG 64-22 binder was selected. This same mix was successfully used in the fall of 2007 at the Memphis International Airport. In its hot mix form, the Type E asphalt is typically produced at temperatures ranging from 300 to 310 degrees F.

The warm mix asphalt experiments of this mix began at 300-degree F mix temperatures, which were methodically lowered throughout the evaluation. According to Nelson, Lehman-Roberts was “shooting for 270 degrees F,” and this target, as well as lower temperatures, was achieved.

Paving conditions were less than ideal. Ambient temperatures ranged from 34 to 37 degrees F, and the subbase was slightly wet. However, the mix went down well for the paving crews. “I was amazed at how long the mix remained workable for shoveling and luting,” says Nelson.

He also noticed that the only visible steam was coming from the drums of the double drum vibratory roller, not from the silo, truck or paver. The breakdown roller immediately compacted the asphalt mat - laid in a 2-inch-thick lift - behind the paver. The roller initially operated in static mode and the operator gradually added vibration to the pattern. A second double drum compactor provided the finish rolling.