R K Hall Construction Co., Ltd., Paris, TX, has emerged as a regional leader in the application of warm mix asphalt. The producer has 10 asphalt plants serving markets in Northeast Texas, southeast Oklahoma and southwest Arkansas, and three of these plants are now capable of producing warm mix asphalt.
Last paving season, about 30 percent of the producer's jobs included warm mix designs. Beginning in April and running through September 2008, the producer mixed and paved more than 135,000 tons of warm mix asphalt; approximately one-third included 20-percent RAP. The lion's share of it was placed on an application in Young County, TX, on U.S. Highway 380. Close to 70,000 tons of a TxDOT Type C surface mix with a 0.5-inch (12.7 millimeter) aggregate and PG 70-22 binder was produced for the job using foamed asphalt warm mix technology.
"Everything tested well in the lab and in the field for this project," mentions Brad Bradford, quality control manager for R K Hall.
"The Texas Department of Transportation (TxDOT) allows warm mix asphalt on a job if the contractor elects to utilize it," says Brad Bankston, operations manager at R K Hall.
The Foamed Asphalt Approach
All three of R K Hall's plants are capable of producing warm mix use foamed asphalt technology to achieve lower mixing temperatures. Foamed asphalt eliminates the need for costly additives, which can increase the asphalt's cost by dollars per ton.
In July, R K Hall equipped its existing Pottsboro Terex E3-300 plant with the company's new warm mix asphalt system. The components were installed a week before a planned July 17, 2008, warm mix asphalt demo held at the plant's location.
Operation of the warm mix asphalt system requires only water and a signal from the plant controls. The system combines heated liquid asphalt with a percentage of water, which results in AC expansion once it is injected into the drum to evenly coat the aggregate. "We built a system that is easy to install and operate, can be quickly adapted to a wide range of plant technologies, and will not significantly impact the producer's cost per ton to make asphalt," explains David Emerson, director of product management for Terex Roadbuilding.
This proven system is an adaptation of foamed asphalt technology available for more than a decade on Terex reclaimer/stabilizers. A patented expansion chamber delivers single-point mixing of water and heated AC prior to entering the drum. A high-pressure, 5-horsepower (3.7-kilowatt) water pump accurately injects a variable percentage of water (based on mix design parameters) into the asphalt.
A simple three-way valve installed on the liquid AC line enables producers to choose between running traditional hot mix and today's warm mix asphalt designs. "It is very easy to switch back and forth between hot and warm mix," says Brad Bankston.
Enlightening Demo
As leaders in the technology, R K Hall teamed with TxDOT last July to teach other contractors, state and local highway transportation employees more about the benefits of using warm mix asphalt.
Nearly 150 guests traveled from as far as Nevada and California to attend the anticipated event. Representatives from private contractors, federal, state, city and county agencies, suppliers, and a local senator watched R K Hall's experienced crew work with the new mix.
For the presentation, R K Hall produced 150 tons (136.1 tonnes) each of two surface mixes: both the Texas and Oklahoma DOT designs. Target mixing temperatures of 270 degrees F were readily achieved with the Terex system.
"Without the warm mix system, we would typically run these mix designs at 325 to 335 F, so we were able to lower mixing temperatures by 55 to 65 F," says Bradford.
According to Bankston, the goal was to reduce mixing temperatures to below 285 degrees F. "At and above this temperature is where the volatiles vaporize and the chance for emissions exists," he explains.
Meeting the desired mix temperatures, the mix went down well and targeted in-place compaction densities were achieved. Attendees were impressed with the mix and its workability at the lower temperatures. "We have found that mix coming out of the plant at 270 degrees F is the right target for achieving compaction," says Bankston. "Once it gets to about 250 degrees F, we have to increase compactive efforts slightly, which negates the dollar savings benefits of running warm mix."
It is these benefits that continue to drive the momentum of warm mix asphalt for producers. "When you can reduce mix temperatures by 50 degrees F or more, you achieve fuel savings and get less oxidation of the mix, which ultimately results in longer life for the asphalt," adds Bradford. And all of these benefits can be realized with a warm mix asphalt system with virtually no increase in the cost per ton of mix produced.
