By Dwight Walker, P.E.
The interest in warm mix asphalt (WMA) is still growing. Some industry leaders predict that 90 percent of asphalt plant production will use WMA technology in five years. With this level of interest, many agencies are working to gain experience with WMA.
A few examples are summarized in this article to illustrate experience with WMA to date. The Asphalt Institute does not endorse any particular product or methodology. Whenever a particular brand is cited or pictured, it is for informational purposes only.
Big Tonnage in Texas
Texas has used so much warm mix that they have stopped tracking quantities. They have placed about one million tons of WMA and have another one million tons under contract.
RK Hall Construction, Texarkana, Arkansas, has been one of the largest Texas suppliers. They have constructed several large projects with multiple WMA technologies, but the asphalt foaming process using the Astec Double Barrel Green System is their most common means of production. They are currently working on a 300,000 ton job on I-30 in the Paris district.
Another one of their projects was on US 380 in Young County. The job was a 26-mile, 2-inch overlay using Texas’ Type D mix with PG 70-22. They produced more than 68,000 tons of WMA. The results were positive. They tried various production temperatures but eventually settled on 270°F. During production, there were some initial concerns about storing WMA, but they found that the mix lost less than 10 degrees and worked just fine. The mix compacted so well that they were able to pull one roller out of the paving train.
The project was a success and TXDOT was happy. The contractor received a significant bonus. The reduced operating temperature resulted in about a 25 percent savings in burner-fuel costs. The ride quality was good. The old pavement averaged an IRI of 95 and the WMA overlay decreased the roughness to an average of 44.
The Pennsylvania DOT (PA DOT) has great interest in warm mix. They established a target of 20 percent of their 2009 asphalt tonnage to be produced using WMA technology.
G.O. Hawbaker, Inc. was the successful bidder for PA DOT’s District 2 trial project constructed in May and June of this year. Four WMA technologies were used, including Sasobit, Advera, McConnaughay’s LEA and Gencor’s foaming process.
Sasobit is a long-chain aliphatic hydrocarbon produced by Sasol Wax of South Africa. Sasobit is completely soluble in asphalt in temperatures greater than 240°F forming a homogenous solution with the base asphalt and resulting in a significant reduction in the asphalt’s viscosity.
Advera WMA uses synthetic zeolite technology to produce time-released foaming of the asphalt. This small amount of foaming, 0.05 percent water by the weight of the mix, allows workability until the mix temperature drops below 212°F.
McConnaughay’s Low Emission Asphalt (LEA) relies on a chemical additive and sequential mixing to produce a 35 to 50 percent reduction in energy consumption over HMA. The LEA additive is combined with the asphalt prior to coating the hot coarse aggregates. The unheated, wet fine aggregate is then introduced, creating a foaming action.
Gencor’s Ultrafoam GX process introduces water into the mixing process to cause the asphalt cement to foam. The foaming occurs when the hot asphalt contacts the water and aids in distribution of the asphalt across the aggregate surface and temporarily lowers the asphalt viscosity, facilitating workability at lower temperatures.
The mix was a 9.5mm surface. Production of the warm mix started under less than ideal conditions. The air temperatures at plant start-up were in the mid- to upper-30s °F when the Sasobit and Advera sections were done, but Hawbaker was able to achieve 92 and 93 percent of maximum theoretical density (MTD), respectively. The FHWA mobile lab was on site to help demonstrate the WMA technology.
Valley Quarries constructed another WMA project for PA DOT on College Street in Carlisle. This job used a terminal-blended PG 64-22 with 0.5 percent Evotherm, a MeadWestvaco product. The binder was supplied by Associated Asphalt from Martinsburg, West Virginia. The mix, a 9.5mm surface containing 15 percent RAP, was shipped at 240 to 250°F. The mat set up well under the breakdown roller and had a uniform texture. An average compaction of 94 to 95 percent MTD was achieved.
The Alaska DOT bid a 25,000-ton warm mix project on Mitkof Island. The project was located in the Tongass National Forest, about 20 miles from Petersburg, Alaska. The contractor was SECON, AK, part of the Colas Group. The project consisted of 8 miles of a two-lane road and a passenger-ferry terminal lot.
Sasobit was the WMA technology applied. The Sasobit-treated asphalt was blended at US Oil’s refinery in Seattle, Washington, during the first week of July of 2008 and barged 1200 miles to Petersburg, Alaska, in 25-ton containers. It was re-heated and used the first week of September 2008.
The Alaska mix was a fine-graded, intermediate mix with 100 percent of the aggregate passing the 19mm sieve and 87 percent passing the 12.5mm sieve. The mix contained 5.0 percent of an SBS-modified PG 58-28 that had been further treated with 1.5 percent Sasobit. The mix was designed using the Marshall Method with 50 blows compactive effort. The mixing plant was a portable drum plant and averaged a production rate of 250 tons per hour.
The haul distance from the plant to the jobsite was only one to four miles and the mix was transported to the project in uncovered belly-dump trucks. Two rollers were available but only one was used due to the mix having relatively low voids and was easy to compact. The project ambient temperature was 51°F. The finished mat thickness was 3 inches. Some problems were encountered with over-compaction but the final ride quality was excellent and both the contractor and Alaska DOT were pleased with the overall work.
Any time a new technology is introduced there is some initial concern about production and performance. The experience with WMA to date has been positive. Each WMA project has brought additional information and increased confidence.
|Dwight Walker is a consulting engineer specializing in asphalt pavement materials and is a contributing editor for Asphalt Magazine.