Road Science Tutorial: Many paths to low-energy asphalt mixes
Better Roads Staff
With interest in WMA growing, Astec began looking for a mechanical means of obtaining the same foaming effects in asphalt that chemical additives were providing. “Adding water to asphalt is the cheapest, easiest way to get that,” says Brock. “The challenge was how much to use, and how to get dispersed evenly.”
They found it in an in-plant foamed system, basically a manifold mounted on a drum plant, with nozzles that precisely meter water into the drum. Injection of water – along with the liquid asphalt cement – causes the liquid asphalt to foam and expand in volume. The foaming action helps the liquid asphalt coat the aggregate at a temperature that normally is in the range of 230 to 270 degrees F, versus the conventional temperatures of 300 to 340 degrees F.
“We found we were not successful introducing water at high rates through one nozzle, so we went to multiple nozzles,” Brock says. “The first unit we came out with had 10 nozzles, but we have adjusted the size and number of the openings, and done machining to get the system to the right point where it will blend asphalt and water properly.” Today’s units have six nozzles.
Tests in 2007 at Astec’s facility, and in the City of Chattanooga, indicated the system would work. “On our test in Chattanooga, the city let us run 50-percent recycle in a surface mix at 265 degrees F, 4,000 tons on a heavily traveled access road,” Brock says. “I just drove it last week and it’s still perfect.”
Now prices of WMA additives have plummeted, Brock says, and all agencies are benefiting from the lower pricing. “In just a few years, we saw a drop from a $6 added cost for a ton of HMA to $1.50 added cost per ton,” Brock says. “It’s amazing how economics plays into the use of WMA. To us, water is the economical winner in this process.”
In the manifold, water is metered through up to six nozzles. The faster the flow rate, the more nozzles that are employed. “The water is atomized, because we are trying to get it evenly dispersed through the mix,” Brock says. “We are adding about 1 pound of water for every 2,000 pounds of hot mix. Much of that evaporates as steam and the amount of water that actually gets into the hot mix is very, very minimal as we create microscopic bubbles. We foam the liquid asphalt to coat the aggregate at lower temperatures and stay workable when it gets to the job site. When you hit the mat with that roller, you will get density very quickly because the microscopic bubbles are ‘popped’ out.”
Of course, the process is more complicated than that. “With foamed asphalt warm mix, if you have cold spots in the mix, you will have density problems,” Brock says. “Inconsistent temperatures behind the paver will lead to compaction problems.”
Other in-plant foamed WMA technologies have appeared since Astec’s Green System. World of Asphalt 2010 this spring exhibited nine such systems. Among the WMA systems requiring plant modification are the Green Machine from Gencor Industries, the AQUABlack system from Maxam, the WMA System from Terex, the Accu-Shear system from Stansteel, and the LEA (Low Emission Asphalt) system from Suit-Kote Corp.
Yet another in-plant foamed process, WAM-Foam from Shell, follows introduction of a “soft” bitumen with a foamed bitumen in the plant. WAM-Foam is a product of a joint venture between Shell International Petroleum Company Ltd., London, U.K., and Kolo-Veidekke, Oslo, Norway.
In WAM-Foam, the binder is formed using two separate binder components in the mixing stage, FHWA reports. By dividing the binder into two separate components, a soft binder and a hard binder in foam form, lower asphalt mixture production temperatures can be achieved.
The soft binder component is mixed with the aggregate in the first stage at approximately 230 degrees F. to achieve full aggregate coverage, FHWA says. The hard binder component is mixed in a second stage into the pre-coated aggregates in the form of foam. As with the other in-plant foamed processes, rapid evaporation of water by injecting cold water into the heated hard binder as it is added to the mix produces a large volume of foam, with plant modification required. The hard binder foam combines with the soft binder to achieve the required final composition and properties of the asphalt product.
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