Better Roads Staff
Another wax – Montan wax – is a complex combination of nonglyceride long-chain carboxylic acid esters, free long-chain organic acids, long-chain alcohols, ketones, hydrocarbons, and resins, the scanning tour report states. It’s a fossilized plant wax, also known as lignite wax or OP wax, obtained by solvent extraction from certain types of lignite or brown coal. Its melting point is 180 to 200 deg F, and Asphaltan-B is a trade name for this type of additive.
Different from the waxes are the surfactants, such as Evotherm and Rediset. We have shown how important surfactants are for manufacturing asphalt emulsions (see Spreading the Wealth: Asphalt Emulsions Mix Oil with Water, June 2012, pp 18-29).
Evotherm is a chemical compound with surfactant activity, which adds lubricity to individual microscopic asphalt particles. The particles or micelles develop “slip planes” that let the asphalt particles move more easily, requiring lower levels of energy. Because the energy is lowered, Evotherm warm mix has the same viscosity properties at lower temperatures as conventional hot mix asphalt. Rediset-LQ is another product.
Water-based, in-plant foamed systems for low energy mixes – such as the Astec Industries, Inc.’s Double-Barrel Green System – use nozzles that precisely meter water into the drum of a drum mix plant. 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.
A much different kind of foamed asphalt incorporate liquid “foamed” asphalt as a stabilizing agent, in which hot-liquid asphalt is foamed with water and air, and is then injected into RAP or aggregate in a mixing chamber, either in a self-propelled recycling machine, or portable plant.
In this cold-mix foamed asphalt process, the recycled aggregate is not completely coated, as is the process with in-plant foamed injection using mostly virgin aggregate. Instead, as 100 percent reclaimed materials are introduced to the pug mill, foamed asphalt is injected into the material stream, and acts as a binding agent to “glue” the reclaimed aggregates together.
This will permit use of less liquid asphalt and much lower mixing temperatures. With 100 percent of existing aggregates used in cold recycling, 2.2 to 2.5 percent liquid asphalt is used to partially coat the RAP and any added virgin aggregate, opposed to in-plant foamed technologies that completely coat the aggregate – as is needed for critical applications like friction courses – that use 5 percent liquid asphalt.
Moisture Sensitivity Enhanced?
At the 2nd International Warm-Mix Conference in October, presentations heralded the spread of WMA from coast to coast.
For example, NAPA director of engineering Kent R. Hansen, P.E., reported that WMA use was growing very rapidly, with a 148-percent increase in use from 2009 to 2010, the most recent year for which firm data were available, rising from 19.2 million tons to 46.7 million tons in one year. That’s drawn from estimated total volume of both HMA and WMA of 358 million tons in 2009 to 360 million tons in 2010. Download the report by Googling NAPA Information Series 138.
As usage grows, WMA has moved from a boutique product to the mainstream, and is coming under increased scrutiny, with the question being raised of whether the physiochemistry of warm mixes makes them more sensitive to moisture damage.
Injection of water, along with the liquid asphalt cement, causes the liquid asphalt to foam and expand in volume.
Moisture damage is a well-recognized phenomenon, reports The Asphalt Institute in its 2007 pamphlet, Moisture Sensitivity: Best Practices to Minimize Moisture Sensitivity in Asphalt Mixtures, with 10 of 50 states reporting fresh hot mixes being treated for moisture damage.
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