June 7th, 2011
WMA JUNGLEPreserve Infrastructure, by Catherine Higgins.
This post is fourth in a series about the Federal Highway Administration’s Every Day Counts Innovation Initiative.
FHWA’s Every Day Counts Innovation Initiative encourages the use of Warm Mix Asphalt.
Using WMA can be a cost and energy saving approach with high quality results. UDOT allows the use of WMA as long as contractors provide a final product that meets the standards specified by UDOT engineers.
Hot Mix Asphalt and Warm Mix Asphalt have essentially the same basic components – asphalt binder and aggregate. Hot or cold, asphalt pavement comes in a variety of mix designs that vary the type, size and amount of asphalt binder and aggregate to determine the strength, thickness and durability depending on many variables, including underlying soil type, expected pavement life, climate and the traffic load carried by the road.
HMA uses heat to decrease the viscosity of the asphalt in order to be able to place and roll the pavement to the correct compaction. Adding Zeolite, surfactants or wax increases the ability of the asphalt to absorb water or makes the mix more workable so correct compaction can be achieved.
- Adding water to hot asphalt during the mixing process causes mix to foam and achieve the viscosity that’s needed at lower temperatures to compact the product after installation. Granite construction has used WMA successfully in Utah for several years by using the water injection method to produce WMA.
- “Zeolite is a mineral sponge used to transport water into the asphalt binder,” says Kevin VanFrank, UDOT’s Engineer for Asphalt Materials. “The foaming is more persistent and therefore the reduced viscosity lasts longer than simply adding water,” and lower compaction temperatures can be maintained for greater periods.
- Waxes are thermosets “that lubricate and decrease viscosity of the mix at temperatures above their liquid phase transition but increase viscosity above the liquid phase transition,” explains VanFrank.
- Surfactants (soap) are added to coat the aggregates to make the binder workable at lower temperatures without changing viscosity
WMA has several advantages. Less energy is expended to heat WMA; HMA is heated to 310 to 350 degrees Fahrenheit while the temperature range for WMA is as much as sixty degrees lower.
The compaction zone, or the temperature range at which asphalt can be successfully compacted, is wider than for HMA, giving workers more time to compact the asphalt once it’s placed.
WMA can be placed in the cooler months of spring or fall, which potentially lengthens the construction season. And, cooler asphalt has a milder odor, which can be good for road users and especially for workers.
WMA was successfully used on Wall Avenue in Ogden. During the project, WMA was placed side by side with HMA. “We couldn’t see any difference,” says Kevin VanFrank, UDOT’s Engineer for Asphalt Materials. The two asphalt mixes were tested extensively. “It all looks the same today,” says VanFrank.
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