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From Near-Solid to Liquid

At ambient temperatures, “liquid” asphalt or bitumen from the refinery is a near-solid material. To lower its viscosity and make it workable, it is kept hot, and classic hot mix asphalt production – in which liquid asphalt coats aggregate – takes place at 275 to more than 330 degrees F, and compaction between 260 and 300 degrees F.

More heat comes from the aggregates. Before mixing with hot-liquid asphalt, fine and coarse aggregates are heated to high temperatures to drive off moisture, to ease coating of the mineral aggregates with the liquid asphalt, and to keep the complete mix fluid enough to be workable during placement.

Low energy mixes introduce chemicals such as organic waxes or surfactants to lower asphalt mix viscosity in lieu of higher temperatures, or water to lubricate the mix.

“WMA technologies can also be classified by type,” the scanning tour report says. “Two major types of WMA technologies are those that use water and those that use some form of organic additive or wax to affect the temperature reduction.

“Processes that introduce small amounts of water to hot asphalt, either via a foaming nozzle or a hydrophilic [water-loving] material such as zeolite, or damp aggregate, rely on the fact that when a given volume of water turns to steam at atmospheric pressure, it expands by a factor of 1,673,” the report says. “When the water is dispersed in hot asphalt and turns to steam (from contact with the hot asphalt), it results in an expansion of the binder phase and corresponding reduction in the mix viscosity.”

Three different types of products comprise the chemical WMA technologies.

Generically they are solid, synthetic zeolites, which release water molecules when mixed with liquid asphalt at the plant, achieving a foamed asphalt binder, an example being Aspha-min; solid organic additives, which are synthetic paraffin waxes that reduce the viscosity of the binder at mixing and compaction temperatures, an example being Sasobit; and surfactant liquid additives, which add lubricity to individual microscopic asphalt particles, enhancing workability at various temperatures, an example being Evotherm.

Asphalt-treated permeable base (ATPB) warm mix for a new taxiway being paved at O¹Hare International Airport.

Zeolites are crystalline hydrated aluminum silicates that have large empty spaces in their structures that allow the presence of large cation groups, such as water molecules. Aspha-min, a synthetic zeolite, contains about 20 percent water of crystallization, which is released when mixed with hot aggregates or asphalt, reports FHWA. The water creates a controlled foaming effect that leads to a slight increase in binder volume, therefore reducing viscosity of the binder.

While the zeolites release water into a mix to decrease viscosity, the waxes melt and lubricate the mix, then stiffen as temperature declines. “The processes that use organic additives (e.g., Fischer-Tropsch wax, Montan wax, or fatty amides) show a decrease in viscosity above the melting point of the wax,” according to Warm-Mix Asphalt: European Practice. “The type of wax must be selected carefully so that the melting point of the wax is higher than expected in-service temperatures (otherwise permanent deformation may occur) and to minimize embrittlement of the asphalt at low temperatures.”

Fischer-Tropsch waxes are long-chain aliphatic hydrocarbon waxes with a melting point of more than 208 degrees F, high viscosity at lower temperatures, and low viscosity at higher temperatures, the report states, adding “They solidify in asphalt between 149 and 239 degrees F into regularly distributed, microscopic, stick-shaped particles. They may be used to modify binder or added directly to the mixture.”