Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85927
Near-solid asphalt or bitumen from the refinery must be heated to make it workable, with production of hot mix asphalt at 275 to over 330 deg F, and compaction between 260 and 300 deg F. But today's low energy mixes reduce the viscosity of their liquid asphalt through other means, and enable the complete coating of aggregates at temperatures 35 to 100 deg F lower than conventional hot mix asphalt, with significant environmen- tal and workplace benefits and blends of Montan wax and fatty-acid amides (Asphaltan-B) Montan wax, fatty acid amides (Licomont BS 100), Fischer-Tropsch wax (Sasobit), Fischer-Tropsch wax is a synthetic aliphatic hydrocarbon wax, such as Sasobit (shown), the molecular lengths of which range from C40 to C120. As the asphalt-wax combination cools, the wax solidifies into regularly distributed, microscopic, stick-shaped particles, reports the FHWA. It's believed that the distribution of stick-like particles provides the framework that increases the viscosity of the Sasobit-modified binder at in-service pavement temperatures IMAGE CREDIT: Granite Rock Montan waxes and their derivatives, according to DAV, the German Asphalt Paving Association, are obtained from processing of lignite and consist of high-molecular hydrocarbons with a melting range from 230 to 284 deg F, and are different from naturally occurring bituminous waxes. Once dissolved in liquid asphalt they reduce the asphalt's viscosity; when cooled, they crystallize, increasing stability and resistance to deformation IMAGE CREDIT: Carmel Wax Because the energy is lowered, Evotherm warm mix has the same viscosity properties at lower temperatures as conventional hot mix asphalt Evotherm is a chemical compound with surfactant activity, which adds lubricity to individual microscopic asphalt particles. The particles or micelles develop "slip planes" which let the asphalt particles move more easily, requiring lower levels of energy. IMAGE CREDIT: Evotherm Time-controlled cleaning and closure of the nozzle In-place foamed low-energy mixes incorporate liquid "foamed" asphalt as a stabilizing agent, in which hot performance-grade asphalt is foamed with water and air, and is then injected into reclaimed materials or aggregate in a mixing chamber, either in a self-propelled recycling machine, or portable plant IMAGE CREDIT: Wirtgen America, Inc. 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 IMAGE CREDIT: Aspha-min An in-plant foamed system for WMA – such as the Double-Barrel Green System – is 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 IMAGE CREDIT: Astec Industries, Inc. melting points greater than 158 deg F. Examples include molecular sizes greater than 45 atoms of carbon (C45) and Viscosity-modifying organic additives change the rheological properties of the binder in such a way that mixing and laying temperatures can be reduced. For example, organic additives typically are waxes with Better Roads August 2012 19