Warm-Mix Asphalt for Rural County Roads: Fid-Bau Portal

Warm-Mix Asphalt for Rural County Roads

Robjent, Lyndon / Dosh, Wayne

Warm-mix asphalt (WMA) is a group of technologies that allow for the placement of asphalt at production temperatures 35°F–100°F lower than traditional Hot-mix asphalt (HMA). Some of the benefits for using WMA are that it lowers fuel consumption, lessens the aging of the asphalt binder, decreases the production of greenhouse gases, allows for the placement of asphalt plants in more restrictive areas, and improves worker conditions. First used in Europe, WMA was introduced to the U.S. with a European scanning tour in 2002. In 2004 the first field tests in the U.S. were constructed, with the first production paving jobs in 2006. Warm Mix Technology is divided into three distinct classifications based on the mechanism used to produce the end product. In one class, foaming, small amounts of water are mixed with the hot asphalt oil. A second process uses organic additives or waxes to decrease the viscosity of the oil. The last class use chemicals or surfactants to aide in the coating of the aggregate at lower temperatures. All three processes have been used in cold weather climates. A surfactant was chosen for Crow Wing County Road 108. Most surfactants require little if any plant modifications when compared to a foaming process in which the modifications could be as simple as adding another bin to a rap feeder or as complicated as adding manifolds and injectors to the plant. The use of a wax was rejected, as one of the ways to get a cost comparable product for this project was to substitute a warm mix product using PG 58-28 oil for a hot mix product using PG 58-34 oil. Research indicated that one of the benefits of warm mix using the foaming or surfactant process is that they generally perform better than the waxes for thermal cracking. On August 19^th of 2008 2913 tons of WMA and 272 tons of HMA were placed on Crow Wing County Road 108 by Anderson Brothers Construction Company of Brainerd. A level 2 gyratory mix design was used incorporating Evotherm 3G in the asphalt binder. The additive was added at the Flint Hills refinery in Minneapolis at a rate of 0.5% by the weight of the binder. The product arrived at the paving plant ready to use, requiring no modifications to the plant or the paving equipment. During production the only change made to the normal testing procedure of the mix was the temperature at which the gyratory samples were made. The oil met all the requirements of a PG 58-28 oil and the mix met all of the requirements of a Level 2 gyratory design. The binder was cut 0.1% from the HMA design to bring the production air voids in the WMA up to the desired levels. The densities of the road cores of the WMA (ave. 92.7%) were found to be slightly below that of the HMA (94.1%) but still at a respectable level for a 2 inch lift on a 5 ton road over gravel. Answers that will come over time are whether or not WMA will perform better than HMA for thermal cracking, will it will have a longer life span and will prices become comparable to HMA. Some other countries currently believe the answers are "yes" and are willing to pay a higher price today to realize cost savings in the future.