Pub. 1 2018-2019 Issue 1

additives) versus traditional HMA. Previous research indicated WMA was inferior to HMA in rutting and cracking performance. The past few years of field testing indicate signif- icant improvement in reduction of rutting, susceptibility to moisture, and cracking. Although WMA is a promising process, long-term adoption will depend on how well it performs over time when compared against HMA. For example, some experts have been concerned about the possibility of increased mois- ture damage and rutting. U.S. research has not found evidence of rutting prob- lems inWMA installations with Highway traffic. As far as moisture damage is concerned, some of the tests on WMA samples produced in a plant have resulted in a lower tensile strength ratio for WMA than for equivalent HMA samples. However, cores extracted from WMA pavements after the pavements have been in use for some time have shown an increased tensile strength ratio. In addition, no moisture damage or stripping had occurred. Empirical evidence has indicated that WMA per forms well on highways. However, the FAA is conducting research to assess the potential ofWMA for being adopted for airfield paving projects. The airfield pavements community lacks guidance and requires more expe- rience with these technologies. It is important to highlight the difference between highway and airfield pave- ments. The main differences in design considerations for highway and airport pavements result from the character- istics of the traffic using them such as load repetitions and distribution, the magnitude of the loads, and the tire pressures. Airfield pavement loads far exceed those applied on the highway. Themaximumsinglewheel load allowed on the road pavement by most highway authorities ranges from 4,000 to 5,000 lb, whereas airfield pavements must support tire loads as high as 50,000 - 60,000 lb. An aircraft wheel tire pressure is about 250 psi more than twice the value of a normal truck tire. These dif- ferences greatly influence the material requirements for pavement design and justify the need for further research and developing guidance on the use ofWMA for airfield pavements. There is a need for writing a compre- hensive specification and a mix design strategy. However, four techniques are currently known to improve the work- ability of mixes to allow the prepara- tion and compaction ofWMA. These are: • Aspha-min, which includes the addi- tion of sodium aluminum silicate or zeolite • Sasobit, which uses foam in the form of a paraffin-wax compound extracted from coal gasification • Evotherm, which includes additives in the formof an emulsion to improve the coating and workability ofWMAmixes • WAM-Foam, which uses a soft binder and a hard, foamed binder added at dif- ferent times during themixing process. Determining which process to use involves consideration of a variety of factors. For example, how many tons are to be produced? Initial equipment costs are higher for some methods than for others, and the same is trueof theaddi- tivecostsper ton. Inaddition, temperature reduction is greater for some methods than for others. Finally, someWMA addi- tives slightly raise binder grades for the final performance grading (PG). However, the way it is used in Utah (and other states) is often as a compaction aid with the same temperatures in the mix as HMA. With the foaming process and or the chemical additive, Evotherm (mostly for Utah), contractors can haul the mix further and still get compac- tion or density. Many states, including Utah, have implemented longitudinal joint specifications, and consequently we have found contractors using more Evotherm to meet the specifications. Temperatures, however, are not being dropped that much. In UDOT, for instance, we allow the con- tractor to use both the foaming and the Although WMA is a promising process, long-term adoption will depend on how well it performs over time when compared against HMA. chemicalWMA. However, they must let us know what they are using and it is part of the mix design process. WMA is clearly an important technology, andone that will certainly continue tobe studied for broader application use. 3 FOR REFERENCE: UDOT SPECIFICATIONS PAGE: http://www.udot.utah.gov/main/f?p=100 :pg:0:::1:T,V:4715 13