Warm mix asphalt (WMA) refers to asphalt concrete mixtures that are produced at temperatures approximately 40 to 70 ºF cooler than typically used in the production of hot mix asphalt. The goal with warm mix asphalt is to produce mixtures with similar strength, durability, and performance characteristics as hot mix asphalt but at substantially reduced production temperatures. Lower production temperatures can also potentially improve pavement performance by reducing binder aging, providing added time for mixture compaction, and allowing improved compaction during cold weather paving (Bonaquist, 2011). When ambient temperatures are consistently below 50 Degrees F, the asphalt mixes are typically produced at near normal or slightly reduced temperatures of hot mix asphalt (depending on technology used, %RAP used and construction conditions) and is intended to facilitate field placement and compaction at colder temperatures (NEAUPG, 2011). In addition, it has been reported that field compaction can generally be achieved faster using WMA which is an important factor for airfield pavement construction which is limited to strict time constraints to minimize service disruptions (Brown, 2007). Several laboratory tests on field cores or other field performance testing have been conducted (or planned) for the multiple WMA-related NCHRP projects (9-43, 9-47, 9-47A, 9-49, 9-49A) and as part of efforts by the WMA Technical Working Group. The full-scale WMA installation in Virginia (Diefendorfer et al., 2007) concentrated heavily on field evaluation between HMA and WMA based on control sections and test sections. The WMA analyzed in the study was produced with Evother and with Sasobit. For each project, testing of WMA trial mixtures and HMA control mixtures were focused on determining properties related to rutting, stripping, and density. Based on variations in mix production reported in many instances from HMA projects, some additional considerations for the sampling plan should include capturing stockpile conditions (moisture levels, environmental conditions, quarry origin and location, on-site versus transported to mobile plant, etc.) and reclaimed asphalt pavement (RAP) source.

In regards to combining WMA and RAP methodologies, the primary concern is whether RAP blends at WMA production temperatures as well as HMA production temperatures. It is generally assumed that a decreased production temperature would result in a decreased level of blending between the RAP and virgin binder. The degree of RAP blending may eventually influence the stiffness of the mixture as a whole. A stiffer mix could help in reducing the rutting potential of a mixture, but could lessen the fatigue resistance and vice versa. The stresses induced under the aircraft wheel load are considerably higher than the wheel loads experienced on highways, where the majority of WMA-RAP mixtures have been built and observed to date. One of the worst damage scenarios for airfield pavements is when high temperatures occur on aprons and taxiways before a fully-loaded plane is 3 ready to push-back from the gate. The influence of the interaction of viscosity reducers, such as WMA with a stiff binder from RAP, on the mechanical properties of the mix is unknown. This uncertainty combined with the high stresses induced onto airfield flexible pavements makes it difficult to predict how these mixtures will perform. Therefore, there is a need to have a better understanding of the viscoelastic properties of the WMA-RAP mixtures and whether they can be designed to withstand the typical high wheel aircraft loading under static conditions, as a starting point.

The overall goal of the project is to enhance the understanding of the performance of WMA-RAP mixtures in airfield flexible pavements in order to help define the viability of its usage with RAP as a common modification. The research team will determine the properties of the high percentage of WMA-RAP and the stresses in the pavement layer. These findings will help to draw conclusions about the appropriate WMA-RAP mixture design (e.g., ideal percentage of RAP and degree of blending required) and appropriate recommendations to the paving industry. These objectives directly address the “Focus Area 2: Infrastructure design, monitoring, inspection, and management to ensure a State of Good Repair” by providing design tools for flexible airfield pavements and tangentially addresses “Focus Area 5: Promoting livable and sustainable communities through quality of life improvements and diverse transportation development” by encouraging an increased use of recycled materials.