Evaluation of Warm Mix Asphalt Concrete Pavement in South Dakota Conditions

The most critical part of constructing the hot mix asphalt (HMA) layer is to obtain a uniform and durable surface layer that can withstand the combined actions of traffic loads and environmental conditions. These desirable properties of the constructed HMA layer have been achieved through uniform mixing and coating during the manufacturing process and effective compaction during the lay-down operation. For HMA mixtures, mixing and compaction at elevated temperatures (275-350oF) have been necessary to achieve the desirable properties of the constructed HMA layer. The cost of constructing the HMA layer has been steadily increasing due to the significant increases in the price of asphalt binder and energy cost. Therefore, by reducing the required energy to produce and construct the asphalt concrete layer, significant cost savings can be realized. The past few years have seen the introduction of the warm mix asphalt (WMA) technology in efforts to reduce the required energy for the production and construction of the asphalt concrete layer. WMA is produced at temperatures that are 30 - 100oF lower than those required for the production of HMA. This reduction in the production temperature of WMA is directly related to savings in the energy required to produce these mixtures. Energy savings brings two major benefits: lower construction cost and reduced emissions which makes the construction of asphalt concrete pavements more affordable and more environmentally conscious. In addition to energy and emissions savings, producing and constructing asphalt mixtures at lower temperatures may allow construction activities to be extended into the lower temperature seasons such as early spring and late fall times of the year. This feature of WMA technology is very attractive in South Dakota where the construction season for HMA is relatively short. The basic principle of the WMA technology is to lower the mixing and compaction temperatures of the produced mixture through lowering the viscosity-temperature relationship of the asphalt binder. Lower mixing and compaction temperatures directly reduce the short term aging of the asphalt binder leading to more durable and flexible mix that is more resistant to cracking and moisture damage. In addition, WMA mixtures may be used in cold weather patching and maintenance activities which was not an option with HMA mixtures. Whatever the economical, practical, and environmental reasons for using the WMA technologies, the produced WMA mixtures must be highly resistant to the failure modes of asphalt pavements that are commonly encountered in South Dakota. In other words, the WMA technology must produce asphalt pavements that are resistant to moisture damage, fatigue and thermal cracking, and permanent deformation when used with the locally available aggregate sources and asphalt binders. Therefore, a comprehensive experimental program must be completed to assess the applicability of WMA technology under South Dakota's conditions as compared to the conventional HMA technology. The experimental program must evaluate the mechanical properties of WMA mixtures produced with the most common aggregates in South Dakota; quartzite, limestone, and natural gravels and must be adaptable to the use of recycled asphalt pavements (RAP) at various percentages. The proposed experimental program is fully described under the research plan. The overall objectives of this research study are listed below as stated in the RFP: 1) Evaluate various types of WMA paving technologies to determine their suitability for use in South Dakota in various applications. This objective will be met by conducting an extensive laboratory experiment to assess the mechanical properties and durability (i.e. resistance to moisture damage) of WMA mixtures produced using South Dakota's aggregate sources and RAP materials. 2) Assess the performance of WMA during construction of test sections and over time. This objective will be met through the construction of test sections using the identified WMA technologies. This effort will evaluate the constructability of WMA mixtures and measures the potential savings in energy and emissions as compared to the conventional HMA technology. Field produced mixture will also be evaluated in the laboratory for their mechanical properties and resistance to moisture damage. Once the test sections are constructed, their long-term performance will be monitored to assess the long-term behavior of the WMA technologies. 3) Obtain Cost versus benefit analyses relative to the constructability, performance, environmental factors, and other considerations. This objective will be met by using the measured properties of the WMA mixtures along with the construction characteristics to assess the cost benefit ratio of using the various WMA technologies as compared to the conventional HMA technology. In other words, this part of the research will combine the results of the laboratory evaluations with the construction data and long-term performance to assess the full benefit of WMA mixtures relative to their added cost. Task 1: Review and summarize literature directly related to the construction and performance of WMA pavements in South Dakota. Task 2: Review the Publication on the National Asphalt Pavement Association website, "Material Test Framework for Warm Asphalt Trails" as this document will be used after modification based discussions between the project Technical Panel and Research Team throughout the course of the work. Task 3: Develop a preliminary laboratory protocol and testing plan including appropriate testing procedures for the WMA mix designs to be reviewed and approved by the Technical Panel. This will be based on the South Dakota AC mix design procedure and should include appropriate WMA admixtures as indicated by the literature review at several dosage rates using typical South Dakota aggregates such as: limestone, quartzite, and natural gravel. Additional, limited testing should also be done using recycled asphalt pavement (RAP) in separate mixtures with quartzite and natural gravel aggregate at 20, 30, and 40 percent by weight of recycled material, and subjected to Asphalt Pavement Analyzer (APA) and tensile strength ratio (TSR) testing, at minimum.