What is WMA(warm mix asphalt)? How does it work?

Warm mix asphalt (WMA) refers to asphalt concrete mixtures that are produced at lower temperatures than the temperatures typically used in the production of hot mix asphalt (HMA) (50°F [28°C] lower or more). The goal with WMA is to produce mixtures with similar strength, durability, and performance characteristics as HMA using substantially reduced production temperatures. There are important environmental and health benefits associated with reduced production temperatures including lower greenhouse gas emissions, lower fuel consumption, and reduced exposure of workers to asphalt fumes. 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. WMA technologies were first introduced in Europe in the late1990s as a measure to reduce greenhouse gas emissions. Since then, a number of WMA processes have been developed in Europe and the United States. At the time that NCHRP Project09-43 was completed, there were approximately20 WMA processes being marketed in the United States. These processes include chemical, wax, and synthetic zeolite additives; plant foaming systems; and sequential mixing processes. The objective of NCHRP Project09-43 was to develop mixture design and analysis procedures that can be used with the wide range of WMA processes that are currently available or that are likely to become available in the future. The research conducted during NCHRP Project9-43 included the following:1.Development of a preliminary procedure based on a review of the literature and research in progress.2.A first phase of testing and analysis to investigate critical aspects of the preliminary procedure including (1) effect of sample reheating, (2) binder grade selection, (3) mixture of recycled asphalt pavement (RAP) and new binders at WMA process temperatures, (4) appropriate short-term oven conditioning for WMA, and (5) evaluation of devices to measure workability.3.Revisions to the preliminary procedure based on the findings of the first phase of testing and analysis.4.A second phase of testing and analysis to evaluate the revised preliminary procedure. This phase included (1) a mix design study to test the engineering reasonableness, sensitivity, and practicality of the revised preliminary procedure; (2) a field validation study that used properties of laboratory- and field-produced WMA to validate the procedure; and (3) a fatigue study to investigate whether lower WMA temperatures improve mixture fatigue properties.5.Final revision of the preliminary procedure based on the findings of the second phase of testing and analysis.