3. Asphalt Materials and Mix Design
3.1 Introduction
Asphalt mixtures have three primary ingredients: binder, aggregate, and air. The asphalt binder, also called asphalt cement or bitumen, is obtained from refining crude oil. Binders can be graded by several methods. Outside the United States and Canada, the most common method is the penetration grading system. In the early 1960s, the Asphalt Institute developed the viscosity grading system to use a more fundamental material property rather than the empirical penetration grading system. In the late 1990s, the United States and Canada adopted the performance-graded (PG) system developed under the Strategic Highway Research Program. The latest binder grading method uses the Multiple Stress Creep Recovery (MSCR) test and specification.
The aggregate used in asphalt mixtures is typically a combination of coarse and fine materials, with mineral filler added as needed. Locally available aggregates from a pit or a quarry are most often used to minimize haul costs.
The mix design process determines the correct proportion of binder and aggregate required to produce an asphalt mix with the properties and characteristics needed to withstand the effects of loading and the environment for many years.
The mix design process determines the correct proportion of binder and aggregate required to produce an asphalt mix with the properties and characteristics needed to withstand the effects of loading and the environment for many years.
Asphalt mixes require air voids to allow for thermal expansion of the binder without bleeding or flushing. Most asphalt mix designs are dense-graded and target air voids in the range of 3 to 4 percent. Open-graded asphalt mixtures are intentionally designed with high air voids (around 20 percent) to allow water to flow through the mix and drain to the side of the mat.
Mix design is performed in the laboratory, generally using one of two methods whose primary difference is the method of compaction. Until the late 1990s, the most common mix design method was the Marshall method, which was used by about 75 percent of State highway departments as well as by the DoD and the FAA. Now almost all State DOTs use the Superpave method of mix design. In this method, samples are compacted with a Superpave gyratory compactor and tested for volumetric properties. Additional tests are often performed to assess the asphalt mixture’s moisture susceptibility and resistance to permanent deformation (rutting) and different types of cracking.
For an asphalt paving project, the mix design is most often developed by either the contractor or a private laboratory, although some government agencies still design their own mixes. Regardless of who completes the laboratory mix design phase of the job, the result of the mix design process is a JMF. The JMF is the starting point for the contractor in producing the asphalt mix for the project. From the JMF, targets are established for acceptance quality characteristics. These targets are then used with tolerances and a quality measure for acceptance of airfield or roadway asphalt materials.
This chapter briefly reviews the properties of the materials used to produce an asphalt mix and the mix design process. Also discussed are some of the differences that can exist between laboratory- and plant-produced mixes and between JMF values and plant test results.