Asphalt mixtures produced in the United States in 2018 contained over 82 million tons of reclaimed asphalt pavement (RAP) and over 1 million tons of reclaimed asphalt shingles (RAS). Public transportation agencies typically specify maximum percentages of RAP and RAS that can be used to produce asphalt mixes, based on factors such as mix type and application. These maximums are needed to ensure good performance of the pavement. These maximum values can be based on total percentage of the RAP or RAS by weight, or on a calculated value such as reclaimed binder ratio. Regardless of the specification method, it is necessary to verify the percentage of RAP or RAS being introduced into the asphalt mix plant in order to determine specification compliance. This is further complicated with the fact that RAP and RAS contain both asphalt binder and aggregate, so, the verification process must include the quantity added as well as the percentages of asphalt binder and aggregate included in each material. Verification of RAS content can be especially challenging because it often does not flow freely when using traditional cold feed bins, so is often mixed with another material (such as RAP or sand) before introduction into the plant. Excess RAP or RAS in a pavement can lead to insufficient new binder and increased stiffness of the mastic, potentially resulting in reduced mixture durability.
States have used various approaches to verify RAP and RAS percentages using techniques such as stockpile measurements or plant recordation. Also, plant manufacturers have introduced new equipment to monitor flow from individual cold feeds or improved plant calibration equipment, but there are no national guidelines in the use of these approaches. In addition to RAP and RAS, materials introduced at asphalt plants include asphalt binder, aggregate, and many additives such as latex, ground tire rubber, warm mix additives, anti-strip additives, fibers, and mineral fillers. Addition of these materials is done with many different types of equipment and control systems. The additions must be accurately controlled and interlocked in order to meet job mix formula requirements and agency specifications. Often, these additives are introduced using systems that do not provide good means to verify the quantity being added. Transportation agencies and asphalt mix producers would benefit from guidelines recommending best practices for verifying quantities of materials including RAP, RAS, and other additives into asphalt mixtures. Improved information will lead to better specification compliance and potentially improved pavement performance.
The objective of the research is to develop recommended practices/standards updates to verify quantities of materials being introduced into asphalt mixtures at production facilities. Tasks include (1) literature review of public transportation agency specifications and inspection procedures related to asphalt mix plant and production requirements and AASHTO M 156, Standard Specification for Requirements for Mixing Plants for Hot-Mixed, Hot-Laid Bituminous Paving Mixtures; (2) survey of public transportation agencies asphalt mix producers and plant manufacturers to determine current procedures for verifying raw material quantities including RAP, RAS, and other additives during mix production; (3) identify best practices for verifying raw material quantities used including RAP, RAS, and additives, as well as any knowledge gaps in this area; (4) develop suggested specification language and inspection procedures, including plant calibrations and procedures, accuracy requirements for various additive types, and documentation/recordation procedures for both batch plants and drum mix plants—staffing, timing, and frequency of these activates should be addressed (there may not be one approach, there may be a variety of approaches from which a state could choose; and (5) prepare a guide for agencies to use to develop specifications and inspection procedures for verification of materials including RAP, RAS, and additive content of asphalt mixes. Develop proposed updates to AASHTO M 156.