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1 Aggregates encompass a wide range of materials including quarried rock, sand, and gravel in addition to recycled/artificial/byproduct materials such as reclaimed asphalt pavement (RAP) and recycled concrete aggregate (RCA), steel furnace slag (SFS), blast furnace slag (BFS), and quarry byproducts (QB). Aggregate type and quality are important factors for determining use and placement within different layers of the pavement structure. There are wide variations in what transportation agencies consider suitable aggregates for specific pavement applications. This synthesis gathered information and presents information related to aggregate quality requirements for pavement construction. Both current agency practices as well as state-of- the-art research findings on sources, locations, standards/provisional testing methods, and ranges for different types of aggregates used in North America are provided. This includes quality assurance methods as well as frequency of sampling and testing established. This syn- thesis also provides information on how aggregate quality has been documented and linked to both structural and functional pavement performance in the field through agency prac- tices and experience. Environmental concerns such as leaching and potential risk to ground or outfall waters are discussed. Finally, knowledge gaps related to characterizations and clas- sifications of different types of aggregates, including lack of performance-based specifications and implementation of effective aggregate quality assurance programs, are identified. The synthesis survey questionnaire (Appendix A) on aggregate quality requirements for pavements included 34 questions posed to transportation agencies in four categories: (1) aggregate sources and properties; (2) aggregate sampling, quality control, tests, and ranges; (3) procedures for approving aggregate sources; and (4) aggregate-related perfor- mance records. Survey questionnaires were sent out to all 50 U.S. Departments of Transpor- tation (DOTs) and 12 Canadian provincial agencies. Appendix B is a list of all U.S. DOTs and Canadian provincial agencies that provided responses. A response rate of 90% was achieved with U.S. DOTs. In total, 45 U.S. DOTs and eight Canadian provincial agencies (Alberta, British Columbia, Manitoba, New Brunswick, Ontario, Prince Edward Island, Saskatchewan, and Yukon) participated in the survey. Twenty-seven U.S. DOTs as well as Ontario and Yukon provided their approved lists of aggregates. A detailed compilation of the compre- hensive survey responses provided by participating agencies may be found in Appendix C. In addition, links to agency-approved lists of aggregates/specifications are provided in Appen- dix D. The appendices are not printed as part of this report but are available for download from the TRB website (www.trb.org) by searching for NCHRP Synthesis 524. Based on the reviews of the survey responses and follow-up communications with agency contacts, differences exist among transportation agencies when it comes to the methods for approving aggregate materials and quality assurance programs. Alaska, Illinois, Indiana, Ohio, Ontario, and Texas were found to have some of the most comprehensive aggregate S u m m a r y Aggregate Quality Requirements for Pavements
2 aggregate Quality requirements for Pavements classification systems with diverse quality classes associated with each aggregate type and pavement layer application. The findings also indicated that only 44% of the agencies classify aggregates to meet certain quality requirements for pavements. Furthermore, 32 U.S. DOTs and seven Canadian provinces indicated that they do not collect information for checking how the quality of aggregate used may affect pavement end performance. More than 50% of the surveyed agencies reported that they do not implement pavement materials charac- terization and performance testing in their specifications to check and control the end per- formances of different qualities of aggregates. Application of RAP in pavement construction was found to be more common when compared with uses of RCA, SFS, BFS, and QB. This was to some extent due to environmental concerns as well as to lack of sufficient knowledge and research findings related to material characterization and field performance of these aggregate sources. The outcome of the survey showed that transportation agencies com- monly blend marginal aggregates and QB with virgin aggregates. Nonetheless, they reported different procedures to control the quality of those blended aggregate products such as for controlling the quality of individual components or the entire blend as one product. Detailed information presented in this synthesis is a compilation of literature review and survey data intended to inform transportation agencies about the common practices and specifications being implemented related to effective classifications of aggregate sources used for pavement construction. Based on the findings of this synthesis, further research and investigation may need to be directed toward developing a comprehensive, consistent, and sustainable aggregate quality management system in North America. Related research activities may focus on establishing implementation strategies for such an aggregate quality management system to achieve satisfactory pavement performance and at the same time reduce detrimental environmental impacts.
