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Properly designed and constructed unbound aggregate layers have the potential to improve pavement performance and longevity while also addressing todayâs issues of the costs of other pavement materials, the need to save energy, and the desire to reduce greenhouse gas (GHG) emissions associated with the construction and reconstruction of pavements. Pavement proj- ects using granular layers will have to be sustainable and cost-effective by (1) making more effective use of locally available materials through beneficiation and use of marginal aggre- gate materials, (2) increasing effective use of recycled aggregate products such as recycled crushed concrete (RCA) and reclaimed asphalt pavement (RAP), and (3) targeting long life and improvement in pavement performance. North American transportation agencies have diverse specifications and construction prac- tices for unbound aggregate base (UAB) and subbase layers. Sharing experiences and effective practices for unbound aggregate layers among transportation agencies would lead to the design and construction of better-performing, more economical, and sustainable pavement systems. The primary objective of this synthesis was to gather information on the current state of practice and the state-of-the-art research findings on the following topics: 1. Materials characterization and quality of natural aggregate and common recycled materials that relate to performance; 2. Properties of unbound aggregate layers that are used in the design of pavements and how they are determined; 3. Influence of gradation and other aggregate properties on permeability; 4. Current practices and innovations in construction, compaction, and quality assurance procedures [such as compaction in thicker layers, use of intelligent compaction (IC) systems, and the use of tests other than density in evaluating in-place modulus, stiffness, and quality]; 5. Performances of different base types, such as the concept of a granular base over a stiff, often cement-treated subbase layer used in inverted pavements, in research pavement sections; 6. Potential to save energy and hauling costs by better utilizing local aggregates and recycled materials; 7. State specifications that lead to how contractors manage storage, transport, and place- ment of materials to minimize degradation of material properties and performance: lessons learned; 8. How states address climatic, subgrade, and drainage considerations in design of aggre- gate base layers. Relevant information was gathered through a literature review, survey of U.S. state and Canadian provincial transportation agencies, industry input, and selected interviews. A total of 46 transportation agencies (including four Canadian provinces) responded to the survey questionnaire. Review of survey responses and subsequent interviews with agency personnel indicated that no common practice exists across agencies as far as the design and construction of unbound aggregate pavement layers is concerned. Most agencies do not have a defined pro- tocol to introduce new and recycled materials into pavement construction. Although numerous sUMMARY PRACTICEs FOR UNBOUND AGGREGATE PAVEMENT LAYERs
2 research and implementation projects over the years have recommended optimum design and construction practices for unbound aggregate pavement layers, there appears to be a signifi- cant delay before such recommendations are adopted into agency practice. Accordingly, this report summarizes important aspects and effective practices related to material selection, design, and construction of unbound aggregate pavement layers. Prevalent agency practices are summarized, and key lessons learned from research studies focusing on unbound aggre- gate pavement layers are highlighted. This information can be used to establish the need for and initiate the development of harmonized protocols for optimum design and construction of better performing, cost-effective unbound aggregate pavement layers.