The performance of unbound, granular pavement layers depends greatly on the properties of the aggregates used. Failures in flexible pavement resulting from poor performance of granular layers are manifested as permanent deformation (rutting), fatigue and longitudinal cracking, depressions, corrugations, and frost heave. Failures in rigid pavements resulting from poor performance of granular layers include pumping, faulting, cracking, corner breaks, and fatigue cracking. Many current aggregate tests were developed empirically to characterize aggregates and may not have any demonstrated relationship to the performance of the final product. The research conducted for this study was undertaken to evaluate existing tests, identify new tests that relate to performance, and develop better procedures for testing and selecting aggregates fur use as unbound pavement base and subbase layers. Factors contributing to distresses in both rigid and flexible pavements, due to the poor performance of unbound layers, include (1) shear strenght, (2) density, (3) gradation, (4) fines content, (5) moisture level, (6) particle angularity and surface texture, (7) degradation during construction and under repeated loads, (8) freeze-thaw cycling, and (9) drainability. Aggregate properties that were determined to affect performance of unbound granular base and subbase layers are shear strenght, frost susceptibility, durability, stiffness, and toughness. For this study, aggregate samples were obtained from different climatic regions of the continental United States. Requirements for test parameters for the aggregate properties were established to evaluate aggregates' suitability for use in a particular climate for a certain traffic level. The research team developed a decision chart incorporating aggregate shear strenght, stiffness, toughness, durability, and frost susceptibility to provide a measure of the performance potential of a particular aggregate. http://gulliver.trb.org/publications/nchrp/nchrp_rpt_453.pdfThe researchers also developed a validation plan to evaluate the research results in the long term. This plan proposes accelerated pavement testing of specially constructed pavement sections and long-term performance monitoring of in-service test pavements. (A) This report may be accessed by Internet users at
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