The project evaluated aggregate tests and, based on the results of laboratory tests, identified performance-related procedures to test and select recycled hot-mix asphalt (HMA) and portland cement concrete (PCC) materials for use in unbound layers for highway pavements.

The use of materials reclaimed from HMA and PCC pavements and structures as aggregates in highway construction has become a widely accepted practice by many state highway agencies. Use of recycled aggregates in unbound pavement layers is one application that should provide technical, economic, and other benefits. Although a great deal of research has been performed on the properties of aggregates used in pavement construction, limited research has addressed the use of recycled aggregates in unbound pavement layers. Research performed under NCHRP Project 4-23, and reported in NCHRP Report 453, Performance-Related Tests of Aggregates for Use in Unbound Pavement Layers, evaluated aggregate tests and, based on the results of laboratory tests, identified a set of aggregate tests that relate to performance of unbound pavement layers. However, the project did not deal with the use of recycled materials, and the validity of the recommended tests for evaluating aggregates from reclaimed asphalt pavement (RAP) and reclaimed concrete pavement (RCP) has not been assessed.  Validity of tests intended for evaluating virgin aggregates could be compromised by the presence of binders and contaminants from deicing chemicals and vehicular traffic spills in the recycled materials, exposure to the elements, and other factors. In addition, the use of recycled materials may pose different constructibility concerns that require evaluation by other tests.  Therefore,  research is needed to assess the validity of existing aggregate tests and, if necessary, to modify available tests or to develop new tests suitable for evaluating and selecting RAP and RCP materials for use as aggregates in unbound pavement layers.

Research included a review of relevant domestic and foreign literature, communications with highway agencies, identification of aggregate properties that influence the performance of pavements, and the conduct of laboratory tests to evaluate the aggregate tests currently used in the United States and other countries as well as potential new tests to measure performance-related properties.  The research also recommended tests for RAP and RCP materials that can be used as predictors of performance, and identified a plan for evaluating validity of these tests by accelerated pavement testing or specifically constructed pavements and long-term monitoring of in-service pavements.  NCHRP Report 598, "Performance-Related tests of Recycled Aggregates for Use in Unbound Pavement Layers," documents the work performed under this project; copies have been distributed to Program sponsors (i.e., state departments of transportation). Appendixes A through C of NCHRP Report 598, which cover the literature review and background information, new aggregate tests, and surface dielectric measurements, were published as NCHRP Web-Only Document 119.