Recycled Unbound Pavement Materials
(TPF-5/129)

In the current global economy, the United States is in competition for non-renewable resources. In this country, and particularly in Minnesota, aggregate materials are being depleted at a rapid rate. Environmental stewardship regulations and permitting criteria will continue to become more restrictive. It is becoming increasingly important to investigate the use of recycled materials in pavement construction.

Minnesota has had a long history of using recycled materials in pavement construction. Recycled materials have been used in all layers of the pavement, from the surface down to the unbound supporting layers. Mn/DOT’s current Class 7 specification (Spec 3138) allows salvaged or recycled HMA, PCC, and glass to be used as part of the granular base materials. However, their material properties (strength, stiffness, unsaturated properties, etc.) are not well understood. Under the current design procedures, Class 7 materials are assigned the same empirical properties as a typical Class 5 (gravel) material. New mechanistic-empirical design procedures require more detailed material properties in order to accurately predict pavement performance. 

In addition, the use of crushed concrete in particular has raised some environmental concerns. These concerns have focused on the relatively high pH of the effluent produced by drainage systems that remove water from untreated recycled concrete aggregate foundation layers. Also of concern is the identification of some constituents (arsenic, chromium, aluminum, and vanadium) that are considered hazardous in drinking water.

Most of the current and former research projects using recycled materials are based on laboratory material characterization and mix design. The research proposed in this pooled fund study seeks to validate many of the previous findings with field performance data. The design guide modeling performed at Texas A&M is also in need of field validation. There is some anecdotal evidence of existing recycled base materials, but this research would provide a controlled field experiment in which quantifiable results could be obtained. This research would also provide a good opportunity to revisit some of the environmental concerns and address ways to mitigate the ill effects of effluent from the recycled concrete base layer. This research will help broaden the application of mechanistic-empirical pavement design methods to the use of recycled materials.

The objective of this study is to monitor the performance of several test cells at the Minnesota Road Research Facility (MnROAD) constructed using recycled materials in the granular base layers, including blended with virgin materials and 100% recycled asphalt and concrete pavement materials. The material properties will be monitored during construction and throughout the pavement life in order to determine their effects on pavement performance. The properties will be used to verify mechanistic-empirical design inputs, especially their variation with changing seasons and moisture regimes.