Waste products as road materials

Recycling part or all of the pavement materials in an existing road during reconstruction is an attractive construction alternative. When reconstructing roads surfaced with hot mix asphalt (HMA), the HMA, underlying base, and a portion of the existing subgrade often are pulverized to form a new base material referred to as recycled pavement material (RPM). Compacted RPM is overlain with a new HMA layer to create a reconstructed or rehabilitated pavement. This process is often referred to as full-depth reclamation. Similarly, when an unpaved road with a gravel surface is upgraded to a paved road, the existing road surface gravel (RSG) is blended and compacted to form a new base layer that is overlain with an HMA surface. Recycling pavement and road materials in this manner is both cost effective and environmentally friendly. However, recycled base materials may contain asphalt binder, fines, and/or other deleterious materials that can adversely affect strength and stiffness. To address this issue, chemical stabilizing agents can be blended with RPM or RSG. Use of industrial material resources for stabilization (e.g., cementitious coal fly ash) is particularly attractive in the context of sustainability. The purpose of this study was to develop a practical method to design local roadways using stabilized RPM or SRSG as the base layer and Class C fly ash as the stabilizing agent. The design method was developed in the context of the "gravel equivalency" (GE) design methodology employed for local roads in Minnesota.

The Minnesota Department of Transportation constructed a low traffic volume road section to determine the environmental fate and effects of chemicals from tire shred materials placed below the seasonal groundwater table. The road base was constructed during November of 2002 with over seven million pounds of tire shreds enclosed in geotextile fabric in the road base. Surface and well water samples were analyzed for pH, temperature, dissolved oxygen, specific conductivity (SpCond), hardness, chemical oxygen demand (COD), total suspended solids (TSS), biochemical oxygen demand (BOD), aluminum, antimony, arsenic, barium, cadmium, total chromium, copper, iron, lead, manganese, nickel, selenium, zinc, mercury, gasoline range organics, diesel range organics, total polynuclear aromatic hydrocarbons (PAHs), and carcinogenic polynuclear aromatic hydrocarbons (c-PAHs) and eight tire specific semi-volatile organic chemicals. Selected parameter data are presented. Elevated levels of barium, iron, and manganese were measured in all of the monitoring wells but not in the background well or surface water. The tire shreds contributed some PAHs to the surface water but in very low concentrations. Four tire material semi-volatile organics were detected in well water but not surface water samples.

Two field projects are described where cementitious fly ashes (10% by dry weight) and water were mixed to stabilize recycled pavement materials and road-surface gravel to form a base during reconstruction of a city street in Waseca, MN, and construction of a flexible pavement in a segment of gravel country road, CR 53 in Chisago County, MN, respectively. Addition of fly ash improves the stiffness and strength of the base materials significantly. A resilient modulus of minimum 50 MPa appears safe to assume irrespective of the base material at the end of construction due to fly ash stabilization. However, moduli of 100 MPa or more can also be achieved. There is no evidence of frost-induced degradation in the field over a single season of winter. Chemical analysis of the draining leachate from the fly ash-stabilized layers showed that the concentrations of trace elements (with the exception of Mn) were below USEPA maximum contaminant levels and Minnesota health risk levels. Longer-term monitoring is needed to fully understand the potential for leaching of trace elements and frost action during the service life. These field cases show that fly ash stabilization provides an effective and economical means of providing a base for asphalt paving using existing roadway materials.

This Mn/DOT Shingles Recycling Project was initiated to help accelerate the implementation of the use of manufacturers' shingle scrap into hot mix asphalt (HMA) in Minnesota. The intent was to help increase demand for manufacturers' recycled asphalt shingles in Minnesota through targeted outreach and technology transfer tools by addressing the information needs of private contractors and local agencies. Therefore, A Guide to the Use of Roofing Shingles in Road Construction was produced as part of a larger outreach and education campaign. Through interviews with HMA producers and other private companies, it was concluded that there was already very good acceptance of shingle recycling technology and that lack of adequate demand was not the key barrier to further market development. Rather, the lack of readily available supply of manufacturers' shingle scrap was determined to be the key barrier to further growth in recycling of this material. Most of the manufacturers' shingle scrap was already committed to a few recyclers and therefore did not allow new business entries into the marketplace. Recommendations for continued Mn/DOT market development support activities include additional research and specification development. The future promise of recycling "tear-off" shingle scrap may help address the limited supply of manufacturer' scrap.

Mn/DOT engineers are increasingly looking to recycled materials as readily available and cost-effective substitutes for natural aggregate and to fly ash as a material that can be used in the stabilization of sub-base soils. These recycled wastes have the potential to contain unacceptably high levels of some chemicals. This project produced chemical data on wastes, non-surface background soils, and natural aggregates for use in a due diligence screening tool in current service by Mn/DOT and developed by the Office of Environmental Services (OES). These data will be used by OES for their internal Mn/DOT due diligence determinations using their streamlined hazard evaluation process. A future Local Road Research Board project will transform the OES streamlined hazard evaluation process into a CD-based product for use by the larger transportation community. Data developed by this current project will be used to populate the future CD-based product electronic database. This project will maintain consistency with the current in use Office of Environmental Services (OES) streamlined hazard evaluation process for waste recycling in Mn/DOT infrastructure projects.