Search Content

The objective of this project was to characterize the properties of crushed recycled concrete (RCA) and asphalt pavement (RAP) as unbound base without being stabilized, to assess how RCA and RAP behave in the field and to determine how pavements can be designed using RCA and RAP. Issues to be considered include variability in material properties, purity of material, climatic effects, how to identify and control material quality, and leaching characteristics. This project included laboratory specimen and large-scale model tests and evaluation of field data from MnROAD test sections constructed using recycled materials. To identify the characteristics of RAP and RCA typically available in different parts of the country, samples were obtained from eight states: California, Colorado, Michigan, Minnesota, New Jersey, Ohio, Texas, and Wisconsin covering a geographically diverse area. A conventional base course was used as a control material. The extensive investigation undertaken on RCA and RAP indicate that these materials are generally suitable for unbound base course applications and they show equal or superior performance characteristics compared to natural aggregates in terms of stiffness, freeze-thaw and wet-dry durability, and toughness. Their typical compositional and mechanical properties and their variability are defined in this study providing a basis for design considerations. Their relative differences from natural aggregate such as temperature sensitivity, plastic deformations, and water absorption and retention characteristics are also well established. It is noted that some RAP may be sensitive to temperature change that may lead to rutting. This aspect needs to be considered in design.

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.

Pavements are constructed on compacted soils that are typically unsaturated. The negative pore-water pressure (soil suction) due to the ingress of water in between soil particles has a significant effect on pavement foundation stiffness and strength. The study characterized the effects of soil suction on shear strength and resilient modulus of four soils representing different regions of Minnesota. The deviator stress in shear strength measurements followed a power function relationship with soil suction. Resilient modulus also followed the power function relationship with suction but these relationships fell within a narrow range. We present models for incorporating suction effects in shear strength and resilient modulus measurements of highly compacted subgrade soils. We also briefly outline a framework for incorporating these models in the resistance factors of MnPAVE. Since soil water content and the resulting soil suction under the pavement varies with season, adjustments are needed to account for increased strength and stiffness of the material as a result of unsaturated soil conditions. These adjustments will not only reflect the more realistic field conditions but will result in more reliable performance predictions than the current pavement design method.

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 report contains the appendices to report number 2009-27: Use of Fly Ash for Reconstruction of Bituminous Roads.