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Development of a Performance-Based Mix Design for Cold In-Place Recycling (CIR)
AdvisorHajj, Elie Y
Civil and Environmental Engineering
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The Nevada Department of Transportation (NDOT) has long used cold in-place recycling (CIR) projects for low and medium-volume roads. Extensive research on CIR mix design has led to the current structure that treats the CIR layer as a stabilized base course followed by a thin HMA overlay or surface treatment. Even though the overall performance of CIR within Nevada has been good, data revealed that select CIR projects did not perform well, possibly due to the lack of a performance-based mix design. As CIR properties are primarily based on the existing pavement's characteristics, it is essential to create specifications based on site characterization rather than pass or fail criteria. Therefore, as a collaborative effort between Nevada state agency, industry partners, and academia, this study's objective was to research a performance-based mix design for CIR of an existing non-traditional pavement. Nevada state route 232 was selected as the optimal location after analysis revealed an asphalt binder content averaging 13% with a relatively soft grade, providing a challenging but common issue within low-volume roads in the state. This study was organized in two phases. Phase 1 began with a preliminary evaluation of the existing pavement to determine the optimal emulsion content (OEM). After discussing with industry, two mix design options were selected with varying emulsion content to be validated through the Hveem mix design methodology. Performance tests, including the Indirect Tensile Strength Test (IDT) and the Hamburg Wheel Track Test (HWTT), were performed on the two designs. The validation concluded that the OEC was 3.0%, and HWTT indicated that the chip seal layer was assisting in rutting resistance. Although the rutting resistance was significantly lower than traditional HMA mixtures, the HWTT on the existing slab indicated that low cycles were expected. To further assist in rutting resistance, cement was included as a recycling additive to investigate. Phase 2 investigated eight other mixtures with varying water, lime, and cement content to address the risk of rutting, cracking, and moisture susceptibility. The quantities of additives were selected through agreement with the agency and industry, acknowledging the benefits and risks of combining lime and cement. Each mix design was verified using CIR's modified Hveem mix design. The recommended and ultimately constructed mix design was selected to balance cracking and rutting resistance while ensuring the mixture would not suffer from water-induced damage. The special provision contract included stability, tensile strength, and density requirements. Further analysis was also conducted on the field data collection and the inclusion of NCHRP 9-62 field testing during construction. This project concluded with recommendations for revisions to the CIR specifications and demonstrated the need for performance-based CIR mix designs.