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Background

In the last years, State Transportation Agencies (STAs) have placed increasing emphasis on incorporating asphalt mixtures using cold-recycling technologies in their rehabilitation strategies. Cold-recycling technologies such as Full-Depth Reclamation (FDR) are effective rehabilitation strategies that are proven to reduce environmental impacts, as well as construction costs and time. FDR involves pulverizing and reusing materials from distressed pavements in place.

Unfortunately, limited information is available on the material properties of these asphalt mixtures to facilitate the structural design of pavements incorporating stabilized base materials produced with FDR. This information is critical in the structural design of pavements using the Pavement Mechanistic-Empirical Design (PMED). Previous studies have pointed out that default values in previous PMED software versions for cold recycled materials were overly conservative and bordered on non representative. This leads to an underestimation of the true performance capabilities of these materials, resulting in a significant loss of potential for cost and resource saving or even reluctance to specify these techniques.

Research Objective

The main objective of this research was to establish standard modulus values for various FDR base materials that can be confidently and effectively used by Colorado Department of Transportation (CDOT) pavement designers as reliable input to designing pavements utilizing the PMED. To achieve this objective, we conducted the following sub-objectives:

• To determine through comprehensive literature review the methods other state transportation agencies are using to assign structural strength to FDR materials and to develop a synthesis of past research completed on this topic.
• To determine the modulus values of various materials used for CDOT pavements built with FDR in 10 projects that were identified by the Study Panel.
• To correlate the back-calculated FDR modulus values obtained by FWD testing with pavement performance data available from Pavement Management Program Unit.
• To establish a range of reliable modulus values for FDR materials that CDOT should use as input to the PMED software program.
• To confirm the validity of the current predictive equations in PMED for CDOT FDR materials.

Research Methods

As part of this project, the research team collected samples from 10 FDR projects constructed throughout Colorado that were tested to determine dynamic and resilient modulus. The research team also collected data on the design, construction, and actual performance of these sections from the CDOT Pavement Management System. Finally, the research team simulated the long-term performance of these sections using the mechanistic-empirical design software (AASHTOWare) and informed recommended values of standard modulus to be used in the design of FDR projects in Colorado.

Findings

This study found that both initial values of the International Roughness Index (IRI) and resilient modulus were found to have a significant impact on M-E predictions and were calibrated in a two-step process. The proposed input parameters lead to a conservative design of FDR projects and result in improved IRI predictions than the ones derived from current design criteria. With the current design parameters, IRI predictions were on average overestimated by 51 in/mile, whereas the proposed input parameters allow to reduce this difference to 17 in/mile.

From the findings of this study, CDOT will have better information for the design, material selection, and construction to improve the performance of future CDOT projects with FDR materials. This research should benefit local entities including cities, municipalities, counties, other government agencies, consultants, and contractors that use CDOT PMED, construction standards and specifications.

Future research is needed to incorporate these findings in Life-Cycle Analysis and to improve current models in M-E pavement design software to adequately model cold-in-place recycled layers such as FDR.

Funding

Colorado Department of Transportation (CDOT).

Publications

• Torres-Machi, C.; Beesam, V.; Galotti, V. Residual Strength of Full-Depth Reclamation. Colorado Department of Transportation, Report No. CDOT-2020-09. 
• Galotti, V.; Torres-Machi, C. Local Calibration of Stiffness Modulus for Full-Depth Reclamation Design. 2019 International Airfield and Highway Pavements Conference, Chicago, IL, USA, July 21-24, pp. 255-262. Organized by: American Society of Civil Engineers (ASCE). DOI: 10.1061/9780784482452.025
• Beesam, V.; Torres-Machi, C. (2021) Input Parameters for the Mechanistic-Empirical Design of Full Depth Reclamation Projects: Aligning Predicted and Actual Deterioration. Transportation Research Record: Journal of the Transportation Research Board, in press, DOI: 10.1177/03611981211017916