CHAPTER 5
Summary of Major Findings and Future Needs

Full-depth reclamation is a rehabilitation option for flexible pavements that utilizes existing asphalt concrete and base layers as the base layer for a new surface wearing course. Stabilizing additives, such as Portland cement, emulsified asphalt, and foamed asphalt, can be blended with the existing materials to produce the FDR layer material. While FDR is a widespread practice across the United States, there is a range of practices in use related to mixture design, construction, and QA. The primary objective of this synthesis is to document the current state of DOT practices and procedures related to the use of FDR. This chapter presents a summary of the major findings from this synthesis project, along with identified knowledge gaps and research needs.

5.1 Summary of the Findings

The responses provided by 49 state DOTs provided valuable insight into current practices with respect to the use of FDR. Detailed interviews were conducted with DOTs in five states. The information obtained in the interview sessions was used to acquire a more precise overview of the effective practices for the use of FDR.

Based on the work carried out in this synthesis, the key findings are

• FDR is used by a majority (85%) of state agencies across the United States, and is typically performed on lower-volume roadways, with less than 50 lane-miles per year in an individual state. Compared to other major rehabilitation methodologies (such as mill and overlay), the use of FDR in the United States is relatively new. The majority of FDR projects are less than 25 years old, and the early FDR trials in the United States are just now reaching the end of their service life. Approximately one-quarter of agencies using FDR have 10 years or less experience with the process.
• Agencies almost always conduct pre-design investigations, which most commonly include determining layer thicknesses and geotechnical classification of unbound materials, with some states also conducting measures of mechanical properties (e.g., DCP or FWD).
• Based on the literature review and survey responses, Portland cement is the most common stabilizing agent used in FDR projects and is typically chosen by the agency based on site conditions (subgrade soil type) and historical experience and performance. Emulsified asphalt is more commonly used as a bituminous stabilizing agent than foamed asphalt.
• The stabilizing agent dosage is most often determined through a mix design performed either by the agency or the contractor. A wide range of mix design methods is used for FDR materials. Unconfined compressive strength for cement-stabilized FDR and indirect tensile strength for bituminous-stabilized FDR are the most common properties used for design thresholds.
• The design of pavements with FDR layers is conducted using a range of methodologies, with empirical design being the most commonly reported approach. Layer properties (structural

• coefficients or modulus values) used in design can be fixed values or measured properties, and a wide range of values is currently used, depending on the stabilization type.
• Most FDR construction specifications utilize similar parameters within the QA processes, and these commonly include in-place density, moisture content, stabilizer content (if used), and gradation. However, there are substantial differences among agencies in terms of sampling and testing frequency and threshold values for these parameters.
• After the construction of an FDR layer, agencies most commonly use a fixed amount of time prior to allowing traffic or an overlay to be placed. The survey and case examples also indicated that some agencies use a loaded vehicle or proof roll to evaluate deformation of the FDR layer prior to allowing traffic or overlay.
• Most agencies indicated that the performance of FDR pavements over time is included within their PMS systems, with the most common measures evaluated being surface distress and ride quality. The case examples indicated that some states formally use this information for tracking and developing deterioration curves, while in some states, the tracking is done more informally at this time.
• Approximately one-third of the responding agencies indicated that they quantify the cost-effectiveness of FDR projects using LCCA, which is most commonly employed and, in some states, is formally included in the decision-making process. Some states use LCA to quantify the sustainability of FDR; however, it has been used for informational purposes only to date. Many states recognize the value of the analysis, and several are in the developmental or planning stages of including it in their processes.
• The survey indicated that the major challenges agencies face in using FDR are (1) lack of agency experience; (2) lack of contractor expertise; (3) lack of funding for planning and pre-construction activities; (4) lack of mix design procedures and criteria to determine stabilizing agent dosage; and (5) lack of consistent guidance or process to suggest FDR as a rehabilitation alternative.

5.2 Identified Gaps in Knowledge and Future Research Needs

Knowledge gaps that were identified from the literature review, surveys, and the case example interviews, and the suggested future activities are the following:

• The literature review, survey, and case examples suggest that a standardized approach is needed for conducting mixture design and selecting the dosage of stabilization agents for FDR materials.
• Characterization methods for FDR materials in pavement design and performance evaluation are currently diverse and under exploration, lacking a unified approach. A broad range of structural coefficients and modulus values is used for FDR materials in pavement design. Knowledge gaps exist about the fundamental, engineering, and empirical characteristics of these materials initially after construction and over time in the field.
• Unified guidance is needed on the appropriate QA processes for FDR materials in terms of the requirements for inspection and acceptance, as well as the frequency of sampling and testing conducted.
• Knowledge gaps exist in the long-term performance of FDR pavements. Formal tracking of FDR sections within PMS and performance analysis over time is needed.
• The quantification of the sustainability of FDR pavements through conducting LCA was identified as a knowledge gap through the survey and literature review. Specifically, there is a lack of existing impact inventories for FDR, as well as limited information on use-phase impact characterization. The information from LCA is not typically included in the decision-making process around rehabilitation strategies.