SUMMARY
Full-Depth Reclamation: Current Practices

Full-depth reclamation is an in-place recycling method used for the reconstruction and rehabilitation of flexible pavements, which utilizes existing asphalt concrete and base layers as the base layer for a new surface wearing course. The environmental and economic performance of pavement can be improved by optimizing the reuse of in-place materials, thus minimizing construction time and associated traffic disruptions. Additional aggregate or stabilizing additives such as Portland cement, emulsified asphalt, and foamed asphalt may be blended with the existing materials to produce the full-depth reclamation (FDR) layer material. While FDR is a widespread practice across the United States, a range of practices related to mixture design, construction, and quality assurance (QA) are also in use. The primary objective of this synthesis is to document the current state of department of transportation (DOT) practices and procedures related to the use of FDR. This section of the report presents a summary of the major findings from this synthesis project, along with identified knowledge gaps and research needs.

The information for this synthesis was gathered through a comprehensive literature review; a survey of the DOTs of the 50 states, Washington, DC, and Puerto Rico; and subsequent interviews with personnel from five states selected for further study. Forty-nine DOTs (47 state DOTs, Washington DC, and Puerto Rico, or 49 of 52) responded and completed the survey, attaining a survey response rate of 94%. After reviewing the literature and detailed survey responses, the project team selected the DOTs of California, Idaho, Minnesota, Vermont, and Virginia for case example interviews.

The following key observations were made based on the survey data, detailed interviews, and literature review.

• Overall, the use of FDR by state agencies is widespread across the United States, but in relatively limited quantities and most often on lower-volume roadways. Most FDR projects are less than 10 years old, and some of the early ones are closer to 25 years old.
• There are no consistent procedures for selecting FDR as a rehabilitation alternative among state agencies. Some agencies have a mature asset management system that specifically suggests FDR. In contrast, others make this decision based on their staffsʼ experience and familiarity of FDR as a treatment.
• Benefits of using FDR compared to other rehabilitation methods include the increased reuse of materials, the in-place nature of the process, energy conservation, decreased construction time, and the requirement for fewer pieces of equipment. Structural and functional deficiencies deeper in the pavement structure can be corrected.
• Agencies almost always conduct pre-design investigations, which most commonly include determining layer thicknesses and geotechnical classification of unbound materials, with some states also assessing mechanical properties (e.g., using dynamic cone penetration or falling weight deflectometer).

• The most commonly used stabilizing agent is Portland cement. This choice is typically based on site conditions (such as the type of subgrade soil), the materialʼs performance, and the DOTʼs historical experience with it. Emulsified asphalt is more commonly used as a bituminous stabilizing agent than foamed asphalt.
• A wide range of mix design methods is used for FDR materials to determine the optimal dosage of the stabilizing agent. Unconfined compressive strength for cement-stabilized FDR and indirect tensile strength for bituminous-stabilized FDR are the most commonly used properties 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 include in-place density, moisture content, stabilizer content (if used), and gradation in their QA processes. However, there are substantial differences among agencies in terms of sampling and testing frequency, as well as 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. Some agencies also use a loaded vehicle or a proof roll to evaluate the deformation of the FDR layer before allowing traffic or an overlay.
• The performance of FDR pavements over time is typically included within most agency pavement management systems (PMSs), with the most common measures evaluated being surface distress and ride quality. However, formal tracking of FDR sections is less common.
• Some of the responding agencies indicated that they quantify the sustainability of FDR projects using a life cycle assessment (LCA), which is currently used for informational purposes only. Other responding agencies indicated that they quantify the cost-effectiveness of FDR projects using life cycle cost analysis (LCCA). In some states, it is formally included in the decision-making process. Many states have recognized the value of these analysis methods, and several are in the developmental or planning stages of incorporating them into their processes.
• The survey indicated that the major challenges agencies face in using FDR are the lack of (1) agency experience, (2) contractor expertise, (3) funding for planning and pre-construction activities, (4) mixed design procedures and criteria to determine stabilizing agent dosage, and (5) consistent guidance or a process to suggest FDR as a rehabilitation alternative.

The following is a list of identified research gaps and suggested future activities:

• There is a need for a standardized approach to 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 moduli 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 on appropriate QA processes for FDR materials—in terms of the requirements for inspection and acceptance, as well as the frequency of sampling and testing—is needed.
• Knowledge gaps exist regarding the long-term performance of FDR pavements. Formal tracking of FDR sections within PMSs and performance analysis over time is needed.
• Quantification of the sustainability of FDR pavements, including conducting an LCA or quantifying economic benefits using an LCCA, was identified as a knowledge gap. The information from these analyses is not typically included in the decision-making process around rehabilitation strategies.