CHAPTER 4
Case Examples of Practices for Use of Full-Depth Reclamation

4.1 Introduction

The state transportation agencies of California, Idaho, Minnesota, Vermont, and Virginia were selected for a more detailed investigation into the current state of practice. There were several reasons that these states were selected as case examples, as discussed in Section 1.2.3 of this report. These states represent a geographic distribution that includes both cold and warm climates and represent a range of agency sizes. All states indicated a willingness to participate in follow-up case example interviews, and the five states also have used FDR with different types of stabilization and on roads with a range of traffic volumes.

4.2 California Department of Transportation Case Example

4.2.1 Current Practice Related to the Use of FDR

The California Department of Transportation (Caltrans) has over 20 years of experience with FDR and indicated FDR is typically used for remedying bottom-up distresses or when an increase in structural capacity of the roadway is needed. The current policy restricts the use of FDR to roadways with less than 5,000 ADT. This policy was developed when FDR was first implemented in 2002 and was based on allowable traffic controls for single lane closures during construction. Caltrans is currently updating its policy. Currently, it is challenging to conduct FDR projects in California because funds for FDR must come from the State Highway Operation and Protection Program (SHOPP). Caltrans is currently exploring additional ways to promote the use of cold recycling in its districts. For example, the project initiation document guidance instructs the reader to document the consideration of recycling within the planning documentation. Caltrans is working to change its policy to allow consideration of cold recycling as an alternative method for dealing with any distress.

Caltrans has a comprehensive site investigation guide (available online at https://escholarship.org/uc/item/6sr85541) that includes suggestions for location and quantity of materials to be collected for classification and testing, as well as data needs for conducting pavement design, mix design, and determining the appropriate stabilizing agent. The depth of FDR is determined based on the information gathered during the site investigation and is typically 10–12 inches thick. Caltrans has a goal to move away from specific FDR or CIR terminology and instead focus on distress-depth-based decisions. This approach will involve cold recycling projects at the bottom of the distress, and the contractor will decide what equipment to use for the work, which could include paver-laid FDR.

Caltrans utilizes the CalME pavement design tool to design pavements with any type of FDR, within the limits specified in the existing Highway Design Manual. As the design manual changes,

CalME has the flexibility to adapt with it. Caltrans uses a default modulus of 45 ksi for unstabilized, 435 ksi for foamed asphalt-stabilized, and 1,450 ksi for cement-stabilized FDR layers in CalME. These modulus values were determined based on FWD testing of existing projects, laboratory testing on field core specimens, and extensive evaluations of accelerated pavement test sections. The designer can change the modulus values if project-specific values are available.

4.2.2 Use of Stabilizing Agents in FDR

Caltrans primarily uses foamed asphalt and cement as stabilizing agents in FDR. Foamed asphalt was the first stabilizing agent used in California and so has been adopted based on historical use and experience. There were some concerns about the emulsionʼs ability to cure in a thick FDR, based on previous literature; however, this is currently being reassessed. The choice of stabilizing agent is based on the Unified Soil Classification System (USCS) soil type classification and gradation information obtained during the site investigation. For high-quality materials (such as well-graded and poorly graded), foamed asphalt is used, while cement is used for other, more marginal materials.

Caltrans has formalized the process for writing specifications through a joint working group composed of agency and industry representatives, as well as academics. Many aspects of their cold recycling specifications originated from ARRA Resources, the Wirtgen manual, national research, and local experience. Caltrans has spent time and effort over recent years to locate the original information in national and international manuals and verify its appropriateness for California. All Caltrans recycling specifications have gone through recent revisions to reflect the latest research, long-term performance, equipment developments, and experience.

4.2.3 Current Practice Related to Post-Construction of FDR

Caltrans requires that the road be open to traffic by nightfall. There is a moisture content specification limit with respect to the time to overlay. However, in practice, it is based on experience and more closely tied to project scheduling, with some sections not overlaid for 2 or more weeks and others only for several days, depending on when the project finishes. Caltrans has not observed any issues with opening to traffic and looks forward to the outcomes of the NCHRP 9-62 project on Rapid Tests and Specifications for the Construction of Asphalt-Treated Cold-Recycled Pavements.

The frequency of QC for FDR projects was established to match the similar quality standards of construction for other pavement layers, such as granular bases and HMA. Caltrans has made a concerted effort to have consistent specifications within pavements. They are currently revising their specifications to accommodate both paver-laid FDR and wheel-driven reclaimer procedures. The Caltrans Independent Assurance (IA) program certifies individual QC technicians on the specific tests they will conduct. The labs conducting the testing need to be certified through AASHTO RE: SOURCE and Caltrans IA programs.

Caltrans currently tracks the performance of FDR projects within its pavement management system (PMS), but the search identifier will change when other work (e.g., patching or maintenance overlay) is conducted. There are a few projects (such as the first one from 2002 and others designated as pilot projects) where regular monitoring and long-term performance are being tracked. The agency would like to develop more formal plans or strategies to track all FDR projects in the PMS.

Designers are required to conduct LCCA as part of the design process for all rehabilitation projects in California, and the information is used to compare with the next best strategy. LCA is not currently required in the decision-making process, but it is used for informational purposes. Caltrans is moving toward LCA being a formal part of the decision-making process, and the

agency is currently benchmarking inputs and evaluating processes to be ready for implementation in the future.

4.2.4 Challenges and Future Plans

Caltrans indicated that the lack of agency training is one of its main challenges in implementing FDR. While FDR started at Caltrans in 2002, the agency staff with the greatest technical knowledge have retired. Recent training for new staff on in-place recycling began about 5 years ago. A 2-day training was conducted in 2021, and there have been five or six additional trainings for districts since then. Caltrans has observed that newer district staff often lack the background knowledge to conduct full site investigations. Caltrans also noted that there are challenges in funding site investigations during the planning stages.

4.3 Idaho Transportation Department Case Example

4.3.1 Current Practice Related to the Use of FDR

The Idaho Transportation Department (ITD) has been conducting FDR projects for over 20 years, and there is no restriction on the roadway traffic volume for which it can be considered a candidate treatment. FDR is considered when the roadway condition determines that preservation is not appropriate, and reconstruction or restoration is appropriate. FDR is used when there is a need to improve the pavement structure due to issues with the base or subgrade layer(s), and additional surface course thickness would not be adequate. ITD primarily uses cement-stabilized FDR, which the agency refers to as Cement Recycled Asphalt Base Stabilization (CRABS) in its manuals and specifications.

The pre-site investigation includes an evaluation of the presence of moisture, ditching issues, the amount of cut and fill required, and the construction history. FWD testing is conducted to evaluate the base and subgrade response, along with a borehole investigation to assess material properties. GPR is used to determine the thickness of layers. ITD staff indicated that they are beginning to see older FDR projects that are nearing the end of their expected service interval. They are exploring various options for handling these, including a second round of FDR with Portland cement or other binders, or completely pulverizing the material. ITD also uses a Traffic Speed Deflection Device to provide continuous information and identify breaks in the base properties along a section of pavement.

Section 530 of the Idaho Materials Manual does not directly address FDR depth. The typical depth used is 8 inches, based on experience and historical equipment constraints. Deeper depths are preferred if the equipment has the capacity. Potential issues with vertical alignment and width restrictions also dictate allowable FDR depths in some areas.

ITD uses AASHTOWare Pavement ME for pavement design and is currently moving to the web-based version of the software. The agency uses a design modulus of 80,000 psi for the CRABS layer based on limited data and experience, and acknowledge that the values being used are likely not sustained throughout the pavement service interval based on field observations after 15 years. ITD is moving toward a more accurate use of FWD confirmation and laboratory testing to define modulus values for use in design. The agency also recognizes that there is considerable uncertainty regarding the treatment of the FDR layer (and CRABS specifically) in PavementME.

4.3.2 Use of Stabilizing Agents in FDR

ITD primarily uses Portland cement in FDR (CRABS), a decision based primarily on historical use and experience. They have done a few trials with foamed asphalt and emulsified asphalt, but do not have a standard specification for FDR using these materials. ITD uses a fixed dosage of

1.5%–2.0% cement, based on the weight of the pulverized material. ITD does not conduct a mix design or perform Atterberg limits or gradations to specify dosage, nor has the agency completed sufficient research to confirm the appropriateness of this dosage and the long-term consistency of the modulus value of the FDR section. The value is based on historical experience and the philosophy of binding the fines in contaminated base or borrow material. ITD has observed that some materials previously stabilized with cement still contain fines after some time, indicating that the long-term viability of the cement stabilization is suspect in some instances. The ITD materials manual indicates that pre-existing fines and uncrushed base materials may reduce the performance of the CRABS base.

4.3.3 Current Practice Related to Post-Construction of FDR

The ITD standard specification indicates that an asphalt overlay must be placed within 48 hours of the CRABS layer construction. The specification indicates that the surface of the compacted CRABS layer needs to be visibly moist at all times prior to overlay. It is common practice to apply an emulsion primer coat for immediate curing.

The specified inspection and QC frequency for FDR is documented in the ITD quality assurance manual and is based on historical experience, generally following other construction specifications. The standard ITD requirements for embankment and base qualification are required for agency personnel. Several projects have required a test for the presence of cement to the required depth when conducting moisture testing. While ITD does not typically require this, the staff believe it is beneficial to confirm the depth and width of the cement distribution.

When highly plastic subgrade soils exist along the project, micro-cracking of the FDR is sometimes specified to alleviate concerns about a brittle layer over a soft subgrade and to avoid transverse cracking at 25- to 30-foot intervals, which has been observed on previous projects. The determination to conduct micro-cracking is based on experience and is specified for the project prior to construction.

ITD has been informally tracking the performance of FDR/CRABS pavements and is looking to conduct more formal tracking moving forward. The purpose of the tracking will be to develop family deterioration curves for FDR within the ITD asset management system and to investigate the potential causes of any issues that arise. ITD would also like to pursue further investigations to evaluate modulus values using FWD on FDR layers prior to overlay placement and to verify PavementME design modulus values during construction.

A deterministic LCCA based on equivalent uniform annual cost is conducted for FDR projects. This information is included in the report, but it is one of many factors considered in decision-making. ITD is looking to research the use of probabilistic analysis, such as FHWA RealCost, in the future.

4.3.4 Challenges and Future Plans

ITD reports that it has observed issues with the uniform mixing of cement to the required depth and across the width of the pavement, with some sections exhibiting extremely variable results. The agency is looking to improve this by providing additional training and inspections to eliminate these issues. ITD is also planning research to evaluate when mix designs would be beneficial instead of the specified standard cement dosage.

ITD plans to continue its program using cement-stabilized FDR (CRABS) as the primary methodology. The agency is interested in further investigation and evaluation of asphalt emulsion and foamed asphalt stabilizers, and planned research may include these within its scope.

4.4 Minnesota Department of Transportation Case Example

4.4.1 Current Practice Related to the Use of FDR

The Minnesota Department of Transportation (MnDOT) has indicated that it has over 20 years of experience with FDR and commonly uses FDR on roadways with ADT of up to 10,000. The agencyʼs pavement management system typically uses the network-level bituminous pavement decision tree (https://edocs-public.dot.state.mn.us/edocs_public/DMResultSet/download?docId=26579260) to suggest projects for reclamation. Pavements with lower ride quality and surface ratings (surface rating is calculated based on various pavement distresses) and those without curbs are considered for reclamation. The ultimate decision to consider FDR as a treatment on existing roadways is made by the districts.

Once a project is suggested for FDR, MnDOT conducts coring on the project. The cores are used to identify various material layers and to establish the reclamation depths. Chapter 2 of the MnDOT Pavement Design Manual (https://www.dot.state.mn.us/materials/pvmtdesign/manual.html) provides guidance on MnDOTʼs site investigations, which primarily utilize auger borings for material classification and layer thickness determinations. Ground-penetrating radar scans are also suggested. Lastly, DCP is occasionally used to measure the mechanical properties of in-place materials after a core has been removed.

MnDOT primarily uses mechanical FDR, with many of these reinforced using geosynthetics. Typically, MnDOT uses a two-pass FDR process, with a depth of 6 inches being the most common for stabilization with asphalt emulsions. Typically, MnDOT FDR projects do not use supplementary aggregate; however, when needed, the agency specifies the use of its standard pavement granular base materials (MnDOT Class 5 or 6). Occasionally, coarse mining tailings (3/8-inch minus) material can also be used in Northern Minnesota (close to the Mesabi Iron Range Mining region).

MnDOT uses the MnPAVE mechanistic-empirical approach for pavement designs with FDR layers. The standard cross-section used is based on the allowable grade raise; the width of the existing shoulders usually governs this, along with the existing pavement structure. If there is not enough granular material under the existing HMA pavement, then the project may shift from mechanical FDR to FDR. The modulus values used by MnDOT in design were determined through field testing.

4.4.2 Use of Stabilizing Agents in FDR

The pavement design process determines whether a project is suitable for mechanical FDR versus stabilized FDR. The current MnDOT practice does not consider the strength gain of stabilized FDR over time. An engineered asphalt emulsion is commonly used by MnDOT when stabilization is needed. MnDOT suggests using the Wirtgen mix design method to determine the stabilizer dosage. The selection of the Wirtgen mix design method was based on previous research conducted by MnDOT, which had favorable experiences with these materials in the field. MnDOT typically also specifies 0.5% cement to support the curing of the FDR; in some instances, 1% cement has also been used, but this is uncommon and is typically used only as a field adjustment. Mix designs for FDR materials for MnDOT projects are commonly done by two consultant laboratories. These designs are typically completed prior to the development of construction plans for the projectʼs letting.

4.4.3 Current Practice Related to Post-Construction of FDR

MnDOT requires paving of the asphalt wearing surface between 3 and 14 days after the construction is complete. The agency does not have a standard method to determine the timing of

wear course application; however, it has used the process of parking a water truck on the FDR surface. If no indent is left by the truck, then the FDR layer is considered to have sufficiently cured for the application of the wear course.

The current QC sampling and test frequencies used by MnDOT are based on input from the subject matter expert on their agencyʼs specification committee. MnDOT requires a control section (test strip) to establish the compaction requirements. A minimum of 97% of the test strip density is required for the full extent of the project. Currently, MnDOT FDR construction specifications do not have requirements for the agency inspector and QC technician certification levels; however, a MnDOT grading and base tester course is required as specified in the MnDOT Technical Certification Policy Manual.

The MnDOT has conducted LCA on select FDR projects using an impact database developed by FHWA. These results were reported in the 2021 MnDOT Sustainability Report (https://www.dot.state.mn.us/sustainability/sustainability-reporting.html).

4.4.4 Challenges and Future Plans

Currently, MnDOT does not utilize LCA outcomes in its decision-making process to select FDR as a pavement rehabilitation treatment, an area that requires future effort. MnDOT has also observed that the number of FDR projects has not changed substantially over the years, with approximately 10–12 projects each year (approximately 10 unstabilized and 1–2 with asphalt emulsion stabilization).

4.5 Vermont Agency of Transportation Case Example

4.5.1 Current Practice Related to the Use of FDR

The Vermont Agency of Transportation (VTrans) has reported an increase in the use of FDR on agency-maintained roadways over the last 20 years. The increase is primarily due to the agencyʼs improved experience with the use of FDR in rehabilitation, as well as the greater availability of contractors with FDR experience. VTrans does not have a fixed traffic threshold for roadways where FDR may be considered as a viable treatment. However, they have not yet undertaken FDR on high-traffic roadways (such as interstate highways and other limited-access highways), as none have yet met the distress and condition criteria that typically justify the use of FDR. The current VTrans PMS utilizes the performance measures of ride quality and pavement distresses (rutting, transverse cracking, and structural cracking) to suggest pavement rehabilitation decisions. When PMS suggests pulverizing and overlaying as a suggested treatment, VTrans typically conducts a geotechnical investigation on the project, which typically includes borings in the travel lanes with depths of up to 5 feet. Cores are also often taken from the existing pavement. The information from the geotechnical investigation is used to specify FDR as a selected treatment. In the future, VTrans is also considering using DCP testing as part of the geotechnical investigation.

Typically, VTrans uses a two-pass FDR process, with the reclamation of existing pavement materials in the first pass, followed by mechanical stabilization only, mechanical and chemical stabilization, or bituminous stabilization in the second pass. Most projects have some amount of new aggregate added. A total FDR depth of 8–12 inches is typical, with stabilization usually applied to the top 6 inches. VTrans determines the exact thickness of the stabilization pass based on the structural pavement design conducted using the AASHTO 1993 empirical pavement design process (using the AASHTO Darwin 3.1 software). VTrans assigns a structural layer coefficient value of 0.2 to the FDR layer for the pavement design. This value is based on prior FWD back

calculations conducted by the agency, as well as laboratory evaluations using the California Bearing Ratio (CBR) test.

4.5.2 Use of Stabilizing Agents in FDR

VTrans uses bituminous emulsions, calcium chloride, and Portland cement as stabilizing agents in FDR projects (in that order of frequency). In past FDR projects, VTrans observed over-cementing and inadequate mixing with cement, which has prompted the more widespread adoption of bitumen emulsion as a stabilization agent in recent projects. VTrans has recently adopted the use of the current AASHTO R 109 standard practice using the AASHTO M 347 specification to determine the amount of stabilizing agent for emulsion-based FDR. The agency restricts the ratio of total added asphalt to the added cement in FDR to a maximum allowable value of 2.5:1. Prior to this adoption, a Marshall stability, flow and quotient value was used for determining emulsion dosage; this practice is being retired due to availability of a new AASHTO specification as well as VTrans ending its Marshall specification for hot-mixed asphalt. VTrans does not have a standard specification to determine the dosage of the stabilizing agent for FDR stabilized with cement. However, the agency has used cement-stabilized FDR on a recent project, and that experience is being used to develop a standard specification. VTrans currently anticipates determining the cement dosage using a 250 psi unconfined compressive strength (UCS) target value. If the strength exceeds 600 psi, micro-cracking will be considered to reduce the in situ strength back to 600 psi or lower.

VTrans FDR projects typically utilize added crushed aggregate within the reclaimed mix. If a project has a high number of curves or geometric updates (such as superelevation changes), the amount of added crushed stone could be high. VTrans is aware that this poses a challenge with respect to the mix design for FDR, and its philosophy is to ensure that material designs have some degree of flexibility to account for material variations along the project.

4.5.3 Current Practice Related to Post-Construction of FDR

VTrans requires the use of a control section (test strip) to establish the target maximum density for a project. Thereafter, the in situ density of the project is required to be between 95% and 105% of the target maximum density. For FDR stabilized with emulsion, VTrans allows for the placement of a wear course when the free moisture content reaches 2% or less, with a maximum of 14 days. These requirements align with the Basic Asphalt Recycling Manual (BARM) suggestions. The construction contractor is responsible for maintaining the roadway until it is overlaid. Projects that use calcium chloride as the stabilizing agent typically get overlaid much sooner due to the fast hydration of this product.

In the past, the VTrans practice for the QA process involved substantial acceptance testing from the agency and limited QC effort from the contractor. This process has matured into the current practice of more process control through QC testing by the contractor. The QC sampling and testing frequencies are based on the agencyʼs experience, ensuring that necessary process control adjustments can be made in a timely manner. VTrans requires that both the agency inspector and contractor QC technicians hold certifications from the Northeast Transportation Training and Certification Program (NETTCP) for soils and pavement inspection.

VTrans has had multiple studies (both in-house and through contractors) that conducted DCP testing of FDR projects to be able to predict modulus values for these pavement layers. The agency has found DCP to be inadequate for obtaining or estimating the modulus value of FDR layers. As part of the PMS network performance data collection effort, VTrans projects with FDR are monitored. At present, VTrans has an ongoing contract to develop family-based deterioration models for their PMS with one of those families being pulverize and overlay.

4.5.4 Challenges and Future Plans

VTrans has encountered some challenges with the use of FDR stabilized with cement, which have been tied to contractor experience and equipment issues. These have typically resulted in inadequate mixing of cement and over-cementing, both of which have led to poor ride quality due to premature cracking and corresponding crack heaving from frost penetration actions. VTrans has not experienced any significant challenges with emulsion-based FDR and mechanical (non-stabilized) FDR. The past challenges of insufficient QC testing to conduct process control have now been alleviated through updated specifications. Additionally, VTrans believes that using the required residual asphalt-to-cement ratio in the specification will reduce the likelihood of encountering projects with “dry” emulsion-stabilized FDR. The significant variation in the supplementary aggregate amount continues to be a challenge, especially on roadways with a significant number of curves.

4.6 Virginia Department of Transportation Case Example

4.6.1 Current Practice Related to the Use of FDR

The Virginia Department of Transportation (VDOT) has over 10 years of experience with FDR, and it is used on all types of roadways, from subdivisions to interstates. It is used if it is determined to be the appropriate process for that particular project. There are maintenance of traffic (MOT) aspects, such as extended lane closures and a quick return to traffic, that minimize the impact of the construction on the traveling public, which are more challenging for higher-volume roadways. VDOT has successfully navigated these challenges where FDR is used.

FDR is primarily used to correct pavement deterioration due to failures in the subgrade or unbound base and subbase layers. Sections that exhibit structural deficiencies will typically be noted as restorative maintenance or reconstruction, which are highlighted as part of the departmentʼs pavement maintenance decision matrices. These notes will serve as the first step in identifying a section that may benefit from a full-depth repair, such as FDR. The decision to use a full-depth repair strategy, such as FDR, is initiated at the district level with support and input from the central office and research. Chapter VI, Section 608 of the Materials Division Manual of Instructions provides guidance on the selection and use of cold recycling techniques for rehabilitating a roadway segment. A formal decision tree process does not currently exist, and VDOT is developing one.

Pre-site investigation for FDR projects includes the standard project-level forensic investigations outlined in the Materials Division Manual of Instructions, Section 601.02. In addition, the use of GPR is suggested to determine the layer thicknesses and consistency of the pavement structure throughout the project. Soil borings are used to classify the base and subbase materials for selecting a stabilizing agent. The extent of pre-site investigation does depend on the type of project, with potentially less extensive evaluation for lower-volume roadways (such as subdivisions).

Historically, VDOT has used a typical FDR depth of 10–12 inches; however, thinner depths, such as 6–8 inches, have also been successfully used. This standard depth range is based largely on prior experience and the desire to capture subgrade materials in thicker pavement structures. Additionally, there was no substantial increase in cost with the greater FDR depths. They require pre-cutting and then a second pass in which all material is stabilized. VDOT is currently exploring the use of a paver-laid FDR process, which will be completed in a single pass.

FDR is considered a form of reconstruction, and as such, pavement design is typically conducted using a mechanistic-empirical design process (MEPDG). VDOT currently uses AASHTOWare PavementME V2.2.6 for designing pavements on Interstate and Primary roadways. During

the flexible pavement design in AASHTOWare PavementME V2.2.6, FDR is modeled as a non-stabilized base with a higher stiffness value of 80,000 psi. This modeling approach is used because the semi-rigid design module in AASHTO PavementME V2.2.6 has not been globally calibrated and therefore has not been adopted in the VDOT ME design process. With the intention of transitioning to the web-based AASHTOWare PavementME process and a globally calibrated semi-rigid model, VDOT is evaluating the feasibility of adopting the semi-rigid module for any cement-treated base layer. Pavement designs for lower-volume, secondary, and subdivision roadways fall under the secondary/subdivision design guide, which utilizes a “Thickness Index” concept developed by VDOT based on the original AASHTO road test and Virginiaʼs design experience. However, secondary roadways with annual ADT greater than 10,000 are considered to be designed using PavementME for VDOT. Before the implementation of PavementME, VDOT used the AASHTO 1993 design approach for designing with FDR using a structural layer coefficient of 0.25

4.6.2 Use of Stabilizing Agents in FDR

VDOT primarily uses cementitious material as a stabilizing agent in FDR projects, although their FDR specification allows for the use of bituminous material. This choice is based primarily on the existing material type, prior experience, and the contractorʼs familiarity and expertise. VDOT used bituminous-stabilized materials on one project in 2008 and recognizes that there may be opportunities to explore further use, such as with thinner overlay sections.

The stabilizing agent dosage is determined based on job mix formula criteria of the VDOT Special Provision for FDR. These criteria depend on the type of stabilizing agent; for cement-stabilized FDR, compressive strength is used, for foamed asphalt, ITS is used, and for asphalt emulsion, Marshall stability is used. The minimum required values of these properties were determined by a task force that utilized a range of available technical resources (e.g., ARRA BARM, Cold Recycling Manual for Asphalt), engaged in discussions with other states, and conducted research.

4.6.3 Current Practice Related to Post-Construction of FDR

VDOT requires that a density of 97% from the approved Job Mix Formula (JMF) be achieved in the trial section constructed prior to full production. VDOT requires that cement-stabilized materials be kept moist prior to overlay and that bituminous-stabilized materials be cured to at least 50% of their optimum moisture content prior to overlay placement. This value was chosen instead of a standard moisture content, recognizing that mixtures can have different optimum moisture contents. The VDOT special provision states that asphalt overlays may be placed with the FDR if it is able to support construction equipment sufficiently; however, no specification language defines any criteria to “sufficiently support construction equipment.” Contractors typically perform a proof roll using a loaded water truck or dump truck.

Most constructed projects used a minimum two-lift asphalt overlay on the FDR layer. In Section 608 of the Materials Manual of Instructions, cold pavement recycling, including FDR, is surfaced with one or more layers of asphalt. Thickness is generally determined by pavement design analysis. On low-volume routes, a surface treatment may be used. However, for all Interstates and routes where the two-way average annual daily truck traffic (AADTT) exceeds 200, a multi-layered asphalt overlay with a minimum combined thickness of 4.0 inches will be used. VDOT is currently conducting research on thinner overlay sections and surface treatments, as well as following other research (e.g., National Center for Asphalt Technology) in this area to document the performance of thinner sections.

The required frequency for QC on FDR projects was determined by task group consensus and closely follows HMA and other pavement material construction specifications. Contractor QC

technicians are required to complete certification courses specifically for FDR through established training at a local community college. Agency personnel involved with FDR also complete the same certification course.

After construction and during its performance period, VDOT has conducted FWD testing to evaluate early strength gains and structural performance parameters on some projects. They also use PMS data to determine deterioration curves for FDR projects.

VDOT required that LCCA be conducted on all projects that fall under the pavement type selection process. The LCCA process used is based on the pavement surface type and is not controlled by the presence of an FDR layer. Sections 606 and 607 of Chapter VI of the Materials Division Manual of Instructions cover the pavement type selection process and the LCCA process.

4.6.4 Challenges and Future Plans

VDOT has not encountered significant challenges with the use of FDR. They have found that conducting just-in-time training for projects has been beneficial. These sessions provide an overview of the FDR process, a review of the project specifications, and experiences and/or challenges from previous FDR projects. Typically, this training includes the contractor and agency personnel, specifically the inspection team that will be on the project. In the past, VDOT has also conducted reviews, or “lessons learned,” on projects and observed benefits in doing so to adjust the process for future projects.

4.7 Summary

Five case examples have been conducted through interviews and review of documents provided by interviewees to further enhance the information on the current state of the practice with respect to FDR. Five state transportation agencies were selected as case examples based on their geographic diversity, experience level with the use of FDR, and types of stabilizers used by them. The case examples demonstrate that the use of FDR in the United States is still relatively new, with the oldest projects being around 25 years old. The case examples also confirm that the practice of using cement-based stabilization for mechanical FDR is longer-standing compared to the use of asphalt emulsions. Only a few agencies have had experience with the use of foamed asphalt as a stabilizer. Regarding the applicability of FDR on highways with varying functionality and traffic volumes, it was found that there is a range of practices, from the use of FDR on interstate highways to local roads. Four of five case example agencies indicated that FDR is available as one of the several rehabilitation techniques within their asset management program, and a responsible entity (either district or central office) can select to choose it amongst other alternatives; one agency has a specific decision tree where the asset management program explicitly suggests FDR as a treatment.

An extensive amount of information was gathered from case examples with respect to pre-site investigation to support FDR selection as well as mix design. The most commonly collected information during a pre-site investigation includes layer thicknesses, geotechnical classifications of unbound and subgrade layers, and measures of materialʼs mechanical properties (such as using DCP or FWD). As noted in the survey findings, there is a range of methods adopted by state transportation agencies in selecting the type of stabilizing agent as well as its dosage. ITS is the most commonly used approach for FDR stabilized with asphalt emulsions, and the unconfined compressive strength is used for FDR stabilized using Portland cement. Between the case example agencies, a wide range of pavement design methodologies is used. They range from the use of the AASHTO 1993 layer coefficient approach, an agency-developed mechanistic-empirical method, to AASHTOWare PavementME.

The construction specifications between the case example agencies have relatively comparable requirements for establishing construction quality, with the use of in-place density, moisture content, stabilizer content, and gradation as the most commonly used process control and acceptance variables within the QA process. While these measures are consistent, there are substantial differences in how the thresholds are established as well as in terms of the sampling and testing frequencies. For example, VTrans requires a test strip to establish the maximum density, whereas VDOT uses the JMF density as the maximum density for controlling in situ density. The case examples also revealed that the majority of agency practices for the QA process of FDR are derived from their current and past QA practices for aggregate bases and HMA.

The current challenges faced by the state transportation agencies in the use of FDR were explored in case examples. The training of agency personnel to familiarize them with FDR, as well as the lack of previous agency experience with FDR and certain stabilizers, has been identified as a challenge restricting the widespread use of FDR. Multiple case example agencies have also encountered overstabilization and inadequate mixing of FDR stabilized by Portland cement. Policy guidance regarding the suggestion of FDR as a suitable treatment, as well as its full integration within asset management systems, has been identified as a challenge to its widespread usage. Last, developing the necessary LCA inventory for FDR and adopting LCA-based rehabilitation treatment selection has been identified as a future step by several case example agencies.