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Full Depth Reclamation (FDR) for In-Place Pavement Recycling

Construction Engineering Services

Pavement Recycling is a general term that involves the re-use of existing asphalt pavement sections for the rehabilitation of existing roadways and parking areas. Numerous techniques are employed for pavement recycling that can be as simple as milling and re-using recycled asphalt in new plant mix asphalt mixtures, to the complete pulverization of the existing pavement section to form a recycled in-place pavement structure. One of the more widely used forms of in-place pavement recycling is Full Depth Reclamation. Full Depth Reclamation (FDR) is an in-place pavement recycling technique in which the asphalt pavement section and the underlying materials (subbase aggregate, subgrade soils or both) are pulverized, blended, and stabilized in place. Depending on the subgrade soil types (if any) that will be part of the mixture, either cement or lime can be used as the additive for FDR. This “Lessons Learned” briefly discusses the various methods of in-place pavement recycling and specific construction practices for the commonly used method of FDR. ASTM specifications (ASTM C-136, D-558, and D-1633) and the AASHTO Pavement Design Guide are excellent references for further information.

Methods of In-Place Pavement Recycling

The most commonly used methods of in-place pavement recycling are mechanical stabilization, chemical stabilization, and bituminous stabilization. Mechanical stabilization incorporates imported aggregate into the pulverized layer to create a stable base or subbase to overlay with asphalt. Bituminous stabilization includes adding an asphalt stabilization agent into the pulverized materials which could consist of an asphalt emulsion or foamed asphalt product. Chemical stabilization includes blending a chemical additive into the pulverized materials which could include Portland Cement, lime, fly ash, or some combination of the above listed items. Mechanical and chemical stabilization can also be combined, as necessary, when one of the listed methods does not provide the required strength for the recycled materials. FDR is a chemical stabilization technique that is used to create a new pavement base prior to the installation of new asphalt layers. Cement is typically the most common chemical additive, and the amount of cement is determined with laboratory testing prior to construction in order to develop the project-specific mix design. Typically, the specifying agency requires a design compressive strength at seven days for the newly stabilized base material and the specific amount (either percentage by weight or volume) of the chemical additive is determined based on attaining the required 7-day compressive strength.

Full Depth Reclamation (FDR) Construction Practices

There are several basic construction practices that should be considered to provide a successful Full Depth Reclamation (FDR) project to the owner:

  • Field samples prior to construction should be taken with appropriate equipment that is capable of fully pulverizing the in-place pavement section (see picture inset above).
  • Mix designs should be determined in the laboratory prior to construction. Depending on the composition of the existing pavement section, several mixtures should be produced in the lab. The mixtures should use variable percentages of the chemical additive, so that an optimal mixture of the materials can be achieved that meets the intent of the project specification for the most efficient cost. If the project design mix is not determined prior to construction, unnecessary delays and costs may be incurred.
  • The contractor chosen to perform the field work for FDR should be well-qualified and capable of showing a history of successful FDR projects. The contractor should have the types of equipment that are necessary to fully and thoroughly mix and blend materials in a consistent manner. Inadequate pulverizing and mixing equipment can result in an inconsistent mixture that will not pass the project Quality Control requirements.
  • A Quality Control (QC) plan is required for most FDR projects and is considered to be an integral part of the FDR project’s success. It outlines the types of inspection and testing required during construction, the intervals required for that testing, and who will be providing the testing.
  • Field testing should only be performed by technicians that are familiar with the FDR process. Although no certification currently exists specifically for FDR testing, the technician providing the testing should have soils certifications from agencies such as VDOT, WACEL, NICET, and specific FDR project experience.

We hope this “Lessons Learned” is helpful to you on a project this summer, or on future projects. For additional information on hot weather concrete, please contact your nearest ECS office. 

ECS Corporate Services, LLC