By Dwight Walker, P.E.
Good practice dictates that pavements be maintained and preserved so that major rehabilitation is not needed. For a variety of reasons, this does not always happen. Sometimes it is not practical to fully rehabilitate a badly deteriorated asphalt pavement. Cracking or other distresses may have progressed to the point that a more aggressive approach is needed. Complete removal and replacement is not necessary; the in-place materials of the old pavement have value and can be reused. Full depth reclamation (FDR) is an efficient means of rehabilitating these pavements.
The Asphalt Recycling and Reclaiming Association (ARRA) defines FDR as, “a pavement rehabilitation technique in which the full flexible pavement section and a pre-determined portion of the underlying materials are uniformly crushed, pulverized or blended, resulting in a stabilized base course.”
FDR destroys the cracks in an old pavement and restores the pavement cross section, while converting the old pavement structure into a stabilized base. ARRA further clarifies that, “FDR is distinguished from other rehabilitation techniques such as cold planing, cold inplace recycling and hot in-place recycling by the fact that the rotor or cutting head always penetrates completely through the existing asphalt layer and into the underlying base, sub-base or sub-grade layers.” Reclaimed thicknesses depend on the depth of the in-place asphalt and aggregate of the old pavement but usually are in the 6- to 12-inch range.
Candidate pavements
FDR is most commonly used on low to medium traffic pavements but is suitable for parking lots, primary roads, interstates and light-duty airfields. In order for FDR to be an effective rehabilitation strategy, a thorough assessment, including sampling and testing of the existing materials, must be performed. FDR is particularly suited to cracked and rutted roads. If the distresses are caused by drainage problems, or by localized sub-grade or base failures, these deficiencies must be addressed prior to the FDR treatment.
Process steps
The primary steps in FDR include the following operations:
• analyzing the existing materials and road conditions,
• performing the pavement design,
• pulverizing the in-place pavement,
• introducing the additive and mixing,
• shaping the mixed material,
• compacting and
• applying the surfacing course.
Pulverization is actually a grinding process using a rotating cutting head, typically within a reclaimer. Stabilization is accomplished by incorporating some type of additive into the pulverized material. Shaping prepares the surface profile.
Compaction generally uses a pad-foot (sometimes called “sheep’s foot”) roller for the primary rolling and a steel-drum roller usually does the finish rolling.
Analyzing the materials
The lab test results of field samples are important information for selecting the appropriate type of stabilizer and developing an effective FDR mix design. The structural design and mix design must be considered together.
Pulverizing the pavement
Pulverization is done either by a single pass or by using multiple passes. In single pass reclamation, the existing pavement and underlying layers are pulverized. At the same time, any stabilizing additives are added and mixed. This type of reclamation is typically used when performing a simple pulverization with no stabilizing additives, when the existing asphalt is relatively thin (six inches or less), and/or when major cross-slope or profile grade corrections are not needed.
In multi-pass reclamation, the existing pavement and underlying layers are pulverized in the first pass, and the material is pre-shaped and compacted. Then the stabilizing additives are applied and mixed with a second pass. This process is applicable when the in-place asphalt is six inches thick or more, when one or more stabilizing additives are being used, and/or when significant cross-slope or profile grade corrections are necessary.
Introducing additive and mixing
The type of stabilization selected is generally based on the existing pavement condition and materials, the availability of contractors, materials, traffic and costs. The in-place materials can be blended and mixed without any additional stabilizers. However, the lab analysis may determine that some improvement is needed. If that is the case, there are three primary means of stabilization – asphalt, mechanical, and chemical. Combinations of stabilizers can also be used.
The choice of stabilizing methodology is dependent on the following considerations:
• thickness of in-place pavement (both asphalt and aggregate),
• properties of the in-place materials,
• amount of structural improvement needed from the stabilization,
•availability of qualified contractor, stabilizing materials, and agency experience
• economics.
Asphalt stabilization
This type of stabilization uses asphalt emulsion, asphalt cement or foamed asphalt. Asphalt stabilized pavements have improved fatigue and load-carrying properties but, with typical voids contents exceeding 10 percent, are not equivalent to HMA. Combining Portland cement or lime with asphalt stabilization can provide additional strength.
When asphalt emulsions are used as stabilizers, additives such as stabilizing agents, coating enhancers, anti-stripping agents, break-controlling materials and/or polymers may be included. The selected emulsion must be compatible with the in-place materials. Emulsion stabilized bases need time to break and cure. Several factors influence curing time, including weather conditions, in-place moisture, characteristics of the emulsion and external pressures from mixing, grading and compaction. Chemical catalysts such as Portland cement or lime can also affect curing time.
Foamed asphalt stabilization works with certain types of reclaimed materials—preliminary testing to determine if foaming is suitable is required. Foaming is accomplished by injecting a small amount (about two percent) of cold water into hot (about 320°F) asphalt cement. The resulting foam increases the surface area and volume of the asphalt, decreases the viscosity, and allows for improved coating of fine materials.
Mechanical stabilization
This type of stabilization adds selected aggregate sizes or RAP to the pulverized existing materials to create a stronger sub-base. Mechanical stabilization can be used in combination with asphalt or chemical modification. It is an appropriate choice for low to medium traffic volume pavements exhibiting surface and minor base defects.
Chemical stabilization
Several types of modifiers are used with this type of stabilization, including Portland cement, lime, fly ash, cement or lime kiln dust, calcium chloride and other chemical products. The additives can be applied ahead of the reclaimer in dry or slurry form placed on the pulverized material or by a spray bar tied to the mixing chamber.
Shaping and compacting
The resulting stabilized base course, after applying and mixing any additives is then shaped and compacted. Proper compaction is critical. A typical compaction sequence consists of initial/breakdown rolling by a vibratory roller or pad-foot compactor, followed by intermediate rolling with a 25 to 30 ton rubber-tire roller or vibratory compactor. A steel-drum roller operating in static mode does the finish rolling.
Final surfacing
The last step is the finish treatment. Typical treatments include chip seals and hot mix overlays. To avoid trapping moisture in the stabilized base, allow three to seven days curing time, prior to applying the surface treatment.
Markey Road project
FDR was used to rehabilitate Markey Road, in Lebanon, Ohio. The existing pavement was about six inches of badly cracked HMA over nine inches of aggregate base, with a subgrade CBR of eight. The road is a two lane, variable width facility, about 0.7 miles in length, with moderate traffic, including some trucks.
Pre-construction sampling was performed to gain information about the variability of the in-place condition of the road, to determine in-place subgrade strength, and to obtain samples for mix design work and emulsion formulation. The mix design was done by Colas Solutions’ Cincinnati lab. The design work indicated that three percent asphalt emulsion was needed.
Base Construction Technologies, Inc. did the construction work. The first day’s effort consisted of milling 2.5 inches in the curbed portion of the project, pre-pulverizing the entire length of the road, then shaping and compacting to form an aggregate-like base. During the second day, an engineered emulsion, supplied by Terry Asphalt Materials, Inc., was injected to a depth of six inches, followed by initial compaction and final shaping, before the final compaction was applied. Curing was allowed to take place for several days before the HMA wearing course was placed. The completed pavement is expected to last more than 20 years.
Conclusion
In summary, full depth reclamation converts a worn-out asphalt pavement into a stabilized base course. FDR removes pavement crack patterns and eliminates reflective cracking. FDR conserves materials by re-using pavement components. FDR reduces future maintenance costs by eliminating cracks, base failures and other distresses.
Dwight Walker is a contributing editor for Asphalt magazine and is a consulting engineer in asphalt materials.