A Full Circle Solution

Full depth reclamation (FDR) is a pavement rehabilitation technique in which the full flexible pavement (asphalt) section and a predetermined portion of the underlying materials are uniformly crushed, pulverized or blended, resulting in a stabilized base course. Additives are sometimes injected into the process to provide further stabilization.

The FDR recycling process is one of several used on a daily basis by the contractor members of the Asphalt Recycling and Reclaiming Association (ARRA), Annapolis, Md. The combination of methods has allowed asphalt to become one of the most successfully recycled materials in North America, both by volume and by percentage of material recycled.

WHY AND HOW THE FULL-DEPTH PROCESS WORKS

All pavement structures, with the passage of time and traffic, wear out. In the past, it has been convenient to overlay these roadways with additional layers of materials, thereby extending their service lives. Eventually, however, due to many factors, these roadways deteriorate to a point that conventional maintenance and/or repair and overlay practices become expensive and do not perform well due to the condition of the underlying structure.

It is at this stage in a pavement’s life cycle that more costly roadway reconstruction is typically specified. However, due to the evolution of high horsepower road reclaimers with state-of-the-art computerized additive systems, FDR is one of the fastest-growing rehabilitation techniques being specified by pavement engineers around the world.

FDR conserves previous investments of in-situ materials while resolving the issues and minimizing the costs of in-situ material disposal common with conventional pavement reconstruction practices. The process has been proven on a wide range of flexible pavement structures to produce quality results at substantially lower costs and considerably shorter construction periods than conventional reconstruction practices.

Full depth reclamation is distinguished from other rehabilitation techniques like cold planning, cold in-place recycling or hot in-place recycling by the fact that the rotor or cutting head always penetrates completely through the asphalt section into the underlying base layers, thereby erasing deep pavement crack patterns and eliminating the potential for reflective cracking. The pulverized layers and additives become a homogeneous well-graded (2 inches/50 mm minus) material with improved structural characteristics.

FDR provides the ideal opportunity to add stabilizing additives to the new base, either solely or in a variety of combinations, to further enhance the characteristics of the reclaimed material. For example, bituminous stabilizing agents, in the form of asphalt emulsion or foamed asphalt, may be added to achieve a flexible bound material with high fatigue resistance.

For added compressive strength, portland cement or fly ash is sometimes blended with the new base. In areas where the freeze/thaw cycle is severe, liquid calcium chloride is often added to the new base. To reduce plasticity and improve load-bearing characteristics, the addition of lime may be specified. If necessary, additional granular material can also be added for improving gradation and/or lessening the asphaltic binder content, thereby increasing the structural characteristics of the base.

City streets and medium-volume roadways are reclaimed often today, mostly due to advances in the design and horsepower of the reclaimers performing the work and the innovative use of specially designed stabilizers. Where frequent cracking, potholes, and patching are present, FDR is a very viable and longer-lasting alternative to an overlay or cold planing and overlay (mill and fill) procedure. With adequate project management, traffic can be maintained-a fact that is appreciated by the traveling public and nearby business and home owners. Another notable point is that due to the improved structural characteristics of the base, a double chip and seal or cape seal may suffice as the surface treatment on low to medium-volume roadways, thereby saving money on expensive imported materials and their costly placement.

Reclaiming machines have also been hard at work on Interstate highways and private and regional airports. Rather than paving directly over cracked or distressed asphalt, the new base or surface asphalt overlay goes on top of a stabilized uniformly compact base. In some cases, the rehabilitation technique also involves the removal of a portion of the asphalt layer, with the remaining asphalt-say three to four inches-being pulverized and blended with the base and applicable stabilizing additives. This is especially useful when curb and gutter reveal or the existing grade needs to be maintained or re-established. With proper design and specifications, the existing pavement distresses are eliminated. This fact, coupled with the contribution of structural integrity provided by a stabilized reclaimed base layer, generally makes it possible to decrease the asphalt overlay thickness.

STEPS IN THE PROCESS

There are four definable steps in the FDR construction sequence, although the sequence can vary when stabilizing additives are used.

Pulverization of in-situ pavement layers is the first step. It is during this step that the specified sizing (gradation curve) is accomplished. Sizing is controlled by the reclaimer operator, who balances the machine’s forward working speed, cutting rotor speed, gradation control beam position, and mixing chamber front and rear door position.

Once the material is pulverized, the next step is to add proper moisture to the pulverized material to aid in compaction. Although moisture can be surface added, generally moisture conditioning is best accomplished through the reclaiming machine’s integrated fluid injection system during the pulverization step to ensure calibrated water consistency throughout the reclaimed material cut depth.

Breakdown compaction, shaping, adding water to the top of the surface, intermediate rolling, final rolling and the addition of a fog seal of cut asphalt emulsion are further steps that can follow pulverization.

Mechanical stabilization involves the incorporation of additional granular materials during the pulverization or mixing pass of an FDR project. Dump trucks and a stone spreader are typically used at this stage.

Mechanical stabilization can improve the grading of the reclaimed material, increasing its structural integrity. By incorporating virgin granular material, the excess bitumen has more surface area in which to coat, thereby decreasing the in-place bitumen content and increasing the mixture’s structural stability.

Adding these materials can also establish increased pavement elevations and improve vertical curves, and road widening can be accomplished without sacrificing section thickness.

Bituminous stabilization involves adding bituminous stabilizing liquids. These liquids can be blended into the reclaimed material through the machine’s injection system either during the initial pulverization pass or in a subsequent mixing pass.

The practice is considered a cost-effective way of improving the strength of a reclaimed material and reducing the effects of water. Bitumen stabilized materials are more flexible than other base course materials and offer superior fatigue resistance. They also work well in combination with other additives such as virgin granular material and/or cement or lime, typically in a slurry state.

Chemical stabilization involves adding dry or wet chemicals to stabilize base material. Chemical stabilization additives such as portland cement, lime, fly ash and blends of these materials are used to provide strength through the cementing of material particles and aggregates in the reclaimed layer.

The additives can be applied by spreading ahead of the reclaiming machine in dry powder form with calibrated spreading units, or can be disbursed in slurry form, either on the ground ahead of the reclaimer or through a suspension material spray bar integrated into the reclaiming machine’s mixing chamber.

Chemical stabilization can include the application of calcium chloride and magnesium chloride in dry or liquid form, resulting in strength gain through cementing. The larger result is the lowering of the reclaimed layer’s freezing point, which helps reduce cyclic freeze/thaw in the layer.

WHEN IS FDR APPROPRIATE?

A thorough pre-project evaluation will point to situations when the full depth reclamation process should be chosen as a rehabilitation technique. Among the distress signs that indicate a need for the process are:

• frequent deep cracking

• reflective cracking

• heavy pothole patching

• severe rutting/shoving

• frost heaves

• parabolic shape

• insufficient base strength.

These signs, when coupled with growing traffic (volume and weight) and shrinking budgets, are increasingly leading designers to cost-effective full depth reclamation techniques.

The first step in a pre-project evaluation is to perform an existing pavement condition survey in order to determine the type of pavement distress and deterioration that has occurred.

Once the distresses are identified, the severity of said distresses can be used to verify that full depth reclamation is the proper recycling method based on the type and number of distresses, as well as the quality of the base and subgrade layers.

Although a review of previous construction documents or a roadway construction history can provide useful information, core samples or test holes/pits are usually required.

A minimum of two samples per mile is typical, with additional locations being selected based upon pavement conditions and variability. This pavement section study, however, should focus on the thickness, type and composition of the pavement layers, as it is critical to know the strength and classification of the supporting sub-grade.

There are several tests that can be performed. The results of these tests can help determine which recycling and paving process to use, as well as what materials to use during the process selected.

There are also recommended atmospheric temperatures and conditions for undertaking FDR procedures, and these can vary depending on the type of stabilizing additive being used (i.e. asphalt emulsion, foamed asphalt, cement, lime, fly ash or combinations thereof).

FDR work is not completed if it is raining or if rain appears imminent. Rain can produce wash-off of asphalt emulsions and premature chemical reaction or wash-off of dry stabilizers spread on the surface. This results in reduced reclaimed material strength. Also, in the case of asphalt emulsions, work normally does not proceed under foggy or other very high humidity conditions because of poor curing.

Full depth reclamation is a process that can boast several advantages, including that it is environmentally sound, gives enhanced roadway performance and saves money for contractors and government entities. C&D

This story has been provided by the Asphalt Recycling and Reclaiming Association (ARRA), Annapolis, Maryland. Those interested in more information on full-depth reclamation or on ARRA activities can reach the organization through its Web site, www.arra.org or at (410) 267-0023.