Full Depth Reclamation (FDR)

Introduction

Roadways are an important part of our infrastructure. As they age they subsequently deteriorate. At some point every road will need either maintenance or total reconstruction to be return to original condition. Although regular maintenance can extend the life of a roadway, many times it is much more like placing a Band-Aid on the pavement instead of fixing it which, in the end, can result in a large waste of effort. Total reconstruction should fix all problems, however it takes considerable funding. This is an on-going problem for many transportation officials, especially when dealing with city or county roads. For certain projects, a solution such as Full Depth Recovery (FDR), which falls between total reconstruction and regular maintenance, may be best.
FDR is a technique used to rehabilitate pavements by recycling the existing roadway [1]. In short, the full flexible pavement section and a portion of the underlying base are crushed and mixed together. The result of this is a homogeneous stabilized base course (SBC), shown below [2]. This resulting SBC is deep and requires minimal new material with exclusion of the required surface coat, making the process cost effect and environmentally friendly. There are many other advantages to using FDR as an in place replacement alternative including cost, energy use, elimination of reflective cracking, and others that will be discussed later in this chapter.
FDR 1.png


Figure 1 - FDR schematic [2]

The remaining contents of this chapter will explain the process by which FDR takes place, materials and equipment needed as well as stabilizer options, candidates for FDR, advantages of using FDR, and finally a comparison of FDR with regard to other pavement rehabilitation techniques.

Road Candidates

Almost any road, parking lot, or small airport runway constructed of flexible pavement can be rehabilitated using FDR. However, the application of FDR is much more common in practice for roads of low to medium volume and parking lots [2]and [3].
When using FDR as a rehabilitation technique many different distresses can be treated or fixed. Many forms of cracking including fatigue, edge, block, and longitudinal cracking can all be treated by way of FDR. Reflective cracking can also be negated by implementing FDR because the process disrupts the current crack path. This makes stops any underlying cracks from further propagating through the newly formed pavement. Ride quality can be improved as permanent deformations as well as surface distresses in the pavement are corrected. These deformations include rutting, shoving, wash board, and any other surface distresses such as raveling, bleeding, potholes etc. Finally, overall structural capacity of the pavement is improved with the correct use of FDR. The FDR process lays down an improved stabilized base followed by an overlay. The combination of these two parts of the pavement results in an increased structural capacity.
It is important to note that roads that exhibit extreme deterioration or deformation may have serious underlying problems throughout the base or in the sub-base. These types of roads could see improvement from FDR, but they could potentially require more investigation and/or base enhancement in order to be structurally sound [3].

Process

The FDR construction process consists of four major steps:
  1. Pulverization and blending of bituminous surface and granular base
  2. Removal of excess material* and grading
  3. Stabilizer addition and final shaping/compaction (stabilization techniques are discussed further in the following section)
  4. Final surface treatment
*Optional, Optional, but typical added
Keep in mind that each project requires variation in pulverization depth, material removal, stabilizer selection, and final surface thickness. These steps are visually displayed in Figure 2.

FDR 2.png
Figure 2 - FDR construction process [4]

This clip shows the entire process of Full Depth Recycling with engineered emulsion. First, two reclaimers inject asphalt emulsion into the road at a depth of 8 inches and mix the material. Then sheep’s foot rollers compact the material until they are "walking out" of the mix. Next, the motor grader takes the sheep’s foot marks out of the pavement and sets final grade and slope. The mixture is then finished with a 25 ton rubber tire roller and a steel wheel roller in static mode. The final product is a structurally enhanced pavement base that is ready for final surfacing [5].

Stabilization Techniques

There are three stabilization techniques that are used with respect to FDR. Their use and benefits are described in detail below [2].

Mechanical Stabilization

Mechanical stabilization involves the addition of granular materials. These materials typically include crushed aggregate, recycled asphalt product (RAP), or crushed concrete but can also come from other sources. The use of mechanical stabilization has a number of benefits including:
  • Increased structural stability
  • Lower bitumen concentrations of in-situ material
  • Profile, cross-slope, and width corrections can be made without affecting section thickness
  • Mechanical stabilization can be used in conjunction with other stabilization techniques

Bituminous Stabilization

Bituminous stabilization involves the addition of bituminous materials in the form of emulsion or foamed asphalt. The bituminous materials can be added by way of a single pass or multiple passes. Multiple passes normally has better results because it equates to a more consistent injection, especially when dealing with thick pavement sections. Benefits of using bituminous material as a stabilizer includes:
  • Cost effectiveness
  • Reduction of moisture effects
  • Increases flexibility and fatigue resistance
  • Less prone to cracking than other stabilizers
  • Works well when combined with other stabilization techniques

Chemical Stabilization

Chemical stabilization typically involves the addition of Portland cement, lime, fly ash, or other cementitious materials. Chemical stabilization makes it possible to rehabilitate roads that would otherwise require substantial repairs or total reconstruction. One other benefit of chemical stabilization is the fact that it allows on-site materials to be turned into a structural base under an asphalt pavement.

Surface Treatments

The final piece of FDR is the surface treatment that is laid on top of the newly blended, stabilized base. Typical surface treatments are:
  • Chip Seal
  • Double Chip Seal
  • Cold Mix Overlay
  • Hot Mix Overlay

Following the surface treatment the road will look brand new as shown in Figure 3 [2].

FDR 3.png


Figure 3 - Final product after surface treatment

Advantages

Using FDR as a reclamation/rehabilitation technique provides many advantages [4]:
  • Energy conservation
  • Material conservation
  • Cost effectiveness
  • Increased Performance
  • Environmentally conscious
By reusing existing materials energy use from hauling, quarrying, and production are reduced. This also leads to less new materials being needed and a drastic reduction in disposal needs. Figure 4, below, compares energy and material usage for FDR and complete new base construction. The benefit of using FDR is clear. The reduction in both materials and energy usage will not only result in substantial savings, usually costing only 30-50% of alternative methods, but also have reduced environmental impact when compared to alternative rehabilitation methods [6]and [7]. In addition to the benefits to the environment and savings, FDR has shown improved performance with respect to frost-penetration, reflective cracking, and load bearing capacity (Figure 5).

FDR 4.png


Figure 4 - Material use comparison [8]

FDR 5.png
Figure 5 - Performance comparison of FDR and traditional methods [9]

Conclusion

According to the Asphalt Recycling and Reclaiming Association, “FDR is a reclamation technique in which the full flexible pavement section and a predetermined portion of the underlying materials are uniformly crushed, pulverized, or blended, resulting in a stabilized base course. Further stabilization may be obtained through the use of available additives. [10]”
Overall, FDR is a sustainable pavement rehabilitation alternative that has many applications. It is also an attractive alternative for transportation officials overseeing city or county roadways on limited budgets. The use of FDR reduces spending, material use, and energy use all while not compromising performance.

Works Cited


[1]
Wikipedia, "Full Depth Recycling," Wikipedia, December 2006. [Online]. Available: http://en.wikipedia.org/wiki/Full_depth_recycling. [Accessed 6 April 2013].
[2]
Mt. Carmel Stabilization Group, AARA, ASCE, "Full Depth Reclamation," [Online]. Available: http://content.asce.org/files/pdf/Ryan.pdf. [Accessed 6 April 2013].
[3]
T. Dziedziejko, "Full Depth Reclamation," 15 November 2009. [Online]. Available: http://www.docstoc.com/docs/52147705/Full-Depth-Reclamation. [Accessed 8 April 2013].
[4]
Ruston Paving Company, "Full Depth Relcamation (FDR)," Ruston Paving Company, 2011. [Online]. Available: http://www.rustonpaving.com/reclamation.aspx. [Accessed 7 April 2013].
[5]
Busy Bee Paving, "YouTube," 15 June 2012. [Online]. Available: http://www.youtube.com/watch?v=dudgZ4Nmjzo. [Accessed 7 April 2013].
[6]
MidState Reclamation and Trucking, "What is Full Depth Reclamation," MidState Reclamation and Trucking, 2013. [Online]. Available: http://www.midstatecompanies.com/index.php/component/content/article/9. [Accessed 7 April 2013].
[7]
Federal Highway Association, "Chapter 16. Full Depth Reclamation," 2013. [Online]. Available: http://www.fhwa.dot.gov/pavement/recycling/98042/chpt_16.pdf. [Accessed 7 April 2013].
[8]
Portland Cement Association, "Pavements," PCA, 2013. [Online]. Available: http://www.cement.org/pavements/pv_sc_fdr.asp. [Accessed 8 April 2013].
[9]
Asphalt Zipper of New Zealand, "Full Depth Reclamation," Asphalt Zipper, 2013. [Online]. Available: http://www.asphaltzipper.co.nz/how-it-works/road-repairs/full-depth-reclamation/. [Accessed 8 April 2013].
[10]
Asphalt Recycling and Reclaiming Association, "ARRA," ARRA, 2013. [Online]. Available: http://www.arra.org/. [Accessed 7 April 2013].