KERCHER 
 ENGINEERING, INC.

  “If a job is worth doing,  it is worth doing right!”

 
 Pavement Recycling - Part 2
To Recycle or to Reclaim? That is the Question!
There are usually several possible solutions to solving any problem. As with many things in life, the previous statement is especially true in rehabilitating asphalt concrete pavements.  In many situations, pavement recycling offers advantages over traditional rehabilitation techniques such as deep patching and/or surface milling that is followed by a hot mix asphalt overlay.  However, one recycling technique is usually a better choice than the other technique, depending on the specific situation.  Although there are no clear-cut guidelines that are correct 100% of the time, this article will provide a summary of general guidelines for project selection. Also, there will be a brief discussion as to the basic issues that must be considered, as well as, summarizing the different types of binding agents that can be utilized with these processes.

Cold In-Place Recycling 
 


Full Depth Reclamation

Cold In-Place Recycling

Cold in-place recycling (CIR) is typically used on thicker pavements with a good base and adequate subgrade strength.  This process is excellent for correcting thermal cracking (transverse and block cracking), reflective cracking and other distresses due to aged pavement such as raveling.  Aged pavements with alligator cracking that are approaching the end of their design life can usually be successfully recycled if the subgrade has sufficient strength (i.e. no pumping, etc.).  Unlike traditional overlays with patching and/or milling, the CIR process produces a uniform base course that is free of distresses.  The CIR base coupled with a properly designed overlay or surface treatment should result in a pavement that performs well for many years.

 

Conventional Patch and Overlay Cold In-place Recycling

If the pavement has localized subgrade/base failure, these areas can be patched.  This method is usually cost effective if the amount of patching needed is less than 10 - 15% of the pavement area.

 

After Patching 

After Recycling


Other pavement distresses that may be corrected by CIR are pavements that have experienced:

  •  Rutting due to “tender” asphalt concrete
  •  Excessive bleeding
  •  Stripping of asphalt from the aggregates


In these cases, additional aggregate with a specific gradation and/or additional binding agents such as portland cement, fly ash or lime may be needed. 
 

Full Depth Reclamation

This process can be used to correct any of the problems that CIR corrects, as mentioned above. Usually the main issue that determines which process to select is the existing pavement structure.  Typically, full depth reclamation (FDR) would be selected for thinner pavements and lighter traffic. When the pavements are relatively thin (less than 3 inches of asphalt with little or no base) or pavement failure has occurred due to wet and/or structurally inadequate subbase/subgrade materials, full depth reclamation (FDR) is probably the best alternative.

The FDR process reclaims the entire pavement and a pre-determined depth of the supporting soil below.  Not only does this process produce a thick uniformly stabilized pavement structure, but also it can help to reduce or eliminate moisture problems such as freeze-thaw by stabilizing fine-grained soils that are susceptible to moisture.   Although the depth of reclaiming typically ranges from 6 - 12 inches, FDR can go deeper if needed, to address freeze-thaw problems.  However, construction techniques must be modified accordingly to assure that the deep base is properly built.  Especially if a thick overlay is applied, the new pavement section should be safe from all but extremely cold temperatures for a prolonged period of time - something that does not happen very often in Delaware. 

In many cases where a thin pavement has a very bad profile and/or cross-slope, FDR can provide an excellent cost-effective solution.  Also, this process can help to rehabilitate low-volume rural roads where the pavement width and thickness vary greatly.
 

Selection Considerations

Before the appropriate rehabilitation technique can be selected, the root causes of the problem(s) must be carefully investigated.  This investigation is crucial to ensuring that the selected technique will properly address the underlying cause or causes of the deterioration in a cost-effective manner.  Some of the reasons why a pavement may require rehabilitation are:

  •  Inadequate structural capacity
  •  Significant distresses are present
  •  Traffic safety problems 
    •  Low skid resistance
    •  Poor alignment
    •  Narrow lane/shoulder width 
  •  Poor cross-section and profile
  •  Poor drainage 


Once the existing conditions are investigated, the engineer must consider the following factors before the design can be finalized: 

  •  Severity of extent of the distresses present
  •  Structural capacity and traffic issues
  •  Drainage
  •  Design life
  •  Life cycle cost analysis


Binder Agents

CIR is used exclusively for stabilizing granular materials in the asphalt pavement and possibly the aggregate base course.  Therefore, emulsified asphalt is usually the stabilizing agent of choice.  The gradation of the material to be recycled and the viscosity of the aged asphalt dictate the type of emulsion to be used.  For a higher early strength mixture, to increase stability or to improve resistance to moisture, portland cement, fly ash and/or lime may be added.

Since FDR is used to stabilize a variety of different types of materials, there are different types of binding agents that can be used.  The actual choice of binder will depend upon several factors including: 

  •  Composition of the existing pavement 
  •  Depth and type of the base/subbase
  •  Subgrade soil
  •  Drainage
  •  Traffic considerations


The type of binders used in FDR include:

  •  Portland Cement
  •  Bituminous Binders
    •  Emulsified Asphalt
    •  Foamed Asphalt
  •  Lime
    • Hydrated Lime or Quick Lime
  • Pozzolan (Fly Ash)
  • Calcium Chloride
In general, granular materials are stabilized using bituminous binders.  Portland cement and fly ash are used for stabilizing clayey soils with a low plasticity index and can also be used with granular soils.  Lime is normally used for stabilizing clayey soils with a high plasticity index.
 

This article has provided a summary of the general guidelines for recycling and reclaiming. These techniques should be considered as possible solutions to improving deteriorated pavements.  It must be emphasized that this information should serve only as a guideline.  To ensure a quality project, the final selection of which technique to use, as well as, the pavement design and preparation of contract documents should be made by someone who is experienced in pavement rehabilitation and recycling.

For more information about these two processes or if you would like technical assistance on a specific project, please contact Kercher Engineering, Inc.