By Dwight Walker, P.E.
Pavement preservation can be described as a proactive approach to protecting and maintaining existing pavements. Today there are numerous preservation techniques available. These techniques are regularly being refined and expanded as owners recognize that pavement preservation is a cost-effective approach and move from emphasizing construction and rehabilitation to focusing on preservation.
When most people think of preservation techniques, treatments such as chip seals, thin overlays and micro-surfacing probably come to mind, but there are several additional options. An important question then arises, “What is the best, or the most appropriate, treatment option?” Unfortunately, there is no easy or exact answer to that question. “It depends,” is a frustrating but accurate summation. At this point in time, many owners rely on applying engineering judgment to select their preservation technique. But there is guidance available in making these judgments.
Maryland’s approach
Many state DOTs have developed guidance documents that assist in their selection of pavement preservation treatments. Typical selection guidelines include treatment descriptions, benefits, applicable pavement conditions and recommended materials and procedures. According to Larry Galehouse, Director of the National Center for Pavement Preservation, one of the better guidance documents has been developed by the Maryland State Highway Administration. The Maryland guidance document is comprehensive and ties into their pavement management system.
The following excerpt from the preface of the Maryland guidance document captures their approach:
“This guide will assist in determining ‘the right fix for the right road at the right time’ when used in conjunction with network-level and project-specific data. Step-by-step instructions on determining treatment options are provided through the use of flow charts, decision trees and treatment tables. At the end of the step-by-step process there will be many treatment options available. One or a combination of treatment options may be selected, depending on project-specific conditions. It is not the intent of this guide to provide a final treatment option(s). This guide will provide a series of options for preliminary consideration by District Offices and the Office of Materials Technology (OMT). It is anticipated that further project specific review, analysis and design will be required…”
Input information
In order to select the appropriate preservation technique, some basic information is needed. A good handle on the current pavement condition is critical, including information on the type and severity of existing distresses. Knowing the causes of the distresses is also important. Distresses may often have more than one cause. Without knowing the cause of the distress, an inappropriate treatment may be selected and may not be effective.
A distress identification manual is a useful resource in determining the type of distress and probable cause. The distress manual developed as part of the Long Term Pavement Performance (LTPP) study in SHRP is an excellent document.
The amount and type of traffic is another important piece of information to consider in selecting a treatment. Another consideration is the availability and experience of contractors for a particular type of treatment. Not all types of treatments may be readily available in all locations. As preservation work is more commonly performed, this concern is becoming less of a problem.
Treatment selection details
The various preservation treatment options have certain advantages, some disadvantages and a few limits associated with their use. The following information provides a brief description of how the more common treatment options may fit a particular pavement.
Crack sealing and filling
Cracking is an almost inevitable form of damage on asphalt and composite pavements. Sealing cracks is a common technique used as a preventative maintenance treatment. Cracks should be promptly treated to prevent water penetration, which accelerates pavement deterioration and results in potholes or base failures. Crack sealing/filling reduces water entering the pavement and lessens future deterioration by keeping debris from entering the crack. Milling is not required; routing is encouraged.
Filling or sealing cracks is not appropriate for a pavement with significant structural problems such as base failures, severe rutting or extensive fatigue (alligator) cracking. Excess sealing application can reduce a pavement’s skid resistance and create a poor visual appearance. Crack sealing/filling does not provide any structural improvement.
Traffic should not be allowed on the sealed cracks until the sealant has cured. Blotting with an application of fine aggregate can protect the sealant from tracking or prevent the surface from losing friction.
Sealing and filling operations should not be done on wet surfaces to avoid adhesion problems between the crack and the sealant. Sealing should be done when the temperature is above 40°F. However, cool temperatures cause the pavement to contract and open the crack, so sealing at temperatures just above 40°F can result in better crack sealing.
Chip seals
Chip seals are used to address aged, cracked and raveled surfaces. They reduce water penetration and can improve frictional characteristics of mildly bleeding pavements. A seal coat will not improve a pavement with structural problems. Seal coats are not effective on cracks wider than 0.25 inches, pavements with lots of potholes or high severity fatigue cracking or severe rutting. Pavements that have stripping problems should not be chip sealed; a chip seal could accelerate the stripping.
Clean, cubical aggregate (chips) are important. Embedment of the chips is critical. Excess or loose chips can result in broken windshields. Seal coats can fail due to aggregate loss and bleeding. Chip loss failures are usually caused by dirty chips, improper selection or application of emulsion or other asphalt binder application, delayed spreading or rolling of chips, and failed traffic control. Bleeding is typically caused by loss of aggregate or spraying too much liquid.
Micro-surfacing and slurry seals
These treatments are very thin emulsion-based treatments placed on pavements with minor deterioration. Micro-surfacing is a mixture of a cationic polymer-modified asphalt emulsion, select aggregate, mineral filler, water and possibly some other additives. Micro-surfacing can be placed in multiple courses (for rut-filling, leveling and surface courses). Micro-surfacing is used to prevent oxidation (aging) and raveling and to improve skid resistance and rideability. It can be used on high traffic roads. It sets quickly and can usually be opened to traffic in about an hour. Micro-surfaces should not be placed when the temperature is below 50°F.
Slurry seals are similar treatments but do not typically use a modified emulsion and are limited to one layer/application. These treatments do not improve a pavement’s structural (load carrying) capacity and should not be placed on pavements with stripping concerns. Crack sealing can be done before placing micro-surfacing or slurry.
Thin overlays
This category includes thin (generally 0.75 to 1 inch thick) HMA (and WMA) overlays and ultra-thin bonded wearing courses (UTBWC). We commonly describe these treatments as overlays, but they can also be placed in mill-and-fill installations.
Thin HMA overlays consist of a thin layer of pavement placed on a milled or leveled existing surface. This preservation treatment has the advantage of being a familiar technique. It can be used to improve skid resistance and rideability and to improve minor pavement cross-section problems (rutting, wear, etc.).
Pavements with minor cracking, weathering, friction loss and rutting problems are good candidates for a thin overlay. These overlays are not recommended for problems such as fatigue cracking, significant rutting or other major pavement deterioration.
An UTBWC is a very thin asphalt mixture layer, about 0.75 to 1 inch thick, which is placed in one pass using a specialized paver that applies a thick polymer-modified emulsion membrane on an existing pavement surface. NovaChip® is a well-known proprietary version of this treatment.
Before applying a thin overlay, cracks wider 0.25 inch should be sealed. Overbanding of cracks is not recommended for thin overlays. These treatments are not appropriate for existing pavements with more than 0.25 inch rut depth.
Conclusion
Selecting the appropriate treatment may take a bit of work, but there is assistance available. The websites of the National Center for Pavement Preservation and FP2 are excellent resources. One or a combination of treatment techniques can provide additional service life for our pavements.