Since 94 percent of U.S. paved roads are asphalt, extending the life of asphalt
roadways can save money and time for the local government agencies who are
responsible for 70 percent of them. An article in Louisiana State University's
Technology Exchange (October, November, December 1995) dealt with methods of
treating asphalt cracking problems. The article's title was "To Seal or Not to Seal?
Crack Treatment in Asphalt Pavements."
The Strategic Highway Research Program (SHRP) has experimented with
methods and materials for treating cracks in asphalt paving. SHRP's research results
helped answer questions roadway maintenance agencies face when deciding whether to
seal cracks and how to go about sealing them.
What Cracks Should be Sealed or Filled?
Pavement cracking can create problems and should be repaired. Cracks allow
water to get into the roadway base and subgrade, resulting in pavement breakup and
potholes. Most cracks are good candidates for filling or sealing. The only exceptions
mentioned in this article were: (1) where there is a failure in the subsurface or (2) where
cracking was not caused by water problems (i.e., the subgrade is sandy soil).
According to the article, the decision on whether to seal or fill a crack would
depend on the amount of horizontal movement:
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Cracks with widths between 0.2 to 0.75 inches should be sealed. Likewise,
cracks showing greater than or equal to 0.1 inch movement should be sealed.
Pavements that have failed because of extensive alligator or block cracking may
not be candidates for crack sealing methods. In such cases, other methods of
repair, such as surface seals or partial depth patching may be more appropriate.
What Materials and Methods Should Crews Use?
Rubberized asphalts are flexible enough to adjust as cracks open and close. The
article mentioned three categories:
- cold-applied thermoplastic materials (emulsions, cutbacks, polymer
modified liquid asphalts)
- hot-applied thermoplastic materials (asphalt cements, fiberized asphalt,
asphalt rubber, etc.)
- chemically-cured thermosetting materials such as self-leveling silicon.
The final decision about what material to use will depend on the specific
properties needed for the intended purpose and on logistical considerations such as
preparation time, workability, and cure time. In a 32-month test, SHRP experiments
showed that rubberized asphalt and asphalt rubber sealants fail only about 10 percent of
the time and silicon sealants fail about 11 percent of the time.
Crews should follow several steps when treating cracks. The article suggested
the most important are "cleaning and drying and material preparation and application."
When repaired cracks will experience great ranges in temperature, the article pointed
out that "crack cutting (routing or sawing), material finishing/shaping, and blotting are
also employed as part of the procedure."
Cold temperatures, dirt, and moisture can prevent proper adhesion between the
sides of the crack and the sealing material. Therefore, "using hot compressed air
followed by a heat lance is recommended."
What Time of Year Should Crews Repair Cracks?
Temperatures affect the timing of crack sealing or filling. Fall or spring
temperatures are generally best. Temperatures of 35 to 50 degrees Fahrenheit (2 to 13
degrees Celsius) allow crack channels to stand partially open and at the middle of the
working range, thus minimizing the amount of expansion and contraction the seal or fill
must tolerate.
