I’ve been working with asphalt for 30-plus years, and there is one problem that I still do not know how to fix. I know some ways to make it better but not how to completely eliminate it.
I’m talking about reflective cracking. Reflective cracks occur due to breaks or cracks in underlying layers because of movement at the original crack. At this location, stresses concentrate and a crack propagates and reflects to the pavement surface over time.
Some people think that reflective cracks only form in asphalt overlays over rigid (concrete) pavements — but that’s not correct. They can also occur in cracked asphalt pavements or asphalt pavements on stabilized bases. Common sources of reflective cracks include joints and cracks in concrete pavements, low-temperature or shrinkage cracks in asphalt pavements, longitudinal joint failures, and fatigue cracks. Reflective cracks can also form due to sub-grade shrinkage and from subsidence over culverts or other utilities.
Reflective cracks are generally not load-related but loading does accelerate the rate and severity of deterioration. These cracks are a problem because they allow water into the pavement which weakens the pavement structure and contributes to premature deterioration of the overlay, usually showing up as spalling (loss of material) at the crack, bumpy ride, etc.
In my opinion and experience, most of the methods and/or materials used to treat reflective cracking only delay or reduce the severity of the cracks which eventually return. Some of the ways I know (a little) about to address the problem include:
• Utilizing mixes with higher cracking resistance. These typically include a modified asphalt binder and may include a gap-graded aggregate structure.
• Constructing thicker overlays. Thicker overlays allow the stresses to dissipate and may reduce the crack width or severity.
• Placing a crack-relief mechanism. These include unbound relief layers, interlayers, geo-synthetic fabrics, and highly specialized asphalt mixtures These techniques can impede the reflection of cracks by minimizing the tension transferred to the overlay.
• Recycling in-place. Some or all of the existing asphalt is reworked to remove the old crack.
• Using saw, cut and seal. In this process, the crack location is identified and a saw-cut is made to form a clean opening which can be sealed to minimize damage.
• Applying one of the fractured slab processes, for overlays on concrete. Rubblization and crack/break-and-seat are effective techniques for reducing and possibly eliminating reflective crackling from old concrete pavements.
For overlays showing early signs of reflective cracking, the typical repair strategy to prolong pavement life is to perform crack sealing to prevent moisture from entering.
After many years, I am still looking for a permanent fix, short of removal, for reflective cracking. If you have experience with a successful approach, I would like to hear about it.
Contact Dwight at talkingasphalt@asphaltinstitute.org
