By Scott Sounart, Contributing Author
Millions of miles in the wake, whether by road or by trail, car or by foot, the aging maltenes are alluring, as they slowly degrade and expose their natural sands. Each single mile of asphalt pavement displays its storied history, one distress at a time. A low severity transverse crack here, widespread raveling there, slowly aging into reconstruction disregard. This is the life of a pavement engineer, understanding the visual signs it portrays, to help extend its life.
The simplicity of learning to identify asphalt pavement distress is vital to implementing best practices for their long-term success to connect to the cities and dwellings they serve. Asphalt pavement needs to be kneaded, serving as a flexible, yet vigorous byway delivering us to our treasured places. Unlike 3D printers, nuclear power plants, or a transoceanic bridge, most people will likely utilize some form of an asphalt pavement multiple times in a day. Asphalt pavements are unique in maintaining their degradation timeline, strong in their infant years, persevering steadily in their middle years, while clinging to avoid falling off the deterioration cliff near the end of its life.
Yet, despite the obvious hazards caused by disregarded or improperly maintained asphalt pavements, roadway stakeholders often are complacent in their attention to their upkeep. This is partly a perception problem. Pavements, like all other materials, and despite their low-key existence, are not maintenance-free. The ability to identify early distress types, their causes, and preventative measures against them, are key to the long service life that often gets taken for granted.
Asphalt pavement undergoes an abundant amount of wear and tear every day from heavily loaded trucks, rain and snow, eroding subgrades, and solar radiation. Therefore, it’s essential to have an assessment and maintenance plan to preserve their aesthetics, ensure safety, and ultimately extend the service life to its intended function.
Understanding basic visual aspects of a pavement’s surface helps minimize long term costs, including costly rehabilitation and repairs.
There are seven primary types of asphalt distress, with varying severity levels and description. The good news is that a pavement evaluation program, developed through the following techniques, can help identify potential problems before they require major repairs, saving taxpayers money and minimizing user delays.
Seven Signs of Trouble
The seven common types of asphalt distress tend to progress chronologically from one to the other. The first sign of trouble typically appears in the form of longitudinal cracks. These cracks often appear along joints, which are the weakest part of the pavement. They can result from poor joint construction or location, or they may be an indication of fatigue within the underlying layers. Even though longitudinal cracks are an early form of deterioration, they can cause major problems down the road by allowing moisture to infiltrate into the subgrade or base.
The next common form of deterioration is transverse cracking, which extends across the pavement perpendicular to longitudinal joints.
Transverse cracking is often related to temperature changes, including low temperature thermal cracking. As temperatures fluctuate, asphalt will expand and contract according to its material properties.
As the materials age and oxidize, weak planes are created resulting in cracks. Transverse cracking can also be attributed to improper asphalt binder (too hard) or a shrinking or cracked base.
As longitudinal and transverse cracking increase in magnitude and density, they form block-like patterns called block cracking. Block cracking is a series of interconnected cracks that appear as rectangular pieces, or blocks. Like longitudinal cracks, block cracking can also allow moisture to infiltrate the asphalt, which can undermine the pavement.
As cracking progresses, an aggressive crack seal program should be implemented. At about a buck per linear feet, crack sealing is a cost-effective way to mitigate moisture infiltration into the subgrade. High severity cracking, or cracks wider than 1-inch should be corrected with full depth patching or sealed with a mastic type mix, including sealant with fine graded aggregates.
When aggregate or asphalt binder materials wear away at the surface, raveling will usually occur. Raveling is essentially the disintegration of the asphalt, causing loss of binder and fine aggregates at the surface. This can lead to gaps within the pavement surface, roughening, and loss of adhesion, which diminishes its protection from moisture. Raveled surfaces should be repaired every other year with an effective seal coat product, including a mineral reinforced polymer modified asphalt emulsion.
While these first four types of deterioration are signs of trouble, they can often be treated with lower cost preventative maintenance procedures including crack seal or seal coat. Nevertheless, higher density cracking and higher severity raveling often require more involved and costly repairs. It’s important to differentiate these less severe distress types from the dreaded pavement rutting, fatigue cracking, and asphalt pavement’s infamous black eye, potholes.
Rutting is a permanent deformation of the pavement that occurs when ruts appear in the subgrade beneath the pavement. Ruts create areas of ponded water, which can lead to vehicle hydroplaning and subgrade failure.
Rutting is often the result of initial construction or design techniques, including inadequate pavement structure or insufficient compaction of base layers during construction. When the subgrade is no longer able to support the pavement structure, fatigue cracking will be forthcoming.
Fatigue cracking, or alligator cracking, resembles an alligator skin’s diamond-shaped pattern. Both rutting and fatigue cracking are indications that, in addition to the obvious surface deterioration, there is also significant corrosion within the asphalt.
Left untreated, these issues cause the pavement to settle or depress, which ends up forming a pothole. The media-friendly term pothole, often asphalt’s chief nemesis, is a depression or hole, shaped like a pot, where the traffic has subsequently removed broken pieces of pavement.
Potholes can cause extensive damage to vehicles suspension systems. Rutting, fatigue cracking, and potholes should only be improved by replacement for widespread distress or patching for isolated areas. Subgrade or subbase repairs should also be anticipated.
Major emphasis should be placed on differentiating fatigue cracking (which usually progress into potholes) from dense block cracking. Fatigue cracking almost always has diamond shaped cracking patterns, often at 30-to-60-degree angles, due to the diminished integrity of the underlying subgrade or subbase materials.
Block cracking is often shaped in squares or blocks with close to 90-degree angles, and is typically a surface defect, with minimal adverse effects from the subgrade. Fatigue cracking should almost always be mitigated with full depth patching or pavement replacement, where block cracking can often be repaired with crack seal or a mill and overlay.
Frequent freeze-thaw cycle areas are more detrimental to pavements due to the inherent properties of asphalt. Annual freeze-thaw cycles greater than 100, typically in the Rocky Mountain and Northwestern areas of the US, require additional pavement layers or stabilized subgrade to counteract the fluctuations in temperatures.
More advanced forms of deterioration typically require mill and overlay rehabilitation, a more expensive form of rehabilitation, or complete reconstruction. Therefore, it is imperative to identify the warning signs and resolve early premature distress before these major—and pricier—problems appear.
Street superintendents should be vigilant about monitoring the condition of the pavement on a regular basis. A regular schedule of inspecting pavement surfaces should be conducted by trained personnel to identify the various types of deterioration by sight.
Pavement responds to variable attributes, including subgrade type, environmental factors, moisture, and traffic. It’s important to have a comprehensive pavement maintenance and repair strategy that considers all the issues that can impact the pavement’s performance and lifespan. When planned and implemented appropriately, a properly maintained asphalt pavement can sustain decades of service life, adding exciting chapters to its history. R&B
Scott Sounart, PE is a Senior Asset Manager with Kimley-Horn and Associates. He can be reached at [email protected].
