By: Dave Bergner, M.A., PWLF, CPWP and Mike Hale, P.E., PLS, M.S.C.E.
Potholes are disruptive, damaging, and dangerous. Impact from a deep pothole can damage tires, wheels, lower engine components, suspension, and steering systems. Furthermore, drivers can lose control of a vehicle when hitting or swerving to avoid potholes, resulting in collisions that injure or kill. They are also a hazard to cyclists and pedestrians.
There are more than 8 million lane-miles of road in the U.S. Over two-thirds are paved, nearly all with asphalt. Due to decades of inadequate funding, deferred maintenance, and increased traffic volumes, much of the system has significantly degraded.
Potholes are a significant result of the poor condition of our roadways. But what exactly is a pothole? Though there is no one universal definition, a commonly accepted description is that a pothole is, “a depression in asphalt (not concrete) with sharp, uneven edges, and typically ranges from 2 in. to 4 in. deep and several inches more in length than width.”
Pothole Formation
Water, temperature fluctuations, and excessive vehicle loads are the primary causes of potholes. All paved roadways are designed to accommodate predetermined loads (weight per axle) and volume. Eventually, traffic stresses the pavement, resulting in small fatigue cracks that allow water to infiltrate into the base and subbase. Oxidation, poor design and construction, and insufficient preventive and corrective maintenance also contribute to pothole formation. When water infiltrates through the upper surface or from the sides of the roadway, the pavement begins to deteriorate. Water softens and, in some cases, displaces the base and the soil underneath.
Most potholes occur in the winter and early spring as a result of freeze-thaw action. As ground temperatures drop below freezing, the water under the pavement becomes ice and expands, heaving the pavement, and leaves small breaks. When temperatures rise, the cracked pavement subsides into the void left by the melted ice. At this point, it may not be readily apparent at the surface that the pavement is about to fail. The weakened surface layer is deflected downward each time a vehicle wheel passes over. With no support underneath, the pavement breaks, and a new pothole suddenly appears. As more vehicles hit this spot, the repetitive impacts enlarge the hole.
However, potholes also occur even when there is no freeze-thaw cycle, such as in Houston, Florida, Southern California, or even Hawaii. In those locations, poor subgrade—and the presence of high water-tables—weaken the pavement.
Precursors
Surface deterioration and subsurface degradation occurs for some time before the surface breaks. Certain defects are precursors to pothole formation; they are indicators of chronic conditions such as overloading, poor construction, material deficiencies, inadequate drainage, and severe environmental impacts.
Training roadway maintenance workers to recognize these defects, identify the causes, and make timely repairs significantly reduces pothole formation. The following defects are described and illustrated in the Distress Identification Manual for the Long-Term Pavement Performance Program:
- Rutting
- Raveling
- Depressions
- Delamination
- Longitudinal cracks
- Transverse cracks
- Edge cracking
- Fatigue cracking (“alligator” or “map” cracking)
- Block cracking
- Slippage cracking
- Shoving
- Failed Patches
Delayed or deferred correction of pavement distresses is the major cause of potholes. Nonetheless, when potholes appear, prompt repairs must be made to keep them from becoming larger and deeper.
Repair Materials and Methods
Selecting the right pothole patching material depends on several factors: prevailing weather; pavement moisture level and temperature; width and depth of pothole; traffic conditions; and availability of material, equipment, and personnel. Additional factors to consider in selecting the proper materials is the age and condition rating according the APWA Pavement Condition Index (PCI).
- Cold mix can be used in any weather and by one person. However, it is a short-term repair. It is used more in winter because it does not need to be mixed, heated, or require tools other than a shovel, rake, and tamper. It is available throughout the year. There are two methods:
- Throw-and-Go is the simplest and fastest repair. Unfortunately, it does not last because of water and/or debris in the hole, insufficient compaction, and lack of adhesion.
- Throw-and-Roll is a more effective technique as the truck tires are used to compact the patch. It typically increases the survival rate from 10% to 25%.
- Hot-mix asphalt requires a trailer or truck-mounted machine that heats the asphalt during transport and application. One drawback is that most asphalt plants will not be producing during the winter.
The typical repair method is cut-and-fill. Because this method takes more time, a follow-truck provides protection for workers and warning to oncoming traffic. This entire process may take from 20 minutes to nearly an hour. The “semipermanent” patch can last several years. During busy periods, crews often skip “cutting the hole” and use throw-and-go or throw-and-roll methods instead. Such repairs will not last as long as “cut-and-fill” patches.
- Spray injection uses a truck or trailer-mounted apparatus consisting of an air compressor, aggregate hopper, tanks of tack oil and asphalt emulsion, and applicator hoses. The process takes just minutes. These patches can last a long time but are best for shallow holes.
- Infrared heating consists of a truck- or trailer-mounted rectangular infrared heater that melts distressed asphalt 2 to 3 in. in depth and then scarified. Rejuvenators can be applied to the old asphalt or new HMA added, then compacted. These patches are permanent.
Other measures:
- Leveling is filling and compacting depressions that have little or no cracking with HMA.
- Partial-depth repair removes asphalt to the base, then replaces with compacted HMA.
- Full-depth repairs correct poor subbase conditions by excavating to the soil. Rock or large aggregate is used to strengthen soil before the base and wearing course are replaced with fresh HMA.
All of the above are ways to repair potholes, but what is more important is to prevent potholes.
Preventive Maintenance Vital for Reducing Pothole Formation
Preventive maintenance is a series of planned, cost-effective treatments that preserves the pavement, retards future deterioration, and maintains or improves the functional condition of the system.
Asphalt pavement begins to deteriorate almost as soon as it is built. Factors that contribute to pavement deterioration include poor drainage, air pollutants, and sun which cause oxidation and hardening, utility cuts, and traffic. Inevitably, potholes occur even on the best constructed and maintained roadways. As cracks develop, water gets in those cracks and freezes, and the cracks get bigger until they become potholes. The obvious solution then is regular, timely maintenance to seal the roadway surface.
The typical asphalt pavement is designed for a 50-year life span. Properly maintained, a street could last for 70–80 years depending on soil and drainage conditions and structural adequacy. With regular preventive maintenance, annual maintenance costs are approximately half what they would be if pavement were neglected. Ideally, agencies should strive to maintain their roadways above a PCI of 55 or “fair” condition.
Preventive Treatments
There is a wide range of preventive maintenance methods that increase in time, cost, and complexity as pavements deteriorate due to age, traffic, and environment. Detailed information is available at the FHWA Pavement Preservation website: https://www.fhwa.dot.gov/preservation/
Sealing
- Narrow crack sealing with rubberized asphalt for longitudinal, transverse, reflection, and block cracks
- Wide-crack sealing with HMA or mastic is for joint with wider separation.
- Rejuvenators restore original properties to aged (oxidized) asphalt binders. Service life is about 2 years.
- Fog seal is a light application of a diluted slow-setting asphalt emulsion to aged pavement.
- Sand Seal is fog seal followed by a thin layer of sand. Fog and sand seals last about 2–3 years.
- Slurry seal is used to fill surface defects as a preparatory treatment for other maintenance treatments or as a wearing course. Used mostly on low-volume roads and streets. Service life is 4–5 years.
- Micro-surfacing is a slurry seal with polymer additives primarily for collector and arterial streets and roads. Service life is 6–7 years.
- Chip seals is a layer of graded aggregate partially embedded in tack oil then compacted with roller and loose chips swept. Reapplications on sound pavement about every 7–10 years.
- Scrub seal is a chip seal with brooms that work the emulsion into the distresses. It is an excellent cost-effective treatment for a heavily distressed road that lasts 5 to 7 years.
- Cape seal is chip or scrub seal followed by the application of slurry seal or micro-surfacing to improve chip retention and smoothness on residential streets and lasts 7–10 years.
Overlays
- Mill-and-overlay removes several inches of the wearing surface, and then an HMA overlay replaces the milled pavement with the same thickness if the base in in good condition.
- Thin overlay is applying an approximate 1-in. layer of HMA to unmilled pavement.
Eventually, more extensive measures such as cold in-place or hot in-place recycling and full-depth reclamation are needed when pavements approach the end of service life.
Drainage
Correcting poor roadside drainage alleviates saturation of the underlying roadway structure. Routine cleaning and periodic regrading of roadside ditches and replacement of inadequate culverts will eliminate water infiltration. Likewise, storm-water inlets on curbed roadways need to be regularly inspected and cleared of debris and snow and ice during the winter. Underground springs can saturate a roadway subbase; subsurface drains must be installed to channel water away.
Conclusion
Regular preventive maintenance is essential to reducing the formation of potholes and other asphalt pavement distresses. But when potholes do appear, prompt response and the use of the proper materials and methods is vital to minimizing the disruption to traffic flow, damage to vehicles, and hazards to pedestrians and cyclists. An aggressive, proactive “find-and-fix” maintenance program year-round is the best approach.
About The Author: Bergner is principal at Monte Vista Associates LLC. Hale retired as supervisory civil engineer for Overland Park, Kansas, after 35 years of managing the pavement program.
