For those who've made their home in south-central Texas all their lives, potholes aren't a normal part of life. Those who've lived abroad - especially the northern areas of Not Texas - know potholes happen frequently this time of year as temperatures fluctuate. For those not accustomed to potholes, this is a description of what they are, how they form, why it takes a while to properly repair them and what can be done about hitting them.
In order to help folks understand more about potholes, we've reached out to a pavement expert (he wanted to be referred to as a "Pavement Buttress", which apparently is funny if you're an engineer) in our local district asphalt and materials lab to discuss the in's and out's of potholes. Here's the Q-and-A:
Go Ahead! Blog: How do potholes form – what causes them?
Pavement
Buttress: Potholes we are most accustomed to are formed by fatigue (cracking) of the road surface. As fatigue
fractures develop into what is known as alligator cracking, chunks of pavement
between the alligator cracks are worked loose and eventually picked out of the
surface by continued traffic loads. This is what we see in areas with heavier truck traffic, at least.
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| This graphic, published in 2013 by the UK's Daily Mail newspaper, shows how most of the potholes drivers are experiencing in the metro areas have formed. |
GA: Why does it seem like we’ve had so many potholes spring up lately?
PB: Potholes become more widespread during San Antonio’s winter months,
due to increased precipitation and increasing the brittleness of the asphalt on
many roadways. However, annual ratings of the San Antonio District’s 11,000+ lane miles of
roadways has stayed fairly consistent, with areas in the energy sector corridor (Atascosa, Frio, Guadalupe, McMullen and Wilson counties) being a well-documented exception. In these areas, numerous projects are planned or underway to repair the road and the in-house maintenance forces are working as diligently as possible to keep up with the issues.
GA: What does it take to repair the potholes?
PB: It depends. If there are a few spot locations, in-house state
maintenance forces will address the potholes by filling with patching material.
Patching material can be kept in containers, in bags, or stockpiled as an open pile. In all of these
cases the patching material is easily portable in standard pick-up trucks. It
is applied by filling the pothole, and tamping it in place (often by simply rolling over the filled pothole with a truck) to create as smooth
of a surface as possible.
If the potholes are more severe, repair could require milling off
multiple inches of the pavement and relaying with stockpiled patching material,
cold-mixed bituminous material, or even laying hot mix asphalt.
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| This stock photo from an asphalt company shows crews milling away several layers of asphalt, probably to repair the road. |
There are also times when the potholes are so severe that
rehabilitation or reconstruction of the roadway could be necessary. This would
require putting out a project to bid.
GA: How long does TxDOT let a pothole sit before we repair it?
PB: TxDOT in-house maintenance forces are available should a roadway concern
arise. There is no set time frame on how long TxDOT may wait
before addressing a pothole. TxDOT’s maintenance forces maintain a constant
supply of patching material on-hand to address any potholes as they arise.
Note: TxDOT leaders have determined that using state resources to patrol all 11,000 miles of highway within the district isn't the best use of state resources or tax money - especially when those resources can be dedicated elsewhere. However, it's a district policy that a TxDOT employee should report a pothole observed, whether the employee is driving on their own time or as part of their normal course of duties. Once a report is received, crews are dispatched to address the issue. TxDOT also depends on road users to report these problems so they may be addressed appropriately.
GA: Why does it seem like these repairs don’t really hold up for very long?
PB: Many pothole repairs have actually lasted multiple
years. In cases where there truly are potholes in constant
need of repair a simple “fill the hole” approach (described above) may not be the right solution. In
these cases the pothole is more a symptom of a larger problem. For example, if the existing soil is so weak that it
cannot support the traffic on the roadway, then patching the pothole will only
mask the problem. However, TxDOT often needs to use a temporary fix - a Band-Aid of sorts - until a more lasting fix can be done. While an in-house patch may not
fix the problem it will provide a safer roadway to drive on until a better solution is possible. Also, an exposed
pothole is a conduit for moisture to enter into the soil, make it weaker, and
make the pothole worse.
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| Highway crews in California fill in potholes as a temporary measure until a long-term fix can be done. This photo comes from the California Department of Transportation. |
Many in-house
patches are methods for TxDOT to hold a pavement together until the appropriate
construction project can be developed and implemented. These projects require clear, dry weather for a stretch of several days, or the project will be nullified by the wet and less-stable material.
GA: What can be done to provide a more lasting fix for the potholes we’re
seeing?
PB: TxDOT is, and has been, a national leader in innovative techniques
and materials. Because of TxDOT efforts pothole repair materials have actually gotten
better over time. For example, historically patching material was only able to
be obtained in bags or containers to keep light-end oils in the material
allowing it to stay workable. Through partnerships within the industry, TxDOT has
developed a patching material that can be stockpiled for 6 months and still
maintain its workability. This has enable larger patches to be addressed and
perform longer.
GA: Is there a roadway material we can use that won’t see potholes? If
so, why aren’t we using it?
PB: Any material
used on a highway will ultimately encounter varying environmental, traffic, and
construction impacts - all of which can have an impact on the roadway’s ability
to resist potholes. Unfortunately, because each one of these
elements is very dynamic, it’s virtually impossible to prevent all the
variables from never causing a pothole.
There are materials and pavement structures that are more resistant
to the formation of potholes than traditional asphalt, but TxDOT has a responsibility to taxpayers
to ensure all roadways are engineered to provide an adequate functional performance. In addition to potholes, TxDOT evaluates rutting,
longitudinal cracking, alligator cracking, spalling, and punchouts.
Functional characteristics can include smoothness, raveling (what folks have seen through February on U.S. 281 near Hildebrand), and flushing. Each
one of these characteristics are considered along with the amount of traffic,
weights, and environment to develop a pavement system that will function
accordingly. To only design a road to account for potholes is not sufficient.
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| Continually reinforced concrete roadways are one option to reduce pothole occurrence, but they are too costly to be feasible in south-central Texas. |
In addition, if TxDOT were only to select materials that were
highly resistant to potholes, a fiscal inequity would develop. For example,
continuously reinforced concrete pavement (CRCP) may be considered a material
that is more resistant to potholes than a thin layer of asphalt and rock (seal
coat), but because CRCPs unit price and quantities are so much higher than a
seal coat it is difficult to justify and prevent other roadways from being maintained or expanded.