Friday, March 13

Potholes - the what, why and how

Monday morning's commute was jarred for many by a rash of potholes that cropped up on I-35 near Rittiman Road, and potholes in other areas have generated some attention and interest.

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.
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.
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.
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.
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.