Science of potholes

 

by Mike Kroll

The Galesburg Zephyr

 

Nobody likes them (except maybe people who sell tires or repair cars) but potholes are an unavoidable fact of life. Furthermore, it doesn't take great knowledge or wisdom to discern that this winter has been very, very good for potholes in Galesburg. Driving anywhere in town and you can experience potholes of varying sizes repeatedly for yourself. And yes, even Galesburg public works director Larry Cox admits that there are definitely more potholes in our city streets this winter than we have seen in recent past winters.

Where do potholes come from and why are they more common this winter? I asked Cox to explain the answer to that question. The normally quiet and reserved public works director actually seemed to brighten up and enjoy discussing this issue even if he too doesn't appreciate potholes themselves.

“Potholes are a function of moisture combined with frequently alternate cycles of freezing and thawing,” explained Cox. “As everyone knows water expands when it is frozen and contracts as it returns to liquid form. When we have lots of moisture during the winter some of it gets into cracks and crevices in the street and finds its way beneath the pavement. When our temperatures drop below freezing this water beneath the pavement expands and pushes up on the road surface expanding the existing cracks and often creating new ones. As it snows more moisture sits on the surface until the temperature goes above freezing and it too melts and finds its way beneath the road surface. This is a cycle that is repeated frequently throughout the winter, more so when we have a long variable temperature winter such as this one.”

Just as a can of pop left too long in your freezer is prone to expansion and even explosion the water that seeps under our streets can cause all kinds of damage to the road surface and subsurface alike. Around manholes or joints between street and curb or along structures is is not uncommon for the water to seep in, expand with freezing pushing the street surface up and then melt with warming temperatures and actually cause erosion in the street base. Between the upward pressure and eroded base a void is created and eventually the street surface will fail.

Cox continued, “Actually winters in central Illinois are worse for the creation of potholes than those in much colder climates such as Minnesota because those colder climates spend more time below freezing temperatures and experience fewer freezing-thawing cycles per year. Some of the worse pothole conditions in the United States is in Flagstaff, Arizona where they experience even longer periods of nighttime freezes and daytime thawing despite their comparatively milder climate.”

Southern states that rarely experience below freezing temperatures are far less likely to have pothole problems but their consistently hotter and longer summers create a different set of problems for roadway maintenance. Wintertime potholes are doubly troublesome due to the difficulty of repairing or maintaining the road surface under winter conditions.

“Most streets in Galesburg are composed of multiple layers including a compacted base of crushed rock, sand and/or soil with multiple layers of concrete and/or asphalt laid above in varying thicknesses,” explained Cox. “When moisture gets under asphalt and freezes it creates voids between the layers and an uneven surface with cracks above. When this happens beneath concrete an entire slab of concrete can move or buckle. As we plow snow on the surface of affected streets the plows themselves will catch on raised portions of the road surface and inadvertently do more damage.”

To make matters worse cold temperatures, even above freezing, make it impossible to properly repair either an asphalt or concrete surface. Both regular asphalt and concrete require warmer temperatures to permit proper application. In the past Galesburg and many similar cities have used a specially designed asphalt substitute known as “cold-patch” to repair potholes in the winter.

Cox explained the city's historical approach to pothole maintenance. “When our street crews aren't plowing snow in the winter months we would send them out in their trucks with cold-patch and some rakes and shovels to repair potholes. They would do their best to clean and dry the hole and then shovel in the cold-patch material and tamp it down. We encouraged them to use the wheels and weight of the truck to really push down the cold-patch. But cold-patch is only a temporary solution and left alone will eventually fail itself or simply be worn away.”

The city street department depends upon reports of potholes or other pavement problems to assign crews to repair them and tries to get to the most serious problems as soon as weather and resources permit but Cox admits that they simply can't know about or fix every single pothole in the city. Even when potholes are fixed the old cold-patch material frequently had a short lifespan on heavily traveled roads.

“We now use a new product called UPM which stands for 'unique paving material' that is designed specifically to address many of the weaknesses of traditional cold-patch materials,” said Cox. “UPM is much more expensive than cold-patch, at least twice the cost, but it is designed to adhere better to the damaged road surface and be less sensitive to moisture and dirt on the surface of the repaired hole. The sales people for this product even demonstrate it by applying it to a pothole literally filled with water. Now we would never use it that way but our experience is that this product does have a much longer lifespan than cold-patch. The sales people say it will last as long as the surrounding road surface but I'm not yet convinced of that. UPM is worth the extra cost if it just survives until warmer weather permits us to get around to the worst damage and properly repair the surface during the spring and summer months.”

Years ago Galesburg initiated a street maintenance program called the paver program that was predicated on as assumption that it is cost-beneficial to do  ongoing street maintenance such that you maximize the useful life of a street before it must be resurfaced entirely. While this program still exists it has been greatly scaled back in recent years. As Cox puts it the Paver program is “a shadow of its former self” due to reduced resources within the street department. Public works was among the most affected city departments when the early retirement program was implemented and savings was realized by not replacing many of those who retired resulting in less manpower.

“Anytime you can prevent the street surface from getting into such bad condition and replacement is the only option you extend the useful life of that street,” explained Cox. “Many of the streets in Galesburg were originally constructed in a less than optimum fashion and are therefore more prone to fail and require replacement or resurfacing. Today we have adopted the IDOT road surface standards and require that all new development adhere to them as new roads are created such as in the new Seminary Square shopping development. Better built roads should last longer and cost less to maintain but available resources still determines just how much street maintenance we can accomplish.”

 

2/14/08