How Road Salt Leads to Rough Pavement (Sometimes)
That pothole your car hit sometime in the past week: can you blame it on salt used this past winter?
If you live in Los Angeles, where potholes-per-mile are among the highest in the nation, the answer clearly is no. Los Angeles does not use salt. Never has, and probably never will. When the roads ice up in Los Angeles County, it is so rare an event that the city has no road salting supplies or methods. Almost always, it melts in a day or two with nary a granule of salt applied. The ubiquitous potholes there form because of water, wear, high temperatures and time.
If you live in the Snow Belt areas that stretch from Texas and parts of the South (even Atlanta had snowy conditions in 2011) up through Denver and across the Upper Midwest and the Northeast, the salt may or may not be at fault for potholes and other forms of rough pavement. Here is why:
- Salt does not affect asphalt pavement until it is already in advanced deterioration. A pothole forms when water gets under the cracks in pavement and undermines the gravel base. In situations where there is significant water accumulation infused with road salt, it might exaggerate the freeze-thaw cycle at lower air temperatures because the salt keeps the water in liquid form below 32 degrees, down to about 15 degrees. With vacillations between, say, 17 degrees and 14 degrees, there are freeze-thaw cycles that would not happen without the salt. But for the most part, salt is not the main culprit in creating asphalt potholes.
- Salt can affect concrete pavement that is reinforced by steel framing (rebars). There is the matter of depacification and pH reduction, as detailed in a report from the Adirondack Watershed Institute (AWI) at Paul Smith’s College in upstate New York. The report, “Review of Effects and Costs of Road Deicing with Recommendations for Winter Road Management in the Adirondack Park,” explains this phenomenon as it affects concrete pavement: “Depacification refers to the break down of the passive layer of ferrous oxide that forms on the surface of steel rebar under alkaline environments. The highly mobile chloride ion [from salt] reaches the steel by leaching through the concrete and replacing some of the oxides in the passive film; without this passive layer, corrosion can occur if moisture and oxygen are available. The pH of the pore solution is reduced when chloride ions react to form hydrochloric acid. The reduction of pH accelerates the corrosion of steel, for example the corrosion rate of steel is five times faster at pH 11.5 than it would be at pH 12.0 (Jones and Jeffery, Environmental Impact of Road De-icing. In D’Itri, F.H. (Eds) Chemical Deicers and the Environment. 1991).
- Salt especially affects bridge deck pavement and concrete parking garages. Because bridge pavement is famously colder and subject to freezing before roads on land, salt use tends to be higher (as is the runoff, which contributes to salt accumulations on underpasses). Bridges can be paved with asphalt or concrete, but all surfaces have steel reinforcement underneath. The AWI report cites work by the Transportation Research B (TRB, 1991) that says in snow belt states, road salt is the “single most important factor in deck repair and maintenance cost, that “15 percent of all bridges are structurally deficient due to corrosion (Koch, 1991),” and that pothole formation, vehicle damage, unsafe driving conditions, and rusting rebar that affects bearings and joints, steel framing and supports all create serious hazards. Road salt aggravates each of these problems by increasing the number of freeze-thaw cycles. It should be noted that the Federal Highway Administration has mandated protection of rebars with an epoxy coating, waterproof membranes and deck overlays and additional concrete in all construction since 1984.
Still, salt is commonly thought to be the culprit. For example, a news story out of the University in West Virginia in 2010 claimed a state department of highways engineer said, “an excessively bad winter, fierce snow conditions and heavy use of salt on the roads have contributed to a large amount of potholes.” Winter conditions in general do typically lead to asphalt pavement potholes, as we now know. Unless they’re talking about steel-reinforced bridge decks, the salt is not to be blamed. But the widespread belief that it does persists.
Salt has its own detractors from the environmental community. Numerous studies show that saltwater runoff is damaging to natural ecosystems, both flora and fauna. Research conducted at the University of Minnesota in 2009 found that 70 percent of the 350,000 tons of sodium chloride distributed in the Minneapolis-St. Paul region is retained in 39 lakes, three major rivers ten tributaries and several observation wells in the region. Increased salinity also affects the area’s drinking water, raising salinity significantly since salting of roads began in 1950. An experiment to reduce salt use methodology by road crews on the University campus since 2007 has already cut that use by 41 percent, suggesting this can be done on a broader scale to greater effect elsewhere.
Road salt certain takes its toll on bridges, other concrete pavement and the environment. But the cause of potholes is largely moisture, hot or cold or frozen, entering cracks in pavement and undermining the roadbed. The aging road system in this country is seeing higher traffic loads than ever before while state and municipal budgets are having a hard time keeping up the maintenance of those roads.
Potholes are inevitable, whether or not you hold the salt.