TRANSPORTATION MATTERS for IOWA

Even when daytime temperatures are above freezing, once the mercury drops overnight, pavement temperatures can reach lows that lead to frost and ice formation. This is especially true on bridges and overpasses that don’t have warmer earth underneath them. The Iowa DOT often treats these areas early in the winter season to prevent unexpected slick spots on bridges.  

So why use brine instead of just plain rock salt? Tina Greenfield, with the Iowa DOT’s Office of Maintenance, said, “We don’t lay down any completely dry material. We always spray granular salt or sand with brine so that it sticks better. When the material is dry, there is too much waste because dry salt and sand bounce off the road. When vehicle tires come in contact with dry material, it tends to just blow off the road surface instead of bonding to the road reducing ice and frost.”

Greenfield continued, “To help prevent icing, we can apply that layer of brine ahead of any forecast frost or ice. The water in the brine will evaporate, leaving a very thin layer of salt that prevents ice from bonding to the road.”

In addition to making the roads safer by preventing slick spots, the process of anti-icing can save the Iowa DOT time and money because it allows the roads to be treated during normal business hours when Iowa DOT garages are at full staff and not incur overtime to bring employees in overnight.

Is all brine the same?

Brine mixing at the Atlantic maintenance garage

For liquid brine, the Iowa DOT uses a salt and water combination 2.2 pounds of salt to one gallon of water for a 23.3 percent brine concentration. Dry salt will also form brine when it mixes with snow and slush on the road, although the concentration will vary. 

“Mixing the brine ourselves allows us to take advantage of brine’s tendency to stick to things,” Greenfield said. “This tendency actually reduces the amount of dry salt we use. Dry salt bounces a lot when it lands on the pavement; some studies estimate up to 30 percent soon ends up in the ditch when applying completely dry material. We spray the dry salt or sand with brine as it is dispersed from the truck so it sticks to the road better. Since more material stays put, we can use less and reduce waste.”

How much does rock salt cost?

The Iowa DOT contracts for a year’s worth of salt at once. Buying in quantity lowers the price. Craig Bargfrede, the Iowa DOT’s winter operations administrator, said this year’s salt price is around $71 per ton. He said, “Because of the way our contracts work, we are also able to offer cities and counties the opportunity to use our volume contract price to buy salt for themselves at lower prices. Especially for smaller cities and counties with limited staff, it eliminates the need for them to go through the bidding process on their own.”

The Iowa DOT looks at the five-year average salt usage to determine an amount to contract every year. Bargfrede said, “For the 2015-2016 winter season, we contracted for 169,425 tons of salt. We work with our field offices to come to an agreement on how much salt each of our 102 garages will need for the season. We work hard every spring to make sure the salt storage is full at the end of one winter season so we’re ready in case next season’s fall frost crops up earlier than expected.”

So now you know. The next time you see an Iowa DOT truck spraying salt on a sunny afternoon, you’ll know to be prepared for possible frost or ice on untreated roads overnight.