IN-DEPTH: How TxDOT’s icy road pretreatment works

Weather & Traffic Q&As

AUSTIN (KXAN) – Texas Department of Transportation crews were out earlier this week, “pretreating” roads ahead of today’s winter storm. You might think this pretreatment actually melts the ice, but it’s a lot cooler than that. It involves a scientific principle called “the freezing point depression.”

First, we need to understand how TxDOT actually preps for icy roads. There are two strategies for doing this: de-icing and anti-icing.

De-icing uses chemicals or salt to melt ice that’s already on the road, whereas anti-icing is the process of pretreating roads before freezing weather. According to TxDOT’s 2017 Snow and Ice Operations manual, anti-icing is the preferred tactic in Texas.

TxDOT’s anti-icing mixture is a brine mixture of salt and water. The salt is basically what you put on your food, but a little less pure. TxDOT trucks spray this mixture on roads BEFORE a freeze. This is very important. If they spray on the mixture after a freeze, it won’t work.

How anti-icing treatments prevent ice from forming

The “freezing point depression” is the scientific principle that explains how this brine prevents ice from forming. Basically, it means adding a substance to water lowers the temperature it starts to freeze at.

Here’s how it works: Think of a puddle of water like a high school dance. When the temperatures are high, it’s like the DJ is playing a fast song. Water molecules act like high school students dancing and bouncing all over the place.

When the temperature drops, it’s sort of like putting on a slow song. Water molecules start partnering up, eventually forming rows and becoming ice. If you ever look closely at ice, you’ll see a crystal structure. These are those water molecules lined up.

Water freezes at 32 degrees Fahrenheit. Technically, even though the water is frozen, the water molecules are still moving at this stage — they’re just slow dancing.

Salt is like adding a chaperone to the dance. The temperatures still drop, the slow music still plays, but then there’s salt blocking water molecules from partnering up.

When salt is mixed, a slushy ice substance forms at 32 degrees instead of solid ice. This is because some of the water molecules can partner up, while others are being blocked by the ice.

At 15 degrees Fahrenheit, the salt is forced off the dance floor and the remaining water molecules can begin slow dancing. At this point, the water turns to ice.

In Central Texas, where temps rarely drop below 30 degrees, a salt water brine is perfect for preventing ice from forming on roads. When TxDOT takes action ahead of a freeze, they usually target bridges, overpasses and flyovers where ice will likely form first.

The City of Austin does not pretreat roads. Instead, they use dolomite, a material made from crushed limestone, after a freeze. Dolomite is spread over icy roads by the city to provide traction for vehicles, according to the city’s Public Works department.

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