Why do bridges ice in winter weather?

Kevin D. Hall, chair of the department of civil engineering, explains:

Bridge-Ices-Before-RoadWe’ve all seen them – the highway signs just before a bridge saying “Bridge Ices Before Roadway.”  This phenomenon mostly boils down to insulation.  A typical highway pavement has only one face exposed to weather – the top.  The bottom of the pavement is somewhat insulated by the base and soil beneath it. Temperatures that freeze the pavement must work their way down through the structure from the top.  A bridge deck, however, is surrounded by frigid air – so that all faces exposed to the air begin to freeze at the same time. This process is accelerated for high bridges, which have greater airflow underneath.  The water that forms ice on the surface of a bridge comes typically from a couple of main sources: any precipitation that falls on the bridge and water that has soaked into the concrete slab. (Rarely are any pavements in our climate completely dry.)



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