As temperatures fall below freezing, we insulate with winter clothing to protect our skin and keep our bodies warm. Pavement doesn’t have the same luxury, being exposed to freezing temperatures, snowfall and ice which create challenges unique to near-polar regions in terms of controlling pavement distress and providing safe driving conditions.
Freeze-thaw cycles occur when temperatures fluctuate above and below freezing, forcing the pavement and supporting materials below to undergo similar temperature changes. Prolonged, freezing temperatures push frost deeper into the ground with warmer temperatures allowing frost to thaw with each occurrence representing one cycle. Winter is hard on pavements with the following forms of distress contributing to this “teeter totter” action of freezing and thawing.
Damage to asphalt binder and reduction in bond strength between binder and aggregate
Most typically controlled through a robust, thoughtful pavement design considering regional weather conditions, traffic loading, drainage of water both above and below the pavement and subgrade properties. Sloped pavements are ideal in that they can quickly direct water off of the pavement surface and into an appropriate collection system.
Water entering existing cracks, freezing, expanding and causing the crack to get worse
Water expands roughly 10 percent in volume when frozen. When temperatures rise above freezing, rainwater and melt water from snow and ice penetrate unsealed cracks within the pavement. Then, as temperatures drop again below freezing, the water within the cracked pavement begins to freeze and expand, causing cracks to expand and grow. Routine pavement maintenance and crack sealing is critical to control this potential form of distress.
Decrease in pavement support from underlying layers when saturated
Ice within the pavement section which melts and surface water able to penetrate the pavement section can lead to a “saturated” condition, decreasing the overall support strength of the pavement’s supporting layers. This is why seasonal weight restrictions (frost laws) are in effect during periods of particularly concerning freeze-thaw cycles, typically early Spring months when “thawing” is most prevalent, to avoid “stressing the pavement out” with heavy axle wheel loads.
Frost heave
Frost heave is the process by which ice lenses form and grow within the pavement section and underlying subgrade resulting in upward movement of the pavement. As water freezes, a negative pore water pressure develops in the various pavement layers which draws water upward from unfrozen layers below towards the depth of frost. For frost heave to occur, three conditions must be met including, with control or elimination of these conditions during design and construction being the primary form of risk control.
- Ground temperatures below freezing
- Presence of water
- Frost susceptible soil
Winter weather is unavoidable, but its impact on pavement doesn’t have to be. Understanding freeze-thaw cycles, water infiltration, and frost heave is key to minimizing damage. With proper design, maintenance, and seasonal precautions, roads can stay strong even in the harshest winters.
