What specific material property change occurs in asphalt pavement that necessitates fog sealing?

As asphalt pavement ages, the asphalt binder, which is the "glue" holding the aggregate together, undergoes a process called oxidation. Oxidation is a chemical reaction with oxygen from the air, causing the asphalt binder to become brittle and lose its flexibility. This loss of flexibility leads to surface cracking and raveling, where small pieces of aggregate become dislodged from the pavement surface. Fog sealing is needed because this oxidation reduces the asphalt binder's ability to properly bind the aggregate, leading to a dry, weathered surface. The asphalt binder becomes less effective at waterproofing the pavement, accelerating the deterioration process due to water infiltration. Fog sealing essentially replenishes the surface with a light application of asphalt emulsion, a mixture of asphalt and water, restoring the asphalt binder content on the pavement surface. This re-establishes the binder's ability to hold the aggregate in place, seals minor surface cracks, and helps to waterproof the pavement, thereby slowing down further oxidation and extending the pavement's service life. Think of it like moisturizing dry skin; the fog seal provides essential oils that have been lost due to weathering.