When I walk to work I notice that, at about 0 °C, the surfaces I walk across freeze differently when wet, sometimes catching me out when I step from one to the other. Water on concrete seems to freeze and become slippery more rapidly than on tarmac. Why is this? Both surfaces appear smooth but porous. Not sure if it makes a difference, but the concrete is yellowy brown while the tarmac is black.
• The surface of concrete can be smoother than tarmac, so when both are covered in a thin layer of ice, the concrete will be slippery because the ice forms a smooth, uninterrupted surface, whereas tarmac will still have rough "peaks" that offer more grip. Tarmac is also slightly more porous, so it will soak up more water than concrete, meaning that there is marginally less water on the surface to freeze.
Lewis O'Shaughnessy, Salisbury, Wiltshire, ...
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