Winter Gritting - How Rock Salt works
Rock salt (plain and treated) is the most commonly used ‘grit’ and it is the substance used on public roads by the highways authority.
Rock salt is used for treating roads, footpaths and car parks and although it has a gritty appearance and the term “gritting” is used, it is not "grit".
Rock salt is a pinky / brown colour, as it is the natural colour of the rock salt, which is mined. The salt contains dirt and impurities that lay in bands throughout the salt beds, which have been deposited over millions of years.
The salt works by lowering the freezing point of moisture on the surface. It is less effective at temperatures below -5°C and has virtually no effect below -10°C.
It is also most effective when it has been ground down, this will obviously take far longer on pedestrian areas than on roads and which is why it should be spread with professional tools.
No research or tests have been carried out, but as a rough guide roads have a rate of approximately 10-15gms/m2 for precautionary salting and 20-40gms/m2 during ice and snow conditions.
The white appearance of a well-salted, dry surface should not be mistaken for frost and it is normally safe. Often, even when dry, residual salt will itself attract moisture and can give the surface a damp appearance, but the salt should prevent the moisture from freezing.
There is no such thing as an absolutely safe surface.
The presence of salt will not always guarantee an ice-free surface so concentration and care is always necessary.