What is road or rock salt and how does it work?

When cold weather arrives you may see in stores big bags of road or rock salt and you may see it sprinkled on pavements and roads to melt ice and snow.

Road salt is a halite, which is the natural mined mineral form of table salt or sodium chloride (NaCl).
While table salt has been purified, rock salt contains mineral impurities, so road salt typically is brownish or gray in color.

Machines mine the salt, which is crushed and packaged for delivery.
Additives may be mixed with the road salt to prevent caking and ease delivery using gritting machines. Examples of additives include sodium hexacyanoferrate(II) and sugar.

How Road Salt Works

Road salt works by lowering the freezing point of water via a process termed freezing point depression. In a nutshell, the salt breaks into its component ions in a small amount of liquid water. The added particles make it more difficult for the water to freeze into ice, lowering the freezing point of the water. So, for road salt to work, there needs to be a tiny bit of liquid water. This is part of the reason why road salt is not effective in extremely cold weather when water would freeze too easily. Usually, there is enough liquid water present, either coating the hygroscopic salt pieces or produced by friction from traffic, that an extra source of water is not necessary. The salt will work down to -10C if there is a high enough concentration of it.

In Stondon we have salt bins at

  • Orchard Way
  • Fakeswell Lane
  • Birch Grove
  • Hillside Road
  • The Pastures Car park
  • Shannon Close/Meadowsweet

The hard crust that sometime forms is where the salt has absorbed water and created a "salt water liquor" and the layer dried and reformed as a crust.

Myth: Salting a road prevents the formation of ice

Salt lowers the freezing temperature of water, which prevents ice or frost forming on the carriageway as it would otherwise, once the temperature of the road or the air falls to zero degrees centigrade.  The higher the concentration of salt, the lower the temperature at which freezing will occur. Generally, on the roads, salt loses its effectiveness once the temperature falls below -10 degrees centigrade.

Pre-salting the road forms a debonding layer so if snow falls, it doesn't freeze onto the road surface and can be ploughed off or churned off by vehicular movements.

Myth: Spreading salt on to ice or snow will melt and remove it quickly without any other actions

Salt comes in grain sizes of 6mm or 10mm and is spread at rates between 10 and 40 grams per square metre depending upon the forecast road surface temperatures and if snow is forecast or is falling.  When spread on top of ice or snow, each grain will begin to melt the surrounding ice working its way outwards. As it melts the ice, it forms a pool of salty water, which in turn helps to melt the surrounding ice and so on. Without any traffic to move the salt and salty water around and mix it into the thawing ice, the melting process can take some considerable time.

Where snow falls on top of salt then it begins to melt the snow from beneath. Again vehicular movements will speed up this process. However the first vehicles over the snow will actually compress the snow into ice in much the same way as a snowball is created. If there is little traffic, or very slow moving traffic, then a layer of ice may form on top of the road until the salt works its way up from below.

Myth: It’s too cold for snow

There is a relationship between the temperature and the amount of moisture the air can hold. However it is only once the temperature gets below -40 degrees centigrade that the air has so little moisture content that snow can rarely occur.

In this country, most rainfall begins as snow in the upper atmosphere throughout the year. As the snow falls through the lower atmosphere the air is warmer and it turns to rain.

In the winter, the air in the lower atmosphere is also cold, and, if it is at or below zero then the snow can make it to the ground. However very slight temperature changes at ground level due to factors like wind and altitude can change the type of precipitation over short distances. This is why weather forecasters are often very cautious and say it could hail, sleet or snow.

Myth: All water freezes at zero degrees centigrade

Except in the case of freezing rain! This phenomenon thankfully occurs rarely and is often associated with the approach of warm air after a prolonged cold spell. Here the precipitation once again starts off as snow in the upper atmosphere, then it passes through a region of warm air which turns it to rain before finally passing through a thin layer of cold air just above the surface. The moisture cools to a temperature below freezing point, but the water droplets do not freeze themselves, and become supercooled.

When the droplets strike the ground or any surface, they instantly freeze and coat everything in a film of ice. This coating will cover the grains of salt rendering them almost ineffective until the air temperature rises and the ice begins to melt. Road travel during a period of freezing rain will be severely disrupted and it is unsafe to send heavy gritting vehicles on to the network as they too will have little, if any, traction.

Myth: You have never refilled the salt bin on my street

The salt in the bins is for use on public roads, pedestrian areas and footways only. It is not there to be used on private properties.
We rely on volunteers to spread the salt as we hope the bins are located in the most needed locations.

We try to check and refill the bins at the start of the winter season. Usage is not monitored and we rely on you to let the Clerk know they are empty.
The Parish Council do not automatically refill them but we will if we can. Each bin holds about 200kg.

We have a small stock available to us provided by Central Bedfordshire Council. Our storage space is limited and we will fill the bins if we have been notified and if resources allow.

Where salt from the bins is seen to be being misused then this can be reported in confidence to the Council.
If it is seen being stolen, then this should be reported directly to the Police along with the make, model and registration number of any vehicle involved.

In Stondon we have salt bins at

  • Orchard Way
  • Fakeswell Lane
  • Birch Grove
  • Hillside Road
  • The Pastures Car park
  • Shannon Close/Meadowsweet

The hard crust that sometime forms is where the salt has absorbed water and created a "salt water liquor" and the layer dried and reformed as a crust.

 

 

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