Slippery when wet

I saw a tweet on Twitter this week by someone from in the Yorkshire area to say that they had freezing rain up there and that he’d been out to spread salt to make it safer.

But does it?

Is ice as slippery as, principally the media would have us believe?

Ooh I can hear you all taking that breath in to shout at the screen and comment but hold it for a minute, let me explain.

Early snow

In the last few weeks we in the British Isles have had quite a bit of early snow. The southeast admittedly has had less than most of the rest of the country, although by the way everything came to a standstill, you wouldn’t believe it.

Anyway we have but what makes roads in particular slippery and does the application of salt make them safer?

I would say not always.

Salt

What salt does is reduce the freezing point of water, even brine (a saturated solution of 23.3% salt to water) will freeze at -21.11⁰C (-6⁰F). So at the dilutions that occur when salt is spread around the roads in the UK it is unlikely that it will prevent ice from forming at temperatures much below -6 or -7⁰C.

For anyone who has watched The Ice Road Truckers they will know that ice (certainly at those temperatures) actually has quite a lot of grip and so does snow. Both ice and snow become slippery when they start to melt, when there is some form of lubricant on the surface or when they break up and become slushy.

My explanation

Let me explain those individually although you will see a common theme as we go through.

Firstly when snow gets near to freezing point and the air around it is at a higher temperature, it will pack together and the wetter it becomes the more it packs together BEFORE it starts to melt. If it does freeze, it goes hard and there is not much grip. However when the air around it warms the surface up again condensation forms on the surface which quickly becomes water and then it is very slippery.

On to the lubricant on the surface, as mentioned above this could be water. It could also be any other liquid, some of which are not water soluble (oil is the obvious one). It could be a powder, even a fresh snow fall. Snow on top of ice or packed down snow is quite simply treacherous, however it is still effectively a lubricant because it creates a layer between the surface of a shoe or a car tyre and completely removes any gripping properties that the shoe or car tyre might have.

Now on to the slushy bit. When snow and ice melt they become soft and slushy, I am sure that most people have experienced stepping on what appeared to be a sound surface, only to have it squelch under your foot. The same happens when you drive on those sort of surfaces, they break up and turn to slush and that slush gets between the tyre and road surface to make driving conditions slippery.

And the theme is?

So the common theme here is a lack of grip between the road and the tyres on a vehicle.

Salt is?

What then is salt, particularly the salt that is so liberally spread around on the roads during the winter?

It is a medium for melting snow and ice.

It does not clear snow, at least not until the snow or ice starts to dissolve the salt and that in turn melts the snow and ice. It is self perpetuating.

So the benefit of spreading salt is that it melts snow, what else does it do?

Apart from the negative environmental effect and assisting the rusting process of our vehicles, the down side is that putting salt on the roads actually reduces the gripping properties of many of the tyres supplied in the UK.

Absorbing salt

The salt absorbs moisture and in doing so creates a lubricating mixture between the tyre and the road surface that means there is less adhesion, a damp salty road has less grip than a wet road. Even when the salt is dry there is still a film of salt between the road and the tyre.

Now if we go back only a few years salt was mixed with grit and sand. The purpose of this was to improve the purchase that could be obtained by adding a rough surface to the slippery one. Therefore if the slush froze into ice there was still some grip to be gained from the aggregate frozen into the surface. Now because of various cut backs those organisations save money by spreading salt alone because the residue does not need to be cleared up afterwards, it just washes away. I believe it used to be collected, mixed with salt and spread on the roads in winter. All very environmentally sound.

Coal ash

In the days of coal burning fires people used to spread the ashes on snow and ice for exactly the same reason, to improve the grip on the frozen snow, slush or ice.

What is the best solution?

I am certainly not advocating covering the road surfaces with anything that is going to reduce traction.

The first and simplest thing with snow is remove it from the road surface, the very same vehicles used to spread the salt nearly always have snow ploughs fitted to them, which rarely, if ever get used.  Once the snow has been cleared then is the time to spread something to improve the grip and melt the snow or ice.

Safety on ice

As with most things in safety removal of the hazard is the first and safest option. Equally as with most things in safety, that is often complicated by the human factor, people either confusing a relatively simple process or finding reasons (excuses) for not doing something that benefits everyone.

So the next time (and I don’t expect it will be long) that the outside temperature drops to freezing or below think about what is between the road and your feet or vehicle tyres and rather than depend on the supposition just because salt has been spread on the roads that it is safe to drive as if they are dry. Remember that anything between the road and the tyres will reduce the grip, so look out for your own safety by driving in a way that suits the conditions because by looking after your own safety you could also be looking after the safety of other people.

What to do!

The biggest cause of major injuries to workers in the UK is slips, trips and falls and the same rules apply to footwear as to tyres, they should be suited to the conditions. These are some of the topics covered in our Level 2 Health & Safety courses for more information call us on 01227 275521 or contact us by e-mail.

Think and be safe