ROAD SALT FOR WINTER SERVICE / MAINTENANCE

CONTENTS

ROCK SALT FOR WINTER SERVICE / MAINTENANCE
THE SPREADING OF SALT, AND SALT SPREADERS
RATES OF SPREAD OF ROAD SALT
POROUS SURFACES
SOURCES OF USEFUL INFORMATION
WET SALTING
ADDITIVES TO ROAD SALT
AVAILABLE "OFFICIAL" INFORMATION ON RATES OF SPREAD OF SALT (New, and with further additions December 2009)

ROCK SALT FOR WINTER SERVICE / MAINTENANCE

ROCK SALT has a, "dry" density of approx. 1.28 tonnes/cu.metre,
and
a "wet" density of approximately 1.44 tonnes per cubic metre.
These figures are a GUIDE, because the density will depend upon the source of the salt, the grading and the moisture content.

It is an extremely quick and easy test to find the actual moisture content and density of the the salt you have delivered or have in stockpiles if you require more accurate figures, you really should be performing this testing on a regular basis.

SALT HAS VERY LITTLE EFFECT IN MELTING SNOW AND ICE BELOW THE TEMPERATURE OF MINUS 9 DEGREES CENTIGRADE

You will get an improvement in grip for vehicle wheels when using rock salt below minus 9 degrees centigrade from the physical action of the gritty nature of the salt improving friction, but not until the temperature rises above minus 9 degrees centigrade (or there abouts) will the ice/snow will start to melt.

ROCK SALT is specified in :-
BS 3247:1991:Salt for spreading on highways for winter maintenance.

Anybody dealing with winter maintenance and buying salt should have a copy of this standard, it covers such points as storing quality, moisture content and grading.

Just one item of note is that SALT FOR SPREADING ON HIGHWAYS shall have a maximum moisture content of 4%, any higher and you are buying water at salt prices.

If the grading of the rock salt is not correct, and just as important of a consistent grading within the specified tolerances, and with a constant moisture content, the rate of spread of the salt from the spreader will alter, even though spreader settings and speed are the same.

The cost of sampling and testing the salt supplied to you is not expensive compared to the cost of the salt, and can reveal "interesting" information.

THE SPREADING OF SALT, AND SALT SPREADERS

The British Standard for salt spreaders and their calibration is found in :- BS 1622 : Spreaders for winter maintenance This specification deals with the various classes of SALT SPREADER, the methods in which to test them for correct distribution of salt, and it sets down the acceptable levels of result to permit the issue of a certificate for a SALT SPREADER. As I have already mentioned, consistency of rate of spread on a particular setting will only remain correct if grading and moisture content of the salt source remains constant.

RATES OF SPREAD OF ROAD SALT

The following rates are a GUIDE to salting, the actual rate will depend upon the road conditions, residual salinity, and severity of predicted weather conditions.

This table of SUGGESTED rates of spread is for impervious road surfaces, using rock salt specified to BS 3247.

Frost or ice after dry conditions.............................10 - 15 grammes/sq. metre
Frost or ice after rainfall/wet conditions................... 20 - 40
Snowfall..................................................... 20 - 40

POROUS SURFACES

It has been widely recognised that the rate of spread of salt on porous wearing/surface courses needs to be increased to give the same degree of protection against ice forming, in relation to the rate of spread needed on impervious road surfaces

It is likely that rates of spread on porous "Thin Surfacing" for "precautionary salting", which is usually the same rate as "Frost or ice after dry conditions" will be double the rate of spread given above, i.e. it will be 20 - 30 grammes/sq. metre.
But you MUST follow the recommendations for salt rates of spread in your Winter Maintenance contract document, not the figures quoted here, the figures here are for guidance only.

SOURCES OF USEFUL INFORMATION

CSS Report No.5/14-1989 : Winter Maintenance Manual and Code of Practice

It may be a little out of date, but it is still an extremely useful source of information on most aspects relating to winter maintenance.

WET SALTING

There are systems now being employed using a finer grade of salt that has been pre-wetted to a uniform standard before it is spread on the road, it is more, quite damp, rather than wet.

I am not familiar with these systems but you ought to be aware of them.

There have been a number of articles in the engineering magazines that will provide more information, or you could get in touch with one of the organisations who are using the system.

There are a number of claims that this method has advantages over the more traditional way, such as :-

• You use less salt, therefore saving money and doing less damage to the environment
  
• The salt stays on the road and does not "bounce" off, or get blown off by the air disturbance caused by passing traffic.
  
• The effect of the salt in melting snow or ice is more immediate.

I do not know if all, or any of these claims are correct, and there are bound to be some disadvantages to the system that are not being so widely broadcast.
I have heard that the actual equipment used for "producing" wet salt is subject to "breakdown" because of the severe nature of the process that it is required to perform. In my experience most times "simple" is best in the overall scheme of things.

So do some homework of your own, I repeat, I have no personal knowledge of this system.

ADDITIVES TO ROAD SALT

There is now on the market an additive to salt based on molasses, a bye product of the sugar refining industry.
The product, and I am aware of only one product being marketed, claims to increase the "stickability" of salt to the road, resulting in a number of benefits.
These include increasing the performance of the salt, and allowing reductions of the rates of spread of salt per square metre.
I have no knowledge of working with this additive, but I am aware that this product has undergone trials in cooperation with TRL, which resulted in a report, which does not seem to be widely available.
I have heard "comments" through the industry that there have been problems with the bulk storage of salt that has been modified with this product, I do not know if these reports are correct or not.
A small trial with this product (or any new product) might be wise before a large scale commitment, and talk to your colleagues in other authorities and contracting organisations.
It is likely that you will find more information on this product in one of the links below.

AVAILABLE "OFFICIAL" INFORMATION ON RATES OF SPREAD OF SALT

This list was compiled for inclusion in the Autumn 2008 Newsletter which is primarily concerned with the recent reduction of the the initial texture depth of new road surfacing, included in the 900 Series of Volume 1 of the Specification for Highway Works published in August 2008.
It follows that if you reduce road surface texture it is likely that rate of spread of salt will be decreased on those new surfaces that do have decreased initial road surface texture.

Whilst browsing through the Highways Agency Research Compendium I could not help but notice the considerable number of Projects that were listed that concerned de-icing, most of the Projects having a greater or lesser connection with "modern surfaces" / "thin surfacing".
I thought I would provide a short list, for those of you who have an interest in Winter Maintenance, as a lot of money has been spent on the various studies, and they would appear relevant, if you could only get to read them.

I list some of the more relevant in order of completion

I am not going to discuss this topic further at this point and leave it to you as to whether you wish to view the introductory information that is available on the above projects, from the Research Compendium of the Highways Agency website.
You may also like to visit  www.ha-partnernet.org.uk section of the HA website so that you may download the, Highways Agency Network Management Manual - Part 5 Winter Service,
this a very comprehensive 116 page document where you will find guidance on most aspects of winter maintenance in the UK, it is recommended reading, and far exceeds the basic information provided on this web page,

New Additions December 2009

Lessons from the severe weather February 2009 - This document, is now available as a downloadable .pdf file from the Roads Liaison Group.
It is well worth reading.
I find the CSS recommendations particularly interesting reading.
Whether there has been time to implement the recommendations included in this document before the recent bad weather of December 2009 I do not know.
But if there has, it would seem the recommendations have not been sufficient to prevent incidents of major disruption to the highway network and its users.

You should also download the recently published (15-12-2009) Complementary Guidance to the document Well-maintained Highways Code of Practice for Highway Maintenance Management.
A large part of this Complementary Guidance, i.e. Section 13 - Winter Service is "new" information relating to providing a Winter Service that is thought to provide a "reasonable" service for the road user.
Also read Appendix H - Winter Service Issues of the document which gives more detailed information on the actual process of providing a Winter Service.

Much of this "new" information is based on the previous document mentioned, i.e. Lessons from the severe weather February 2009.
If you are involved with the highway network of the UK, either as a "provider" or a "user" I suggest that you download both these documents and study them.
If it does nothing else it will give the reader a clearer picture of what is/should be provided.

I believe it is once again important for the reader to realise there is what could be considered two road networks in the UK.
One being the motorway and trunk road network that is maintained by private contractors under the direction of the Highways Agency, and consists of about 5% of the network but carries the bulk of the heavy commercial traffic and a large proportion of private vehicles. You could liken these roads to the main "arteries" of the body.
And there is the "local" network that is still mainly maintained/serviced by a local authority, although in a few cases this duty has been outsourced to large contractors/consortia.
You could liken this part of the network to the smaller arteries of the body and the capillary network that supplies the blood to the organs that actually need it.
This "local" network makes up 95% of the actual highway network in the UK and it has to be realised that it is just not possible, or economically appropriate, for local authorities to keep all their network ice/snow free in severe weather by the application of salt.
This is why it is very important on local networks to utilise surfacing materials and surface treatments that respond best to frost, ice and snow.

It is still my opinion that the effect of mainly proprietary porous / negatively textured thin surface course system surface courses is still not being fully recognised with regard to the lower road surface temperatures these bituminous mixtures will exhibit during cold weather, on any highway network.
I am not going to go into any lengthy explanation on this matter, I do not want anybody taking my word on anything, I want them to undertake serious study of their own and form their own views on the matter.
So, I leave you to "Google" appropriate keyword combinations such as, porous asphalt, winter, ice formation, winter service, etc. and let you read the articles produced from such a search.
However I would suggest that you give precedence to the articles from recognised highways institutions, not the ones that tend to be in glossy presentations from indeterminate sources.

But one important point, just because a surface course bituminous mixtures are not called "porous asphalt", "pervious macadam",  "open graded macadam", etc., does not mean they do not have very similar aggregate gradings to these particular mixtures, the "porous effect" will still be the same.
The "porous effect" depends" upon the aggregate structure of the material grading and the binder/mastic content, not the product name.
Not all Thin Surface Course System bituminous mixtures are of a porous nature, but many, perhaps most, are.
I keep repeating on this website, when writing about bituminous mixtures, know the nature/characteristics of the bituminous mixture that you are purchasing. If you do not understand this part of highway engineering consult an engineer who does.

But I will leave you with a bit of schoolboy physics,
1) An impervious, dense, material will allow better conduction of "heat" still present lower down in the road pavement to rise to the surface thus allowing the prevention/reduction of ice formation on untreated roads, perhaps entirely at temperatures around zero, and will also hasten the thaw of a road surface as ambient temperatures increase.
2) "Heat" that is radiating from lower down in the road pavement will not be transmitted as efficiently through a voided material.
3) It is also likely that the phenomena of "latent heat of evaporation" will cause at least some of this rising "heat" to be utilised in evaporation of the water contained in the porous structure of the surfacing material, again causing the road surface to remain colder for longer, or become icy more quickly.

A wind blowing across a porous surface course containing evaporating water will stay colder, even become colder, it is physics, it is what happens.
This will not happen to the same degree with an impervious surface, which will dry much more quickly, and there will not be a "reservoir" of water available to feed the effect.
Take notice of the changes in colour/condition of road surfaces as you drive along them on a bright, but cold, sunny Winter's day. The roads that become dry and white with salt first will be the impervious surfaces.

I am thankful that the use of porous thin surface course systems, as surface course, have not been embraced as widely in my local area as has been the case with some authorities further south.
I also appreciate travelling along the frosty country lanes, which I have to quite often, and which do not receive priority salting, on the good rugous surface dressings that have been applied, driving appropriately I have not run out of road yet, thanks lads.

I try to refer to as few commercial sites as possible in compiling my site, but when a site offers particularly useful information about a subject I make an exception.

For more information on how salt melts ice, press ------------------------------------------------------> HERE

Good information on the WINTER MAINTENANCE of roads, but from the USA, press ---> HERE

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