[ Top ] Contents Page

The Idiots' Guide to Highways Maintenance
Copyright 2000/14, C.J.Summers

ROAD SALT FOR WINTER SERVICE / MAINTENANCE

ROCK SALT FOR WINTER SERVICE / MAINTENANCE SOURCES OF USEFUL INFORMATION RELATING TO ROAD GRITTING / SALTING
THE SPREADING OF SALT, AND SALT SPREADERS SPREADING "WETTED" SALT ON TO THE ROAD SURFACE
RATES OF SPREAD OF ROAD SALT ADDITIVES TO ROAD SALT
SALTING RATES OF SPREAD ON POROUS (OPEN TEXTURED) ROAD SURFACES LINK TO - WEBSITE OF THE NATIONAL WINTER SERVICE RESEARCH GROUP (NWSRG)
(This website has been much improved and contains a wealth of information, the bad news is that you do have to look for it, the good news is most of the information can be downloaded.)
   
AVAILABLE "OFFICIAL" INFORMATION ON RATES OF SPREAD OF SALT (Added December 2009)
FURTHER AVAILABLE "OFFICIAL" PUBLICATIONS APPLICABLE TO WINTER SERVICE/MAINTENANCE (New references added January 2013)
 
A BRIEF EXPLANATION OF THE TWO HIGHWAY NETWORKS, AND THE IMPLICATIONS TO THE WINTER SERVICE

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. (This has been superseded.)
BS 3247:2011:Salt for spreading on highways for winter maintenance, is current.

I have not seen the 2011 edition, so I do not know if there have been any significant changes, until I have the opportunity to study the 2011 document I will continue to recommend the 1991 standard for specifying and guidance.

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.

You will now find most of the salt for road salting/gritting is being sold under proprietary trade names, in my opinion it should still comply with the British Standard to ensure consistency of the product.

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


Current recommended precautionary salting rates are likely to be lower than the above, and I would suggest that you read the recent TRL Reports and associated technical literature.

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.

Since this page was first written and since the Winters of 2008/9 and 2009/10 these figures have been challenged, downwards, but they are still included in many "official" publications.

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 :-

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.

"Pre-wetted Salt Dosage Advice"
Prepared for the Highways Agency ( Report Date October 2008)
(This is a study prepared for the Highways Agency and is a fairly detailed review of world wide practice with preparation and application of pre-wetted salt and brine, it also outlines potential costs and savings of the various systems.)

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.

"ABP (Agricultural Bye Product) treated de-icing road salt"
Highways Agency Framework - Contract 3/387 for Research and Development Services - Task Ref:212(387) WSPB (Report Date July 2007)
(Well worth reading in my opinion, and it could save you money.)


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

Ref. Title Completion Date Published Project Value
3/145 De-icing of modern surfaces 02/2002 No 84,000
3/261 Investigate de-icing using wetted salt 06/2002 No 116,000
YY91866 Salt spread rates for thin surfacings 06/2004 No 20,000
Y204284 Review of residual salt level detection methods 05/2006 No Not indicated, cost included in Y206792.
Y206791 Skid resistance resulting from de-icing 05/2007 No 260,000
Y623913 Winter service treatment for negatively textured surfacing 04/2007 No 41,000
It is possible to download from the above project reference the "Review of Winter Service Treatment for Negatively Textured Surfaces", prepared by the Highways Research Group (HRG).
It is interesting reading, but whether you will obtain any actual guidance for your particular situation/s is another matter, but it does indicate many of the various considerations that need to be taken into account when determining actual rates of spread of salt.
Y206794 ABP (agricultural bye product) treated salt spread rate research Current Current 41,300

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.

New Publication Additions, November 2010 and March 2012

This list was originally compiled for inclusion in the Autumn 2010 Newsletter.

The publications that I suggest your read are all available from the Highways Agency Research Compendium, once you find them, they are there, or they where until quite recently, I checked.
However publications that I recommend for download and perusal have a habit of disappearing once I have mentioned them so I would not tarry in finding them if you do have an interest in this subject.
The Reports are not difficult to find once you have become acquainted with the recent changes in the manner in which Highways Agency Research Compendium has bee restructured.
I am not going to "spoon feed" you these documents you are either interested or you are not, but I have been able to download them all.

ABP (Agricultural Bye Product) treated de-icing road salt
Highways Agency Framework - Contract 3/387 for Research and Development Services - Task Ref:212(387) WSPB (Report Date July 2007)
(Well worth reading in my opinion, and it could save you money.)

De-icing Materials and Corrosion - Laboratory Testing Report
Highways Consultancy Group - Highways Research Group - Task Reference : 628(387) MTSC (Report Date March 2009)
( I would have thought that they could have, should have, used generic rock salt for this testing rather than using a proprietary branded rock salt, in my opinion the results would have been very similar. I feel that this may confuse the cost effective purchasing of rock salt. )

Pre-wetted Salt Dosage Advice
Prepared for the Highways Agency ( Report Date October 2008)
(This is a study prepared for the Highways Agency and is a fairly detailed review of world wide practice with preparation and application of pre-wetted salt and brine, it also outlines potential costs and savings of the various systems.)

XRWIS - Next Generation Road Weather Information System Trial
Highways Agency - National Framework for Technical Consultancy Services ( Report Date June 2007)
( This is not my type of report but it is essential reading if you are involved in providing Winter  Service as this is the way the industry will endeavour to take the "business" of applying salt to highway networks, how reliable it is I leave for you to decide. This view does not mean I am opposed to technology providing further information for experienced operatives to make the necessary decisions. )

Please note that all these reports have been prepared for the Highways Agency but the information that they contain is no less useful to those in charge of Winter Maintenance working for local authorities.

There still appears to be a number of reports related to the subject of Winter Service that I have mentioned on my website that still have not been published, or should I say that I could not find them.

A useful report that has been available for some time and appears to have been re-published, is,
Review of Winter Service Treatment for Negatively Textured Surfacing
Highways Consultancy Group / Highways Research Group - Contract Ref : 3/387 (R&D)
(Report Date March 2007, but appears to have a new publish date of September 2010 on the Highways Agency Research Compendium)
( This is a very informative report with references to other useful documents. This was prepared for the Highways Agency but the information will also assist local authorities who have embraced the use of proprietary Negatively Textured Surfaces (NTS's).)

I am afraid that some of the more comprehensive documents on providing the Winter Service are no longer available for download by ordinary mortals. I am glad I downloaded my copy while it was available.
One particular document I refer to is the,
Highways Agency Network Management Manual - Part 5 Winter Service,
perhaps because if you had a copy you would realise more fully the nature of the failure to provide an adequate service by many authorities during the Winters of 2008/09 and 2009/10.

TRL Published Project Report - PPR 392 Precautionary salting trials on the M62 at Goole (added March 2012)

"Two salting trials were carried out on a three-lane section of the M62 near Goole.
Trial 1 was with untreated dry 6.3mm rock salt and Trial 2 was with dry 6.3mm rock salt treated with an additive based on an agricultural by-product.The de-icer was spread with the trial site open to traffic. The site was closed to traffic 2 hours after spreading and the residual salt was collected from four 1m long strips. After the salt recovery, the site was reopened to traffic until 25 hours after spreading. The site was then again closed to traffic and the residual salt was collected from another four strips. It was estimated that 14.6g/m of the untreated salt and 17.0g/m of the treated salt was spread to Lanes 1 to 3. After 2 hours of trafficking, the salt loss in these lanes was estimated to be 59 per cent for the untreated salt and 51 per cent for the treated salt. After 25 hours, the loss of salt was estimated to be 73 per cent for both de-icers. Most of the residual salt that was present 25 hours after spreading was trapped in the surface voids of the negatively textured surfacing."

Author      RW Jordan and MG Evans     Pages  56
Date  09/12/2011      Reference  PPR392       ISBN  978-1-84608-990-9       ISSN    0968-4093     Hard Copy Price  45.00      PDF Price  35.00


 I have not yet been able to read the full report, but the abstract included here, and I acknowledge the copyright of the TRL, suggests it is a very useful and interesting report and well worth study by any engineer responsible for providing cost effective Winter Maintenance (sorry Service) of a highway network.
It is likely the full report contains more in depth information on the nature of the materials/products used in the trial and the characteristics of the road surface on which it was spread.
At 35:00 from the TRL this seems an inexpensive purchase, and I am not on commission from the TRL.
But do not forget that the TRL is now self funding and at this time I am unsure as to who paid for the trail, it may have been the Highways Agency, it may have been the company selling the particular products.
However I usually find most TRL reports useful and informative if you read them fully, sometimes by what  not included, as well as what they do contain, so I still suggest it is better to have this report and study it rather than to ignore it.
 

TRL Published Project Report - PPR512 The feasibility of brine spreading on the Highway Agency's road network (added March 2012)

This is a comprehensive 145 page report on the feasibility of brine spreading as a form of providing Winter Service (Maintenance) on motorways and trunk roads.
It is extremely well presented and will be useful to anybody considering this form of ice prevention/removal on their highway networks.
However my main reason for including reference to this report here is to draw your attention to the various sections that refer to the retention of salt on the various bituminous mixtures that constitute the road surface course, it is interesting and has practical relevance.
I found that if you navigated the appropriate pages on the "Research Compendium" section of the Highways Agency's website you can download a free copy of this report, my usual advice prevails, if you want a copy get one quickly before the opportunity "disappears".
 

TRL Published Project Report - PPR387 Precautionary salting : A review of spread rates and their effectiveness (Draft) (added March 2012)

This is another comprehensive and interesting report (71 pages) from the TRL relating to Winter Service and salt spreading rates for the motorway and trunk road network, and draws on work done in a number of European countries.
I again refer you to the section that relates to the effectiveness of spread rate on particular road surfaces of differing types of bituminous mixture, i.e. porous and none porous materials.
This report has a copyright date of January 2011, but no "signing off" date and each page is headed "draft", so I would imagine this is still a work in progress, unless there is a final version of this report that I am not aware of.
However copies of the draft are available to download from the "web" if you search appropriately, so I can only draw the conclusion that somebody wishes the report to be widely available for discussion, but I suggest that you seek a copy without delay.

TRL Published Project Report PPR459 - Proposed precautionary spread rates (of salt) for the Highway Agency's road network (added January 2013)

I include here the summary (abstract) provided by TRL on their website, and in doing so acknowledge their copyright.

Spread rates for precautionary treatments with dry and pre-wetted salt were proposed for the Highway Agency's road network. A literature review was carried out to determine,
(i) the spread rates used for the precautionary salting of roads in Europe and,
(ii) the factors that need to be taken into account when determining rates.
Road trials were carried out on the M62 and the M180 to determine the effect of trafficking on residual salt levels after spreading dry untreated salt, dry treated salt and pre-wetted salt.
The effect of trafficking on water film thicknesses was determined in road trials and from a literature review.

The proposed spread rates were calculated using the sodium chloride - water phase diagram to determine the freezing point temperature of the brine formed from the salt and water assumed top be present at road surfaces after trafficking.
Lower spread rates were calculated for heavily trafficked areas than for lightly trafficked areas because of the different salt losses and water film thicknesses assumed.
The proposed rates compare favourably with those used elsewhere in Europe at road surface temperatures not less than - 4C, but they tend to be higher at lower temperatures.
Further guidance was recommended for inclusion in the Network Management Manual.

The very good news is that there is no reason why you do not obtain a copy of this report and read it, now that TRL provide access to .pdf downloads of their reports free of charge, this means most of their recent reports, after 1995, and indeed some from before 1995. Read the Winter 2012-13 Newsletter for details.
In my opinion it is good reading for the experienced and the "new comer" involved in winter maintenance, sorry winter service.


A BRIEF EXPLANATION OF THE TWO HIGHWAY NETWORKS, AND THE IMPLICATIONS TO THE WINTER SERVICE

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 these roads will receive the greater effort to be kept open.

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.
Local authorities will endeavour to keep their main highway network open, and on their rural network at least one route into and out of all larger villages, more precise information can usually be found on individual authority websites.

Click to enlarge this 20mm. "porous asphalt" that is a HAPAS approved product.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 proprietary 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


[ Top of Page ]