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The Idiots' Guide to Highways
SURFACE DRESSING BY PICTURES - SOME BASIC KNOWLEDGE ON THE PROCESS
|INTRODUCTION||SURFACE DRESSING CHIPPINGS|
|THE FINISHED PRODUCT (SURFACE DRESSING)||DEPOT TRAY TEST (Checking performance of the sprayer.)|
|SPRAYING BINDER||SPREADING CHIPPINGS|
|THE BINDER ( BITUMEN EMULSION or CUTBACK BITUMEN)|
|EXAMPLES OF POOR PERFORMANCE||POSSIBLE CAUSES OF POOR PERFORMANCE|
When you read much of what I write on surface dressing it may seem a daunting process, with possible dire consequences, so you may find it strange to hear that I like the process of surface dressing. It is a most cost effective process, and a well established surface dressing provides an impervious, durable and safe road surface. But, it has got to be performed correctly on suitable existing road surfaces, and the more people know about the process the more successful it is likely to be.
It is also likely that as much as 75% of roads in rural counties, and this will include some principal "A" roads, are likely to be well established surface dressings that the motoring public no longer associate with the somewhat "unpopular" initial laying process.
It would certainly be a lot less "unpopular" if ALL members of the motoring public would help by abiding by the advisory speed limits at the time of, and soon after, laying.
Probably the biggest rule of surface dressing is not to surface dress sites that are not suitable for surface dressing.
But this type of decision is part of highways maintenance, not just relevant to surface dressing, i.e. you would not put a thin, paver laid, bituminous mixture surfacing on a road that had underlying structural weakness, not if you wanted it to obtain "best value", as the process would be expensive and its' life would be short.
Surface dressing is definitely not a "slap dash" process as most of the general motoring public perceive it, there is a great deal of engineering knowledge and practice that is combined together to produce a successful surface dressing.
This page is a concise bringing together of the fundamentals of the surface dressing process, for a more in depth description and explanation of the process access the,
Practical Guide to Surface Dressing,
where the process is discussed in some depth, along with a list of relevant publications where you can obtain a large amount of further information.
PRODUCT (SURFACE DRESSING)
The object of surface dressing is to create a stable mosaic of chippings securely attached to the road surface.
This is achieved by spraying the correct amount of bitumen onto the road surface followed by the appropriate amount of the correct size of chippings according to the softness of the road surface.
Once you have established the hardness of the road surface, and you know the amount and type of traffic, you need to consult TRL Road Note 39 to design the appropriate dressing for the site.
BS 63:Part 2 : Specification for single-sized aggregate for surface dressing.
Sets limits for "chippings'' e.g. grading, flakiness index, aggregate strength.
See Note (2) below
Most aggregate properties ( polished stone value, PSV, aggregate crushing value, ACV, aggregate abrasion value, AAV ) will be related to site conditions, e.g. amount, weight and speed of traffic coupled with the characteristics of the actual site, e.g. tight bends approaches to junctions etc..
This process does allow the most economic use of high specification (high PSV) aggregate on perceived potential accident areas where the process is suitable, i.e. bends on roads in rural areas.
Guidance on the required properties of the aggregate for a particular site can now be found in the latest edition of,
TRL Road Note 39, ( the 6th. Edition was published early in 2008, keep up to date with this publication )
Design Manual for Roads and Brides - Volume 7:Pavement Design and Maintenance - Section 5:Surfacing and Surfacing Materials - HD 36/99:Part 1:Surfacing materials for new and pavement construction, (in particular, Tables 3.1, and 3.2)
The image above shows the edge of a surface dressing application, showing the original surface, the sprayed layer of bitumen binder, and the applied layer of surface dressing chippings, in this case 6mm. chippings.
The surface course receiving the surface dressing in this case is a hot rolled asphalt (HRA) and 20mm. precoats, and although this small area is in good condition the surfacing was of some age, over 15 years old, and the overall surface was starting to fret but without serious failure of the matrix of the 40mm. layer. Therefore surface dressing is the appropriate coat effective maintenance process to seal and bind the fretting surface, and if applied at the correct time to provide another 10 years of life from this surfacing material.
Some would question the use of 6mm. chippings, but in this case the surface had a high rate of spread of 20mm precoats, and had been in place for 15 years without any sign of wheel tracking. So you did not really even have to perform the hardness test, to know this was a hard surface and that there would be little embedment of chippings.
However the penetration/hardness test was performed and confirmed it as being a hard surface, with 6mm. chippings being an option.
If the surface course material matrix, i.e. the HRA, continued to retain its durability the 6mm. surface dressing could be used as a "pad coat" for a larger chipping in the future.
You could have chosen a 10mm. racked in 3mm. if you wished, but we regarded the increased cost as unnecessary.
10MM racked in 3mm. would not have been wrong but may cause vehicle tyres to generate slightly more noise.
The organisation that I worked for had done serious trials/research to indicate that 6mm. surface dressing on roads with good ride quality was a surface that was amongst the "best" surfaces for causing lower levels of tyre generated noise.
The binder is sprayed either as a cutback bitumen, or more usually as a bitumen emulsion.
Spray bars are becoming more and more automated with regard to spraying width, but there are still a lot of sprayers about where the spraying width is controlled by the operator manually switching individual spray nozzles, on the spray bar, on and off to achieve the correct spraying width.
The rate of spray from the spray bar will be at a constant rate dependent upon the viscosity / grade of the particular binder and the temperature and pressure in the tank holding the bitumen.
Actual rate of spread of binder deposited on the road will be dependent upon the forward speed of the sprayer.
THE BINDER (BITUMEN EMULSION or CUTBACK BITUMEN)
BS 434:Part 1 : Specification for bitumen road emulsions.
Composition and properties of emulsions, including those used for surface dressing.
BS 3690:Part 1 : Specification for bitumens for roads and other paved areas.
Composition and properties of bitumens, including cut-back bitumen for surface dressing.
BS EN 12591 : 2000 : Bitumen and bituminous binders : Specification for paving grade bitumens ,
has superseded BS 3690, but reference to BS 3690 may still be present in some contract documents.
SURFACE DRESSING CHIPPINGS
The chippings need to be of the correct size appropriate to the softness of the road surface and the nature of the traffic that drives over the road. (Consult Road Note 39)
It is also most important that you select the aggregate source from which the chippings are produced to provide the required Polished Stone Vale (PSV) and the Aggregate Abrasion Value (AAV), these values will depend upon the speed of the traffic on the length of the road, the nature of the site (e.g. tight bends), and the type of traffic.
If you have not yet acquainted yourselves with the change in the way surface dressing chippings are specified I feel you should do so as quickly as possible.
With the introduction of,
BS EN 13043 : 2002 : Aggregates for bituminous mixtures and surface treatments for roads, airfields and other trafficked areas
PD 6682-2 : 2003 : Aggregates - Part 2 : Aggregates for bituminous mixtures and surface treatments for roads, airfields and other trafficked areas - Guidance on the use of BS EN 13043,
replacing BS 63, although BS 63 was withdrawn in June 2004, it has brought a significant potential change in the nature of the chippings you could be receiving, especially with regard to increased amounts of "oversize" chippings.
|DEPOT TRAY TEST (Know the amount of binder you are spraying, and its evenness of distribution.)|
|This is the depot tray test where the sprayer is run for one minute at standard temperature and pressure settings, in to the main tray which consists of a number of individual and separate long narrow strips. By dipping each strip the evenness of distribution can be determined and by addition of all individual dips, and knowing the tray dimensions, the overall rate of discharge of the spray bar can be found.|
1707 : Hot binder distributors for road surface dressing.
Gives definition of what is required on the binder "sprayer'' to fulfill specification, details of Depot Tray Test, sampling recording sheets, etc..
However I believe the Depot Tray Test is soon to be discontinued as British Standard, see Note(1) above, so I am no longer too sure how you obtain Binder Sprayer certification / accreditation.
It is necessary to know the rate of discharge of the spray bar for each grade of binder it is proposed to use so that you can establish a "speed chart" for the sprayer to achieve particular amounts of binder sprayed on each square metre.
Speed (metres per min.) = rate of binder delivery in litres per min. / [spray width (metres) x binder depth (mms.)]
N.B. 1 litre per sq. metre = 1.0mm. depth of binder | 1.5 litres per sq. metre = 1.5mm. depth of binder | ETC.
A test to replace the depot try test is the carpet tile test, although the carpet tile is now an absorbent pad, this is a test performed on site and reduces the amount of time the sprayer is not actually spraying, personally I prefer the Depot Tray Test.
This method of spreading the chippings means you can still be traveling forward, and with the wheels of the spreader traveling over chippings not bitumen.
The chipping spreader actually tows the lorry containing the chippings and the chippings are slowly discharged into the hopper of the chipping spreader.
They are then conveyed up to the spreading mechanism and spread onto the bitumen in front of the spreader wheels.
There are still a few "tailgate" spreaders which are actually attached to the back of the lorries that transport the chippings.
The lorry has to travel continually in reverse with an operative walking by the side of the tailgate controlling the rate of discharge with a ratchet mechanism attached to the tailgate.
This is not a particularly accurate way of controlling the rate of spread of chippings.
OF A SUCCESSFUL 6MM.DRESSING
The image to the left is a photograph of a successful 6mm. surface dressing, about one year after dressing, click the image to produce an enlargement.
I hope that you will notice that the use of a "good" 6mm. chipping does produce significant macrotexture, but do not fail to choose a chipping of appropriate polished stone value (PSV), and appropriate aggregate abrasion (AAV) for the site conditions.
Do not offer a general poor criticism of 6mm. surface dressings just because you have designed them badly, or applied them to the wrong surface, or the wrong site.
In my opinion a 6mm. chipping is the best option for the first dressing on most "hard" surfaces, and that includes 20mm. binder course as a strengthening running surface, on rural roads.
|THESE ARE EXAMPLES OF WHAT YOU DO NOT WANT TO HAPPEN|
There are many reasons
for failure, but directly above is an example of what is likely when there is not
sufficient binder to retain the chipping on the road surface, was the design
incorrect or was there a failure to spray the amount of binder
picture on the left shows serious "fatting up" failure, so much
so that it can be seen that areas have become so binder rich and
"sticky" that they have been picked out by vehicle wheels.
This can be for a number of reasons, but one of the most common being that a particular road has received a number of surface dressings over many years and with a particular hot spell of weather will cause the accumulated binder to "flush" to the surface in the wheel tracks and cause the problems shown.
POSSIBLE CAUSES FOR SURFACE DRESSING FAILURE
SURFACE DRESSING fails for many reasons and it MUST be said most of the causes of failure can be controlled and avoided.
Let me first say failure is RARE, it really is, but if you know why failure may occur hopefully you will have procedures in place to prevent it happening, (this includes the regular sampling of supplied binder and chippings).
It is not a bad idea to have further procedures on "standby" in case of emergencies, e.g. the availability of clean grit and the means to spread it if a site has catastrophic "pick up" failure under traffic conditions, these are the failures the newspapers always likes to report.
Get the exposed / fresh binder covered with grit or more chippings as soon as possible, worry about remedials after you have made the site acceptable for immediate trafficking, even close the road if possible.
For those of you with little experience of SURFACE DRESSING I would recommend you first read and digest Road Note 39, and TRRL Report 627, and other reference publications.
If you do not have immediate access to them you need to build up your library, and seek help from a knowledgeable and experienced INDEPENDENT source.
BUT I will list as many factors as possible for you to consider, BUT in some cases (and ONLY some) the failure can be obvious:-
 Road hardness assessed wrongly.
 Wrong size chipping used.
 Wrong rate of spread of BINDER chosen.
 Different rate of spread of BINDER actually sprayed to that stated in the design,
a) Speedometer of sprayer inaccurate.
b) Incorrect "rate of spread speed chart" for a particular sprayer.
c) Tank pressure incorrect
d) Viscosity of binder used was not that specified
 Wrong BINDER VISCOSITY specified for the ambient temperature.
 Badly formulated BINDER e.g. unsuitable BASE BINDER.
 Poor traffic control, i.e. traffic allowed to go too fast or make turning movements on a dressing before it has had time to become stable.
 Site not suitable for surface dressing, e.g. areas of "screwing" heavy traffic tearing dressing.
 Ambient temperature too cold, binder not bonding to road surface, chipping not bonding to binder layer.
 Ambient temperature too hot, binder not able to cool sufficiently to achieve a viscosity sufficient to hold the chippings in place.
 Not enough attention paid to the weather forecast, caught out by thunderstorms on fresh dressings.
 Pre-dressing patching has been carried out using too soft a binder in the bituminous mixture, e.g. a cutback binder for ease of use at the time of patching.
 Dirty/dusty chippings, chippings unable to establish a physical bond with the binder, dressings using dusty chippings can fail quite some time after laying, especially after rain when water tends to follow the dust coating around the chipping, and chippings can strip out under trafficking.
For a Practical Guide to
Surface Dressing, where the process is discussed in some depth,
and a list of publications where you can obtain a large amount of further
information press, HERE.
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