Let us introduce a bit of honesty into the subject of failure of any road treatment straight away. In my opinion there are very few failures that just happen, there is usually a reason, or an accumulation of small reasons, that contribute to a failure. I am aware that in some circumstances things may occur that cannot have been anticipated and prepared for, but I believe these situations are not common if you employ people at ALL levels who are knowledgeable and experienced in what they are doing. It is also my belief that we all need to pay attention to the many small boring bits of any process for the "whole" work to succeed. This starts with an appreciation of site conditions, but also includes a regular calibration of all machinery, and the routine observation and testing of the materials used in the various highways maintenance processes.  And, it has to be faced that some highways maintenance processes are not suitable for some highway situations, this does nor mean to say the process itself is not a good treatment for other appropriate situations. It is the responsibility of the highways maintenance professional to ascertain an appropriate course of action for each site, subject of course to the budget that is available.  May I also stress that surface dressing failures are rare, they really are, but when they do happen, for what ever reason, they usually make the local press, and the miles and miles of successful dressings providing cost effective and safe road surfaces go ignored.  I would "guesstimate" that maybe 70% of all none motorway roads we drive on in the U.K. are surface dressed, and remember approximately 95% of the road network in the U.K. is none motorway or trunk road.

CONTENTS

THE PURPOSE OF SURFACE DRESSING
A STUDY OF THE TYPE OF FAILURE ILLUSTRATED IN THESE PICTURES
POSSIBLE CAUSES OF "FATTING UP" OF SURFACE DRESSED AREAS
CONCLUSION

THE PURPOSE OF SURFACE DRESSING

The object of surface dressing is to create a stable mosaic of chippings securely attached to the road surface, this provides a comprehensive seal to prevent the ingress of water in to the road pavement, and a fresh hard wearing, well textured, skid resistant 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.

A STUDY OF THE TYPE OF FAILURE ILLUSTRATED IN THESE PICTURES

	"FATTING UP" The condition know as "fatting up" whatever the cause is demonstrated by the image to the left where you can see "free" binder on the surface of the road with little or no texture. What aggregate is showing is flush with the surface. I have pushed the chippings in to the surface to demonstrate the considerable amount of binder that can be present on a severely "fatted up" road. In some situations, and only some, this "free" binder can be utilized to perform a "Sandwich Dressing" to rectify the fatting up problem. This is a process that needs to be performed by experienced engineers/contractors  in accordance with Road Note 39. It is often more appropriate to consider the option of a thin bituminous overlay using a bituminous mixture equivalent to a BS 4987 10mm. size close graded macadam, but from 01/01/2008 specified in accordance with BS EN 13108:Part 1. It will cost a little more but you gain the benefits of adding a small amount of strength and the improvement in ride quality that comes with "paver" laid bituminous mixtures. And it is a more or less a guaranteed process subject to the ability of your paving contractor.    Your Highways Maintenance Engineer/Materials Engineer should be able to advise you on these and other options.
	FATTED UP SURFACE DRESSING IN RURAL AREAS This image is fairly typical of "fatting up" in rural areas, in as much as the "fatting" occurs in fairly well defined channelised wheel tracks. This "fatting" can be for a number of reasons, the main one being just a sheer build up of binder over a number of subsequent dressings over a long period. Each dressing will have been designed to retain chippings over the whole width of the road, therefore the wheel tracks have likely received too much binder to enable the chippings to be retained in the less frequently trafficked areas. In the course of time and especially during periods of extremely hot weather (we do get them from time to time in the UK) the chippings will be pushed in to the, now, binder rich matrix displacing binder to the road surface. In some areas you can see evidence that patches of surface have actually been pulled from the road surface because it has "stuck" to the wheels of passing traffic. It is roads such as these where paving with a bituminous mixture equivalent to BS 4987 10mm. close graded macadam, but now specified to BS EN 13108:Part 1, is a suitable option if funds permit.

POSSIBLE CAUSES OF "FATTING UP" OF SURFACE DRESSED AREAS (Including errors in the design process) Wrong assessment of road hardness Too high a rate of spread of bitumen, and there can be a number of reason for this other than poor design Incorrect speed chart for binder sprayer Incorrectly calibrated "fifth wheel" or speedometer of spraying vehicle Incorrect tank pressure Change in the spraying viscosity of the binder Change in the base viscosity of the binder Using a smaller chipping than the road hardness testing indicated Roads where surface dressing as been the only maintenance treatment for many years. Roads where the underlying bituminous material has a low viscosity (i.e. high penetration grade or cut-back binder, this can often show itself in patches where a bituminous mixture with a cutback binder has been used for ease of laying by hand.
	FATTED UP SURFACE DRESSING IN URBAN AREAS The photograph shows a not unusual occurrence when faced with urban streets. It is necessary to balance the of rate of spread of applied binder for roads which will have two distinct areas, i.e. areas of trafficking and areas that will mainly be parked cars. It is often not possible to establish a rate of spread that will retain the chippings on an area that has little traffic as well as not be too high a rate of spread for the trafficked areas, such that the binder flushes to the surface. BUT, this may not be a binder problem but a size of chipping issue.  A larger chipping may have been unsuitable to achieve embedment in the parked area. The choice of a smaller chipping meant that it was unable to prevent embedment on the trafficked area.   The fact is it is very difficult to surface dress sites that have areas of differing traffic and surface conditions.
	UNDERLYING SURFACING MATERIAL AFFECTING SURFACE DRESSING This photograph clearly shows a difference in hardness of road surface that has been surface dressed. The area to the left shows a trench reinstatement by a utility company that has been surfaced with bituminous material that is "softer" than the existing road pavement. The chipping size for the main area of the road has been correct and the dressing retains good texture without stripping. The chippings on the reinstated area have been pushed in to the road surface by the traffic, and the surface has become binder smooth. I cannot say whether the surfacing material used by the utility contractor was a permitted option in the Specification for the Reinstatement of Openings in Highways, it probably was because there are a number of permitted materials and binder viscosities. Often, to the "utility" surface contractor ease of laying, and compaction, provided by the use of mixtures containing cutback binders, is more important at the time of laying, than thinking about possible future implications of his choice of material.

CONCLUSION

The last thing I want is for this page to be used as a reason for not using surface dressing as an extremely cost effective option in maintaining highways.
Surface dressing failures are rare, they really are, if you employ a competent contractor with a proven history of good work.
However I thought a few photographs and a bit of a discussion would be useful to alert everybody to the fact that surface dressing is quite an intricate process and not the "slap-dash" operation the public seem to think it is.
The process needs good supervision from a knowledgeable contractor and client, and I believe the cost of a client supervisor working alongside the contractor is money very well spent considering the cost of the average local authority surface dressing contract.
More understanding of the surface dressing process can be achieved by accessing the pages indicated below, but you need to have actually been involved for a few years to really get a "feel" of what is involved.

It is also a fact that some sites are best not surface dressed, they do not lend themselves to the process. 
It may be that the budget of the authority does not allow for anything other than surface dressing, then so be it.
But, in my opinion surface dressing contractors do not do themselves or their industry any favours by seeking any type of site as part of their current contract to increase that years profits.
I often give the (fictitious) example that if you had ten miles of superb surface dressing and one hundred metres of failed dressing, the only bit the public would see would be the failed area, so perhaps it is best not to get those extra few square metres of work if they stand a good chance of discrediting all the good dressing you have achieved.

For further information on surface dressing failure where chippings have "stripped" from the road surface press, -----------------------> HERE

For a Practical Guide to Surface Dressing and a list of publications where you can obtain a large amount of information press, -> HERE

For a list of individual items of information all related to surface dressing press, ---------------------------------------------------------------------------> HERE

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