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 rare if you employ people at ALL levels who are knowledgeable 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 includes a regular calibration of all machinery, and the routine testing of the materials used in the various highways maintenance processes.

CONTENTS

THE PURPOSE OF SURFACE DRESSING
A STUDY OF THE TYPE OF FAILURE ILLUSTRATED IN THESE PICTURES - LOSS OF CHIPPINGS
A DISCUSSION RELATED TO POSSIBLE CAUSES OF THIS TYPE OF SURFACE DRESSING FAILURE
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 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 - LOSS OF CHIPPINGS

A DISCUSSION RELATED TO POSSIBLE CAUSES OF THIS TYPE OF SURFACE DRESSING FAILURE

• Q - Did the existing surface, which itself was a surface dressing , need to be surface dressed again ?
  A - I would suggest it did not, it appears a good dressing with significant texture.
  
• Q - If it was to be surface dressed was the correct size chipping chosen ?
  A - I happen to know the traffic details of this site, which were quite light, this also appears to be a hard stony surface and I would not have thought anything larger than a 6mm. chipping would have been appropriate for this site. 
  
• It is my opinion the chipping used here is a 14mm. or at least a "bold" 10mm. chipping.
  
• You also need to ask yourself if any hardness tests had been performed on this surface. 
  
• Q - Was a sufficiently high rate of spread of binder used on this site ?
  A - Ignoring my previous comments I see little evidence of a rate of spread of binder that would have been necessary to "hold" this size chipping on the existing surface.
  The stripped areas do not show any evidence of "blacking up" which I think would be be present if chippings had been lost through a wet weather failure soon after laying, in fact there appears to be remarkably little binder remaining on the old road surface.
  

Of course the above are just the initial questions that carry you on to further study, as why certain situations may have come about, such as why was there insufficient binder, was it :-

• 1  -  Incorrect functioning of the speedometer of the sprayer.
• 2  -  An incorrect speed chart for the particular sprayer,
• 3  -  A change in type of binder used that had not had a change of spray rate, i.e. sprayer speed, indicated to the sprayer driver.
• 4  -  A plant malfunction such as blocked spray bar jets or change in the pressure maintained in the bitumen tank.

If you have Quality Assurance procedures in place you should have trace-ability, and you should, in theory, be able to determine the cause of the failure. 

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.
A more complete understanding of the surface dressing process can be achieved by accessing the pages indicated below.

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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