LAYING A THIN BITUMINOUS WEARING COURSE ( FOUR YEARS LATER )

CONTENTS

Introduction
What is failure
Background
Discussion by photographs
Conclusion
Personal Comment

This site, at the time of laying the surface course
This site, six years later, and the adjacent site eight years later, summary

INTRODUCTION
This is the same site four years later showing how rubberised 6mm. medium textured surface course laid 20mm. thick, or less, has coped with the traffic on a very busy urban road with a great deal of traffic making a sharp right turn, often from a stationary position.
There follows a number of photographs to illustrate a number of separate points.
I do know how to take a photograph to show a road surface at its worst, i.e. when a damp road has almost dried out but with moisture still remaining in all the cracks. 
On a completely dry day much of the failure that you can see here would not be immediately apparent, and none of the failure can be regarded as hazardous to the motoring public, i.e. there are no deep potholes, that is after one small area of material loss having received a temporary repair with "tubbed" material that has proved to be permanent.

It is also VERY important to determine what is failure in a particular situation, and what has been its cause.

Although this site does not look very "pretty" it is a fact that if you added up all the areas that show failure, for what ever reason, it would probably amount to only 10% of the site, not an insignificant figure, but we have not yet discussed the reasons for the failure. 
It is also interesting to note that although some areas have a significant amount of cracking in them the surface course has not become detached from the old road surface, now will you believe me how important a good tack/bond coat is.

WHAT IS FAILURE
I looked in a number of dictionaries to obtain a suitable meaning of failure and the one I thought was the best available for this article was quite simply an "unsuccessful thing", but then  of course we have to decide what is success.

Therefore I have decided to determine my own meaning with regards to what can be called success in relation to the use of a particular bituminous mixture that is laid thinly, i.e.  20mm. or less.
This definition is :-
" A bituminous mixture intended to be laid thinly shall be formulated such that it will last at least seven years in areas not subject to underlying road pavement weakness."
In my opinion a bituminous surfacing is failing, has failed, when there is loss of surface course material, or deformation of the surface course, for no other reason than the action of vehicle wheels trafficking the surface.
Now before some people start getting upset there will be exceptions, but I believe I have stated a reasonable definition to judge success or failure in most "normal" conditions.

BACKGROUND
Before reading on it might be a good idea to read the page discussing this site, and the use of a truly thin overlay, at the time of laying,
LAYING A THIN BITUMINOUS WEARING COURSE,
especially the section on points to be considered when taking this course of action.

DISCUSSION BY PHOTOGRAPHS
 I am not going to write a long text based discussion on various problems on this site, I am going to leave it at the stage of notes attached to the following photographs to elaborate various topics.
It is my opinion this is not a failed site, i.e. the total area laid does not need to be removed and replaced at this time.
However it is a site that has areas of failure that will need to be addressed fairly quickly to prevent progressive deterioration of the surface..
This site is just one of a number of sites that have received this very cost effective treatment, and is the one showing the most problems, but it is also one of the most highly stressed.
So, I have chosen the worst case scenario to make you think before adopting this course of action, I could have course shown you a number of sites where the process has been completely successful, but there would not have been as much to learn from them.

Photograph 1  This is an overview of the majority of the site although there is about 50 metres behind the camera that is in pretty good condition. Particularly look at the area in the centre of the picture where standing traffic turns sharp right in to the road seen on the left. Also note the area where standing traffic turns left as it exits the road on the left. These are areas of concentrated turning and scrubbing from vehicle wheels and we do have small areas of surface course loss.

Photograph 2 This is a more detailed picture of the central area of the photograph above showing the small area of complete loss of the surface course, with the old road surface visible. The thickness of the surface course is the 20mm. nominal thickness required, but no thicker. It is also possible to see straight line cracks that will be associated with underlying utility works. I personally do not regard the cracks as failure of the surface course material.

And you will find a disclaimer to this effect with most if not all HAPAS approved Thin Surfacing bituminous mixtures, relating something along the lines of, "the two year guarantee excludes failure caused by underlying pavement weakness". In all honesty I do not disagree with this disclaimer as long as the "small print"  was pointed out to the purchaser of the product at the time of recommendation or sale.

Photograph 3 This picture illustrates an area of failure induced by underlying weakness that can be attributed to poor quality reinstatement of utility works. This may be due to layers of base, binder course, and wearing course less than is specified in the Specification for the Reinstatement of Openings in Highways. But as often as not the layer thickness will be adequate, but the compaction of the layers especially the general backfill and sub-base leaves a lot to be desired.

But supervision of trench reinstatement is a complete subject in itself that I will leave for another time, but everybody knows it is probably the biggest cause of road failure in urban areas, even if few will openly admit it.

Photograph 4 This picture is interesting because it shows "fretting" failure of an area of surface course material that abuts to a "rip" of surface that has not failed. Is this failure because the material was laid against a length of material that was cold, with the paver screed plate riding on the cold material so that full tamping and full compaction by the rollers was not achieved. So that this area would be poorly compacted and fail early.  Or does this small area consist of substandard material, for what ever reason, compared to the bituminous mixture adjacent to it.

I was present at the time of laying of the bituminous material on this site, and it is a fact that one lorry load of material was rejected because it did not contain the rubber latex modifier. This was after some tonnage had passed from the lorry body on to the road and it is quite possible this area is that which received the unmodified bituminous material. This is now four years after laying, before problems are becoming apparent, so trying to pursue some sort of a claim would be quite silly, you have to put it down to "one of those things", and be ever vigilant with work that is happening now. But again it does show how important good site supervision is, and without the technician who noticed the behaviour of the material being laid, i.e. it did not behave as if it had a latex modified binder, all of the substandard material would have been laid. An experienced person really  can tell when a supposedly rubberised material does not have the rubber added, it is that obvious to the experienced eye.

Photograph 5 This area of failure  is absolutely classic, "alligator", "scale", "plate" road pavement failure, you must use which descriptive term you think most appropriate. This appearance indicates general underlying road pavement weakness and it will be interesting to dig trial holes in this area in the future to determine the cause of the failure. Fortunately this type of failure is very localised and occurs where the new (although now about 15 years old) major estate road access joins the long established main road, such areas can always be a potential problem.

Photograph 6 This is a picture of a similar junction just 300 metres further down the same road, and it was surfaced just prior to the previously described junction. It also has a right turn lane where traffic waits before turning, traffic here includes a significant number of HGV's avoiding the town centre. The previous junction takes almost all private vehicles to large residential estates.  This site was overlaid in a similar manner to the previously shown junction and is still completely sound, with the exception of one or two "straight line" cracks from underlying utility trench work.

Photograph 7 This picture is concentrating on the turning area, showing there is no failure at the present time. It is good to point out that excellent work practices at the time of laying has created an almost perfect joint between the new thinly laid material and the existing hot rolled asphalt wearing course on the road that abuts the new surface. The quality of this joint is emphasised by the highly stressed situation in which it is located. There is little excuse for poor joints in bituminous material, failed joints have not been constructed correctly.

Photograph 8 This picture shows, in detail, the nature of the majority of the road surface on both sites. It is now a completely exposed aggregate surface.  The small surface voids that you can see are typical of a 6mm. medium graded bituminous mixture.    You may wish to refer back to the photograph of the newly laid material, on the original page, as an interesting  comparison between newly laid material and an established surface.  It will be apparent that the surface has actually tightened, although some voids will be filled with road detritus.

CONCLUSION
I leave the conclusions to you, I have talked over all the problems that can be encountered with this system, you have to decide if laying a bituminous mixture thinly is suitable for your sites.
But let me once again go over the reasons why this process was adopted, with particular reference to these two junctions.

• The remainder of the main road was to receive a surface dressing to extend its life.

• The nature of these junctions would have made it unwise to have surface dressed them, the probability of failure of the surface dressing due to turning/ screwing traffic would have been high and would have put at risk the rest of the surface dressing due to carry over of exposed bitumen.

• The junctions did need maintenance.

• We could have planed off a full 40mm. and replaced with hot rolled asphalt (HRA) or stone mastic asphalt (SMA) but that would have doubled the cost of each site.

• This would have reduced the number of other sites that received required maintenance that year, budgets are fixed and tight, decisions are made to make the best use of funds available to keep the highway network in as good overall condition as is possible.

Finally, and I hope it was apparent from the photographs, these were not "easy option" sites, they were chosen to put the material and how it was laid to the test. I have also taken one of the oldest, and the least well performing sites to highlight the problems that can be encountered.
I hope you are able to learn from this information.

PERSONAL COMMENT
I would just like to make one more observation which is not particular to this site, but relates to all urban road networks.
It seems to me that choosing the truly long life (20 years) option of a 40mm. surface course of HRA or SMA in urban areas may not be the most sensible course of action when streets seem to be continually dug up by utility companies with little prior notice. 
Two of the recent sites using a 6mm. thin overlay had major trench works in them within 18 months of  the surfacing, as far as I am aware we had not received prior notification that this work was to take place.
What had been two very successful areas of surfacing now look spoilt, and will almost certainly be weaker than before the trench work took place.

For more information on British Standard bituminous mixtures that can be laid thinly, press ----------------> HERE

For more information on proprietary "Thin Surfacings", press ----------------------------------------------------------------> HERE

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