An Early SMA Case Study

HRA Surface Course

EME2 - Enrobé à Module Élevé

Surface Dressing 2009

Newt News

London Marathon

Motto of the Month

Introduction
I said I would return when the daffodils were in flower, and I am as good as my word.
I have been prompted into action a little sooner than I had anticipated because of a piece of excellent news that has come to my attention and I wished to share it with you, immediately.
This is so that you may secure your places as soon as possible.
The IHIE (Institute of Highway Incorporated Engineers) have arranged two courses with the title, "How to get more durable surface courses". 
I believe that this is a most commendable arrangement and one which you know, if you have been keeping up to date with my newsletters, I am in full support.  
It is good to report that one of the major engineering institutions is prepared to take up this challenge.
The speaker is Jeff Farrington, an excellent speaker able to draw upon his vast experience as a long serving former qualified County Materials Engineer. Jeff was also much involved in representing British "highway engineering interests" in the "Europeanisation" of many of our most commonly used and most important highway engineering British Standards, e.g. BS 594 and BS 4987.
( If you are still struggling with the new BS EN standards for bituminous mixtures may I suggest that you rely on,  
P.D. 6691:2007:Guidance on the use of BS EN 13108 Bituminous mixtures -  Materials specifications
to keep you safe. In my opinion, there are defined mixtures that will supply all you needs for a local highway network, but take care to select the most appropriate material. However I digress.)
I used to really enjoy Jeff's "Materials" newsletters which he produced many years ago for the "Northern - Soils and Materials Group", part of the "real" County Surveyors Society, possibly where I obtained my idea for my newsletters, first in my former employment, and now on the web.
Details of the courses to take place in July and October can be found on the IHIE website at,
www.ihie.org.uk/events/courses/how-to-get-more-durablesurfac/
However, I do have concerns, that is why I suggested that you purchase your place as soon as possible.
My concerns being that the bulk of the places will be purchased by the "industry", they often "dominate" so called highway engineering "gatherings" with their presence and turn them into public relation exercises, rather than allowing them to remain a forum for discussion on highway engineering/materials matters. This is why I went to very few during my career.
In my opinion I would like to see the IHIE offering preference to local authority, and consultant, engineers and technicians, as I feel these courses are going to be filled quite quickly.
If these organisations feel they do not need staff with the guidance offered in these courses I hope that it is noted by those awarding the contracts.
Sadly I did hear through the "grapevine" from a "reliable source" that one such similar course that was going to be presented for a local authority was "cancelled" due to contractor influence.
So, purchase your place now, if both dates are sold out quickly hopefully we may get some additional courses and venues, which would be nice.

An Early SMA Case Study
However in the nature of a contribution to discussing "real" performance of "new" bituminous mixtures let me say that I can confirm that I am aware of at least one stone mastic asphalt (SMA) site in my "patch" that is 14 years old (i.e. laid in 1995) that is still in overall good condition. 
It has not been surface dressed, but in my opinion there are early signs that a surface dressing will be necessary in the next few years to considerably prolong the life of this surface course
This was a mixture supplied to the authority as a proprietary 10mm. nominal size "Masterpave"  material, however the authority had required that the material also complied with authority specification for 10mm. stone mastic asphalt which had in turn been taken from, 
TRL Project Report 65:Evaluation of stone mastic asphalt (SMA):A high stability wearing course material, by M E Nunn

"Listen very carefully, I will say this only once", stone mastic asphalt is not necessarily a proprietary bituminous mixture that is part of a "Thin Surfacing Course System" (TSCS). 
"HAPAS approved" proprietary TSCS's became the compulsory surface course for Motorways and Trunk Roads in England in 1998 at the instruction of the Highways Agency (Government), and there are a number of basic designs for numerous "approved" TSCS mixtures offered by many suppliers, one of the basic designs will be an SMA "type".
To try and understand what bituminous mixture surface courses are permitted for use on motorways and trunk roads in England, Wales, Scotland and Northern Ireland, you need to download, 
Part 1 of HD 36-06 : Surfacing Materials for New & Maintenance Construction 
from www.standardsforhighways.co.uk and study Tables 2.2E, 2.2W, 2.2S AND 2.2NI.
(You will note that you can use hot rolled asphalt without restriction on flexible pavements in Scotland and Northern Ireland.)
It may seem boring stuff but it is the "intricacies" of such tables and other "specifications" that can have a profound effect on what bituminous mixtures you are able to use for your surface course, and other pavement layers.
I could not help but notice that there was no mention at all of generic SMA in the tables for England and Wales, thus probably allowing the "public relations" boys and girls to say that no SMA is used on Motorways and Trunk Roads in these areas, which I believe is "playing" with words. 
I hope you are beginning to understand what a "convoluted" situation the industry has created for itself, keeping up to date is not easy unless you have a Materials Engineer with the time, knowledge and experience to keep your engineers and technicians briefed.

This study is of what I would consider a "genuine" SMA, and I will not continue the "Thin Surfing" issue any further in this newsletter, it is discussed comprehensively elsewhere on this website.
But do not forget we now have, 
BS EN 13108-5 : 2006:Bituminous mixtures. Material specifications -  Stone mastic asphalt 
for specifying SMA in its many "modern" forms, hopefully you will opt for a high binder content, low void, mixture similar to the SMA described in this case study.
Also there are several good, basic, defined SMA mixtures in Table D.1 of PD 6691:2007, with the 10mm. nominal size mixture probably being of most general use.

The work on this site was part of a fairly comprehensive study contracted to the soils and materials laboratory of a neighbouring authority, a large facility that was probably one of the best in the UK at that time. 
Sadly after a brief period in consultancy ownership it has now ceased to be, it is no more, with few of the previous engineers and technicians now actively involved in soils and materials testing. So have gone, over recent years, many excellent laboratories, and their collective expertise, and the information that they can provide to the industry. 
My former "bosses" in the former structure of the authority where I was employed wished to "know" about this "new" bituminous mixture before it was considered for general use. 
I as the Materials Engineer had the responsibility, as part of my duties, for the co-ordination of the work and the dissemination of the results.
So there should be, sat in a dusty filing cabinet, somewhere, a full report of this material and site, along with several other similar reports of other sites where the "new" bituminous mixture SMA was laid, studied and compared.

The SMA surface course was laid a nominal 40mm. thickness, as I recall. Over the laying period 4 samples were taken and analysed for composition, all samples met the specification requirements.
Now here comes the really interesting bit, the binder contents of the samples from this site were 6.5%, 6.4%, 6.9% and 6.3% giving an average of 6.53%, the specification requirements being 6.5% +/- 0.6%.
I was only on site for short periods each day assisting with the sampling but I did not see any evidence of excess binder / binder drainage.
Perhaps this is the secret of why this material has remained in such good condition for so long.
Do not forget that binder percentages are quoted by weight, but to determine the volume i.e. the amount of "binding" bitumen, you need to multiply by approximately 2.5 to allow for the difference in densities between bitumen and aggregate.
So that every time you decrease the binder content (weight) in the mixture, you decrease the actual amount of bitumen present in the mixture by much more.
If you do not understand it will take too long to explain here, take the matter up when you are on the course, or with your Materials Engineer. 
The finished surface had an initial texture depth "in the region of" 1.5mm. (sand patch) certainly above 1.2mm., and this improved with trafficking.
The material was supplied from a local quarry at Mancetter, using "in house" aggregate with a polished stone value (PSV) stated at 63. This would mean that all "fine" aggregate incorporated in the mastic coating of the surface exposed, predominantly single sized 10mm., aggregate would also have been from the same source and hence a 63 PSV.
A factor, in my opinion, very important for the early life skid resistance of SMA.
From 1995 to 2006, when I retired, I was not made aware of any skidding problems on this length of surfacing.
All this work had on site supervision from an up and coming Highway Engineer wishing to embrace the introduction of "new" surfacing options into his highway maintenance options.
I would suggest that the industry have let such highway engineers down badly in the subsequent years, after early successful trials, mainly because they have not supplied the same materials, with the same binder contents, laid at the same thickness, therefore comparisons are meaningless.
So, a simple, factual case study, for you to discuss at your next "gathering", but it would be so much better if you were able to bring your own contribution, so increasing the pool of engineering knowledge for all highway engineers to draw upon.

I have chosen this image as a general indicator of the good current condition of the surface course, as junctions are where you will find early indications of problems, and from local knowledge I know that many large car transporters turn here to access a large, "new car" storage facility, at a very large former WW2 airfield.

I apologise for the wet conditions but it was not raining when I left home to travel to this site. But it does show the nature of texture working to provide paths for the water to escape and allow tyre to aggregate contact to provide optimum wet skidding resistance. This image reflects the good quality of the majority of the road surface. It is worth pointing out once again that the original "negative texture" of the surface of an SMA has become a uniform positively textured surface.

You will note that all traces of surface binder/mastic have been removed and vehicle wheels are  traveling on aggregate, thus indicating the importance of aggregate properties.

To the left is an image of, currently, a small proportion of the surface course, in some areas of the wheel track. You will observe that there is some loss of aggregate particles due to oxidation and hardening of the surface binder and shear wear from trafficking. There is no cracking that would indicate failure in the road pavement, this is an early sign of failure, or perhaps more honestly, the end of the life of the surface course. Once this process has begun it will develop fairly rapidly and needs to be judiciously monitored by "real" eyes.

I do not know how SCANNER would interpret this, compared to the texture on the satisfactory  areas of surface course. 
But then I, personally, do not know how SCANNER  interprets most road surface conditions.
I favour the use of engineers, technicians and inspectors with the experience of knowing what they are looking at. But then I am an "old fart", but an "old fart" who does know what he is looking at with regard to road surfaces.
I worry that "scientists" are programming SCANNER, and what is their experience of actual highway maintenance, and what are the parameters that define "pass" or "failure" of a road surface, note road surface.
But I do like Deflectograph used correctly, with correct road pavement information, to determine the strength of the road pavement. However you do not believe Defletograph all the time. It is only a tool to be used and interpreted by actual engineers, even the developer/designer of the instrument has made that statement on a number of occasions, and in a number of reports.
In my opinion, if politicians, and "strategic managers" think they are able to replace qualified and experienced highway engineers and engineering technicians with "clever machines" they are sadly mistaken, but they will not answer back. 
And you can reset the level of failure/success to meet the requirements of the public relations "people", somebody mentioned this has already happened at least once, but it is "grapevine" talk so I cannot verify it.
It may also have been to "correct" earlier errors, my point is that once you start playing around with parameters, what is correct and what is not. In my opinion there has not yet been sufficient use, and follow up of suggested treatments, for this new technology to be stated as proven, either in engineering terms or the cost effective use of the budget.

I thought that I would include this image to give you an indication of the appearance of the original surface. This is possible because this image is of the surface in an adjacent layby, the surfacing material being the same as the main highway. It is drier because it was the area beneath my parked car during the rain. You can still see the considerable amounts of bitumen/mastic between the aggregate particles as there has been little trafficking to remove it.

And finally, just a simple image to show you the reinstatements to a group of cores that were taken soon after laying to determine bulk density and air void content. Information gathered for the report I mentioned earlier. The image shows that care was taken in the reinstatement  of the core holes for it to have lasted successfully for fourteen years. It also shows the integrity of the matrix of the SMA surface course for no deterioration to have emanated from the core holes.

So often you see that where cores have been taken from poor quality surface course, usually a high void content mixture with a low binder content, that the area around the core holes quickly fails and breaks out. This phenomena, to me, is an an indication of the poor durability of the surface course bituminous mixture.

HRA Surface Course
Let me just remind you that in supplying the above factual item in support of "genuine", "original", stone mastic asphalt surface course, that my preferred surface course bituminous mixture, where traffic conditions allow it to be used, will always be hot rolled asphalt (HRA) with a 20mm. or 14mm. high PSV precoat.
In my opinion it has better engineering properties and is therefore a more cost effective surface course over time, and it allows more economical use of scarce high PSV aggregate.
The image to the left being of a large local contract being carried out this April, just a couple of weeks ago.
Whatever "they" tell you there is still a large quantity of HRA surface course being laid around the UK, on local highway networks, and I am led to believe the amount is increasing.
"Stories" of the demise of this excellent bituminous mixture are much exaggerated, its decline in tonnage used being largely due to the fact that HRA surface is not a Highways Agency permitted surfacing option for motorways and trunk roads.
But I can see it making a return on these roads, at least on islands and highly stressed junctions, because the "modern" proprietary bituminous mixtures do not have the durability without decreasing texture where you need it most, and you just cannot keep causing major disruption to road users every few years replacing the surface course, not to mention the cost.
May I just say that while we have gangs around of the quality that were on this contract you need not have any fear of the quality of job produced, the chipping gang was amongst the best I have ever observed.
The road surfaced is a long feeder road to a large housing estate, carrying a large amount of traffic, often HGV’s that cannot get under the low railway bridge on the main road out of Hinckley.

EME2 - Enrobé à Module Élevé
And, I am sorry, but casting a last glance over the details of the IHIE course I cannot ignore the mention of EME2. 
In my opinion this is still an unproven bituminous material, (remember the problems with the last "stiff" base / binder course the "marketing people" recommended).
Laboratory determined "stiffness" with the associated increase in load carrying ability and reduction in layer thickness is pointless if it will not "survive" more than a year or two before the whole road pavement needs refurbishment.
One TRL trial, and published report, 
TRL Report TRL636 - The application of Enrobé à Module Élevé in flexible pavements,
does not make a whole genre of bituminous mixtures successful, although it will give you an insight into the nature of the material and particular designs, methods of work, and problems, that may be associated with particular sites and working conditions.
So, perhaps you would like to take a look at an established "stiff" bituminous mixture, i.e. a 50/14 or a 50/20 hot rolled asphalt base / binder course, bituminous mixtures with some binder in them, and therefore durable. 
Stiffness is not everything, how about a bit of elasticity, a very important engineering property, especially when surfacing over evolved local highway networks.  
In simple terminology, bituminous mixtures that will "give a bit", even have some elastic recovery, not materials that crack at the first sign of stress.
Many of us have used these British Standard  (BS 594) bituminous mixtures very successfully for decades, and they are very similar to the "original" French EME's, apart from the use of a standard 50pen. binder, instead of a 15pen., 25pen, or 35pen..
The use of these stiffer binders, in my opinion, definitely providing problems with laying and compaction.
You will find current example specifications of these HRA materials in, Table C.1 of, 
P.D. 6691 : 2007 : Guidance on the use of BS EN 13108 Bituminous mixtures -  Materials specifications,
And if you are maintaining a local authority highway it is very unlikely that you will need a binder stiffer than a 50pen. 
(Which will, by the way, be stiffer than a 50pen by the time the bitumen has been in hot storage for a while before mixing, then at elevated temperatures during mixing, and while the "bitumen/aggregate mixture" has been held in hot storage bins, and finally during the time of transportation to site.)
Well, a final "there you have it", a couple of "items" to bear in mind when you attend the IHIE course, something "real" to talk about. 
It is so much more interesting than discussing your next "free" golf tournament, I still do not play golf, and I am not a member of "any" clubs.

(And, as a complete aside, except it is on the subject of compaction of bituminous mixtures, I was fortunate to attend, very recently, a demonstration of an impressive hot mix mobile asphalt recycling plant, which I will perhaps discuss another day. that is not the point of this aside.
But present at the demonstration was a professional photographer with a thermal imaging video camera.
If proof were ever needed to show how important it is to lay material quickly and get it rolled quickly, to achieve full compaction, you only needed to watch the process through this new technology to see how quickly heat is lost, on a not particularly cold April day.
A senior figure from one of the largest utility companies in the country was hoping that a training video could be produced using the video footage to show to laying gangs, to actually show them the heat being lost.)

Surface Dressing 2009
Before I go, do not forget that the 2009 surface dressing season will soon be commencing, it usually starts somewhere in the country in the third or fourth week of April, weather permitting. 
Regular readers know that I like surface dressing as an excellent cost effective road surface treatment. But it does have to be  performed correctly, with good materials, and people who know what they are doing, and such contractors do not come cheap, and you must, yes must, have engineering supervision on site.
But having said all that it is an excellent, safe, surface once established. 
I know that I have said this a number of times before, but I will say it again because I think that it is important. 
In most rural authorities something like 70% of their highway network will have a surface dressed surface, and the motoring public are quite happy with it. In fact they do not even know that it is a surface dressing once it is established. The moral for the contractor is to work as "tidily" as possible ensuring that there are good sweeping procedures in place for the dressings early life, putting the motorist to as little inconvenience as possible during the actual dressing process and the early weeks. Do not upset your "real" customer.

Considering the harsh winter we have just experienced, I have seen no failures in my locality, from last seasons dressings, an amount of "shedding" yes, but nothing serious, keep up the standards lads.  You know there are those who would phase out surface dressing if they got their way.

Newt News (A particularly poor, and expensive, "local" joke)
This newsletter may be continued, I need some photographs of the hot rolled asphalt islands at each end of the newly opened local bypass.
I do know the reasoning (pressure) behind the choice of the surface course for the bulk of the bypass, but perhaps if a few more engineers and even planners had gone on an IHIE course, a different bituminous mixture may have been used.
Ten years ?, my opinion is not, but it is possible, as the traffic levels are not high.
But at least the authority concerned still has the option of a suitable surface dressing, to prolong the life of the surface course, when needed.

There is an awful lot in this news letter to take on board and consider, and I have "gone off at a tangent" on more than one occasion to mention topics that I think are important to highways maintenance.
So I may call it a "wrap" until the Summer Edition, it will come soon enough, take time to enjoy the Spring.
However if you do take the opportunity to attend one of the IHIE "How to get more durable surface courses" courses, you may be able to discuss at least some of the issues mentioned amongst like minded engineers and engineering technicians.

And as a celebration for the small, independent, newsletters, of the "old days", I have produced, for your convenience, this newsletter in .pdf format, just click,  Spring 2009 - Newsletter.  

London Marathon (Inspection of Hot Rolled Asphalt and Red Aggregate Precoated Chippings, Surface Course)
It is remiss of me to not have included this brief item earlier, but I forgot, so not many of you will read this newsletter in time for the annual inspection. 
If you are reading this before Sunday perhaps you will tell you friends and colleagues.
By this I mean it is the 2009 London Marathon this coming Sunday, the 26th. of April.
This will be your opportunity to obtain a close up of the hot rolled asphalt (HRA) and red precoats, surface course (now without surface binder), along The Mall and outside Buckingham Palace.
You really do get a good view of the surface when the camera man provides the classic "feet pounding road" shot that they are so fond of.
Lat year the bulk of the surface still looked in very good condition to me, and I do not expect it to have changed dramatically. 
This is a durable surface which has been providing, and continues to provide a subtle and pleasing permanent red colouration to this area by the use of a surface applied natural red aggregate.
The durability of this particular HRA surface course mixture, as I recall, being improved by the use of a 50/50 50pen. bitumen and TLA (Trinidad Lake Asphalt) binder.
A binder that is little used now with the introduction of polymer modified bitumens, but all the same a binder that is probably one of the best available, if a tad expensive these days.
For the record you can still purchase TLA in "handy" pellatised form for addition to any "standard mixture" whose engineering properties you wish to improve.
So if you are watching the London Marathon for all the right reasons, marveling at the endeavour of all those runners raising money for charity, take a few minutes to do a little highway surfacing study.
I believe this surface must be approaching 20 years old, if not older, and it is a credit to those that laid it, and the fact that the engineers chose a HRA surface course and a natural red aggregate precoat.
Only the best for the lady who lives in Buckingham Palace, their Royal Highnesses do not want to be standing on the balcony looking out at potholes, can they put me in the Tower for such comments, I hope not.

Motto of the Month
"A teacher affects eternity, he can never tell where his influence stops."