Agency Press Release
What is a Thin
What is a Stone Mastic Asphalt
Another One bites the Dust
of the Month
I had intended to give myself a break in March, as there
seemed little fresh to talk about, and I do not like repeating myself
for the sake of it. But there is so much incorrect reporting and
comment on the subjects of stone mastic asphalt and thin surfacing
that I could no longer sit on the sidelines without making a comment.
The silence of the
industry and those who supported (encouraged) the introduction and use
of proprietary thin surfacing is deafening in response to what is
being portrayed on radio and television.
The number of research projects, working groups, and consultant
contracted out "surveys" on the above topics is
proliferating yet we never seem to have anything published.
It begs the question do the initial results need further work to
obtain the answers required.
I am aware some organisations have already done some of their own
investigations which do not suggest promised outcomes are being
achieved with some of the "new" surfacing techniques.
At this point in time I would still suggest to those involved in
looking after the 95% of the road network, that is not motorway or
trunk road, to remain with proven bituminous surfacing options that are
specified to British Standards.
However, you still need to know which bituminous mixture you
require for the site in question, and this is where an independent
(not selling any particular product or service) qualified and
experienced Materials / Road Pavement Engineer is a particularly
useful member of any team.
If you are unable to secure the services of experienced engineers and
technicians you do not have to wait for "new" guidelines on
how lay bituminous mixtures 40mm. or less (50mm. to match existing
surface course thickness), just purchase, study, and follow the
specification and advice in BS
4987 and BS
Agency Press Release
There has been a
development since I wrote the above introduction, the Highways Agency
have issued a news
item in response to the concerns about early life skid
resistance of stone mastic asphalt.
I suggest you use the link I have provided and form your own opinion as to whether it satisfies your concerns.
The item, it appears, is to particularly address the worries of motor
I find the content very interesting, but then I know more than most on
the subject and I personally cannot agree with all the statements
made, others may, but it is a well crafted piece of work for the
But I do think that the suggestion that there is no SMA on Highways
Agency roads is playing with words, and may not be well received by
some suppliers who include the term "SMA" in marketing some of
their proprietary bituminous surfacing mixtures.
I also think it is worth noting that the item is written/prepared by
"DeHavilland Information Services" a company that provides
"news information service for latest current affairs and
political news", and not an Engineer, or Engineering Consultancy,
an Engineering Journal, or even a spokesman for the Quarry Products Association.
Perhaps we are getting closer to the reason for the introduction of
the exclusive use of proprietary Thin Surfacings on motorways and
trunk roads in England than you think.
But to try and provide some clarity in what has become an extremely confusing
- There is no one single stone
- There is no one single hot
- There is no one single close
graded bitumen macadam
- There is no one single
- There is no one single open
graded bitumen macadam
- Etc. for all forms of road
All these bituminous mixtures
are the result of designing and blending aggregates, from sources of
aggregate (quarries) all over the UK, with differing amounts of bitumen.
The aggregates will have differing physical properties, and will be proportioned to different designs and mixed in different
types of batching plants, with different bitumen viscosities supplied
from several major suppliers.
The supplied bitumens are likely to be sourced from differing crude
oil stocks and may be modified with various synthetic or
natural modifiers. The bitumen must be stored and used within certain
time and temperature constraints to ensure the bitumens do not degrade
before incorporation in any bituminous mixture, or indeed after mixing
whilst in storage bins.
So, what am I saying is that there is the potential for hundreds,
possibly thousands, of combinations to produce the sum of the various bituminous mixtures
I have named above, and that includes SMA, generic or proprietary.
The properties of the bituminous mixture will depend upon the quality,
blending and mixing of its components, not on the name it may carry.
Just as a rose will smell as sweet if you call
it a turnip, you will not get the same fragrance from a turnip if you
call it a rose.
You probably know what I am going to say next, but I will say it
anyway, you need a qualified and experienced Materials Engineer / Road
Pavement Engineer to help you decide on the appropriate bituminous
mixture for particular surfacing requirements, not a "news
information service for latest current affairs and political
How many hours/days/years have they spent by the side of a paving
machine, in a materials laboratory or in a dusty, wind swept, quarry
If you wish to know more about proprietary Thin Surfacings, Stone
Mastic Asphalt and bituminous road surfacing mixtures in general there
are plenty of pages on this website and links to other websites to
provide you with more information than you really want to be bothered
with, and you should also try a little Googling
using "sma early life skid resistance" as your keywords.
One website that is to be highly recommended to increase your
knowledge on the topic of bituminous mixtures for roads is that of R.J.Maxwell
& Son Ltd., it has an excellent "education"
What is a Thin
Design Manual for Roads and Bridges – Volume 7 – Pavement Design and Maintenance –
HD 37/99 : Bituminous Surfacing Materials and Techniques - Chapter 6 : Thin Wearing Course Systems
“6.1 Thin wearing course systems, or thin surfacings
as they are more commonly described, are
proprietary systems in which a hot bituminous bound mixture is machine-laid with a controlled screed paver onto a bond or
tack coat to form, after compaction and cooling, a textured wearing course generally less than 40mm. in thickness.”
It would be sensible to read all of HD 37/99 if you want a better understanding of bituminous surfacing materials
and techniques, after all the Department for Transport have gone to the trouble of publishing it, and you can
download it free of charge from the “web”.
It is also stated in Clause 942 of Volume 1 of the Specification for Highway Works
(SHW) that Thin Surface Course Systems shall have a British Board of Agrément
HAPAS (Highways Authority Product Approval Scheme) Roads
and Bridges Certificate applicable to the combination(s) of traffic level and site classification given in Appendix
7/1 of your contract document, in accordance with the Specification for
There are other conditions included in Clause 942 appropriate to the use of a bituminous mixture that has been
produced under a Thin Surface Course System that has a HAPAS
certificate, e.g. texture depth.
I personally do not regard an actual 40mm. thickness of surface course as “thin” but a standard thickness that will
provide durability and contribute load spreading strength to the road pavement.
It is also necessary to make reference to the “Guidelines Document for the Assessment and
Certification of Thin Surfacing Systems for Highways”, which should be
obtainable from the British Board of Agrément
It is the criteria in this document that determines whether a Thin Surfacing System achieves certification.
The British Board of Agrément introduced this document in 1997, in draft form.
The draft copy I have is dated January 2000 and I believe it is still in draft form at this time, although it may be a more recent draft.
With regard to skidding resistance of a Thin Surfacing, a pre-wetted skid resistance test is not a mandatory test (Table 1) for HAPAS approval, what is required is the
appropriate combination of polished stone value (PSV) of the aggregate, and texture depth.
After removal of surface bitumen this combination should/will achieve the skid resistance appropriate to the site
A pre-wetted skid resistance test (SCRIM) can be specified as an optional test (Table 2), but required performance
levels are not specified in the draft Guidelines Document that was still current on the 26/2/2004.
It has to be noted that HAPAS approval is not an approval of an individual product, it is an approval of a system,
and that system will comprise a series of mixtures with varying blends of binder, filler, fibres, coarse and fine
aggregate dependent upon site specific requirements.
So, it might be quite a complicated, and expensive, process to obtain individual pre-wetted SCRIM values for each
individual bituminous mixture covered by a particular Thin Surfacing System.
What is a
Stone Mastic Asphalt
There are a number of ways of defining SMA, the description I think most appropriate is to be found in :
TRL Report 314 – Road trials of Stone Mastic Asphalt and other thin surfacings – by J. C. Nicholls
Chapter 2 – Para 2.1 : Stone Mastic Asphalt
The description is quite comprehensive and I would suggest that you read it, along with the rest of this excellent
But to quote just a small section, which basically summarises what an SMA should be, “The single sized nature of
the aggregate skeleton leaves a relatively high void content between the aggregate particles which is partly filled
with a binder rich mastic. As such, the aggregate grading is similar to that of a porous asphalt, but with the voids
filled with mastic.”
I would also suggest you read,
TRL Project Report 65 – Evaluation of stone mastic asphalt (SMA) : A high stability wearing course material.
This report will give you a background on the introduction of SMA into the UK from Germany, and an insight into its
development, production and use in Germany.
In order that an SMA can be used on motorways and trunk roads in England it must be produced to a “system” that
has HAPAS certification and then it can be regarded as a proprietary Thin Surfacing.
( This is where I believe "people" have
been playing with words over whether SMA has or has not been used on
Highways Agency supervised road networks. )
Generic SMA is a “Not permitted” option for use on motorways and trunk roads in England, see
Design Manual for Roads and Bridges – Volume 7 – Pavement Design and Maintenance –
HD 36/99 : Surfacing Materials for New and Maintenance Construction – Chapter 2 : Surfacing Options –
Table 2.2E (England) : Permitted Pavement Surfacing Materials for New and Maintenance Construction
I am aware that work on early life skid resistance of SMA by TRL started early in 2002
soon after the reported problems in Derbyshire. Considering the potential seriousness of this issue you would have thought that at least an interim publication of achieved data might have been appropriate, but I believe
this data is still unpublished.
/ Road Noise
from the topic above, we have to remember that one of the prime reasons
for the introduction of proprietary Thin Surfacings was to reduce tyre
generated road noise.
It is important to remember that the tyre generates the noise
(especially wide, high pressure, super singles) not the road surface, but
the nature of the road surface plays a significant role in how much
noise the tyre will generate.
So you may like to purchase a recently published report from the
Transport Research Laboratory (TRL).
It is :
Published Project Report PPR023 : Comparison of Tyre/Road Noise
for a Range of Surfaces
I believe it costs
just £15:00, from the TRL Publications Unit, and I regard it as an
The work was undertaken by the TRL, under a commission from a local
authority, using the newly developed TRITON, a purpose built vehicle
containing an advanced system for monitoring the noise emitted from
tyres as they run over a road surface.
I believe this is the first published work of the use of TRITON,
although I am aware that it has done work for other agencies/organisations that is not yet
All the road surface types, of which there is a wide range, including
hot rolled asphalt, stone mastic asphalt and surface dressing are
generic bituminous mixtures.
It is my opinion
that TRITON is an excellent addition to the current range of road survey
equipment that is being developed.
It would just be nice to see its
wider use and the publication of the results. It would then be possible
to make better judgments on the choice of road surfacing materials, in
relation to road noise
based on evidence.
I think you will be surprised by the results.
But what it does clearly
indicate, as I have described above, you cannot make simple
statements that one broad material classification is "noisy"
or "quiet" just because of the general description/name that
the bituminous mixture/process carries.
Another One Bites the Dust
It is with real
sadness that I have recently heard of the ensuing demise of a large
"testing laboratory" facility, of possibly the largest
highways maintenance consultancy in the UK.
This former local authority soils and materials laboratory has
tremendous respect in the industry, and is one of the few that has UKAS
accreditation for a full range of materials and soils testing procedures.
I hope this report is incorrect, or that the decision can be reconsidered,
or influenced by the wider industry, who, in my opinion, have no idea
how much they need facilities such as this in the increasingly
commercial highways maintenance industry.
this newsletter will not make you an expert. I am not an expert, I
have some knowledge on the subject of highways materials because of
the years I have spent in the profession. I wish I knew a lot
more than I do, but it is unlikely my knowledge will increase much
more in the current situation the industry has got itself into, i.e. everything
being commercially sensitive and hence secret.
There are few well equipped independent local authority Soils and
Materials Laboratories left, if any, that will openly exchange the
type of information that needs to be shared.
I can remember when it was expected that at least 1% of the total
highway budget would be spent on various types of sampling and
testing, does not sound a lot does it, but not any more.
Perhaps this months effort will at least stop people making rash
statements, and will at the same time allow them to ask more relevant,
and searching, questions of individual problems on particular sectors
of the highway
I hope the considerable amount of work I have put into this newsletter
will indicate that selecting, specifying, designing, producing, and
storing bituminous surfacing mixtures is not a simple process, and I
have not mentioned transporting, laying and compaction.
All organisations responsible for looking after highway networks, should not be tarred with the same brush, (no
pun intended), some do quite a good job considering the
"pressure" they work under.
Finally, if I can make a plea for some sanity in the industry. Stop setting up groups with
no road engineering knowledge to organise "us". If "we"
are unable to perform our function, i.e. maintain highway networks, give us
retirement deals and let us go, and let the organisers do the work.
obviously far better at highways maintenance than we are, because
"they" are the ones telling us what to do, I think it is
fortunate that some of us are not programmed to blindly follow.
Motto of the Month
none so blind as those that will not see"