TOPICS
Re-inventing
the Wheel
10mm.
Size Close Graded Surface Course
Motto
of the Month
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Introduction
After the newsletter of last month I had hoped I could move on
from the subject of Thin Surfacings but it appears I cannot.
There are so many articles being published on this topic, by so many
authors, to try and clarify the situation all that is happening, in my
opinion, is that it is getting more confusing.
So in true "Idiot's Guide" fashion I thought it might be a
good idea to throw a few facts in to the argument that might be of
benefit to those of you out there with the responsibility for
maintaining a local highway network.
Re-inventing
the Wheel (and calling it the "Thin Wheel")
It is always a good idea to start
at the beginning, so what is a "Thin Wearing Course
System".
I quote from the Design Manual for Roads and Bridges, Volume 7,
Section 5, Part 2 - HD 37/99
Section 6. Thin Wearing Course Systems :-
"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 (yes, it does say
wearing course) generally less than 40mm. in thickness."
Now I know this paragraph goes on
to describe the mixture constituents and state that the Thin Surfacing
may have a modified bond coat and it may have a modified binder,
(please obtain a copy of HD 37/99 and read it, it is well worth it,
you can download most of the
Design Manual for Roads and Bridges
from the web).
But if you drop the word proprietary from the above sentence you have
a pretty accurate description of the majority of the surface course
mixtures specified in,
BS 594 : Hot rolled asphalt for roads and other
paved areas - Part 1 : Specification for constituent materials and
asphalt mixtures. (more
information)
and,
BS 4987 : Coated macadam (asphalt concrete) for
roads and other paved areas - Part 1 : Specification for constituent
materials and for mixtures. (more
information)
So let us stop using terminology that causes confusion, and use the
one word that truly distinguishes Thin Surfacings from bituminous
mixtures laid thinly, i.e. proprietary, it seems simple enough to me.
So we just call proprietary bituminous mixtures, proprietary
bituminous mixtures, no confusion, except for you, not me,
because you have to
decide which is the best one to use from the dozens that are now
available.
I think this is especially good advice when Thin Surfacings, sorry
proprietary mixtures are being suggested up to seventy millimetres
thick, incorporating the binder course and surface course in one
layer, to better utilise the finer fractions of High Specification Aggregate
(HSA) in a more sustainable manner.
Can you really call them Thin Surfacings when they are laid that
thick, I think not.
I have no qualms over this thicker concept at all, but if I require a material
to perform that function I think I will stick with,
BS 4987 : Part 1 : Clause 6.2 0/32mm. size
single course,
or possibly,
BS 4987 ; Part 1 : Clause 6.5 0/20mm. size dense
binder course "recipe" mixture
I have found both these materials, with an appropriate grade of
bitumen, to be excellent at "shaping", adding strength, and
providing a running surface for rural roads, usually followed in a
year or two by a surface dressing.
Equally valid is the argument to produce six millimetre proprietary
bituminous mixtures to also utilise the finer fractions remaining
after processing HSA's to produce the 10mm. and 14mm. proprietary
bituminous mixtures currently demanded on motorways and trunk roads in
England.
But before you go rushing off to purchase your branded product perhaps
you would like to consider,
BS 4987 : Part 1 : Clause 7.6 0/6mm. size medium
graded surface course (preferred mixture)
as this will give you a reasonable texture for urban
situations, I personally would not recommend using it on
roads that allow speeds above 40mph..
Any of the above materials can have the modified binder of your choice
specified, if you think it is necessary, not the modifier that comes
as part of a HAPAS system.
I understand trials are being conducted on proprietary 6mm. materials
to determine their suitability for motorways and trunk roads. I do not
know the nature of the material that is being trialed but perhaps it
is similar to the Novachip process, that does exhibit a high texture
depth as it can be described as a coated surface dressing process.
An excellent description of this process can be found courtesy of the Washington
State Department of Transport.
This process or one very similar
was marketed in the UK ten to twelve years ago under the name Safepave,
and was featured in many articles. I have no personal experience of
this process but there were reports, in the highway engineering press
of the time, of cases of significant failure.
I do not know if any of this material remains as a surface course in
the UK, and I am not aware of it still being marketed.
I hope the above examples serve to show why I chose the title of this
news item, it is my view that there is little that is truly new in
bituminous mixture design, even the bulk of the bitumen modifiers have
been around for the length of my time in the industry, and some such
as rubber latex and Trinidad Lake Asphalt were used successfully prior
to that, and still are.
So may I suggest that you explore fully what generic bituminous
mixtures are available, when deciding what is the most cost effective
solution to each highway surfacing situation. If you feel that you are
not able to make that decision employ an independent Materials or Road
Pavement Engineer to advise you.
If you are currently involved in maintaining a local highway network
you still have the choice of which bituminous mixtures you procure, it
would be nice to think that this choice will remain available to
engineers and engineering technicians in the future.
This matter is further complicated by whether the engineers and
engineering technicians are employed by a "partnering
consortium" or directly by a highway authority.
10mm.
Size Close Graded Surface Course
I was recently reading a draft
document of some importance that contained the statement that bitumen
macadams were unsafe at speeds over 20mph.. I do not know who included
that statement as the draft was compiled from many sources, and it may
have been an error, but the text that accompanied the statement
suggested it was not. My comments on the draft, and this item, have
been returned to the report compiler. I would assume comments from
some sections of the industry would have been stronger than mine.
I cannot speak for the whole of the UK because I have spent the whole
of my working life in this locality, but this does allow me to say
that 10mm. close graded bituminous macadam has successfully played a
significant part in the maintenance of residential and rural highways
in this area over many years, at least thirty.
The use of this material is very cost effective to the purchaser and of
benefit to the supplier because the mixture uses a considerable amount
of crushed rock fines, also the contractor needs quite limited
resources to lay it successfully.
So to dispel myths of its safety I include a few images to let you
make your own judgments,
subject to the polished stone value (PSV) of the aggregate being
suitable for the site in question, but that criteria would apply to
all surface course mixtures.
These pictures are of quite normal, in specification, material
supplied and laid to BS 4987.
These pictures are of a quite
recently laid surface, so it is a fair assumption that more bitumen
will be removed and skid resistance performance will increase further
to that which is indicated in these photographs.
I could describe in more detail the nature of the material you are
looking at but I would rather you talk to your Materials Engineer, if
you have access to one with "hands-on" experience of
bituminous mixtures.
Motto of the Month
"A man who
reviews the old so as to find out the new is qualified to teach others."
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