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The Idiots' Guide to Highways
'Aspho-Chip' & 'Aspho-Chipless'
The two proven bituminous surfacing materials of the 70's, 80's
and 90's (and in many authorities the 21st. century).
Comprising of a layer of durable and impervious bituminous material with a surface applied layer of high polished stone value (PSV) aggregate chippings, with a low aggregate abrasion value (AAV).
The size of the chippings usually being 20mm. or 14mm. nominal size, precoated with a layer of 50pen bitumen.
The bituminous material having 30/35% aggregate content of a 10mm. or 14mm. nominal size aggregate, the bulk of the matrix being a good "asphalt" sand, approximately 10% limestone filler and approximately 8.0% bitumen (usually 50pen. straight run bitumen)
Comprising of a layer of durable and impervious bituminous material incorporating 55% aggregate of 10mm. or 14mm. nominal size with an appropriate PSV for the site conditions.
If you have not guessed already there are no such "branded" materials as "Aspho-Chip" and Aspho-Chipless", well not just yet, but give the marketing boys time, especially with the increased complication of specifying the materials that you require to the BS EN 13108 family of standards for specifying bituminous mixtures.
However, the above statements are the excellent reasons for, and the drawbacks against, using hot rolled asphalt wearing course and precoats, as formerly specified in,
BS 594 : 2005 : Hot rolled asphalt for roads and other paved areas :
Part 1 : Specification for constituent materials and asphalt mixes
I am a great believer in this bituminous mixture for more reasons than I care to bore you with, and I am not going to stop supporting it because for some very strange (well not really very strange, it is very durable and lasts a long while) reason it is no longer "fashionable", and of course the the Highways Agency (Government) will not permit its use in England and Wales on motorways and trunk roads.
It seems the practical Highway Engineers have more influence in Scotland and Northern Ireland.
Although it is still far more used In England and Wales than the "engineering journals" and public relation "boys and girls" would have you believe, but on the local authority highway networks.
However you will now (from 1st. January 2008) need to refer to,
BS EN 13108 : Bituminous mixttures - Material specifications - Part 4 : Hot Rolled Asphalt,
BS EN 13108 : Bituminous mixtures - Material specifications - Part 20 : Type testing
BS EN 13108 : Bituminous mixtures - Material specifications - Part 21 : Factory Production Control
so that you may define the target specification of the bituminous mixture (hot rolled asphalt) that you wish to be supplied with.
This specification (recipe) will take the form of a "CE" conformity marking, an example of which can be found in,
BS EN 13108 : Bituminous mixtures - Material specifications - Part 4 : Hot Rolled Asphalt, Appendix ZA.3 CE marking and labelling.
It is entirely possible to specify, for practical purposes, the same materials that it has always been possible to purchase, you just need to put in a little more effort, initially, until you have defined a number of specifications/recipes to cover the range of HRA's that you wish to employ on your highway network.
But you may wish to defer to the suggested "sample" bituminous mixtures for hot rolled asphalt in,
PD 6691 : 2007 : Guidance on the use of BS EN 13108 Bituminous mixtures - Materials specifications,
these "sample" specifications are to be found in "tables" in the appendices at the back of the document, they are remarkably similar to BS 594 specified mixtures.
I will bore you with just one justification for my support of HRA materials equivalent to BS 594 specifications/recipes, that is BS 594 : Col. 6/4 : 30%/14mm. hot rolled asphalt wearing course, has a binder content target of 7.8%.
modern proprietary "Thin Surfacings" have binder
targets of between 5.0% and 5.5%, all these figures have binder
tolerances of at least plus or minus 0.5% at present, with a plus or minus 0.3%
coming in with most generic bituminous mixtures from the 1st. of January 2008,
but often coupled with a 0.1% reduction in binder content.
It is important to note these figures are by mass not by volume , aggregate is approximately 2.7 times more dense than bitumen.
So a material with a binder content of 7.8% by mass has about a 20% binder content by volume, and in my experience durability of bituminous material is dependent on an adequate bitumen content of the appropriate grade and properties.
( Your friendly Materials Engineer will explain it to you in more depth.)
The evidence for my support of this material is on roads all around me, with many, many miles of this type of road surface over twenty years old with no subsequent treatment, and some just receiving a surface dressing.
If this surfacing process is good enough, and quiet enough, to surface The Mall outside Buckingham Palace, I am not too proud to keep recommending it for general use, but without the red chipping, (except in traffic calming areas).
At my last "London Marathon -
2009" inspection of the Mall as I watched the event
on television, the surface still looked excellent to me, as you do get a good
view of the surface when the camera angle follows the feet of the
Those of you that may be interested in looking at this excellent HRA surface, you will need to wait for the "2010 camera survey".
Although I cannot exactly remember when it was laid, I think it is likely be at least 15 years old, probably nearer to 20 years, and looks good for another 15 to 20 marathons, if it does not experience too much "utility" work.
Even with utility work, providing you have a supply of the original chippings a "match" with the original surface should not be difficult to achieve.
I do know where there is a considerable length of HRA and 20mm. precoats that is almost 40 years old and still in a perfectly acceptable condition.
However the binder was a 50/50, i.e. 50% 50pen bitumen and 50% TLA (Trinidad Lake Bitumen) blend, and the lower roadbase (now called base) was seven inches of lean concrete, with a HRA upper base and a bitumen macadam (asphalt concrete) basecourse (now called binder course) on top of it.
So there you have an example of "modified" bitumen with a stiff base (but it is concrete) that "everybody" says is what they want, but nobody uses this design any more, strange.
I am not going to tell you where it is, it might go missing, but even if this surface course was replaced, or more likely overlaid, tomorrow, as long as its performance remains documented the total pavement strength of this design cannot be disputed.
For more information on
hot rolled asphalt and precoated chippings as a surface course press, HERE.
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