INTRODUCTIONAfter the conference, "Highway Maintenance 2006" organised by the industry journal Surveyor on the 6th. of June at the East Midlands Conference Centre, Nottingham, the document,
"Best Practice Guidelines for Specification of modern Negative Textured Surfaces (NTS) on Local Authority Highways"
has been published and is available to download as a .pdf file on the website,
www.roadscodes.org
I suggest that you obtain this document and read it thoroughly, it contains a wealth of good information, and observation on road surfacing practices.
However, and there is always a however with my comments, it is my opinion that in order not to cause offence to both the "traditionalists" such as myself with the, "if it ain't broke don't fix it attitude" to the bituminous surfacing of local authority highway networks, and the "modernists" who wish to pursue a more "marketing" approach to all bituminous mixtures offered for all road surfacing, the document is almost contradictory in some of its statements.
With the publication of these "Guidelines" we now know what NTS's are, apart from another set of initials we have got to remember, "the NTS family comprises a suite of proprietary surfaces, known collectively as thin surfacings, and generic stone mastic asphalt."
I read this to mean that if a particular proprietary bituminous mixture is laid on a motorway or trunk road it is a thin surfacing (TS), if the same material is laid on a local authority highway it is a negative textured surfacing (NTS).
Or, are the suppliers going to develop two ranges of materials, one for Highway Agency work and another for Local Authority work.
So this gives us the situation that we can specify generic bituminous mixtures that are very similar to those that were able to be specified in BS 4987 and BS 594 that will have the characteristic of negative texture but cannot be regarded as a NTS because they are not proprietary bituminous mixtures, this is the same situation as being able to lay British Standard bituminous mixtures at or less than 40mm. and they cannot be called Thin Surfacings.
IMPORTANT NOTE : Bituminous mixtures (recipes) that were able to be specified using BS 4987 and BS 594 will need to be specified using the appropriate part/s of BS EN 13108 : Bituminous Mixtures-Material specifications from the 1st. January 2008.
Information relating to the introduction of the BS EN 13108 family of standards can be found in the,
Summer 2007 Newsletter.
"One" has to wonder if somebody is deliberately trying to confuse the hard working highways maintenance engineer who has a host of other things on his mind.
Purchasers will be spoilt for choice and will not know what to choose, and this document does say the client is responsible for the selection of material, I think, so if it all goes wrong it is your fault.
I am still not happy about the explanation of what is a HAPAS approved system and how it relates to the production of individual proprietary bituminous mixtures, but I am very pleased these "Guidelines" include a revised copy of the CSS "Advice Note for the Specification of Thin Surfacing", (Report Eng/2003).
If you do decide to purchase proprietary materials use this specification, all of it, but especially the sections "Declaration of Design", and "Cold Weather Working".
It would also have been a good opportunity to have included a copy of the BBA document,
"Guidelines Document for Thin Surfacing Systems", but I believe it is still in draft form, after nine years, so perhaps this was not possible.
I could continue at length and highlight individual statements in the document that I do not like, such as, "The 0/10 close grade mixture is unlikely to produce a texture depth suitable for areas with a speed limit above 30mph", but I will not, other than to say the considerable use of this material in the area I have worked does not support this statement, perhaps because we paid appropriate attention to the polished stone value (PSV) of the aggregate.
May be our local suppliers just happen to produce a particularly well formulated 10mm.CGM Surface Course within the tolerances included in BS 4987 that does provide sufficient surface texture when laid.
10mm. CGM seems to fill an important role in the maintenance of our rural, and urban, road network, and use a considerable amount of the embarrassing stockpiles of "fines" left over from producing proprietary thin surfacings.
(Also 14mm.CGM Surface Course, and 20mm.DBM Binder Course laid as a "running surface", although this will be laid thicker to add strength, are good materials for rural road networks that successfully include large percentages of suitable quarry fines, but I digress from "thin" and "negative texture", although remaining with the theme "Local Authority Highways".)
"Best Practice Guidelines for Specification of modern Negative Textured Surfaces (NTS) on Local Authority Highways", is a good document, download it and study it, all of it.
However if you are to have a comprehensive understanding of the full implications of the introduction of proprietary bituminous mixtures for road surfacing you must also study the other relevant official documents (some of which are mentioned later on this page) relating to this topic of highways maintenance.
Many of the relevant official documents can be downloaded from the website, www.standardsforhighways.co.uk
It is not sufficient to just read the promotional publications on these materials.
But do not think the available documents will replace a knowledgeable and experienced Materials Engineer, or Road Pavement Engineer. It is likely you will need such a professional to explain the documents to you, as well as advise on the most cost effective bituminous mixtures to employ on local authority highway networks, while you still have a choice.
You will also need such an Engineer to provide advice and guidance on the specifying of generic bituminous mixtures, from the 1st. of January 2008, with the introduction of the,
BS EN 13108 : 2006:Bituminous mixtures. Material specifications, family of specifications.
DEFINING A "THIN SURFACING"
Defining an individual "Thin Surfacing" is not easy, so I will start by stating the definite bits about a "Thin Surfacing System" definition.
"Thin Surfacing Systems" are proprietary materials, i.e. they will be sold as a brand named product.
This is stated in several places and includes,
Notes for Guidance on the Specification for Highway Works, Clause NG 942.
and
Part 1 of HD 36/99 : Texture and Aggregate properties : Para 3:36 : Thin Wearing Course Systems
But it is also stated in,
Part 2 of HD 37/99 : Chapter 6 : Thin Wearing Course Systems : Para 6.1, where you will find the definition,
"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."
There are three types of "Thin Surfacing Systems" ,
Type A, less than 18mm. thick
Type B, 18mm. to 25mm. thick
Type C, 25mm. to 40mm. thick
The above definitions are stated in HD 37/99 : Chapter 6 : Thin Wearing Course Systems.
But, and it is a big but, where a "Thin Surfacing System" is,
"to replace by inlay an existing worn out surfacing layer, then the increase in thickness should be limited to not more than 25% of the nominal maximum thickness given above and a three year guarantee should be obtained from the proprietor of the system to replace the two year requirement in sub-Clause 26."
So, the 40mm. max. thickness can become a 50mm. max. thickness, i.e.
40mm. ÷ 100 x 125 = 50mm.
The above is taken from Clause NG 942.
So, let's recap,
All our current British Standard surfacing materials, including 30%/14mm. Hot Rolled Asphalt will meet the 40mm. thickness criteria, and even 35%/14mm. Hot Rolled Asphalt will comply with the inlay requirements.
SO YOU SEE, "THIN SURFACING SYSTEMS" ARE NOT NECESSARILY THIN !
If you want to know more than this guide will tell you, and you should, then read the many references I am quoting, especially,
HD 37/99 : Chapter 6 : Thin Wearing Course Systems,
and
Clause 942 of the Specification for Highway Works, with its associated Notes for Guidance
HIGHWAYS AGENCY APPROVAL
For a "Thin Surfacing System" to be used on a trunk road or motorway it used to need "Highways Agency Approval".
The material received type approval after being assessed and found to comply with all stages of the,
Highways Agency 5-Stage Procedure for Evaluating New Materials
The five stages consist of :-
STAGE 1:Desk Study,
Assess and evaluate existing information on the material.
STAGE 2:Laboratory Study,
Test the mechanical properties to allow theoretical predictions to
be made of their performance.
STAGE 3 : Pilot-Scale Trials,
Evaluation of construction and performance of materials in small scale
trials.
STAGE 4 : Full-Scale Trials,
Full-scale trial on a trunk road to establish whether the previous assessments obtained from Stages 2 and 3 are realised.
STAGE 5 : Highways Agency Specification Trials,
This stage is necessary to carry out further evaluation of the material and to test the specification under contract conditions.
Notes
(1)
Stages 1 to 4 are financed by the manufacturer of the material.
For stage 5 the additional cost, if any, of the material is borne by the manufacturer.
(2)
Stages 1 to 4 can be carried out by the TRL or other independent organisation, in the latter case the reports are appraised by the TRL.
(3)
In all cases, the new materials are compared with conventional materials to obtain comparative performance.
HIGHWAY AUTHORITY PRODUCT APPROVAL SCHEME, (HAPAS)
The Highways Agency has passed on responsibility for approval of "Thin Surfacing Systems" to HAPAS, and there are now many "Thin Surfacing Systems" that have received approval from the BBA under HAPAS.
Current information on products that have HAPAS approval / certification can be accessed by pressing ------> HERE
In my opinion there is a significant difference between "product approval", i.e. the approval of a single documented product, and the approval of a "Thin Wearing Course System" that may include a number of products with differing nominal size aggregate, with differing binders and aggregate source all under the same approval.
In fact I quote from,
NG 942 : Thin Wearing Course Systems : Para 4
"This specification for thin wearing course systems is not intended to be an exhaustive, binding specification for the use of proprietary type mixtures, but rather to form the basis of a document for Contractors to tender for work."
So the question is, does a "Thin Surfacing" need HAPAS approval, and if it does can "they" change the approved mixture about after it has received approval, because if they can there was not much point in getting approval in the first place.
Of course the argument will be that if the bituminous mixture "performs" that is all that matters, everything these days is about performance specifications.
But I thought the 5 stage approval process was to establish that a particular bituminous mixture did perform and therefore did receive approval.
If you change the constituents how do you know if it will still perform without going through a new approval process.
The guarantee is only two years, and many "Thin Surfacings" have not lasted that long.
There had been "talk" that this two year period would be increased to five years with the publication of,
"Best Practice Guidelines for Specification of modern Negative Textured Surfaces (NTS) on Local Authority Highways"
but it did not happen.
Possibly because if this had have occurred the producers would have increased the price of Thin Surfacing bituminous mixtures as an "insurance" against possible failure, again one could speculate that a further substantial increase in price of proprietary bituminous mixtures would have destroyed the market for these materials.
The public only got to hear about the surfacing failure (porous asphalt) on the Newbury Bypass because it was such a high profile site, there have been many other similar but unreported failures around the country concerning other materials.
In my area alone the history of the use of Thin Surfacing bituminous mixtures on the short M69 has not been without its problems, but as far as I am aware they have only been reported in the local press.
And, I did indicate there was a possible problem via a photograph in the Autumn 2007 Newsletter.
THE STRUCTURE OF THE HIGHWAY AUTHORITIES PRODUCT APPROVAL SCHEME (HAPAS)
This is a recently introduced organisation to "approve" new products for use in highway maintenance and construction, so that individual authorities do not need to undertake their own trials.
It has been set up by the Highways Agency (HA), CSS (formerly the County Surveyors Society) and the British Board of Agrement (BBA).
I am naturally suspicious about this type of "national" approval without any local input from the various distinct regions of the UK.
I am also concerned, when it appears that the approval of one product, that has been produced in a particular production plant, to a particular design/recipe with particular components, that is laid in one situation, in particular weather conditions, by particular plant, and a particular gang, gives approval to a whole "system" of materials, not necessarily having the same design or the same constituents as the bituminous mixture that received approval, or laid on a similar site in a similar manner.
(A lot of "particulars", but I hope that you understand the nature of my concern, but just the disparity of site and weather conditions could explain some reported failures that have occurred very soon after laying.
Bituminous mixtures that you have laid successfully in "trial conditions" on a Summer's day on a "protected" site may not be appropriate for the middle of winter on the top of a motorway embankment, even within the permitted bounds of ambient temperature and wind chill.)
The development of HAPAS is administered by the BBA, closely advised by the Highways Technical Advisory Committee (HiTAC), certificates of approval are issued by the BBA acting on recommendations from HiTAC.
Specialist groups, with a strong industry representation, are created for each product type.
Specialist Group 3, is the group with responsibility for assessing Thin Surfacing Systems.
Products and processes are put forward for consideration by the group who will report back its findings to HiTAC, who will in its turn make recommendations to the BBA as to whether the product / process should receive HAPAS approval.
Full details of the particular schemes and the criteria for assessment of products are available from the British Board of Agrement as a Guidelines document, and unless things have changed quite recently the Guidelines document for Thin Surfacing Systems is still in draft form after many years, with many of the test procedures included in it not to be regarded, at this stage, for specifying purposes.
If you are seriously interested in highway materials and you have some understanding of highway materials testing these documents make very interesting reading.
Until a few years ago only a few products were receiving HAPAS approval in what I regard as the Highways Maintenance sector, these products being High Friction Surfacings, and they did tend to be products rather than systems.
However, many "Thin Surfacing" systems have now received HAPAS certificates, check the BBA web-site.
I repeat these are certificates are for systems for producing products, in many cases not the actual product that you will receive.
In my opinion you will need an engineer / technician with suitable materials knowledge to study the certificate with care to be able to determine the suitability of an individual product of the system to meet the requirements of the site you wish to surface.
For further information relating to HAPAS, press ------------------------------> HERE
THE IMPLICATIONS OF INTRODUCING PROPRIETARY "THIN SURFACINGS"
The true impact of this move is delivered in,
HD 36/99 : Chapter 2 : Surfacing Options
It is here in Table 2.2E (England), that it states, that the only permitted "surfacing option" for use without restriction on motorways and trunk roads is "Thin Surfacing".
Which in turn means you are having to buy proprietary materials at proprietary material prices at the insistence of the Highways Agency.
It is likely, if the CSS (which is not the County Surveyors Society, although it did replace it) are successful in exerting their influence over local authorities a similar policy will be adopted in county situations, refer to the document,
"Best Practice Guidelines for Specification of modern Negative Textured Surfaces (NTS) on Local Authority Highways"
mentioned above.
Producers of proprietary "Thin Surfacings" claim it is the engineering properties of these products which make them so popular with highway maintenance engineers.
I think this claim is foolish, because thanks to Highways Agency policy no other surfacing materials but "Thin Surfacings" are allowed on motorways and trunk roads in England, which is the largest single market for bituminous materials in the U.K.
It also explains why "Thin Surfacings" have suddenly taken such a large share of the bituminous materials market.
GUIDANCE IN SPECIFYING PROPRIETARY THIN SURFACING
CSS Report ENG 1/2003 - Advice Note for the Specification of Thin Surfacing
The purpose of this document is to advise specifying engineers in the scheduling of proprietary thin surfacing systems for maintenance and new works.
This Advice Note was published in July of 2003.
This is an excellent Advice Note and its content relates to the specifying of "Thin Surfacing Systems".
I believe it is necessary reading if you are considering the use of any of the range of the proprietary bituminous mixtures now offered by the industry, these being known as "Thin Surfacing Systems", rather than individual unique products.
Copies are available from :-
CSS Honorary Secretary and Treasurer, Lincolnshire County Council, City Hall, LINCOLN, LN1 1DN
Tel. 01522 553098 Fax. 01522 512335
Cost is £5:00 to members and £10:00 to non-members
This Advice Note can now be found in the document,
"Best Practice Guidelines for Specification of modern Negative Textured Surfaces (NTS) on Local Authority Highways"
and is available to download as a .pdf file on the website, www.roadscodes.org
CONFUSION
Be very careful when ordering Stone Mastic Asphalt (SMA).
A "genuine" SMA had the original concept/design of a load bearing aggregate skeleton with voids filled with bitumen, and it was/is meant to be impervious.
The words Stone Mastic Asphalt are now being applied to some "Thin Wearing Course Systems", I believe quite incorrectly.
Materials that are used on motorways and trunk roads, until recently, still needed to meet the 1.5mm. texture depth ( by sand patch ) requirement, and when not applying a surface layer of chippings this usually means an open graded bituminous mixture to be able to achieve this surface characteristic.
This confusion does mean that if you order SMA from a supplier without making absolutely clear what you require him to supply he will almost certainly defer to a "Thin Surfacing" SMA, as it contains less expensive bitumen and it is less difficult to design and mix, it is also a less durable material.
Creating a genuine Stone Mastic Asphalt is quite a delicate balancing act, which also relies on having a good mixing plant.
Be aware that from the 1st. of January 2008, and the introduction of the BS EN 13108 family of standards for specifying bituminous mixtures, there is now a standard for specifying stone mastic asphalt (SMA),
BS EN 13108-5 : 2006:Bituminous mixtures. Material specifications - Stone mastic asphalt
and in fairness it has to be said that you can, in my opinion, produce many generic types of SMA that will have significantly different engineering properties from the "original" requirements of an SMA , e.g. you will be able to specify a generic SMA with a high void content, if this is an engineering characteristic of the surfacing material that you require.
Important Note :
From the 1st. of January and the introduction of Interim Advice Note 101/07, which is a complete, revised edition of,
900 Series of the Specification (MCHW1) for Road Pavements - Bituminous Bound Materials,
there are differing texture depth requirements for differing types of bituminous surfacing materials.
Clause 921 - Surface Macrotexture of Bituminous Surface Courses : Table 9/2 Requirements for Initial Texture Depth
states that the revised surface texture, on high speed roads, for a, "Thin surface course systems to Clause 942 with an upper (D) aggregate size of 14mm. or less, is not less than a 1.3 average per 1000m. section".
The BS 598:Part 105 (now withdrawn) method can still be used for routine monitoring, so these figures refer to millimetres of average texture depth.
The texture depth requirements for hot rolled asphalt, in the same situation, remains the same at 1.5.
(So it is going to be difficult to compare tyre generated noise on these two road surface types on a like for like basis, I just thought I would mention it.)
PERSONAL NOTE
It would clearly be
quite wrong of me to condemn all "Thin Surfacings",
there are already too many of these products on the market to
make such a sweeping statement, and a high proportion of them will be good materials if used
appropriately.But how you go about choosing the correct product for a particular site I am not sure, you must judge performance against cost.
The fact that materials have Highways Agency approval, now HAPAS approval, does not make them all equal, many have quite distinct properties for distinct site requirements, this being a quite separate issue from the actual quality of the product.
I realise I am regarded as somewhat of a "throw-back" in modern highways maintenance procedures, but with my background it is relatively easy for me to specify a particular British Standard product to suite a particular site and the budget that is available, and if I want a modified binder I can choose the binder type or modifying agent I think most suitable, not the one that automatically is part of the proprietary "Thin Surfacing System".
May I suggest that when comparing costs of bituminous mixtures that you compare cost per tonne of material and not cost per square metre.
It is likely to make a far greater engineering contribution to the road pavement to lay 50mm. thickness of a generic bituminous mixture than to lay a 30mm. thickness of a NTS with a modified binder, for approximately the same cost.
I hope you noted that I said "and the budget that is available", gone are the days (if they ever existed) when you are able to specify the best option for a particular site, budgets have been cut too severely, so it follows that it is my opinion that a move to more expensive branded products in bituminous mixtures is not a good move.
I would much rather see a move to re-establishing the role of the Materials Engineers and Materials Technicians in the "educated client" role of the highways maintenance industry, but then I would, would I not.