INTRODUCTION TO "THIN SURFACING"
After 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 comprise of, 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 amongst the range of proprietary bituminous mixtures available, and will not know what to choose, and this "Roads Codes" document does say the client is responsible for the selection of material, I think, which I take to mean if it all goes wrong it is the Engineer's fault.

But there is help for those who are not familiar with specifying bituminous mixtures in the form of,
PD 6691:2010 (supersedes the 2007 edition) - Guidance on the use of BS EN 13108 Bituminous Mixtures - Material specifications, with special reference to the "example" specifications in the various Annexes B,C & D at the end of the document, which may help the highway engineer / engineering technician with little experience and without the backup of a Materials Engineer.

I have provided some guidance to the selection of generic bituminous mixtures for surface course on the page, British Standard Bituminous Mixtures that can be Laid Thinly.

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".

I could continue at length and highlight individual statements in the "Best Practice Guidelines for Specification of modern Negative Textured Surfaces (NTS) on Local Authority Highways" 
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".)

All the above generic bituminous mixtures receiving an appropriate surface dressing when required, usually after about five years.

"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 TSCS and NTS proprietary materials, in my opinion you need to know exactly what these bituminous mixtures are if you are to make an engineering judgement on their use.

I would also suggest that sampling and testing of supplied bituminous mixtures is a very important engineering element of any surfacing programme, something that always used to be performed on the supply of generic bituminous mixtures to British Standards, and I cannot understand why Clause 942 of the Specification for Highway Works actually states, " the composition of thin surface course systems produced under a sector scheme shall not be tested under this contract other than for audit purposes, unless some obvious variation in production occurs (e.g. binder drainage is observed for the first time"
So as long as it looks alright everything is okay, my experience tells me that this is not always the case, and it is far more difficult to "spot" low binder contents than high ones, that is until it starts fretting out.

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 Surface Course Systems (TSCS's) 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 Notes for Guidance on Specification for Highway Works - Clause NG 942.

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 a "product approval", i.e. the approval of a single documented product, and the approval of a "Thin Surface Course System" (TSCS) that may include many 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 surface 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, if 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  HAPAS approval 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 adversely affected  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.

In my opinion this lack of reporting of highway surfacing failures is due to the, now, total absence of any "independent" highway engineering journals.
All these journals, even the ones with the respected titles are owned by publishing, or public relations companies, all heavily dependent on advertising revenues and "customer" business  to obtain their incomes.

It is now possible to obtain/specify a five year guarantee by reference to the appropriate clause in the 900 Series of the Specification for Highway Works.
As I understand it if you do not include this relevant clause in your contract document you will only be receiving the two year guarantee period, read the "box" below for details.

However, you may like to read Para 15 (08/08) of Clause 942:Thin Surface Course Systems of the Series 900 - Road Pavements - Bituminous Bound Materials, the August 2008 amendment. There has been a major revision from the previous version of Clause 942, i.e. in the Amendment 2004 of the Series 900, please check out the differences Clause 942 now states, "On the trunk road including motorway network the Contractor shall guarantee the integrity of the surfacing and the workmanship for a period of five years from the date of opening to traffic, unless otherwise specified in Appendix 7/1. Para 16 (08/08) states, "The five-year guarantee shall include for defects such as fretting, ravelling, stripping and loss of chippings. The guarantee shall exclude defects arising from accidental damage or damage caused by settlement, subsidence or failure of the underlying carriageway on which the surfacing material has been laid." Whether this will bring about a change to the "guarantee period" stated on BBA certificates for the Thin Surface Course Systems that have gained approval I am not sure, it may be just as Para 15 (08/08) states and be site/contract specific.  What I would suggest is that this extended guarantee period may indicate that the Highways Agency (Government) are not happy with the performance of some proprietary bituminous mixtures that have been supplied under the Thin Surface Course System approval scheme. Please do not blame the failure of these materials just on workmanship. I knew, still know, many of the men in the laying gangs, they do their best with what they are supplied, they really do.

THE STRUCTURE OF THE HIGHWAY AUTHORITIES PRODUCT APPROVAL SCHEME (HAPAS)

This is a fairly recently  organisation, introduced in the late 1990's, 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 (bituminous mixture), 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 Agrément as a Guidelines document. As stated above, things have changed quite recently and the Guidelines document for Thin Surface Course Systems is now available to be downloaded, after existing in "draft" form for approximately ten years. However I believe many of the test procedures included in it are still 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 Surface Course 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

DRAFT - INTERIM ADVICE NOTE (IAN) - THIN SURFACE COURSE SYSTEMS - INSTALLATION AND MAINTENANCE

(This has now been released as Interim Advice Note 157/11 Thin Surface Course Systems - Installation and Maintenance see the following item, but the information here is still pertinent.)

The above draft document  appears to be widely available through various "groups" but is not yet present on the Highways Agency website www.standardsforhighways.co.uk.

I strongly recommend to those of you who are involved in specifying/purchasing bituminous mixture for surface course both on motorways and trunk roads, and local authority highway networks, to obtain a copy of this document and study it thoroughly.

I include local authority engineers in this advice because any document covering proprietary Thin Surface Course Systems, has a direct effect on Negative Textured Surface bituminous mixtures, which are also proprietary materials, and marketed for use on local authority highway networks.

To better understand this IAN may I suggest that you also download the current, March 2011, "Guideline Document for the Assessment and Certification of Thin Surfacing Systems for Highways" from the BBA/HAPAS website, (look under HAPAS ---> Product Sectors).

Maybe I should feel some guilt for saying this, but I do not, but I suggest that you download this document to read the last paragraph of "Terms and Conditions of Use" on page 3.
Now in fairness there is a similar disclaimer in the front of most British Standards, i.e. "Compliance with a British Standard does not in itself confer immunity from legal obligations."
Neither does having BBA/HAPAS approval for a bituminous mixture, and its method of laying, automatically imply that the bituminous mixture is "suitable" for the inexperienced engineer to purchase and that his decision will be endorsed by referral to the "approval" certificate.

In reading both documents I am becoming increasingly confused between the approval of a "product" (a particular bituminous mixture laid under particular conditions) and the approval of a whole "system" of products (bituminous mixtures) laid under very varied conditions.
I had thought, still do think, that it is only "systems" that are approved and that an individual product (bituminous mixture) achieves indirect "approval" by complying with aspects attributable to a "system".
I understand that relatively few actual distinct bituminous bituminous mixtures have been trialled by the BBA, and the ability of these relatively few bituminous mixtures to perform satisfactorily gives approval to the "system" which then gives indirect approval to any "product" that is produced in accordance with the parameters of the "system".
In my opinion it is necessary to study carefully the nature of these parameters to understand how the various "products" can differ in their nature and engineering properties.

I am not going to discuss the topic in any detail here, as there are many pages and newsletters already covering this issue on this website. But I will repeat my recommendation that you obtain copies of these documents and study them thoroughly.
If this IAN is a true draft it should still be open for discussion and there should still be opportunity to forwarded any concerns to the Highways Agency before the publication is included in a new HD 37, as is the intention.

It has to be observed that most concerns have been already highlighted in the document in some detail.

With so much detail included in this IAN regarding the shortcomings and difficulty in successfully laying TSCS bituminous mixtures I find it difficult to make a judgement as to whether the document is designed to promote the compulsory use of these proprietary materials on motorways and trunk roads, or to bring the practice to an end.
If anyone is in doubt as to my opinion, it is to return to the open practice of allowing knowledgeable and experienced highways engineers to make the appropriate decision in these matters, i.e. return to the use of correctly specified British Standards bituminous mixtures.

Hot Rolled Asphalt (HRA)surface courses where not subject to wheel tracking because they were HRA, it was because there was some aspect of the design or production that was lacking in that particular supply, perhaps the mixture included "extractor dust" rather than limestone filler with its hydraulic binding and stiffening properties.
If anybody knows of any titles of detailed investigations into HRA wheel tracking I would be pleased to receive their titles so that I may study the information..

If there are not enough knowledgeable and experienced highway engineers, meaning engineers with a real knowledge of bituminous materials, to make these decisions, whose fault is that.

It is my opinion that with so much unnecessary complexity coming into the specifying and supervising of highway surfacing with bituminous mixtures, in its many forms, a knowledgeable and experienced engineer purely to oversee this aspect of highway engineering is a significant asset to any highways department, we could call them "materials engineers".
 

INTERIM ADVICE NOTE 157/11 THIN SURFACE COURSE SYSTEMS - INSTALLATION AND MAINTENANCE   (INTRODUCED DECEMBER 2011, AND IS AN IMPORTANT INTRODUCTION)

The Interim Advice Note 157/11 Thin Surface Course Systems - Installation and Maintenance has been published and is available for download from the Highways Agency website
www.standardsforhighways.co.uk .

I strongly recommend that all who are responsible for purchasing, laying and maintaining proprietary Thin Surface Course System (TSCS) bituminous mixtures as surface courses on motorways and trunk roads,
or indeed proprietary Negative Textured System (NTS) bituminous mixtures for use on local highway networks, to download a copy of IAN 157/11 and study it thoroughly.

This is an excellent document, and contains much useful information and advice that is relevant to the laying and maintaining of TSCS and NTS bituminous mixture surface courses, and it does state that the "recommendations" should be implemented immediately subject to already existing contractual arrangements, except where compliance would incur significant additional expense.

However, as good as this document is in actually providing additional relevant information that is important when purchasing and laying these proprietary materials, in my opinion, it contains little specific information relating to purchasing and laying bituminous mixture to  TSCS.
This lack of specific information is because the bituminous mixtures referred to are proprietary materials that are supplied to BBA/ HAPAS system approval.

It follows that you must know the particular temperature and laying requirements that pertinent to the particular TSCS proprietary bituminous mixture that you are purchasing and laying, there is no uniform criteria for specifying and laying these materials as you will find with generic bituminous mixtures specified to British Standards, that is unless you include your bituminous mixture requirements in the contract document.

This document is what it says it is, it is an Advice Note, and there is much good advice contained within it, download it and read it, and take note of what is not included, and therefore what you may want included in your own contract documents.

It is not a specification, but if you want more control over the properties of the the TSCS bituminous mixture that you are purchasing you may like to read the item above "Guidance in Specifying Proprietary Thin Surfacing", the advice being provided by the CSS, now rebranded as ADE(E)PT

This Advice Note can be also be found in an appendix of 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 were 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.

For an external website that gives a fairly comprehensive description of the "original" German stone mastic asphalt, its development and specification, press, HERE.
 

INITIAL TEXTURE DEPTH REQUIREMENTS (AMENDED CLAUSE 921 OF SERIES 900 - AUGUST 2008)

IMPORTANT NOTE : From August 2008 there is a revised edition of,
900 Series of the Specification (MCHW1) for Road Pavements - Bituminous Bound Materials,   
there are now differing texture depth requirements for differing types of bituminous surfacing materials, you must study the revised/amended

Clause 921 - Surface Macrotexture of Bituminous Surface Courses : Table 9/2 Requirements for Initial Texture Depth

E,g. The initial texture depth for a "Thin Surface Course System to Clause 942 with an upper (D) aggregate size of 14mm.or less has been reduced to "Not less than 1.3mm.".
And, there are other important changes to initial texture depth requirements for particular highway locations and lower speed roads, again I say that you must study these revisions/amendments of required initial texture depth.

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.

(As an aside, it will now be difficult/impossible to compare tyre generated noise on Hot Rolled Asphalt and Thin Surfacing (Stone Mastic Asphalt) road surfaces on a like for like basis, as they now have differing initial texture depth requirements, 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 Surface Course 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.

Other useful pages on this website relating to laying bituminous mixture /asphalt surface courses "thinly".

For information on British Standard bituminous mixtures that can be laid thinly,  press ---------------------------------> HERE

For further information on laying bituminous materials as a thin surface layer, with photographs, press -------> HERE

For access to a table comparing and contrasting many bituminous surfacing materials, press -----------------------> HERE 

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