BRITISH STANDARD BITUMINOUS MIXTURES THAT CAN BE LAID THINLY ( i.e. 40mm. or less)
USING TACK COATS, AND BOND COATS TO ENSURE ADHESION

IMPORTANT
From the 1st. of January 2008 the bituminous mixture (materials) descriptions have changed, for further information click ------> HERE
I include pre and post January 1st. 2008 material descriptions in the table below.
This does not mean the "old" bituminous mixtures will not be available, but it does mean that you must be aware of the new description, and more importantly that you have defined/specified the "target composition" of the bituminous mixture description that you want to receive when you purchase that particular mixture.
I fear there could well be a substantial amount of confusion in the whole (supply and use ) of the bituminous mixture industry, with the purchaser/user experiencing the greater difficulty, with possibly "alternative" and "improved" materials being offered, at "improved" prices.

I have now updated the table below to provide new information on particular bituminous mixtures that are able to be currently specified by reference to,
PD 6691 - 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.
It is up to you the specify and purchase bituminous mixtures in a manner to avoid confusion so that both you and your Supplier / Contractor completely understand the bituminous mixture required for the particular site.
It is then up to the supervising Engineer to ensure that he receives that particular bituminous mixture in the nature and condition stated in the specification / contract.
I would suggest that you always sample (representatively) and test at a rate of at least one sample per hundred tonnes, or from any load that gives you concern.
In my opinion sampling and testing is cost effective even though you may not see the benefit for a number of years. Not only does this procedure provide you with information on a particular material on a particular site, it will also build up your information on individual suppliers and you can support those the provide the better bituminous mixtures, i.e. mid-point gradings and mid-point binder content.
This of course is only possible when you have a specification for the bituminous mixture ordered.

! ! !  THIS INFORMATION IS FOR GUIDANCE ONLY - YOU KNOW YOUR LAYING SITES - OR YOU SHOULD - YOU MAKE THE DECISIONS  ! ! !
 

CONTENTS

TABLE OF BRITISH STANDARD MIXTURES THAT CAN BE LAID THINLY
NOTES FOR LAYING BITUMINOUS MIXTURES THINLY
TACK COAT AND BOND COATS
PROPRIETARY MATERIALS ( "THIN SURFACINGS" SUPPLIED TO A THIN SURFACE COURSE SYSTEM)
THE APPROPRIATE BRITISH STANDARDS
LAYER THICKNESS
CONCLUSION
 

TABLE OF BRITISH STANDARD MIXTURES THAT CAN BE LAID THINLY

NOTES FOR LAYING BITUMINOUS MIXTURES THINLY

Three of the materials I include in the table above were actually described as ''Preferred mixes'' in latter editions of BS 4987 and BS 594.
''Preferred mixes'' having  been introduced into BS 4987 to encourage rationalisation of coated macadam and hot rolled asphalt compositions, and the British Standard quotes :-
''Mixtures suitable for most uses could be selected from this reduced range and purchasers were encouraged to adopt their use wherever possible''.

I have quoted the normally used binders for the materials mentioned in the table, for stiffer materials and increased stability the viscosity of the binder in the mix can be increased accordingly.

For less stiff materials, and increased workability at lower laying temperatures reduce the viscosity of the binder, but be aware this will reduce the stability of the material in use.

A good example of the use of a less stiff binder are the Medium Temperature Asphalts (MTA's), which are in fact High Stone Content Asphalts, (HSCA), with a 100pen or 200pen bitumen according to the traffic conditions of the site.

The 6mm. macadams with 200/100pen binders when machine laid are cost effective alternatives to slurry macadam where site conditions permit, i.e. if you can lay the increased thickness that you will obtain with a 6mm. macadam you will achieve a thicker layer to the road pavement for your money.

Bituminous mixtures laid thinly cool very quickly on laying, ensure compaction takes place while the material is still hot.
The stiffer mixtures can ''drag'' when laid, especially if the material is to the bottom end of the laying temperatures, and the paving machine is going too fast for the material workability.
Instruct/request the supervisor to decrease the paver speed if dragging is occurring.

TACK COATS AND BOND COATS

Tack Coat (i.e. the spraying of a suitable generic bitumen emulsion)

It is important that all bituminous mixtures laid thinly have good tack coats applied to the existing surface prior to laying to ensure bonding of the thin layer to the existing road.
This means a complete wetting of the existing surface with a K1-40 emulsion, to produce a complete coating of bitumen once the emulsion has ''broken''.

BS 594 stated 0.35 to 0.55 litres/metre², and this will ensure the complete coating as shown in these images.

A black wiggly line disappearing into the distance, or a mist spray so thin you can see through it, are not to be regarded as a tack coat. The images above and to the left show the level of coverage you should be obtaining with a correctly applied, but basic, tack coat. In the majority of surfacing this level of tack coat will be quite sufficient to ensure the complete bonding of the surface course to the underlying road surface, whether it be a new binder course or an existing surface course that is being overlaid. In my opinion it is only necessary to use a proprietary bond coat where the surface course is pervious/porous in nature to prevent the ingress of water into the binder course and subsequently the road pavement. This is more likely to be the case with many proprietary "Thin Surface Course System" bituminous mixtures (motorways and trunk roads), and proprietary Negative Textured Surfacings" (local highway networks). The bituminous mixtures included in the table above can be regarded as practically impervious except were stated, especially if the correct rate of spread of tack coat is applied completely sealing the existing surface. The exceptions being the 6mm. medium textured wearing course and the 10mm. open graded wearing course, and these are suitably qualified in the text, a bond coat may be the preferred option with these materials.

"Open" mixtures, whether generic or proprietary will allow passage of water through the material matrix to a greater or lesser degree depending upon their grading.
This may or may not be a problem but should be considered and the possible use of a thicker bond coat to ensure an impervious layer between surface course and binder course if necessary.
It is my experience that providing you obtain the specified rate of spread for generic tack coat this is quite sufficient to ensure bonding of surface course to binder course in all situations where the binder course is a generic "dense" binder course.

The exception to all this advice on tack coats and bond coats is if you are constructing a SUDS (Sustainable Urban Drainage) road pavement or car park, then you would only use a mist spray of tack coat, if at all, as you want the surface water to drain through bituminous mixture layers.

The difficulty with the tack coat and bond coat procedure arises when the weather is wet and you are faced with the dilemma whether to use any tack coat / bond coat at  all, because you do not want the emulsion washing down drains and gullies, and blocking them.
Not permitting laying is an easy enough decision to make while sitting in your office, when you are on site with all the pressures of work on real roads, in real situations, at real costs, it is not so easy.

Bond Coats (Spraying of a proprietary bitumen emulsion, possibly polymer modified)

The use of tack coats and bond coats is comprehensively covered in,
Section 5.5 of BS 594987 : 2007 : Asphalts for roads and other paved areas - Specification for transport, laying and compaction and type testing protocols.

Where bond coats are part of a "Thin Surface Course System" their use is generally regarded as part of the "system".

However I believe that in the course of time bond coats may come on to the market that will be regarded as individual products and their nature and use will be "governed " by,
BBA HAPAS SG4 Guideline Document for bond coats, this may now already have happened.

Images of a modern self propelled tack coat / bond coat sprayer are shown above, these are steered by a simple "jockey" wheel at the front and usually provide a more reliable coverage of tack/bond coat than a hand sprayer.
You can see by the tow bar that this particular sprayer is towed to site behind a works truck, but some more compact types are hydraulically lifted on and off the back of a large lorry.

On larger sites surface dressing sprayers will may be used to apply the tack/bond coat, this is an expensive option on small sites. It is still most usual, on small sites, to see hand pulled sprayers used to apply the tack coat. All systems are capable of applying K1-40 at the required rates.

Good information on bond coats, that is able to be downloaded, can be found on the website, www.rea.org.uk/tech_data/sheet5.htm

IMPORTANT - Be aware it is quite possible to lay too much tack coat or bond coat that will produce its own problems.
These problems, especially where there is "ponding" in "low" areas can result in "bleeding through" into the surface course, especially with pervious/porous bituminous mixtures.
Even more bizarrely, with too thick a layer of  tack coat / bond coat, usually in hot weather, in areas of severe vehicle braking, the surface course can "slide" on the binder course causing ripping of the surface. This is not as common as "bleeding through" but I have seen it.

PROPRIETARY MATERIALS ("THIN SURFACINGS" SUPPLIED TO A THIN SURFACE COURSE SYSTEM)

There are an increasing number of proprietary materials coming on to the highways maintenance scene, and they are being very forcefully marketed.
Most of these products are excellent materials when you choose the correct material for the job in hand.

But, they are expensive compared to a similar British Standard material.

They are not always as comprehensively specified as a British Standard material, so you may not know what you are actually buying.

Exaggerated "marketing" claims are being made for some of these materials especially in view of the fact they are still relatively recent introductions to the UK market, not having the many years of proven use of generic bituminous mixtures.
Also, the similarity between British Standard bituminous mixtures and proprietary bituminous mixtures is sometimes remarkable, a BS material in many instances will comply with the proprietary specification.

I once received ''sales'' literature for a new proprietary material from one of the very large bituminous material suppliers, sold under a single particular brand name.
The company offered any size grading I should require, with a range of polished stone values (PSV's), coupled with any viscosity of binder I needed all under the same proprietary product name.
This practice is known as a "Thin Wearing Course System", and you still need to know which mixture in the "system" is most suitable for your surfacing need.
Why not just find a suitable generic bituminous mixture for a particular site from PD 6691 (or BS 4987 or BS 594 if you still have copies) and have funds left in your budget to help surface another site.

 THE APPROPRIATE BRITISH STANDARDS

The popular standards for bituminous mixtures that have been superseded,

BS 594:Hot Rolled Asphalt for Roads and other Paved Areas :-
Part 1:Specification for constituent materials and asphalt mixtures
Part 2:Specification for the transport, laying and compaction of rolled asphalt

BS 4987: Coated Macadam for Roads and other Paved Areas :-
Part 1:Specification for constituent materials and mixtures
Part 2:Specification for transport, laying and compaction

and the new specifications that were introduced on 1st. January 2008

BS EN 13108-1  : 2006:Bituminous mixtures. Material specifications. Asphalt Concrete
BS EN 13108-2  : 2006:Bituminous mixtures. Material specifications. Asphalt Concrete for very thin layers
BS EN 13108-3  : 2006:Bituminous mixtures. Material specifications. Soft asphalt
BS EN 13108-4  : 2006:Bituminous mixtures. Material specifications. Hot rolled asphalt
BS EN 13108-5  : 2006:Bituminous mixtures. Material specifications. Stone mastic asphalt
BS EN 13108-6  : 2006:Bituminous mixtures. Material specifications. Mastic asphalt
BS EN 13108-7  : 2006:Bituminous mixtures. Material specifications. Porous asphalt
BS EN 13108-8  : 2005:Bituminous mixtures. Material specifications. Reclaimed asphalt
BS EN 13108-20 : 2006:Bituminous mixtures. Material specifications. Type testing
BS EN 13108-21 : 2006:Bituminous mixtures. Material specifications. Factory production control

and, published in 2007,

PD 6691:2007:Guidance on the use of BS EN 13108 Bituminous mixtures. Materials specifications,

BS 594987:2007:Asphalts for roads and other paved areas - 
Specifications for transport, laying and compaction and type testing protocols,

The above British Standards are comprehensive WORKING DOCUMENTS and if you have any involvement in the specifying, supervision, or purchasing of bituminous materials you MUST have access to these specifications, the information they contain is excellent.
In fact many of the more reputable proprietary materials even deferred to Part 2 of either BS 594 or BS 4987 for transport laying and compaction.
Any reasonable sized organisation whether Supplier, Contractor and definitely Engineering Consultant should have a Technical Library containing these documents or subscribe to an "on line" company providing such access.

The "NEW" bituminous material industry really does not like people being aware that all these British Standards exist, they would much rather market a proprietary material at an enhanced price that has a lot less onerous specification, i.e. their own specification/recipe.
But it is not beyond the bounds of possibility that the "industry" will introduce a range of proprietary, "branded" mixtures that were/are the well used "preferred mixtures" of BS 4987 and BS 594, at proprietary prices.

If you do not have copies of BS 4987 and BS 594 I would advise that you obtain them, and use the mixtures contained in them as reference mixtures, the days of these standards have passed as a specifying option, but the information they contain will always be useful and allow you to design/specify, for practical purposes, the same proven bituminous mixtures. 
Although the "sample" bituminous mixture contained in PD 6691 are, in most cases, very similar to the proven mixtures contained in BS 4987 and BS 594.

I will repeat, you should also be aware that the "NEW" proprietary bituminous mixtures, (and that is all they are, mixtures of bitumen and aggregate in varying formulations ) are being marketed as "Systems" so in many cases one proprietary name will apply to a number of distinctly different materials, with different engineering properties.
This applies to bituminous mixtures supplied to a "Thin Surface Course System" for motorways and trunk roads, and "Negative Textured"  bituminous mixtures for none motorway and trunk roads, both "sets" of materials will be proprietary, although in reality they are likely to be the same "sets" of materials.

So if you do choose the option of  procuring proprietary bituminous materials for constructing and maintaining your highway network be very sure you know what you are buying, and in this respect you will find more information on the page,
Proprietary "Thin Surfacing" & "Negative Textured Surface" Bituminous Mixtures

And, take on board the advice that can be found in, CSS Report ENG 1/2003 - Advice Note for the Specification of Thin Surfacing.
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" 
which is available to download as a .pdf file on the website, www.roadscodes.org

LAYER THICKNESS

The thickness of layer for the various aggregate sizes in the table above are based upon, but modified by a little personal experience, figures that were stated in tables contained in BS 4987:Part 2, and are such that the material will have a fairly dense nature when laid, in line with procedures that have been standard practice until recently.
More comprehensive tables regarding layer thickness can be found in, Tables 6A, 6B AND 6C of,
BS 594987:2007:Asphalts for roads and other paved areas - Specification for transport, laying and compaction and type testing protocols

The bituminous mixtures, nominal aggregate sizes and layer thickness combinations quoted in the table above, have been recommended because dense layers of bituminous materials will better prevent the ingress of water into the road matrix and also reduce the rate at which bitumen oxidisation occurs.
Hydraulic scouring as water is forced through porous surface courses and increased oxidation both being conditions that will reduce the life of open graded surface course materials.
If water can progress into the lower layers of the road pavement then the life of the road pavement will be also be reduced.

With the recent introduction of proprietary Thin Surface Course Systems, with a predominantly open graded/ single sized aggregate content, there was a body of thought to lay mixtures in thinner layers to provide a high surface texture.
(Although the original high initial texture requirements, still required for HRA and precoats, has recently been substantially reduced for Thin Surface Course Systems.)
This means that depending on the nature and thickness of the mixture it is likely that the material layer will be porous in nature and will not provide a physical barrier to the ingress of water into the road pavement, and the rate of oxidation of the bitumen will be increased.
This in turn will mean a reduced life compared with Hot Rolled Asphalt Wearing Course, or other dense bituminous mixtures used for the surface course layer of a road pavement.

CONCLUSION In my opinion, in all but the most exceptional cases, on the none trunk road and motorway network, i.e. 95% of the national highway network, one of the bituminous mixture options included in the table above will be appropriate for laying at a thickness of 40mm. or less. The option of choosing a British Standard defined bituminous mixture means that you can go to all suppliers and expect to receive a "standard" material that falls within the limits of the specification, thus encouraging price competition, on an equal basis, for the material you want Unfortunately this process has been negated in many instances by organisations opting to enter contracts with single suppliers/contractors through which all materials must be purchased, it is understandable that such suppliers/contractors will always favour their own products. But may I just point out that you do not have nine bituminous bituminous mixtures to select from, you have twenty, as most of the mixtures mentioned have at least two options of binder/bitumen viscosity, depending upon site trafficking. Each change in binder will provide a bituminous mixture with different engineering properties, and characteristics, e.g. a mixture with a stiff binder will provide a "stiffer" surface course more resistance to rutting/deformation, but it will be more difficult to lay, especially if you are laying it by hand, and it will need rolling/compaction at a higher temperature. I prefer the option of selecting the bituminous mixture to suit the site conditions rather than using a modified binder to enhance a mixture so that it may be suitable for particular stress situations. In very difficult situations, and these will be few, on the networks I have already defined, you have the option of specifying your own preferred modified bitumen to further enhance the engineering characteristics of the material, not all modified bitumens offer the same degree of improvement. You must also be aware of the increased mixing, storage and laying criteria when using a modified bitumen to ensure maximum benefit from its inclusion and the extra cost incurred. If you read any of the literature provided by the modified bitumen supplier you will almost certainly find a section stressing the importance of they way their product is stored both before and after mixing, with special importance placed on time of storage and temperature of storage. And, of course, you will need to consider the aggregate source for the material matrix, or applied chipping, to make sure that you have a laid material surface with the required skidding resistance, but this will not change the composition of the aggregate structure. Be aware if you wish to lay a thicker surface course, there are also other excellent British Standard bituminous mixture options. I would still use the British Standard expression "preferred" for these bituminous mixtures, to be found in PD 6691, or refer back to BS 4987 and BS 594, and transfer those particular specifications into current terminology, some suppliers offer more help than others in this process. You will find reference to these thicker options on other pages, this page is intended for bituminous mixtures laid 40mm. thick or less. Oh dear!, I can feel it all getting complicated again, I can see the "suited" engineers reaching for their glossy brochures, or talking to the various marketing "suits", or marketing "suit" if you are allied to only one supplier/contractor. They could always seek guidance from their Materials Engineer, or Road Pavement Engineer, if they had one.

For further information on laying a thin bituminous surface course, press --------------------------------------------------------> HERE

For a table comparing the various thin asphalt / bituminous surfacing materials that are available, press ------>HERE

[ Top of Page ]