HOT ROLLED ASPHALT AND PRECOATS
A number of photographs showing the laying of hot rolled asphalt wearing course with a paving machine and then the spreading of the precoated chippings on to the surface, and finally the chippings being rolled in to the surface during compaction of the surface course asphalt.

But first a bit of basic specifying information.

BRITISH STANDARDS

The specification of these materials and how they should be transported, laid and compacted was comprehensively covered in :-

BS 594 : Hot rolled asphalt for roads and other paved areas

Part 1. Specification for constituent materials and asphalt mixtures
and
Part 2. Specification for the transport, laying and compaction of rolled asphalt.

If you are using hot rolled asphalt bituminous mixtures I would suggest that you retain  copies of the above as reference documents until such time that you are fully conversant with,  
BS EN 13108 : 2006 : Bituminous mixtures - Material specifications - Part 4 : Hot Rolled Asphalt

This recently published standard has superseded BS 594 from the 1st. of January 2008. 

This new standard is quite different in presentation and operation from the superseded BS 594.
It does allow the specification requirements to be stated for mixtures of the mix group Hot Rolled Asphalt for use on roads, airfields and other trafficked areas.
A mixture specification derived from this standard can be used either to declare the conformity of a mixture with known requirements or to make known what those requirements are.
As I read this document, at this moment this standard specifies empirical requirements only, performance requirements may be introduced at a later date.
This standard includes requirements for the selection of the constituent materials. 
BS EN 13108-4 is designed to be read in conjunction with EN 13108-20 and EN 13108-21, and with suitable study you will be able to specify, for practical purposes, the same bituminous mixtures (hot rolled asphalts) that you have already been supplied, and that you have found meet the engineering requirements that you require in your material.

PD 6691 Asphalt - Guidance on the use of BS EN 13108 bituminous mixtures - Material specifications
is now published and will assist engineers and technicians to understand the changes in the way you need to specify the bituminous mixtures you currently know and understand, and wish to keep receiving.
This standard is needed because some of the new bituminous mixture standards are not that "user friendly" in the opinion of many in the industry, and I included BS EN 13108 Part 4 : Hot Rolled Asphalt in that category.
In my opinion the changes in specifying hot rolled asphalt are significant, however if you know and understand bituminous mixtures you will be able to specify and order exactly what you are used to receiving at the present time, so EN 13108-4 is a good thing, if we are not able to retain BS 594 because of "European Legislation".

The "Part 2's" of BS 594 and BS 4987 have been combined into,
BS 594987:2007:Asphalts for roads and other paved areas -
Specifications for transport, laying and compaction and type testing protocols

However if you do not understand bituminous mixtures, and direct "translations" of the existing mixtures that you utilise are not included in PD 6691, the engineer or technician will be confronted with a number of tables of options relating to binder content, filler content, void content etc. for him/her to choose from, to specify the bituminous mixture that they require.
Engineers and technicians may find this rather daunting, so, if you wish to continue using hot rolled asphalt in a "traditional" manner and are not particularly knowledgeable of bituminous mixtures, may I suggest that you employ, either directly or indirectly, the services of an experienced independent Materials Engineer to guide you in the transition from BS 594 to BS EN 13108:Part 4.
I repeat, at this time, and perhaps for some time to come, that  it might be a good idea to retain your copies of BS 594 and BS 4987 in your technical library, as they contain a lot of exceedingly good information and guidance, which is not that easy to come by these days.
Just because they have been superseded, and therefore cannot be used in new contracts and agreements, does not mean the information contained in them is not useful, remember these standards have been used successfully, with some slight modifications, since the 1960's.

HOT ROLLED ASPHALT AND PRECOATS IS BY ITS NAME A "TWO LAYER" PROCESS The diagram shows that there is a layer of hot rolled asphalt wearing course laid by a paving machine, then a layer of chippings which are precoated with a 50pen bitumen is applied to the HRA matrix at a rate of approximately 10/14kgs. per square metre. But it is the surface texture that is more critical than rate of spread of chippings. A surface texture of 1.5mm., by "sandpatch" was the usual criteria for trunk roads and motorways whether it is Thin Surfacing or HRA and precoats, however TSCS's have been decreased to 1.3mm. Although this has been decreased to 1.2mm. for low speed roads, see, Clause 921-Surface Macrotexture of Bituminous Surface Courses, of the Specification for Highway Works, Amendment August 2008. Explained in further detail on page, Skid Resistance and High Friction Surfacing

The chippings are rolled into the asphalt "mat" with a smooth wheeled roller without vibration to ensure a smooth ride quality of the road surface.
This process allows the most economical use of scarce resources of high specification, high polished stone value (PSV) aggregate in relation to laying bituminous mixture surface course, as the premium aggregate is only required as a surface applied layer.
A lower quality, but above 45PSV, ( see BS 594 or appropriate part of BS EN 13108 ) aggregate can be used in the matrix of the layer.

THE LAYING PROCESS IN PICTURES WITH SOME ATTACHED NOTES

	Hot rolled asphalt paver, chipping spreader, and "three point" roller, making up a "paver train, you will see there is a also a second roller.
It is always wise to check the temperature of delivered bituminous material, before laying if possible. This is not always possible with modern health and safety legislation, but it certainly should be performed as the lorry tips into the hopper of the paver. Overheating at the time of production and during storage is one of the major causes of poor performance of bituminous road surfacing materials, and can reduce the life of the road surface considerably. Specified temperatures for mixing and storage of bituminous mixtures were able to be found in BS 594, until the 1st. January 2008, when this standard was superseded by,  BS EN 13108-4 : 2006 : Bituminous mixtures - Material specifications - Part 4 : Hot Rolled Asphalt and, BS 594987:2007:Asphalts for roads and other paved areas - Specifications for transport, laying and compaction and type testing protocols This particular load was in specification at approximately 170°C (50pen. HRA surface course has a maximum temperature of 190°C) Overheating is also quite likely to render any polymer modifier inert, so not not providing any improvement to the bituminous mixture that was intended.  Ways of testing for the performance of polymer modifiers are found in the Specification for Highway Works (SHW), Volume 1.

	The board, being positioned beneath the chipping spreader on the left hand side is to provide a chip free drainage channel against the kerb.
The height of the tamping/vibrating "bed" at the back of the paver is now controlled by hydraulic rams, but many years ago it was actually controlled by a coarse threaded screw mechanism, and to this day the operator who controls the height of the "bed" is known as the "screwman". The driver will operate the augers that supply the the bituminous material from the centre of the machine out to the edges of the mat so that the amount of material along the length of the bed is constant. Fluctuating amounts of material at the "bed" and along the "bed" can cause the bed to raise and lower hence producing a poor ride quality.  The "screwman" can check the thickness of the laid mat quite simply by pushing a sharp probe (usually a screwdriver) through the laid material to existing hard road surface below, marking it with his thumb and then measuring it against a rule, isn't technology wonderful. Of course any supervising person can also perform this simple task to ensure correct thickness of material is being laid, but be aware thickness tolerance is allowed in the Specification for Highway Works (SHW). If you are paying for work on a square metre basis rather than tonnage it is not uncommon for laid thickness to be less than the nominal thickness specified, but may well be within the specified tolerance. I always recommend paying for work on a laid tonnage basis, to remove the temptation to "lay to tolerance", and hence ensuring a thicker, more durable mat.

	Slight adjustments being made to the laid mat by the "rake hand", perhaps where the paver stopped temporarily. You will have to take my word for it, but the chap in the "all yellow" by the telegraph pole is the all important, "Engineer's Representative" on site supervision I am always recommending.
Precoated chippings being laid on to the already laid hot rolled asphalt (HRA) wearing course. The majority of criticism laid at HRA and precoat chippings as a road surface is a result of poorly maintained "chipping spreaders" and short staffed or  inexperienced chipping gangs operating them. So I am pleased to say these pictures show a gang showing how precoat spreading should be done.

The picture above shows a gang member "back chipping" small areas of light chipping, even though he chipping spreader is performing well.	A picture of a well chipped hot rolled asphalt mat, I would suggest a 10kg. to 12kg. rate of spread which in most instances is quite adequate. You do not need to over chip, which in the early days of texture depth specifying caused some problems with ride quality and tyre noise generation. This was largely "overplayed" by the "low noise" Thin Surface Course System "people", picking out particularly poor examples of very high textured, over chipped, HRA surfaces. Also note the chipping free drainage channel.
There is nothing at all wrong with the hot rolled asphalt and applied precoat process, it produces first class road surfaces that are very long lived, skid resistant and makes optimum use of scarce (and costly) high specification aggregate. It is always wise to check that the chipping spreader works correctly, and that the rate of spread of chippings is set correctly, and uniformly across the mat, before you start laying the hot rolled asphalt. If you have started to lay the asphalt, perhaps 20 to 30 metres, before you commence using the chipping spreader and then find it is not working correctly, then you are then you are already in trouble because the asphalt waiting to receive chippings is already cooling, and who knows how long it is going to be before the chipper is working properly.

It is usual for a "3-point" roller to reverse onto the newly laid mat and precoated chippings, this provides maximum compactive effort it applied, i.e. the greatest weight spread over the smallest area of roller. However if the roller driver decides that the mat is perhaps a little thicker than usual or the hot rolled asphalt is quite hot he will operate with the smaller (lighter) front wheel being the first compactive force on the precoats. There is a great deal of skill in every aspect of laying a good hot rolled asphalt and precoat surface course. Just sitting in the roller doing nothing is often the right course of action, even if that seems a contradiction.	The chippings must be rolled in to the HRA layer as soon as possible after laying, in order that the precoats become securely attached to the surface. This is because it is necessary for the bitumen coating around the chipping to melt and fuse with the bitumen in the asphalt so that a firm and complete physical bond is formed between the asphalt and the chipping when the new road surface cools to ambient temperature. The above, in my opinion is a good HRA and precoats surface, one could say it is even "negative textured", it is my experience it will not cause excessive tyre generated noise when compared fairly with other bituminous mixture surface course providing similar surface texture. I have been involved with TRITON in performing comparative tests on various surface course materials that have included a range of road surface materials.
Be careful of wind chill, high winds on a day of "moderate" temperature can cause more cooling of the asphalt surface than cold temperatures on a still day. It is good practice to not allow the paver to advance any faster than the chipping spreader can follow. If the asphalt surface chills before the chipping is applied to the surface, there will not be sufficient heat to melt the precoated binder layer on the chipping thus preventing a positive "melt" bond between asphalt and chipping. In the worst case scenarios the chipping is seen to sit in a "depression" in the asphalt without any asphalt surrounding it, these chippings are quickly lost under the action of traffic.

The "three point" roller, with cutting attachment, cutting a vertical joint on the fully compacted, but still plastic hot rolled asphalt mat.	Cutting the vertical joint, this will be cut so that the joint is in fully compacted material. This operation is performed after the mat has been fully compacted but while the material is still warm and plastic.

The vertical joint is coated with hot, liquid, 50pen. bitumen from a "pitch pot", if this is still allowed. I am not sure whether the "health and safety" stance by some contractors is laid down in "law" any where, or if it is their own decision to avoid using the established and proven pitch pot process. I am sure somebody will respond to these comments. The "modern" alternative is to use a proprietary tubbed thixotropic bitumen to paint joints. I am afraid I have no personal experience of the success of painting joints with this material. The "debris" from the cutting process will be removed by sweeping and/or a fore-end loader scraping over the binder course/old surface course. When the next mat of hot asphalt is laid the bitumen joint coating will melt and form a permanent sealed bond with the already laid surface. It cannot be stressed too highly how important it is to create good, "tight", joints between mats. You only have to drive round the highway network to see joints failing prematurely and creating serious problems, which can be very expensive to repair, that is if they can be successfully repaired.

It is also a fact that many proprietary Thin Surface Course Systems cannot, indeed must not, have a cut and coated joint as described here, usually meaning that for successful joints there needs to be no joints by laying separate mats in echelon so that fresh hot material bonds with fresh hot material. However on many sites this is not possible and you will have hot material laid against cold material with no form of joint preparation other than the sweeping of loose material from the joint. There are, or where, pieces of add on plant to a paver known as a joint heater, but I have not seen one in a long time on a "local" surfacing contract, whether they are still used on motorway and trunk road work I cannot say.

The above is a picture of an established HRA and precoat surface, still in excellent condition after 15 years, and showing an excellent rate of spread of chippings.

A close up of the surface to the left showing the nature of the surface of the actual "asphalt" which  you will see even better after you have magnified the image. Some Engineers and "writers" on the subject only class the surface of the aggregate as having microtexture, others would claim that the exposed "sharp" sand in the asphalt matrix is also to be regarded as microtexture. Either way the exposed "sharp" sand would exhibit high friction characteristics in dry/damp conditions. However a complete, similar, surface would fail to provide drainage channels for dispersal of water under vehicle tyres in wet conditions, or hysteresis dispersal of energy by tyre deflection in braking that larger macrotexture provides.

An example of a 20mm. precoated chipping. The use of a layer of precoats of high polished stone value (PSV) aggregate allows cost effective use of scarce and expensive aggregate resources. It is only this top layer of aggregate that needs to be of expensive, scarce, aggregate. Where as with Thin Surface Course Systems the whole of the layer must be of expensive, scarce, high PSV aggregate to ensure the exposed surface aggregate has the required friction characteristics.

For further information relating to hot rolled asphalt and precoats as a surface course press -------------------------> HERE

For close-up pictures of a hot rolled asphalt wearing course and other bituminous surfacings press --------------> HERE


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