[ Top ] Hot Rolled Asphalt and Bituminous Macadam

The Idiots' Guide to Highways Maintenance
Copyright 2000/14, C.J.Summers

HOT ROLLED ASPHALT AND PRECOATED CHIPPINGS
 

BRITISH STANDARDS FOR HOT ROLLED ASPHALT AND PRECOATED SURFACE COURSE THE CHIPPING SPREADER THAT LAYS THE REQUIRED RATE OF PRECOATED CHIPPINGS
THE TWO LAYERS - HOT ROLLED ASPHALT AND THE APPLIED HIGH PSV PRECOATS COMPACTION / ROLLING OF THE LAID ASPHALT MAT AND APPLIED PRECOATED CHIPPINGS
EXAMPLE OF STABLISHED HOT ROLLED ASPHALT, AND PRECOAT SURFACES CUTTING THE CENTRE JOINT (BETWEEN MATS) IN THE LAID HOT ROLLED ASPHALT
THE PAVING MACHINE THAT LAYS AND CONTROLS THE THICKNESS OF THE ASPHALT IMAGES OF A NEWLY LAID HRA AND PRECOATS SURFACE

BRITISH (BS) AND BRITISH EUROPEAN (BSEN) 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This recently published standard has superseded BS 594 from the 1st.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. 

The hot rolled asphalt and precoat process, showing paver, chipper and roller operating in unison, with an excellent "gang".This new standard is quite different in presentation and operation from the superseded BS 594.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 6691:2010: 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 published document (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 594987:2010 (supersedes the 2007 edition) : 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.



THE TWO LAYERS
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 12/14kgs. 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 was Thin Surfacing or HRA and precoats.
Although this has been decreased to 1.2mm. for low speed roads.
See,
Clause 921-Surface Macrotexture of Bituminous Surface Courses, the August 2008 amendment, for full details of the recent changes as they are significant in relation to proprietary Thin Surface Course Systems.

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.

ESTABLISHED HOT ROLLED ASPHALT AND PRECOAT SURFACES

The picture above shows the surface where a 14mm. precoated chipping has been spread.
The use of a 14mm. chipping gives a less rugous surface, but will still give adequate texture in most situations.
When a 14mm. chipping is used it is only necessary to lay a 30mm. thick layer of a 30% / 10mm. aggregate size hot rolled asphalt wearing course.
It is not widely known that this material is available.
This particular photograph is of a busy street in the centre of my home town and is at least 25 years old and still in excellent condition. 
This picture shows a HRA surface that has the more usual 20mm. precoated chippings.
The coin is a 1 coin in both photographs.
HRA and precoats has been the standard wearing course surface for many, many years, and has rendered excellent value for money.
Failures of this material I have been involved in investigating have been due to poor specifying of the correct material for the site in question, material not produced to specification, or poor laying practice.

 

THE PROCESS

THE PAVING MACHINE
The paving machine will lay the HRA to the required thickness to meet the line and levels set out for the site.
The mat thickness will need to be appropriate to the size of the chipping and the percentage of coarse aggregate in the bituminous mixture.
E.g. a 35%/14mm. HRA wearing course to receive 20mm. precoats will be laid 45/50mm. thick.
THE CHIPPING SPREADER
It is the quality of the chipping spreader and the gang that operate it that will largely dictate whether a HRA and chips wearing course is regarded as an acceptable surface with regards to its appearance and ride quality.
If you have a poorly maintained and "sloppy" chipping spreader it is going to be very difficult for even a good gang to produce a good spread pattern of chippings.
An evenly spread 12kgs. per square metre is usually sufficient to provide a 1.5mm. texture depth, providing the hot rolled asphalt is not supplied and rolled too hot, when the precoated chippings will be lost into the material.
On rare occasions, usually after heavy rain on sand stockpiles, you may experience "wet sand" in the asphalt. This can cause loss of stability in the asphalt and loss of chippings into the matrix. It can also cause poor ride quality as the paver is unable to control the thickness of layer of the unstable asphalt.

 

COMPACTION
The compaction HRA and precoats is quite a skilful process.
Roll the mat too soon and you could have the precoats disappear in to the asphalt if the material is at the top end of the temperature range, or the material thickness is greater than it should be.
Allowing the precoats to "sit" on the mat without immediate rolling when the asphalt is at the bottom end of the temperature range, especially if the ambient temperature is low, and you are going to find it difficult to press the chippings into the mat.
With a worst scenario of there will insufficient heat in the asphalt mat to melt the bitumen on the precoat to form a positive "glued" bond between asphalt and chipping.
Without this actual bitumen bond chippings will detach at a later date, even if "dented" into the asphalt surface.
Be vary careful of the wind chill factor produced from high winds associated with permitted laying temperatures. In these conditions always keep paver, chipper and roller working closely together.

 

CUTTING THE JOINT
Employing the correct procedure in cutting the joint is extremely important.
It is good practice to create a vertical joint sufficiently set into the mat to remove all loose and uncompacted material. 
The fully compacted joint will later be completely painted with hot (liquid) 50pen. bitumen to produce a complete bonding when the next rip of hot asphalt is laid adjacent to it, melting the applied bitumen.

If you are able to lay against recently laid material that is still hot, above minimum rolling temperature, the cutting of joints can be omitted.
A full description of cutting joints in laid hot mix bituminous materials can be found in both BS 594, and BS 4987, the Part 2's, now superseded by 
BS 5949871:2007.

 

It is usually recognised that you need to cut in to the mat a distance equal to the depth of the mat to "find" fully compacted asphalt.

A correctly cut and painted joint will last indefinitely as an enormous number of examples on the highway network will show.
Unfortunately there are almost a similar number of failed centre joints that have received or will need remedial treatment.
In some cases joints can be so poor that they will be the reason for early resurfacing of the site.

Without strong, experienced on site supervision you will not know to what standard the joint has been produced, until it is too late, as all but the poorest of joints will last longer than the "guarantee" period, which is normally one, but possibly two years.

A correctly designed, manufactured, stored, transported and laid hot rolled asphalt and precoat surfacing has the potential for a thirty year life.
I know of no other bituminous mixture that has this potential, not even "German" designed stone mastic asphalt, and I could demonstrate many examples.

 


THE NEWLY LAID SURFACE

The above photographs indicates what a newly laid surface should look like after the precoated chippings have been applied and after they have been rolled in to the asphalt.
There are areas of surface where the rate of spread of chippings is almost "book" perfect, i.e. more or less shoulder to shoulder cover without chippings being laid on chippings.

There are also a few areas of surface that need a few more precoats, but generally these photographs show a quite  acceptable surface for a newly laid mat.

Over chipping, with precoats resting on precoats, will give chippings loss and a poor ride quality and appearance. 
In my opinion it has been over chipping on motorway and trunk road work for fear of not achieving the 1.5mm. texture depth (sand patch) that has resulted in a macro-texture far above what is necessary, and useful.  
It is my belief that it is these over chipped surfaces that have given good HRA with precoat surfaces the reputation of being a road surface that generates unacceptable levels of road noise when compared to other surfaces.
With the introduction of IAN 101/07 texture depth requirements on low speed roads have been decreased to 1.2mm., by volumetric patch method, which should help address this situation in some areas.

 
Surface texture, in the picture shown above, will improve as the asphalt gets worn away by the action of traffic as the photographs at the top of the page indicate.
It is necessary to remember that freshly laid surface of any material is not usually the surface the material will have for most of its service life, ( the exceptions being surface dressing and cold applied high friction surfacing ).
E.g. you will not benefit from the high Polished Stone Value (PSV) of the aggregate from which the precoats are manufactured until the binder coating has been worn off by traffic.
This does not mean the initial Skid Resistance Value (SRV) of the surface will be unacceptably low, but that it will be substantially higher when the bitumen coating has worn off.
And do not discount the "sharp" microtexture supplied by the sand/fine aggregate in the areas between the applied chippings, a good HRA and precoats surface course has almost a "dual function" surface, in my opinion.

This process of surface binder removal applies to all bituminous mixture surface courses, ( and some resin bound materials ), with the materials that have a greater "build" of bitumen around the aggregate surface usually taking the most time for the actual aggregate, and its associated engineering properties, to be exposed to vehicle tyres.


Further information relating to hot rolled asphalt and other bituminous mixtures, press ------------------> HERE

Further information on the rates of spread of precoated chippings, press -----------------------------------------> HERE

"Aspho-Chip" and "Aspho-Chipless", press -----------------------------------------------------------------------------------------> HERE


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