RUBBERISED HOT ROLLED ASPHALT WEARING COURSE

CONTENTS

INTRODUCTION TO RUBBERISED ASPHALT
SPECIFICATIONS
RUBBERISED HRA SURFACE COURSE (WEARING COURSE) TO RESIST CRACKING
RUBBERISED HRA SURFACE COURSE (WEARING COURSE) TO RESIST WHEEL TRACK RUTTING
PRACTICAL TIPS

Personal Note

INTRODUCTION TO RUBBERISED ASPHALT

Rubber is a naturally occurring polymer, and a very good one, when added to bitumen it will increase its elasticity, decrease its brittle point and increase its softening point.

The benefits the addition of rubber imparts to a bitumen binder are well documented, and these benefits will be passed on to the bituminous mix the rubberised bitumen is incorporated in. However subtle changes in mix design can create a material that is primarily used for resistance to cracking, or conversely a material to increase rut resistance of the wearing course.

Although this page was prepared when hot rolled asphalt (HRA) surface course (wearing course) was still a surfacing option on motorways and trunk roads in England ( only "Thin Surfacings" are a permitted surface course at present ) you are of course still able to use HRA/SC's on roads not funded by the Highways Agency.
I believe that In Northern Ireland or Scotland you are still be able to use HRA/SC's on motorway and trunk roads, and you may modify it by the addition of rubber if you wish.
The information and process of adding the rubber is more or less the same for any bituminous mixture.

Rubber is usually added to the bitumen part of a material in the form of a liquid latex metered into the mixing drum by a pump system, or it should be, I have heard tales of it being added from a container by hand, very naughty.
The latex is supplied in metal drums of varying sizes according to what quantities are being used, the latex has quite a long storage life while in the sealed drums (and frost free) several months at least.
Because rubber is added to the bituminous material in this way, only as much latex is used as is needed, and means small tonnages of modified material are able to be produced at reasonable cost.

When using proprietary polymer modified bitumens it is usually the case that a supplier will have to purchase at least a fifteen tonne tanker of binder, this can rule out small tonnage jobs because the cost of the modified mixture has to allow for the cost of buying the tanker load of bitumen, also pre-modified bitumen is not able to kept in storage for long periods.
Conditions of storage, and length of time of storage will vary with the type of bitumen modifier that has been incorporated in the bitumen.

Although I now know of at least one company that can supply the polymer Styrene Butadiene Rubber in latex form similar to the natural rubber product, and it can be added to the mix in the same way as rubber latex.

I have read articles that have suggested some synthetic polymers are not compatible with bitumen from all sources, but that natural rubber appears to be able to blend with bitumens derived from all crude oil stocks.
This comment may be worth keeping to mind in case you have unexplained problems, however if you can be assured the source of the bitumen you are going to receive in your material is based on Venezuelan crude you should not have any problems from incompatibility.

SPECIFICATIONS

A description of a procedure for the addition of rubber in a mixing plant used to be described in :-
Clause 938 of the DOT Specification for Highway Works, which is the Clause relating to Porous Asphalt
All Clause 938 now contains is a reference to,
BS EN 13108-7   : 2006:Bituminous mixtures. Material specifications -  Porous asphalt,
whether this new standard contains the same reference to the use of rubber as a bitumen modifier you will have to check for yourself.

Rubber latex is added normally at a rate of 4% to 5% of the weight of the bitumen component, this is actual rubber, so you add rather more latex because the latex is only 70% rubber.

The production and specifying of rubberised asphalt is well documented in TRRL Reports :-

TRRL Report LR 308 : Resistance to cracking of rubberised asphalt : Full-scale experiment on the A6 trunk road in Leicestershire
and
TRRL Report LR 370 : Full-scale road experiments using rubberised surfacing materials

The production of rubberised asphalt has its own Road Note :-

Road Note 36, Specification for the manufacture and use of rubberized bituminous road materials and binders.

It is 30 years old but still appropriate, and was referred to in, Clause 931 of the current Specification for Highway Works, until quite recently.

I include all this boring stuff to demonstrate that there was/is more published, ‘‘open’’ information on rubber as a polymer additive than many of the more recent arrivals.

And for anybody, ‘‘suit’’ or otherwise, to just tell you that a bituminous material has got a polymer additive in it without telling you which one it is and how much has been added and what temperature conditions are necessary for its production, storage and transport is not even telling you half a story, but I digress from the rubber theme.

The information presented in glossy advertisements in engineering journals is usually woefully inadequate, if not to say misleading, for an engineer to make a technical assessment of how the product will affect a particular bituminous material.

However, the documents,  
Clause 928 : Determination of the Complex Shear (Stiffness) Modulus (G*) and Phase Angle (ð) of Bituminous Binders using a Dynamic Shear Rheometer (DSR)
and
Clause 939 : Determination of Cohesion of Bitumen and Bituminous Binders
contained in the,
Interim Advice Note 101/07 - Revised Manual for Contract Documents for Highway Works (MCHW) - 
Specification for Highway Works 900 Series : Road Pavements - Bituminous Bound Materials
set down procedures and references for the testing of all "fresh" and reclaimed binders in an equal manner. 
Note, this is a testing of binders to be used or have been used in bituminous mixtures, this is not a testing of the complete bituminous mixture, which is covered elsewhere in the "900 Series" in relation to various bituminous mixtures.

The difference in performance that a binder has on the bituminous mixture can only truly be compared if the mix constituents and proportions are the same with only the binders being different. If mix designs are also different you will not know what stability is achieved from the actual design (and its constituent parts) and what stiffness/elasticity is related to the particular binder, modified or not.

The storage of the modified binder, or binder modifier, prior to mixing, 
the performance of mixing (drum/batch mixer, duration of mixing time), 
the temperature of mixing, 
the temperature of storage of the mixed material, 
the manner of storage, (heating and insulation of the storage bins)
the length of time of storage (including time held on the lorry)
can all have a significant effect on the engineering performance of exactly the same bituminous mixture designs and constituents with exactly the same binder.
These factors will have a similar effect on generic straight run bitumens, but the reduction in potential engineering performance of a modified binder is likely to be greater than that of a straight run binder.

Further information on Polymer modified binders can be accessed by pressing --------------------> HERE

RUBBERISED HRA SURFACE COURSE (WEARING COURSE) TO RESIST CRACKING

Rubber is generally added to an asphalt when we are laying it over areas that we are know are liable to reflective cracking, whether it be from joints in concrete roads, failure of lean concrete base (roadbase) or underlying movement in the road pavement.

A correctly formulated and manufactured rubberised asphalt surface course (wearing course) laid a minimum thickness of 40mm. will resist cracking quite successfully compared to a normal asphalt.

Where reflective cracking is severe and deep seated through the bituminous layers overlaying concrete roads or lean concrete base (roadbase) it is quite effective to remove all bituminous material over the cracked concrete and replace it with a rubberised HRA binder course (basecourse) material, the most popular material being 50%/20mm. HRA basecourse, (which is a relatively binder rich bituminous mixture).

Rubberised asphalt, binder course (basecourse) and surface course (wearing course), over joint cracking is a cost effective alternative to other more complex systems found in the "market place" for controlling reflective cracking in bituminous material surfacing over concrete roads.
TRL trials have shown that you need to produce quite a rich asphalt combined with the rubber to produce a material best suited to prevent cracking.

The richness of the mix is not usually a problem in use as the rubber will increase the softening point of the bitumen and offset any tendency for the richer material to be prone to wheel tracking.

Please note the following description/notes will refer to the superseded,
BS 594-1:Hot rolled asphalt for roads and other paved areas - Part 1 Specification for the constituent materials and asphalt mixtures
I do this because it is entirely possible to specify very, very, similar bituminous mixtures (HRA) using, 
BS EN 13108-4   : 2006:Bituminous mixtures. Material specifications -  Hot rolled asphalt 
the standard that superseded BS 594 on the 1st. of January 2008.
I leave you to determine the Target Mixture Composition using BS EN 13108-4 that will be the same as the BS 594 specification that I give as an example.

This usually means a BS 594 ‘‘Recipe’’ 30%/14mm. asphalt with a Schedule 1A bitumen target of 7.8%, plus 4% rubber, which gives a target of 8.1% +/- 0.6%.

Now we have to start out-thinking the production plants !

A material in the 8.0% to 8.5% range, including the rubber has proved to be a successful mix formulation.
But, if as an example, we specify a target of 8.1% we will receive material in the binder range 7.5% to 8.1%, i.e. the bottom half of the specification, modern bituminous plant technology is that good it allow suppliers to do this quite successfully, and they do, I have seen enough results in my career to know this is so, and they are quite within their contractual rights to carry out this practice.

Note, the specification does allow +/- 0.6% tolerance either side of the specified binder content.
But if we specify a target of 8.6%, (i.e. Schedule 1B plus the rubber percentage, for those of you who know your BS 594), we will receive material in the binder range (including rubber) of 8.0% to 8.6%, which is what we want.

Yes it will be a rich ‘‘sticky’’ mix, and it will be difficult to lay, and it will stick to everything, or it should do, and the workmen who rake and shovel the material and clean the paver will complain, if you are not getting this sort of reaction on site something is wrong.

You may also have a slight fatting of the surface after rolling.
But do not panic! Correctly apply your precoated chippings and do not over roll, let the material cool to ambient temperature, the increased binder softening point will prevent rutting and you will have a surface that will resist cracking successfully.

All this trouble is why you pay approx. £25 to £30 a tonne, laid price, more for this material than a conventional recipe mix, it is known as the ‘‘buggeration’’ factor, and of course the cost of the latex which is not cheap.
The cost of rubber latex does tend to be a bit erratic depending on the world commodity price of rubber which itself fluctuates due to climatic factors in rubber producing countries affecting crop production.

RUBBERISED HRA SURFACE COURSE (WEARING COURSE) TO RESIST WHEEL TRACK RUTTING

All the points mentioned above need to be taken into account in a similar manner, but starting from the point of initially specifying a 35%/14mm. "Design" HRA Surface Course (Wearing Course) with a 8kn. stability requirement but with a minimum Target Binder Content of 7.2% plus the added rubber.

A material with 35% percent aggregate will naturally be a more stable mix than a 30% mixture, and to decrease the bitumen content a little, compared to a "Recipe" mix is a good idea, but you do not want a "dry" mix or there will not be a sufficient modified binder coating on the aggregate to impart the improved characteristics of the binder to the material.
Never forget that the stability of a hot rolled asphalt is largely dependent upon quality of the sand used in the mixture, and it is known that some sources are far better at producing a "good" hot rolled asphalt surface course and they do carry a premium.

PRACTICAL TIPS

1) Know what mix properties you require and specify it fully.

2) Check that the mixing plant is capable of accurately dispensing and distributing the latex into the mix.

3) Ensure mixing temperatures are strictly observed and keep the length of time before laying as short as possible.

4) Try and employ a laying contractor who is experienced in the laying of rubberised material, if he does not have this experience take the time to inform him of possible difficulties.

An example of laying a thin bituminous surfacing that has a bitumen modified by the addition of rubber, 
can be accessed by pressing, ----------------> HERE

Personal Note

Many of the points made on this page will apply to other good quality bitumen modifiers, but the bulk of my experience with modified bituminous mixtures has been with natural rubber as the modifier, so I cannot comment on these other products.

Also, most of the information here will be appropriate to modifying most bituminous mixtures, indeed a recently Highways Agency approved ( pre HAPAS ) Thin Surfacing had natural rubber as the binder modifier.

Take note that with HAPAS approval of "Thin Wearing Course Systems" your preference for a particular bitumen modifier may not be provided in the proprietary bituminous mixture you have decided to specify / purchase.

I am not even sure in the information I have read to date whether a supplier can obtain approval for a product whilst using one bitumen modifier and then change the modifying agent in the bituminous mixture and the HAPAS "Thin Surfacing System" approval is still valid.

It is interesting to note that in IAN 101/7,
in, Clause 937:Stone Mastic Asphalt (SMA) Binder Course and Regulating Course
it states in Para 4, "Without BBA HAPAS certification, polymer modified binders, binder modifiers or additives shall not be used without the approval of the Overseeing Organisation".
As far as I am aware unless there has been quite recent changes, Specialist Group 4 - Modified Binders of the Highways Technical Group (HITAC) of HAPAS, never got around to developing procedures for the approval of individual binder modifiers, or modified bitumens carry proprietary titles.

Further information on BBA HAPAS approved materials can be accessed by pressing ----------------> HERE

Personally, I take this to mean that if you are the Overseeing Authority you can decide which binder modifier, or modified binder, you wish to specify in your bituminous mixtures where you need an increase in engineering performance of the bituminous mixture of your choice.
But nothing is that simple because HAPAS approved proprietary bituminous mixtures, Thin Surfacings, will have been approved whilst containing a particular (usually a proprietary polymer) bitumen modifier.
This could well be the case for motorways and trunk roads where the use of proprietary Thin Surfacings as the surface course is compulsory.
But this need not be the case with Local Authority highway networks, unless of course you are using proprietary Negative Textured Surfacing instead of generically specified bituminous mixtures.

Confused, do not blame me, but I think that you may need to talk to your Materials Engineer. 

I apologise if my retirement has not allowed me to keep up to date on these matters, but unless things have changed dramatically and very recently I believe my assumptions are correct.

Personally, I find the approval and use of "Thin Wearing Course Systems" and "Negative Textured Surfacing" quite confusing, and I hope as time passes the way this process works will be clarified. 
Until that time I would suggest caution, and invest some time in finding out what the material you are ordering, actually consists of. 
This is not an unreasonable request because, believe it or not, the supplier must have a design ( "recipe" ) on his database to be able to constantly produce a particular bituminous mixture, 
and,
the supplier of the modified binder must know the nature of his "stock" straight run bitumen and the nature of the polymer that has been added, how much has been added, and the engineering properties of the resultant modified binder, at least in the laboratory if not in the road pavement. 

This knowledge may be more readily acquired by using generic bituminous mixtures specified to the various parts of BS EN 13108 : 2006:Bituminous mixtures. Material specifications, rather than with "commercially sensitive" proprietary products.

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