[ Top ] Hot Rolled Asphalt and Bituminous Macadam

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

MODIFIED BITUMEN AND BITUMINOUS MATERIALS

Introduction to Modifying Bitumen and Bituminous Materials HAPAS (Highways Authority Product Approval Scheme)
What is a polymer Consideration of the total mixture / material chosen
Synthetic polymers Possible problems
Natural Rubber Specifying
The basic properties of bitumen British European Standards Relating to Modified Binders 
The ways addition of polymers may modify bitumen Cost effectiveness
The names of polymers you may need to know Recycling
Some of the publications/information available related to polymer modified bitumen Comment
   
Useful websites  

INTRODUCTION TO MODIFYING BITUMEN AND BITUMINOUS MATERIALS

Modified bituminous materials can bring real benefits to highway maintenance/construction, in terms of better and longer lasting roads, and savings in total road life costings.
But the choice of what materials to choose and how they perform has to be said is a bit of a minefield at present with little truly independent advice available, and this guide may help in making the necessary decisions.

This guide is an attempt to provide some fairly practical information on modified bituminous materials, particularly materials where the bitumens have been modified by the addition of a polymer.
I will outline what may be achievable by the addition of various modifiers, and the broad action of how it is achieved.

I will also outline some of the problems that can be encountered in the storage of modified binders, the production of bituminous material, and the storage of mixed material prior to laying.
It has to be said at the outset almost all modified bituminous materials are proprietary materials, this causes problems in determining the benefits of different materials on offer, and the ability to directly compare one material with another.
It is unrealistic, and is not what a highway engineer is paid for, to just believe the claims of sales representatives.

The exception to the growing number of proprietary modified materials on offer is rubberised material, this is able to be specified using ,
Road Research Laboratory, (R.R.L.), ROAD NOTE 36 and an appropriate generic bituminous mixture specified as a result of using the appropriate part of the BS EN 13108 : 2006:Bituminous mixtures. Material specifications family of standards.
The generic bituminous mixture specified can be, for practical purposes, identical to familiar specifications that are/were found in the
British Standards BS 594 or BS 4987.

NOTE : The British Standards BS 594 and BS 4987 were superseded by the
BS EN 13108 : 2006:Bituminous mixtures. Material specifications, family of standards on the 1st. of January 2008.

Then all suppliers of rubberised material will have to comply to the  same specification in the production, transport, and laying of this material, which will rule out the bother of having to consider the individual properties of proprietary materials.

WHAT IS A POLYMER, (polymers being the most common bitumen modifier)

The term "polymer" does not automatically mean a synthetic material.
It basically means a combination of a large number of similar small molecules or "monomers" into large molecules or "polymers".
The polymer will have different properties to the monomer.
There are a large number of naturally occurring polymers, these can be organic or mineral substances.
Such natural examples of polymers include hair, rubber, diamonds and sulphur.
Even bitumen could be regarded as a polymer because of the long-chain nature of some of the organic molecules that are the constituent parts of bitumen.

SYNTHETIC POLYMERS

These are polymers that have been manufactured in a chemical process to combine particular molecules in a way that would not occur naturally.
And although various synthetic polymers have been capable of being produced since the early part of this century it is the more recently developed polymers that are now being used to modify bitumens and produce the "new" bituminous binders.
The new polymers being the result of research and development by the large petro-chemical industries.
But when I say "new" most of the synthetic polymers used in the modification of bitumen have been around for 30 years or more.
What may be "newer" is the way that they are "mixed" / "blended" with the base bitumen.

NATURAL RUBBER

Rubberised asphalt, mainly surface course (wearing course) but also binder course (basecourse), has been used with a fair degree of success for over 40 years.
Rubber is a natural polymer and its action in a bituminous mix is similar to that of the synthetic thermoplastic rubbers (TR's).
Quite a lot of the original trial work including rubber in bituminous mixes was conducted in Leicestershire in conjunction with the TRRL and the rubber companies.
These trials are very well documented in TRRL reports and are an excellent source of information for anybody proposing to use natural rubber as a bitumen modifier.
Suggested reports to read are :-
TRRL REPORT LR 308 and TRRL REPORT LR 370.
These trials led to the production of a specification for rubberised asphalt,
i.e. ROAD NOTE 36, published by the T.R.R.L..

For further information on producing rubberised bituminous materials press -------------> Here

BASIC PROPERTIES OF BITUMEN

Bitumen is so useful in the road making and road maintenance industries because of its basic thermoplastic nature, i.e. it is stiff/solid when cold and liquid when hot, (well with penetration grade bitumens anyway).
(The modifying polymers used in bitumen are also thermoplastic in nature.)

The basic properties of bitumens can be modified by the addition of flux oils or volatile oils to produce bitumens of various grades.
These grades are specified by their viscosity, (penetration), and their softening point, this information, along with other physical characteristics is specified in,
BS EN 12591 : 2000 : Bitumen and bituminous binders  - Specification for paving grade bitumens 

This standard superseded BS 3690:Part 1, which is still often referred to.

The above ways of altering the characteristics of bitumen are really ways of decreasing the stiffness of the binder and increasing the workability of bituminous mixtures at lower temperatures, e.g. hand-lay work.
Of course penetration grade bitumens modified with flux oils or volatile oils  will have a lower performance in use.
It is a good policy to always try and use the highest/stiffest grade of bitumen in a particular mix consistent with being able to lay and compact it efficiently.

For further information on the viscosity requirements of bituminous mixtures, press ------->
Here

THE WAYS THE ADDITION OF POLYMERS MAY MODIFY BITUMEN

The polymer additives do not chemically combine or change the chemical nature of the bitumen being modified, apart from being present in and throughout the bitumen.
What polymers will do is change the physical nature of bitumens, and they are able to modify such physical properties as the softening point and the brittleness of the bitumen.
Elastic recovery/ductility can also be improved.

This in turn will alter the properties of the aggregate / bitumen mixture in which the modified bitumen is used.
These criteria are important in a mix with regard to problems such as wheel track rutting at high temperatures and fatigue cracking at low temperatures due to the brittleness of the mix.

The basic laying workability of the asphalt or macadam you are using will still be governed by the viscosity of the grade of bitumen you have specified.
It is usually the stiffer 50pen, or possibly 70pen bitumen that is modified.

The way the additive/polymer usually influences the bitumen characteristics is by dissolving into certain component fractions of the bitumen itself, spreading out its long chain polymer molecules to create an inter-connecting matrix of the polymer through the bitumen.
It is this matrix of the long chain molecules of the added polymer that modifies the physical properties of the bitumen.

Because of the thermoplastic nature of the polymers, some polymers will actually break up into their constituent molecular blocks at the high temperatures, during mixing and laying, and recombine into their polymer chains at lower temperatures, i.e. ambient temperatures.

What has to be ascertained in practice is the degree of modification that takes place, and whether the degree of improvement achieved in the overall qualities of the bituminous mix is worth having, and is it cost effective.

POLYMER NAMES YOU MAY NEED TO KNOW

Thermoplastic Rubbers, (TR's)

This may be regarded as a group name / description for a number of polymers/copolymers used in the modification of bitumen.
A copolymer is a polymer that has more than one type of molecule incorporated in the polymer.
These polymers are made up of many thousands of individual monomers/molecules built up into chains by the various polymerisation processes developed by the large chemical industries.

Styrene Butadiene Styrene, (SBS)

This is a thermoplastic rubber.
SBS is a copolymer that you will come across in bitumen modification, it was originally developed for use in the production of tyres and the soles of shoes, but is suitable for the modification of bitumen.

Ethylene Vinyl Acetate, (EVA)

This is not regarded as part of the thermoplastic rubber group but is still thermoplastic in its nature.
One of the uses for this type of polymer are the "hot melt" glues, the sticks of which you may be familiar with in "D.I.Y" hot melt adhesive guns.

The most common grade of EVA for bitumen modification, for road pavement materials, is the classification "150/19".
This classification means it has a melt flow index of 150 and a vinyl acetate content of 19%, how much you include in the bitumen to be modified for optimum benefit can be debatable, but 5% by weight is a commonly quoted figure.

EVA modified mixes have been around for some time now, I can remember them being used 25 years ago, and at that time EVA incorporation was claimed to make to make HRA wearing course more workable in cold weather.
In fact EVA was being added to 70 pen binder to produce a wearing course asphalt which indeed was much more workable in cold conditions.

The theory was good, because at higher mix temperatures the EVA does not increase the stiffness of the mix but at lower temperatures the EVA polymer recombines to increase the stiffness of the asphalt above that expected from a 70pen. bitumen.

But, it did cause all kinds of problems with rolling, one of the complications being chippings "lost" in the mat due to the initial reduced stiffness of the material, the other being differential cooling so that in some areas of the laid bituminous mat the EVA had recombined and in other areas it had not.
The differing rates of cooling produced different levels of stiffness in the mat and this in turn has been known to cause "tearing" in the mat whilst rolling 

PUBLICATIONS /INFORMATION AVAILABLE  RELATING TO POLYMER MODIFIED BITUMENS

The Shell Bitumen Handbook
As the name suggest a book produced by Shell, but if you make an allowance for that and its slight bias to the industry it is an excellent book, without being too technical, on bitumens and bituminous products, including a section on bitumen  modification.

Shell Bitumen Review 66, May 1992, (Bitumen/Polymer Special Edition)
Again a Shell publication therefore with a bias to Shell and industry, but it does explain, relatively simply, the production of the different polymers and how they may improve bituminous mixes.

Encyclopaedia of Materials Science and Engineering, Supplementary Vol.3
There is in this volume a very informative section on "Bitumens, Modified", unfortunately this is the sort of reference book you will have to get from a large library because you will not be able to afford to buy it.

TRRL Research Report 323 : Trials of porous asphalt and rolled asphalt on the A38 at Burton
Various modified bitumens were used on trial sections, including natural rubber, and there is information on these materials and how they performed.

TRL Project Report 109: EVATECH H Polymer modified bitumen
Well worth reading if you are considering using this bituminous binder in your material.

TRL PROJECT REPORT 61 : Assessment of Multiphalte, the Shell multigrade bitumen
Well worth reading if you are considering using this bituminous binder in your material.

TRL REPORT 157 : Rheological properties of polymer modified binders for use in hot rolled asphalt wearing course 

TRRL Research Report 122
This report relates to trials documenting the effect of EVA modified bitumens on hot rolled asphalts containing different fine aggregates, it gives good basic information on the addition of EVA to HRA surface course (wearing course).

CSS ENG/3-94 - POLYMER MODIFIED BINDERS FOR SURFACE DRESSING 
The purpose of the report is to investigate the use and performance of polymer modified binders used in surface dressing.
It is an informative report and well worth reading.

Draft for Development - DD 248 : 1999 : Determination of the hot storage stability of modified bitumen binders
( Identical with IP PM CH/1999 )

This standard specifies a method for the determination of the susceptibility of a preblended modified bituminous binder to separation or instability during prolonged periods of storage at high temperature.
It is applicable to modified binders which are stored at elevated temperatures for more than one hour prior to being incorporated into a hot asphalt mixture or otherwise used as a binder.
It is not applicable to modified binders produced by adding the bitumen and polymer separately to an asphalt mixture, blending the modified binder immediately prior to mixing, or where continuous agitation of the modified binder prevents separation from occurring.

BS ISO 13741-1 : Plastics / rubber - Polymer dispersions and rubber latices : Determination of residual monomers and other organic components by capillary column gas chromatography : Part 1 : Direct liquid injection method

Specification for Highway Works - Volume One - 900 Series : Road Pavements : Bituminous Bound Materials:-
Clause 939 - Determination of Cohesion of Bitumen and Bituminous Binders
(Vialit Pendulum Test)
Clause 941 - Modified Binder Storage Test

Both the above tests are now to be found in the, 
Interim Advice Note 101/07, 

which is a complete, revised
 
900 Series of the Specification (MCHW1) for Road Pavements - Bituminous Bound Materials   
Introduced on the 1st. of January 2008 to provide the revision needed to support the changes produced by the introduction of,
BS EN 13108 : 2006:Bituminous mixtures. Material specifications family of standards.

DTP DESIGN MANUAL HD 37/99:(Surface Dressing - Binders - SMA - Thin Wearing Courses)
This DESIGN MANUAL FOR ROAD BRIDGES updates HD 37/97 and now includes information on a variety of surfacing materials and processes, including a section on, binders and binder modifiers.

HD 37/99 now includes information on :-

Laying bituminous surface courses,
Binders and binder modifiers,
Hot rolled asphalt, porous asphalt,
Thin wearing courses,
Stone mastic asphalt,
High friction surfacing,
Slurry surfacing,
Retexturing.


There are of course many other books/papers/reports on this subject, these are just a few of those I know about, at a level that may help individuals similar to myself who just require a reasonable background into the subject.

I would however suggest caution in reading some "promotional" technical literature.
Although it may not tell any lies, it sometimes presents information in a way that can mislead the reader, unless you have some deeper understanding of bitumen/bituminous mix qualities.

HAPAS (HIGHWAYS AUTHORITY PRODUCT APPROVAL SCHEME)

HAPAS is administered by the British Board of Agrement (BBA), closely advised by the Highways Technical Advisory Committee (HiTAC), certificates are issued by the BBA acting on recommendations from HiTAC.
Specialist groups (SG) are created for each product type put forward for consideration, and the group will report back its findings to HiTAC.
SG4:- Modified Binders
A more difficult Specialist Group which itself has been split into two Working Groups, a lot still to be sorted as I understand the situation.
Although this group exists I am not aware that an individual polymer/modifier has achieved an appropriate HAPAS certificate. 
The bituminous mixture production industry preferring to achieve HAPAS approval for a "system" that uses a bitumen modifier, however with "system" approval the modifier may be changed, be aware of this. 
Therefore because of the differences in the composition of any individual bituminous mixture, and indeed the amount/percentage of modified bitumen in the mix it is difficult to determine the performance of the modified bitumen by the performance of the mixture that it is a part of.

CONSIDERATION OF THE TOTAL MIXTURE / MATERIAL CHOSEN

By this I mean what type of mix, (aggregate make-up), are you thinking of modifying, or more to the point what type of modified mix is somebody trying to sell you.

Is it ?

What we might call a usual 30%/14mm Hot Rolled Asphalt surface course (wearing course), but with a modified binder.

A material similar to a High Stone Content Asphalt.

A well graded asphalt concrete (macadam) similar to a 14mm or 10mm Close Graded Macadam Surface Course (Wearing Course).

A macadam surface course (wearing course) of an "open" nature such as Pervious Macadam/Porous Asphalt.

Or, a proprietary "Thin Surfacing" supplied under a Thin Surface Course System (TSCS) approval, and many distinct mixtures may be are covered by a single system approval.

All the previously noted mixes/aggregate combinations will have certain inherent properties of stability/durability/flexibility/texture/skid resistance, porosity, etc. that will be independent of the bitumen modifier.
It may be a far better engineering judgment as well as a financial one to select a different material more appropriate to site conditions than to chose the wrong material and try and rectify its shortcomings by modifying the binder.

POSSIBLE PROBLEMS

The possible problems with modified bitumens are mainly in the storage of the bitumen, mixing temperatures, and the length of time the material is held at elevated temperatures before laying.

The blending of bitumen and polymer is not an easy process, so modified bitumen is usually purchased by the quarry in a ready blended form from the bitumen supplier.
This means the quarry normally has to take a 20 tonne tanker load, and with an 8% binder content asphalt this will produce 250 tonnes of asphalt.

Of course the lower the bitumen content/percentage the more tonnage of mixed material produced per tonne of modified bitumen, but this begs the question will the bituminous mixture not acquire increased engineering benefits  from a design that has a higher, improved, binder content.
I know my opinion is countered by the argument that because the bitumen is "improved" you can use less of it in the bituminous mixture, but I thought I would mention it any way, as it is a matter that I think does need consideration.
On the occasions I recommended the use of a modified bitumen I always insisted on the same binder content, or indeed in the case of the use of natural rubber the normal binder content plus the addition of the rubber added as latex.

Wherever/ whenever I had the authority I did not approve the lowering of binder contents, the polymer modifier was an addition to the binder content, not a replacement for a reduction in binder.


However, the need for pre modified binder does usually rule out the production of  small tonnages of most modified bituminous materials, as it is not financially feasible to buy in the modified binder for a small amount of work.

It is usually necessary for the modified bitumen to be held in a tank that is capable of being agitated in some way, as the polymers being of a different density to the bitumen tend to separate if kept in storage for prolonged periods

The polymer additive can be destroyed by too high a temperature in mixing, or by being held at a high temperature for a long period of time after mixing, even the binder storage times should be kept as short as possible or deterioration of the polymer may take place.

It is difficult to be specific because conditions will vary for different materials and additives, but site staff who are taking temperatures should be aware of these possible problems.

You do not want to spend a lot of money on a bituminous material that has a modified bitumen, for the incorporated modifying agent, polymer, to have been rendered inert, thus removing the engineering benefits that it would have imparted to the mixture.

SPECIFYING

As previously stated most materials containing modified binders are sold as proprietary materials with certain claims for their properties.

A tender document incorporating a specification for proprietary materials would have to be based on a number of measurable performance criteria common to all materials, so that suppliers of different materials that are able to meet these performance criteria will be able to submit tenders.
This is the only way various suppliers could be treated equally.
But this raises the question of being able to perform rather complicated and costly performance type testing on supplied material to ensure the material complies with the specification.

The number of
possible tests for determining the type of polymer, the percentage included in the "base" bitumen and the benefit "on the road" are increasing.

Tests for determining the improvement in the bituminous mixture attributable to binder modification include,

the Indirect Tensile Test, using the Nottingham Asphalt Tester to test for Elastic Stiffness,
and
the Wheeltracking Test in accordance with BS 598:Part 110.

Tests on the actual binder include,
Clause 939 - Determination of Cohesion of Bitumen and Bituminous Binders (Vialit Pendulum Test), which can be performed on reclaimed binder.

But to repeat myself the performance of the laid bituminous mixture will depend upon many more factors than just the inclusion of the modified binder, so unless all other parameters are "equal" it is difficult to state that any improvement in the laid surface is due to the use of a polymer modified binder.


BRITISH EUROPEAN STANDARDS RELATING TO MODIFIED BINDERS

A specific polymer can be incorporated in a bitumen and that particular modified can be used to replace what we may regard as a standard binder in a "British Standard" material.
This way of specifying a modified bitumen would be relatively easy, similar to the method of specifying rubberised binder, and allow all regular suppliers of coated material to tender on an equal footing in a normal manner, and it would ensure you were able to use the polymer modifier you believed most suitable for the material and site conditions. 

Quite recent British European Standards include a number of specifications relating to the use, storage and testing of modified binders, such as,

BS EN 13398 : 2003 : Methods of test for petroleum and its products - BS 2000-516 : Bitumen and bituminous binders - Determination of the elastic recovery of modified bitumen
(Identical with IP 516-2004) 

This standard specifies a method for the determination of the elastic recovery of bituminous binders in a ductilometer at a given temperature. It is especially applicable to bituminous binders modified with thermoplastic elastomers (polymers).


BS EN 13399 : 2003 : Methods of test for petroleum and its products - BS 2000-517 : Bitumen and bituminous binders - Determination of storage stability of modified bitumen
(Identical with IP 517-2004) 

This standard specifies a method for measuring the storage stability of modified bitumens/binders at high temperatures.


BS EN 13589 : 2003 : Methods of test for petroleum and its products - BS 2000-520 : Bitumen and bituminous binders - Determination of the tensile properties of modified bitumen by the force ductility method 
(Identical with IP 520-2004) 

This standard specifies a method for determining the tensile properties of a bituminous binder, in particular those of polymer modified bitumens by means of a force ductility test.

BS EN 13632 : 2003 : Methods of test for petroleum and its products - BS 2000-518 : Bitumen and bituminous binders - Visualisation of polymer dispersion in polymer modified bitumen 
(Identical with IP 518-2004)
 
This standard specifies a method for visualisation of the polymer distribution in a polymer modified bitumen by fluorescent microscopy.
The method is applicable for most of the commercially used polymers, but before the method is used it should be examined whether the test is applicable for the actual polymer. This method should only be used for identification purposes, i.e. in conjunction with production control.
Sample preparation and treatment have an important influence on the test results and it is essential to follow strictly the method described to achieve comparable results.


BS EN 13702-1 : 2003 - Methods of test for petroleum and its products - BS 2000-513 : Bitumen and bituminous binders - Determination of dynamic viscosity of modified bitumen -
Part 1 : Cone and plate method ( Identical with IP 513-2004)

This standard specifies a method for determining the dynamic viscosity of a modified bituminous binder over a range of temperatures by means of a cone and plate viscometer. 

BS EN 13702-2 : 2003 : Methods of test for petroleum and its products - BS 2000-514 : Bitumen and bituminous binders - Determination of dynamic viscosity of modified bitumen - Part 2 : Coaxial cylinders method - 
(Identical with IP 514-2004) 

This standard specifies a method for determining the dynamic viscosity of a bituminous binder / modified bitumen over a range of temperatures by means of a coaxial viscometer. Although the method has been developed for modified binders, it is also suitable for other binders.

BS EN 14023 : 2006 : Bitumen and bituminous binders - Framework specification for polymer modified binders
This standard provides a framework for specifying the characteristics and relevant test methods for polymer modified bitumens which are suitable for use in the construction and maintenance of roads, airfields and other paved areas.
The framework covers the following four characteristics :-
"Consistency at intermediate service temperature" (surrogate characteristic : penetration)
"Consistency at elevated service temperature" (surrogate characteristic : softening point)
"Cohesion", and "Durability" of consistency
The cohesion property has been included as a means of discriminating between polymer modified bitumens and other bituminous binders. 
The other essential requirements, "adhesion" and "setting ability" are indicated by tests carried out on the finished asphalt mixtures.

A guidance document relating to this standard can be downloaded from the website of the Refined Bitumen Association (RBA), it is to be found by selecting "Bitumen" from the toolbar at the top of the homepage and then selecting "Testing and Standards" from the list of items in the drop down menu, the guidance document is at the bottom of the page, or it was.

Draft for Development - DD 248 : 1999 : Determination of the hot storage stability of modified bitumen binders
( Identical with IP PM CH/1999 )

This standard specifies a method for the determination of the susceptibility of a pre-blended modified bituminous binder to separation or instability during prolonged periods of storage at high temperature.
It is applicable to modified binders which are stored at elevated temperatures for more than one hour prior to being incorporated into a hot asphalt mixture or otherwise used as a binder.
It is not applicable to modified binders produced by adding the bitumen and polymer separately to an asphalt mixture, blending the modified binder immediately prior to mixing, or where continuous agitation of the modified binder prevents separation from occurring.


COST EFFECTIVENESS

Bituminous materials containing modified bitumen binders are very expensive, especially the proprietary materials.   
In some cases the extra cost will be justified on heavily stressed sites, but there are a wealth of very good "conventional'' materials available that are quite capable of satisfying most highway requirements, if you know what to specify.

RECYCLING

There is a great deal of "plastic" waste being generated from household and agricultural waste, e.g. plastic bottles and poly' tunnels. It is quite feasible for some of this material to be incorporated in to bituminous mixtures, both as an "aggregate" bulking material, and as a modifier/improver of existing bituminous mixtures.
The plastics/polymers with the most obvious potential are low and high density polyethylenes, (LDPE and HDPE).
As collection and sorting of these materials becomes more established and mechanised it is possible that "mountains" of reclaimed material will reduce the cost of LDPE and HDPE to the point where the inclusion of these materials in bituminous mixtures will be commercially driven.

Considerable research work, world wide, is being performed on the inclusion of "rubber crumb" from old tyres to find an environmental answer to disposing of used tyres, whilst improving the properties of the bituminous mixtures that contain this rubber.
There are now a number of well documented technologies relating to rubber crumb inclusion in hot bituminous mixtures, and a quick search on the internet will find them, however at this time it seems they are relatively expensive however "green" that they may be.
It is likely that the cost will come down as the technology develops, the production costs of the rubber crumb come down, and the problem of adding rubber crumb into the mixing process in a very hot environment are over come.

COMMENT

This is a fairly rudimentary guide, and I do not claim to be an engineer of the chemistry of bituminous binders.
It is a gathering together of information that is available, and that may be considered before the decision to choose or use modified bituminous materials is made.
But I have to say that there is always doubt in my mind, that the engineer/technician may not always receive the benefit he is hoping to achieve, and is paying for, when using polymer modified bituminous mixtures.
I have personally experienced polymer modified bituminous mixtures that have performed extremely well, more than paying for the increased cost in terms of performance (e.g. resisting wheel tracking) and durability.
Therefore there is no doubt that correctly designed, specified, manufactured, stored and laid polymer modified bituminous mixtures will deliver engineering and cost benefits, in highly trafficked and difficult situations.
However, I have also experienced the use of polymer modified materials that I regard as having failed miserably after only a few years, but outside any two year guarantee period, with some of the successful and failed mixtures being supplied and laid to exactly the same specification / contract requirements.
Bituminous mixtures recommended for particular site conditions do need to last considerably longer than two years in my opinion, in whatever situation they are employed.
Unless more certainty can be brought into the field of supplied bituminous mixtures containing polymer modified binders I find it difficult to recommend these mixtures for use on local authority highway networks, even if they are felt to be required, in most cases they will not.
I am concerned with the manner in which these products are "marketed", I would suggest caution, and demand evidence of performance, and details of how it can be monitored. 
In my opinion it would be good to see a greater role played by the producers/suppliers of polymer modified binders in the performance and testing of the laid bituminous mixtures, and I believe the RBA, and indeed some individual bitumen producers may have already taken steps in this direction.


USEFUL WEBSITES
I try to refer to as few commercial sites as possible in compiling my site, but when a site offers particularly useful information about a subject I make an exception, this is not an endorsement of the company or product.


For further information on polymer modified bitumens, press --------------------------------------------> HERE (.pdf format)


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