PICTURES OF CORES THROUGH BITUMINOUS ROAD PAVEMENTS
AND
NOTES ON HIGH MODULUS (STIFF) DENSE BITUMEN MACADAM BASE (ROADBASE)

CONTENTS

Whole life costing
Road pavement failure
Hot rolled asphalt lower base (roadbase)
Stiff bituminous base (roadbase)
High modulus (stiff) bituminous macadam base (roadbase) and binder course (basecourse)
Real life aspects of laying stiff bituminous base (roadbase) and binder course (basescourse)
Binder content of dense bitumen macadam base (roadbase) 
Published information on stiff bituminous base (roadbase) and binder course (basecourse)
Road pavement trials using high modulus dense bitumen macadam base (roadbase)

IMPORTANT NOTE
I have been updating this page to show that basecourse and roadbase are now called binder course and base, and have been for some time, indeed younger engineers and technicians may not be aware of any other terminology. 
Whilst performing this process I also note that the term macadam is used quite often in the text below, however the bituminous mixtures previously known as "Bitumen Macadam" are now (from 1st January 2008) known as "Asphalt Concrete".
So you do need to be aware of the introduction of the BS EN 13108 : Bituminous mixtures  ,family of standards that were introduced on the 1st. of January 2008, and the changes in terminology and more importantly the major changes in the way that bituminous mixture are specified, resulting from the replacement of BS 4987 and BS 594 by the appropriate parts of BS EN 13108.
At this time I have not made any changes regarding the term macadam, in the text, as many of the reports that I refer to have the word macadam in their title, as it was the appropriate term at the time the study was undertaken.
I will leave you with the fact that whatever terminology that you use to describe a particular bituminous mixture it is its actual composition ("recipe"), i.e. aggregate proportions and bitumen content (and grade), that will determine the performance of the mixture in the road pavement, assuming that it has been mixed correctly, at the appropriate temperature, and that it has also been stored, transported and laid in the correct manner.
Not forgetting that the actual physical characteristics / qualities of the aggregate will play a very important role in the performance of the surface course (wearing course), and to a lesser degree in the binder course (basecourse) and base (roadbase).

I thought a few pictures, with a few accompanying notes might be useful to demonstrate what a road pavement can actually look like, consist of, and how long a life it might have.

WHOLE LIFE COSTING
Before the current popularity of analytical road design with the subsequent use of stiffer bituminous base (roadbase) mixtures and binder course (basecourse) mixtures, and the resulting thinner layers of bituminous materials, this depth of construction in areas of high traffic volume was quite common.
On roads were a high proportion of the traffic was heavy goods vehicles (HGV's) a deep road pavement was necessary to spread the considerable load from HGV wheels over a large area before the load was imparted to the subgrade.

As the scale is in 10mm. graduations, this particular road pavement is, more or less, 500mm. thick.
This therefore is initially an expensive design, but I am aware there are many road pavements of this type and thickness in our motorways and trunk roads that are already thirty years old and still in a strong condition with what could be an indefinite future life.

The binder in the dense bitumen macadam roadbases and basecourse in these cores would have been 100pen at the time of mixing.

This initial 100pen. after some initial hardening during the heating of the mixing and laying process will have further hardened, to a small degre, insitu and maybe in the region of 60/70 pen after a number of years, this hardening process increases the stiffness in the road pavement and is in fact beneficial to the strength of the road pavement.

A good number of these "thick" road pavements are many years old already, and indications are they still have a long life before them providing the integrity of the surface course is maintained.
The current "in" phrase is "whole life costing", and it is not just the cost saving relating to road reconstruction taking place at a later time, there is also the saving in inconvenience, and the cost this inconvenience imposes, on the road user.
And, a road pavement that has a long life is far "greener" than a road pavement that continually needs attention, and unfortunately there have been a number of those constructed/reconstructed recently in the name of so called progress.

See reference to, TRL Report 250 : Design of long-life flexible pavements for heavy traffic, below.

ROAD PAVEMENT FAILURE
I thought I would include this photograph to indicate what a core can show you, but having determined the crack is a major failure a trial hole and further site investigation is necessary.
We now have the debate is the crack propagating from the surface down, or rising from the subgrade, various engineers tend to favour one or other of the options, I am afraid I have to "sit on the fence", over this matter.

It is my experience where I have investigated serious failure in lean concrete base (roadbase) that it has been the result of poor earthworks, either in the choice of soil type, moisture content or compaction, that has resulted in voids forming under the lean concrete.
The lean concrete will bridge the voids for a time but will ultimately fail and crack and that crack will then quickly propagate to the surface.

But I have also been part of investigations relating to cracks in road surfaces, i.e. surface course (wearing course) and binder course (basecourse) that have ceased at the lean concrete layer, these cracks I would suggest have been initiated at the wheel / surface course (wearing course) interface.
Surface cracking being the result of the increasing heavy load applied by the tyres of modern heavy goods vehicles (HGV's), and the ageing and hardening that occurs to the bituminous binder of surfacing materials over time, areas of comparative weakness in the underlying bituminous layers being the areas where the cracks will form.

I would suggest that if you are experiencing road failure from serious cracking that you investigate the cause thoroughly before deciding upon the cause of the failure and a course of action, the cost of detailed investigation is likely to be money well spent.

Personal Note
I am aware there are some very clever pieces of apparatus about these days that can supply you with all types of analytical information, which can be very useful.

But, it is my opinion there is no substitute to also digging some decent size trial-holes and having a good look, and taking samples of the materials you excavate for laboratory testing, it is quite surprising what you sometimes find out.
Be on site at the time of the trial hole excavation, just how hard the gang have to work to excavate the hole will tell you a lot about the condition of the materials making up the road pavement. Simple "stuff" but true.


HOT ROLLED ASPHALT,
LOWER BASE (ROADBASE)
I am still unsure when the whole road pavement is of a bituminous nature whether cracks begin at the bottom and work up, or start at the top and work down.

But for those who favoured the bottom up theory it was reasoned that if the lower roadbase was a hot rolled asphalt the cracking would be much delayed or even prevented because of the gap graded nature of the material and the significantly higher binder content when compared to a dense bitumen macadam roadbase or basecourse.

The binder in a hot rolled asphalt is also a 50pen. bitumen, which is stiffer than a conventional DBM roadbase or basecourse.
The increase in binder content of a hot rolled asphalt roadbase tends to make the material more plastic in nature and able to accommodate some movement without cracking, and the binder stiffness will increase its load spreading properties.

The upper roadbase and basecourse were dense bitumen macadam, this material is less expensive than hot rolled asphalt that has a higher binder content, but still has excellent load spreading ability through the mechanical interlock of the aggregate particles.
Well that is the theory which leads to the type of core in this particular photograph.

STIFF BITUMINOUS BASE (ROADBASE)
It is currently popular to talk of stiff roadbases and the reduction in road pavement thickness you can achieve when you use an analytical approach to the design.
If it is stiffness you want in a roadbase you cannot get much stiffer than a lean concrete, and I personally favour this option over the use of bituminous roadbases that have low binder contents and very stiff bitumen binders, e.g. 15pen., 25pen.and 35pen..


So if you do favour a stiff roadbase to reduce construction thickness remember the lean concrete option.

HIGH MODULUS ( STIFF ) BITUMINOUS MACADAM BASE (ROADBASE)

It is able to be proved analytically, in the laboratory, that thin layers of roadbase material that contain stiff bitumens, are better able to spread load over the underlying layers because of their increased stiffness.

This argument continues, that if stiffer roadbases are more efficient at spreading load and are therefore "stronger", thinner layers of this stiff material are equivalent to thicker layers of conventional dense bitumen macadam roadbase.

Obviously if this assumption proves correct in practice, there is a considerable cost saving to be made in road pavement construction, and this potential saving has not escaped the notice of contractors when submitting bids for new work.

However the increased stiffness of material with lower penetration grade bitumens will only prevail while the roadbase retains its integrity, if the aggregate particles do not remain tightly bound by the binder the material will gradually begin to function as a granular layer and the failure of the road pavement will be quite swift.

REAL LIFE ASPECTS OF LAYING STIFF BITUMINOUS BASE (ROADBASE) AND BINDER COURSE (BASECOURSE)

It is unfortunate that too few people have experienced and understand the real world of bituminous material production and laying, and all that it entails.

E.g.'s,

1) Continuous supply of material to the paver, with no standing of the paver waiting for the next load to arrive with subsequent chilling of material before rolling.

2) Bituminous material supplied at a high (in specification) temperature for optimum compaction, lower temperature material may well be in specification but could be difficult to achieve full compaction.

3) Wind chill conditions on site that verge on the, "do we lay or not", the contractor may not want to lay but the producer will not want to stop production.

4) Enough rollers of the correct size to allow for possible breakdowns, or rollers not being available for compaction while they are refilling with water.

5) Any other on site problems that mean lorry loads of material have to "sit" on site losing heat.

6) No segregation in material during hot-bin storage, transportation and laying, 40mm. dense bitumen macadam roadbase is a material that is prone to segregation, and segregated areas will be full of voids and difficult to compact.

The above situations can cause enough problems with a conventional 100pen. bitumen, but with a 15pen., 25pen. and even a 35pen. bitumen it is serious, and if temperature is lost in this type of material who is going to be "brave" enough to send material back or remove poorly compacted areas.

BINDER CONTENT OF DENSE BITUMEN MACADAM BASE (ROADBASE)

If you are thinking of laying a 150mm. layer of dense bitumen macadam roadbase it is likely you will choose a 40mm. aggregate size, this will have a binder content target of 3.5% +/- 0.6%, so you could have a bituminous roadbase with as little as 2.9% binder and it would still be in specification.

Even if the binder target was continually met it is still not a lot of binder to "cement" a roadbase together that is designed to be laid in a relatively thin layer and withstand the load of a high number of heavy goods vehicles.

If you select a 28mm. nominal size dense bitumen macadam roadbase it will have a 4% binder target +/- 0.6%, which is still not a lot of binder.

It is also considerably more difficult to lay and compact very stiff bituminous materials, especially in the winter, and good compaction is absolutely essential with high modulus roadbases if they are to perform as the design intended.

It is because of this necessity for complete compaction that the French road builders use 35 tonne deadweight pneumatic rollers.

PUBLISHED INFORMATION ON STIFF BITUMINOUS BASE (ROADBASE)

I have brought together a number of TRRL/TRL reports on this subject for those of you who wish to learn more on the subject of stiff bituminous roadbases, especially dense bitumen macadam roadbases and basecourses.

TRRL Report 308 : Overlay design for improved macadams

This report describes the possible benefits of using IMPROVED/STIFF MACADAMS.
These improved macadams being known as :-
1) HEAVY DUTY MACADAM, (HDM) ----- ( 50pen. binder but with a higher, 7/11% filler )
2) DENSE BITUMEN MACADAM 50PEN, (DBM50) ---- ( 50pen. binder with standard grading )
The basic concept behind the use of these materials being that they are much stiffer than conventional (100pen. bitumen) macadams and are therefore possess better load spreading properties, meaning thinner layers of material or increased design life.
Increased resistance to rutting is also a possibility with these materials.

( This is an earlier report regarding increasing the stiffness of dense bitumen macadam roadbase and basecourse from what is regarded as the conventional viscosities of 100pen. and 200 pen. bitumen to a stiffer 50pen bitumen, with the option of increasing the filler to increase stiffness. )

Using a 50pen. bitumen has allowed some decrease in road pavement thickness and has been employed successfully in the design and construction of a number of road pavements.
This is not to be unexpected because the option of using hot rolled asphalt roadbase or basecourse with a 50pen. bitumen has been around for many years and has been used very successfully in road pavements, but it does contain a higher binder content and therefore it does cost a little more, but is less likely to fatigue.

TRL Project Report 66 - Evaluation of "Enrobe a Module Eleve" EME :
A French high modulus base (roadbase) material

( This report precedes TRL Report 231, and in my opinion TRL Report 231 should not be read in isolation, TRL Project Report 66 must also be studied. )

In particular I would draw your attention to the grading and binder content of the material the French used in their early use of stiffer (15pen.) base (roadbase) materials, the success of which promoted interest in stiffer base (roadbase) materials in the U.K..

TRL Report 231 : Road trials of high modulus (stiff) base for heavily trafficked roads.

This report describes trials of high modulus (stiff) base (roadbase) materials in road pavement construction, 15pen., 25pen., and 35pen., bitumen binders were used in the trials.

The increased stiffness of these materials increases the ability of the base (roadbase) to spread load and therefore thinner pavements can (in theory) be designed, thus providing a saving in cost of road construction.

( It is this report more than any other that has influenced the use of high modulus bitumen roadbases and basescourse, again I suggest you read TRL Project Report 66 and the other reports I mention before deciding whether the cost saving benefits at the time of construction will endure over the whole life of the pavement. )

TRL Report 250 : Design of long-life flexible pavements for heavy traffic

This report is a study of existing and past methods of flexible pavement design to assess how successful the various designs have been after many years of trafficking.
It indicates that flexible road pavements that were initially well constructed have maintained their strength, and in some cases gained strength.
Therefore it suggests well constructed flexible pavements will have a long structural life providing deterioration of the surface course receives remedial treatment at the appropriate time.

( This is a most excellent report and must be studied if you wish insight into long life bituminous road pavements. )

TRRL Report 633 - The effect of mix variables on the fatigue strength of bituminous materials

The work for this report was carried out at the University of Nottingham from November 1968 to October 1972.
The report was published in 1974, the main authors being K.E. Cooper and P.S. Pell.

( This report may seem quite old but it is a comprehensive work and I believe it is relevant to the subject of high modulus base (roadbase) as it not only deals with the dynamic stiffness of bituminous materials but equally relevant their fatigue. )

TRL Project Report 87 - Accelerated and field curing of bituminous roadbase

The report is a study of the curing, (hardening), of bituminous binder used in base (roadbase), and the development of a simple accelerated curing test able to be performed in the laboratory that reflects the on site curing characteristics of binders.

TRL Research Report 358 - Modified dense bitumen macadam base (roadbase)

This report covers trials where the bitumen binder has been modified in some way, e.g. polymer and metallic modifiers.
The report concluded that there was no significant improvement in the base (roadbase) incorporating polymers, but there was some improvement with the use of metallic modifiers, although metallic modifiers in other trials have given variable results, including early failure.

Also bitumens that were stiffer than conventional, i.e. 50pen., rather than a 200pen. or a 100pen., where included in the trials.
The work with a straight run 50pen. bitumen did confirm an improvement in roadbase performance using harder grades of bitumen.
It did however point out these materials require mixing and rolling at higher temperatures to ensure adequate coating and compaction.

ROAD PAVEMENT TRIALS USING HIGH MODULUS DENSE BITUMEN MACADAM BASE (ROADBASE)

The use of 15pen. and 25pen. in dense bitumen macadam base (roadbase) and binder course (basecourse) has currently been suspended by the Highways Agency while the cause of failure in road pavements where these materials were being trialed is being investigated.


Personal Note
It is my belief there of five groups of people involved with the provision of adequate highways.
a) Those who pay for the roads to be constructed.
b) Those who design roads.
c) Those who construct roads to the design provided, or alterations to the design approved by the designer.
d) Those who maintain the road during the course of its life.
e) Those who pay for the road to be maintained.

It is my opinion that in the long term groups (a) and (e) are broadly speaking the same purse.

It would therefore seem reasonable to ensure that groups (b) and (c) are encouraged in thinking about (d) whilst performing their respective tasks.
I am beginning to think that in an effort to hold down the cost of road building, which is commendable, some basic concepts regarding material fatigue and hence durability are being ignored.

That is my theory, which is mine (apologies to "Monty Python"), and I would love people to take the time to read the information that is available ( not just the references I suggest ) and arrive at theories of their own, and at the same time learn something about road building and maintenance.

[ Top of Page ]