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 most importantly what effect new "stiff roadbase
(base)" thinking
may have on the life of a road pavement.
WHOLE LIFE COSTING
Before the current popularity of analytical
road design with the subsequent use of stiffer bituminous base (roadbase) and
binder course (basecourse), 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 road
pavement life, with only the surface course receiving attention as and when
required.
The binder in the dense bitumen macadam roadbase (base) and basecourse (binder
course)
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 in situ 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.
HOT ROLLED ASPHALT,
LOWER ROADBASE (BASE)
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 (base) 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 (base) or
basecourse (binder course).
The binder in a hot rolled asphalt is also a 50pen. bitumen,
which is stiffer than a conventional DBM roadbase (base) or basecourse (binder
course).
The increase in binder content of a hot rolled asphalt roadbase (base)
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 (base) and basecourse (binder course) 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 ROADBASES (BASE) It is currently popular to talk of stiff roadbase (base) 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 (base) you cannot get much stiffer than a lean concrete, and I personally favour this option over the use of bituminous roadbase (base) that have low binder contents and very stiff bitumen binders, e.g. 15pen., 25pen.and 35pen.. So if you do favour a stiff roadbase (base) to reduce construction thickness remember the lean concrete option. |
HIGH MODULUS (
STIFF ) BITUMINOUS MACADAM ROADBASE (BASE)
It is able to be proved analytically, in the laboratory, that
thin layers of roadbase (base) 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 roadbase (base) 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 (base).
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 ROADBASE (BASE) AND BASECOURSE (BINDER COURSE)
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.
Examples :-
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 ROADBASE (BASE)
If you are thinking of laying a 150mm. layer of dense bitumen
macadam roadbase (base) 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 (base) 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 (base) 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 (base) it will
have a 4% binder target +/- 0.6%, which is still not a lot of binder, especially
at the bottom end of the specification, which is where it is likely to be.
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 roadbase (base) 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 ROADBASE (BASE)
(INCLUDING ENROBÉ À MODULE ÉLEVÉ
- EME, "ORIGINAL" AND EME2)
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.
Note - I still refer to 100pen., and 200pen. viscosity grades in this item
because that was what was used in the older road pavements I am referring to,
but be aware that the 100pen. grade has now been replaced by a 125pen. grade,
and the 200pen. grade has been replaced by a 190pen. grade.
This changed could have an effect on the bituminous mixtures using 125pen.
viscosity and a change to a 50pen. binder may be appropriate on higher
trafficked local authority road networks).
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.
New !
TRL Report TRL636 - The application of Enrobé à Module
Élevé in flexible pavements
("EME2")
This excellent report provides much good information on recent work that has been
performed to achieve good results in trials with an EME that has been produced based on
French design principles.
However TRL636 then proceeds to worry me a little by appearing to suggest that
options are being explored to produce proprietary EME materials that are different to those that
have been successfully trialled, i.e. undercover in test facilities with a huge
amount of supervision, for use in the UK.
I think people should recall the chequered history that the introduction of
stiff bases (roadbases) into the UK has had to date before they start departing
from the bituminous mixture designs that appear successful in the described
trials, and have a proven past history in France.
Personally, I think it would have been appropriate to have included a Hot Rolled
Asphalt binder course (basecourse), possibly a "50/20" mixture as part of the trial, perhaps
with a stiffer binder than 50pen, this being a bituminous mixture closer in
nature / characteristics to an EME than a coated macadam, and a material with
which laying contractors in the UK have successful experience.
In my opinion this report needs to be studied completely
before you make your decision to employ an EME, now referred to as "EME2" specified or proprietary, in your heavy duty road pavement
design.
And, you only need these materials in road pavement designs intended for heavily
trafficked roads, you really do not need them in the road pavement of the
majority of local authority highway networks.
The correct generic bituminous mixture will provide the engineering
properties that you require. You can find "examples" of appropriate bituminous
mixtures in the Appendices at the back of,
PD 6691 : 2007 : Guidance on the use of BS EN 13108 Bituminous mixtures -
Material specifications.
TRL
Project Report 66 - Evaluation of "Enrobe a Module
Eleve" EME :
A French high modulus 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.) roadbase materials, the success of which
promoted interest in stiffer roadbase materials in the UK. The French EME
mixtures have designs closer to UK hot rolled asphalt binder courses rather than
dense bitumen macadams.
TRL
Report 231 : Road trials of high modulus (stiff) base for heavily
trafficked roads.
This report describes trials of high modulus (stiff) 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 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 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 roadbases, 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 roadbases
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 roadbases incorporating polymers, but there was some
improvement with the use of metallic modifiers, although metallic
modifiers in other trials have given variable results.
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 ROADBASE
The use of 15pen. and 25pen. in dense bitumen macadam roadbases
and basecourse has currently been suspended by the Highways
Agency while the cause of failure in road pavements where these
materials were being trialled 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.
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