Bituminous bases and surfacings for
low-cost roads in the tropics
L. S. Hitch and R. B. C. Russell
I 1977 J Digest SR284
BITUMINOUS BASES AND SURFACINGS
L S Hitch and
Authors -- -- .--,,
BITUMINOUS BASES AND SURFACINGS FOR LOW-COST ROADS IN THE TROPICS*
L S Hitch and R B C Russell
Much of the accumulated knowledge gained from the construction of roads over the last
150 years relates to the use of mechanically stable roadmaking materials such as crushed
rock. These materials are not always available, particularly in the remoter parts of
many developing countries, but in recent years the development of the technique of soil
stabilisation provides practical alternative roadma!yy: ‘ayers
and design life in terms of cumulative
Mlnlmum soaked CBR values of 80 for crushed stone bases
and 100 for cement and lime stabilised material are normally specified.
A problem arises, however, in the application of these criteria when
constructing bases with bitumen-stabilised soils. Bituminous mixtures 13
behave viscoelastically and are temperature susceptible. In 1954 Nijboer
proposed the concept of “stiffness” of a mix, which reflects loading time,
acceptable strain (1 per cent max) and maximum temperature experienced in
practice. It was shown that this stiffness could be expressed as a ratio
of the stability to the flow value (load and strain at failure respectively)
measured in the Marshall test . A high ratio may ensure resistance to
deformation under traffic but at the expense of a brittle mix. The problem
of selecting an appropriate ratio for surfac+ng materials is considered
later (Section 5.1). The application of the concept to bituminous
stabilisation is perhaps less important and its relevance probably depends
upon the type of traffic using the road and the thickness of the
surfacing. The stiffness concept proposed by Nijboer is “rational”;
empirical methods, notably the Marshall, Hubbard-Field and the cone
penetrometer methods, have ~~so been used for the design of bitumen-stabilised
soil bases. A bibliography has been compiled which provides some examples
of the use of different design methods and the design criteria adopted.
3.2.2 A full-scale experiment in Africas: Abrasion-resistant sand-bitumen
surfaces with relatively high bitumen contents have already been referred
to in contrast to the leaner sand-bitumen materials for bases which are more
common today. The conflict between abrasion resistance and stability has
been recognised for many years. In 1960 the Tropical Section (now
Overseas Unit) of tie T- participated in the construction of experimental
sections on a road in Northern Nigeria. These sections were incorporated
into a new bitumen- ~gabilised road, the construction of which has been
described elsewhere . Itwa~ found that cut-ba:k bitumen with a viscosity
of up to approximately 8 x 10 .centistokes at 60 C (S.125) could be mixed
in a simple paddle mixer with theowell-graded sand available. The
ternerature of the sand was 26-28 C and of the binder when added was 8
120 C (ie at a viscosity of approximately 200 centistokes).
These findings raise the question of whether it is necessary to use
sophisticated mixing plant in order to heat sand simply to enable a
viscous penetration grade bitumen to be used, even though the superior
st~ility of such mixes on cooling is unquestionable.
The experimental mixes were laid without aeration to remove volatiles
and were surface dressed before curing could occur. Materials stabilised
with MC2 grade cut-back failed quickly but were the only sections to fail.
The remainder have, in general, performed as satisfactorily as those with
the hot mix material.
The road has carried mainly light traffic in its 14 years of use and
the number of heavy commercial vehicles probably does not exceed 100 per
day; this volume of traffic, however, is characteristic of many developing
It is interesting to record that samples of bitumen removed from
slabs and taken from several of the experimental sections and from the
hot-mixed main contract material after 13 years were all found to have
essentially the same viscosity, ie 10-20 pen. A limited number of small
samples taken after 18 months showed that the cut-back bitumens had, even
at that early stage, cured to the consistency of 8O-1OO pen.
3.2.3 Research at TW on bituminous stabilisation: The principal
objectives of the research undertaken by the Overseas Unit were:
1. To evaluate different design methods for bituminous stabilisation.
2. To establish design criteria.
3. To study the behaviour of a variety of sand-bittien mixtures
under different shear conditions.
The first two of these have so far been studied in the full-scale ex~~riment in Nigeria and tie results have been reported in full elsewhere .
Briefly, sand-bitumen mixtures were prepared using the sand from the
Nigeria experi~ent and three different binders. Mixtures were cured for
one year at 45 C and four tests were used go determine stability. Table 2
shows the results of stability tests at 60 C after one year and Tab&e 3
shows some inter-relations of test values for material tested at 45 C.
Of the five design methods originally inclu~~d in the study, two 18
(Marshall and Htibard-Field) are well documented . Alexander and Blott
proposed the use of the cone penetrometer for sand-bitumen mixtures and
it has been frequently used although, as with some other methods, design
criteria have been little more than recommendations. It was found that
the tes”tdid not correlate well ith three other methods exmined. 1~ The
deformation wheel-tracking test , designed at TRRL and used mainly for
studying deformation resistance in surfacings, was the fourth method
studied. Tracking rates were found to correlate well with the Marshall
and Hubbard-Field methods. The fifth method (CBR) was rejected as
inapplicable early in the study.
The following design criteria for sand-bitumen bases for lightly-trafficked
roads were deduced from the above study:
Sttiility, Marshall at 600C ................. 100 kg(min)
Stability, Hubbard-Field at 600C ............ 300 kg(min)
In a recent investigation of the causes of some failures in a road
carrying heavy traffic in the Middle East it was found that the type of
sand used for the sand-bitumen base varied considerably from place to
place along the road and the sand particle texture varied from very rough
to very smooth. A limited initial investigation appeared to show a link
between failure and bitumen content, and the low residual bitumen contents
found (2-4 per cent) were especially noteworthy in relation to the fineness
of the sands, most of which passed a 300 pm mesh sieve. The bitumen film
thicknesses were thus minimal, and mixes were suspected of being prone to
The shear properties of the sands used in this road when stabilised
with different percentages of bitumen were studied using a shear box.
The effects of binder viscosity and shear rate were also studied. Fipre
2 illustrates a typical stress/strain relationship for three sands, showing
how the behaviour becomes more plastic as the normal load decreases, ie
witi increase in depth of cons~~ction. Inspection of site failures
confirmed Prmdtl-type failure, and Fig 3 illustrates the effect of
binder viscosity and shear rate on Prandtl bearing capacities for one
of the sands.
Calculations based upon the most severe conditions likely to b
experienced in a sand-bitumen base showed that the Coulomb equation
$= C+ 1035
where ~= shearing
c = apparent
and 6 = angle of
cohesion (kN/m )
shearing resistance (degrees) .
The results obtained using the shear box are in good agreement with
Figure 4 shows the relationship between $and estimated bitumen film
thickness (EBFT) for three sands. The above formula is much simpler to
use and to express graphically than the formula derived by Prandtl:
Prandtl bearing capacity = c
‘(sin d +1) Xtan 4
tan~ (sin@ -l).e -11
where c = apparent cohesion
$ = angle of shearing resistance from the Coulomb equation.
It was found that all of the sands possessed optimum ~ values at an EBFT
of 0.7 to
where x =
0.8 microns, given by
bitumen content (% wt)
Specific surface area (cm*/g).
recent work has also shown that optimum # values for di~~erent
related to Hveem’s centrifuge kerosene equivalent (Cm) ~,the
the Efflux Rat?g ~~s~~~a~~ sand.
angle of sheari measured by the Angle of Repose metiod”, and
Results for three sands are plo~ted
in Fig 5.
~ilst this study has been by no means exhaustive, the following
conclusions relating to sand-bitumen bases for medium to heavily trafficked
roads can be tentatively drawn:
(a) For natural dry sand, an angle of shearing resistance of at least
30 degrees and/or Cm of at least 1.5 is required, and
(b) for the sand-bitumen mix, ~n EBFT of O .7 to 068 microns witi
a # value of at lea~~ 120QlkN/m is required at 25 C and rate of
strain of 2.22 x 10 sec .
Comparative criteria for the Marshall and Hubbard-Field stabilities
are as follows:
Marshall stability of 300 kg(min) at 60°C, or
Hubbard-Field stability of 700 kg(min) at 60”c, both accompanied by
an EBFT of 0.7 to 0.8 microns.
The mix-in-place process
It is worthwhile perhaps to consider carefully what is involved in
any mixing process: the requirement may perhaps be expressed in two parts:
1. the components must be brought together
2. an adequate mixing action must be provided.
The first statement may seem obvious but deserves discussion. The stabilisation process involves natural soil as the major component and the
stabilizing agent less than 10 per cent of the final product. It should
therefore be rational to bring the stabilizing agent to the natural sand;
for example, in cement stabilisation the stabiliser is very often spread
on the soil and mixed in by machine in one or more passes. This type of
process, mix-in-place, often requires only simple plant, eg trucks,
graders and water bowsers, and high output is possible. There are several
drawbacks to this process however; in particular, difficulties are often
experienced in controlling bitumen content, completeness of mixing and
processing depth. ~ilst these difficulties are lessened by the use of
purpose-built single-pass stabilisation machines, mechanical failure
usually results in complete stoppage of work. For this reason simpler
multi-pass equipment has much to commend it.
Single-pass machines require the soil to be spread in a windrow such
that it can be picked up by the machine as it travels slowly forward; the
stabilizing binder is added to the soil and mixed within the machine and
is discharged from the rear, usually again as a windrow, ready for subsequent spreading and compaction.
An arrangement used on one contract in the Arabian Gulf consisted of
a tractor-mounted mixer equipped with a spray bar above the tines inside
the mixing hood and supplied with cut-back bitumen by a tanker which
preceded it. A metered quantity of bitumen was thus delivered to the soil
under the mixing hood but mixing was incomplete during the first pass:
subsequent passes of the machine, without the bitumen supply, and assisted
by a blade grader were necessary to complete the mixing process. In its
simplest form multi-pass work must often be done using only a bitumen
distributor and blade grader.
3.4 The premix process
Mix-in-place work is only possible where low viscosity binders, ie
cut-backs or emulsions, are to be used. If it is necessary to stabilise
with penetration grade binders, premix plant is required, although techniques are now available for st~~lising with foamed penetration grade
bitumen; Mobil Oil (Australia
static processes and Bowering
}4 describe equipment for both mobile and
discusses the properties and behaviour
of foamed bitmen mixtures.
Several points should be made concerning the role of static premix
plants for bituminous stabilisation:
(a) Continuous type mixers, which are often capable of high outputs, are
well suited to this type of work, ie production using a cold feed of constant
gradation; materials are metered by volume. For stabilisation work the -– — -
process becomes one of soil and binder only in most cases. Production of
uniform quality is therefore more feasible than for surfacing materials,
which demand close control. Batch-type mixers are essential for surfacing
work and can naturally also be used for stabilisation.
(b) Pre-mixed stabilised material can be laid by paver to uniform depth
and regularity; shaping by blade grader is thus cut to a minimum.
(c) Modern mixing plants are often constructed as several mobile units,
cap&le of disconnection and re-assembly within a few hours. Such plants
can follow the progress of work, thus keeping haul distances to a minimum.
The thickness of bitumen coating in soil-bitumen systems is relatively
thin, conferring cohesion rather than providing lubrication, and in this
respect such systems resemble dense continuously-graded surfacing materials,
such as asphaltic concrete, in’which the interlocked mineral particles
resist compaction. In common with these materials, soil bitumen is
compacted most effectively by ‘the kneading action of rubber or
4. SURFACINGS FOR LOW-COST ROADS
4.1 The role of surfacings
Irrespective of its type, a surfacing is basically any treatment which
can withstand the abrasive effects of traffic, and many processes will
satisfy this definition. The structure of a low-cost road may take several
(a) a compacted and shaped in-situ soil structure;
(b) a cement/lime stabilised base or a mechanically stable base;
(c) a bitumen stabilised base.
Surfacing must be provided in all cases and must moreover be impervious,
even in areas of very low rainfall. Base materials are designed to retain
sufficient strength in a soaked condition but subgrade/sub-base must be
protected. It follows therefore that although pervious open-textured
bituminous materials are used for resurfacing on some minor roads, in the
UK for example, they must be laid over structures which are known to be
adequately sealed by former surface dressings. Where such materials are
used as surfacings on new roads, surface dressing is indispensable.
4.2 Surface treatments
The term ‘surface treatments’ is used here to describe processes in
which,bitumen is applied directly to the surface of a road. Surface
treatments commonly used include:
1. Dust-laying processes
3. Surface dressing.
Treatments 1 and 2 are described briefly and surface dressing in rather
4.2.1 Dust-laying processes: These processes are used to abate the dust
nuisance resulting from loose surface mat
roads in the tropics. An ~. publicatio~gi~;;~ ~~;ti~~~~3flR~d
the subject; low viscosity2cut-backs of the MC230 (MCO/MCl) type may be
applied at 0.55 - 1.35 l/m (0.1 - 0.25 gal/yd ) or, less preferably, crude”
oil, bunker grade fuel oil, etc may be used at heavier application rates.
These simple applications are not very durable and, since loose material
is being treated, they cannot be considered as equivalent to prime coats.
Their effectiveness will be governed mainly by the residual bitumen content
of the binder and the application rate. In conclusion, dust-laying must
be regarded as a palliative and not a permanent solution to the problem:
the latter is obtained by more expensive methods, eg mix-in-place stabilisation of the loose material so as to provide a coherent layer of at least
4-5 cm. The so-called road-mix method of mix-in-place st~ilisation is
referred to in Section 5.3.
4.2.2 Priming: A prime coat prepares the surface of a new road base for
superimposition of a surface dressing or bituminous ‘premix’ surfaci~g: a
low viscosity cut-b~ck, MC30 (MCO-MC1), is applied at 0.4 - log l/m
(0.08 - 0.20 gal/yd ) to the brushed and slightly damp surface. It should
penetrate to at least 4-5 mm leaving a matt dry surface in 24 hours.
If surface dressing is to follow, binder can be applied directly to
the dried prime coat. However, if a premixed surfacing is applied, a tack
coat may be necessary to ensure interlayer adhesion, especially if the mix
is of a dense type with no large stones to give a mechanical ‘key’ to the
base. The engineer must judge the need for a tack coat in each case; a 26
useful guide on prime and tack coats for use in Africa has been published .
4.3 Surface dressing
Surface dressing, also known as a ‘spray and chip’ process, is a surfacing treatment of the highest importance not only in developing countries
but also in many industrialised countries of Europe. It can be used as an
economical first surfacing, either as an improvement of an existing road
or as part of the initial construction of a new road. Further surface
dressings can be applied as a maintenance process: the life of all forms
of premixed bituminous surfacing is extended by the p~~}~~~~g:~~ of surface
dressing as a maintenance treatment. Some references are
given to the extensive literature on surface dressing. A surface dressing
seals the road structure against surface water and presents a rugous stone
layer to vehicle tyres. ,The base is thus protected against attrition but
the single layer of stone chippings applied cannot restore riding quality
to an irregular pavement or contribute significantly to pavement strength.
4.3.1 Surface dressing design: Although the most important feature of
surface dressing is the provision of a continuous impervious film of binder,
the success in the design of the dressing begins with selection of the
appropriate sizes for the stone chippings, which protect the film from
damage by vehicle tyres. Selection depends upon the weight and nature of
traffic and penetrability (softness) of the existing surface. That choice
having been made, an appropriate thickness of sprayed binder can be
The following stages are necessary:
Select a nominal chipping size for the job; the most frequently used
sizes are 14 mm (approx % inch) and 10 mm (approx % inch). The former size
is preferable for roads carrying large numbers of heavy commercial vehicles
and the latter for lightly-trafficked roads often used by fast-moving
vehicles. Extreme cases are soft bituminous surfaces carrying very heavy
vehicles (20 mm) and very hard, eg concrete, surfaces (6 mm); these cases
will seldom be encountered in developing countries but are included for
completeness. Table 4 shows the recommended nominal size of chippings in
relation to hardness of surface and traffic loads. Table 5 gives the rates .
Road Note ~9~7four nominal
of spread fo sizes of chippings when using cut-back binder.
in which this Table appears, stresses the need to consider
the shape and ~pecific gravity of chippings in addition to size when
selecting the rate of spread.
Jackson30 discusses the effect of the shape of chippings and provides
a design method based upon the Average Least Dimension of chippings. His
procedure may be summarised as follows:
(a) Measure the least dimension of approximately 200 chippings to determine
the Average Least Dimension (ALD) of the stone. Chippings of a given
nominal size can have different ALDs. These differences affect the
application rates of chippings and binder. Refer to Figure 6; take
the intercept of ALD and line AB and read off appropriate rate of
spread from upper scales.
(b) Select from Table 6 an appropriate constant from each of the four sets
of conditions listed. Sum the four constants to obtain an overall
factor and refer to Figure 6, take intercept of ALD and overall factor
line determined above and read off binder application rate from lower
4.3.2 Choice of stone: Aggregate should be clean dust-free and should
comply with requirements of the type given in BS 63 . Chippings should
be roughly cubical; flaky and elongated stones tend to be broken under
the roller or to be picked out by traffic. Rounded aggregates require more
binder than ngular ones and offer less skid resistance.
is the main factor which affects the skid resistance of
surface dressings and recowended values for different site conditions in
the UK have been published (see Table 7).
Aggregates of high crushing value sometimes show a tendency to polish.
Low strengt may abrade and develop surfaces of poor skid
such aggregates are particularly susceptible to crushing
when rolled with a steel-wheeled roller, and pneumatic or rubber-tyred
rollers are preferable. Occasionally coarse sand is the only cover aggregate available and can be used for seals under light traffic. In other
situations, such unconventional aggregates as lightly broken cockle shell
or coral have been successfully used.
Some authorities specify different stone sizes for the first and
second layers of double surface dressings, eg 14 mm for the first layer
and 10 mm for the second; other users occasionally reverse the order.
Experience at TRRL has shown that there appears to be no advantage in
changing the size of stone for the second layer. When applying a double
surface dressing to concrete, however, a small size, eg 6 mm, would be
used for the first layer followed by a 10 mm chipping for the second layer.
The sprayed binder film rapidly wets the surfaces of clean chippings.
Dusty chippings are less easily wetted and adhesion will be delayed.
Traffic then ‘whips off’ the chippings: disastrous failure is especially
likely to occur if rain falls within two or three hours of chipping.
Where water to wash the chippings clean is in short supply, an alternative
often used is to pre-treat the chippings with a very small amount (0.5 per
cent by weight) of bitumen: these are referred to as ‘lightly coated’ in
order to distinguish them from the ‘precoated’ chippings applied to rolled
asphalt (see Section 5.1). Chippings must be capable of being spread by
standard gritting machinery and must therefore not be tacky and liable to
agglomerate. In the UK a light coating of bitumen ortar is applied to
chippings in most surface dressing jobs, and abost always when treating
heavily-trafficked roads which must be opened to vehicles immediately upon
completion of the process. Coating temperatures at the mixing plant are
somewhat higher than normal in order to produce a lacquer on the chippings;
this ensures rapid adhesion to the film of sprayed binder. If coating
plant is not available, a light spray of diesel oil, kerosene or very fluid
cut-back can be used. The above treatments are not recommended when
surface dressing with bitumen emulsion.
The above reference to pre-treating chippings is included for completeness, although it is appreciated that such treatments may not be feasible
within the context of low-cost roads and are, indeed, seldom necessary
provided that attention is paid to correct choice of binder (see below),
aggregate size and construction procedure.
4.3.3 Choice of binder: Selection of binder is often dictated by
availability; nonetheless, an understanding of binder behaviour can prevent
disasters. The simple rule fo~ surfacedr ~ssing binders is that they should
have a viscosity of between 10 and 5 x 10 centistokes at the prevailing
road temperature. At higher viscosities, stone will not be wetted by the
binder and will be lost by whip-off: at lower viscosities wetting will
occur but the binder will be too fluid to hold the stone.
Figure 7 shows the viscosity/road temperature relationship for a wide
range of binders. It will be clear that for most conditions in the tropics
suitable surface dressing binders will be MC3000 (MC4/MC5) or soft penetration grades up to 80/100 grade. The use of a more viscous bitumen than
80/100 is not recommended and should not be necessary. Bitumen of 60/70
penetration grade is generally available in the Arabian Gulf area and can
be cut-back with diesel at a rate of 5 gal/100 gal of 60/70 pen to give
a bitumen in the rang~ 300-400 pen suitable for surface dressing at road
temperatures of 40-65 C approximately. Occasionally chippings have to be
spread manually instead of by mechanical gritters; in such cases it may
be advisable to use a slightly less viscous binder than that theoretically
The role of bitumen emulsion in surface dressing should be discussed
at this stage. The usefulness of the commonest and cheapest form of bitumen
emulsion - anionic emulsion - is limited by several features:
1. Poor adhesion
2. The effective
to dry dusty surfaces.
bitumen content (seldom exceeding 55 per cent).
3. Suitable source of supply, transportation and storage.
Conditions in the Middle East are predominantly dry and dusty and do not
therefore favour the use of anionic emulsions, which are better suited to
cool and damp conditions. For this reason special cationic emulsions of
higher viscosity and containing 7Q-75 per cent bitumen have been developed
in recent years, for example, for surface dressing in the early spring months
in the UK. It is stressed, however, that these emulsions are sprayed at 2G
requires heating to
equipment is lost.
surface dressing in
break occurs: cover
lost, whilst excess
650per cent cationic emulsion as used in South Africaa”
70 C before spraying and the advantage of use in unheated
The low viscosity of some emulsions is a disadvantage in
that they tend to drain from the crown of the road before
aggregate is therefore poorly held at the crown and is
binder accumulates at the edges and leads to bleeding.
Apart from considerations of bitumen content, the grade of base bitumen
present in an emulsion is imp~~tant and may need to be varied according to
season and traffic conditions . Finally, coagulation can occur when some
emulsions are transported or stored in unsuitable conditions.
It is tempting to assume that when hot binder is not available, bitmen
emulsion applied at ambient temperature is a simple alternative. Indeed,
emulsions are often used for successful surface dressings - but the Engineer
should be aware of the problems cited above that can arise. Careful
attention to technique of application is needed.
4.3.4 Construction procedure: C s uction methods for surface dressing
are adequately described elsewhere ; attention to detail and good
planning will ensure success. Excellent equipment is now available for
surface dressing. Recent developments in the UK have included the following:
1. Constant-rate-of-spread distributor (binder metering pump coupled to
2. Extending spray bar (spray width adjustable whilst spraying: for use
with 1 above).
3. Forward-control, self-propelled metering gritter (tows chipping lorry;
operator has excellent view of work).
The process selected should be suitable for the function expected of
it; it is appreciated that cost considerations often affect the choice.
Generally a double surface dressing is preferred on new bases; single
dressings are sometimes used however, particularly on lightly-trafficked
roads and as temporary seals. Single dressings are used for road maintenance
and are particularly valuable for treating badly polished slippery surfaces
often found at road junctions, traffic lights, etc, in cities.
The use of adhesion agents in surface dressing (and bituminous premix)
overseas is sometimes advised. These are correctly used to prevent traffic
damaging a new surface dressing when rain may be expected to fall within two
or three hours of construction, ie before chippings are thoroughly wetted by
the binder and orientated into a matrix by traffic; fast vehicles are
particularly damaging in these cases. tien needed, these agents are
usually blended with the binder in small amounts, often only 1-2 per cent
by weight, but are rapidly decomposed by heat and the treated binder must
be used within 1-2 hours of blending. Some agents in solution can be
sprayed on to the aggregate before chipping commences or even directly on
to the sprayed binder film. Normally, however, provided that clean chippings
are used with the appropriate grade of bitumen, adhesion agents are not
required. Men it is difficult to obtain dust-free chippings, the lightcoating treatment for chippings described earlier (section 4.3.2) may be
an economical alternativeto the use of adhesion agents.
4.3.5 Surface dressing as a maintenance treatment: Surface dressing is
undoubtedly the cheapest and most widely used process for the maintenance
of road surfacings generally. A new road surfaced with a dense and expensive
wearing course will require maintenance, sometimes within the first three
years. The effects of traffic, climate and constituent materials, often
combined with those of errors in mix production, can produce several
(a) Lean ‘hungry’ surface
(b) Fatting up or bleeding
(c) Surface hard but
Bitumen content too low
Poor mix design
Poor process control
Binder hardened and
Binder content too high
Poor mix design
Poor process control
Exposed hard stone
polishes under traffic
stone lost, ravening
may occur (damage to
Possibility of excess
binder on surface;
Pushing and/or rutting
especially when wet
These conditions occur regularly and can usually be treated by surface
dressing, provided that local depressions, pot-holes, etc are rectified
beforehand; it is customary in the UK to burn off any excess binder from
bleeding areas using specialised equipment. Surface dressing
maintenance treatment providing a waterproof seal, additional
and an anti-skid surface at a cost which is 20-30 per cent of
thinnest practicable premix overlay.
Rolled asphalt, which is dealt with in Section 5.1, is a
premix used. in the UK for the surfacing of all main roads and
is a valuable
that of the
has a life of
20-25 years. The layer of pre-coated chippings (20 mm) applied as an antiskid measure during laying may remain effective for some 10-15 years (more
or less depending on numbers of heavy vehicles) . Thereafter surface dressing
is done at 4-5 year intervals. Traffic on motorways canbe exceptionally
heavy and surface dressing is then required more frequently.
4.4 Slurry seals
These materials may be considered as intermediate between surface
treatments and premixed materials with respect to cost and performance.
They are basically mixtures of fine aggregate, water, bitumen emulsion and
occasionally cement. Where new seals have to be opened to traffic soon
after laying, chemicals are added to break the emulsion within a controlled
period. Some UK treatments can be trafficked within 2-3 minutes of laying;
a typical composition would be:
combined aggregate (5 mm down) 70-80% weight
water 7-lo% weight
emulsion 13-20% weight
Anionic or cationic emulsions may be used. Anionic slurries may be
premixed in a static mixing plant, but, because emulsions in cationic
slurries break relatively quickly, these slurries have to be prepared in
a purpose-built mixing and laying machine.
Slurry seals are of fluid consistency when applied and can thus penetrate
and seal cracks and surface voids. The resulting layer, some 3-4 mm thick,
improves the riding quality of the road to some extent depending upon the
laying technique. This can range from simple squeegees to modern mixerspreader units; costs are higher than for surface dressing but may be
justified for old badly-cracked or lean bituminous surfacings. In South
Africa slurry seals are sometimes applied to new surface dressing, thus
furnishing a superior type of seal, often referred to as ‘cape seal’.
They are particular ~y useful for re36 sealing aerodrome runways and
standard reference works are avail~le. The runways of two international
airports in the Arabian Gulf area have been successfully re-sealed using
the slurry seal process. One of these was badly cracked and abrasion loss
was occurring; in the touch-down areas particularly, disintegration was
imminent due to embrittlement of the binder. A double seal, 2 x 3 mm, was
applied at these areas and a single seal over the major portion of the runway.
The treatment has been entirely successful at both airports.
5. PREMIXED BITUMINOUS SU~ACINGS AND T~ LOW-COST ROAD
In the context of low-cost roads, premixed bituminous surfacings would appear
initially to be both unnecessary and economically unjustified. The following
points, however, should be considered:
(a) New roads are invari~ly found to generate traffic; the estimated daily
usage is often exceeded early in the life of a road.
(b) The low-cost road of today can become the important route of tomorrow.
~~~~~~ ‘o what were
formerly inaccessible areas encourages development.
has concluded from available evidence that most forecasts of r ral Y
traffic in developing countries are subject to considerable error: a - 50
per cent confidence interval is likely.
The design history of many low-cost roads in developing countries,
notably those in Africa, may be summarised simply as follows:
(i) Initial construction Subgrade CBR = 25 (rein)
(100 mm (rein)sub-base, CBR = 25 (rein),where
subgrade has CBR less than 25 at equilibrium
moisture content) .
150 mm Base a) Stabilised, soaked CBR = 100
or b) Mechanically stable, soaked
CBR = 80 (rein).
Double surface dressing
(*Does not apply to bitumen-stabilised sand)
(ii) Strengthening overlay 50 mm (rein)bituminous premix.
It is imperative that the benefits expected at Stage (ii) should be realised
in full, particularly in view of the high cost of bituminous materials.
Bituminous overlays often fall short of their objectives for various reasons,
among which are:
(a) inappropriate premix type;
(b) inadequate materials and manufacturing equipment;
(c) inadequate control of manufacture.
The characteristics of different mix types and their application are
5.1 Bituminous pretix types
Bituminous mixes may be considered as being of two main types, the
performance of which depends upon:
(i) the mechanical interlock of aggregate
(ii) the stiffness of a bituminous mortar.
‘escrfied in Bs ~~,,~~se ‘ate
Type (i) comprises t rials known generally as macadams, eg as
; bitumen of very high viscosity is not required in
these mixes since stability is obtained by the interlock of particles which
must, however, be l~ricated during the compaction process. The well-known
‘asphaltic concrete’ is a very dense continuously-graded macadam. The
Marshall mix design method is generally used to determine the ‘optimum’
binder content, which must reflect the condition of optimum packing of
aggregate (and therefore optimum lubrication) together with optimum cohesion.
This is the condition for maximum stabi~~ty which is often given attention
at the expense of flexibility. Nijboer introduced the idea of mix
‘stiffness’ and it was subsequently concluded that an
could be calculated as follows:
Stiffness = Marshall st~ility (lb)
Marshall flow (0.01 in) = 1.2 x tyre
Certain-assumptions and approximations were made in arriving at this
simple relationship; nonetheless, it appears to have been supported
reasonably well by the results of field and laboratory tests. Experience
at T~L is that mixes produced in the developing countries often have an
unnecessarily high stiffness. Marshall design criteria for road surfacings
in the tropics have been calculated for three types of traffic on the basis
of assumed tyre pressures, permissible flow values and the stiffness
calculated as shown above. The results are given. in Table 8.
Type (ii) mixes are bituminous mortars, ie sand is coated with a very
viscous bitumen which has been further stiffened by the addition of filler.
Stone is usually added as an extender and enhances stability. In the UK
these stone-fi~~ed mortars are known as rolled asphalts, and are described
in BS 594:1973 . The gradings of stone and sand are specified separately
and a discontinuity appears in the overall grading: rolled asphalts are
therefore often referred to as ‘gap-graded’ mixtures. Compositions have
evolved from experience and the resulting recipes consistently define
materials with lives of 20-25 years, requiring only routine re-texturing
by surface dressing. Initially, rolled asphalts containing less than
approximately 45 per cent stone are given, as an anti-skid measure, an
application of coated chippings during laying.
Ex~rience and research
Macada and rolled asphalt compositions for use in the UK have evolved
many years of experience without the need for mix design procedures.
In contrast, asphaltic concrete has come into general use in the developing
countries, especially in the tropics: this is understandable because of high
road temperatures. It is usually found that asphaltic concrete correctly
designed, mixed and laid performs well under fairly continuous traffic,
the tyres of which tend to keep the surface ‘closed’. Troubles are common
when such surfacings are very lightly trafficked, particularly by fast
moving cars. Aggregate particles in the surface tend to be picked out
instead of being re-embedded into the matrix, for which slow, high-pressure
tyres are required. Picking can develop rapidly into severe ravening:
this can and does occur very easily with materials made to specifications
which result in low ‘optimum’ bitumen contents. Table 9 contains two
recommended specifications for asphaltic concrete. The aggregate particles
in asphaltic concrete are very thinly coated: mixes tend to be brittle and
to develop cracks which are not self-healing.
In recent years the Overseas Unit, TM, has been examining the relative
performances of different premix types in tropical and sub-tropical conditions.
At one experimental site in the Arabian Gulf the performance of asphaltic
concrete, surface dressing, open and dense macadams and rolled asphalts is
being compared. ~ilst there is no noticeable deterioration of any of the
sections after three years of use, it is encouraging to note that the rolled
asphalt sections show no signs of rutting. A second nd more extensive 41
group of experimental sections constructed in Turkey is devoted almost
entirely to rolled asphalt surfacings but is relatively new: further
opportunities for trials are being sought, the objective being to est~lish
suitable surfacing specifications for given environments. Gap-graded
compositions are already in4~ se in South Africa and Table 10 shows a
specification used in Natal .
tiere dotit exists as to a suitable surfacing specification, a ‘recipe’
such as the ‘Group 3 (14 mm) wearing course shown in Table g can be adopted:
if necessary this would be sealed with a surface dressing. For lightlp
trafficked roads it will be advisable to use binder contents at the higher
end of the appropriate range. Compared with aspha,ltic concrete such a mix
will have higher voids and will be permeable; however~ the aggregate will be
well-coated and the risk of bleeding (resulting from poor production control)
will be lessened. The risk of picking and abrasion will be small and the
mix will have greater flexibility than asphaltic concrete, cracking being
therefore 1~gs likely. Table 9 includes three further specifications taken
from BS4987 .
5.3 Construction tectiiques
Excellent construction equipment is available for all bituminous road
processes. Recent developments in surface dressing equipment were mentioned
earlier (Section 4.3.4) ; this equipment is relatively cheap compared with
sophisticated premix plant.
Bitumen distributors can be used for work other than surface dressing
and priming. mere expensive mixing plant is not availabl~3it is often
possible to manufacture premix by the so-called ‘road mix’ process:
1. Aggregates are windrowed on a suitable mixing site; this is often a
new base, old road or a levelled piece of ground which has been sealed
with a bitumen spray.
2. The required quantity of cut-back bitumen is sprayed by a distributor
in several applications and a blade grader mixes the material between
3. The mixed material is then spread or, if necessary, aerated for some
time to remove volatiles before spreading.
A range of gradations can be mixed by this technique which is limited
only by the stiffness of the resulting mix; for this reason the cut-back
grade will be determined by prevailing aggregate temperatures. The process
is used extensively in bituminous stabilisation for bases and will doubtless
be applicable to remote areas in the developing world. Control of mix
proportions in this process is poor. Nonetheless, some examples of effective
application have been seen in the Arabian Gulf area.
Available premix plant varies in complexity from simple paddle mixers
producing approximately 10 tph to fully automatic asphalt plants producing
25o tph. Simple mixers have no aggregate drying unit and are restricted to
mixing at ambient temperatures; thus bitumen-stabilised soils can be produced
and occasionally coated materials, for which a cut-back bitumen or slowbreaking (stable) emulsion must be used. Aggregates must be pre-wetted when
mixing with emulsion, which must be particularly stable when large amounts,
of fine material are present.
The production of well-controlled premix requires appropriate mixing
plant. For high-grade surfacings, particularly those like asphaltic concrete
for which binder content control is so critical, only a weigh-batch type
plant is permissible. Continuous mixers are susceptible to variations in
hot bin level and serious departures from the design grading and bitumen
content may occur with these plants, which are better employed for preparing
single-size aggregate mix”tures or for soil stabilisation. Weigh-batch
plants normally have four hot aggregate bins and a filler bin; all ‘fractions
are pre-weighed before discharge to the pugmill. Binder proportioning is
almost invariably by weigh-batching, which has been found to be more accurate
than volumetric devices.
In the UK it is customary to produce macadams in batch-heater type
mixers; stone fractions are pre-weighed in the cool state and then combined
before loading to the batch-heater. This system is much simpler than that
of the more familiar asphalt plant and is well suited to macadams and their
lower drying/heating demand. Dense mixtures such as rolled asphalt and
asphaltic concrete require much higher heat inputs for drying and involve
higher temperatures since the binders used are more viscous than in macadams.
The continuous dryers in asphalt plants are better suited to these requirements and, although macadams can be produced effectively with such plants,
it is not operationally feasible to produce dense mixtures using batch-heater
Asphalt plant operation
Several points can be made concerning asphalt mixing and paving plant:
Where cold feeders at the mixing plant are not charged with several
screened materials of different sizes, the gradations of the materials
in the hot bins must be checked frequently. Too often an all-in
material is cold fed; the contents of the hot bins will then depend
entirely upon variations in the stockpile. Materials such as asphaltic
concrete cannot be produced under these conditions.
Plant output must be adequate for paver laying capacity. The latter
usually exceeds production, with the result that the paver stops and
restarts frequently, leaving a series of irregularities in the carpet;
the problem can be aggravated when insufficient trucks are used.
Mix temperatures must be checked frequently; it is not unknown for
producers to keep temperatures high in order to speed up compaction,
thus increasing output. Appropriate binder temperatures are given in
The paver must be well maintained, particularly the hydraulic system.
Trucks must not be permitted to reverse into direct contact with the
The laying gang should not be allowed to back-blind the spread material;
this is commonly done for little or no reason and can affect the riding
quality. Hand rakes are frequently used to remove excess material at
the longitudinal joint: exce=ive raking invariably segregates coarse
stone, leaving open-textured areas.
Roller operators can, and will, ruin any laying operation if inexperienced
or badly controlled. Correct overlapping of rolled passes by a half
width, staggering of stop points, cleanliness of
efficiency of cleaning mats and water spray bars
rolls (or tyres) ,
are particular points
There is adequate publisQe$4w$~k4~n4$he operation and maintenance of
bituminous construction plant ‘ ‘ ‘ ‘
The work described in this Report was carried
and Road Research Laboratory.
out in the Overseas Unit of
P T. The properties of cement stabilised materials.
of Transport, RRL Report LR205. Crowthorne, 1968 (Road
ROAD RESEARCH LABORATORY. Notes on the cement treatment of Ghana
soils. Department of Scientific and Industrial Research, Road Research
Overseas Bulletin No. 8. Harmondsworth, 1958 (Reprinted 1962) (Road
ROAD RESEARCH LABORATORY. Lime stabilisation of soils for use as road
foundations in Northern Rhodesia. Ministry of Transport, Road
Research Overseas Bulletin No. 9. Crowthorne, 1969 (Road Research
JACKSON G P and D BRIEN. Asphaltic concrete. London, 1962 (Shell
International Petroleum Co Ltd).
WILLIAMS F H P. Bitwen-sand mixes for road bases. Ministry of
Transport, RRL Report LR 239. Crowthorne, 1968 (Road Research
TILLEY A C. Stabilizing sandy soils in Nebraska. Proc. llth Nat.
Asph. Conf. New York, 1938 (Asphalt Institute), pp 49-55.
BECKHAM W K. Soil stabilisation with asphaltic materials in South
Carolina. Proc. llth Nat. Asph. Conf. New York, 1938 (Asphalt
Institute), pp 61-68.
WEATHERS H C. S.tabilising sandy soils with asphaltic materials in
Florida. Proc. llth Nat. Asph. Conf. New York, 1938 (Asphalt
Institute), pp 43-47.
SHELL CO~ANY OF EGYPT LTD. Plastic roads
(Shell Co of Egypt), 2nd edition.
PORTER O J. The preparation of subgrades.
Wash. , 1938, 18 (2), 324-31. —
T~NSPORT AND ROAD RESEARCH LABO~TORY. A
and runways. Cairo, 1939
Proc. Highw. Res. Bd.
guide to the structural
design of bitumen-surfaced roads in tropical and sub-tropical countries.
Department of the Environment, Road Note 31. London, 1977
(H M Stationery Office), 3rd Edition.
LIDDLE W J. Application of AASHO Road Test results to the design of
flexible pavement studies. Proc. internat. Conf. struct. Design Asph.
Pavements. Ann Arbor, 1963 (University of Michigan), pp 42-51.
NIJBOER L W. Some considerations of the ’Marshall test method for
investigating bituminous masses (in German) . Strasse und Autobahn,
1957, ~, 210-4. ..
THE ASPHALT INSTITUTE. Mix design methods for asphalt concrete and
other hot-mix types. Manual Series No 2 (MS-2). College Park,
Maryland, 1969 (The Asphalt Institute) , 3rd edition.
HITCH L S. Bibliography on bituminous stabilisation of soils.
Ministry of Transport, Road Research Laboratory Library Bibliography
NO 99/LSH. Crowthorne, 1966 (Road Research Laboratory).
JOHNSON C M and J J GANDY. Construction of the Maiduguri-Bama road,
Northern Nigeria. proc. Instn. civ. Engrs, 1964, 29 (6814), 1-18. —
HITCH L S and R B C RUSSELL. Sand-bitmen for road bases; an examination
of five methods of measuring stability. Department of the Environment,
T~L Report LR 717. Crowthorne, 1976 (TransPort and Road Research
ALE~NDER P and J F T BLOTT. Factors affecting the structural stability
of sand carpets. J. Sot. them. Ind., Lend., 1945, 64, 89-101. —
BROOME D C
and A P~ASE. The use of mechanical tests in the design of
road surfacing mixtures. II St~ility tests on rolled
J. appl. Chem., Lend., 1958, ~, 121-35.
On the resistance to indentation (hardness) and the strength
materials. Z. angew. Math. Mech., 1921, —1, 15-20.
OF SCIENTIFIC AND INDUSTRIAL RESE~CH, ROAD RESEmCH
Stresses in soils and bearing capacity of ground.
Soil Mechanics for Road Engineers. London, 1952 (H M Stationery
Office), pp 451-63.
HANSON J M. Particle interaction and the stability of mixes. Faculty
of Engineering of the University of London~ MSC Thesis. London, 1966.
MOBIL OIL AUSTRALIA LIMITED. The foamed bitumen process as a soil
stabilisation technique. Technical Bulletin No 5. Melbourne, 1969
(Mobil Oil Australia Limited).
BOWERING R H. Properties and behaviour of foamed bitumen mixes for
road building. Proc. Fifth Conf. Aust. Rd Res. Bd, 1970, 5 (part 6),
ROAD RESEARCH LABO~TORY. Dust-laying on unsurfaced
roads. Department of the Environment, Road Research
No 14. Crowthorne, 1971 (Road Research Laboratory).
earth and gravel
26. NATIONAL INSTITUTE FOR ROAD RESEARCH. Guide on prime coats, tack coats
and temporary surfacings for the protection of bases. South African
Council for Scientific and Industrial Research, TRH 1. Pretoria,
1970 (National Institute for Road Research).
27. TRANSPORT AND ROAD RESEARCH LABORATORY. Recommendations for road
surface dressing. Department of the Environment, Road Note No 39.
London, 1972 (H M Stationery Office).
28. SWAMI S A. Australian method of surface dressing. J. Inst. Highw.
Engrs, 1965, 12 (1), 22-31. —
29. NATIONAL INSTITUTE FOR ROAD RESEARCH. Bituminous surface treatments
for newly constructed rural roads. South African Council for
Scientific and Industrial Research, TRH3. Pretoria, 1971 (National
Institute for Road Research).
30. JACKSON G P. Surface dressing. London, 1963 (Shell International
Petrolew Co Ltd).
31. BRITISH STANDARDS INSTITUTION. Single-sized roadstone and chippings.
British Standard 63:1951. London, 1951 (British Standards Institution).
32. BRITISH STANDARDS INSTITUTION. Methods for sampling and testing of
mineral aggregates, sands and fillers. British Standard 812:1975.
London, 1975 (British Standards Institution).
33. SALT G F and W S SZATKOWSKI. A guide to levels of skidding resistance
for roads. Department of the Environment, TRRL Report LR 510.
Crowthorne, 1973 (Transport and Road Research Laboratoq).
34. HOSKING J R. The effect of aggregate on the skidding resistance of
bituminous surfacings: factors other than resistance to polishing.
Department of the Environment, TRRL Report LR 553. ‘Crowthorne, 1973
(Transport and Road Research Laboratory).
35. NATIONAL INSTITUTE FOR ROAD RESEARCH. The use of bitumen emulsions in
the construction and maintenance of roads. South African Council for
Scientific and Industrial Research, TRH7. Pretoria, 1972 (National
Institute for Road Research).
36. HAWKEN N. Slurry seal - practical application and current use. Slurry
seal Symposium. Rds Rd Constr., 1967, 45 (531), 60. —
37. THE ASPHALT INSTITUTE. Asphalt surface treatments and asphalt penetration macadam. Manual Series No 13 (MS-13). College Park, Maryland,
1964 (The Asphalt Institute).
38. HOWE J D G F. The sensitivity to traffic estimates of road planning in
developing countries. Department of the Environment, TRRL Report
LR 516. Crowthorne, 1973 (Transport and Road Research Laboratory).
39. BRITISH STANDARDS INSTITUTION. Coated macadam for roads and other
paved areas. British Standard 4987:1973. London, 1973 (British
BRITISH STANDARDS INSTITUTION. Rolled asphalt (hot process) for roads.
and other paved areas. British Standard 594:1973. London, 1973
(British Standards Institution).
HITCH L S. Bituminous surfacings for the tropics: the laying of
experimental sections on a road in Turkey. Department of the
Environment, TRRL Report SR 142UC. Crowthorne, 1975 (Transport and
Road Research Laboratory).
GROTH P J. Overlay design in Natal. Proceedings of First Conference
on Asphalt Pavements for Southern Africa. Durban, 1969 (Secretary
Birds (Pty) Ltd, Durban).
THE ASPHALT INSTITUTE. Asphalt mixed-in-place (Road mix) manual.
Manual Series No 14 (MS-14). College Park, Maryland, lg65 (The Asphalt
HATHERLY L W and P C LEAVER. Asphaltic road materials. London, 1967
(Edward Arnold (Publishers) Ltd).
DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH, ROAD RESEARCH
LABOWTORY. Bituminous materials in road construction. London, 1962
(H M Stationery OffiCe).
THE ASPHALT INSTITUTE.
(MS-3). College Park,
THE ASPHALT INSTITUTE.
(MS-8). College Park,
Asphalt Plant Manual. Manual Series No 3
Ma~land, 1967 (The Asphalt Institute).
Asphalt Paving Manual. Manual Series No 8
Maryland, 1965 (The Asphalt Institute).
Mortar type mixes
(includes rolled asphalt)
Some features of different bituminous processes and materials
binds surface of new base in
preparation for surfacing
provides bond between existing
surface and bituminous premix
dense, very fine
(very open - very dense]
may include stone
Properties of sand-bi~umenmixes after 1 year (+) storage at 45%
stmility tests at 60 c (all compacted densities, CDM, in Mg/m )
(Sand: as for construction of experimental sections, Maiduguri-Bamaroad)
Hubbard-Field Marshall Properties of recovered binder
(poises x 10 )
at 25°C Binder s
CDM CDM CDM
at 45°C I
1.91 0.O36 1.91 293
1.93 206 2.0 5.89 I
1.87 92 0.66
1.92 0.098 1.92 1.93 170
1.8 1.89 88
3 2.7 1.8 0.9 1.91 0.036 1.91 1.92 337 2.0 1.89 58
1.92’ 0.O38 1.92 1.93 327
2.0 1.91 65 18.5 I 2.2
1.90 0.094 1.90 1.91 2.0 16.7 I
1.88 44 4.0
1.92 0.041 1.92 1.93 219 2.0 1.91 42 16.0 I 1.8
,,,,,,,,,,,,,, ,, ,, ,,, ., ,. ,,, ,,,, ,,
Hubbard-Field/Marshall stability ratios and cone stability/equivalent tyre_pressure
(Sand: as for construction
uncured and cured material; tested at 45°C
of experimental sections, Maiduguri-Bama road)
Cured 1 year (+) at 45°C kinder
stabilit (S) Y
(% Wt) (kg)
198 79 2.51
197 44 4.48
3.0 42 4 126
3 587 473 1.24 9.6 136
112 F 85
420 I 232 I 1.81 2.7 38 7.9
2.69 3.4 48 4 586 391 1.50 6.o
Recommended nominal size of chippings (millimetres)
Lane Traffic Categogy
Approximate number of commercial vehicles currently
Type of surface carried per day in the lane under consideration
(1) (2) (3) (4) (5)
Over 2000 1000-2000 200-1000 20-200 Less than 20
Very hard (10) 10 6 6 6
Hard 14 14 10 6 6
Normal 20+ 14 14 10 6
soft * 20+ 14 14 10
Very soft * * 20+ 14 10
The size of chipping specified is related to the mid-points of lane
traffic category ranges 2-5: lighter traffic conditions may make the next
smaller size of stone more appropriate.
+At the discretion of the Engineer, 20mm chippings may be used for remedial
treatment where traffic speeds are low. Very particular care should be
taken when using 20mm chippings to ensure that no loose chippings remain
on the surface when the road is opened to unrestricted traffic as there is
a high risk of windscreen breakage.
*Unsuitable for surface dressing.
Rate of spread when using cut-back bitumen
Lane Traffic Category (See Table 4)
1 2 3
Surface Chipping Binder Chipping Binder Chipping Binder Chipping Bind&r Chipping Binder
Size Rate Size Rate Size Rate Size
(litre/m2) (mm) (litre/m2)
(litre/m2) (mm) (litre/m2) (mm) (litre/m2)
Very hard I 10 1.0* 6 1.0 6 1.1 6 1.2
Not recommended 14 l.l+ 10 1.0 6 1.0 6 1.0
Normal 14 l.O+ 14 1.0 10 1.0 6 1.0
soft Conditions not 1
20# 14 0.9 14 1.0 10 1.0
Very soft suitable for surface dressing
20# 0.9 14 0.9 10 0.8
* Rubberised cut-back bitmen only is recommended.
+ Evidence is being sought for successful use of bitumen in these categories.
# At the discretion of the Engineer, 20mm chippings may be used for remedial treatient where traffic speeds are low.
Very lean bituminous
Very rich bituminous
Veh/day Constant Type of chippings
0-100 +3 Round/dusty
100-500 +1 Cubical
500-1000 0 F1aky
1000-3000 -1 Pre-coated
+6 Northern Europe (wet and cold) +2
+4 Tropical (wet and hot) +1
o Temperate o
-1 Semi-arid (dry and hot) -1
-3 Arid (very dry and very hot) -2
PSV of aggregate necessary to achieve the required skidding resistance
in bituminous surfacings under different traffic conditions
PSV of aggregate necessary
not greater than 12 ; not greater than 10
SFC = sideway force coefficient
SFC values in these traffic conditions are sometimes
achievable with aggregates of extreme hardness and
very high resistance to abrasion, such as certain
grades of calcined bauxite
Suggested Marshall design criteria for road surfacings in the tropics
flow valu s
-5 (in x 10 )
–9 obtained in Marshall test
Flow (in x 10–”)
to nearest 5
Suggested range of Marshall stabi$ity I
and flow values
lb inx10 kg lb -2 kg# kN
m — —
inx10 m m
7w-1200 12-20 ‘ 315-545 3.0-5.1 35-1oo 60-180 0.6-1.8
.lw-1600 11-18 455-725 2.8-4.6 55-145 100-250 1.0-2.5
1200-1900 10-16 545-86o 2.5-4.1 75-190 135-340 1.3-3.3
-., ,,,, ,,
Aggregate grading (% wt passing)
(total aggregate) .
Bitumen Content Crushed rock
(% wt total mix) Limestone
Typical specifications for bituminous premix
80 - 100
60 - 80
35 - 56
28 - 44
20 - 34
15 - 27
10 - 20
4.8 - 6.1
80 - 100
54 - 72
42 - 58
34 - 48
26 - 38
18 - 28
12 - 20
5.7 - 7.0
95 - 100
70 - 94
56 - 76
44 - 60
32 - 46
!.9 - 4.1
British Standard 4987
(28 mm nominal)
90 - 100
71 - 95
58 - 82
44 - 60
32 - 46
4.1 - 5.3
course (14 mm
95 - 100
70 - 90
45 - 65
30 - 45
15 - 30
4.4 - 5.4 (A)
4.6 - 5.6 (B)
4.1 - 5.1 (A)
4.4 - 5.4 (B)
Coarse cold asphalt
(10 mm nominal)
95 - 100
70 - 90
40 - 60
15 - 30
4.9 - 5.9 (A)
5.1 - 6.1 (B)
4.7 - 5.7 (A)
4.9 - 5.g (B)
A)>300 cvpd) in one
Gap-graded overlay specification, Natal, South Africa
BS sieve size (per
(1 in) 26.5 mm
(+ in) 19 mm
(+ in) 9.5 mm
(3/16 in) 4.75mm
(No 7) 2.36 mm
(No 25) 600 pm
(No 52) 300 Am
(No 72) 200 pm
(No 100) 150 @
(No 200) 75 Km
90 - 100
60 - 80
25 - 45
15 - 30
95 - 100
70 - 80
45 - 65
25 - 45
Coarse aggregate (ret 7 mesh) 47 : 5% Wt
Fine aggregate (7-200 mesh) 40 ~ 5% Wt
Filler (pass 200 mesh) 7.5 : 2% Wt
Bitumen (60/70 pen) 5.5 - 0.3% Wt
Filler/bitumen ratio I
between 1 and 2*
Marshall StabilltY (rein) 340 kg (750 lb)
Marshall flow value 2- 4.5 mm (8-18x10-2in)
Voids in mix 4- 10% Vol
Suitable for compacted course 30 to 65 mm (1% to 2% in)
Sand equivalent value of >45
(AASHOT.176 - 65)
Aggregate crushing value
(BS 812:1967) I <30 per cent
*in the authors’ view a filler/bitumen ratio of between 1 and
1.5 is likely to be the optimum ratio in most cases.
Application temperatures for bitumens and cutbacks
(1) Owing to the inflammable nature of the solvent in RC type cutbacks
the application temperatures of these grades should be kept to the
lower end of the ranges quoted.
(2) The temperature range given for each grade is necessarily rather
wide because local climatic conditions and the type and condition
of the equipment affect the optimum spraying temperature.
50mm (2in) of bituminous
150mm (6in) of base
150mm (6in) of base 200mm (8in) of base
Minimum thickness of sub-base of 100mm (4in) to be used with
subgrades of CBR 8 to 24 per cant. Material used in this zone to
have CBR of not less than 25 Per cent
0.05 0.1 0.2 0.3 0.4’0.5 0.75 1.0 1.5 2.0 2.5
Cumulative number of standard axles in one direction ( x 106)
If it is dasired to provida at the time of construction a pavement capable of carrying
more than 0.5 million standard axles, the d=igner may choose either a 160mm (6in)
base with a 50mm (2in) bituminous surfacing or e 200mm (Bin). base with a doubla
surface dressing. For both of thasa alternatives, the recommended sub-base thickness
is indicated by tha broken line.
Alternatively, a base 150mm (6in) thick with a double surfaca dressing mav be laid
initiallv and the thicknaee increased when 0.5 million standard axles hava been carried
The extra thickness mav consist of 50mm (2in) of bituminous surfacing or at Iaast
75mm (3in) of crushed stone with a double surface dressing. The largest aggregata size
in the crushed stona must not exceed 19mm (%in) and the old surface must be prepared
bv scarifying to a depth of 50mm (2in). For this stage construction procedure, the
recommended thicknass of sub-base is indicatad bv the solid Iina.
Fig. 1 PAVEMENT DESIGN CHART FOR FLEXIBLE PAVEMENTS
— Sand A
— — Sand B
---- Sand C
Strain rate = 1.67 x 10-4sec-’ 629
Binder viscosity 6 x 103poiSeS
~ 629 (Normal pressure
~ - 315
-— ___ _
---- —— 315
0 2 4 6 8 10 12 14
Strain (per cent)
Fig. 2 TYPICAL STRESS/STR&N RELATIONSHIPS FOR
SAND/BITUMEN IN SHEAR BOX TEST
A 6x 103
1.67 X 10-4
2.22 x 10-2
1.67 X 10-4
2.22 x 10-2
1.67 X 10-4
2.22 x 10-2
0 2 4 6 8 10
Binder content (per cent wt)
Fig. 3 PRANDTL BEARING CAPACITY VALUES FOR TYPICAL SAND/BITUMEN
, Sand A
“ Sand B
0 0.5 1.0 1.5 2.0
Estimated bitumen film thickness @m)
Fig. 4 RELATIONSHIP BEWEEN EBFT AND~ AT 2.22 x 10-2 SEC-~ 106 POISES
0 Centrifuge Kerosene
,,\ o 1 2 3
Centrifuge Kerosene equivalent (CKE)
I 1 1 I
25 30 35 40
I 1 I I 1 1 I i I I [
34 32 30 28 26 24 22 20 18 16 14
Efflux rate (ml/see)
Fig.5 RELATlONSHIPBEWEEN *opt AND SOME PHYSICAL
CONSTANTS OF THREE SANDS (DRY)
Chipping application rate
45 50 60 70 80 90 100 120 150 200 yd2/ton
I 1 I 1
I 1 I
‘~ I ~
27 25 23 21 19 17 15 13 11 g 7 5kg/m2 ~> ,g g
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 41itre/m 2J
0.6 0.7 0.80.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1,8 1.9 2.0 2.1 2.2 kg/m2
1 1 1 I 1 1 1 I 1 I J
9876 .5 4 i.5 3 2.5yd21gal
Binder application rate
Fig. 6 SURFACE DRESSING DESIGN CHART
✚✚✚✚✚✚✚✚✚✚✚✚✚ ✚✛✚✚❊✎✛ ❑ ~M Too stiff (poor wetting)
✎✎:~~~~:~:~:~:. Too fluid (drainage and whip-off) . . . . . . . .
................ ., . . . . . . . .
. . . . . . . .
............. . . . . . . . .
10 20. 30 40 50 60 70 80 90
1 1 1 I 1 I I
50 77 100 122 140 160 180
Fig. 7 SURFACE TEMPERATURE / CHOICE OF BINDER FOR SURFACE DRESSING
(911) Dd443233 1,000 6/77 HPLtd So’ton G1915
PRINTED lN ENGLAND
BITUMINOUS BASES AND SURFACINGS FOR LOW-COST ROADS IN THE
TROPICS: L S Hitch and R B C Russell, BSC MPhil MIHE FGS:
Department of the Environment Department of Transport TRRL
Supplementary Report 284: Crowthorne, 1977 (Transport and Road
Research Laboratory). Mechanically stable materials for road
bases are often not obtainable in developing countries and the
technique of soil stabilisation has therefore been developed.
In the Middle East, aggregates are often scarce but oil products
are readily availtile. The region has therefore provided some
of the earliest examples of bituminous stabilisation, which
originally consisted of thin running surfaces over compacted
sand. Bituminous stabilisation can also enable local sand to
be used for base construction, and various tests and design
criteria have been proposed for such applications.
The report describes full-scale experimental trials
supported by laboratory research, which have enabled acceptance
criteria for bitumen-stabilised sand bases for light/medium
traffic to be proposed. Construction methods for bitminous
stabilisation are also described.
Details are given of methods of surface dressing, which
is important both as an initial running surface on new bases
and as a maintenance treatment.
Premixed bituminous materials, both as bases and surfacings,
might perhaps be considered as inadmissible for low-cost roads.
Such roads, however, usually require progressive improvement
because of the traffic growth which accompanies development.
There is a growing use of strengthening overlays and the report
briefly discusses premixed materials and their application.