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8/13/2019 Bitumen Emulsion in Sprayed Seals_Muller
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van Zyl, Muller, Sadler
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BITUMEN EMULSION IN SPRAYED SEALS: EXPERIENCE AND CURRENT BEST PRACTICE IN
SOUTH AFRICA
GD van Zyl, Mycube Asset Management Systems, Cape Town, South Africa
J Muller, Sasol Technology, Rosebank, South Africa
D Sadler, ex Tosas (Pty) Ltd, Wadeville, South Africa
ABSTRACT
Bitumen emulsions, in various forms, are used extensively in South Africa as prime coats, for sprayed seal
construction and rejuvenation, as well as slurry seal application. Emulsion types include cationic and anionic
emulsions, inverted emulsions as well as polymer modified emulsions.
Several lessons have been learnt through use of emulsions in different climatic conditions, with different
aggregates, seal types, from low to high volume traffic spectrums and with different construction
methodologies.
This paper endeavours to collate the current best practice in South Africa for using bitumen emulsions in
sprayed seals.
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INTRODUCTION
Background
Bitumen emulsions were first manufactured in SA in 1929. They soon became the preferred binder for several
of the Municipalities and Provincial Administrations construction and maintenance units in South Africa.
Government drives towards privatisation since 1980 resulted in sealwork being contracted out to the privatesector. They in turn, preferred to apply hot binders due to cost factors and production rates. Currently, new
drives towards more environmental friendly products, safety and the need to maximise sealwork throughout theyear could result in emulsions again becoming the preferred choice.
Scope and layout
The paper provides a short background to the components and manufacturing of bitumen emulsions, different
types of emulsions available in South Africa and applications related to sprayed seals. The paper then focuses onthe benefits of using emulsions in sprayed seal operations and discusses good practice and lessons learnt.
BITUMEN EMULSION COMPONENTS AND MANUFACTURING
Components
Emulsions are droplets of immiscible liquid suspended one in another. Polar liquids like water and organic non-
polar liquids like oil are examples of such immiscible systems. Mechanically the two liquids can be suspended
but will separate completely in two different layers, like when cooking oil and water is shaken into suspension.
The droplets can however be kept in suspension by surface-active agents. The suspension is made possible by
the fact that these surface-active agents has both organic (non-polar) and polar functional groups present in its
chemistry and will orientate themselves in such a way that the droplets will stay in suspension. Milk is a typical
example emulsions found in nature and surface-active agents in soap allows us to dissolve fatty substances in
water.
Bitumen is organic in nature and is largely non-polar and will therefore by its nature not mix with
water. Furthermore bitumen is a liquid only at elevated temperatures above the boiling point of water. Yet by
careful selection of physical, chemical and mechanical conditions it is possible to manufacture bitumen
emulsions.As bitumen can only be applied at elevated temperatures typically above 100C, emulsified bitumen
enables us to apply bitumen at ambient temperature conditions, reducing the risk of burns, the requirement to
work at such dangerous temperatures.
Manufacturing
Manufacturing of bitumen emulsions requires sophisticated equipment and a thorough understanding of the
physical properties of bitumen, the chemistry of bitumen and emulsifier systems and that of water. Bitumen, the
organic non-polar phase, is typically heated above 120C in order to reduce the viscosity to such a level that it
can be mechanically sheared into fine droplets. Emulsifiers are reacted and activated chemically in the water(inorganic and polar) phase. The two phases are introduced into the colloidal mill separately. The emulsified
bitumen will then be allowed to cool down and depending on the droplets size, the type of emulsion and
emulsifier system used in the process, the bitumen emulsions will have a range of stability, reactivity andphysical behaviour required in various applications.
Specifications
Bitumen emulsion are classified depending on the chemistry of emulsifier systems into either anionic
(electronegative), non-ionic (neutral) or cationic (electropositive) emulsions. Specifications for bitumen
emulsions are therefore set in order to distinguish in terms of the chemistry and the physical nature of the
product required in the application.
The bitumen emulsion properties related to the stability, electric charge and behaviour of the bitumen
emulsion in the application is regulated to distinguish between the types of emulsion, break rate and set rate of
the product.
The most important factor in emulsion specification is related to the viscosity which is mostly depend on the
binder content but can also be controlled by the addition of visco-enhancing technologies.
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TYPES AND PURPOSE OF BITUMEN EMULSIONS USED IN SOUTH AFRICA
Emulsion Classification
Two types of emulsions are used in road construction in South Africa.
Anionic bitumen emulsions are regulated by the South African National Standards for Anionic
Emulsions (SANS 309) (1) and the requirements for Cationic Bitumen Emulsions are regulated in the SANS548 specification (2).
Depending of the stability and reactivity of the emulsion, the types of emulsions are classified in termof set and break rate into three different categories; Stable Set, Medium Set and Rapid Set emulsions.
Depending on the application, the grading and type of the aggregates, the type and the class of the bitumen
emulsion are selected. The break rate and set rate of anionic emulsion are largely dependent on evaporation
whereas cationic emulsions will also be influenced by the chemical break rate of the bitumen emulsion
Emulsion Selection
Aggregate properties are also split into electro positive or electro negative types. Most of aggregates in South
Africa are electro negative. Granite, dolerite and quartz-like aggregates contain siliceous compounds in the
mother rock which introduce a negative charge on the aggregate. Electro positive aggregates, like dolomite,
contain calcareous deposits and exhibits. In principle, since opposite charges attract, an emulsion with theopposite charge to that of the aggregate will exhibit the best adhesion.
Although the general perception is that the Anionic Bitumen Emulsions work better with
Electropositive aggregates and Cationic Bitumen Emulsions work better with Electronegative aggregates, other
factors and chemical components in the mother rock will attribute to the successful application of emulsion with
a similar charge in slurry seal, fog spray and sprayed seal applications.
Cationic Bitumen Emulsions are generally preferred for sprayed seal applications in South Africa and
anionic emulsions are largely used in slurry seals and the construction of emulsion treated bases.
Polymer Modified Emulsions
Polymer modified cationic emulsions have been developed to reduce the temperature susceptibility of the base
binders used in the manufacturing of emulsion. Modification of bitumen emulsions results in reduced
temperature susceptibility. Emulsification of modified bitumens increases the complexity of the emulsificationprocesses as a result of physical properties requiring higher temperatures. A simpler way to produce polymer
modified emulsions is to add the polymer in the form of latex emulsion after the bitumen emulsification.
Pressure units and heat exchangers are therefore required to prevent the emulsion temperature exceeding the
boiling point of water.
Inverted Bitumen Emulsion Primes
When water droplets are suspended in an organic phase, the resultant emulsion is called an inverted emulsion.
Inverted bitumen emulsions are manufactured using cutback bitumens. With a further reduction in the viscosity
of the binder by the addition of organic cutters, it is possible to manufacture an inverted emulsion. Theseinverted bitumen emulsions are used primarily in prime applications with difficult base materials where
conventional cutback bitumen primes are not performing well in terms of penetration and drying time.
Other Emulsion Primes
In recent years emulsified cutback bitumens were developed to overcome penetration problems experienced
with cutback bitumen primes. In South Africa the use of tar primes were discontinued in 2006. This lead to the
development of bitumen emulsion primes.
These emulsion primes are classified as more environmentally friendly than the tar prime. These
emulsion primes contain between 15 and 40% water resulting in much reduced net binder in the final
application. Experience and attention is required to ensure the correct application rate being used for the
emulsified cutback primes to perform adequately. The use of non-crude derived cutters in such systems adds to
complexity and cost and although the organic cutters used in such products are sourced from renewable sources,
the impact on environment from carbon footprint perspective is not fully understood by the end-users and
authorities specifying the Environmental Friendly Primes.
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TERMINOLOGY
Terminology for bituminous surfacings and specifically sprayed seals vary throughout the world. For purposes
of this documentFigure 1provides a classification of sprayed seals in the context of surface dressings.
Notes:
Prime refers to a bituminous binder applied at 0.8 1.2 l/m2on a newly constructed granular base. Tack coat refers to the first binder application of a sprayed seal Penetration coat refers to the second binder application when a multiple aggregate seal is constructed. Blacktop spray (cover spray or fog spray) refers to an emulsion or diluted emulsion application on the
final aggregate layer of a single or multiple aggregate seal.
Emulsion Treated Bases and Dust Palliative application are not classified as surface dressings but areoften trafficked as if they are final surfacings
FIGURE 1 Classification of bituminous surfacings
BITUMINOUS SURFACINGS
Asphalt overlays
Sprayed
Seals
Surface
dressings
Slurry
Seals
Combination
Seals
Micro
Surfacing
Single Stone
Seal
Sand/ Grit Seal
Graded
Aggregate Seal
Multiple Stone
Seal
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TYPICAL SURFACE DRESSINGS USED IN SOUTH AFRICA
Figure 2and
Figure 3 show the typical surface dressings applied in South Africa. Bitumen emulsions are used in the tack
coat, penetration coat and as fog sprays.
FIGURE 2 Most common surface dressings in South Africa
Single Seal
Double Seal
1 " Seal
Cape Seal
Slurry Seal
Sand and
Grit Seal
fog spray optional
stone
tack coat
existing substrate
fog spray optional
stone
tack coat
existing substrate
fog spray optional
tack coat
existing substrate
1st layer - stone
2nd layer - stone
penetration coat
fog spray optional
tack coat
existing substrate
1st layer - stone
2nd layer - stone
penetration coat
fog spray optional
tack coat
existing substrate
1st layer - stone
2nd layer - stone
penetration coat
fog spray optional
tack coat
existing substrate
1st layer - stone
2nd layer - stone
penetration coat
fog spray optional
tack coat
existing substrate
1st layer - stone
2nd layer - stone
penetration coat
tack coat
existing substrate
1st layer - stone
2nd layer - stone
penetration coat
tack coat
existing substrate
stone
fine slurry (1 or 2 layers)
fog spray
tack coat
existing substrate
stone
fine slurry (1 or 2 layers)
fog spray
tack coat
existing substrate
sand or grit
tack coat
existing substrate
sand or grit
existing substrate
2ndlayer -n of slurry-
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FIGURE 3 Other surface dressing applied in South Africa
Note: The Split Seal in
Figure 3is gaining popularity due to its ability to accommodate high traffic volumes
OTHER APPLICATIONS
Rejuvenation sprays
Periodic rejuvenation of dry and open textured surfacings (e.g. stone seals) is still considered standard practice
within some road authorities in South Africa. The preference is to use anionic stable grade emulsion, normally
diluted with water (50/50) and applied at approximately 1 litre/m2
(This relates to approximately 0.3 0.35litre/m
2 residual bitumen). Should the surface still require additional binder, a second application could be
considered.
For the rejuvenation of dense textured surfaces (e.g. airport runway edges), proprietary products
marketed as rejuvenators (typically consisting of cut-back inverted emulsions, containing high boiling point
aromatic oils), are also locally available. Experience with these products resulted in the recommendation only to
be used on very low volume roads and with low application rates. The typically 0.40.5 l/m2application may
take from 5 hours to 4 days before the surface can be opened to traffic.
Practitioners further recommend application during the dry season and not to use spray gradeemulsions for rejuvenation purposes.
Additional binder to prevent stripping soon after construction
Sealwork done on high trafficked roads in South Africa require that binder application rates are kept on the lowside to minimise the risk of bleeding and to retain high macro texture for purposes of skid resistance. This
Geotextile
Seal
Split Seal
Choked Seal
Inverted
double seal
Graded
aggregate
seals (Otta
seals)
stone
tack coat
existing substrate
thin layer of aggregate
penetration coat
stone
tack coat
existing substrate
thin layer of aggregate
penetration coat
tack coat
existing substrate
1st layer stone
penetration coat
2nd layer - stone
3rd layer - stone
tack coat
existing substrate
1st layer stone
penetration coat
2nd layer - stone
3rd layer - stone
tack coat
existing substrate
1st layer stone
2nd layer - stone
tack coat
existing substrate
1st layer stone
2nd layer - stone
tack coat
existing substrate
1st layer stone
2nd layer - stone
penetration coat
tack coat
existing substrate
1st layer stone
2nd layer - stone
penetration coat
tack coat
existing substrate
graded aggregate
sand
tack coat
tack coat
existing substrate
graded aggregate
sand
tack coat
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strategy results in sprayed seals being sensitive to adverse weather conditions and colder micro climatic areas
(e.g. shady areas) soon after construction. Recommendations from practitioners are:
Add a cover spray of cationic spray grade emulsion (normally diluted) in shady areas or when coldnight temperatures are expected (addition of 0.2 to 0.4 litre/m
2 net bitumen), and if considered
necessary to accommodate early traffic, blind with coarse sand (no fines)
Provide for additional diluted cationic cover spray in contracts when sealwork is expected to be doneclose to winter time. Monitor adhesion and potential stripping and apply fog spray, if necessary.
Longitudinal joints often cause problems. Additional binder applied in the form of an emulsion coverspray (only on the joint), reduces the risk of joint stripping.
PHOTOGRAPH 1 Emulsion spray on longitudinal joint
Pre-treatment before resurfacing
Old bituminous surfacings, especially multiple stone seals, are often dry/brittle and porous. Good practice
suggests application of stable grade diluted emulsion to fill capillaries within the existing surfacing before
resealing.
PHOTOGRAPH 2 Brittle and porous surfacing
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BENEFITS OF EMULSIONS IN SPRAYED SEALS
General Benefits
Benefits of bitumen emulsion when compared to hot bituminous binders are well known and include:
Environmental friendliness and safety Reduced energy as a result of low temperature application No harmful solvents released into the atmosphere Worker safety as a result of low temperature and no hazardous fumes
Practical aspects
Increased working time via extension of construction window Reduced risk of stripping due to seal structure Improved adhesion Labour intensive seal work More forgiving transverse distribution Ease of additional binder application
Ease of construction (Less rolling required)
The abovementioned practical aspects are briefly discussed in the following section.
c
Working time
Specifications in South Africa calls for road surface temperature of minimum 25C when using 80/100Pen Bitumen and standard summer grade modified binders. Emulsions allow working at minimum road surface
temperature of 10C, which result in much longer working time and increased production.
Reducing risk of stripping due to seal structure
Several binder types are used in South Africa for seal construction and include straight run bitumen, cut
back bitumen, polymer modified bitumen, bitumen rubber as well as bitumen emulsion and polymer modified
emulsion.Application of emulsion cover sprays increases the aggregate surface area in contact with the binder
and, therefore increasing the bond strength, without a significant risk of bleeding. Observations and two-
dimensional scanning shows the filling of the voids and the additional bond between the adjacent aggregate
particles. The effects of emulsion cover sprays are shown inFigure 4 andFigure 5.
FIGURE 4 Emulsion filling voids (Typically achieved with stable grade anionic emulsion)
FIGURE 5 Bond between aggregate particles (Typically achieved with spray grade cationic emulsion)
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The ionic charge and setting rate of different emulsion types, result in different effects. Anionic stable grade
emulsion (Photograph 3- left lane) and cationic spray grade emulsion (Photograph 3- right lane) were applied as
cover sprays, at the same application rate, on a newly constructed double seal. Whereas some stripping occurred
on the anionic emulsion treated lane occurred, no stripping could be observed after a year on the cationic
emulsion treated lane.
PHOTOGRAPH 3 Effect of Anionic stable grade versus Cationic spray grade emulsion as fog sprays
Improved adhesion
Several aggregate sources produce dusty aggregate, requiring washing and or precoating before use to improve
adhesion. Emulsions are known to improve adhesion more than most hot binders. Although it is not common to
precoat aggregate in South Africa when using emulsions, good experiences have been recorded. Practitioners
recommend the dampening of the aggregate and road surface before sealing with emulsion.
The majority of seal aggregates in South Africa are negatively charged, resulting in better initial adhesion whenusing cationic emulsions, however, if the stone is precoated, the effect of ionic attraction is minimised.
Labour intensive sealwork
Stable grade emulsions are commonly used for labour intensive sealwork. Although cationic stable grade
emulsion is recommended, several cases have been recorded where seals have been constructed with hand
labour using anionic stable grade emulsions.
Particular conditions where anionic emulsion is favoured are:
High atmospheric temperatures When cationic emulsion still breaks to fast When improved penetration is required (lower viscosity)
Safety
The temperature required for the spray application of hot binders varies between 130 200C depending on the
binder type. Emulsion can be sprayed at between ambient and 60C, thus they are the preferred choice,
especially with inexperienced contractors.
Transverse distribution
The success and good performance of a sprayed seal is highly dependent on the uniform application of the
binder. The low viscosity of emulsion, lower spray application temperature and road surface temperature result a
much more even distribution of binder.
Although blocked nozzles are not acceptable, the end result is more forgiving when using low viscosity binders
such as emulsions. Photograph 4 shows the effect of a too low spray bar setting when using a high viscosity hot
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binder. Photograph 5 shows a blocked nozzle and the more forgiving effec t due to the easy flow of the
emulsion.
PHOTOGRAPH 4 Sensitivity of high viscosity binders to transverse distribution
PHOTOGRAPH 5 Emulsion flow - even with blocked nozzle
Ease of compaction
Experience has shown that seals constructed with hot high viscosity binder require at least double the number of
roller passes to obtain the same aggregate /binder contact area. In addition to this, the binder temperature rapidly
reduces to the prevailing road surface temperature, requiring a high degree of control and short following
distance between the spray tanker, chip spreader and rollers, when using hot binders.
Ease of additional application
Evaluation of the binder content in the sprayed seal after rolling could result in the need for additional binder.
The minimum hot 80/100 Pen bitumen that could be accurately applied is considered to be 0.65 l/m2. Making
use of diluted emulsions allows application of down to 0.24 l/m2net bitumen. This is valuable when adding
additional binder to longitudinal joints or in shady areas.
GOOD PRACTICE AND LESSONS LEARNT
Principles of emulsion in sprayed seals
One of the most important recommendations when using emulsion in sprayed seals is to split the binder intomore than one application e.g. for a single seal the binder is applied as a tack coat (first application) and a cover
spray (second application).
When splitting the binder into two applications, the second spray is normally applied at a higher rate
than the first. This strategy reduces the run-off potential of the low viscosity emulsion. The first stone layer is
bound with the minimum amount of binder to allow construction traffic. This results in a much coarser surface
texture, which can then accommodate the higher application rates without the risk of run-off. Example: On a
flat grade with, say 0,7mm macro texture, the maximum emulsion application rate before run-off occurs couldbe in the order of 1.3 l/m2. If a 65% emulsion is used the residual bitumen on the road would be 0.85 l/m
2. From
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Table 1, as recommended in the South African National Roads Agency (SANRAL) (Technical
recommendations for highways; TRH3), the maximum stone size recommended would be a 13,2 mm stone (3).
TABLE 1 Minimum quantity of net cold binder required for tack coat (TRH3 Table 7-5) (3)
Aggregate size 9,5 mm 13,2 mm 19,0 mmOnly construction traffic 0,5 /m
2 0,7 /m
2 1,0 /m
2
Recent winter seal experiments using emulsions at different application rates, with and without an emulsion or
diluted emulsion cover spray, confirmed much improved performance when splitting the binder into two
applications, due to increased aggregate surface area in contact with the binder and bonding between adjacent
aggregate particles.
PHOTOGRAPH 6 Single seal without and with emulsion cover spray
Even though the total net bitumen for the two seals in Photograph 6 is the same, the seal without the cover
spray soon started to show signs of aggregate loss (stripping).The effect of the emulsion cover spray (normally cationic spray grade emulsion), from observations
and stone retrieval, is similar to the effect shown in Figure 5.
Run-off due to steep grades
Current manuals in South Africa recommend a maximum application rate for emulsion (typically 65% Bitumen)
of 1.5l/m2
{in TRH3 (3)} and of 1.75 l/m2
{Western Cape Materials Manual (4)}. However, this latter rate can
only be applied on a coarse textured surface where the road grade is relatively flat. Combining the opinions of
practitioners, resulted in the following recommended maximum application rates.
TABLE 2 Maximum emulsion application rates (65% Emulsion)
Grade Macro texture
< 0.7 mm 1.0 mm >2.0mm< 4% 1.0 1.5 1.7
46% 1.0 1.3
6 - 8 % 0.8
Notes:
Grade refers to the maximum gradient/cross fall combination Viscosity of the binder is dependent on the bitumen content and temperature Porous surfacings will allow higher application
Road surface temperature and ionic charge difference between the aggregate and emulsion will influence the
setting time and therefore also the potential run-off
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Sensitivity to wind
Due to the low viscosity of emulsions, spraying at wind speeds in excess of 30 km/h is not recommended.
Changing the overlap configuration (typically triple overlap applied in South Africa) to a double overlap by
lowering the spray bar reduces the sensitivity to wind (ReferFigure 6)
PHOTOGRAPH 7 Effect of wind on emulsion spray
FIGURE 6 Reducing wind sensitivity by lowering spray bar
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Slow breaking due to low temperature and humidity
Most emulsions rely on the evaporation of water to break. Low road surface temperatures and high humidity
(typically > 70%) could result in slow breaking of the emulsion and delays in the opening of the road to traffic.
Cover spray or rejuvenation spray stays tacky and pick-up often occurs.
The main reasons identified are: Presence of cutter in the emulsion Too high road temperature >50C Too dense / Fine textured surfacing Dust on surface Road opened too early Polymer modified emulsions Too high binder application
Polymer modified cationic emulsion tends to stay tacky much longer than conventional cationic emulsion. In the
case of cover sprays, this could result in delays opening the road to traffic. Problems to accommodate traffic
have been overcome by:
Diluting cover sprays Blinding with coarse sand Spraying water on the surface, especially when the road surface temperature is higher than the binder
softening point
False breaking
False breaking occurs when the emulsion forms a skin on the surface, thus delaying the evaporation of water
underneath. This problem has mainly been experienced with polymer modified emulsion.
PHOTOGRAPH 8 The effect of false break
Corrugation effect
Corrugations on the road surface could be the result of:
Chip spreader near empty and high inflated tyres (Hopping effect), resulting in uneven spread ofaggregate
Applying small aggregate into the wet emulsion e.g. sand, resulting in wave forming
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PHOTOGRAPH 9 Wave forming due to sand in wet emulsion
Poor penetration of rejuvenation sprays
Poor penetration could be the result of:
Too dense surfacing Dirty surfacing Rapid breaking emulsion (Prefer anionic stable grade diluted emulsion) Too low application
PHOTOGRAPH 10 Cationic emulsion fog spray (Polymer modified) on old quartzitic aggregate
PHOTOGRAPH 11 Poor penetration due to dirty surface
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Penetration coat (2nd
spray) on open aggregate layer
Multiple seals such as the split seal shown in
Figure 3are used on high traffic volume roads in South Africa with great success. Following the first binder and
large aggregate application, a layer of smaller but dry aggregate is placed to fill the voids between the large
aggregate particles as shown in Photograph 12. Thereafter the second binder layer is applied, followed byanother layer of smaller aggregate with a final application (cover spray), consisting of an emulsion or diluted
emulsion (normally cationic spray grade emulsion).
Note: The second binder application (on the dry aggregate layer) must consist of a high viscosity binder.
Emulsion is not appropriate for this application
PHOTOGRAPH 12 Dry aggregate layer
Effect on precoated aggregate
Manuals in South Africa still caution against the use of emulsions with precoated stone. Although the authorshave experienced problems with the breaking of emulsions when using tar-based precoating fluids, recent
experiments using highly cutback bituminous precoating fluid did not significantly affect the expected breaking
time of either stable grade anionic emulsion or spray grade cationic emulsion.
Cutters in emulsion tack coats
Cationic spray grade emulsions, as supplied in South Africa, often contain small percentages of cutters (1% -
4%). Experience with the performance of Cape seals and Geotextile seals suggests that these cutters be omitted
from the emulsion, when applied as the first binder application.
Binder selection
Table 3 from the recently published by the South African Bitumen Association (SABITA) Manual 10 -Bituminous Surfacings for Low Volume Roads (5), provides some guidelines for the selection of appropriate
binders for initial construction surfacings.
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TABLE 3 Binder suitability for initial road construction (SABITA Manual 10) (5)
Binder description
80/100 - 80100 Pen bitumenMC 3000 - Medium Cut-back bitumen (Typically 80-100 Pen bitumen with 12% cutter)
S-E1 - Hot polymer modified bitumen (Typically less than 3% polymer)
S-E2 - Hot polymer modified bitumen (Typically more than 3% polymer)S-R1 - Rubber crumb hot modified binder (20% Rubber crumbs used in South Africa)
Spray grade emulsion - Typically 65% Cationic Spray Grade bitumen emulsionSC-E1 - Polymer modified emulsion (Typically 5% SBR polymer)
Stable grade emulsion - Cationic or Anionic stable grade emulsion
Relative
binder
Cost
Ratio ( c )
Very
HighHigh Med Low
Very
LowHigh Medium Low 10
High
>70%Low 45
Hand
spray
Mech
sprayL ow Medium High per m
2
>50001000 -
5000
500 -
1000
100 -
500
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CONSTRUCTION
Sequence of activities with sprayed seals using emulsion
Although there are several schools of thought amongst practitioners, the following sequence of activities should
result in a well performing seal:
Cleaning of the road surface Moistening of the road surface Application of the tack coat (first binder application) Application of the aggregate
o Single size aggregate, preferably moist, applied onto the wet emulsiono In case of Sand Seals, the moist sand is applied when the emulsion starts to break
Initial rolling using a light steel wheel roller (two passes recommended) Back-chipping, if required Pneumatic rolling (only when the emulsion has broken/ cured). Dependent on the emulsion type and
climatic conditions, this could even be the next day, when road surface temperatures increase
(Preferably above 25C).
Note: With low volume roads and high binder volumes (high percentage of voids filled with binder),
two passes with a pneumatic roller could be sufficient. For high volume roads, where the minimum
voids are filled with binder, experience suggests a minimum of eight roller passes .
Dragbrooming, if necessary, must take place as early in the mornings as possible on the previous daysseal work, when the road temperature is low and the binder sprayed the previous day is cold and stiff.
The broom is dragged to and fro on the surface. Thereafter, when the road surface temperature
increases, the seal is rolled with a light steel wheel roller, followed with more pneumatic rolling.
Triangular, T-shaped and Z-shaped dragbrooms are effective to move surplus aggregate in places toareas where there is a lack of aggregate to fill open gaps.
PHOTOGRAPH 13 Z-Type dragbroom
Rotary brooming to remove excess aggregate (in case of single sized aggregate) is recommended whenthe first binder application has cured properly and stones are well adhered. Thereafter, the final cover
spray can be applied.
The road should only be opened to traffic when proper adhesion between the binder and the stone hasdeveloped.
Consideration must be given to prevailing and expected temperatures, as well as the traffic volume. The cover spray should be cured. In case of tackiness of the binder on top of the aggregate and at
intersections or colder micro-climatic areas, a light application of coarse sand will allow quicker
opening to traffic
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In case of a double seal;
The penetration coat (second binder application) is preceded by a light spray of water The second aggregate layer is applied onto the wet emulsion, followed by light steel wheel rolling as
well as back chipping, if required
Pneumatic rolling when the emulsion has cured (with dragbrooming, if required) Rotary brooming to remove excess aggregate (in case of single sized aggregate) Light steel wheel roll preferred at this stage by several practitioners Final cover spray, when the second binder application has cured properly and stones are well adhered Opening to traffic should only be allowed when the emulsion cover spray is properly cured. The speed
of the traffic on the completed seal should be limited during the day of application and the following
night. If rain falls on a newly constructed reseal, traffic should if possible, be kept off until the rain has
stopped, the seal has dried and road surface temperatures increase to above 25C).
QUALITY ASSURANCE
Quality assurance on site is essential. Although not discussed in this paper, some of the key aspects to ensure
good initial performance are:
Control over preparatory work Stockpile management Equipment checks Trial section construction to verify operational aspects which includes
o Aggregate packingo Rolling procedureso Joint constructiono Adhesion developmento Verifying applicability of the design
Safety and traffic accommodation Sampling and testing, which includes
o Aggregate propertieso Binder properties
Binder application rate Aggregate spread rate Sufficient rolling and correct sequence
ACKNOWLEDGEMENTS
Several practitioners have been consulted to obtain opinions on good practice. Although not possible to list all
names, the following people are acknowledged for providing specific information and photographs.
T Distin
TJ Lewis
JG LouwJ van Jaarsveld
G Forward
REFERENCES
1) South African National Standards SANS 309 Specification for Anionic Bitumen Emulsions
2) South African National Standards SANS 548 Specification for Anionic Bitumen Emulsions
3) South African National Roads Agency (SANRAL). 2007. Design and construction of surfacing seals .
(Technical recommendations for highways; TRH3). Pretoria. South Africa
4) Western Cape Department of Transport and Public Works. 2010.Materials Manual, Volume 2, Volume
6,Cape Town, South Africa5) Southern African Bitumen Association (SABITA). Bituminous surfacings for low volume roads.
Manual10. Cape Town. South Africa