Bitumen Emulsion in Sprayed Seals_Muller

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

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Bitumen Emulsion in Sprayed Seals_Muller