AP-T107/08

Bitumen Emulsions

AUSTROADS TECHNICAL REPORT

Bitumen Emulsions

Bitumen Emulsions First Published August 2008

Austroads Inc. 2008

This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without the prior written permission of Austroads.

Bitumen Emulsions ISBN 978-1-921329-80-7

Austroads Project No. TT1220

Austroads Publication No. APT107/08

Project Manager John Worrall

Prepared by Christina Chin

Published by Austroads Incorporated Level 9, Robell House 287 Elizabeth Street

Sydney NSW 2000 Australia Phone: +61 2 9264 7088

Fax: +61 2 9264 1657 Email: austroads@austroads.com.au

www.austroads.com.au

Austroads believes this publication to be correct at the time of printing and does not accept responsibility for any consequences arising from the use of information herein. Readers should

rely on their own skill and judgement to apply information to particular issues.

Bitumen Emulsions

Sydney 2008

Austroads profile Austroads purpose is to contribute to improved Australian and New Zealand transport outcomes by:

providing expert advice to SCOT and ATC on road and road transport issues facilitating collaboration between road agencies promoting harmonisation, consistency and uniformity in road and related operations undertaking strategic research on behalf of road agencies and communicating outcomes promoting improved and consistent practice by road agencies.

Austroads membership Austroads membership comprises the six state and two territory road transport and traffic authorities, the Commonwealth Department of Infrastructure, Transport, Regional Development and Local Government, the Australian Local Government Association, and New Zealand Transport Agency. It is governed by a council consisting of the chief executive officer (or an alternative senior executive officer) of each of its eleven member organisations:

Roads and Traffic Authority New South Wales Roads Corporation Victoria Department of Main Roads Queensland Main Roads Western Australia Department for Transport, Energy and Infrastructure South Australia Department of Infrastructure, Energy and Resources Tasmania Department of Planning and Infrastructure Northern Territory Department of Territory and Municipal Services Australian Capital Territory Department of Infrastructure, Transport, Regional Development and Local Government Australian Local Government Association New Zealand Transport Agency The success of Austroads is derived from the collaboration of member organisations and others in the road industry. It aims to be the Australasian leader in providing high quality information, advice and fostering research in the road sector.

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SUMMARY

This is a literature review on overseas and Australian experiences in using bitumen emulsions for road surfacings. Emphasis is placed on reviewing emulsion road trials conducted in Australia and New Zealand between 1993 and 1997. Specifically, the review focussed on whether the use of emulsions could:

extend the sealing season lower the energy consumption reduce the emission of greenhouse gases provide an alternative to the use of hot cutback bitumen.

Bitumen emulsions are made up of three components: bitumen, water and emulsifier. It is a two phase system consisting of two immiscible liquids. The emulsifying agents maintain the bitumen droplets in a stable suspension and control the breaking time (time taken for the water phase to separate from the bitumen phase and evaporate) of the emulsion.

The predominant use of bitumen emulsions in Australia is for sealing works. They are an alternative treatment to hot cutback bitumen on low to medium trafficked roads during the cooler months. Bitumen emulsions are less preferred to hot cutback bitumen because:

hot cutback bitumen is more cost-effective as it eliminates the manufacturing process between the refinery and road

bitumen emulsions have run off (low viscosity) problems of the delay between application and opening to traffic to ensure that the emulsion has

broken

of the lack of knowledge and understanding of emulsion technology.

Current high binder content emulsions, emulsified polymer modified binders (PMEs), tailored emulsifying agent etc. have managed to overcome some of the problems mentioned above. High binder content bitumen emulsions address the run off and slow breaking problems. PMEs provide the same elastic properties as conventional PMBs as well as having the decreased viscosities and low spraying temperatures of an emulsion. PMEs, with their lower viscosity, are better than conventional PMBs at coating the sealing aggregate and so reduce the risk of early aggregate stripping problems. The lower operating temperatures also reduce the risk of potential damage to the product during storage and handling. High binder content PMEs also reduce the amount of polymer additives and use up to 30% less water.

Solvents such as kerosene are used as cutback and are added to bitumen to decrease the viscosity of the binder, hence making it more workable. Unlike bitumen emulsions which are water based, cutback bitumen needs to be kept at high temperatures (160180 C). The evaporation of kerosene is an energy intensive process and it emits greenhouse gases such as carbon dioxide and volatile organic compounds which are harmful to the environment. Staff working under such a high temperature environment are exposed to a variety of safety hazards such as burns, explosions, etc. The fumes and odours released from the solvent have also been known to affect the workers making them feel nauseous.

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In conclusion, it was observed that the demand for emulsions is increasing overseas. The main reasons for this are the possibility of extending the sealing season, lower energy consumption and, particularly, the elimination of kerosene use which is seen as detrimental to the environment.

Reduction of greenhouse gases is often cited as a major advantage of emulsions but some studies have suggested that the benefits may be reduced because of the energy involved in emulsion production and the transport of the extra water incorporated in emulsions to the work site.

Emulsions can provide an alternative to the use of hot cutback bitumen but currently in Australia it is only used as an alternative treatment during the cooler months. Emulsions is not preferred in Australia because of the loss of aggregate and tight traffic control required at the early stage of sealing.

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CONTENTS

1 INTRODUCTION ............................................................................................................ 1 1.1 History............................................................................................................................. 1 1.2 Development from Anionic to Cationic............................................................................ 2 1.3 Development from Low to High Residue ........................................................................ 2 2 EMULSION CHEMISTRY............................................................................................... 3 2.1 Breaking of Bitumen Emulsions...................................................................................... 4 2.2 Curing of Bitumen Emulsions ......................................................................................... 4 2.3 Factors Affecting Breaking and Curing ........................................................................... 4 3 CURRENT DEVELOPMENTS ....................................................................................... 6 3.1 High Bitumen Content..................................................................................................... 6 3.2 Emulsifier ........................................................................................................................ 7 3.3 Particle Size Distribution................................................................................................. 7 3.4 Applications .................................................................................................................... 7 4 POLYMER MODIFIED BITUMEN EMULSIONS............................................................ 8 4.1 Introduction..................................................................................................................... 8 4.2 Manufacture of Polymer Modified Bitumen Emulsions ................................................... 8 4.3 Applications .................................................................................................................... 9

4.3.1 Introduction ....................................................................................................... 9 4.3.2 Sprayed Seal Binder in High Stress Areas (New Zealand) ............................ 10

4.4 Advantages................................................................................................................... 11 4.4.1 Improved Properties ....................................................................................... 11 4.4.2 Comparison with Non-emulsified PMBs ......................................................... 12 4.4.3 Environmental Aspects ................................................................................... 12

4.5 Disadvantages.............................................................................................................. 12 5 ENVIRONMENTAL COMPARISON BETWEEN HOT CUTBACK AND BITUMEN

EMULSIONS ................................................................................................................ 13 5.1 Introduction................................................................................................................... 13 5.2 Occupational Health and Safety ................................................................................... 13 5.3 Energy Consumption .................................................................................................... 13 5.4 Greenhouse Effect........................................................................................................ 13 6 FACTORS AFFECTING QUALITY AND PERFORMANCE OF BITUMEN

EMULSIONS ................................................................................................................ 16 7 OVERSEAS EXPERIENCE.......................................................................................... 17 7.1 New Zealand................................................................................................................. 17

7.1.1 Introduction ..................................................................................................... 17 7.1.2 Applications .................................................................................................... 17 7.1.3 Other Technical Issues ................................................................................... 19

7.2 Europe .......................................................................................................................... 19 7.2.1 Introduction ..................................................................................................... 19 7.2.2 French Road Network..................................................................................... 19 7.2.3 Applications .................................................................................................... 20

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7.3 South Africa .................................................................................................................. 217.3.1 Introduction ..................................................................................................... 21 7.3.2 Emulsion Treated Bases................................................................................. 21 7.3.3 Other Applications .......................................................................................... 22

8 CHALLENGES FACED IN AUSTRALIA ..................................................................... 23 8.1 Introduction................................................................................................................... 23 8.2 Road Trials ................................................................................................................... 23

8.2.1 Precoating of Aggregate on Site with Low Binder Content Emulsions ........... 23 8.2.2 Precoating of Aggregate in the Quarry with Low Binder Content

Emulsions ....................................................................................................... 24 8.2.3 Sprayed Seals Using High Binder Content Emulsions ................................... 25 8.2.4 Weather Conditions ........................................................................................ 26 8.2.5 Priming Grade Emulsions ............................................................................... 27 8.2.6 Primersealing Grade Emulsions ..................................................................... 27

8.3 Other problems faced ................................................................................................... 28 8.3.1 Storage and Handling ..................................................................................... 28 8.3.2 Stability of Emulsions in Cold Conditions ....................................................... 28

9 CONCLUSIONS ........................................................................................................... 29

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TABLES

Table 1.1 : Seven highest users of bitumen emulsions ...................................................... 1 Table 4.1: Trial site details and performance.................................................................. 11 Table 5.1: 1992 emission estimates (CO2 equivalent)..................................................... 14 Table 5.2: Carbon dioxide generated per tonne of sealing bitumen sprayed .................. 15 Table 7.1: Emulsion use on NZ State Highways ............................................................. 18 Table 8.1: Site details and inspection report.................................................................... 27

FIGURES

Figure 2.1: Schematic diagram of a bitumen emulsion....................................................... 3 Figure 3.1: Relationship between viscosity and residue content in the emulsion ............... 6 Figure 4.1: Post addition of latex ........................................................................................ 8 Figure 4.2: Co-milling of latex ............................................................................................. 9 Figure 4.3: Latex addition to bitumen.................................................................................. 9 Figure 7.1: Total consumption of bitumen in South Africa ................................................ 21

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1 INTRODUCTION

1.1 History Emulsions were first developed at the beginning of the 20th century. The use of bitumen emulsions for road construction only started in the 1920s (Asphalt Institute & AEMA 2006). The earliest uses of bitumen emulsions were mainly in sprayed sealing applications and to prevent dust from the increasing traffic (Le Coroller 1999).

After World War II, the use of bitumen emulsions was greatly reduced. This was a result of the increasing use of hot mix asphalt to counteract rising traffic loads and volumes. However, this did not persist and bitumen emulsions began to experience an increase in demand. Some of the contributing factors were (Asphalt Institute & AEMA 2006):

The energy crisis in the early 1970s prompted the conservation of oil by the U.S Federal Energy Administration. Bitumen emulsions became favoured over hot cutback bitumen since they contain less cutter and use less energy.

There were rising concerns about atmospheric pollution. Bitumen emulsions emit little to no greenhouse gases.

Bitumen emulsions are capable of coating damp aggregate surfaces which reduces the amount of fuel required for heating and drying of aggregates.

Cold materials are able to be used at remote sites.

Emulsions can be used on slightly distressed pavements.

At the end of the 20th century, the world production of bitumen emulsions was estimated to exceed 7 million tonnes (Le Coroller 1999). Table 1.1 shows United States as the main producer while France is the highest user of bitumen emulsions. Today, an estimated 5-10% of the world paving grade bitumen is used for emulsions and the United States remains the worlds largest emulsion producer (James 2006).

Table 1.1 : Seven highest users of bitumen emulsions

Country Annual production of bitumen

(Millions of metric tonnes)

Consumption per head (kg/ person)

Bitumen used in emulsions (%)

United States 2.26 8.53 4.8 France 1.01 17.4 24.8 Mexico 0.51 5.65 37.2 Brazil 0.41 2.54 18.8 Spain 0.35 9.00 17.5 Japan 0.32 2.63 5.1

United Kingdom 0.16 2.72 4.7

Source: Le Coroller (1999)

When bitumen emulsions were first introduced in Australia, they were very much overshadowed by the hot cutback bitumen process which was more economical. The performance of bitumen emulsions was affected by a lack of understanding and the state of bitumen technology at that time.

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1.2 Development from Anionic to Cationic The continuing development of emulsifying agent used for detergent has contributed significantly to the improvement of bitumen emulsions.

In the early days, the standard type of emulsions used in Australia were anionic with a bitumen content of 55% (Austroads 2003). In 1958, cationic emulsions were introduced in Australia six years after they had been used in France (AAPA 2004).

Cationic emulsions became increasingly popular in the 1950s due to their affinity to a wider range of mineral aggregates. They promote better adhesion of bitumen to aggregate (mostly electronegative aggregates) and are effective for use in all weather conditions (Gorman et al. 2004).

1.3 Development from Low to High Residue Two of the main concerns faced by many roadmakers when using bitumen emulsions were the run off problem and the necessary delay during breaking.

Bitumen emulsions have low viscosities at ambient temperatures and are capable of running off the pavement surface. They are highly susceptible to being washed off the pavement during inclement weather should this occur soon after spraying. Good weather conditions during construction and relatively high pavement temperatures for at least one month are necessary to ensure that adequate curing of the emulsions is achieved (Austroads 2003).

High residue bitumen emulsions were introduced to overcome the two problems mentioned above. They were developed based on the knowledge that emulsion breaks and cures faster as the percentage of water is decreased. These types of emulsions soon became the mainstay of sealing in Europe, South Africa and many parts of the United States (Austroads 2003). In 1988, Australia incorporated these products in the Australian Standard, AS 1160 (Standards Australia 1988). High binder content emulsions contain a minimum binder content of 67% as compared to the conventional bitumen emulsions with 60% or less.

Over the years, the development of high residue emulsions continued to progress. Emulsions with binder contents as high as 78 to 80% have been used (Remtulla & Swanston 2000).

Bitumen Emulsions

2 EMULSION CHEMISTRY Bitumen emulsion is made up of three components i.e. bitumen, water and an emulsifying agent. It is a two phase system consisting of two immiscible liquids. The bitumen is dispersed in the continuous aqueous phase in the form of discrete globules, typically 0.1 to 50 m in diameter. It is held in suspension by electrostatic charges stabilised by an emulsifier.

Emulsifiers consist of polar (hydrophilic) and non-polar (hydrophobic) groups. This unique arrangement enables the emulsifier to orientate itself amongst the bitumen soluble group which is hydrophobic and the water soluble group which is hydrophilic.

The functions of an emulsifier in the bitumen emulsion system are (Gorman et al. 2004):

to reduce the interfacial tension between bitumen and water to form an emulsion to stabilise the emulsion and provide long and short range stabilising forces when it is cooled to support the adhesion between bitumen and mineral aggregates.

There are two fundamental types of emulsifiers available, anionic and cationic. Anionic emulsifiers were the first to be developed in the 1920s. They were derived from fatty acids - a long hydrocarbon chain that terminates with a carboxyl group which is activated in an alkaline solution (Gorman et al. 2004). These emulsions are negatively charged. Hence, they work most effectively with positively charged aggregates, such as limestone and marble. Anionic emulsifiers also work best in warm and dry conditions.

Cationic emulsifiers are fatty amine salts derived from reaction of fatty amines (e.g. diamines, imidazolines and amidoamines) with acidic solution. They are positively charged and work well in all kinds of weather conditions. This emulsion reacts effectively with predominantly negatively charged aggregates, giving a more rapid break and better adhesion. This is an important characteristic which enables the cationic emulsion to be less dependent on the evaporative breaking mechanism. Figure 2.1 depicts a schematic diagram of a cationic emulsion. The bitumen droplet is suspended in the continuous water phase by the positively charged emulsifier.

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Bitumen droplet (dispersed phase)

Water (continuous phase)

Emulsifying agent (positive surface charge)

Figure 2.1: Schematic diagram of a bitumen emulsion

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2.1 Breaking of Bitumen Emulsions For the bitumen emulsion to behave like a binder, the water phase must be separated from the bitumen phase. This phenomenon is termed breaking. Breaking commences once the emulsion comes into contact with the aggregates, gravels or pavement surface. If the charges on the bitumen particles are opposite to that of the aggregate surface, electrical attraction will take place. The bitumen particles will then start to migrate to the aggregate surface. This migration will cause the emulsion to break and start to separate into its original components: water and bitumen (Gaughan 1992).

The breaking of emulsion is highly dependent upon the type and concentration of the emulsifying agent. Various methods have been designed to accelerate the breaking process. Some of the techniques are (Remtulla & Swanston 2000):

Use of surface sprayed chemical breaker. This method applies a layer of anionic mist after the application of emulsion and before the aggregate is spread. However, this method poses skinning problems, especially with modified bitumen emulsions, and the modification of spraying equipment is fairly expensive.

Use of anionic precoat solution. This method precoats the aggregate with a mild anionic solution prior to the application of the emulsion. This is to ensure a uniform break between the aggregate-emulsion interface.

Use of an in-line blended breaking agent. This method involves in-line blending of a chemical additive to break the emulsion within a controlled time frame. The additive neutralises the pH of the emulsion, causing the water phase to separate/ break and promotes the formation of cohesive bonds. This technique allows a more flexible breaking system with time.

2.2 Curing of Bitumen Emulsions The emulsion is fully cured when the water and/ or any volatile oils have evaporated and cohesive bond strength is established between the binder and aggregate. The water can be removed not only through evaporation, but also by pressure (rolling) and by absorption into the aggregate. Water evaporation is highly dependent upon the weather conditions, and it is difficult for bitumen emulsions to properly cure when subjected to high humidity, low temperatures or rainfall soon after application (Asphalt Institute & AEMA 2006).

2.3 Factors Affecting Breaking and Curing Some of the factors affecting the breaking and curing process of bitumen emulsions include (Asphalt Institute & AEMA 2006):

Weather conditions breaking favours warmer temperatures. However hot weather can cause skin formation on sprayed seals, trapping water and delaying curing. Recently, some chemical formulations had been developed to break rapidly at cool temperatures.

Surface area higher aggregate surface area, particularly excessive fines or dirty aggregates, speeds up the breaking of emulsion.

Emulsion and aggregate temperature high temperatures favour breaking, especially in micro-surfacing.

Type and amount of emulsifier determines the breaking characteristics of a seal and the mixing grade emulsions.

Water absorption a rough textured, porous aggregate absorbs water faster, hence speeding up the setting time.

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Aggregate moisture content high moisture content will slow the curing process as longer time is required for evaporation.

Mechanical forces slow moving traffic helps to force the water out of the emulsion to attain mix cohesion, cure and stability.

Surface chemistry the intensity of the aggregate surface charge and the emulsifier charge plays an important role in the setting rate.

Bitumen Emulsions

3 CURRENT DEVELOPMENTS

3.1 High Bitumen Content As discussed in Section 1.3, high residue emulsions are capable of addressing problems associated with run off and slow breaking. High residue emulsions contain between 78 to 80% of binder content depending on the bitumen source (Remtulla & Swanston 2000). Figure 3.1 shows the relationship between the residue content and the viscosity of the emulsion. The viscosity changes exponentially as the residue content increases.

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

68 69 70 71 72 73 74 75 76 77 78 79 80 81 82

Residue (%)

Visc

osity

(mPa

.s),

250 C

Figure 3.1: Relationship between viscosity and residue content in the emulsion

With a minimum amount of water in the system, the closely packed emulsion is still mobile enough to flow as a cluster whilst maintaining the non-Newtonian behaviour. Any small displacement of water will result in an immediate coalescence and partial inversion of the emulsion, hence driving the breaking mechanism of the emulsion. The time required for a high residue emulsion to set was also proven to be one-third of that needed for a conventional low residue emulsion (Remtulla & Swanston 2000).

Advantages of a high residue emulsion include:

high viscosities which allow for high application rates. Even though at high viscosities, this material can still be sprayed easily without streaking problems

rapid break because of the low water content. Therefore, traffic can be opened within an hour of application

greater independence from weather conditions. However it is not applicable when it is raining or rain is forecast soon after the emulsion application

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the ability to adhere to wet aggregates at low pavement temperatures allowing the use of high polymer (e.g. styrene- butadiene- styrene, SBS) content by up to 8%

in sealing applications without the risk of aggregate stripping and polymer degradation

being environmentally friendly as it does not contain any cutter, compared to most conventional low residue emulsions which have a very small amount

being easy to handle and requiring less heating.

3.2 Emulsifier The emulsifier is the critical element of a bitumen emulsion. It stabilises the emulsion and keeps the bitumen droplets in a stable suspension. Current innovation focuses on the chemistry of the emulsifier, especially formulations for slow setting (SS) emulsions (James 2006).

Hydrochloric acid is often used to ionise the cationic emulsifier to enhance performance. However, hydrochloric acid has gradually been replaced by phosphoric acid especially in emulsions for microsurfacing and cold mix in Europe, Asia and in the United States. Phosphoric acid is claimed to be more user friendly as it allows a wider range of bitumens to be used in the emulsion (James 2006).

3.3 Particle Size Distribution The stability, application rate, breaking rate and curing rate of an emulsion is highly dependent on its particle size distribution. The particle size of a bitumenemulsifier system is determined by the mill shearing effect and residence time.

Various methods have been devised to formulate and adjust the bitumen chemistry in order to improve the formation and distribution of the bitumen particles by the mill and stabilise the resulting emulsion. These methods include improving the dispersed phase, doping the binder with surfactants, tailoring bitumen compositions and optimising the manufacturing conditions (Baumgardner 2006, Holleran et al. 2005).

3.4 Applications New emulsions based technologies for road construction and maintenance have been developed over the last 20 years. These include (James 2006):

ultrathin hot mix friction courses with modified emulsion bond coat spray-applied crack seal with emulsion seal surfacing scrub seal with modified emulsified binder (scrub seals are a process where the membrane

of the modified binder is pressed or scrubbed into a cracked and aged surface)

glass fibre-reinforced chip seal trackless tack coats warm mix asphalt.

Bitumen Emulsions

4 POLYMER MODIFIED BITUMEN EMULSIONS

4.1 Introduction Emulsified polymer modified bitumen (PMEs) combine the high performance properties of the polymer with the unique properties of an emulsion such as the decreased viscosities and low spraying temperatures. This material is environmentally friendly and energy efficient.

Latex is a common additive used for modified emulsions in Australia. It can be added either by post addition to a CRS emulsion or by direct addition during the milling process of the emulsion. Often, latex can also be added to the bitumen through specialised equipment to gradually vent the water. Latex modified bitumen emulsions are generally more stable than a conventional CRS emulsion. They contain a residual content of 6065% and demonstrate low viscosity behaviour (Remtulla & Swanston 2000).

Increasing the binder content of emulsions would inevitably increase the viscosity. However, this poses a problem to the sprayer the sprayer faces difficulty in achieving an even distribution of the emulsion. Nonetheless, a high binder content PME (containing up to 80% binder) has been developed. This material has proven to be a success and demonstrates good spraying performance. According to Read and Whiteoak (2003), the spraying performance of a high residue PME is as good as a standard bitumen emulsion.

High binder content PME not only reduces the amount of water used by up to 30% but also the polymer additives. The polymer content could be as low as 1%. The dramatic reduction in water content is highly beneficial to the evaporative breaking system of the emulsion after application. In addition, this is also a very cost-effective method in terms of the storage and transport of water (Read & Whiteoak 2003).

4.2 Manufacture of Polymer Modified Bitumen Emulsions PME is manufactured through a two stage process. The first step is the manufacture of PMB followed by the emulsification of PMB. The emulsion type (cationic or anionic) and pH issues are both very important aspects of the manufacturing process (Holleran 1998).

Various types of PMBs can be emulsified. The polymers used include styrene-butadiene-styrene (SBS), ethyl vinyl acetate (EVA), polybutadiene (PBD), polyethylene (PE) composites, atactic polypropylene (ethylene), epoxies, urethanes and tyre rubber (Holleran et al. 2001).

Generally the polymers can be blended with the bitumen via three systems:

1. post addition of latex

+ Latex emulsion Emulsion mixture Emulsion

Figure 4.1: Post addition of latex

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Bitumen Emulsions

2. co-milling of latex by direct injection into the binder emulsion dispersing phase (soap phase)

Soap (water phase)

Latex

Bitumen PME

Figure 4.2: Co-milling of latex

3. direct latex addition to bitumen

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Steam +

Figure 4.3: Latex addition to bitumen

The post addition method gives a weak suspension of latex in the emulsion mixture (Figure 4.1). If the emulsion mixtures were not thoroughly mixed, the inconsistency of the mixture would be seen when it is applied on the road. Therefore it is essential to ensure thorough mixing is achieved to attain uniform distribution of the polymer (Remtulla & Swanston 2000). The polymer is more homogenously distributed in the co-milling process (Figure 4.2) and has a higher viscosity (Holleran 1998). Nonetheless, the direct addition of polymer into bitumen is claimed to be the most effective system to ensure complete solubility of polymer prior to emulsification (Figure 4.3).

The addition of ground tyre rubber is not a simple process. Ground solid crumb rubber may be added as a dry ingredient into slurry mixes through the post addition approach. The rubber will then become a part of the aggregate phase and mainly acts as a filler. This method is typical in the United States (Holleran et al. 2001).

In the emulsion design, it is important to acknowledge that latex contains its own emulsifying system. If this is not considered and compensated for, there will be an increase in mixing times and slow setting.

4.3 Applications 4.3.1 Introduction The applications of PMEs for road maintenance have increased considerably over the years. Since the mid-eighties, polymers have been successfully incorporated into the cationic emulsion system of bituminous slurry surfacing to produce a quick set which allows opening to traffic relatively quickly. It was claimed to provide effective rutting and bleeding resistance, minor improvement to shape correction, improved aggregate retention and moderate resistance to low temperature cracking (AAPA 2004).

Bitumen Latex emulsion PMB (latex) +

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The typical types of polymer used for slurry surfacing include SBR, EVA, natural rubber and SBS (AAPA 2004). Although each of the polymers appears to improve the mix performance in critical applications, there is still insufficient information to grade the polymer types based on performance.

4.3.2 Sprayed Seal Binder in High Stress Areas (New Zealand) Treatment with conventional polymer modified seals were perceived as suitable to overcome the aggregate retention problems faced in New Zealand (Patrick 1994). However, this benefit is often offset by its poor wettability characteristics.

Emulsified PMBs were later claimed to be a better choice of material. Their unique characteristics are capable of overcoming the wettability problem even in damp and cool conditions and require a lower temperature (< 100 C) for storage and spraying. Further elaboration on the comparison between PMBs and PMEs is discussed in Section 4.4.2.

In 1993, road trials were conducted by Transit New Zealand and the private sector to test the emulsified PMBs ability in handling high traffic stresses. The PMEs used contained a high binder content of 77%, hence reducing the amount of water required. The construction system used the racked in method with precoated aggregates. The weather condition during construction was in the range of 1427 C, with pavement temperatures from 16-43 C (Patrick 1994). Controlled traffic was allowed within 15 minutes of spraying. The thixotropic characteristic of the emulsion prevented any run-off and the racked in system (with one application of binder followed by two aggregate sizes) was faster to construct than traditional double/double seals.

The performances from the road trials were recorded four months after construction and before the onset of winter in New Zealand (Table 4.1).

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Table 4.1: Trial site details and performance

Location Site Traffic (v/d) Aggregate sizes

Site details Performance

New Plymouth

Waiwakaiho Hill

Cemetery Hill

16,700

6,400

3/5

4/6

State highway open road State highway open road

Excellent condition. Slight flushing over previously flushed areas Slight flushing in wheeltracks of uphill lane

Auckland Coronation Road

Hastle Avenue

Roundabout

4,100

600

4,500

3/5

3/5

3/5

Residential with one bend subject to buses Industrial and residential traffic with industrial crossing 33 m diameter servicing commercial and light industrial crew

Excellent condition Slight flushing at industrial crossing but no aggregate loss Some chip loss and rollover on inside wheeltrack, less than 5% of area

Wellington Newlands Roundabout

Crawford Road

7,500

8,000

3/5

3/5

7m diameter on route to landfill area Winding grade on trolley bus route at 10% gradient

Slight flushing on hand spray areas. Scabbing on inside wheeltrack close to centre island. Majority of area in good condition Aggregate loss inside the bends with higher traffic speeds. Major intersections in excellent condition. Control double/double seal in good condition

Christchurch Porter Pass 940 3/5 Area subjected to snow grading in winter at 9.8% gradient

Excellent condition, double/double seal in excellent condition

Source: Patrick (1994) The performances shown above have demonstrated PMEs ability to withstand high traffic stresses on variable surface texture. Patrick (1994) claimed that the emulsified PMBs are a more efficient treatment than the conventional New Zealand single/single or double/double seals. PMEs also reduce the risk of polymer degradation because of the low storage and application temperatures, and have good adhesion with the aggregates.

4.4 Advantages 4.4.1 Improved Properties A series of tests was conducted by Holleran et al. (2001) on some American emulsion slurries under the International Slurry Surfacing Association (ISSA) guidelines. The results showed an improvement in the properties when using a PME as compared to a standard PMB.

The improved properties were:

higher resistance to stone loss which could potentially reduce the binder content an increase in cohesion strength which improved the deformation resistance and surface

abrasion resistance (Forbes et al. 2001)

improved crack resistance.

Unlike conventional bitumen emulsions, PMEs are suitable for cool climatic conditions. The polymer additives improve aggregate adhesion and provide early green strength during curing (Austroads 2002). Green strength is the cohesive strength developed between the application and complete cure of the binder (Tredrea 1998).

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4.4.2 Comparison with Non-emulsified PMBs PMEs have been seen as an alternative to conventional PMBs. Polymers are generally heat sensitive. Any heating to high temperatures required during spraying, pumping and/ or other practices can potentially damage the quality of the polymer. The addition of polymers into an emulsion does not only offer the advantages of a PMB but can also extend the sealing season of a PME (OHara 1994).

Remtulla and Swanston (2000) report that high binder content PMEs may be the solution to the polymer degradation problem faced by conventional PMBs transported over long distances. The quality of PMB delivered through this method could then be guaranteed.

Conventional PMBs are often faced with difficulties attaining an even film thickness offered to the aggregates because of its high viscosity. PMEs, on the other hand, have a lower viscosity and will consistently achieve a more even film.

4.4.3 Environmental Aspects PMEs are safe to use and environmentally friendly. They contain little to no cutter and can be handled easily. There is potential for the use of recycled tyres and rubber materials in the manufacturing process of PMEs. This is desirable in terms of cost and the environment.

4.5 Disadvantages The disadvantage of using PME was the change in morphology/ structure of the composites after the addition of the polymer (Holleran et al. 2001).

In road trials conducted in Australia and New Zealand using a PME, problems related to the materials breaking rate were recorded. The cause of the problem was later attributed to the extra stability built into the emulsion to allow for its transportation from Australia. Gillespie (1994) concluded that this problem could have been eliminated if it was manufactured in New Zealand. The breaking rate of this material was also found to be affected when it is used during cold damp conditions (e.g. pavement temperatures in the range of 5-12 C). In an email correspondence with Steve Halligan from Main Roads Western Australia on 14 June 2007, he reported that PMEs are faced with very rapid skinning when the pavement temperature is in the 30 C range. A thick skin is formed on the surface with the unbroken emulsion underneath. When aggregate is applied, the skin tears apart and the fluid binder squirts to the top of the aggregate resulting in pickup of the aggregate.

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5 ENVIRONMENTAL COMPARISON BETWEEN HOT CUTBACK AND BITUMEN EMULSIONS

5.1 Introduction Bitumen is mixed with (0-8%) petroleum solvents such as kerosene to produce a cutback bitumen. The solvent is used to decrease the viscosity of the bitumen, hence making it more workable. Conversely, bitumen emulsions can be easily applied onto the road surface with or without the use of any solvent. According to BP bitumen, standard grades of emulsion may contain up to 2% cutter (Leach 1998).

The increasing pressure on the environment, energy conservation and health related issues have led to an increased interest in the use of bitumen emulsions. For example, approximately 90% of the sealed roads in the United Kingdom (UK) were constructed using emulsion (Slaughter 2004). Government legislation such as the UK Environmental Protection Act of 1990 was enforced to tighten emission controls from bitumen manufacturing plants (Read & Whiteoak 2003). However, in Australia, the use of hot cutback bitumen is still the predominant option for most sealing works.

5.2 Occupational Health and Safety Bitumen emulsions are generally water based and therefore not flammable or explosive. Further heating is not required during application (although they are manufactured at high temperatures). Since emulsions have a water based nature, they pose minimal health risk to workers.

Unlike bitumen emulsion, cutback bitumen needs to be kept at high temperatures (160180 C) to enable it to be sprayed onto the road surfaces. At such elevated temperatures, the flash point of the solvent at 61.5 C is exceeded i.e. the temperature at which the vapours will burn in the presence of air and an ignition source (AAPA 2003).

Staffs working in such high temperature environments involving the solvents are exposed to a variety of safety hazards such as fires, burns and explosions. Burns can occur at any stage of the working process i.e. blending, transferring of cutback, spraying, etc.

The high volatility of the solvent can cause nuisance fumes and odour during the manufacture, transportation and the use of the material. Some workers handling it may even feel nauseous.

5.3 Energy Consumption In Australia, up to 7 or 8% of kerosene or gas-oil is added to the binder. According to the Asphalt Institute, almost 40,000 kJ of energy is required to process one litre of cutter. In comparison only 1151 kJ of energy is required to process one litre of bitumen emulsion (Holleran & Reed 1996).

Spraying a litre of emulsion at ambient temperature could save up to 99% of energy. Before cutback was phased out in the USA, the annual energy loss was estimated to be sufficient to fuel 588,000 cars (Holleran & Reed 1996).

5.4 Greenhouse Effect Greenhouse gases such as carbon dioxide (CO2) and volatile organic compounds (VOC) are major products from the combustion of fossil fuel. The production, heating and exhaust emitted from the transportation of both cutback bitumen and emulsions will collectively generate CO2 into the atmosphere.

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Various studies have been done on the emission of CO2 from hot cutback bitumen and bitumen emulsions. Data from one Australian study is summarised in Table 5.1. This report examined the emission of CO2 from the manufacturing, transporting and application process of the binders. The information in the report was summarised in terms of CO2 equivalents.

The report found that the emission of CO2 from both binders did not differ significantly. The main difference is from the extra energy required to transport water for the emulsion (Leach 1998).

The secondary effect shown in Table 5.1 is related to the higher cost incurred for the use of bitumen emulsions without any increase in road funding. Consequently, fewer kilometres of road would be sealed or maintained and the overall condition of the network would decline (Leach 1998).

Table 5.1: 1992 emission estimates (CO2 equivalent)

Process Actual emissions using hot and cutback bitumen

Hypothetical emissions using only emulsions

Manufacture: Bitumen Cutter & flux

30.22 2.22

32.83 0.02

Total: 32.44 32.85 Transport: Product Solvent/ water

10.33 0.18

10.33 4.28

Total: 10.51 14.61 Application: Evaporation Heating

-

4.00

- -

Secondary effects (fewer sealed roads) - 4.2 Grand total 46.95 51.66

Source: Leach (1998)

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Table 5.2: Carbon dioxide generated per tonne of sealing bitumen sprayed

Cutback (kg/tonne) Emulsion (kg/tonne)

Transportation 2 7

Production/ heating 60 16

Total 62 23

Source: Slaughter (2004)

In New Zealand, a similar study was done by Slaughter (2004) (Table 5.2). Since the measurement units used in both studies were different, a quantitative comparison cannot be performed. Nonetheless, it can be deduced that the NZ report favoured the use of emulsion over hot cutback bitumen. Table 5.2 showed that the greenhouse gas emission from the production/ heating of cutback bitumen greatly outweighed the emulsion. Slaughter (2004) explained that this can be attributed to the heating needs for NZ sealing conditions and the higher use of hydroelectric power in their bitumen industry.

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6 FACTORS AFFECTING QUALITY AND PERFORMANCE OF BITUMEN EMULSIONS

There are many factors which can affect the performance and quality of a bitumen emulsion including (Asphalt Institute 2006):

Bitumen

chemical properties hardness/ viscosity quantity bitumen particle size in the emulsion.

Emulsifier

concentration and chemical properties.

Manufacturing process

manufacturing conditions such as temperature, pressure, shearing rate, etc. order of addition type of equipment used addition of chemical modifiers and polymers properties and type water quality hardness.

These factors cannot be graded or singled out as being the most significant. However, they can be varied to tailor - make an emulsion binder to meet the available aggregates or construction conditions.

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7 OVERSEAS EXPERIENCE

7.1 New Zealand 7.1.1 Introduction Traditionally in New Zealand, hot cutback bitumens were predominantly used for chipseal surfacings (known as sprayed sealing in Australia), while bitumen emulsions were used only for special small-aggregate seals (Gundersen 2006). More recently, the use of bitumen emulsions has expanded to include every application used by hot binders, whether straight, cutback or fluxed bitumen binders (Austroads 2003). The proportion of road network in NZ covered by bitumen emulsion is between 10-15%, whereas 85-90% is covered by hot or hot cutback binders (Gundersen 2006).

The road construction and maintenance industry has claimed that bitumen emulsions are a high performing material which has demonstrated improvements in terms of safe handling and the environment. The performance related improvements include (Austroads 2003):

greater control over binder application rates thinner bitumen films required reduced bitumen oxidation which is related to better aggregate retention, high cracking

resistance in seals and ravelling mixes.

7.1.2 Applications Sealing

There is an increasing use of bitumen emulsions in NZ. In 1992, 6 - 8% of the total volume of bitumen in NZ was used in the form of emulsions (Simpson 1992). In 2006, this figure increased to 10 15% (Gundersen 2006).

Single coat and double coat seals are used extensively in NZ, regardless of whether cutback has been added or not. The use of double coat seals has been considered to be cost effective - when it is compared to the decreased life of a single coat and with the use of hot bitumen binders. The benefits of using emulsion seals include (Simpson 1992):

less dependent on weather conditions which allows the sealing season to be extended less sensitive (greater control) towards application rates reduced flushing safer handling and more conducive to the environment no excessive delays associated with binder heating.

Even though the practice of using bitumen emulsions is on the rise in NZ, contractors were still faced with a number of limitations (Simpson 1992):

sealing work could not be conducted during unreasonably poor weather i.e. imminent rainfall, extreme cold weather, etc.

difficulty in establishing optimum binder application rates using the conventional sand circle method for measuring surface texture. Ultimately, this might cause ravelling of sealing coats

rainfall immediately after sealing might cause run-off polluting the adjacent environment

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bleeding was seen occurring over smooth patches where uniform double/double emulsion seals had been placed over non-uniform surfaces.

Maintenance

Maintenance applications such as resealing, texturising and void filling with emulsions are on the rise in NZ. Table 7.1 shows average road length where emulsion used over a period of 10 years (1991-2001) and data for 2000/2001. However, the use of this application is still very much influenced by the contractors as they revert to using hot cutback bitumen to facilitate the efficient use of their equipment.

Similarly, bitumen emulsions used for such purposes demonstrate flexibility in application rates, are energy efficient and safer to handle. Nonetheless, it is still highly weather dependent and inappropriate handling of the material can potentially lead to frothing and plugging of spray tips (Simpson 1992).

Table 7.1: Emulsion use on NZ State Highways

Average yearly length (km) (1991 2001 data)

Length (km) (2000/2001 data)

Asphaltic concrete 0.07 0.18 Bicouche/ Sandwich 2.04 13.51

Locking coat seal 0.61 0.00 Open graded emulsion mix 0.57 0.00

Open graded porous asphalt 1.88 1.47 Other material type 0.46 0.00

Prime and seal 0.23 0.00 Single coat seal (1st coat) 3.96 0.32 Single coat seal (2nd coat) 15.92 8.27 Single coat seal (reseal) 61.35 39.23

Slurry seal 10.78 0.96 Texturising seal 36.80 8.01

Two coat seal (1st seal) 7.95 6.26 Two coat seal (2nd seal) 0.91 3.77 Two coat seal (reseal) 22.64 52.21

Void fill seal 33.44 46.56 Total road length : 199.61 180.75

Total State Highway network in NZ: 10,600 km (Austroads 2000) Source: (Austroads 2003)

Tack coat

This is an effective way to apply low rates of bitumen prior to patching or overlay (Austroads 2003). No problems were cited with the use of emulsion tack coats.

Slurry mixes

The advantages of slurry mixes are:

good control of the finished texture can be used over flushed areas

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provide good dense seals like those of conventional seals reduce loose aggregate associated with conventional seals effective levelling and skid resistance in one layer.

Emulsion mixes

Emulsion mixes which include open graded emulsion mixes and friction courses are appropriate corrective work for the following reasons (Simpson 1992):

reduce tendency for bleeding good overlays for flushed areas safe handling, transport and practical storage open graded mixes have a slightly elastic nature economic good storage stability versatile.

However, difficulties associated with this application are:

establishing an optimum binder application rate on an open textured surface ravelling poor durability.

7.1.3 Other Technical Issues Emulsions used for the first sealing coat were reported to have poor adhesion to the base course substrate. This is due to the emulsions inability to penetrate and wet the fine dry dust layer formed after the final preparation of a base course surface. This can be rectified by using a coarser base course and altering the technique in order to prepare a clean stone mosaic final surface.

Streaking due to unevenness in the transverse spray distribution may happen when low shear viscosity emulsion is increased to very high levels to prevent the run-off scenario. The solution for this is to use two-application seals. A low binder application rate is used for the first application to avoid run-off and a heavier rate for the second application after the aggregate has been spread.

7.2 Europe 7.2.1 Introduction Bitumen emulsion was first discovered and used in Europe. Europe remains as the major bitumen emulsion user (Table 1.1). France utilises almost 50% of the total European production (Le Coroller nd).

7.2.2 French Road Network The main reason for the extensive use of bitumen emulsion in France is due to the structure of the French road network. Three-quarters of the network is made up of secondary and rural roads, and therefore costs, reliability, flexibility and maintenance become the focal point for the choice of construction work.

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Building on the experience gained by the French contractors and road authorities on these secondary roads, the use of bitumen emulsions gradually extended to higher volume traffic roads. As a result, more sophisticated technology, better defined specifications and mix designs were developed through collaboration of the private sector and government authorities. An agreement was signed in 1996 between the Department of Roads and the Association of Road Industries (USIRF) to develop a method for characterising the behaviour of cold mix asphalt for base courses (Le Coroller nd).

One of the direct consequences of this joint venture was the development of Grave Emulsion. Grave emulsion was originally a continuously graded 20 mm material. However, modern technology has now enabled the material to be reduced in nominal size to 14 mm with a bitumen content of 4 - 4.5%. The advantages of this material are it can be stockpiled, compacted at ambient temperature and can be easily laid using conventional paving equipment (Read & Whiteoak 2003).

7.2.3 Applications Bitumen emulsions are used in various applications in Europe:

Sprayed sealing

Modern surfacings such as the racked in or sandwich sprayed sealing played an important part in the successful use of emulsion in this application. Le Coroller (1999) observed that the use of this treatment is comparatively higher in countries with high emulsion production.

The use of emulsion for sprayed sealing in France has increased by 50% between 1967-1997. One of the reasons for this is the lack of funding for road maintenance which promoted the use of sprayed sealing instead of asphalt overlay. Another reason is due to the lack of interest for the traditional surface dressing on the secondary road network and is now treated with three coarse sprayed seals (Le Coroller nd).

Tack coat

Tack coat is becoming more popular amongst European countries as the wearing courses become thinner and thinner. Countries such as Italy use 85% of their emulsions for tack coats (Le Coroller nd).

Slurry surfacing

This technique holds a less significant position in the European market as it is gradually taken over by micro-surfacing.

Bitumen Emulsions

7.3 South Africa 7.3.1 Introduction Bitumen emulsions are commonly used in South Africa. Almost 27% of the total volume of bitumen in South Africa is used in the manufacture of bitumen emulsion (Emery & OConnell 1997) (Figure 7.1).

Emulsions (modified)

5%

Asphalts52% Bitumens

21%

Emulsions (unmodified)

22%

Figure 7.1: Total consumption of bitumen in South Africa

South Africa experiences large climatic variations. It can be extremely hot during summer (over 40 C) and cold during winter (down to 0 C). The warm to hot temperatures are believed to be one of the main driving forces towards the development of fogsprays and rejuvenators as maintenance treatments. Cationic emulsion is the most common emulsion type, taking up 65% of the market share. Nonetheless, the use of anionic emulsion is still very strong (Emery & OConnell 1997)

7.3.2 Emulsion Treated Bases Emulsion treated bases (ETBs) are widely used. They are made up of reclaimed aggregate treated with less than 2.5% of bitumen emulsion to produce a base course quality type material. For the past 30 years, ETBs, as part of the pavement structure in South African roads, have a good performance record (Verhaeghe et al. 1997).

In 1981, an experimental trial was conducted using the milling and recycling procedure on a cracked cement treated base. The trial was divided into two sections. Section 1 was treated with a low percentage emulsion (1.4%), while Section 2 was re-compacted without the addition of any emulsion. Heavy Vehicle Simulator (HVS) tests were performed and the conclusions were (Horak et al. 1984):

Section 1 had better compaction, less permanent deformation and greater resistance to shear forces.

Section 1 had a lower moisture sensitivity as a result of lower permeabilities and the binding of fines.

Section 1 had a higher resistance to reflective cracking. Section 1 could withstand heavy traffic for extended periods before being sealed.

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Section 1 required less construction time and eliminated the need for priming. Therefore, it was a more cost-effective rehabilitation option.

In situ material could be used to its full benefit in Section 1. It eliminated the need to remove existing materials and the replacement with new materials, thus reducing material costs, handling costs, haulage costs and time.

Following the 1981 road trials, the Southern African Bitumen Association (Sabita) continued to invest in the research and development of bitumen emulsions in the road construction industry. For example:

1988-1992 Sabita sponsored a research program to develop nationally acceptable mixing, testing and evaluation methodologies for Granular Emulsion Mixes (GEMs)

1993 Sabita launched a design manual equipped with recommended design procedures for stabilised and modified GEMs

1996 Sabita launched a project to broaden the range of ETB applications as a base course material in both rural and urban areas by expanding the design scope of the current technology.

7.3.3 Other Applications Some of the applications of bitumen emulsions in South Africa include (Emery & OConnell 1997):

fogspray as final spray in new seal construction fogspray as a maintenance application stone and sand double/double seal with SBR modified CRS slurry/ microsurfacing broom slurry.

Fogspray is a diluted emulsion with 30% binder content often used for new surface sealing construction. It is generally used as a final spray for double/double seals. This method improves the adhesion between the second layer aggregate with the seal as well as providing a uniform surface.

Broom slurry is a 1-2 mm thick slurry applied on an existing asphalt surface, after which all excess slurry is vigorously broomed off. This practice is to ensure only the voids and interstices of the asphalt are filled with fine slurry. It uses an anionic stable grade 60% emulsion combined with very fine dust.

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8 CHALLENGES FACED IN AUSTRALIA

8.1 Introduction Emulsion sprayed seals have always been treated as an alternative to the use of hot cutback sealing treatment on low to medium trafficked roads in Australia, especially during the cooler and wetter periods of the year (May August). The emulsion sprayed seal treatment is not popular because of a number of factors (Parfitt 1999):

high costs incurred with the manufacturing and transporting of emulsions prolonged set-up period to achieve good aggregate retention under normal traffic conditions emulsions are dependent upon the traffic and weather conditions during the initial stages of

construction

lack of knowledge and understanding of the characteristics of this treatment.

In November 1991, the Austroads Bitumen Emulsion Project Group was formed. This group is made up of representatives from SRAs, ARRB and the industry. In 1992, the Group introduced the Code of Practice for Bitumen Emulsion which contained information on the proper use and handling of this material (Gaughan 1992). Later in 2002, it was superseded by the Guide to the Selection and Use of Bitumen Emulsions (Austroads 2002). One year later, the group produced a report on The State-of-the-art of Bitumen Emulsions in Australia and New Zealand (Austroads 2003).

A series of road trials were carried out in Australia and New Zealand between 1993 and 1997. The aims were to develop guidelines, introduce new technologies and identify different applications of bitumen emulsions based on laboratory and field results. These road trials are further elaborated in Section 8.2.

8.2 Road Trials The field trials focused on the following issues:

8.2.1 Precoating of Aggregate on Site with Low Binder Content Emulsions The concept of precoating aggregate with emulsion is not new. In the past, many problems have arisen from the instability of transporting and storing emulsion, and the uncertainty of the breaking and curing time. Trials using high binder content (60-65%) emulsion to precoat the aggregate have shown a large inconsistency in performance i.e. within a short period of time, stripping was evident; while some low trafficked roads went eight to ten years without exhibiting any distress.

Between October 1995 and July 1996, field and laboratory trials were conducted on aggregates precoated with cationic low binder content (30-35%) emulsions (Parfitt 1996).

There were two parts to these road trials:

Trial 1: Precoating wet and dry aggregate on site for immediate use Trial 2: Precoating wet and dry aggregate on site for future use.

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There were no visual differences in the seals observed between the emulsion precoated aggregates and the standard distillate based precoated material. Good surface mosaic was evident. The wet aggregates were found to be easily precoated compared to the dry aggregates. A more uniform film of binder coverage could be achieved with the emulsion based products. As a result, less aggregate was picked up by traffic. Another observation was that the emulsion precoated aggregates appeared to have better adhesion than the distillate precoated aggregates.

The advantages of precoating aggregate on site are:

It is environmentally friendly, with little or no hydrocarbon evaporation and reduces the leaching of distillate based agents into the aground at the stack site.

Damp aggregate can be precoated.

The disadvantages are:

In the event of rainfall, emulsion could be washed off from the stockpiled aggregate before the break time is achieved.

Emulsion precoated aggregate did not readily exit from the back of the truck. Higher costs are incurred due to the increase in quantity of emulsion precoat required. An unkempt aggregate stockpile site is left behind as a result of the excess use of lower

viscosity emulsion during the precoating process.

In trial 2, where the aggregates were precoated on site, restacked and left for future use, free emulsion had fallen off from the aggregate loader and was found to have hardened and required mechanical means to clean up the site. Loose lumps of excess binder were also evident at the site.

8.2.2 Precoating of Aggregate in the Quarry with Low Binder Content Emulsions In the trial reported in Section 8.2.1, a 30-35% binder content was used. In Parfitt (1997b), an even lower binder content (15%) of emulsion was used to precoat wet aggregates at the quarry.

In this trial, wet aggregates were precoated at the quarry, transported to the site and stacked for three months before being used. During the stockpiling period, there was extensive rainfall for two months. The aggregates were inspected and found to be dry on the inside. The precoating material did not leach out of the stacksite into the environment.

Numerous concerns were raised with regard to the time limit for stockpiling of precoated aggregates prior to use. The contamination by dust or deterioration of the binder might also affect the performance of the aggregate.

An inspection was conducted six months after the precoated aggregates had been stockpiled. The materials were observed to have some agglomeration but could be readily broken apart. Nonetheless, the aggregates still demonstrated sufficient adhesion to form near vertical faces for a short while. The stockpile of precoated aggregates was less dusty than the uncoated aggregates.

The advantages of precoating aggregate in the quarry are (Parfitt 1997b):

It is environmentally friendly, with little or no hydrocarbon evaporation. Wet aggregates can be precoated. Therefore, the aggregates can be precoated immediately

after washing in the quarry, saving time and costs.

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No contamination was seen at the stack site and there was no leaching of the emulsion precoat.

There was no contamination of roller and vehicle tyres from the emulsion precoated. aggregate sections. This eliminated problems related to early life low levels of skid resistance associated with some of the distillate based precoat.

However, the length of storage time for the precoated aggregate is still unknown.

8.2.3 Sprayed Seals Using High Binder Content Emulsions A series of road trials were conducted to investigate the use of high binder content emulsions at various application rates and with two different sizes of aggregate (size 10 and 14) (Parfitt 1997a).

Trial 1: Western Highway - Two different grades (from different suppliers) of high binder content (70 - 80%) emulsion and size 10 aggregate were used. The controlled section of the trial used a standard cutback bitumen.

Trial 2: McIvor Highway - One grade of high binder content (70%) emulsion with a larger size aggregate (size 14) were used for this trial. The controlled section used a standard cutback bitumen.

Trial 3: Moorooduc Highway A high binder content emulsion was used (the aggregate size was not disclosed) (Parfitt 1999).

After two to three hours, the emulsion seals appeared to be weak and may encounter an early aggregate loss. To avoid stripping, a size 7 aggregate was applied as a rack-in coat to pin down the emulsion and controlled seals (Parfitt 1997a). Long term inspection showed that there was no residual evidence of the rack-in coat on the emulsion section in Trial 2.

Once the emulsion had attained the curing time, the original viscosity of the binder was returned, unlike the cutback bitumen which required warm temperatures to evaporate the cutter. This may take longer than in winter. In the presence of little (or no) cutter, emulsions can also reduce the flushing of binders to the surface. From the point of view of occupational health and safety, fewer risks are involved in emulsion spraying as a lower spraying temperature is required and less solvent is used.

However, if the emulsion sealed road is subjected to high speed heavy traffic before the full breaking and curing time has elapsed, the seal will encounter an early loss of aggregate. Therefore it is very important to ensure that tight traffic control is implemented especially during the initial stages of sealing and/ or a rack-in coat is applied.

An increase of 0.2 L/m2 in the application rates was found to demonstrate better sealed surfaces. Although this improved the texture of wheel paths which appeared to be hungry, flushing may still be possible during extended hot temperatures.

Advantages of emulsion sprayed sealing include:

Emulsion is more environmentally friendly than cutback bitumen, with little or no hydrocarbon evaporation.

Once the emulsion attains its curing time, the original viscosity of the binder is returned. (unlike cutback bitumen which requires warm temperatures to evaporate the cutter).

In the absence of cutter, emulsion reduces the risk of binder flushing to the surface. Lower spraying temperatures are required.

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Greater texture depth is achieved.

Disadvantages of emulsion sprayed sealing include:

During the first two to three hours of sealing, emulsion seals appeared to be weak and may encounter an early loss of aggregate.

During the early stages, traffic control is required to monitor the traffic speed. The length of breaking/ curing time is too long. This may also lead to possible early loss of

aggregate.

There is a possibility of long term stripping outside wheel paths.

8.2.4 Weather Conditions During cooler periods, cutback is added into hot bitumen to reduce the viscosity and improve the adhesion between binder and aggregate. When there is a high proportion of cutback in the binder, the chances of aggregate stripping from the seals are greater; especially when the aggregate is damp or wet weather occurs during or soon after sealing. However, if bitumen emulsions are used under these conditions, chances of failure can be considerably reduced. Emulsions can coat damp aggregates and are less susceptible to being washed out by rain shortly after application (Walker 1985).

Although emulsions can be used at lower temperatures than hot cutback bitumen the risk increases as temperature decreases (e.g. < 15 C). It is therefore important to have traffic and speed control on newly laid emulsion surfacing.

In May 1988, trials were run on the Calder Alternate Highway and the Bass Highway to compare the performance between emulsion and cutback sprayed seals applied during the cooler and wetter seasons of the year in Australia (Austroads nd). Only precoated aggregates were used for the cutback section of the road trial. Ten years later, the sites were revisited and the inspection report is shown in Table 8.1.

Stripping was found between the wheel paths of the emulsion seals where non-precoated aggregate was used. Nonetheless, the overall comments also suggested that emulsion sprayed seal performed better than cutback bitumen sprayed seal on a long term basis.

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Table 8.1: Site details and inspection report

Trial site Description Comments Calder Alternate Highway

Emulsion CRS 70% binder (non-precoated aggregate)

Stripping was evident in the areas outside the wheelpaths. Surface texture in the wheelpaths was good Patches were evident beneath the seal and although the surface in places appears full, it had not flushed

Cutback bitumen (6 parts cutter) distillate based precoated aggregate

Minor bleeding within wheelpaths.

Bass Highway Emulsion CRS 70% binder (non-precoated aggregate)

Performing well, with a good texture and mosaic.

Cutback bitumen (12 parts cutter) distillate based precoated aggregate

The section has been resurfaced.

Source: Parfitt (1999)

8.2.5 Priming Grade Emulsions An emulsified primer is different from a conventional bitumen emulsion which does not penetrate the pavement surface. A priming grade emulsion needs to have low viscosity properties for surface penetration, coating of fine particles, sealing surface pores and bonding between pavement layers (Austroads 2002). NSW has carried out laboratory testing and road trials to develop and evaluate various priming grade bitumen emulsions. The primed surface on Jenolan Caves Road was later spray sealed and was found to perform well. However, its good performance could not be attributed specifically to either the prime or the seal.

According to Austroads (2002), a pavement should be dampened before it is primed to optimise primer penetration. If spraying is done during hot weather or where the base is highly absorbent, it is advised that the emulsion be diluted up to 50% and applied with a higher application rate.

8.2.6 Primersealing Grade Emulsions A two application road trial was conducted on the Hume Highway at Broadford using a high float emulsion with 70% binder content. High float emulsions are specially formulated to allow a thicker binder film coating without the danger of runoff. The high float emulsifying agents orientate themselves to form a network gel structure to keep the cured binder from flowing.

The sequence of this three day trial was (Austroads nd):

Day 1 high float emulsion (70% binder content) was applied at a rate of 1.3 L/m2 over a bed of unprecoated but damp size 10 aggregates. This was done when the top 100 mm pavement had reached a moisture content of 0.5% below optimum. Trafficking was not allowed except for the continuous rolling by the multi-wheel rollers.

Day 2 high float emulsion (same as Day 1) was applied at 0.7 L/m2 and covered with size 5 or 7 aggregate. No traffic was allowed.

Day 3 once the seal was observed to have set-up, slow traffic (maximum 60 km/h) was allowed. If the seal was observed to be damaged, traffic was advised to be delayed by another day.

The primerseal emulsion was seen to have set after 1 2 days and was not easily damaged by traffic. However, aggregates did not adhere very well except in areas where more surface moisture had been retained such as cement stabilised areas.

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8.3 Other problems faced 8.3.1 Storage and Handling The storage and transport of bitumen emulsion needs to be done in full containers. However, many emulsion consumers in Australia have a low rate of usage which does not justify bulk storage and handling facilities. Manufacturers have difficulty cleaning and disposing waste product left in the drums returned. Likewise, consumers find it tedious to dispose the drums as many Councils and waste disposal authorities refuse to accept large drums because of the bulk.

8.3.2 Stability of Emulsions in Cold Conditions Even though precautions were taken by covering the tanks during cold frosty conditions and tanks are periodically agitated, abnormal settlement of emulsions in tanks still happen occasionally during cold weather. Manufacturers may need to adjust the formulation or use additives to improve the situation.

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9 CONCLUSIONS This report has reviewed the development of overseas and Australian experience with emulsion technologies. Although the use of bitumen emulsions is still rather limited the demand for this material is increasing in most European countries, New Zealand, South Africa, etc. Some of the contributing factors for the increasing use overseas include:

1. environmental awareness from the emission of greenhouse gases from hot cutback bitumen

2. reduced risk of staff exposure to safety hazards such as burns and explosions while handling the solvents used in cutback bitumen under high temperatures

3. ability of emulsified PMBs to reduce problems related to the transport and storage of conventional PMBs

4. good performance from the material

5. extended sealing season

6. lack of funds for asphalt overlays (France).

In 1993 1997, a series of road trials were conducted in Australia and New Zealand. The findings from these road trials were:

1. No visual differences were observed between seals constructed with emulsion precoated aggregates or with the standard distillate based precoated aggregates which had been prepared on site.

2. Damp or wet aggregates can be precoated easily with emulsions.

3. Emulsion precoated aggregates showed no contamination of roller and vehicle tyres. Thus reducing problems related to early life low levels of skid resistance.

4. Emulsion precoated aggregates prepared in the quarry which had been stockpiled for six months demonstrated sufficient cohesion and adhesion to form near vertical faces.

5. Tight traffic control during the initial stages of sealing is essential to avoid early loss of aggregate.

6. Size 7 aggregate was applied as rack-in coat to avoid stripping.

7. Length of breaking/ curing is too long. This could lead to early loss of aggregates.

8. An additional 0.2 L/m2 in the application rate was suggested because of the hungry appearance showed on the area outside the wheeltrack.

9. Emulsion sprayed seals performed better than cutback bitumen sprayed seals on a long term basis when applied during the cooler and wetter seasons of the year.

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REFERENCES

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Gaughan, R 1992, Code of practice for bitumen emulsion, Surfacings workshop, Australian Road Research Board Ltd (ARRB) Conference, 16th, 1992, Perth, Australia, ARRB, Vermont South, Vic., 3pp.

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Horak, E, Myburgh, PA & Rose, DA 1984, Rehabilitation of a cement treated base pavement, Conference on Asphalt Pavements for Southern Africa, 4th, 1984, Cape Town, South Africa, National Institute for Transport and Road Research, Pretoria, South Africa, vol 1, pp. 316-326.

James, A 2006, Overview of asphalt emulsions, in DR Salomon, (ed) Asphalt Emulsion Technology: Transportation Research Circular E-C102, Transportation Research Board, Washington DC., pp. 1-15.

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INFORMATION RETRIEVAL

Austroads (2008), Bitumen Emulsions, Sydney, A4, 42pp, AP-T107/08

Keywords:

bitumen, emulsions, emulsifier, breaking, curing, emulsified polymer modified binders, sprayed seals, cutback bitumen, road trials, environmental

Abstract:

This report reviewed overseas and Australian experience in using bitumen emulsions. Increasing economic and environmental pressures have induced an increase in demand for the use of bitumen emulsions overseas. However, emulsion sprayed seals are only an alternative treatment to the use of hot cutback bitumen on low to medium trafficked roads in Australia.

Bitumen emulsions use less solvent, are claimed to consume less energy, are environmentally friendly, extend the sealing season and have no problems with storage and handling.

AP-T107/08 BITUMEN EMULSIONS1 INTRODUCTION1.1 History 1.2 Development