Paving with SBR Latex Modified Asphalt

Koichi Takamura

BASF Corporation, Charlotte Technical Center

11501 Steele Creek Road, Charlotte, NC 28273, USA e-mail: takamuk@basf.com

K. Takamura, “Applications for Asphalt Modification” in “Polymer Dispersions and Their Industrial Applications”, Edited by Dieter Urban and Koichi Takamura, Wiley-VCH, PP. 301-327 (2002).

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13 Applications for Asphalt Modification Koichi Takamura

13.1 Introduction

The annual worldwide consumption of asphalt was over 90,000,000 metric tons in 1995 and the U.S. used approximately one-third of that total [13-1]. The global asphalt consumption in 1996 is represented in Figure 13-1 [13-2]. Greater than 85% of 30,000,000 metric tons consumed in USA was used to maintain and improve more than 3,000,000 km (2,000,000 miles) of asphalt roads (Figure 13-2) with an annual road budget of $85 billion [13-2]. Federal Highway Trust Fund Authorizations reached nearly $21 billion in 1996. There are more than 6,000,000 km (4,000,000 miles) of roads, and asphalt roads account for 94% (3,270,000km) of the paved roads in the U.S. The other 6% (200,000km) are paved with Portland cement concrete, which is primarily used for the heavy traffic area of the interstate highway [13-1]. Asphalt production is not very uniform throughout the U.S. The Midwest has the highest production (40%), followed by the Gulf Coast (25%), the East Coast (17%), the West Coast (11%), and the Rocky Mountains. Over 90% (16% of total asphalt consumption as in Figure 13-2) asphalt used in the U.S. for non-paving purposes is sold to the roofing industry. Two thirds of that is consumed in the manufacture of shingles for houses, and the other third is used in commercial built-up roof.

Almost 90% of U.S. paving asphalt consumption is for hot mix. The remaining 10% of the paving asphalt usage (approximately 7% of the total asphalt consumption) is comprised of asphalt emulsions, primarily used for preventive maintenance and rehabilitation techniques such as chip seal, slurry seal and microsurfacing. Emulsified asphalts are also used for construction in recycling of old paving materials.

Global Asphalt Consumption

Europe

Asia/Australia

North America

Others

Others

Asphalt Cement for

Paving

Emulsions for Paving

Roofings

Figure 13.2. Use of asphalt in the U.S. Figure 13.1. The annual global asphalt consumption. North America and Europe consume two-thirds of total 90,000,000 tons.

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The use of polymer modified asphalts for hot mix and asphalt emulsions has grown significantly in the U.S. during the last 10 years. The National Center for Asphalt Technology (NCAT) has published a list of reasons for the use of asphalt modification [13-3,4]. Asphalt has been modified to: • stiffen binders and mixtures at high temperatures to minimize rutting and reduce the

detrimental effects of load induced moisture damage • soften binders at low temperatures to improve relaxation properties and strain tolerance,

thus minimizing non-load associated thermal cracking • improve fatigue resistance, particularly in environments where higher strains are imposed

on the asphalt concrete mixture • improve asphalt-aggregate bonding to reduce stripping, • reduce raveling by improving abrasion resistance, • minimize tender mixes, draindown, or segregation during construction, • rejuvenate aged asphalt binders, • replace asphalt cement as an extender, • permit thicker films of asphalt on open-graded aggregates for increased durability, • reduce flushing or bleeding, • improve resistance to aging or oxidation, • stiffen hot mix asphalt (HMA) layers to reduce required structural thickness, • improve pavement durability with an accompanying net reduction in life cycle costs, • replace Portland cement concrete with asphalt construction methods that reduce lane

closure times and user delay costs, and • improve overall performance as viewed by the highway user. According to the article by King et al [13-3], patents for modifying asphalt with natural and synthetic polymers were granted as early as 1843 [13-5]. The polymers are added to alleviate pavement problems and to realize economic, environmental, energy, application and/or performance benefits. Test projects were placed in Europe beginning in the 1930’s [13-6]. In North America, neoprene latex was introduced in the 1950’s and found a small but steady market, primarily in Canada and the Western United States [13-7]. Natural rubber latex, one of the materials mentioned in the earliest patents, is still being used today, primarily in water-

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based emulsion applications such as microsurfacing. Neoprene modified asphalts have been used for many years, but have more recently been replaced by other types of elastomeric polymers such as styrene-butadiene-styrene, SBS block co-polymers. Water based styrene-butadiene rubber (SBR) latex has found wide usage as an additive to asphalt emulsions to improve chip retention. The introduction of polymers into asphalt emulsions allows them now to be successfully used for almost any paving application. Recent emphasis on sustainable development and the concern about global warming has encouraged further development of cold paving technology using asphalt emulsions. Asphalt emulsions for paving already account for more than 40% of total asphalt consumption in France and nearly 30% in Spain [13-2]. Polymer modified asphalt accounts for less than 10% of total asphalt consumption for paving in the US, thus corresponding to approximately 3,000,000 metric tons a year in the U.S. The polymer content in these modified asphalt is, on average, 3% by weight, resulting in less than 100,000 metric tons of polymers being used for this application. Although the exact figure is not available, the synthetic and natural latices account for approximately 30% of the total polymer annually consumed in the United States. Approximately 1,000,000 metric tons of polymer modified asphalt were used in Europe in 1996 and 70% of these are modified with elastomeric polymers. 13.2 Hot Mix Asphalt Paving Hot mix paving and cold paving with asphalt emulsion are the two types of paving technologies used for producing asphalt-based pavements [13-3]. In hot mix paving, aggregates are heated to a temperature above 200°C to remove residual water and mixed with molten asphalt, which is at a temperature as high as 160-180°C [13-3,8]. The aggregate-asphalt mixture is then transported to the job site, spread and compacted. The mix has to be sufficiently hot to be compacted adequately within the specified density. In general, the job site has to be within a 1-hour transportation distance from the mix plant. Because of the need for the aggregate and the asphalt to be at high temperatures, there are considerable energy requirements and cost associated with the hot mix process. In fact heating aggregates accounts for nearly 90% of the total energy usage of hot mix paving. The typical cold paving method includes mixing aggregates with asphalt emulsion and thus it does not involve heating the components used to produce the asphalt-based formulation. Asphalt emulsions will be discussed in 13.3.

13.2.1 Asphalt Specification Asphalt used for paving has been graded with regards to its viscosity at 60°C (140°F) conforming to American Association of State Highways and Transportation Officials, AASHTO M226 specifications, or 25°C (77°F) penetration graded asphalt conforming to AASHTO M20. These specifications are based on properties at one specified temperature and do not necessarily predict asphalt performance over the wide range of climatic conditions that the pavement is subjected to in its lifetime.

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Changes in crude sources and refinery processes caused deterioration of pavements during the energy crisis in the late 1970’s to early 1980’s. The problem was recognized by the Federal government and led to the development of the Strategic Highway Research Program (SHRP) to examine the entire paving technology for both Portland cement and asphalt based pavements. For asphalt paving technology, these studies included the entire road construction procedure, aggregate specification and compaction method. The study also defined the laboratory testing procedures and specifications for both the asphalt-aggregate mix, (such as a compaction method for core sample preparation), rutting, fatigue and cold fracture testing of prepared samples. The SHRP study also developed Superpave Performance Graded (PG) asphalt binder specifications based on the pavement’s temperature range [13-3,4,8]. The specification is based on the rutting, fatigue and cold fracture resistance of the asphalt binder and defined by two numerical values representing the upper and lower temperature limits of particular asphalt in °C (e.g. PG64-22). The asphalt binder becomes too fluid during hot summer days under strong sun resulting in permanent deformation, which is called rutting. In contrast, the binder becomes brittle during cold winter nights. A perpendicular crack across the pavement lane develops when the stress generated by thermal contraction exceeds a critical value. The freshly applied pavement is most susceptible to rutting, but it becomes more susceptible to fatigue fracture when the asphalt binder is oxidized and loses its flexibility during usage. Therefore, rutting and fatigue resistances are based on the fresh asphalt and after the rotating thin film oven test, RTFOT. This test simulates asphalt heat aging during the hot mix process. Cold characterization is based on the asphalt binder after the pressurized aging vessel, PAV, test which is intended to reproduce 7-10 years of oxidative aging of the asphalt binder on the road. All Superpave specifications are based on the SI unit, emphasizing importance of international recognition. The World Road Association (PIARC) in France has been active in developing similar international specifications [13-9,10]. PG Grading The Superpave asphalt mix design system includes the performance grade (PG) asphalt binder specification. The Superpave asphalt binder tests try to determine physical properties that can be directly related to field performance in terms of rutting, fatigue cracking and low temperature cracking. Superpave characterizes asphalt at the actual pavement temperatures it will experience, and at the periods of time when distresses are most likely to occur. A part of the Superpave binder specification is shown in Table 13-1. Detailed specifications as well as test apparatus, procedures and the PG specification can be found in [13-8, 11].

Pumping and Handling: The maximum viscosity of the unaged binder should be below 3Pas at 135°C to ensure that the binder can be pumped and handled at the hot mix facility. A rotational viscometer (ASTMD4402) is used.

Permanent Deformation: The rutting resistance of the binder is represented by the stiffness of the binder at high temperatures that one would expect in use. This is represented by G*/sin(δ), where G* is the complex shear modulus and δ is the phase angle determined by the

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dynamic shear rheometry, DSR, measured at 10rad/s (1.59Hz). The complex modulus can be considered as the total resistance of the binder to deformation under repeated shear, and consists of elastic modulus, G’ and loss modulus, G” (recoverable and non-recoverable components). The relative amounts of recoverable and non-recoverable deformation are indicated by the phase angle δ. The asphalt binder will not recover or rebound from deformation if δ=90°. In practice, to minimize rutting, G*/sin(δ) must be a minimum of 1.00 kPa for the original binder and 2.20 kPa after aging the binder using the RTFO procedure. To address rutting resistance, the Superpave specification promotes the use of stiff, elastic binders. Fatigue Cracking: The Superpave specifies G*sin(δ)<5.00MPa, thus promotes the use of compliant, elastic binders to address fatigue cracking. Since fatigue generally occurs at low to moderate pavement temperatures after the pavement has been in service for a period of time, the specification addresses these properties using binder aged in both RTFO and PAV. Low Temperature Cracking: When the pavement temperature decreases, asphalt pavement shrinks. Since friction against the lower pavement layer inhibits movement, tensile stresses buildup in the pavement. When these stresses exceed the tensile strength of the asphalt mix, a low temperature crack occurs. The bending beam rheometer is used to apply a small creep load to a binder beam specimen and measure the creep stiffness. If the creep stiffness is too high, the asphalt will behave in a brittle manner, and cracking is more likely to occur. To prevent this cracking, creep stiffness has a maximum limit of 300MPa.

Table 13-1: Example of Superpave binder specification

Performance Grade PG64 PG70-10 -16 -22 -28 -34 -40 -10 -16 -22 -28 -34 -40

Average 7-day maximum pavement design temperature, °C <64 <70Minimum pavement design temperature, °C >-10 >-16 >-22 >-28 >-34 >-40 >-10 >-16 >-22 >-28 >-34 >-40

Original PavementFlash Point Temp., Minimum °C 230Viscosity

Maximum 3Pas,Test Temp., °C 135Dynamic shear

G*/sin(δ), Minimum 1.00kPa, Test temp. at 10 rad/s, °C

64 70

Rolling Thin Film Oven ResidueMass Loss, Maximum, % 1.00Dynamic shear

G*/sin(δ), Minimum 2.20kPa, Test temp. at 10 rad/s, °C

64 70

Pressure Aging Vessel ResiduePAV aging temperature, °C 100 100(110)*Dynamic shear

G*sin(δ), Minimum 5.00MPa, Test temp. at 10 rad/s, °C

31 28 25 22 19 16 34 31 28 25 22 19

Creep stiffness,S, Maximum, 300MPa, m-value Minimum, 0.300, Test temp., at 60 sec, °C

0 -6 -12 -18 -24 -30 0 -6 -12 -18 -24 -30

* 110°C PAV for the desert climate

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The rate that the binder stiffness changes with time at low temperatures is regulated through the m-value. A high m-value is desirable since this leads to smaller tensile stresses in the binder and less chance for low temperature cracking. A minimum m-value of 0.300 after 60 seconds of loading is required by the Superpave binder specification. Since low temperature cracking usually occurs after the pavement has been in service for some time, this part of the specification addresses these properties using binder aged in both the RTFO and PAV. As a part of the Superpave activities, chemical and physical properties of more than 70 asphalt samples commonly used in the United States were analyzed. These asphalt samples were available through the Material Research Library (MRL) for researchers in the field [13-12]. An example of the Superpave analysis of one of the MRL asphalt, AAA-1 (Lloydminster) modified with 3% BUTONAL NS175 (SBR latex from BASF Corporation) is shown in Table 13-2. The rotational viscosity measured by the Brookfield viscometer is 1.11Pas at 135°C, which is within the specification of <3Pas. G*/sin(δ) of the original (unaged) asphalt were 1.91 and 0.59kPa at 64 and 70 °C, respectively, which allow us to estimate G*/sin(δ)=1.0kPa at 67°C. The same measurement for the RTFO residue gave G*/sin(δ)=3.47 and 1.89kPa at the same temperatures, thus G*/sin(δ)=2.2kPa at 69°C. The upper limiting (rutting resistance) temperature of this asphalt is 67°C and thus this modified asphalt meets specification of PG64.

The Superpave analysis specifies the PAV temperature of 100°C for 20 hours for this sample (see table 13-1). The DSR analysis of the PAV residue gave G*sin(δ)=2.98 and 2.38 MPa at 7 and 10°C, respectively. These values are significantly lower than the requirement for the PG64 asphalt.

Viscosity at 135°C, Pas 1.11Rutting Resistance by DSROriginal Aspahlt

G*/sin(δ) at 64°C, Pa 1.91at 70°C, Pa 0.59

After RTFOG*/sin(δ) at 64°C, Pa 3.47

at 70°C, Pa 1.89Fatigue Resistance by DSRAfter RTFO and PAV

G*sin(δ) at 7°C, Mpa 2.98at 10°C, Mpa 2.38

Low Temp. Crack Res. by BBRAfter RTFO and PAV

Creep Stiffness S at -24°C, Mpa 186at -18°C, Mpa 108

Rate of S Change, m-value at -30°Cat -24°C 0.298at -18°C 0.364

Temp. at G*/sin(δ)=1.0kPa, °C 67Temp. at G*/sin(δ)=2.2kPa, °C 69Limiting high temperature, °C 67

Temp. at S=300MPa, °C -29Temp. at m=0.30, °C -24

Limiting low temperature, °C -34PG grading, °C 64-34

Table 13-2: Example of Superpave characterization of SBR modif ied asphalt.

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Polymer modification improves the rutting and fatigue resistance mostly through enhancement in the recoverable, elastic component, G’ of the asphalt as seen in Figure 13-3. The elastic and loss modulii, G’ and G” of the unmodified asphalt were 0.035 and 0.80kPa, respectively with phase angle δu=88° at 64°C, resulting G*/sin(δ)=0.80kPa. G’ increased nearly 11x to 0.37kPa upon modification with 3% SBR latex but non-recoverable, viscous component, G” showed only moderate increase of 2.3x to 1.80kPa, which gives the phase angle δm=78° at the same temperature. This resulted in G*/sin(δ)=1.91kPa as shown in Table 13-2. The phase angle δ alone makes only a limited contribution for determination of the rutting resistance since sin(δ)=0.999 and 0.981 at δu = 88° and δm=78°, respectively, for the unmodified and modified asphalt.

The bending beam rheometry (BBR) of the PAV residue was conducted to establish the low limiting temperature of the modified asphalt. The creep stiffness values were 186 and 108 MPa, and the m-values were 0.298 and 0.364 determined at -24 and -18 °C, respectively (see table 13-2). Thus, S would be 300MPa about at -29°C and m would be 0.300 at about -24 °C. Taking the higher value, -24°C is the low temperature limit of this asphalt determined at a condition of 60-second loading. The desired value of creep stiffness was originally developed as a correlation between thermal cracking of in-service asphalt pavement and binder stiffness values estimated at two hours loading time. However, using the concept of time-temperature superposition, it was confirmed that by raising the test temperature 10°C, equal creep stiffness could be obtained after a 60-second loading. Thus, -34°C would be the low temperature limit from the BBR

Figure 13-3. Complex modulus G* of unmodified and 3% SBR latex modified asphalt measured at 64°C. The polymer modification results in 11x increase in the elastic component, G’ with moderate increase in viscous component, G”.

0.01

0.1

1

10

0.01 0.1 1

G' (Elastic Behavior), kPa

G"

(Vis

cou

s B

ehav

ior)

, kP

a

δ u

Unmodified

SBR Modified

G *

G*

δ m

8

measurement. We conclude that this asphalt meets the PG64-34 specifications as shown in Table 13-1. Improvement in both the rutting and cold fracture resistance of the asphalt with the polymer modification are demonstrated in Table 13-3 where Superpave analysis of unmodified and modified with 3% BUTONAL® NS175 for AAA-1 and AAK-1 (Boscan) asphalt are compared. In addition to the Superpave analysis, values of conventional measures of unmodified and modified asphalt, such as ductility (ASTM D113), penetration (ASTM D5), softening point (ASTM D36) and absolute viscosity measured at 60 °C (ASTM D2171) are also included.

The results of modifying asphalt with additives are highly dependent upon the concentration, the molecular mass, the chemical composition, and microscopic morphology of the additive as well as the crude source, the refining process and the grade of the base asphalt used. Superpave binder specifications are successful in predicting the rutting resistance and cold fracture resistance of the unmodified asphalt. A new DSR procedure is under development for the better prediction of fatigue resistance. Integration of the bending beam rheometry data and direct tension measurement in the near future will provide a better description of the benefits of the polymer-modified asphalt.

One of the primary benefits of polymer modified asphalt binders is a reduced susceptibility to temperature variation [13-13]. Because many Performance Grade asphalt specifications can only be met with polymer modification, it is expected that the use of polymer modified binders will increase as these specifications are implemented during the late 1990’s and the early 2000’s. A 1997 survey of state highway agencies found that 35 agencies reported that they will be using greater quantities of modified binders; 12 agencies reported they will be using the same amount of modified binders; and no agency reported they will be using less modified binder [13-14].

Table 13-3: Superpave analysis of unmodified and SBR modified asphalts

Asphalt AAA-1 AAK-1Properties Unmodified Modified Unmodified Modified

Brookfield viscosity at 135°C, mPas 280 1100 560 2000Temp at G*/sin(δ)=1kPa, °C 58 67 63 79

Temp at G*/sin(δ)=2.2kPa after RTFO, °C 58 69 65 78Temp at S=300MPa, °C -21 -29 -14 -19

Temp at m=0.300, °C -24 -24 -17 -14Limiting high temperature, °C 58 67 63 78Limiting low temperature, °C -31 -34 -24 -24

PG Grading 58-28 64-34 64-22 76-22Ductility at 4 °C >150 >150 28 86

Penetration dmm at 25 °C 160 110 67 45Softening Point, °C 44 56 49 63

Absolute viscosity at 60°C, kPas 0.086 0.34 0.33 1.6

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Storage Stability A polymer-modified asphalt is a two phase system, forming a continuous fine polymer network, that is highly swollen with aromatic components in the asphalt. The polymer is mixed in the asphalt and stored at elevated temperature, which could cause chemical reaction within polymer chains and with some components in the asphalt. The degree of swelling, and thus the microscopic morphology of the polymer phase, varies widely dependent on the crude source, the refining process and the grade of the base asphalt [13-15,16,17]. When the chemical and physical properties of the polymer and asphalt are not matched to each other, a polymer rich phase could develop near the surface of the asphalt when stored at 160-170°C for a few days without agitation as reported by Brûlé et al with SBS modified asphalts [13-16]. The asphalt composition in the polymer rich phase is vastly different from the original asphalt.

One of their results with Asphalt E modified with 5% SBS polymer is shown in Figure 13-4. The aromatic and saturate components preferentially partition to the polymer phase, thus concentrating the asphaltenes and polar resin fractions in the asphalt phase. The majority of asphaltenes are retained in the asphalt phase, resulting in an increase in the asphaltenes/aromatic ratio. This potentially leads to reduced swelling of asphaltenes, which would have negative effects on low temperature flexibility of asphalt.

The phase separation during storage can be visualized with hot stage optical microscopy, which allows us to observe changes in the polymer morphology at the mixing and storage conditions. Here, the other MRL asphalt, AAB-1 (Wyoning Sour), was modified with 3% BUTONAL® NS175. Photomicrographs shown in Figure 13-5 illustrate the presence of a fine polymer network in the freshly mixed sample observed at 110°C at x200 magnification. The polymer phase transfers to macroscopic polymer globules without agitation when the

Original Asphalt E

Aromatic

Resin

Asphaltenes

Saturate

Asphalt phase

Polymer phase

Figure 13-4. Difference in asphalt composition among original asphalt and the polymer rich and asphalt phases developed during storage.

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sample is slowly heated to 170°C. These polymer blobs migrate to the top due to the density difference.

V. Wegan et al [13-17] reported observing similar macroscopic polymer globules and/or a polymer layer surrounding the aggregate surface in the paved asphalt mixtures, even though only fine structures existed in the modified binder observed at room temperature using fluorescence microscopy, which is the traditional method of studying polymer morphology [13-15,16,17].

The photomicrograph shown here (Figure 13-5) demonstrates that polymer modified asphalt behaves as a dispersion consisting of two immiscible fluids; a highly visco-elastic fluid dispersed in a less viscous one. The dispersed phase elongates to fine fluid columns under agitation. When the agitation is removed, these elongated columns transfer to a series of spherical droplets as minimizing the total surface area and thus the total energy.

Numerous inventions are reported in the literature to overcome the polymer incompatibility in the modified asphalt, which often involve introduction of a controlled cross-link reaction in the polymer phase. Cross-linking reduces solvent swelling and increases the visco-elasticity

Figure 13-6. BUTONAL® NX1129 maintains stable, fine polymer network even at 170°C.

Figure 13-5. Photomicrographs of conventional SBR modified asphalt taken at 110 and 170°C.

200µm

110°C 170°C

200µm 110°C 170°C

110°C 170°C

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of the polymer phase. BUTONAL NX1129 is an example of the new type of SBR latex. As shown in Figure 13-6, a fine polymer network remains even when the modified asphalt is observed at 170°C for 10 min. Stable polymer structures of this latex also extend the low temperature limits of certain modified asphalts, as determined by the direct tension measurement. 13.2.2 In-line Injection (pump-in) Preblending infers that the latex and asphalt have been mixed at a central location using a batch process as discussed above. In- line injection (also known as pump-in) implies that the latex and asphalt are blended immediately before being applied to the aggregate at the hot-mix plant. This process eliminates potential separation of polymer and asphalt during transportation and storage of incompatible materials, and the need for an asphalt storage tank for the polymer modified asphalt, thus reducing handling costs.

With the preblending process, polymer and asphalt are thoroughly mixed and the binders can be tested and certified before application to the aggregates. Recent advancement in quality control at the mixing process guarantees adequate mixing and performance of the asphalt produced by the in- line injection process. An optical photomicrograph demonstrating polymer networks in the asphalt prepared by the direct injection process is shown in Figure 13-7.

13.3 Paving with Asphalt Emulsion Asphalt emulsions used in road construction and maintenance are either anionic or cationic, based on the electrical charge of the asphalt particles, which is determined by the type of the emulsifying agent used. The asphalt contents of these emulsions are, in most cases, from 55 to 75% and prepared using a high shear mechanical device such as a colloid mill. The colloid mill has a high-speed rotor that revolves at 1,000-6,000rpm with mill-clearance settings in the range of 0.2 to 0.5mm. A typical asphalt emulsion has a mean particle size of 2-5µm in diameter with distribution from 0.3 to 20µm. A photomicrograph and typical size distribution

Figure 13-7. Photomicrograph demonstrating the presence of polymer networks in the asphalt prepared by the direct injection (pump-in) process.

200µm

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of an asphalt emulsion are shown in Figure 13-8. Asphalt emulsion properties depend greatly upon the emulsifier used for their preparation.

A latex modified asphalt emulsion can be prepared using several methods: addition of the latex in the aqueous emulsifier solution, direct injection in the asphalt line just ahead of the colloid mill or post-addition to the pre-manufactured emulsion, as schematically shown in Figure 13-9. Addition to the aqueous phase is the most commonly used method. The direct injection process often helps to produce an emulsion with a desired high viscosity for chip seal application (see 13.3.1). This is due to the narrow partic le size distribution of the asphalt emulsion produced with this process.

ColloidMill

AsphaltWaterEmulsifier

Acid or Base

Latex

Latex

Latex

Storage

Figure 13-9. Schematic illustration for latex modified asphalt emulsion production.

Figure 13-8. Particle size distribution and photomicrograph of a typical asphalt emulsion.

20µm

0

2

4

6

8

0.1 1 10 100

Particle Size, µm

Vo

lum

e %

13

Asphalt emulsions are classified with their charge and on the basis of how quickly the asphalt will coalesce, which is commonly referred to as breaking, or setting. The terms RS, MS and SS have been adopted to simplify and standardize this classification. They are relative terms only and mean rapid-setting, medium-setting and slow-setting. An RS emulsion has little or no ability to mix with an aggregate, an MS emulsion is expected to mix with coarse but not fine aggregate, and an SS emulsion is designed to mix with fine aggregate. The emulsions are further subdivided by a series of numbers and letters related to the viscosity of the emulsions and the hardness of the base asphalt cements. The letter “C” in front of the emulsion type denotes cationic. The absence of the “C” denotes anionic. For example, CRS-2 is a cationic rapid setting emulsion typically used for chip seal application. ASTM and the American Association of State Highway and Transportation Officials (AASHTO) have developed standard specifications for the grades of emulsions, shown in Table 13-4 for anionic and 13-5 for cationic emulsions. The “h” that follows certain grades means that harder base asphalt is used. The “HF” preceding some of the MS grades indicates high-float, as measured by the Float Test (ASTM D139 or AASHTO T 50). High float emulsions have a specific quality that permits a thicker asphalt film coating on the aggregate particles.

Table 13-4. Selected requirements for anionic asphalt emulsion (ASTM D977)

Table 13-5. Selected requirements for cationic asphalt emulsion (ASTM D2397)

Test Rapid-Setting Medium-Setting Slow-Setting

CRS-1 CRS-2 CMS-2 CMS-2h CSS-1 CSS-1hTest on emulsion

Viscosity, Saybolt Furol at 25°C, s 20-100 20-100

Viscosity, Saybolt Furol at 50°C, s 20-100 100-400 50-450 50-450

Minimum residue by distillation, % 60 65 65 65 57 57Test on residue from distillation

Penetration at 25°C 100g, 5s, dmm 100-250 100-250 100-250 40-90 100-250 40-90

Ductility, 25°C, 5cm/min, cm <40 <40 <40 <40 <40 <40

Test Rapid-Setting Medium-Setting Slow-Setting

RS-1 RS-2 HFRS-2 MS-1 MS-2 MS-2h HFMS-1 HFMS-2 HFMS-2h SS-1 SS-1hTest on emulsion

Viscosity, Saybolt Furol at 25°C, s

20-100 20-100 100+ 100+ 20-100 100+ 100+ 20-100 20-100

Viscosity, Saybolt Furol at 50°C, s

75-400 75-400 20-100 100+ 100+ 20-100 20-100

Minimum residue by distillation, %

55 63 63 44 65 65 55 65 65 57 57

Test on residue from distillationPenetration at 25°C 100g, 5s, dmm

100-200 100-200 100-200 100-200 100-200 40-90 100-200 100-200 40-90 100-200 40-90

Ductility, 25°C, 5cm/min, cm

<40 <40 <40 <40 <40 <40 <40 <40 <40 <40 <40

Float test, 60°C, s 1200 1200 1200 1200

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13.3.1. Applications of Asphalt Emulsions The Cold-mix recycling operation, which utilizes milled old asphalt pavement mixed with asphalt emulsion, is gaining popularity for rehabilitating deteriorating roadways. In this method, the old asphalt pavement is crushed, often in place. An in-place aggregate base can also be incorporated or new aggregates can be added to the old materials and asphalt emulsion added. Then, materials are mixed together, spread to a uniform thickness, and compacted. Slow setting SS and CSS asphalt emulsions are used currently without polymer modification. Surface treatments applied to an existing pavement for preventive maintenance are the most significant application of polymer modified asphalt emulsion. They are economical, easy to place, resist traffic abrasion and provide a long lasting waterproof cover over the underlying structure. There are several types of surface treatment, but in this chapter, we will limit our discussion to chip seal and slurry surfacing. Detailed descriptions as well as recommended performance guidelines of various paving technologies using asphalt emulsions can be found in [13-18,19]. Chip Seal: This treatment involves spraying asphalt material (heated asphalt or asphalt emulsion) followed immediately by a thin (one stone thick) aggregate cover as schematically shown in Figure 13-10. The aggregate is immediately rolled with a pneumatic roller and a light brooming may be necessary to remove any excess aggregate. Cutback asphalts have been used in the past for this purpose but asphalt emulsion is now preferred due to environmental and safety (fire hazard) concerns associated with cutback asphalt. A rapid setting RS, HFRS or CRS is usually used, though a medium setting MS, HFMS or CMS asphalt emulsion could be used (ASTM D977 and D2397 for anionic and cationic emulsions, respectively). The cationic asphalt emulsion often provides better asphalt adhesion to the aggregate. The polymer modified asphalt emulsion (2-4% polymer by weight of asphalt) improves chip retention and enhances pavement durability (see 13.3.5).

Slurry Seal: A slurry seal is a homogeneous mixture of well-graded fine aggregate, asphalt emulsion, water and mineral fillers applied to a pavement as a surface treatment. Slurry seal is usually applied in a thickness of 3 to 6 mm. A small amount of mineral filler, hydrated lime, limestone dust, Portland cement or fly ash, aids in setting the slurry. The Slurry comes directly from a traveling mixing plant into an attached spreader box that spreads the slurry by a squeegee-type action as shown in Figure 13-11. The machine used for production of slurry

Figure 13-10. Schematic diagram illustrating chip seal paving.

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seal is a self-contained, continuous-flow mixing unit. The asphalt emulsion used in the slurry mix may be SS-1, SS-1h, CSS-1 or CSS-1h. Quick-setting (QS) asphalt emulsion is used when early opening to traffic is necessary.

Microsurfacing : A new slurry technique, microsurfacing, takes advantages of polymer modified asphalt emulsions. It can be applied at greater thicknesses than conventional slurry seals, allowing its use for rut- filling and is maintains a friction resistant surface throughout the service life. The microsurfacing mix has to set quickly enough to accept traffic within one hour after placement [13-20]. Polymer modified CSS-1h asphalt emulsion (ASTM D2397 and AASHTO M208) is used with a minimum polymer level of 3%. The international Slurry Seal Association has established recommended performance guidelines A105 and A143 for the slurry seal and microsurfacing, respectively. A careful mix design (ISSA TB-139, TB-09, TB-114, TB-100, TB-147A, TB-144 and TB-113) confirming compatibility of the aggregate, polymer modified asphalt emulsion, mineral filler, and other additives is essential for successful slurry seal and microsurfacing operations. 13.3.2. Asphalt Emulsion Tests Standard tests and procedures for testing asphalt emulsions are specified in ASTM D244 and AASHTO T59. These include particle charge, viscosity, storage stability, demulsibility and others. A distillation or evaporation test is used to recover the asphalt (emulsion residue) from the emulsion. In these tests, the asphalt emulsion is subjected to a maximum of as high as 260°C for the distillation method or 167°C for the evaporation. The most common tests run on the recovered residue include penetration, softening point, ductility, elastic recovery and torsion recovery. These tests are meant to be used as a quality control tool, (e.g. to confirm a designed polymer level in the modified asphalt), but are not designed to correlate the binder performance for each application [13-21]. During the residue recovery process, excess heat

Figure 13-11. Schematic diagram of a typical microsurfacing paver. Courtesy AKZO NOBEL Asphalt Applications Inc.

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applied to the polymer modified asphalt emulsion causes formation of macroscopic polymer globules that are as large as a few mm in diameter. A minor difference in the temperature and length of the distillation would cause variation in the polymer morphology, which explains the poor reproducibility reported by the AEMA Materials Committee Round Robin Studies on emulsion residue characterization [13-22]. 13.3.3. Polymer Honeycomb Structure in Cured Asphalt Emulsion. Modified asphalt emulsion with latex is not just an emulsion of polymer-modified asphalt, but rather an emulsion containing dispersed latex particles in the aqueous phase, as schematically shown in Figure 13-12. Menisci of water containing latex particles (and Portland cement particles for microsurfacing) form among asphalt particles when water starts to evaporate from the asphalt emulsion. The SBR latex for asphalt modification is designed to create a polymer film without coagulum formation; promoting early strength development. The majorities of latex particles migrate together with water and accumulate in the menisci, and thus act as “spot welding” of asphalt particles to ensure maximum binding power, as shown in the right side of Figure 13-12. To form the finest honeycomb structure the asphalt emulsion should not break (coalesce) during the process.

Scanning electron microscope observation of the microsurfacing pavement confirmed the presence of the polymer honeycomb structure [13-22]. Here, a sample of the freshly applied pavement sample was treated with OsO4 and the asphalt was extracted with MEK (methyl ethyl ketone) solvent. The treatment with OsO4 makes the SBR polymer insoluble to the organic solvent and also improves the contrast for the scanning electron microscope, SEM, observation.

Figure 13-12. Left: Schematic illustration of latex modified asphalt emulsion showing that latex particles remain in the aqueous phase. Right: Latex particles transform to a continuous polymer film surrounding asphalt particles, which cures to form the honeycomb structure.

Asphalt

Latex Film

Asphalt

Latex

Latex Modified Emulsion Cured Asphalt Emulsion

17

A series of SEM photographs of the fractured surface were taken and shown in Figure 13-13. These photographs, especially (b) and (c) demonstrate the honeycomb structures of the SBR polymer formed around asphalt particles. Some latex polymers are also adhering on the aggregate surface as seen in (c). It is important to realize that the latex polymers should remain in the aqueous phase, not in the asphalt, and transform to a continuous polymer film during the curing process. Since Portland cement particles also remain in the aqueous phase, the flexible polymer-cement complex creates these honeycomb structures. In contrast, the honeycombs made only with Portland cement would also be very brittle and this would be the case when the polymer is present in the asphalt phase. Do these honeycombs strong enough to maintain their structure under repeated poundings by heavy weight truck tires running at above 100km/hour througho ut the lifetime of the pavement? Pavement samples were taken from Texas State Highway 84 near Waco. This highway was treated with the microsurfacing in 1998. Samples were taken from the wheel path as well as the shoulder of the pavement. As seen in Figure 13-14, the honeycomb structure with the SBR latex polymer-cement complex is flexible enough to withstand repeated stresses after three years service at the highway condition. Advantages of this flexible honeycomb structure with SBR latex will be discussed later in the emulsion residue characterization.

(a)

(a) (b)

(b)

(c)

(c)

1µm

25µm

10µm

5µm

Figure 13-13. A series of scanning electron microscope photographs of the cured microsurfacing specimen demonstrating (a) and (b) SBR polymer honeycomb formed around asphalt particles. (c) Some polymers also adhere on the aggregate surface.

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An optical microscope observation simulating chip seal was also conducted using SBR latex modified CRS-2 emulsion [13-23]. When the emulsion is placed on sand particles, which are placed on the microscope slide glass, spontaneous formation of the polymer network was observed as shown in Figure 13-15.

Figure 13-14. SEM photographs of microsurfacing pavement taken under the wheel path; Texas State Highway 84 near Waco, paved in 1998, and samples taken in 2001.

50µm

Latex Polymer Network

Figure 13-15. Photo-micrograph demonstrating spontaneous formation of polymer network upon curing of the CRS-2 asphalt emulsion modified with 3% cationic SBR latex.

5µm

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13.3.4 Asphalt Emulsion Residue Characterization The need for an appropriate residue recovery procedure for asphalt emulsion has been recognized in both Europe and the US. The forced airflow drying method, which dries the emulsion at ambient temperature, provides a sufficient amount of residue samples within 3-5 hours for the Superpave binder characterization [13-22]. An example of estimating the rutting resistance temperature, Tr (temperature at G*/sin(δ)=1kPa) of microsurfacing emulsion residue is shown in Figure 13-16. A typical microsurfacing formulation consists of 100g aggregates, 12g of 65% asphalt emulsion containing 3% latex polymer, 10g water and 1g Portland cement. The formulation used for this study is the same but without the aggregate and 10g water.

As seen in Figure 13-16, the unmodified asphalt emulsion was made with a PG64 asphalt and the rutting resistance temperature increased slightly from 66°C to 68°C after one month. The sample with Portland cement shows a gradual increase in Tr to 71°C within three weeks. This increase in Tr is mostly due to stiffening of the asphalt as the phase angle of the residue increases from 82° to 88° at Tr. The value of Tr showed a rapid increase to 76°C within the first 3 days of curing when 3% of the SBR latex is also present in the mix. Two PG grades improvement in the rutting resistance was achieved after two weeks of curing. The phase angle at Tr remained nearly constant at 77-78° throughout the curing, confirming that SBR modified asphalt binder maintains the elasticity. Differences in the phase angle of these three samples are also summarized in Figure 13-16.

64

70

76

82

0 10 20 30 40 50

Curing Time, day

Tem

p. a

t G*/

sin

(δ)=

1kP

a, °

C

Emulsion Only

Emulsion+Cement

Emulsion+Cement+SBR Latex

30 days Cured

60

65

70

75

80

85

90

Emulsiononly

+ Cement +3% Latex

Ph

ase

An

gle

at G

*sin

(δ)=

1kP

a, d

egre

e

Figure 13-16. The rutting resistance temperature, Tr, of microsurfacing emulsion, emulsion plus cement and emulsion, cement and 3% SBR latex. Two PG grade improvement can be observed with the polymer-cement system, which maintains elasticity of the residue as seen with the low measured phase angle.

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To evaluate potential benefits on performance during its lifetime, an accelerated curing test was also designed. Here, the emulsion was dried one day under the forced airflow, and transferred into an oven at 60°C, and so simulating pavement temperature during the daytime.

Two different latex polymer levels of 3 and 5% were studied. Three PG grades improvement (from PG64 of the unmodified asphalt) with 3% latex polymer takes only 10 days of curing, as seen in Figure 13-17, demonstrating the rut filling capability of the microsurfacing system. The European Standard for emulsions of pure and polymer modified bitumen including a residue recovery procedure and characterization of the recovered residue is currently under preparation. 13.3.5. Application Tests for Chip Seal and Microsurfacing Microsurfacing: D. Jones et al [13-24,25] analyzed the performance of seven polymer-modified asphalt emulsions for microsurfacing application. The objective of their studies was to examine effects of different polymers on microsurfacing performance. The same asphalt, surfactant and aggregates were used to eliminate all other variables from the mix design. Results of the Wet Track Abrasion Test, WTAT, and Loaded Wheel Test, LWT, are reproduced in Figure 13-18. The authors concluded that SBR latex continues to perform well in virtually all the laboratory tests to which it has been subjected. They also recognized that the materials which were received as latices, tended on average to outperform the solid polymers. These conclusions, especially LWT results, can now be understood in the light of the formation of the polymer honeycomb structure providing excellent rutting resistance of the asphalt emulsion residue. This is demonstrated in Figure [13-13,14,15,16].

Figure 13-17. Accelerated curing of the microsurfacing residues at 60°C after drying under forced airflow for one day.

BUTONAL NX1118

64

70

76

82

88

94

100

0 10 20 30 40 50

Curing Time at 60°C, day

Rut

ting

Res

ista

nce

Tem

pera

ture

, °C

3% Polymer

5% Polymer

21

Chip Seal: Loose chips from a freshly paved road are the major safety concern for chip seal operation, and several attempts were reported in the literature to develop a laboratory procedure to simulate the field experience. A modified fretting test (also know as the abrasion cohesion test Esso, ACTE) appears to be the most successful [13-26,27]. In this test, a known amount of CRS-2 asphalt emulsion and aggregates are spread on a roofing felt, and then rolled with a 30kg rubber roller. The sample is subjected to the shearing action of a horizontal steamhose, which is attached to a Hobart sun and planet mixer, and the percentage of retained chips is recorded as a function of curing time.

0

20

40

60

80

0 30 60 90 120

Curing time, min.

Ch

ip r

eten

tion

, %

Latex Modified

Unmodified

Figure 13-18. The wet track abrasion test and loaded wheel test of cured microsurfacing specimen prepared with five different polymers reported by D. Jones [13-23,24].

0

20

40

60

SBR

Natural SB

SEV

A

Neopre

ne

Wet

Ab

rasi

on

Lo

ss, g

/ft2

0

5

10

15

Wh

eel T

rack

Def

orm

atio

n, %

Figure 13-19. Results of modified fretting test demonstrating advantages of the early chip retention with cationic SBR latex modified emulsion.

22

An example of the test results is shown in Figure 13-19, which demonstrates early cohesion development with the latex modified asphalt emulsion [13-28]. J. Marchal et al [13-27] report that the maximum chip retention does not exceed 80% even with a fully cured asphalt emulsion, and approximately 50% chip retention is considered to be strong enough to be open to traffic. Use of a specially designed brush appears to reduce a problem of chip build-up around the steamhose [13-29].

13.4. Eco-Efficiency Analysis Recent study by Queiroz, et al [13-30] demonstrated a statistically significant relationship between a country’s economical development and its road infrastructure (Figure 13-20). A well-developed and well-maintained highway system is credited for improvements in access to goods and services, education and employment opportunities. A person living in Australia has, on average, access to 27- lane meters of paved road. In comparison, it is only 16- lane centimeters for people in China! Improvement in cold mix technology to provide durable pavements would result in significant impact on the well being of people living in these developing countries. A cold mix plant, using asphalt emulsion, requires less initial capital investment and lower energy consumption than a hot mix plant.

For developed countries, environmental focus has shifted from pollution prevention to sustainable development. BASF developed a so-called eco-efficiency analysis, as an internal decision making tool, to help in evaluating products and processes for sustainable

Figure 13-20. A linear correlation with R2=0.76 exists between a country’s economical well-being and road infrastructure. Here, PGNP = GNP/Capita in $ and paved roads in km/million inhabitants are plotted for 98 countries [13-30].

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development. The eco-efficiency analysis takes equal account of both the ecological and economic aspects and compares pros and cons of each choice. The main goal of the eco-efficiency analysis is “To offer customers the best possible alternatives with the least environmental impact at the best cost”. It has been realized that preventive maintenance of existing roadways is the most financially effective use of available resources [13-3,31,32]. The eco-efficiency analysis was applied to compare three different paving methods of hot mix, polymer modified hot mix and asphalt emulsion based microsurfacing [13-33]. The study integrates environmental impact analysis and economical consideration. The base study assumes a 7-year life for the microsurfacing treatment (8-12mm thick), a 10-year life for the thin (4cm) hot mix overlay and a 13-year life for the polymer modified hot mix overlay. The environmental impact analysis is based on the life cycle analysis [13-34], which evaluates environmental aspects and potential impacts throughout a product’s life cycle (e.g., cradle-to-grave evaluation) from raw material acquisition through production, use and disposal. For asphalt emulsion based paving, this analysis includes not just for production of the asphalt emulsion and paving operations, it starts from the crude oil production, refinery process, chemical additives and aggregate production. It also includes waste production, recycling operation, and transportation and distribution of all these activities.

These environmental impacts are classified into five parameters: raw materials consumption, energy consumption, emission, potential health effects, and risk of accident and misuse. When all factors were considered, microsurfacing had a lower environmental “footprint” as shown in Figure 13-21. The thicker hot mix layer let to a greater use of natural resources, as well as higher energy consumption and emission involved in its manufacture and transportation.

0,00

0,50

1,00Energy

Emissions

Risk potential

Raw material

Cold-mix microsurfacing

Hot-mix asphalt

Figure 13-21. Environmental profiles for microsurfacing and thin hot mix overlays. Microsurfacing has a lower environmental “footprint” than two other alternative treatments.

Potential health effects

Modified hot-mix asphalt

24

These environmental impacts were weighed according to how surveys said the public viewed their relative importance. When this result is combined with the annual costs of the treatments, the overall conclusion is that microsurfacing provides a better balance between cost-effectiveness and environmental impact than does a thin hot mix overlay as shown with the eco-efficiency portfolio of the preventive maintenance in Figure 13-22. Here, all costs and environmental impacts were averaged over each year of the life of the treatment. The study also suggests that future improvement in microsurfacing techniques could lead to additional cost and environmental advantages [13-33]. 13.5 Concluding Remarks Asphalt roads account nearly 95% (3,300,000 km) of the paved roads in the U.S. Addition of as little as 2-3% of polymers in the asphalt improves rutting resistance, and prevents premature fatigue and cold fracture crack formation. The latex can be used for both hot mix and emulsion based paving. Recent studies [13-33] on eco-efficiency analysis clearly demonstrate economical and ecological advantages of the asphalt emulsion based microsurfacing for preventive maintenance. Latex, because it is an aqueous dispersion, is the ideal polymer for modification of an asphalt emulsion. Commercial availability of the cationic form makes SBR latex ideal for chip seal, slurry seal and microsurfacing applications, which are predominantly used for preventive maintenance.

Cold-mix micro-

surfacing

Hot-mix asphalt

Modified hot-mix asphalt

0,2

1,0

1,8

0,21,01,8Costs (relative)

Env

iron

men

tal i

mpa

ctHigh eco-effciency

Low eco-effciency

Figure 13-22. Eco-efficiency portfolio combines environmental impact with costs of treatments. Results demonstrate that microsurfacing is more “Eco-Efficient” than hot mix overlays.

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ACKNOWLEDGEMENT The author is grateful to Glynn Holleran of Valley Slurry Seal Co., Drs. Alan James and Julia Wates of Akzo Nobel Surface Chemistry LLC, Dr. Per Redelius of AB NYNÄS Petroleum, Jeremy Kissock of BASF New Zealand and Mike Taylor of BASF Corporation for their valuable comments and advice. REFERENCES [13-1] The Asphalt Institute, The Asphalt Handbook, Manual Series No. 4 (MS-4). [13-2] Symposium of world road bitumen emulsion producers, Bordeaux, Sept. 1997. [13-3] F. L. Roberts, P.S. Kandhal, E.R. Brown, D.Y. Lee, T.W. Kennedy, “Hot Mix

Asphalt Materials, Mixture Design and Construction,” NAPA Research and Education Foundation Textbook, 2nd Edition, 1996.

[13-4] G. King, H. King, R.D. Pavlovich, A.L. Epps and P. Kandhal, “Additives in

Asphalt,” J. Association of Asphalt Paving Technologist, 75th Historical Review and Index of Journals 1975-1999, 68A, 32-69, 1999.

[13-5] T. Hancock, British Patent No. 9952, November 21, 1843. [13-6] S. Shuler, and J.A. Epps, presented to the Rubber Division, American Chemical

Society, Philadelphia, PA, 1982. [13-7] D. C. Thompson, and J.F. Hagman, Assoc. Asphalt Paving Technol., 1958, 55. [13-8] Construction of Hot Mix Asphalt Pavements, Manual Series No. 22, Second Edition,

Asphalt Institute, Lexington, KY. [13-9] “Use of Modified Bituminous Binders, Special Bitumens and Bitumens with

Additives in Pavement Applications”, World Road Association (PIARC) Technical Committee Flexible Roads (C8), Laboratoire central des Ponts et Chaussées, September 1999.

[13-10] World Road Association, www.piarc.icpc.fr [13-11] Superpave Binder Manual, Superpave Series No. 1 (SP-1), Asphalt Institute,

Lexington, KY. [13-12] D. A. Anderson, D.W. Christensen, H. U. Bahia, M.G. Sharma, C.E. Antle and J.

Button, “Binder Characterization and Evaluation Volume 3: Physical Characterization”, Strategic Highway Research Program (SHRP-A-369), National Research Council, Washington, DC 1994.

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[13-13] W. Arand, O. Harder, B. Herr, “Asphalt Containing Conventional and Polymer-

Modified Bitumens in High and Low Temperature Conditions,” PIARC XIXth World Road Congress, Marrakesh, Sept. 1991.

[13-14] H. Bahia, W. Hislop, H. Zhai, and A. Grangel, “Classification of Asphalt Binders

into Single and Complex Binders,” Association of Asphalt Paving Technologist, 67, 1998.

[13-15] L. H. Lewandowski, “Polymer modification of paving asphalt binders”, Rubber

Chemistry and Technology, 1994, 67, 447-480. [13-16] B. Brûlé, Y. Brion, A. Tanguy, “Paving asphalt polymer blends: relationships

between composition, structure and properties”, J. Asphalt Paving Tech. 1988, 57, 41-64.

[13-17] V. Wegan, B. Brûlé, The strycture of polymer midified binders and corresponding

asphalt mixtures, J. Assoc. Asphalt Paving Tech. 1999, 68, 64-88. [13-18] AEMA Recommended Performance guidelines, second edition, Asphalt Emulsion

Manufactures Association, Annapolis, Maryland. [13-19] A Basic Asphalt Emulsion Manual, Asphalt Institute Manual Series No. 19, Second

Edition, Lexington, KY. [13-20] R. Hassan, State-of-the-practice design, construction and performance of micro-

sufacing, 1994, FHWA-SA-94-051, Federal Highway Administration, Washington, D.C.

[13-21] L. D. Coyne, Evaluation of polymer modified chip seal coats, J. Asphalt Paving

Tech. 1988, 57, 545-575. [13-22] K. Takamura, “Comparison of Emulsion Residues Recovered by the Forced Airflow

and RTFO Drying”, AEMA/ISSA Proceedings, 2000, 1-17. [13-23] K. Takamura, W. Heckmann, “Polymer network formation in the emulsion residue

recovered by forced air drying”, Proceedings of International Symposium on Asphalt Emulsion Technology, 1999, 185-194.

[13-24] D. R. Jones, AEMA Annual Meeting, Nov. 1988. [13-25] D. R. Jones, and A. C. Ng, ISSA Annual Meeting, Feb. 1989. [13-26] E. Cornet, “Esso Abrasion cohesion test, A description of the cohesive breaking of

emulsions for chip seals”, Proceedings of International Symposium on Asphalt Emulsion Technology, 1999, 346-355.

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[13-27] J. L. Marchal, P. Julien, N. Boussad, “Bitumen emulsion testing: towards a better

understanding of emulsion behavior, 1990, ASTM symposium on asphalt emulsion. [13-28] J. Wates and A. James, AKZO NOBEL Internal results. [13-29] L. Barnat, “Predictive capabilities for maintenance products”, AEMA/ISSA

Proceedings, 2000, 19-49. [13-30] C. Queiroz, R. Haas, and Y. Cai, “National economic development and prosperity

related to paved road infrastructure”, Transportation Research Record 1455, 1994. [13-31] M. S. Mamlouk and J.P. Zaniewski, “Pavement Preventive Maintenance:

Description, Effectiveness, and Treatments”, Symposium on Flexible Pavement Rehabilitation and Maintenance, ASTM STP 1349, 121-135, 1999.

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Treatments Used in Texas”, Symposium on Flexible Pavement Rehabilitation and Maintenance, ASTM STP 1349, 136-150, 1999.

[13-33] K. Takamura, K.P. Lok and R. Wittlinger, AEMA/ARRA Annual Meeting, Feb.,

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Asphalt and Steel-Reinforced Concrete Pavements”. Transportation Research Record 1626, 105-113, 1998.