- Email: [email protected]
Polymer modified asphalt binders
Construction and Building
MATERIALS
Construction and Building Materials 21 (2007) 66–72
www.elsevier.com/locate/conbuildmat
Polymer modified asphalt binders Yetkin Yildirim
*
Department of Civil Engineering, University of Texas at Austin, 3208 Red River CTR 318, Austin, TX 78705, USA Received 19 August 2004; received in revised form 5 July 2005; accepted 21 July 2005 Available online 19 September 2005
Abstract This paper is a review of research that has been conducted on polymer modified binders over the last three decades. Polymer modification of asphalt binders has increasingly become the norm in designing optimally performing pavements, particularly in the United States, Canada, Europe and Australia. Specific polymers that have been used include rubber, SBR, SBS and ElvaloyÒ. Specifications have been designed and pre-existing ones modified to capture the rheological properties of polymer modified binders. The elastic recovery test is good at determining the presence of polymers in an asphalt binder, but is less successful at predicting field performance of the pavement. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Polymer modified binder; Asphalt; Binder specifications; Elastic recovery; SBR; SBS; Elvaloy; Rubber
1. Introduction
2. History, use, and benefits
The addition of polymers, chains of repeated small molecules, to asphalt has been shown to improve performance. Pavement with polymer modification exhibits greater resistance to rutting and thermal cracking, and decreased fatigue damage, stripping and temperature susceptibility. Polymer modified binders have been used with success at locations of high stress, such as intersections of busy streets, airports, vehicle weigh stations, and race tracks [1]. Polymers that have been used to modify asphalt include styrene–butadiene–styrene (SBS), styrene–butadiene rubber (SBR), ElvaloyÒ, rubber, ethylene vinyl acetate (EVA), polyethylene, and others. Desirable characteristics of polymer modified binders include greater elastic recovery, a higher softening point, greater viscosity, greater cohesive strength and greater ductility [1,2].
Processes of asphalt modification involving natural and synthetic polymers were patented as early as 1843 [3]. Test projects were underway in Europe in the 1930s, and neoprene latex began to be used in North America in the 1950s [1]. In the late 1970s, Europe was ahead of the United States in the use of modified asphalts because the European use of contractors, who provided warranties, motivated a greater interest in decreased life cycle costs, even at higher initial costs. The high preliminary expenses for polymer modified asphalt limited its use in the US [4]. In the mid-1980s, newer polymers were developed and European technologies began to be used in the US [5,6]. At the same time, the prevalence of a long-term economic outlook in the country increased [1]. In Australia, the current National Asphalt Specification includes guides and specifications regarding polymer modified binders [7]. The United States Federal Highway Administration (FHWA) has developed a life cycle cost analysis approach, which can be used to evaluate the life cycle costs
*
Tel.: +1 512 232 1845; fax: +1 512 475 7914. E-mail address: [email protected].
0950-0618/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2005.07.007
Y. Yildirim / Construction and Building Materials 21 (2007) 66–72
of pavement containing asphalt rubber binders as well as other treatments. The findings indicated that asphalt rubber is cost effective as it is used, for example, in Arizona and California, although the estimated life of the pavement is based on interviews and engineering judgment, and can be refined as the pavement ages and long-term field performance is included in the model [8]. A 1997 survey of state departments of transportation in the United States found that 47 states of the 50 reported that they would be using modified binders in the future, 35 of them saying that they would use greater amounts [9]. Several research teams around the world have worked on evaluating the benefits of polymer modification on pavement performance, and tests and specifications for binders are continually being developed. In a 2001 study for the Ohio Department of Transportation, Sargand and Kim [10] compared the fatigue and rutting resistance of three PG 70–22 binders, one unmodified, one SBS modified, and one SBR modified. It was found that the modified binders were more resistant to both fatigue and rutting than the neat binder, even though all three had the same performance grade. According to a 2003 Nevada study, the viscosity of polymer modified binders tends to be significantly greater than that of non-modified binders at 60 °C, although penetration changes only slightly at all temperatures [11]. In 2003, Newcomb [12] discussed the concept of perpetual pavements in Hot Mix Asphalt, claiming that it is a misconception that fatigue cracking is inevitable. Many full-depth hot mix asphalt (HMA) pavements built 30–40 years ago have yet to exhibit any fatigue cracking, and Newcomb claims that research shows that increasing polymer modified binders at the bottom of the asphalt layer may raise the fatigue limit of the pavement. A 2003 US Army Corps of Engineers study [13] points out that for optimal economy, it is desirable to choose an asphalt modifier that resists multiple distresses, such as rutting, fatigue, thermal cracking and water damage. It was found that the choice of polymer may have a significant impact on fatigue properties, and that the mixtures boasting the highest fatigue life contained reactive styrene–butadiene crosslinked polymer. Other polymers tested were a chemically modified crumb rubber, SBR, linear block SBS and a proprietary modified SBS.
67
tegic Highway Research Program (SHRP). Bahia et al. in their 1998 article for the Journal of AAPT note, however, that this blanket testing method failed to test the extreme grades required by the new, modified binders, resulting in the initiation of new testing protocols for modified binders [14]. New test protocols include measuring the softening point using a ring and ball apparatus (ASTM E 28) to determine the resistance to flow at high temperatures and a force ductility test that measures tensile properties [15]. Several tests have been developed to look at elastic recovery, one of the major areas of improvement in elastomer modified asphalt. Thompson and Hagman developed a torsional recovery test, included in California specifications for identifying the presence of elastomers [1]. The elastic recovery test using a ductilometer, described later in this paper, is included in the Task Force 31 Specifications and is used in the US and Europe [1]. King et al. [1] point out that many tests exist to identify whether modification is present, such as the IR, low temperature ductility and torsional recovery. The West Coast User Producer Group tried to use performance based asphalt (PBA) specifications, which involve a high temperature viscosity test and low temperature penetration and ductility tests, as specifications for modified asphalt, but they were not as good at predicting performance with modified asphalt as they were with neat asphalt [1]. In 1998, Blankenship et al. [16] conducted field and laboratory tests in Kentucky and found that PG 70–22 made using different methods of modification gave different results for laboratory tests. They used the Dynamic Shear Rheometer (DSR) and Bending Beam Rheometer (BBR) tests to identity five different PG 70–22 binders, two SBS-modified, one SBR modified, one chemically modified, and one neat and compared their behavior in various tests. These binders were found to differ as far as rutting, moisture damage and modulus testing, although the rutting difference was no more than 10 mm between the binders. In 2004, Yildirim et al. [17] utilized a design method for determining the modification level of asphalt binders using waste toner, which contains styrene acrylic copolymers. Binder designs were performed including blending time, performance grading, storage stability and, mixing and compaction temperature calculations. Test results indicated that the stiffness of the blend increases as the percentage of the toner content increases.
3. Test methods 4. Specific modifiers As discussed by King et al. in the 1999 Journal of the AAPT [1], there are several test methods that have been developed or altered for modified binders. Previously, both modified and unmodified binders alike were tested according to the same methods, supported by the Stra-
4.1. Rubber ‘‘Crumb rubber modifier’’ (CRM) and ‘‘asphalt-rubber’’ are terms that refer to applications in which ground
68
Y. Yildirim / Construction and Building Materials 21 (2007) 66–72
recycled rubber and paving asphalt are combined [1]. Characteristics of asphalt–rubber are dependent on rubber type, asphalt composition, size of rubber crumbs, and time and temperature of reaction [1]. Usually, the rubber is recycled from used automotive tyres, which has the additional benefits of saving landfill space that would otherwise be occupied by tyres and reducing cost [1,18]. Natural rubber modification results in better rutting resistance and higher ductility but the modifier is sensitive to decomposition and oxygen absorption. Due to its high molecular weight, it has problems of low compatibility [18]. Recycled tyre rubber reduces reflective cracking, which increases durability. There are some practical problems in using natural rubber: it needs high temperatures and long digestion times in order to be dispersed in the bitumen [18]. In 1991, the Intermodal Surface Transportation Efficiency Act (ISTEA) section 1038 was passed into law in the USA. In its Declaration of Policy, ISTEA states ‘‘It is the policy of the United States to develop a National Intermodal Transportation System that is economically efficient, environmentally sound, provides the foundation for the Nation to compete in the global economy, and will move people and goods in an energy efficient manner’’ [19]. The act required that, starting in 1994, 5% of roads built with federal funds must use pavement made with crumb rubber, processed recycled tyres, or modified asphalt. By 1997, 20% of roads built with federal funds were required to use recycled tyres in the pavement [20]. On the other hand, the Used Tyre Working Group [21] describes the United Kingdom as still being in the process of evaluating a pilot project involving road surfacing that contains recycled tyres. 4.2. Styrene–butadiene–styrene Styrene–butadiene–styrene (SBS) is a block copolymer that increases the elasticity of asphalt [18]. According to a 2001 review in Vision Tecnologica by Becker et al. [18], it is probably the most appropriate polymer for asphalt modification, although the addition of SBS type block copolymers has economic limits and can show serious technical limitations. Although low temperature flexibility is increased, some authors claim that a decrease in strength and resistance to penetration is observed at higher temperatures. Nonetheless, ‘‘SBS is the most used polymer to modify asphalts, followed by reclaimed tire rubber’’ [18]. The Danish Road Directorate [22] found that an SBS-modified binder course showed no superior rut resistance compared to other Danish asphalt courses. Asphalt cores taken from the job site indicated that separation had occurred, and that the polymer phase was not homogeneously distributed, which might have
been the cause of the poor performance of the pavement. As reported in the Journal of Material in Civil Engineering, transmission electron microscopy was used in 2002 to better understand the behavior of SBS in asphalt binders [23]. Depending on the sources of asphalt and polymer, morphology varies: there can be a continuous asphalt phase with dispersed SBS particles, a continuous polymer phase with dispersed globules of asphalt, or two interlocked continuous phases. It is the formation of the critical network between the binder and polymer that increases the complex modulus, an indication of resistance to rutting. In 2003, in the Journal of the AAPT, Mohammed et al. [24] looked at the possibility of recycling SBS modified asphalt for resurfacing pavement. They found that the impact of the extraction and recovery process on the binder was minimal. Eight-year-old SBS modified binder was recovered from Route US61 in Louisiana, and was found to have experienced intensive oxidative age hardening. At low temperatures, the binder was quite brittle. Blends of virgin and recovered polymer modified binder were found to be stiffer than anticipated at both low and high temperatures. It was also found that as the percentage of recovered binder increased, rutting resistance increased, while fatigue resistance decreased. In 2004, the Florida Department of Transportation and FHWA published a report [25] looking at the effect of SBS modification on cracking resistance and healing characteristics of Superpavee mixes. They found that SBS benefited cracking resistance, primarily due to a reduced rate of micro-damage accumulation. SBS did not, however, have an effect on healing or aging of the asphalt mixture. The possibility of using SBS-modified binders in India has been investigated recently [26]. Calculations indicated that the surface life of the Delhi–Ambala expressway would be almost doubled while the thickness of the bituminous layers would be reduced, although the cost per km would be greater for polymer modified binders. 4.3. Styrene–butadiene–rubber Styrene–butadiene–rubber (SBR) has been widely used as a binder modifier, usually as a dispersion in water (latex). An Engineering Brief from 1987 available at the US Federal Aviation Administration website [2] describes the benefits of SBR modified asphalt in improving the properties of bituminous concrete pavement and seal coats. Low-temperature ductility is improved, viscosity is increased, elastic recovery is improved and adhesive and cohesive properties of the pavement are improved. The benefit of latex is that the rubber particles are extremely small and regular. When they are exposed to asphalt during mixing they disperse
Y. Yildirim / Construction and Building Materials 21 (2007) 66–72
rapidly and uniformly throughout the material and form a reinforcing network structure. According to Becker et al., SBR latex polymers increase the ductility of asphalt pavement [18], which allows the pavement to be more flexible and crack resistant at low temperatures, as found by the Florida Department of Transportation [25]. SBR modification also increases elasticity, improves adhesion and cohesion, and reduces the rate of oxidation, which helps to compensate for hardening and aging problems [25]. In a 1999 laboratory test at the Texas Transportation Institute, it was found that coating smooth, rounded, siliceous gravel aggregates with cement plus SBR latex for use in HMA increased stability according to Hveem and Marshall standards, as well as tensile strength, resilient modulus and resistance to moisture damage. Coated aggregates have greater resistance to rutting and cracking [27]. Water-based SBR latex has been widely used to improve chip retention in emulsions, but SBS has gradually replaced latex because of its effect of greater tensile strength at strain, and because it is compatible with a broader range of asphalts [1]. Elastomers such as SBR and SBS have a significant effect on the results of the ductility test at both 4 and 25 °C; while SBR modified asphalts have high ductility at all temperatures, SBS modified asphalts tend to have lower ductility [1]. 4.4. ElvaloyÒ The Duponte website [28] describes ElvaloyÒ as an ethylene glycidyl acrylate (EGA) terpolymer that chemically reacts with asphalt. As a result of the reaction, problems with separation during storage and transportation are avoided. Roads using ElvaloyÒ have been in use since 1991. In 1995 Witczak, Hafez and Qi [29] studied the laboratory performance of asphalt modified with ElvaloyÒ at the University of Maryland. Two different grades of asphalt were each modified by 0%, 1.5% and 2.0% ElvaloyÒ by weight of binder. The susceptibility of the mixtures to moisture damage was found to be greatly decreased by the addition of ElvaloyÒ. In addition, an
% Recovery ¼
69
ElvaloyÒ in combination with granite had a significantly higher (poorer) fracture temperature than with diabase, limestone or granite aggregate treated with hydrated lime. At the DuPont Institute, Babcock et al. [31] devised a lap shear test for high temperature binder properties, which appears to agree with high temperature DSR measurements. The results indicated that binder failure at temperatures above 6 °C tends to be cohesive failure, due the loss of integrity within asphalt. On the other hand, around 6 °C and colder, failure is adhesive, from a loss of adhesion between the binder and the substrate. Since this indicates that cold temperature failure of a road may be the result of loss of adhesion to the aggregate, a chemically reactive polymer is expected to perform better, and reactive elastomeric terpolymer does in fact perform better in this test than SBS or the control neat bitumen.
5. Elastic recovery test Elastic recovery (or elasticity) is the degree to which a substance recovers its original shape following application and release of stress. A degree of elastic recovery is desirable in pavement to avoid permanent deformation. ‘‘When a tire passes over a section of pavement, it is desirable for that pavement to have the ability to ÔgiveÕ, but it is equally important for it to recover to its original shape,’’ according to the Asphalt Institute website [32]. 5.1. Measurement and calculation The elastic recovery of asphalt is measured with the aid of a ductilometer, which is used to elongate an asphalt specimen at a constant rate. After a period of time, the elongated specimen is cut and then allowed to rest. After the period of rest is complete, the distance between the ends of the cut specimen is measured [33]. The elastic recovery is the ratio between the difference in elongation between cutting and the end of the rest period, and the total elongation applied [33].
Initial elongation Observed elongation after rejoining sample 100 Initial elongation
analysis of repeated load permanent deformation behavior showed that increasing concentrations of ElvaloyÒ resulted in a marked decrease in deformation. In a study on low-temperature rheological properties of polymer modified binders, the FHWA [30] found that
5.2. Binder characterization The elastic recovery test is used to test polymer modified binders by the departments of transportation of several states in the US and several other countries, as well
70
Y. Yildirim / Construction and Building Materials 21 (2007) 66–72
as by researchers around the world. According to the Spring 2002 edition of the Asphalt Technology News [34], Kansas, Louisiana and Texas require use of the elastic recovery test to ensure that binders have been modified. Michigan also uses it, although it does not require it, and Kentucky uses it to test PG 76–22 binders. It is also used to characterize polymer modified binders in Quebec, Sweden, Finland and Switzerland [35]. In 1981, Oliver developed the elastic recovery test for the Australian Road Research Board to measure deformation response of rubber modified binders [36,37]. He found that binders with natural (truck-tyre) rubber showed greater elastic recovery than synthetic (car-tyre) rubber. In 1997, it was reported to the Australian Asphalt Pavement Association that even low concentrations of SBS caused an increase in elastic recovery, softening point, viscosity and cohesive strength [38]. In 1990, Valkering and Vonk [39] compared SBS and EVA modified binders to neat binders and found that SBS modified binders had significantly higher elastic recovery than neat binders. Compared to SBS, EVA modified binders showed a lesser degree of improvement in elastic recovery and also lost ductility and elastic recovery much more rapidly. Braga and Corrieri [40] used the elastic recovery test to compare the resistance to thermal degradation of SBS and heterophasic polyolefin (TPO) modified binders. Aged SBS polymers following showed a lower resistance to thermal degradation than aged TPO binders. Several studies have investigated the relationship between measurements of elastic recovery and other measures of performance, in both laboratory and field tests. In the Transportation Research Record 1996, Bonemazzi et al. [41] compared the performance of binders modified with an array of polymers (atactic propylene– ethylene copolymer, low- and high-density polyethylene, ethylene/propylene rubber, ADFLEX, ethylene methacrylate copolymer, EVA, thermoplasticpolyolefinic terpolymer, and SBS linear and radial block copolymers) in tests of penetration and elastic recovery as well as the rheometer dynamic test. All the tests were shown to be good measurements of polymer contribution to binder performance. In 1997, Oliver [42] examined the relationship between the rheological properties of asphalt mixes and rutting resistance using the wheel tracking test. While a relationship between polymer consistency and rut resistance was found, no relationship was apparent between rut resistance and elastic recovery or softening point. In the Journal of the AAPT, 1998, Bahia, Perdomo and Turner [9] compared five modified binders, measuring elastic recovery, ductility and resilience. They found that these conventional measurements were inconsistent in ranking the suitability of polymer modified binders. Specifically, rankings changed as strain level changed.
Superpavee testing results were equally inconsistent in ranking modified binders. In a 1988 Iowa Department of Transportation report, Lee and Demirel [43] compared viscosity, penetration, softening point, force ductility, elastic recovery and several other characteristics of binders with SBS, polyolephins, neoprene, SBR latex and hydrated lime. There was no correlation between the different types of measurement. John DÕAngelo, on the Asphalt Institute web page [44], points out that the literature suggests that most tests of modified binders may only measure whether a polymer modifier is present, not its effect on the field performance of the modified binder.
6. Conclusions In the 1980s, polymer modified asphalts began to be used in the US and by 1997 all but three states were already using modified binders or intended to use them in the future and federal regulations supported their use. Pavements made with modified binders are more resistant to fatigue, thermal cracking, rutting, stripping, and temperature susceptibility than neat binders. Polymer modified binders tend to exhibit increased viscosity and elastic recovery, although penetration does not appear to be influenced by modification. An ideal modifier will increase binder resistance to multiple types of distresses. Modification is not without its drawbacks, however, since compatibility between an asphalt and a modifier is not assured, and separation during storage or application, if not addressed, can result in poorly performing pavement. Since Superpavee specifications were designed for neat binders, they are inappropriate for polymer modified binders. In fact, asphalts modified with different polymers can behave very differently even when they have the same performance grade. Test methods that have been developed or altered for modified binders include measuring the softening point and elastic recovery, and a force ductility test. There is disagreement about whether bending beam rheometer (BBR) tests, developed for Superpavee, are acceptable for polymer modified binders. In general, it seems that the results of rheological tests are not indicative of the performance of polymer modified binders. Several tests exist to identify whether modification is present, such as the IR, low temperature ductility and torsional recovery. In 1991, the ISTEA required that an increasing proportion of roads use modified asphalt. Different polymers impact characteristics of asphalt to differing degrees. Natural rubber improves rutting resistance and ductility, but is sensitive to decomposition and often has problems of compatibility.
Y. Yildirim / Construction and Building Materials 21 (2007) 66–72
The use of tyre rubber as an asphalt modifier is environmentally responsible and results in decreased rutting and reflective cracking, but special conditions, such as high mixing temperatures and long digestion times, need to be maintained to prevent separation from the asphalt binder. The addition of SBR to asphalt improves low-temperature ductility, increases viscosity, improves elastic recovery and improves the adhesive and cohesive properties of the pavement. Water-based SBR latex was used commonly to improve chip retention in emulsions. SBS has been replacing SBR due to the formerÕs wider compatibility and greater tensile strength under strain. SBS is now the polymer most used to modify asphalt. SBS increases the elasticity of asphalt and SBS modified asphalt can be recycled. SBS modified binders have been found to perform better at low temperatures than neat binders or binders modified with chemically reactive polymers. ElvaloyÒ is a modifier that forms a chemical bond with the asphalt, avoiding problems of separation during storage, transportation and application. It increases pavement moisture resistance and results in modified asphalt performing better in high temperature DSR tests. Elastic recovery of asphalt, a measurement widely used to test polymer modified binders, can be measured by elongating an asphalt sample, cutting it, allowing it to rest, and determining the degree to which the elongated specimen returns to its original length. The elastic recovery test has been shown to be a good measurement of polymer contribution to binder performance, although no relationship appears to exist between rut resistance and elastic recovery. Elastic recovery and other conventional measurements are inconsistent in ranking polymer modified binder performance and may only measure whether or not a modifier is present in an asphalt specimen, not its contribution to the asphaltÕs performance. Polymer modified binders have had proven success in the field and the laboratory, and a continuing effort is being made to develop a correlation between results from laboratory tests and field performance.
References [1] King G et al. Additives in asphalt. J Assoc Asphalt Paving Technol A 1999;68:32–69. [2] Bates R, Worch R. Engineering Brief No. 39, Styrene–butadiene rubber latex modified asphalt. Federal Aviation Administration, Washington, DC, 1987. Available from: http://www.faa.gov/arp/ engineering/briefs/eb39.htm. [3] Thompson DC. Hoiberg AJ, editor. Bituminous materials: Asphalt tars and pitches. Robert Krieger Publishing Co.; 1979.
71
[4] Terrel R, Walter J. Modified asphalt pavement materials: The European experience. J Assoc Asphalt Paving Technol 1986;55:482. [5] Brule B. Polymer-modified asphalt cements used in the road construction industry: Basic principles. Transportat Res Record 1996;1535. [6] Collins J. Thermoplastic block copolymers for the enhancement of asphaltic binders in paving applications. Presented at the Paving and Transportation Conference, Albuquerque, NM, 1986. [7] Australian Asphalt Pavement Association. National asphalt specification, 2nd ed. Kew, Australia, 2004. [8] Hicks RG, Epps JA. Life cycle cost analysis of asphalt–rubber paving materials. In: Proceedings of the First International Conference on the World of Asphalt Pavements, Sydney, Australia, 2000. p. 69–88. [9] Bahia HU, Perdomo D, Turner P. Applicability of Superpave binder testing protocols to modified binders. Transportat Res Record 1997;1586:16–23. [10] Sargand SM, Kim SS. Performance evaluation of polymer modified and unmodified Superpave mixes. In: Second International Symposium on Maintenance and Rehabilitation of Pavements and Technological Control, Auburn, AL, 2001. [11] Sebaaly PE, Bazi GM, Vivekanathan Y. Evaluation of new pavement technologies in Nevada, University of Nevada, 2003. [12] Newcomb D. Limit the strain at the bottom of an asphalt pavement, and what do you get? A perpetual pavement. Hot Mix Asphalt Technol 2003;8(6):30–2. [13] Partl MN, Newman JK. Flexural beam fatigue properties of airfield asphalt mixtures containing styrene–butadiene based polymer modifiers. In: Proceedings of the Sixth International Rilem Symposium, Zurich, Switzerland, 2003. p. 357–63. [14] Bahia H, Hislop WP, Zhai H, Rangel A. Classification of asphalt binders into simple and complex binders. J Assoc Asphalt Paving Technol 1998;67:1–41. [15] Chaverot P. The force ductility test: A simple means of characterization of polymer modified asphalt. In: Proceedings of the Fourth Eurobitume Symposium, Madrid, Spain, 1989. p. 96–100. [16] Blankenship P et al. Are All PG 70-22Õs the same? Lab test on KY I-64 field samples. J Assoc Asphalt Paving Technol (AAPT) 1998;67:493–552. [17] Yildirim Y, Hazlett D, Davio R. Toner-modified asphalt demonstration projects. Resources, conservation and recycling, vol. 42. Amsterdam: Elsevier; 2004. pp. 39–50. [18] Becker Y, Me´ndez MP, Rodrı´guez Y. Polymer modified asphalt. Vis Technol 2001;9(1):39–50. [19] Camph DH, Siwek S, Forestieri F. How to keep America moving: Report on the US Department of TransportationÕs outreach on reauthorization of the Intermodal Surface Transportation Efficiency Act (ISTEA). In: Elles-Boyle A, editor. Office of Governmental Affairs, Washington, DC, 1997. [20] Blumenthal M. Changes impacting the ground rubber industry. Rubber Manufacturers Association, Montreal, Canada. 2002. Available from: http://www.rubberassociation.ca/pdf_docs/ RR2002/Michael2.pdf. [21] Used Tyre Working Group. Recycling and recovery. No date. Available from: http://www.tyredisposal.co.uk/ disposal_routes. asp. [22] Wegan V, Nielsen BC. Microstructure of polymer modified binders in bituminous mixtures. Report Number 87-90145-85-8. Danish Road Directorate, Roskilde, Denmark, 2001. [23] Chen JS, Liao MC, Shiah MS. Asphalt modified by styrene– butadiene–styrene triblock copolymer: Morphology and model. J Mater Civil Eng 2002;14(3):224–9. [24] Mohammed LN et al. Investigation of the use of recycled polymer modified asphalt binder in asphalt concrete pavements
72
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
Y. Yildirim / Construction and Building Materials 21 (2007) 66–72 (With discussion and closure). J Assoc Asphalt Paving Technol 2003;72:551–94. Roque R, Birgisson B, Tia M, Kim B, Cui Z. Guidelines for the use of modifiers in Superpave mixtures: Executive summary and volume 1 of 3 volumes: Evaluation of SBS modifier. State Job 99052793. Florida Department of Transportation, Tallahassee, FL, 2004. Shukla RS, Singh VKP, Bhanwala RS. Polymer modified bitumen for construction of heavy traffic density corridors. Indian Highways 2003;31(4):55–66. Kim MG, Button JW, Park DW. Coatings to Improve low-quality local aggregates for hot mix asphalt pavements. Report SWUTC/ 99/167405-1, Texas Transportation Institute, Texas A&M University, College Station, TX, 1999. DuPont. About DuPonte ElvaloyÒ RET reactive elastomeric terpolymer. DuPont website. 2004. Available from: http:// www.dupont.com/asphalt/link1.html. Witczak MW, Hafez I, Qi X. Laboratory characterization of ElvaloyÒ modified asphalt mixtures: vol. I – Technical report. College Park, Maryland: University of Maryland. 1995. Available from: http://www.dupont.com/asphalt/link5.html. Chappat M, Poirier JE, Robert M, Durand G. Appreciation by rheological and tensile tests methods of the impact of ageing of polymer modified binders toward low temperature behaviour. Revue Ge´ne´rale des Routes 2000:22–4. Babcock GB, Statz RJ, Larson DS. Study of asphalt binders using lap shear bonds. Wilmington, DE: DuPonte website. No date. Available from: http://www.dupont.com/asphalt/ lapshear/lapshear.html. Asphalt Institute. Individual asphalt binder tests. Lexington, KY. 2003. Available from: http://www.asphaltinstitute.org/ai_pages/ Lab_Services/Lab_Services_Indiv_Binder_Tests_List.asp. Hobson K. No date. Elastic recovery testing in Oklahoma. Available from: http://www.utexas.edu/research/ superpave/articles/er.html. Asphalt forum responses. Asphalt Technology News 2002;14(1):12–13. Available from: http://www.eng.auburn.edu/ center/ncat/newsl/newssprng_02.pdf.
[35] Zubeck H, Raad L, Ryer J. Constructability of polymer-modified asphalts and asphalt-aggregate mixtures in Alaska: Final report. Alaska: State of Alaska Department of Transportation and Public Facilities. 1999. Available from: http://www.dot.state.ak.us/stwddes/research/assets/pdf/fhwa_ak_rd_99_1.pdf. [36] Oliver JWH. Modification of paving asphalts by digestion with scrap rubber. Transportat Res Record 1981;821:37–44. [37] Oliver JWH. Enhancing bitumen products with scrap rubber. Presented at 19th ARRB Regional Symposium. Vermont South, Australia, 1982. p. 117–31. [38] Vonk W. Polymer modification of bitumens: Manufacture and application of premium binders for improved spray seal performance. Presented at 10th AAPA International Flexible Pavements Conference, Kew, Australia, 1997. [39] Valkering CP, Vonk W. Thermoplastic rubbers for the modification of bitumens: Improved elastic recovery for high deformation resistance of asphalt mixes. In: Proceedings of 15th Australian Road Research Board Conference, Vermont South, Australia, 1990. p. 1–19. [40] Braga V, Corrieri R. A comparative study of SBS and TPO based PMB. In: Proceedings of the Papers Submitted for Review at Second Eurasphalt and Eurobitume Congress, Breukelen, Netherlands, 2000. [41] Bonemazzi F, Braga V, Corrieri R, Giavarini C, Sartori F. Characteristics of polymers and polymer-modified binders. Transportat Res Record 1996;1535:36–47. [42] Oliver JWH. Development of PMB specifications for asphalt rut resistance. In: Proceedings of AAPA International Flexible Pavements Conference, vol. I, Kew, Australia, 1997. [43] Lee DY, Demirel T. Beneficial effects of selected additives on asphalt cement mixes. Final Report. ISU-ERI-Ames88070 Report Number IDOT Project HR-278. Ames, Iowa: Engineering Research Institute, Iowa State University, 1988. [44] DÕAngelo J. Modified binders and Superpave Plus specifications. Federal Highway Administration. No date. Available from: http://www.asphaltinstitute.org/upload/Modified_Binders_SP_Plus_ Specification.pdf.
MATERIALS
Construction and Building Materials 21 (2007) 66–72
www.elsevier.com/locate/conbuildmat
Polymer modified asphalt binders Yetkin Yildirim
*
Department of Civil Engineering, University of Texas at Austin, 3208 Red River CTR 318, Austin, TX 78705, USA Received 19 August 2004; received in revised form 5 July 2005; accepted 21 July 2005 Available online 19 September 2005
Abstract This paper is a review of research that has been conducted on polymer modified binders over the last three decades. Polymer modification of asphalt binders has increasingly become the norm in designing optimally performing pavements, particularly in the United States, Canada, Europe and Australia. Specific polymers that have been used include rubber, SBR, SBS and ElvaloyÒ. Specifications have been designed and pre-existing ones modified to capture the rheological properties of polymer modified binders. The elastic recovery test is good at determining the presence of polymers in an asphalt binder, but is less successful at predicting field performance of the pavement. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Polymer modified binder; Asphalt; Binder specifications; Elastic recovery; SBR; SBS; Elvaloy; Rubber
1. Introduction
2. History, use, and benefits
The addition of polymers, chains of repeated small molecules, to asphalt has been shown to improve performance. Pavement with polymer modification exhibits greater resistance to rutting and thermal cracking, and decreased fatigue damage, stripping and temperature susceptibility. Polymer modified binders have been used with success at locations of high stress, such as intersections of busy streets, airports, vehicle weigh stations, and race tracks [1]. Polymers that have been used to modify asphalt include styrene–butadiene–styrene (SBS), styrene–butadiene rubber (SBR), ElvaloyÒ, rubber, ethylene vinyl acetate (EVA), polyethylene, and others. Desirable characteristics of polymer modified binders include greater elastic recovery, a higher softening point, greater viscosity, greater cohesive strength and greater ductility [1,2].
Processes of asphalt modification involving natural and synthetic polymers were patented as early as 1843 [3]. Test projects were underway in Europe in the 1930s, and neoprene latex began to be used in North America in the 1950s [1]. In the late 1970s, Europe was ahead of the United States in the use of modified asphalts because the European use of contractors, who provided warranties, motivated a greater interest in decreased life cycle costs, even at higher initial costs. The high preliminary expenses for polymer modified asphalt limited its use in the US [4]. In the mid-1980s, newer polymers were developed and European technologies began to be used in the US [5,6]. At the same time, the prevalence of a long-term economic outlook in the country increased [1]. In Australia, the current National Asphalt Specification includes guides and specifications regarding polymer modified binders [7]. The United States Federal Highway Administration (FHWA) has developed a life cycle cost analysis approach, which can be used to evaluate the life cycle costs
*
Tel.: +1 512 232 1845; fax: +1 512 475 7914. E-mail address: [email protected].
0950-0618/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2005.07.007
Y. Yildirim / Construction and Building Materials 21 (2007) 66–72
of pavement containing asphalt rubber binders as well as other treatments. The findings indicated that asphalt rubber is cost effective as it is used, for example, in Arizona and California, although the estimated life of the pavement is based on interviews and engineering judgment, and can be refined as the pavement ages and long-term field performance is included in the model [8]. A 1997 survey of state departments of transportation in the United States found that 47 states of the 50 reported that they would be using modified binders in the future, 35 of them saying that they would use greater amounts [9]. Several research teams around the world have worked on evaluating the benefits of polymer modification on pavement performance, and tests and specifications for binders are continually being developed. In a 2001 study for the Ohio Department of Transportation, Sargand and Kim [10] compared the fatigue and rutting resistance of three PG 70–22 binders, one unmodified, one SBS modified, and one SBR modified. It was found that the modified binders were more resistant to both fatigue and rutting than the neat binder, even though all three had the same performance grade. According to a 2003 Nevada study, the viscosity of polymer modified binders tends to be significantly greater than that of non-modified binders at 60 °C, although penetration changes only slightly at all temperatures [11]. In 2003, Newcomb [12] discussed the concept of perpetual pavements in Hot Mix Asphalt, claiming that it is a misconception that fatigue cracking is inevitable. Many full-depth hot mix asphalt (HMA) pavements built 30–40 years ago have yet to exhibit any fatigue cracking, and Newcomb claims that research shows that increasing polymer modified binders at the bottom of the asphalt layer may raise the fatigue limit of the pavement. A 2003 US Army Corps of Engineers study [13] points out that for optimal economy, it is desirable to choose an asphalt modifier that resists multiple distresses, such as rutting, fatigue, thermal cracking and water damage. It was found that the choice of polymer may have a significant impact on fatigue properties, and that the mixtures boasting the highest fatigue life contained reactive styrene–butadiene crosslinked polymer. Other polymers tested were a chemically modified crumb rubber, SBR, linear block SBS and a proprietary modified SBS.
67
tegic Highway Research Program (SHRP). Bahia et al. in their 1998 article for the Journal of AAPT note, however, that this blanket testing method failed to test the extreme grades required by the new, modified binders, resulting in the initiation of new testing protocols for modified binders [14]. New test protocols include measuring the softening point using a ring and ball apparatus (ASTM E 28) to determine the resistance to flow at high temperatures and a force ductility test that measures tensile properties [15]. Several tests have been developed to look at elastic recovery, one of the major areas of improvement in elastomer modified asphalt. Thompson and Hagman developed a torsional recovery test, included in California specifications for identifying the presence of elastomers [1]. The elastic recovery test using a ductilometer, described later in this paper, is included in the Task Force 31 Specifications and is used in the US and Europe [1]. King et al. [1] point out that many tests exist to identify whether modification is present, such as the IR, low temperature ductility and torsional recovery. The West Coast User Producer Group tried to use performance based asphalt (PBA) specifications, which involve a high temperature viscosity test and low temperature penetration and ductility tests, as specifications for modified asphalt, but they were not as good at predicting performance with modified asphalt as they were with neat asphalt [1]. In 1998, Blankenship et al. [16] conducted field and laboratory tests in Kentucky and found that PG 70–22 made using different methods of modification gave different results for laboratory tests. They used the Dynamic Shear Rheometer (DSR) and Bending Beam Rheometer (BBR) tests to identity five different PG 70–22 binders, two SBS-modified, one SBR modified, one chemically modified, and one neat and compared their behavior in various tests. These binders were found to differ as far as rutting, moisture damage and modulus testing, although the rutting difference was no more than 10 mm between the binders. In 2004, Yildirim et al. [17] utilized a design method for determining the modification level of asphalt binders using waste toner, which contains styrene acrylic copolymers. Binder designs were performed including blending time, performance grading, storage stability and, mixing and compaction temperature calculations. Test results indicated that the stiffness of the blend increases as the percentage of the toner content increases.
3. Test methods 4. Specific modifiers As discussed by King et al. in the 1999 Journal of the AAPT [1], there are several test methods that have been developed or altered for modified binders. Previously, both modified and unmodified binders alike were tested according to the same methods, supported by the Stra-
4.1. Rubber ‘‘Crumb rubber modifier’’ (CRM) and ‘‘asphalt-rubber’’ are terms that refer to applications in which ground
68
Y. Yildirim / Construction and Building Materials 21 (2007) 66–72
recycled rubber and paving asphalt are combined [1]. Characteristics of asphalt–rubber are dependent on rubber type, asphalt composition, size of rubber crumbs, and time and temperature of reaction [1]. Usually, the rubber is recycled from used automotive tyres, which has the additional benefits of saving landfill space that would otherwise be occupied by tyres and reducing cost [1,18]. Natural rubber modification results in better rutting resistance and higher ductility but the modifier is sensitive to decomposition and oxygen absorption. Due to its high molecular weight, it has problems of low compatibility [18]. Recycled tyre rubber reduces reflective cracking, which increases durability. There are some practical problems in using natural rubber: it needs high temperatures and long digestion times in order to be dispersed in the bitumen [18]. In 1991, the Intermodal Surface Transportation Efficiency Act (ISTEA) section 1038 was passed into law in the USA. In its Declaration of Policy, ISTEA states ‘‘It is the policy of the United States to develop a National Intermodal Transportation System that is economically efficient, environmentally sound, provides the foundation for the Nation to compete in the global economy, and will move people and goods in an energy efficient manner’’ [19]. The act required that, starting in 1994, 5% of roads built with federal funds must use pavement made with crumb rubber, processed recycled tyres, or modified asphalt. By 1997, 20% of roads built with federal funds were required to use recycled tyres in the pavement [20]. On the other hand, the Used Tyre Working Group [21] describes the United Kingdom as still being in the process of evaluating a pilot project involving road surfacing that contains recycled tyres. 4.2. Styrene–butadiene–styrene Styrene–butadiene–styrene (SBS) is a block copolymer that increases the elasticity of asphalt [18]. According to a 2001 review in Vision Tecnologica by Becker et al. [18], it is probably the most appropriate polymer for asphalt modification, although the addition of SBS type block copolymers has economic limits and can show serious technical limitations. Although low temperature flexibility is increased, some authors claim that a decrease in strength and resistance to penetration is observed at higher temperatures. Nonetheless, ‘‘SBS is the most used polymer to modify asphalts, followed by reclaimed tire rubber’’ [18]. The Danish Road Directorate [22] found that an SBS-modified binder course showed no superior rut resistance compared to other Danish asphalt courses. Asphalt cores taken from the job site indicated that separation had occurred, and that the polymer phase was not homogeneously distributed, which might have
been the cause of the poor performance of the pavement. As reported in the Journal of Material in Civil Engineering, transmission electron microscopy was used in 2002 to better understand the behavior of SBS in asphalt binders [23]. Depending on the sources of asphalt and polymer, morphology varies: there can be a continuous asphalt phase with dispersed SBS particles, a continuous polymer phase with dispersed globules of asphalt, or two interlocked continuous phases. It is the formation of the critical network between the binder and polymer that increases the complex modulus, an indication of resistance to rutting. In 2003, in the Journal of the AAPT, Mohammed et al. [24] looked at the possibility of recycling SBS modified asphalt for resurfacing pavement. They found that the impact of the extraction and recovery process on the binder was minimal. Eight-year-old SBS modified binder was recovered from Route US61 in Louisiana, and was found to have experienced intensive oxidative age hardening. At low temperatures, the binder was quite brittle. Blends of virgin and recovered polymer modified binder were found to be stiffer than anticipated at both low and high temperatures. It was also found that as the percentage of recovered binder increased, rutting resistance increased, while fatigue resistance decreased. In 2004, the Florida Department of Transportation and FHWA published a report [25] looking at the effect of SBS modification on cracking resistance and healing characteristics of Superpavee mixes. They found that SBS benefited cracking resistance, primarily due to a reduced rate of micro-damage accumulation. SBS did not, however, have an effect on healing or aging of the asphalt mixture. The possibility of using SBS-modified binders in India has been investigated recently [26]. Calculations indicated that the surface life of the Delhi–Ambala expressway would be almost doubled while the thickness of the bituminous layers would be reduced, although the cost per km would be greater for polymer modified binders. 4.3. Styrene–butadiene–rubber Styrene–butadiene–rubber (SBR) has been widely used as a binder modifier, usually as a dispersion in water (latex). An Engineering Brief from 1987 available at the US Federal Aviation Administration website [2] describes the benefits of SBR modified asphalt in improving the properties of bituminous concrete pavement and seal coats. Low-temperature ductility is improved, viscosity is increased, elastic recovery is improved and adhesive and cohesive properties of the pavement are improved. The benefit of latex is that the rubber particles are extremely small and regular. When they are exposed to asphalt during mixing they disperse
Y. Yildirim / Construction and Building Materials 21 (2007) 66–72
rapidly and uniformly throughout the material and form a reinforcing network structure. According to Becker et al., SBR latex polymers increase the ductility of asphalt pavement [18], which allows the pavement to be more flexible and crack resistant at low temperatures, as found by the Florida Department of Transportation [25]. SBR modification also increases elasticity, improves adhesion and cohesion, and reduces the rate of oxidation, which helps to compensate for hardening and aging problems [25]. In a 1999 laboratory test at the Texas Transportation Institute, it was found that coating smooth, rounded, siliceous gravel aggregates with cement plus SBR latex for use in HMA increased stability according to Hveem and Marshall standards, as well as tensile strength, resilient modulus and resistance to moisture damage. Coated aggregates have greater resistance to rutting and cracking [27]. Water-based SBR latex has been widely used to improve chip retention in emulsions, but SBS has gradually replaced latex because of its effect of greater tensile strength at strain, and because it is compatible with a broader range of asphalts [1]. Elastomers such as SBR and SBS have a significant effect on the results of the ductility test at both 4 and 25 °C; while SBR modified asphalts have high ductility at all temperatures, SBS modified asphalts tend to have lower ductility [1]. 4.4. ElvaloyÒ The Duponte website [28] describes ElvaloyÒ as an ethylene glycidyl acrylate (EGA) terpolymer that chemically reacts with asphalt. As a result of the reaction, problems with separation during storage and transportation are avoided. Roads using ElvaloyÒ have been in use since 1991. In 1995 Witczak, Hafez and Qi [29] studied the laboratory performance of asphalt modified with ElvaloyÒ at the University of Maryland. Two different grades of asphalt were each modified by 0%, 1.5% and 2.0% ElvaloyÒ by weight of binder. The susceptibility of the mixtures to moisture damage was found to be greatly decreased by the addition of ElvaloyÒ. In addition, an
% Recovery ¼
69
ElvaloyÒ in combination with granite had a significantly higher (poorer) fracture temperature than with diabase, limestone or granite aggregate treated with hydrated lime. At the DuPont Institute, Babcock et al. [31] devised a lap shear test for high temperature binder properties, which appears to agree with high temperature DSR measurements. The results indicated that binder failure at temperatures above 6 °C tends to be cohesive failure, due the loss of integrity within asphalt. On the other hand, around 6 °C and colder, failure is adhesive, from a loss of adhesion between the binder and the substrate. Since this indicates that cold temperature failure of a road may be the result of loss of adhesion to the aggregate, a chemically reactive polymer is expected to perform better, and reactive elastomeric terpolymer does in fact perform better in this test than SBS or the control neat bitumen.
5. Elastic recovery test Elastic recovery (or elasticity) is the degree to which a substance recovers its original shape following application and release of stress. A degree of elastic recovery is desirable in pavement to avoid permanent deformation. ‘‘When a tire passes over a section of pavement, it is desirable for that pavement to have the ability to ÔgiveÕ, but it is equally important for it to recover to its original shape,’’ according to the Asphalt Institute website [32]. 5.1. Measurement and calculation The elastic recovery of asphalt is measured with the aid of a ductilometer, which is used to elongate an asphalt specimen at a constant rate. After a period of time, the elongated specimen is cut and then allowed to rest. After the period of rest is complete, the distance between the ends of the cut specimen is measured [33]. The elastic recovery is the ratio between the difference in elongation between cutting and the end of the rest period, and the total elongation applied [33].
Initial elongation Observed elongation after rejoining sample 100 Initial elongation
analysis of repeated load permanent deformation behavior showed that increasing concentrations of ElvaloyÒ resulted in a marked decrease in deformation. In a study on low-temperature rheological properties of polymer modified binders, the FHWA [30] found that
5.2. Binder characterization The elastic recovery test is used to test polymer modified binders by the departments of transportation of several states in the US and several other countries, as well
70
Y. Yildirim / Construction and Building Materials 21 (2007) 66–72
as by researchers around the world. According to the Spring 2002 edition of the Asphalt Technology News [34], Kansas, Louisiana and Texas require use of the elastic recovery test to ensure that binders have been modified. Michigan also uses it, although it does not require it, and Kentucky uses it to test PG 76–22 binders. It is also used to characterize polymer modified binders in Quebec, Sweden, Finland and Switzerland [35]. In 1981, Oliver developed the elastic recovery test for the Australian Road Research Board to measure deformation response of rubber modified binders [36,37]. He found that binders with natural (truck-tyre) rubber showed greater elastic recovery than synthetic (car-tyre) rubber. In 1997, it was reported to the Australian Asphalt Pavement Association that even low concentrations of SBS caused an increase in elastic recovery, softening point, viscosity and cohesive strength [38]. In 1990, Valkering and Vonk [39] compared SBS and EVA modified binders to neat binders and found that SBS modified binders had significantly higher elastic recovery than neat binders. Compared to SBS, EVA modified binders showed a lesser degree of improvement in elastic recovery and also lost ductility and elastic recovery much more rapidly. Braga and Corrieri [40] used the elastic recovery test to compare the resistance to thermal degradation of SBS and heterophasic polyolefin (TPO) modified binders. Aged SBS polymers following showed a lower resistance to thermal degradation than aged TPO binders. Several studies have investigated the relationship between measurements of elastic recovery and other measures of performance, in both laboratory and field tests. In the Transportation Research Record 1996, Bonemazzi et al. [41] compared the performance of binders modified with an array of polymers (atactic propylene– ethylene copolymer, low- and high-density polyethylene, ethylene/propylene rubber, ADFLEX, ethylene methacrylate copolymer, EVA, thermoplasticpolyolefinic terpolymer, and SBS linear and radial block copolymers) in tests of penetration and elastic recovery as well as the rheometer dynamic test. All the tests were shown to be good measurements of polymer contribution to binder performance. In 1997, Oliver [42] examined the relationship between the rheological properties of asphalt mixes and rutting resistance using the wheel tracking test. While a relationship between polymer consistency and rut resistance was found, no relationship was apparent between rut resistance and elastic recovery or softening point. In the Journal of the AAPT, 1998, Bahia, Perdomo and Turner [9] compared five modified binders, measuring elastic recovery, ductility and resilience. They found that these conventional measurements were inconsistent in ranking the suitability of polymer modified binders. Specifically, rankings changed as strain level changed.
Superpavee testing results were equally inconsistent in ranking modified binders. In a 1988 Iowa Department of Transportation report, Lee and Demirel [43] compared viscosity, penetration, softening point, force ductility, elastic recovery and several other characteristics of binders with SBS, polyolephins, neoprene, SBR latex and hydrated lime. There was no correlation between the different types of measurement. John DÕAngelo, on the Asphalt Institute web page [44], points out that the literature suggests that most tests of modified binders may only measure whether a polymer modifier is present, not its effect on the field performance of the modified binder.
6. Conclusions In the 1980s, polymer modified asphalts began to be used in the US and by 1997 all but three states were already using modified binders or intended to use them in the future and federal regulations supported their use. Pavements made with modified binders are more resistant to fatigue, thermal cracking, rutting, stripping, and temperature susceptibility than neat binders. Polymer modified binders tend to exhibit increased viscosity and elastic recovery, although penetration does not appear to be influenced by modification. An ideal modifier will increase binder resistance to multiple types of distresses. Modification is not without its drawbacks, however, since compatibility between an asphalt and a modifier is not assured, and separation during storage or application, if not addressed, can result in poorly performing pavement. Since Superpavee specifications were designed for neat binders, they are inappropriate for polymer modified binders. In fact, asphalts modified with different polymers can behave very differently even when they have the same performance grade. Test methods that have been developed or altered for modified binders include measuring the softening point and elastic recovery, and a force ductility test. There is disagreement about whether bending beam rheometer (BBR) tests, developed for Superpavee, are acceptable for polymer modified binders. In general, it seems that the results of rheological tests are not indicative of the performance of polymer modified binders. Several tests exist to identify whether modification is present, such as the IR, low temperature ductility and torsional recovery. In 1991, the ISTEA required that an increasing proportion of roads use modified asphalt. Different polymers impact characteristics of asphalt to differing degrees. Natural rubber improves rutting resistance and ductility, but is sensitive to decomposition and often has problems of compatibility.
Y. Yildirim / Construction and Building Materials 21 (2007) 66–72
The use of tyre rubber as an asphalt modifier is environmentally responsible and results in decreased rutting and reflective cracking, but special conditions, such as high mixing temperatures and long digestion times, need to be maintained to prevent separation from the asphalt binder. The addition of SBR to asphalt improves low-temperature ductility, increases viscosity, improves elastic recovery and improves the adhesive and cohesive properties of the pavement. Water-based SBR latex was used commonly to improve chip retention in emulsions. SBS has been replacing SBR due to the formerÕs wider compatibility and greater tensile strength under strain. SBS is now the polymer most used to modify asphalt. SBS increases the elasticity of asphalt and SBS modified asphalt can be recycled. SBS modified binders have been found to perform better at low temperatures than neat binders or binders modified with chemically reactive polymers. ElvaloyÒ is a modifier that forms a chemical bond with the asphalt, avoiding problems of separation during storage, transportation and application. It increases pavement moisture resistance and results in modified asphalt performing better in high temperature DSR tests. Elastic recovery of asphalt, a measurement widely used to test polymer modified binders, can be measured by elongating an asphalt sample, cutting it, allowing it to rest, and determining the degree to which the elongated specimen returns to its original length. The elastic recovery test has been shown to be a good measurement of polymer contribution to binder performance, although no relationship appears to exist between rut resistance and elastic recovery. Elastic recovery and other conventional measurements are inconsistent in ranking polymer modified binder performance and may only measure whether or not a modifier is present in an asphalt specimen, not its contribution to the asphaltÕs performance. Polymer modified binders have had proven success in the field and the laboratory, and a continuing effort is being made to develop a correlation between results from laboratory tests and field performance.
References [1] King G et al. Additives in asphalt. J Assoc Asphalt Paving Technol A 1999;68:32–69. [2] Bates R, Worch R. Engineering Brief No. 39, Styrene–butadiene rubber latex modified asphalt. Federal Aviation Administration, Washington, DC, 1987. Available from: http://www.faa.gov/arp/ engineering/briefs/eb39.htm. [3] Thompson DC. Hoiberg AJ, editor. Bituminous materials: Asphalt tars and pitches. Robert Krieger Publishing Co.; 1979.
71
[4] Terrel R, Walter J. Modified asphalt pavement materials: The European experience. J Assoc Asphalt Paving Technol 1986;55:482. [5] Brule B. Polymer-modified asphalt cements used in the road construction industry: Basic principles. Transportat Res Record 1996;1535. [6] Collins J. Thermoplastic block copolymers for the enhancement of asphaltic binders in paving applications. Presented at the Paving and Transportation Conference, Albuquerque, NM, 1986. [7] Australian Asphalt Pavement Association. National asphalt specification, 2nd ed. Kew, Australia, 2004. [8] Hicks RG, Epps JA. Life cycle cost analysis of asphalt–rubber paving materials. In: Proceedings of the First International Conference on the World of Asphalt Pavements, Sydney, Australia, 2000. p. 69–88. [9] Bahia HU, Perdomo D, Turner P. Applicability of Superpave binder testing protocols to modified binders. Transportat Res Record 1997;1586:16–23. [10] Sargand SM, Kim SS. Performance evaluation of polymer modified and unmodified Superpave mixes. In: Second International Symposium on Maintenance and Rehabilitation of Pavements and Technological Control, Auburn, AL, 2001. [11] Sebaaly PE, Bazi GM, Vivekanathan Y. Evaluation of new pavement technologies in Nevada, University of Nevada, 2003. [12] Newcomb D. Limit the strain at the bottom of an asphalt pavement, and what do you get? A perpetual pavement. Hot Mix Asphalt Technol 2003;8(6):30–2. [13] Partl MN, Newman JK. Flexural beam fatigue properties of airfield asphalt mixtures containing styrene–butadiene based polymer modifiers. In: Proceedings of the Sixth International Rilem Symposium, Zurich, Switzerland, 2003. p. 357–63. [14] Bahia H, Hislop WP, Zhai H, Rangel A. Classification of asphalt binders into simple and complex binders. J Assoc Asphalt Paving Technol 1998;67:1–41. [15] Chaverot P. The force ductility test: A simple means of characterization of polymer modified asphalt. In: Proceedings of the Fourth Eurobitume Symposium, Madrid, Spain, 1989. p. 96–100. [16] Blankenship P et al. Are All PG 70-22Õs the same? Lab test on KY I-64 field samples. J Assoc Asphalt Paving Technol (AAPT) 1998;67:493–552. [17] Yildirim Y, Hazlett D, Davio R. Toner-modified asphalt demonstration projects. Resources, conservation and recycling, vol. 42. Amsterdam: Elsevier; 2004. pp. 39–50. [18] Becker Y, Me´ndez MP, Rodrı´guez Y. Polymer modified asphalt. Vis Technol 2001;9(1):39–50. [19] Camph DH, Siwek S, Forestieri F. How to keep America moving: Report on the US Department of TransportationÕs outreach on reauthorization of the Intermodal Surface Transportation Efficiency Act (ISTEA). In: Elles-Boyle A, editor. Office of Governmental Affairs, Washington, DC, 1997. [20] Blumenthal M. Changes impacting the ground rubber industry. Rubber Manufacturers Association, Montreal, Canada. 2002. Available from: http://www.rubberassociation.ca/pdf_docs/ RR2002/Michael2.pdf. [21] Used Tyre Working Group. Recycling and recovery. No date. Available from: http://www.tyredisposal.co.uk/ disposal_routes. asp. [22] Wegan V, Nielsen BC. Microstructure of polymer modified binders in bituminous mixtures. Report Number 87-90145-85-8. Danish Road Directorate, Roskilde, Denmark, 2001. [23] Chen JS, Liao MC, Shiah MS. Asphalt modified by styrene– butadiene–styrene triblock copolymer: Morphology and model. J Mater Civil Eng 2002;14(3):224–9. [24] Mohammed LN et al. Investigation of the use of recycled polymer modified asphalt binder in asphalt concrete pavements
72
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
Y. Yildirim / Construction and Building Materials 21 (2007) 66–72 (With discussion and closure). J Assoc Asphalt Paving Technol 2003;72:551–94. Roque R, Birgisson B, Tia M, Kim B, Cui Z. Guidelines for the use of modifiers in Superpave mixtures: Executive summary and volume 1 of 3 volumes: Evaluation of SBS modifier. State Job 99052793. Florida Department of Transportation, Tallahassee, FL, 2004. Shukla RS, Singh VKP, Bhanwala RS. Polymer modified bitumen for construction of heavy traffic density corridors. Indian Highways 2003;31(4):55–66. Kim MG, Button JW, Park DW. Coatings to Improve low-quality local aggregates for hot mix asphalt pavements. Report SWUTC/ 99/167405-1, Texas Transportation Institute, Texas A&M University, College Station, TX, 1999. DuPont. About DuPonte ElvaloyÒ RET reactive elastomeric terpolymer. DuPont website. 2004. Available from: http:// www.dupont.com/asphalt/link1.html. Witczak MW, Hafez I, Qi X. Laboratory characterization of ElvaloyÒ modified asphalt mixtures: vol. I – Technical report. College Park, Maryland: University of Maryland. 1995. Available from: http://www.dupont.com/asphalt/link5.html. Chappat M, Poirier JE, Robert M, Durand G. Appreciation by rheological and tensile tests methods of the impact of ageing of polymer modified binders toward low temperature behaviour. Revue Ge´ne´rale des Routes 2000:22–4. Babcock GB, Statz RJ, Larson DS. Study of asphalt binders using lap shear bonds. Wilmington, DE: DuPonte website. No date. Available from: http://www.dupont.com/asphalt/ lapshear/lapshear.html. Asphalt Institute. Individual asphalt binder tests. Lexington, KY. 2003. Available from: http://www.asphaltinstitute.org/ai_pages/ Lab_Services/Lab_Services_Indiv_Binder_Tests_List.asp. Hobson K. No date. Elastic recovery testing in Oklahoma. Available from: http://www.utexas.edu/research/ superpave/articles/er.html. Asphalt forum responses. Asphalt Technology News 2002;14(1):12–13. Available from: http://www.eng.auburn.edu/ center/ncat/newsl/newssprng_02.pdf.
[35] Zubeck H, Raad L, Ryer J. Constructability of polymer-modified asphalts and asphalt-aggregate mixtures in Alaska: Final report. Alaska: State of Alaska Department of Transportation and Public Facilities. 1999. Available from: http://www.dot.state.ak.us/stwddes/research/assets/pdf/fhwa_ak_rd_99_1.pdf. [36] Oliver JWH. Modification of paving asphalts by digestion with scrap rubber. Transportat Res Record 1981;821:37–44. [37] Oliver JWH. Enhancing bitumen products with scrap rubber. Presented at 19th ARRB Regional Symposium. Vermont South, Australia, 1982. p. 117–31. [38] Vonk W. Polymer modification of bitumens: Manufacture and application of premium binders for improved spray seal performance. Presented at 10th AAPA International Flexible Pavements Conference, Kew, Australia, 1997. [39] Valkering CP, Vonk W. Thermoplastic rubbers for the modification of bitumens: Improved elastic recovery for high deformation resistance of asphalt mixes. In: Proceedings of 15th Australian Road Research Board Conference, Vermont South, Australia, 1990. p. 1–19. [40] Braga V, Corrieri R. A comparative study of SBS and TPO based PMB. In: Proceedings of the Papers Submitted for Review at Second Eurasphalt and Eurobitume Congress, Breukelen, Netherlands, 2000. [41] Bonemazzi F, Braga V, Corrieri R, Giavarini C, Sartori F. Characteristics of polymers and polymer-modified binders. Transportat Res Record 1996;1535:36–47. [42] Oliver JWH. Development of PMB specifications for asphalt rut resistance. In: Proceedings of AAPA International Flexible Pavements Conference, vol. I, Kew, Australia, 1997. [43] Lee DY, Demirel T. Beneficial effects of selected additives on asphalt cement mixes. Final Report. ISU-ERI-Ames88070 Report Number IDOT Project HR-278. Ames, Iowa: Engineering Research Institute, Iowa State University, 1988. [44] DÕAngelo J. Modified binders and Superpave Plus specifications. Federal Highway Administration. No date. Available from: http://www.asphaltinstitute.org/upload/Modified_Binders_SP_Plus_ Specification.pdf.