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End-of-life management of scrap tyres using crumb rubber modified TLA.

Introduction

Most developing countries struggle to properly manage their solid waste due to inadequate service coverage, limited utilisation of recycling activities and inadequate landfill disposal sites [1]. Waste tyres are among the most problematic type of municipal solid waste to dispose of, largely due to their high durability and long life span [2]. Moreover, waste tyres pose environmental, health and fire risks because they provide breeding grounds for rats, mice, vermines and mosquitoes [3]. In addition, scrap tyres waste valuable landfill space because they contain approximately 75% void space, and as such landfilling is not a good end-of-life disposal technique [2]. Landfilling is also becoming unacceptable because of the rapid depletion of available sites for waste disposal [4]. As such, the reuse of bulky wastes is considered the best environmental alternative for solving the problem of disposal [5]. However, most of the developing third world countries have yet to raise their awareness regarding recycling of waste materials and have not developed effective legislation with respect to the local reuse of waste materials [6].

While there are no accurate data on the number of scrap tyres currently stockpiled in Trinidad and Tobago, the problem of disposal is worsening due to the steady increase in the number of imported vehicles, the import of tyres to be used as replacement for scrap tyres, as well as the absence of an export market for tyres. For the period 2003 to 2007, approximately 1.5 million tyres were imported into Trinidad and Tobago each year, while only 15% of this amount was exported (1). This data suggests that, on average, 85% or 1.275 million tyres remain in the country each year (2). The abundance of waste tyres on the island therefore requires adequate end-oflife disposal techniques. Siddique [4] outlines innovative solutions to meet the challenge of tyre disposal such as: use of tyre rubber in asphaltic concrete mixtures; incineration of tyres for the production of steam; and reuse of ground tyre rubber in a number of plastic and rubber products. Scrap tyres have also been used as a fuel for cement kilns, feedstock for making carbon black and artificial reefs in marine environment [7]. According to Sunthonpagasit and Duffey [8], one major end-of-life technique for disposing of waste tyres is the application of crumb rubber or ground rubber.

Many researchers have studied the use of recycled tyres relating to applications such as asphalt pavement, waterproofing systems and membrane liners [9]. This study explores the use of scrap tyre rubber in an asphaltic mixture as a possible solution for dealing with the disposal of scrap tyres on the Caribbean island of Trinidad and Tobago. Previous studies conducted on the performance of asphalt and polymer mixtures (such as tyre rubber) conclude that there exists a clear relationship between the differences in the quality of asphalt from different sources and the resulting performance characteristics [10 - 13]. Research investigating the influence of tyre rubber on the mechanical properties of Trinidad Lake Asphalt (TLA) is limited and this paper attempts to fill this gap. Therefore, the main aim of this study is to investigate the influence of tyre rubber on the rheological properties of TLA, with a view to its potential use as an end-of-life management option for the disposal of scrap tyres in Trinidad and Tobago.

Background

The necessity for effective management of the vast volumes of used car tyres worldwide has led to the development and utilisation of several methods for recovering some of the added value in these products, reducing at the same time the negative environmental effects of uncontrollable disposal or landfilling [16]. Such methods include energy recovery, recycling, retreading and direct reuse in secondary markets or applications [16]. One promising disposal method is incorporating scrap tyre rubber (in the form of crumb rubber) in different applications. Markets for crumb rubber include asphalt modifications, molded products, sports surfacing, construction applications and animal bedding. Sunthonpagasit and Duffey [8] provide an estimate of crumb rubber markets in North America, showing that the largest reuse markets for scrap tyres are for tyre-derived fuels (approximately 33%); civil engineering applications in which tyres are shredded for applications such as leachate collection in landfills (15%); and crumb rubber or ground rubber (approximately 12%).

The Scrap Tire Management Council [2] defines crumb rubber as 'rubber that has been reduced to a particle size of 3/8 in or less'. The process of making crumb rubber involves shredding the tyres and removing the steel and polyester fragments. The remaining rubber chips are further reduced to crumb-like particles, which can then be used in many different applications, from road paving to crack sealants. Epps [17] explains that crumb rubber is made either mechanically or cryogenically (using very low temperature to change the tyre material properties), although due to cost, mechanical sizing by chopping and grinding is most often used.

When crumb rubber is used in asphalt paving it is termed 'crumb rubber modifier' [18]. Crumb rubber modifier (CRM) has been incorporated in asphalt mixtures and in a number of asphalt paving products for many years. In fact, polymer modification of asphalt binders has increasingly become the norm in designing optimally performing pavements, particularly in the United States, Canada, Europe and Australia [19]. Chiu [20] points out that using tyre rubber in pavements is not only economically beneficial in that it improves pavement performance, but it is also environmentally favourable by offering a better life-cycle for scrap tyres. The performance of road pavements containing crumb rubber has been discussed in several studies such as [17] and [18]. In general, the addition of tyre rubber to asphalt has been shown to improve the durability of the asphalt, resulting in increased resistance to cracking, reduce temperature susceptibility, improved oxidation and aging resistance, as well as improved resistance to permanent deformation. However, the size, shape and texture of the CRM have a significant effect on the performance of the asphaltic mixture [18]. The performance also depends on other factors such as the crumb rubber particle size, the chemical/physical properties of the bitumen, as well as the bitumen source [21]. Moreover, the performance of asphalt rubber (3) depends on the quality (composition) of the asphalt itself, as asphalt from different sources give different performance qualities as documented by researchers such as [10 - 13]. Thodesen et al. [22] point out that accurately predicting the properties of crumb rubber modified binders has proven difficult as these properties tend to vary with changing crumb rubber concentrations and temperatures. Although it may seem economically and environmentally beneficial to combine rubber and asphalt, the compatibility of both materials is vital. Lesueur [23] explains that ways to predict whether a particular polymer (e.g. tyre rubber) will be compatible with a given bitumen (such as asphalt) are not well defined and usually depends on laboratory experiments, rather than theoretical predictions. It is within this framework that we seek to use laboratory experiments to investigate the influence of scrap tyre rubber on the mechanical properties of Trinidad Lake Asphalt. With the availability of TLA and an abundance of scrap tyres in Trinidad, there is the possibility that crumb rubber modified TLA may be prove to be a win-win situation for the country - providing a feasible end-of-life disposal method for scrap tyres while improving the physical properties of the asphalt rubber mixture. However, because there is a paucity of research work investigating the effects of modifying TLA with scrap tyre rubber, this paper is a starting point for further investigations that will provide information for the effective reuse of waste tyres locally.

Area of study

Trinidad Lake Asphalt is possibly the most famous source of natural bitumen [24]. It occurs naturally in the form a 100 acre 'lake' in La Brea, in the southwest of Trinidad in the West Indies. The material comprises a mixture of bitumen (53-55%), water, and very fine mineral matter (36-37%) [25]. TLA is well known for its consistent properties, stability and durability, and is widely used for bridges and airports applications where high stability surfaces are required [26]. TLA is a well established commercial product and typically a 50:50 blend of TLA and bitumen is adopted in the production of TLA modified asphalt [27]. TLA has also given consistent performance under varying environmental conditions, in addition to providing significant savings as a result of enhanced pavement cycles [28]. These savings result from enhanced physical properties provided by TLA such as increased resistance to fatigue and cracking, enhanced durability and improved temperature susceptibility [28]. According to Asphalt Associates [24] TLA can easily be added to any asphalt mixture, in any quantity, at any time, and mixed at elevated temperatures with no degradation or loss of performance.

Materials and Methods

Mix design

In this study, laboratory experiments were used to study the rheological properties of 3 different blends of crumb rubber modified TLA mixtures, each with a different rubber concentration. Rheology is the science used to measure and analyse the viscosity, complex modulus and phase angle of polymer-modified bitumen [27]. For our study, complex modulus and phase angles were studied. Complex modulus (G*) is a measure of a material's stiffness or resistance to compressive deformation [23, 27]. Phase angle is used to demonstrate the viscoelastic response of bituminous materials. Higher values for phase angle indicate a tendency towards more viscous behaviour, whilst lower phase angles indicate more elastic response. The elastic behaviour (lower phase angle) is generally associated with high stiffness and increased brittleness, while the viscous response (higher phase angle) reflects high ductility and low stiffness. These two properties were selected for this study because according to Navarro et al. [29], these are the parameters that have the largest impact on a material's durability and susceptibility. If asphalt rubber is to be used in road pavement applications, because of increased traffic volumes and vehicle loads, there is a need to improve conventional bitumen properties, particularly the resistance to rutting (permanent deformation of road pavement in the form of ruts or corrugations) and thermal cracking (fracture of road pavement due to the lack of flexibility at low temperatures) [30]. As such, 3 blends of different crumb rubber to TLA ratio (by weight) were made and the complex modulus and phase angles of each blend were compared to the unmodified TLA (0% crumb rubber), which was used as the control mixture. The asphalt rubber blends contained 2%, 5% and 10% crumb rubber (by dry weight of the total mixture). This is consistent with similar studies; for example, asphalt mixtures containing rubber content of 1%, 2% and 3% by weight of total mix are used in [31]; and rubber concentrations of 0%, 5%, 10%, and 15% are used in [22].

Laboratory experiments

A scrap 'Dunlop' car tyre was obtained and shredded. The steel was removed and the pieces of rubber immersed in liquid nitrogen. On achieving the requisite rigidity the rubber strips were grounded. The resulting crumb rubber of various particle sizes was then dried and sieved to attain mean diameters of 400um (3/8 in). Aluminium cans of approximately 500[cm.sup.3] were filled with 250-260 g of TLA and heated to 200[degrees]C using a thermoelectric heater (Thermo Scientific Precision Model 6555). A digital high shear mixer (IKA Model RW20D) was then immersed in the can and set to 3000 rpm. The crumb rubber was then added gradually (5 g/min) while keeping the system at a temperature of 200[+ or -]1 [degrees]C. The 3 different crumb rubber-TLA blends were mixed, each weighing 10g. At the end of mixing, the blends were transferred to an oven heated to 200 [degrees]C, under static conditions and in an oxygen-free environment. After curing the cans were removed and the molten mixtures cast into a ring stamp with 25 mm diameter and 1 mm thickness. Before testing, the samples were cooled to room temperature and stored in a freezer (Fisher Isotemp) at -20 [degrees]C for subsequent rheological testing. The complex modulus and phase angles of the 3 crumb rubber TLA blends and the unmodified TLA were studied using an oscillatory dynamic shear rheometer (ATS RheoSystems) operated within the linear domain under strain control. The test geometries were plate-and-plate (diameters 25mm and 1mm gap). The maximum strain was kept below the limit of the linear viscoelastic region.

Viscosity measurements were conducted in the temperature range 80[degrees]C -140[degrees]C and frequency range was 0.1 - 15.91 Hz. The temperature range represents low and high temperature respectively and is similar to those used in similar research, for example, [30]. Moreover, the range is appropriate since the softening point of TLA is between 93[degrees]C and 99[degrees]C, at which point it begins to soften and change from a glassy solid state to a liquid hot mix [27; 28]. At temperatures lower than 80C the mixture will bind the plates, making it is difficult to get any meaningful measurement.

Results and discussion

The experimental results obtained for the 3 different crumb-rubber-TLA blends and the unmodified TLA (0% crumb rubber) with respect to G* and phase angle are presented in this section.

Effect on complex modulus (G*)

Figure 1 shows the variation of G* at different frequencies and a constant temperature of 80[degrees]C for the 4 mixtures.

[FIGURE 1 OMITTED]

The results seem to indicate that the complex modulus (G*) increases with the addition of crumb rubber. This increase in G* corresponds to an increase in the stiffness of the material. As such, adding crumb rubber to TLA results in a stiffer material. Similar results were obtained for different temperatures (100[degrees]C, 120[degrees]C and 140[degrees]C) [32]. Figure 1 also shows that G* increases with an increase in frequency. This increase in stiffness increases as the load frequency decreases. For example, at a load frequency of 15.9 Hz, 5% asphalt rubber mixture produces a complex modulus that is 25 times more than the unmodified TLA. At a lower frequency of 0.1Hz, the complex modulus for the same blend is approximately 95 times that of the unmodified TLA. From the figure, it can be seen that the maximum G* occurs at around 5% crumb rubber addition, after which it appears to plateau.

Effect on phase angle

Figure 2 shows the effect of the addition of crumb rubber to TLA on the phase angle.

[FIGURE 2 OMITTED]

The results show that the viscosity of the material increases (higher phase angle) with the addition of 2% crumb rubber. However, further addition of crumb rubber results in a decrease in viscoelastic response (lower phase angle), with an apparent plateau around 8% crumb rubber. The effect on phase angle becomes more pronounced as the frequency increases. For example, the 5% asphalt rubber blend results in an almost 26% decrease in the phase angle compared to the unmodified TLA. At 15.9Hz the decrease is approximately 60%.

The above results indicate that a blend of crumb rubber and TLA can produce a material with varying degree of stiffness or viscosity, based on the percentage of crumb rubber added. As such, it is possible to produce a range of materials with different performance characteristics by changing the percent of crumb rubber. For example, the addition of between 1%wt - 2wt% crumb rubber results in maximum phase angle and a reduction in G* compared to unmodified TLA. Such a blend results in a more ductile, flexible and crack resistant material, and will fit applications for use in which high ductility and flexibility are required. On the other hand, the addition of 5wt% crumb rubber to TLA results in maximum G* and a reduction in phase angle compared to 'pure' TLA. This blend therefore produces a more elastic material with higher resistance to deformation.

Using crumb rubber to modify TLA will not only address the end-of-life management of scrap tyres in Trinidad and Tobago, but will also expand the uses of TLA. For example, for applications in which resistance to crack is important such as road pavements and crack sealants in roofs, a 2% CR-TLA mixture will give better rheological performance than 'pure' TLA. CR-TLA blends with more than 6% crumb rubber will produce a flowable mixture that can be pumped and used, for example, to repair potholes on road surfaces. The ability to pump this mixture has the potential to reduce mobilisation effort (time and money) as small pumps can perhaps be used to replace traditional large asphalt paving equipment (such as Barber Greene).

Although similar studies have used crumb rubber concentrations of greater than 10%, in our study this was not possible due to excessive stiffness at concentrations greater than 10%. Therefore we were unable to determine the exact crumb rubber concentration or range that resulted in a plateau.

Conclusion

The end-of-life management of consumer products is an important facet in the quest for sustainability. To enable this, innovative ways to recover products that have come to the end of their useful lives have to be consistently explored. Finding alternative uses for scrap tyres has been the focus of much research, partially based on the proliferation of this 'waste' product in many countries. According to Stutz et al. [33] in a Tellus report, 281 million scrap tyres were generated in the year 2001 alone. However, of this quantity 218 million scrap tyres were recovered for beneficial use. In addition to the significant waste volume, scrap tyres have potential environmental and health impacts as they provide breeding sites for disease-carrying mosquitoes and other insects. If tyres are burnt they produce oil runoff when doused with water and emit toxic pollutants as they burn [33]. Long-term problems following an open-tyre fire include the contamination of soil, groundwater, and surface water. Moreover, because they are bulky, scrap tyres in landfills are problematic as they take up large amounts of space and do not compact. This is a concern for many developing countries like Trinidad and Tobago, where the main landfill has officially reached its maximum storage capacity and is presently scheduled for closure [34] and space for new landfills is limited. As such we sought to explore the possibility of utilizing scrap tyres in the local natural asphalt, TLA, as one way of helping to reduce the disposal problem. Previous research has shown that the interaction of an additive (such as tyre rubber) is composition dependent on the asphalt-bitumen mixture. This means that asphalt and bitumen from different sources, whether natural or manufactured, will behave differently with different additives, which will in turn result in different performance characteristics of the resulting mixture. Naturally occurring TLA has unique properties [26] and the effects of modifying the asphalt with crumb rubber have not been previously explored. Our laboratory study has shown that the addition of crumb rubber to TLA results in changes in the rheological properties and as such, confirms previous findings [10 - 13] that the rheology of asphaltic materials in indeed composition dependent. Asphalt rubber blends with relatively lower complex modulus (G*) and higher phase angles are more ductile and flexible, resulting in more crack resistant materials, whereas blends with relatively higher complex modulus (G*) and lower phase angles are more elastic and deformation resistant. Our results show that waste tyre rubber can have significant potential to be used a modifier of TLA, and a wide range of materials can be produced depending on the particular application and desired performance parameters. These results should encourage local authorities and policy makers in Trinidad to further explore and support the use of crumb rubber modified TLA as an environmentally-beneficial way of dealing with a large percentage of waste tyres accumulated in the country. However, several other factors need to be considered.

Firstly, the conclusions of our study are based on laboratory experiments and it is therefore necessary to undertake field tests to evaluate the performance of the crumb-rubber-modified-TLA under actual climate/traffic conditions. For instance, if this blend is to be used in paving applications then it is important to observe several parameters such as ride, rutting, cracking, skid, splash/spray, fatigue and aging [18]. Huang et al. [35] explain that characteristics such as friction, strength, noise, and the ability to drain off surface water are essential to vehicles' safety and riding quality. Therefore in the future, with the assistance of the Ministry of Transport and Works in Trinidad, a test pavement will be constructed and actual performance data of the asphalt rubber blend collected and analysed.

Another consideration is the environmental impacts of the CR-TLA blend, which is unknown at this time. However, several researchers have looked at the environmental impacts of both crumb rubber and crumb rubber modified asphalt. For instance, Liberty Tire Recycling [36] provides information related to 52 studies and reports that give a comprehensive collection of crumb rubber health and environmental impact studies conducted between 1994 and 2008. Carlson and Zhu [37] point out that in the US, fume emissions have been studied extensively in a number of asphalt-rubber projects since 1993 and in all cases have been determined to be below the National Institute for Occupational Safety and Health (NIOSH) recommended exposure limits. FHWA and EPA [18] conclude that there 'is no reliable evidence indicating that the manufacture, application, or use of asphalt pavement containing recycled rubber substantially increases the threat to human health or the environment as compared to the threats associated with conventional asphalt pavements'. In addition, studies on leachate from crumb rubber (such as [38]) show no deleterious effects on the environment. One objective environmental assessment tool that can be used to quantify the environmental burdens associated with crumb rubber modified TLA is a life cycle assessment (LCA), which takes a holistic view of the entire life cycle of a product in order to prevent problem-shifting. An LCA will ensure that additional transportation and processing of the scrap tyres, which can imply additional energy use and emissions, are considered. Moreover, an LCA will include the use phase and maintenance options of the asphalt rubber mixture, depending on the application, in order to accurately quantify the environmental impacts.

Apart from the technical and environmental considerations there are economic issues that need to be explored. One such issue is the typically higher initial cost of asphalt rubber than conventional asphalt. Carlson and Zhu [37] point out that although the initial cost of asphalt rubber may be higher savings can be achieved when using asphalt rubber as maintenance costs are reduced, for example, in road pavements when these resist cracking. Since rubber asphalt has a longer lifetime than traditional pavement maintenance costs can be significantly reduced. With respect to plant modification, Carlson and Zhu [37] believe that little, if any, modification to a standard asphalt plant is necessary to facilitate the blending of asphalt and rubber. A comprehensive feasibility study will help to determine what would be required to make the large scale production of asphalt rubber in Trinidad profitable for potential manufacturers. Sunthonpagasit and Duffey [8] present a starting point for public and private ventures interested in exploring the economic feasibility of the scrap tyre recycling industry.

Even though our study provides some useful results, several issues require further work. In previous work, Chiu [20] compares truck and car tyres and concludes that truck ground rubber tyre differs from car ground tyre rubber by a higher natural rubber content. As noted, since the performance of asphalt rubber is dependent on many factors, for future work we intend to investigate the effects of crumb rubber from different tyre sources on the physical properties of TLA. Further work will also seek to investigate the effect of crumb rubber size, shape and texture on the properties of crumb rubber modified TLA.

Acknowledgements

The authors would like to thank Dimple Singh-Ackbarali for her technical assistance in the laboratory.

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Suzana N. Russell *, Rean Maharaj and Aidan St. George

University of Trinidad and Tobago, O'Meara Campus, O'Meara Industrial Park, Arima, Trinidad and Tobago, West Indies. E-mail: suzana.russeH@utt.edu.tt, rean.maharaj@utt.edu.tt, aidanstgeorge@hotmail.com * Corresponding Author E-mail: suzana.russell@utt.edu.tt

(1) Data was collated by the authors from import and export data from the Central Statistical Office (Trinidad and Tobago) and corroborated with data from UN Comtrade Database.

(2) This data seems to be fairly consistent if, according to Reschner [14], we assume that scrap tyres are generated at the rate of one passenger car tyre equivalent per person per year. Trinidad and Tobago has a population of approximately 1.3 million people [15] which translates into roughly 1.3 million scrap tyres generated per year.

(3) Asphalt rubber (AR) is defined as an asphalt cement binder that has been modified with CRM and can be used in a number of asphalt paving pr
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Author:Russell, Suzana N.; Maharaj, Rean; St. George, Aidan
Publication:International Journal of Applied Environmental Sciences
Date:Feb 1, 2011
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