Scrap tires are causing environmental hazards in many countries due to inappropriate disposal. They are unquestionably one of the waste products that have been causing environmental issues in Sarawak and other states of Malaysia over the past few years. Therefore, scrap tires should be recycled for use in various construction applications. The crumb rubber extracted from scrap tires can be used in hot-mix asphalt (HMA) mixes to improve their various properties for durable and sustainable application on the roads. Past research has revealed that the incorporation of crumb rubber into asphalt mixes can increase the rutting resistance, reduce the fatigue cracking, and enhance the durability of flexible pavements against traffic loads [1
]. With the rapid increase in the number of vehicles on the roads today due to economic growth, it is necessary to construct more durable and sustainable pavements, wherein crumb rubber obtained from scrap tires can play a significant role.
Crumb rubber is a commodity that is produced during the reprocessing of scrap tires composed of natural and synthetic rubbers. Although in Malaysia the use of crumb rubber in modified asphalt pavement is relatively uncommon, the United States of America (USA) started using it back in 1950 [5
], and since then it has been implemented widely across the different states of the USA. In the 1960s, crumb rubber was also used in the Swedish pavement industry [6
]. From more environmentally friendly and sustainable engineering perspectives, there is a necessity for the application of crumb rubber on the roads of Malaysia—particularly in Sarawak state, with its unique local resources, such as aggregate and asphalt materials.
According to a survey conducted by the Natural Resources and Environment Board (NREB) of Sarawak, in collaboration with the Danish Cooperation for Environment and Development (DANCED), back in 2001, the annual production of scrap tires reached a total of 150,000 units [7
]. With the rapid increase in the number of vehicles today, it is certain to say that the amount of scrap tires has increased proportionally. The State Government of Sarawak has an environmental program to collect, manage, and recycle scrap tires throughout Sarawak via a waste tire storage and recycling center, as well as to deport tire waste statewide as part of its collection network. Companies collecting scrap tires have been set up in a bid to ease the problem of handling scrap tires in recent years. Their main purpose is to retrieve the fiber and steel contained in tires, which make up only a minor part of tire composition. The majority of scrap tire composition is rubber; a scrap tire typically contains 70% recoverable rubber [8
]. This rubber is used as an alternative burning material. In many countries, scrap tires are co-fired with coal or other fuels in cement factories [9
], but this still creates environmental pollution. Furthermore, as the number of vehicles on the roads has increased, scrap tire disposal has become a greater environmental concern to the municipal and public health authorities, as sanitary landfills will soon be unable to accommodate the stockpiles of scrap tires, which are also hazardous to human health. Hence, proper recycling and utilization of scrap tires are necessary. Since roads constitute a major part of the infrastructure in any country, a vast amount of scrap tires could be utilized in pavement construction.
Roads play a pivotal role in the daily activities of people, mainly transporting passengers and goods to different points. In East Malaysia, the Trans-Borneo Highway (Pan Borneo Highway) is the major road that not only links cities and small towns throughout Sarawak, but also connects Sarawak and Sabah with Brunei and the Kalimantan region of Indonesia, covering a total length of approximately 2083 km [10
]. This highway was officially launched on 31 March 2015 [11
]. Thereafter, the road’s condition deteriorated rapidly due to its high and heavy usage, mainly by heavy trucks and buses delivering goods and passengers between cities. Millions of MYR (Malaysian Ringgit) are being spent annually on the maintenance and upgrading of the Trans-Borneo Highway for the safety of the road users. Therefore, the application of high-durability pavement to this highway is urgently required in order to slow down its deterioration.
Many investigations have been conducted for finding alternative materials to be used as modifiers in asphalt mixes in order to improve their properties; crumb rubber is one such option among the alternatives. The application of crumb rubber in HMA mixes has been established in advanced countries—most notably in Sweden and the USA [6
]—as they aimed to tackle the number of scrap tires generated every year without compromising the quality of the roads, rather improving their performance. To date, the USA is the leading country in crumb rubber modified asphalt pavement technology. In Malaysia, this technology is less established, and uncommon in the road construction industry. According to Hassan [13
], the use of crumb rubber as a modifying agent in HMA mixes for road construction is not widespread in Malaysia. Hence, there is a need for research in order to evaluate the performance of crumb rubber modified HMA mixes in Malaysian road conditions.
This study aims to examine the effects of crumb rubber when it is used as a modifier of asphalt in HMA mixes. Several modified asphalt binder and HMA mixes were produced incorporating crumb rubber as 5–19% weight replacements of asphalt, and their performance characteristics were examined based on JKR (Jabatan Kerja Raya—Department of Public Works) standards [14
], compared with the unmodified asphalt binder and HMA mixes. Specifically, the objectives of this research were to evaluate the performance of the crumb rubber modified HMA mixes compared to the conventional HMA mixes in terms of Marshall stability and flow, as well as major volumetric properties such as bulk density, voids in the total mix (VTM), voids in mineral aggregate (VMA), and voids filled with asphalt (VFA). In addition, the stiffness of the HMA mixes was determined based on their Marshall stability and flow values. The penetration resistance (as an indirect measure of viscosity) and softening point (as a direct measure of temperature resistance) of the modified and unmodified asphalt binders were also studied.
6. Environmental Benefits
Ever since concerns about environmental pollution due to waste materials have been raised, humans have been looking into innovative solutions to see if some of these materials could be reused or recycled in order to reduce their impact on the environment. The development of the HMA mixes incorporating crumb rubber extracted from scrap tires is such an innovative effort.
The issue of the disposal and handling of scrap tires has become a worldwide problem over the past few decades. This is due to their rapid increase, proportionately with the upsurge of new vehicles on the roads. In many countries, a large portion of scrap tires is improperly disposed of in the environment. Serious environmental problems are caused by the improper disposal of scrap tires—such as burning, land filling, etc. [18
]. These have led to innovations in the recycling of scrap tires for reuse in various fields—mainly rubber and plastic blends, automotive parts, landscaping, running tracks, and playground surfaces.
The use of crumb rubber derived from scrap tires could be a sustainable solution to the environmental problems caused by the unwanted or discarded tires. Although there are some practices to recycle and reuse scrap tires for energy recovery [43
], more than 50% of scrap tires are still being discarded without any proper treatment [21
]. The utilization of scrap tires could be intensified if the crumb rubber extracted from these tires is incorporated into the HMA mixes for constructing flexible pavements. This is because a huge amount of HMA mix is generally required for such construction work. The results of this study indicate that the crumb rubber obtained from scrap tires can be used sustainably in producing the HMA mixes for flexible pavements, thus reducing various forms of environmental pollution, including soil contamination. Particularly, it could be very useful to resolve the environmental problems caused by scrap tires over the years in Malaysia, especially in Sarawak region.
The environmental issues (e.g., air pollution, noise pollution, waste generation) caused by the repair and maintenance activities on pavements will be reduced if the roads can serve for a longer time without any major damage, as the frequency of such activities will be lessened significantly. This can be achieved if the crumb rubber modified HMA mixes are used to construct durable pavements. Furthermore, the quantity of the end-of-life waste generated from the damaged pavements will be lower and the landfill sites will be less overloaded if the roads can sustain the combined effects of traffic and weather for a longer time. In addition, the consumption of the virgin asphalt and aggregate will be reduced in the case of more sustainable and durable flexible pavements. Consequently, the use of raw materials, and thus the depletion of natural resources, will be decreased. Based on the life cycle assessment of sustainable pavement materials, Praticò et al. [47
] stated that the extraction and supply of construction materials are responsible for critical environmental consequences, which could be reduced if crumb rubber is incorporated into the HMA mixes for flexible pavements.
Air pollution will be lower if crumb rubber is used in the production of HMA mixes. The emissions of CO (carbon monoxide) and CH4
(methane) can be reduced by 39.7% and 61.7%, respectively, during the production of the rubberized asphalt mixes [48
]. The environmental noise performance of flexible pavements will also be improved if crumb rubber is incorporated into HMA mixes. Wang et al. [48
] reported that the rubberized asphalt mixes can reduce the tire–pavement noise by 40–88%. They also stated that the leachate from crumb rubber modifier does not pose a measurable threat to the environment.
In Malaysia, the estimated amount of motorcar scrap tires generated annually was 8.2 million units—or approximately 63,263 tons—more than a decade ago [49
]. The number of scrap tires has certainly increased to date, thus creating a large environmental load. About 60% of the scrap tires are disposed of through unknown routes, and they are neither categorized as solid waste nor hazardous waste in Malaysia [49
]. Inappropriate scrap tire management brings forth adverse impacts on the environment. The open burning of scrap tires causes air pollution by releasing harmful contaminants (hydrocarbons, nitrogen oxides, furans, dioxins, etc.). Moreover, the dumps and stockpiles of scrap tires cause aesthetic pollution, enhance mosquito breeding, and become habitat for pests, such as snakes and rats. The dumping of scrap tires in ditches and watercourses may have other impacts, such as changes in the hydrological systems. These environmental problems occur due to the absence of a well-coordinated scrap tire management system. There is a lack of producer responsibility for scrap tire management in Malaysia. The tire manufacturers do not care about the final disposal of the end-of-life tires. They leave it entirely on their dealers to confront this problem. The absence of producer responsibility makes scrap tire management problematic.
The environmental issues caused by scrap tires can be alleviated through recycling of these out-of-use tires. The crumb rubber obtained from the recycling of scrap tires will contribute to environmental sustainability if it is used in the production of the HMA mixes for flexible pavements. The incorporation of crumb rubber into the HMA mixes will allow a large amount of scrap tires to be recycled in Malaysia, particularly in Sarawak, and thus help in reducing air pollution, tire–pavement noise, and other environmental problems.