Crumb Rubber: Comparison
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Please note this is a comparison between Version 2 by Rita Xu and Version 1 by Munder Bilema.

The crumb rubber (CR) recycled from waste tyres could be a viable alternative in achieving green pavements that offer exciting new markets to global investors. Adding CR into flexible pavements enhances their performance and ensures environmental sustainability.

  • crumb rubber (CR)
  • characteristics
  • mechanical performance
  • greenhouse gas emissions

1. Introduction

The road construction industry should follow other related sectors in utilising recycled and waste materials or by-products, for example, glass furnace dross, ashes from the incineration of municipal waste, crushed brick, plastics, glasses and crumb rubbers from waste tyres, to produce asphalt mixtures. However, their applications are still underexplored since few studies have investigated the potential of these materials [1,2][1][2].
Generally, crumb rubber is the rubber recycled from automotive and truck scrap tyres. Waste tyre disposal management is challenging because tyres have a long lifespan and are non-biodegradable [3,4][3][4]. The United States is the single-largest market for ground rubber, with an annual consumption of 12 million waste tyres (more than 100,000 tons). The conventional waste tyre disposal methods, namely stockpiling, illegal dumping or landfilling, are temporary solutions. The urgent need to recycle waste tyres is apparent given the substantial amount of waste tyres generated (up to nine million tons each year globally. In some countries, the volume of waste tyres reaches 220 thousand tons), limited landfill space and pollution issues. Generally, gathering waste tyres for dumping in landfills is costly and not an environmentally viable disposal method. Therefore, it is imperative to find sustainable solutions for recycling waste tyres to effectively deal with the massive amounts of waste tires produced globally [5].
One way to recycle waste tyres is by using crumb rubbers in asphalt binder modification. Waste tyres have thermo-mechanical, chemical and physical properties that make them suitable for the asphalt construction sector [6]. Crumb rubbers are safe because they are lightweight, durable, non-toxic and inert [7,8][7][8]. In addition to using crumb rubber as a chemical de-vulcanisation feedstock and incorporating it into the bitumen as asphalt sealants and roadway laying [9], it has been used as an asphalt modifier for over 40 years. Studies have demonstrated that asphalt–rubber pavements reduce road pavement thickness, traffic noise, pollution and maintenance costs and extend the lifespan of road pavements while reducing refraction and reflection [10,11][10][11].

2. Crumb Rubber

CR has been used to modify asphalt mixtures and binders for decades. In 1840, natural rubber was first used in asphalt pavement to increase the durability of conventional asphalt [12], and the paving industry has been using CR since 1950. The research by McDonald’s to determine the best method for developing an ideal combination found that a mixing time of 45 min to an hour produced an asphalt mixture with the best engineering characteristics [13]. In 1975, researchers successfully incorporated CR into asphalt mixes, and in 1988, the American Society for Testing and Materials (ASTM) recommended incorporating 15% ground tires into the original asphalt to produce asphalt binders [14]. Between the early 1970s and mid-1980s, South Africa and Australia used bitumen rubber as a sealant and asphalt binder [15]. Two Australian territories (New South Wales and Victoria) began using rubberised asphalt binder for limited application, primarily as a crack-resistant layer through spray sealing applications [16]. In 1991, the United States established federal rules and regulations for the CR asphalt used in stress absorption interlayers, HMA and joint sealants. Since then, researchers have begun exploring new methods to improve CR-modified asphalt manufacturing techniques [17]. Portugal, Spain, Italy, the Czech Republic and Sweden use CR asphalt the most, and Taiwan uses CR-modified asphalt for rehabilitation projects [18]. The Rubber Research Institute of Malaysia (RRIM) and the Malaysian Public Works Department (PWD) investigated the effectiveness of using CR asphalt for road construction. The researchers in the 1950s focused on developing CR asphalt techniques and constructed a 91 m road between Kota Bharu and Kuala Krai by incorporating 5% CR into the asphalt mixture. Between 1988 and 2003, the states of Melaka, Negeri Sembilan, Kedah, Johor and Perlis constructed rubberised paving as part of their research. Unfortunately, the results of these experiments were never published [19].

2.1. Production Methods

Crumb rubber is recycled rubber from the scrap tyres of cars and trucks. The two methods for making crumb rubbers are cryogenic grinding and ambient mechanical grinding [3], and the crumb rubber is ground repeatedly to obtain finer crumb rubber particles [20]. Figure 1 shows the ambient mechanical grinding method for producing crumb rubber by breaking up the scrap tyres at or above the average room temperature (25 °C). This process comprises several steps and uses whole truck tyres to produce rubber shreds or chips. The first step is separating the metals, fabrics and rubber; the next step is shredding the scrap tyres to obtain the chips fed into a granulator that grinds them and removes any remaining steel or fibre using a combination of magnetic separation, shaking screens and wind sifters. The third step grounds the discarded tyres into smaller rubber pieces through the secondary granulators and high-speed rotating mills [3]. Ambient plants use extruders or screw presses and cracker mills for fine grinding.
Figure 1. Tyre recycling plant.
Cryogenic grinding uses liquid nitrogen or other industrial refrigerants to ground the scrap tyres at about −80 °C. This process uses truck tyres in the form of chips or ambiently formed granulates as feedstock. Even though cryogenic grinding consumes more energy than ambient mechanical grinding, it produces high-quality crumb rubber. This process comprises four stages: initial size reduction, chilling, separating and grinding. In the first stage, the scrap tyres are placed in a freezing chamber containing −80 to −120 °C liquid nitrogen to reduce the rubber’s flexibility. A hammer mill then separates the metals from the fibres, and in the last stage, the granules pass through magnetic screens and sifting stations to remove harmful materials [3].

2.2. Physical and Chemical Properties

Tyres are categorised into different rubber compositions and other components to ensure a safe function under various challenging conditions. Figure 2 shows the materials required for tyre production, including natural rubber, artificial polymers, metals, fabric, fillers (such as carbon black and crystalline-precipitated silica), anti-oxidants and curing agents (such as sulphur and zinc oxide).
Figure 2. The composition of light truck tyres.
Figure 3. SEM analysis of CR: (a) ×100; (b) ×150; (c) ×400; (d) ×600.
Researchers investigated the physical and chemical characteristics of four CRs, two produced by the cryogenic method and the others using the ambient method.

2.3. Size and Contents

There is a grading system for crumb rubbers with varying particle shapes and sizes, and the particle size of crumb rubbers can be as small as 0.075 mm. The typical CR gradation in rubberised asphalt pavements ranges between 2.0 and 0.075 mm. The crumb rubber size is the screen or mesh size that crumb rubber passes through during manufacturing. Finer screens or meshes have more apertures or holes per linear inch; for example, a 30-mesh screen has 30 openings or holes per inch [29,30][21][22]. The permeability coefficient of an asphalt mixture decreases markedly with bigger CR particle sizes and higher CR contents [31][23]. Cao et al. [32][24] reported a marked change in the penetration, softening point and ductility of the asphalt binder added with 15% 80-mesh CR. Wong and Wong [33][25] discovered that the asphalt mixture containing 0.6 mm CR had a higher rutting resistance than that with 0.3 mm CR. Researchers experimented with varying CR sizes and found that larger particle sizes affected the mixture’s stiffness and indirectly increased its tensile strength [34][26]. Another study [35][27] demonstrated that adding different crumb rubber sizes of 30-mesh 0.6 mm CR, 30-mesh 0.3 mm CR and 40-mesh 0.15 mm CR in varying percentages of 0%, 5%, 10% and 15% by weight of the base binder had a minor effect on the moisture sensitivity of rubberised asphalt. The researchers concluded that the CR sizes and contents influenced asphalt mixture performance. The 50-mesh CR has a better low-temperature performance than the 14-mesh crumb rubber [36][28]. However, Liu et al. [37][29] concluded that adding 60-mesh and 80-mesh crumb rubbers to asphalt binder did not considerably impact its performance. Adding 0.15, 0.3 and 0.6 mm CRs to asphalt mixtures resulted in a slight performance difference [33][25].
Figure 4. Crumb rubber with varying particle sizes.

2.4. Blending Methods

The mixing of crumb rubber and asphalt uses the wet or dry process. The dry process mixes the CR and aggregates in an asphalt mixture, whereas the wet method mixes CR and asphalt binder at a specific temperature [39][30]. The latter enhances the asphalt mixture’s rutting resistance, resilience modulus and fatigue cracking [40][31]. Moreno et al. [41][32] observed that the dry method was more effective for producing CR asphalt, while Losa et al. [42][33] noted that the rubberised asphalt mixtures produced by wet and dry methods showed similar indirect tensile strength (ITS), although those produced through wet mixing have a higher resilient modulus. The rubberised asphalt binders mixed using the wet method had the following characteristics.
  • The optimal shearing temperature for ductility at 5 °C ranged from 170 to 180 °C [43][34], where low shearing temperatures reduced the asphalt’s fluidity (higher consistency), making it unsuitable for CR adsorption and swelling, while too high shearing temperatures caused ageing.
  • The optimum shearing time ranged between 30 [44][35] and 60 min [45][36]. The rubber particles were not sheared properly when the shearing time was too short, but an exceedingly long shearing time accelerated asphalt ageing.
  • The recommended shearing rates for rubber-modified asphalt are 700 rpm [35][27], 1200 rpm [46][37] or 5000 rpm at 180 °C for 45 min [45][36]. The shearing outcome was poor when the shearing rate was too low, but an excessively high shearing rate increased the rubber particle temperature rapidly.
According to the American Society for Testing and Materials (ASTM), asphalt–rubber is a mixture of asphalt binder, aggregates, scrap tyres and additives, where the rubber content should be at least 15% of the total mixture weight and react sufficiently with the asphalt binder to ensure swelling of the CR particles. [47][38]. Increasing the blending time from 30 to 60 min increased the asphalt binder’s rutting resistance and elastic recovery [40][31]. The longer blending duration and higher blending temperature caused the CR asphalt binder to have a higher failure temperature and viscosity at 135 °C [48][39]. Liu et al. [45][36] concluded that time, shear and temperature influenced asphalt binder performance, and they recommended mixing at 5000 rpm and 180 °C for 45 min to achieve the optimal rubberised asphalt binder performance. The viscosity of CR asphalt binder decreased considerably with higher mixing temperature and time [32,49][24][40]. Figure 5 shows the wet and dry methods for producing rubberised asphalt mixtures. Laboratory experiments and field studies revealed that CR mixtures produced via the dry process showed marginal improvement compared to the wet process. Numerous laboratory investigations have determined the suitable aggregate gradation, optimal bitumen content and appropriate mixture preparation to enhance the consistency and performance of the blends produced via dry mixing. The findings showed that the mechanical characteristics of mixtures produced via dry mixing were more susceptible to varying rubber concentrations. The critical parameters for formulating a CR mixture for both mixing methods are aggregate gradation, bitumen content and air void proportion [50,51][41][42].
Figure 5. The dry and wet methods for producing rubberised asphalt mixture.

2.5. Morphology

A previous investigation found that rubberised mixtures require 1–2% higher bitumen percentages than the original asphalt mixture [53][43]. A study by Bilema et al. [54][44] used the Superpave Gyratory Compactor with varying crumb rubber percentages of 5, 10 and 15% asphalt mixture and found that higher crumb rubber contents required 0.25% higher bitumen content since the rubber particles absorbed some of the bitumen in the rubberised asphalt mixture. Researchers employed modern methods for examining asphalt structure to study the morphology of CR asphalt binders. The compatibility between CR and asphalt binder strongly determines the properties of a CR asphalt binder. Crumb rubber absorbed the lightweight component in the asphalt binder, which expanded the mixture and created a gel-like layer. The CR particles were connected by the gel film surrounding them. A better integration of crumb rubber in the base binder enhanced the asphalt binder’s characteristics [9]. Wang et al. [55][45] classified the interaction between the asphalt binder and CR particles into four stages, as shown in Figure 6. The first stage mixes the asphalt binder with the rubber particles. In the second stage, the rubber particles swell as they absorb the light bitumen fractions, forming a gel layer close to the bitumen–rubber interface. In the third stage, the rubber granules expand, causing the polymer chains and crosslinked network to break down as the chemical reaction occurs. The destruction of the network structure causes the swollen rubber particles to break into smaller components. The deterioration of the rubber particles continues in the fourth stage until they are fully incorporated into the bitumen structure, creating a homogeneous asphalt binder.
Figure 6. The interaction stages between the rubber particles and asphalt binder.
Xu et al. [56][46] reported that the smooth surface of the crumb rubber particles caused a poor absorption of the lightweight components in the asphalt binder. Researchers used an ambient grinding method to generate CR using untreated CR and CR with polymer-enhanced surface treatment. The largest particle diameter for both CRs was 800 μm. The environmental scanning electron microscope (ESEM) analysis showed no apparent difference between the CR particles, although the surface of the treated CR particles was more porous and open. The chemical activation improves the interaction with the asphalt binder [26][47]. The AFM images in Figure 6 show asphalt binders with varying percentages of crumb rubber powder and the characteristic changes in the catana phase. The clustered and floating micro-rubber powder had more impact on the interfacial tension than the apparent structural change [55][45]. Cong et al. [57][48] used a fluorescence microscope to examine the morphology of rubberised asphalt binders, particularly the discontinuous and continuous phase distribution and observed that the chemical composition of the asphalt binder and CR swelling influenced their low-temperature properties. Zhang et al. [58][49] examined the morphology of rubberised asphalt binders using a scanning electron microscope after microwave treatment, and they concluded that the CR surface had a strong reactivity, permeability and affinity at the interface with epoxidised soybean oil, which entered the asphalt structure and restored and enhanced the asphalt binder’s performance after ageing. The scanning electron microscope (SEM) imaging revealed that the high concentrations of CR powder created an excellent network connection with virgin asphalt. The microscopy of asphalt with high rubber contents showed minor differences before and after ageing, and the compatibility and stability remained satisfactory even after extended ageing [26][47].

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