Critical Review on The Development of Rubberized Asphalt Pavement Properties Incorporating Recycled Tire Rubber Waste Modification

Abstract

Over the course of years, considering recycled tire rubber waste as a valuable source of secondary raw material and reprocessing it into construction applications, especially into asphalt pavement has been attaining more interests. The growth of this interest is due to ecological, economic and engineering intentions which include the valuable opportunity to eliminate and recycle large quantity of tire waste in asphalt road networks, to reuse tire waste as a substitute of raw materials or natural aggregates, and to take an advantage from the high compatibility between tire rubber waste and asphalt binder to improve the strength and quality of asphalt. Although tremendous effort has been done on this recovery principle, the limited discoveries on the physical and mechanical performance of rubber asphalt positions this concept in its early stages. Since the properties of asphalt pavement relies mainly on the properties of bitumen binder, sever fluctuations in environmental conditions and loading rates highly affect the bituminous asphalt and lead to rutting which is the one most critical deformation problems in asphalt pavement. The reported reduction in the asphalt pavement resistance in recent years against temperatures and direct loads from vehicles result massive costs in the maintenance of roads. Hence, investigating efficient and cost-effective methodologies to improve the durability and extend the serviceability of pavement is needed. Adding additive materials such as recycled tire rubber waste (granules, crumb, ground or ash rubber) in asphalt mixture improves the viscoelastic properties of bitumen which delays the form of thermal cracks and rutting, increases the dynamic load resistance and as a result enhances the long-term asphalt performance and stability of asphalt pavement against deteriorations.

Key words: Recycled tire rubber waste; natural aggregate; bitumen; eco-efficient asphalt pavement; weathering; dynamic load; thermal cracks; rutting; deterioration resistance.
1. Background
1.1. Problem Statement

The vast growth in the worldwide production and consumption of industrial materials accelerates the need of innovating long-term and sustainable plans to recycle the discarded waste especially in engineering applications. The reported depletion in the natural resources in recent years increases the urgency of protecting the earth’s raw materials by substituting the waste material with sand or aggregate. The valorization of these waste materials in infrastructures has been a major concern in research industry to achieve the needed sustainable, cost effective and green construction targets.

Tire waste is considered as one of the most complicated type of waste to manage which causes challengeable environmental issue due to its toxic, non-biodegradable components (Segre and Joekes, 2000), complex material, volume and large space this waste takes. The landfill areas in global receive in total around 1 billion of end-of- life tires (ELTs) per year (Thomas et al., 2016), which leads to a critical threat to the overall health, ground and underground resources (Bulei et al., 2018). In fact, the landfill sites of tire piles are suitable breeding areas for mosquitos, rats and snakes that carry diseases (Jr, 2013).

According to Somayaji (2001), around 2 to 3 billion tires were already collected and landfilled overseas. Another study reported that the same removal process was applied for almost 4 billion tires (Messenger, 2013). By 2030, the expected total number of stockpiled tires will reach at least 1.2 billion tires, and starting from the same year, 5 billion tires will be discarded in a regular basis. Therefore, finding alternative methods than burning and storing this amount of tire waste is seriously needed to reduce the ecological damage and the depletion of the available sites (Garrick, 2005; Benazzouk et al., 2007; Onuaguluchi and Panesar, 2014; Su et al., 2014; Thomas et al., 2015a, b).
1.2. Objectives of Recycled Tire Rubber Waste’s Utilization in Asphalt Binder

In the last 30 years, tremendous effort has been put into effect to develop useful tools to recover the waste tire and its compositions (Table 1) in order to build new approaches toward increasing their applications in structural and non-structural targets, which subsequently leads to a significant reduction in the quantity of tire waste in the landfill sites. Recycling tire waste in asphalt pavement is found to be acceptable and gained wide interest due to the remarkable interaction between rubber and asphalt binder portions.

This compatibility led to various improvements in the overall property and performance of asphalt mixture. In addition, pavement can consume large quantity of tire rubber waste, which therefore, helps protecting the environment, creating sustainable construction, improving the quality (Lintz et al., 2009), and the safety conditions by absorbing the elastic behaviour of asphalt (Oda and Fernandes, 2001). Despite its sustainable feature, rubberized asphalt has not been extensively applied in pavement infrastructures. This refers to its limited performance (Table 2) and cost effectiveness in pavement construction.

However, the high traffic volume increases the need to improve the asphalt surface pavement structure and its mixture (Figure 1), as rubber modified asphalt concrete showed an excellent fatigue resistance and durability (Takallou and Takallou, 1991). The high viscosity of rubberized bitumen and the high temperature required to produce rubberized asphalt are also reasons that narrowed the applications of rubber asphalt (Mohammadi and Khabbaz, 2012). There is a need to enhance the viscoelastic property of asphalt as its consistency and adhesion mechanisms are directly affected by the temperature and load capacity. Therefore, the improper mix design of the asphaltic mixture leads to severe failures including fatigue, cracking, rutting and potholes, as asphalt in high temperature behaves as a viscous material, while it behaves as an elastic solid in low temperatures (Khan et al., 2016).

Furthermore, studies confirmed a better performance of rubberized asphalt than conventional asphalt mixture as rubber asphalt showed better skid resistance and longer serviceability than normal asphalt especially at high temperature. Tire waste is found to be an efficient material as a filler material in roads and railways. Recycling tire rubber waste in asphalt’s modification process enhances the overall viscosity of asphalt mixture when rubber particles are blended with asphalt binder (Patrick,.2006; Lee, 2003). In terms of noise reduction, many roads in United States and India experienced significant noise reduction from 4 to 10 decibels after applying rubber asphalt pavement in the examined roads (Tagayun, 2001).
1.3. World’s Reaction Toward Recycling Tire Rubber Waste in Asphalt Pavement Construction

According to Heitzman (1992), the first practice of mixing natural rubber and bitumen was in the 1840s. The purpose of this task was to examine the natural flexibility of rubber with asphalt in creating durable pavement surface. However, the outcome of this practice was not significant as the cost effectiveness and long serviceability features were not achieved (Carlson and Zhu, 1999). Since that year, enormous investigations had been conducted until the 1930s when rubber asphalt was initially applied as a joint sealer in patches and membranes. In 1950s, the Bureau of Public Records of the State of California investigated the effect of rubber applications in pavement by mixing recycled tire rubber powder in asphalt mixtures. In the early1960s, “Asphalt Institute” organized the first scientific workshop to discuss the innovation of recycling tire rubber waste in asphalt pavement (Sacramento County Department of Environmental Review and Assessment SCDERA, 1999).

In the mid-1960s, the asphalt wet process in which recycled tire rubber partially reacts with asphalt binder was firstly developed and explored by Charles H. McDonald. This innovation enhanced the rubber asphalt applications significantly for crack sealants, spray applications and hot mix binder purposes (Hicks, 2002). This followed by remarkable developments were reported in the pavement industries in US and Sweden, as companies developed rubber asphalt mixtures including crumb rubber as an aggregate portion. Moreover, this process was further applied in chip seals, interlayers and high modifier asphalt in both warm climate in California, Arizona, Florida and Texas (SCDERA, 1999) and in cold region in China and Scandinavia (Cheng et al., 2011).

As a result, this method greatly impacted on the recycling target, as nearly 50% of the manufactured tires in the US were reprocessed (Turer, 2012).

In the 1970s, department of transportation in California started testing asphalt rubber for spray applications and began evaluating asphalt rubber on hot asphalt mixtures in the 1980s. This development helped the department to publish their design guide of rubber asphalt with a thicker feature than conventional asphalt (Van-Kirk, 1992,1997). In the late 1980s and early 1990s, the US Department of Transportation (USDOT) and Federal Highway Administration (FHWA) established critical studies on the utilization of recycled tire waste in the highways (Heitzman,1992; Federal Highway Administration,1993; Epps, 1994). In 1991, the US congress proposed a mandatory law which requested the highway administrations funded by federal government to consider recycling tire waste in their projects (Public Law,1991).

In 1990, the Canadian government drastically prioritized waste tire recycling across its entire provinces. The government wanted to re-evaluate the consumption rates and waste tire landfills after a massive fire incident occurred in Hagersville, Ontario province. The event gained international recognition after a total of 12.6 million tire piles caught fire and displaced 1,700 people from their homes. Serious measures were taken by the Ministry of Environment towards sustainable practices which were to be implemented in all the provinces (St. Pierre, 2013). In 1995, the Canadian Technical Asphalt Association conducted a study on the rubberized asphalt in British Columbia province. The study concluded that there is a need of full scale reevaluation and improvement of asphalt concrete pavement in the entire province (SCDERA, 1999; CATRA 2006).
1.4. Recycled Tire Rubber Waste Impacts on The Engineering Properties and Applications of Asphalt

There is a global demand of durable, sustainable and cost-effective road infrastructures (Figure 2) to fulfill the future demand of regional, economic and transportation developments. (Schafer, 1998; Wendell, 1966; United Nations Environmental Program, 2009). The severe climate change and the sharp growth of urban areas, populations and number of vehicles on the roads soar the possibility of road damages. Around 95% of the world’s pavements are mainly constructed using asphalt, which throughout the time suffer different failures from traffic loads and weathering (Huang,1993), which these factors reduce the performance and the life span of pavement (Figure 3). Also, these factors increase the need of rehabilitations and maintenances, which subsequently, increase the cost of pavement construction as reported in Canada, where the cost of road construction reaches 150 million dollars per year (Shafabakhsh et al., 2014).

Hence, civil engineering industries proposed many approaches to improve the conventional asphalt properties using recycled tire rubber waste such as crumb rubber as an additive material in bitumen. This asphalt rubber type of modification proved its crucial roles on extending the service life and improving the performance of asphalt pavements (Heitzman, 1992). The quality of bituminous mixture containing rubberized asphalt enhances the resistance to fatigue, deformation, ageing and water damage. In addition, utilizing this type of bituminous and rubberized asphaltic mixture provide such a promising skid resistance and noise absorption properties, which these features impacted remarkably on the maintenance and rehabilitation treatments of the roads (Hicks, 2002; State of California Department of Transportation, 2005). Furthermore, this modification enhances the viscoelastic properties of the bitumen leading to superb resistance to a higher temperature and dynamic loading (Technical Guide I, 2007) which as a result, provide a longer life span of asphalt surfacing for up to 100% (Rokade, 2012).

In spite of that, the American Society for Testing and Materials Committee (ASTM D34.15) still urges the difficulty of studying the physical properties of the tire waste as an engineering material tool, and further studies are need in addition to the recommended guideline ASTM D6270-98 (The Guidelines for the Use of Scrap Tires in Civil Engineering Applications).
2. Production of Bituminous Asphaltic Mixture
2.1. Bitumen Properties

Bitumen is a viscous, sticky and black liquid obtained from a crude petroleum mixture. The bituminous chemical compound is 95% carbon and hydrogen and 5% of sulfur, nitrogen, oxygen and metals. Bitumen is considered as the heaviest portion of crude oil as it consists of nearly 300-2000 chemical complex. Thus, its boiling point is one of the highest by 525ºC (977 ºF). In pavement, it is essential that bitumen (Table 3) provides an acceptable cohesive, adhesive, water repellant and thermoplastic properties. There are factors reduce the life of roads such as, oxidation and bleeding at high temperature which both drastically damage the roads. Also, wet weathering weakens the bond between bitumen and aggregates, which leads to severe segregation between and multiple potholes on the roads (Gawande et al., 2012).

Hence, innovating new methods to improve the properties of petroleum bitumen is needed. One of the well-known methods of improving the quality of bitumen binders is the addition of rubber (Shunin et al., 2002; Ongarbaev et al., 2001). It can be described that the effect of rubber modification in the overall properties of asphalt is as effective as the addition of additive materials in concrete (Huang et al.,2007). The American Society of Testing and Materials (ASTM D8-18) defines rubber modified bitumen as “a blend of asphalt cement [bitumen], reclaimed tyre rubber and certain additives, in which the rubber component is at least 15% by weight of the total blend and has reacted in the hot asphalt cement sufficiently to cause swelling of the rubber particles”.

The specification of bitumen varies and depends on the desired application (Issa, 2016). In Sweden for instance, ground tire rubber was utilized in the asphalt mixture in many of road projects as a replacement of mineral aggregates in order to produce a modified bitumen with high asphaltic durability and resistance against snow and studded tires. Since then, many researched have been done to improve the quality of rubberized bitumen which led to remarkable developments in the asphalt binders and stress absorbing layers of asphaltic rubberized bituminous pavement (Patil et al., 2016).The manufacture of rubber asphalt paving mixtures (Table 4) is processed through either by wet process or dry process (Figure 4):

a) The wet process which is also historically known as “Asphalt Rubber” has been widely recognized and extensively used overseas in the last 35 years. The production of this process begins with blending the recycled tire rubber waste modifier particles (crumb rubber or ground rubber) with asphalt cement, extender oil (if needed) and bitumen at 5-25% (Nguyen and Tran,2018), 15-20% (Oikonomou, and Mavridou, 2009), or 18-25% (Licitra et al., 2015) by weight, followed by adding the bituminous rubber portion with aggregates (Table 5) at elevated temperatures between 160ºC and 200 ºC (320 ºF and 392 ºF) (Nguyen and Tran,2018) or 190ºC and 218ºC (375ºF - 425ºF) for 1-2 hours to produce a suitable mixture for high modified asphalt construction (HMA) (Holubka and Salaiová,2013). This allows the asphalt and recycled rubber binder to swell and highly absorb a viscous gel. This reaction enhances the overall rubberized asphalt binder viscosity and stiffness (Heitzman, 1992). Wet process is well recognized method of its high consumption of tire rubber waste which is environmentally beneficial. However, one of the reasons prevent this methodology from wider application is its storage stability issue, as the incorporated recycled tire rubber waste moves to the surface of the container and leads to segregation (Chehovits, 1989; Heitzman, 1992; Rahman, 2004; Presti,2013; Airey et al.,2003). Nowadays, wet process is the only process approved by the Department of Transportation (Caltrans) according to Sacramento County Department of Environmental Review and Assessment report (1999).

b) The dry process in contrast requires blending the recycled tire rubber waste with the under-heat aggregate in a range of 1-3% by weight of the total mix prior to adding bituminous asphalt. This technique is well known of its low consumption of bitumen but highly consumes recycled tire rubber waste. Dry process improves the resistance properties to deformation at high temperature and cracking at low temperature. (Cao, 2007). Furthermore, the mixture in dry process requires 1.5-2% asphalt which is more than the asphalt in a conventional mix. This method was invented and firstly practiced in Sweden in the late 1960s and known as “Rubit” system (Nguyen and Tran,2018). The dry method was secondly used in the US in 1978 and recognized as “Plus Ride” system (Heitzman, 1992). However, the unpopularity of dry process refers to the need of a special graded aggregate in order to incorporate the recycled tire rubber leading to construction difficulties and road surfacing failures, which these factors increase the cost of construction (Hunt, 2002).
2.2. Recycled Tire Waste Reactivity with Asphalt Mixture

The nature of the recycled tire rubber waste and bitumen reaction is not widely investigated, and there is not enough finding in the industry on this aspect. However, it is claimed that the natural reactivity of rubber-bitumen is not chemical (Heitzman,1992). The enhancement of rubber in asphalt mixture (Figure 5) leads to partial digestion and adsorption into bitumen. In addition, the absorption reaction of the existing bituminous oil in the asphalt mixture swells and softens the rubber particles (Cheovits et al.,1982). This reaction reduces the oily fraction and increases the rubber particles size, which as result, forms gel structure that enhances the viscosity of mixture (Heitzman,1992).

The type of recycled tire rubber waste used in the asphalt mixture affects the overall asphalt performance. It is highly recommended to use recycled ground tire rubber with particles size less than 1mm, as the utilization of this recycled rubber with this type and size, provides more stiffness, frost resistance, fatigue and deformation than larger particle (Oikonomou, and Mavridou, 2009). Thus, the serviceability of rubberized asphalt mixture is better than conventional mixture (Tortum et al., 2005). However, the low bulk density and modulus of elasticity of tire rubber waste increase the compaction difficulty of asphalt mixture. Hence, the interaction of rubber, bitumen and aggregate becomes tough (Khalid and Artamendi, 2002). The mixing and transporting of the asphalt mixture results promising properties as rubber highly reacts with bitumen, leading to a better bond and rigidity than in normal asphalt mixture (Airey et al., 2003).

The addition of tire rubber waste with particle size ranges between 2-8mm reduces the stiffness of rubberized asphalt mixture. Therefore, enhancing large particle size of recycled tire rubber in the mixture create voids which weaken the asphaltic matrix and structure. However, many studies agree that enhancing rubber particles with less than 2mm sizes in the asphalt mixture highly fills the air voids (Kettab and Bali, 2004), provides better strength, compaction and fatigue within the granular skeleton (Rahman et al., 2004). The addition of tire rubber at 10-15% by weight of bitumen may cause a penetration issue into bitumen and reduces softening point but improves the viscosity of the asphalt (Khalid and Artamendi, 2002). Consequently, rubberized asphalt mixtures perform better in wide varieties of hard weather changes from extreme high temperatures to the freezing temperatures.
2.3. Applications of Rubber Modified Asphalt Pavement Layers

Rubber modified asphalt has been applied in dense graded, gap-graded and open graded asphalt as; hot mix asphalt concrete (HMAC or HMA), in which rubber asphalt is used as a binder for hot mixtures, porous friction course (Figure 6) , in which rubber asphalt functions as a binder for open graded porous friction course (OGFC), spray applications as stress absorbing membrane interlayer(underseal), in which rubber asphalt works as a waterproof layer of an existing asphalt pavement and as a delayer of reflective cracking, and finally as stress absorbing membrane (chip seal coat) which has been widely utilized due to its high asphalt rubber and aggregate incorporations ,low rock loss mitigation, durable asphalt surface and high seal coating and finishing property (Tahmoressi, 2001).

The placement of HMA is not recommended during rainy weather and when temperature is below 13 ºC (55.4 ºF) to avoid poor compaction and early raveling. HMA must be highly compacted prior to opening the roads to traffic to avoid early surface failures. This HMA can replace an existing pavement as a deterioration treatment caused by sever, weathering, oxidation and raveling. Also, HMA has been commonly used on concrete pavements, bridge foundations, and can provide cost effective feature as it can be produced with lower thickness layer than in conventional hot mixture. Chip seals on the other hand has been extensively applied on an existing pavement as surface seal or as an interlayer rehabilitation to an asphalt concrete leveling course. HMA layer can be coated with up to 50 mm (2 in) of chip seals and chip seals can be used as an interlayer in 2 to 3 layers to prevent cracking, improve skid resistance and durability of pavement. Studies recommend that chip seals should be placed within a clean and dry surface, and the ambient temperature should not be higher than 40 ºC (104 ºF) or higher.

Dense graded mixtures (DGM) can be used as a supporting layer of HMA. DGM is known of its impermeable feature which ascribed to its fine and coarse well graded aggregate mixtures. Hence, DGM provides a compressive strength property which can be used to resist traffic conditions. Also, HMA can be utilized as a stone matrix asphalt (SMA) forming a gap graded HMA (Figure 7). SMA has been commonly used in Europe to enhance rutting resistance and durability under heavy traffic. This high resistance attributed to the high content of coarse aggregate which can be filled with bitumen and fibre to create an asphaltic structure with high interlock skeleton, resistance to deformation and drainage preventive properties (Hicks, 2002).
3. Physical Properties and Hazards of Bituminous Rubberized Asphaltic Concrete Pavement
3.1. Rubber Asphalt Modification Properties

The properties of bituminous binder are highly impacted on the final performance of pavement. The addition of recycled tire rubber waste as a modifier proportion highly affects the overall asphalt rubber binder properties (Table 6), which lowers thermal susceptibility (Liang et al., 2015; Presti, 2013), reduces penetration, enhances softening point temperature and as a result, increases the stability and durability of the asphalt production (Al-Hadidy and Yi-Qiu,2009; Hinislioglu,2004). Also, at low temperature, rubber modification enhances the deformation and fatigue resistance (Navarro et al., 2002). However, asphalt is known of its high brittle characteristic in cold condition and its soft during in warm condition, which these factors prevent from achieving an efficient pavement (Yu et al., 2009). Besides, the rough surface of the selected recycled tire rubber waste has its major effect on building the cohesion property within the asphalt mixture (Wang et al., 2011). The intensive concentration of recycled tire rubber waste in the binder creates a cohesive force within the asphaltic mixture materials. Hence, with the high concentration of rubber in the asphalt binder, the asphalt mixture exhibits stiffer behaviour than conventional asphalt (Agrawal, 2014). Studies highlighted the notable rheological properties of the asphalt rubber modifier in enhancing the complex modulus, which was found to be higher than in the modulus of non-asphalt rubber modifier (Kim et al., 2010).
3.2. Physical Interaction of Recycled Tire Rubber Waste Particles in Asphalt Pavement

Numerous studies concur the unavailability of an accurate scientific finding on the physical interaction of rubber particles and asphalt. This ascribes to the immediate mixing between the aggregate and asphalt proportions. Studies also illustrate the direct mixing and transporting as another reason, as the rubber particles react with asphalt causing major interaction and changing in the properties, shape and rigidity of the utilized rubber. The performance of the rubber asphalt modifier changes significantly (Marvridou et al., 2010; Oikonomou and Mavridou, 2009; Airey et al., 2003). Thus, the physical interaction of the incorporated rubber and asphalt mixture is foundational since the mechanism of this interaction relies on the swelling and dissolution of the rubber modified asphalt mixtures (Heitzman, 1992; Zanzotto and Kennepohl, 1996; Abdelrahman and Carpenter, 1999). In addition, the absorption process of the added rubber into the heated asphalt binder is a physical reaction rather than chemical. This process swells the rubber particles into two to three times of their regular volume and forms a gel material (Heitzman, 1992; Heitzman, 1992) and shrinks the particle distances, as a result, increases the binder viscosity (Figure 8) (Heitzman, 1992; Abdelrahman and Carpenter,1999; Bahia and Davies, 1994; Airey et al.,2002).

The size of the rubber particle also affects the level of the viscosity in the asphalt mixture, as adding large particle size enhances the viscosity (Cong et al., 2013). Hence, rubber asphalt modified mixture exhibits higher viscoelastic and viscosity than conventional asphalt, resulting a better deformation resistance (Liang et al., 2015). Further, large particle surface areas ease the absorption process in the binder and enhances the digestion of the added rubber. The surface texture of rubber particles supports the skid resistance of the rubber asphalt pavement during freezing condition. This high ice resistance attributes to the rigidity of the rubber particles in the binder and the flexibility of the rubber asphalt modified mixture under traffic load. The rigidity of this rubberized pavement develops a non-adhesive surface between the rubber modified asphalt pavement and the ice layer causing multiple breakdown in the icy surface (Takallou and Takallou,1991).

Accordingly, the natural interaction between the added rubber and bitumen physically originates by diffusion. This physical interaction with the asphalt binder increases the asphalt coating and extends the pavement surface against aging (Wang et al.,2013). Also, the mechanism of the rubber asphalt collaboration reduces the depth of asphalt rutting and delays the surface thermal sensitivity of the asphalt (Shafabakhsh et al., 2014). Consequently, these physical properties of asphaltic rubber pavement expand the modulus and therefore develops high stiffness. Simultaneously, further studies are needed to investigate the fatigue resistance, when the rubber asphalt layers are subjected to high strain and deformation, which impacts negatively on the fatigue resistance of pavement (Moreno-Navarro et al.,2014).
3.3. Deterioration of Asphalt Pavement

The physical and mechanical designs of the asphalt concrete pavement must be highly put into consideration. In general, the design of asphalt pavement should highlight several basic components includes the quality of materials, construction status, traffic loading, road geometry and environment. The investigation of these factors prior to paving the roads reduces the chance of early asphaltic deterioration behaviors such as rutting, top-down fatigue and thermal cracking. Hence, incorporating modified materials such as recycled tire crumb or ground rubber enhances the pavement life, reduces the deterioration damages (Karacasu et al.,2015) and improves the compaction quality due to the hard compaction and heat loss when the internal heat of asphalt on site or during transporting of asphalt is below 160 ºC (320 ºF) (Karacasua et al., 2012).

The appearance of deterioration in the asphalt pavement is due to the severe interactions the harsh weathering, traffic loading, and aging (Isa et al., 2005). Thus, the ability of the asphalt pavement to resist strains caused by the environmental and traffic loads is critical. In addition, the resistance to the physical deterioration of the asphalt pavement depends on the pavement indirect tensile strength, pavement thickness, and mix design parameters (Krishnamoorthy et al., 2016). These parameters are thoroughly affected by condition and stress loadings. Therefore, improving the properties of binder materials integrated in the hot asphalt mixture and developing advance laboratory and field investigations are paramount to prevent such pricey pavement and maintenance (Huang, 1993).

The subgrade layers of asphalt concrete pavement are designed to extend and support the pavement structural capacity, while the hot mix asphalt in the pavement works as a stress distributer and as a protector of sublayers. Hence, to construct eco-efficient pavement, the careful design and selection of the added asphalt materials to protect the strength and the stability of the pavement layers under vehicles loadings (Tabatabaei, 2005), especially the upper asphalt layers where rutting phenomenon or what is known as wheel track groove initiates (Rabbira, 2002). This type of deterioration is an impact of the consolidation and compaction process of asphalt mixture. Accordingly, this has been an important research topic in the construction of high modified asphalt to overcome this plastic deformation failure and to develop pavement with flexible engineering properties. The improvement can be achieved by widely investigating and improving modified bitumen and modified asphalt mixture properties (Kandhal, 1992).
4. Rubber Asphalt Modified Concrete Pavement Performance
4.1. Design of Rigid and Flexible Rubber Asphalt Pavement Materials

Road infrastructures nowadays must meet significant safety, functional and economical standards. To achieve these demands, several fundamental parameters which involve asphalt mix constituents, traffic and environmental loads need to be highly considered in the design of pavement (Peralta, 2009; Mahrez, 1999; Mashaan,2012). In pavement, bituminous asphalt works as holder of aggregates and as sealant of the entire mixture against heat and humidity. The durability of bitumen drops with aging and fatigue failures (Mahrez, 1999).

Hence, the modification of asphalt pavement with waste such as recycled tire waste overcomes the durability challenges of pavement. In terms of load resistance, the severe damages which road structures have experienced from high and sudden loads such as in airport roads, have remarkably reduced the service life of these pavement. This escalates the importance of designing such an exceptional asphalt rubber concrete pavement (Table 7) to extend the serviceability of the constructed road (Shafabakhsh et al., 2014). Considering bitumen modification reinforcement in asphalt pavement design enhances the consistency at high temperature, increases flexibility and elasticity at low temperature, enhances adhesion with aggregates, and develops homogenous binder (Larsen et al., 1988). Thus, designing an asphalt concrete pavement layers include surface course, base course and subbase is needed to obtain these factors and in order to maintain the pavement function to grip compressive, tensile and shear stresses are needed. Consequently, these layers develop high resistance and such a promising flexible concrete pavement structure (Hamed, 2010).

This efficient design leads to high resistance against fatigue cracking which is generated in the bottom of asphalt layer (Shaw,1980). Hence, researchers explain that the rheological properties of the normal asphalt need to be intensively modified, in order to resist the crack behaviour initiated from the bottom layers of asphalt pavement structure. This is found to be a common issue in non-modified asphalt, which has weaker rheological property than modified bitumen. As a result, modified bitumen pavement acquires more durable and cost-effective properties than non-modified bitumen pavement (Mashaan, and Karim, 2012).
4.2. Distress Distribution

The resistance of asphalt pavement against distress distributions relies mainly on the properties of the bitumen binder and asphalt mixture. The durability and dynamic properties of bitumen and asphalt proportions including modifier materials such as rubber particles, significantly control the resistance limit of pavement structure against permanent failure (Mashaan, 2012). The careful selection and design of asphalt mixture provides an economical pavement, stability, workability, less voids and segregation, besides resistance to high temperature and deformation (Hamed, 2010; Mahrez, 2008). In terms of rubber asphalt modified mixture properties, both crumb or ground rubber and bitumen have a thermoelastic and viscoelastic characteristics, hence, both proportions are affected by temperature and strain changes. At low temperature, rubber particles behave more ductile than bitumen (Mahrez, 1999; Mashaan, 2012). The ductile behavior of rubber in the asphalt mixture reduces the tendency of asphalt rubber modified pavement to fatigue failure (Ali et al.,2013).

Distress mostly occurs when asphalt pavement becomes less durable. Durability is typically defined as the degree of resistance to change in Physiochemical properties of pavement surface materials with time under the action of weather and traffic. Thus, factors such as mix design, properties of binder, mix design and asphalt construction methods affect the overall durability of pavement (Mahrez, 1999). The rapid hardening process of asphalt after paving is an indicator of a durable asphalt mixture. In hardening process, asphalt mixture exhibits brittle property and subjects to distress during oxidation (age hardening), volatilization (evaporation of light components during production of hot mix asphalt at elevated temperature), polymerization (an increase in the brittleness due to the combination of resins and asphalt) and thixotropy (an increase in viscosity over time) (Peterson,1984).

The expansion of distresses in the pavement is also due to the passing vehicle loads and thermal contraction associated with temperature fluctuations. First, vehicle loading results different distresses in the pavement surface temperatures. This causes the asphalt binder to be more fluid which prevents the asphalt to resist the shearing load of the passed vehicle. However, at low temperature, the asphalt behaves more brittle. This is a common phenomenon in viscoelastic materials, such as bitumen and recycled tire rubber. The reason of this viscoelastic behavior is due to the “Normal stresses” or “Wiesenberger effect”, which attributes to the developed forces perpendicular to the shear direction. For instance, when an asphalt pavement subjects to a distress, the vertical applied load from the vehicle forces the bituminous asphalt to broaden horizontally to overfill the existed cracks (Oliver, 1981).
4.3. Fatigue and Failure Mechanism

Number of studies approve the critical impact of rheological properties of asphalt binder on the mechanism of fatigue cracking (Bahia and Davies,1994). The changes in the high and low temperature develops dramatic elastic behaviors, as bitumen behaves fluidically viscous in high temperature degrees and behaves solidly elastic at low temperature degrees. (Van der Poel,1954). In Aflaki and Memarzadeh (2011) study, the modification of asphalt using recycled tire rubber waste resulted a notable fatigue resistance due to the development in the rheological properties of the rubberized asphalt at different low, intermediate and high temperatures.

Fatigue is still one of the most urged topics in pavement structure research, the repeated loads and temperature changes escalate the need to enhance alternative asphalt modifications to lower the resulted fatigue damages. Rubberized asphalt has shown better performance than conventional asphalt, as rubber provides significant rheological properties to the asphalt mixture. Fatigue cracking is classified into thermal cracking and load -associated fatigue cracking. Thermal cracking occurs as a result of a combination of thermal tensile stress alongside with the stress applied by the passed traffic. Load associated fatigue (Figure 10) arises when repeated or fluctuated stresses which cause pavement to flex, and the base of bituminous layer reaches its maximum tensile strain, which results various pavement surface fractures and fatigues. Therefore, the fatigue cracking resistance depends on to the tensile strength and elastic properties of bituminous asphalt mixture (Mahrez, 2008).

Prior to failure or rupture, asphalt pavement layer experiences two crack generations which are crack initiation and crack propagation. Crack initiation is a multiple microcracks, which initiates as a result of macrocrack formations caused by the tensile strains. This crack behavior results gradual weaknesses in the overall structural strength of pavement (Majidzadeh et al., 1983). The repeated initiations of these microcracks concentrate stresses and generate additional crack propagation. Consequently, the crack mechanism is a result of crack initiation and propagation when the tensile stresses exceed the tensile strength limit (Mahrez, 2008).
4.4. Rutting Resistance

Rutting or permanent deformation is caused by consolidation or the displacement of materials in the pavement subgrade due to repeated traffic load. The excessive penetration of moister, stress and basic asphalt design error cause rutting failure mode. This type of deterioration regularly occurs when road is freshly paved and when the temperature soars to high degrees which soften the newly placed asphalt. However, hardening and aging of asphalt reduces the appearance of rutting. Studies show that the addition of recycled tire rubber in asphalt mixture enhances the protection of asphalt against rutting even at high temperature, as high as 60 ºC (140 ºF) (Shin et al., 1996).

The development of rutting (Figure 11) ascribes to the shear load occurs close to the surface of pavement which is the contact point between the tire and the pavement. This shear load is the main mechanism of rutting during the service life of asphalt pavement (Tayfur et al., 2007). Also, factors related to truck tire pressures, axle loads, and volume of traffic arise the occurrence of rutting. Thus, studies highly suggest that applying rubberized bitumen in the asphalt pavement design play an important factor on absorbing and resisting the rutting deformations which affect the serviceability life span of pavement. Studies also agreed that consolidating rutting and instability rutting are common types of rutting, as the extreme consolidation of pavement with the decrease in the air voids as a result of the wheel path or permanent deformation of the subgrade. While instability rutting is due to the properties of asphalt mixture which control the level of asphalt pavement resistance against rutting (Mahrez,1999; Harvey et al., 2001).
5. Conclusion

1- Treating waste tire as a valuable resource and recycling this waste as a potential choice into construction greatly helps achieving environmental and economic targets.

2- By reducing the environment threat and substituting natural aggregates to reduce the depletion in the natural resources, this recycling waste concept significantly leads to accomplishing environmentally friendly and economically products of construction.

3- The recent interest in the paving industry toward recycling tire waste is due to better properties rubber asphalt showed than conventional asphalt such as, high elasticity, better skid and rutting resistance and serviceability.

4- Although the inclusion of tire waste in asphalt mixture design reduces the asphalt content, the air voids become high and therefore increases permeability which impacts negatively on the durability of asphalt. Further studies are needed on this case.

5- Rubber pavement is mostly designed for areas to reduce the rut depth and where the risk of failures and dynamic loads are high.

6- The utilization of rubber in asphalt helps protecting the strength and quality of asphalt, and increases resistance against pavement distresses include rutting, fatigue cracking and low temperature cracking.

7- The addition of tire waste in asphalt mixture enhances the penetration index, which as a result, decreases the temperature sensitivity and improves the viscoelastic properties of the binder. The enhancement asphalt viscosity boosts the deformation resistance and reduces cracks.

8- The drastic increase in the traffic loading and sever weathering deteriorate the asphalt pavement of roads and elevate the possibility of pavement structures to fail rapidly, and thus, more investigations on building efficient and green roads are still required.
   
                                               







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