Editing-Recycled Tire Rubber In Asphalt Pavement-edited

Critical Review on The Development of Rubberized Asphalt Pavement

Properties Incorporating Recycled Tire Rubber Waste Modification

Abstract

Throughout years, seeing recycled tire rubber waste as a treasured source of secondary raw

material and reusing it into construction solicitations, particularly into asphalt pavement has been

getting 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 expand the strength and quality of asphalt. Although remarkable effort has been

made on this salvage principle, the limited findings on the physical and mechanical performance

of rubber asphalt positions this concept in its early stages. Since the properties of asphalt pavement

rely mainly on the properties of bitumen binder, severe fluctuations in environmental conditions

and loading rates profoundly 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 in

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, upsurges the dynamic load resistance and as a result, enhances the

long-term asphalt performance and stability of asphalt pavement against deteriorations.

Keywords: 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 for 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 significant concern in the research industry to achieve the needed sustainable, cost-effective

and green construction targets.

Tire waste is considered as one of the most complicated types of waste to manage which causes a

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). 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 initiated 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 disposed of on 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 (Error! Reference source not found.) 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 broad 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. Besides, pavement can consume a 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 behavior of asphalt (Oda and

Fernandes, 2001). Despite its sustainable feature, rubberized asphalt has not been thoroughly

implemented in pavement infrastructures. This refers to its limited performance (Error! Reference

source not found.) and cost-effectiveness in pavement construction.

However, the high traffic volume increases the need to improve the asphalt surface

pavement structure and its mixture (Error! Reference source not found.), a 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 acts 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

standard asphalt, especially at high temperature. Tire waste is found to be an appropriate 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 mixed with

asphalt binder (Patrick,.2006; Lee, 2003). Regarding noise reduction, many roads in the United

States and India experienced significant noise reduction from 4 to 10 decibels after applying rubber

asphalt pavement in the examined paths (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 inherent flexibility of rubber with asphalt in

creating durable pavement surface. However, the outcome of this practice was not as significant

as the, and extended serviceability features were not achieved (Carlson and Zhu, 1999). Since that

year, enormous investigations had been conducted until the 1930s when rubber asphalt was there

before utilized as a joint sealer in patches and membranes. In the 1950s, the Bureau of Public

Records of the State of California investigated the effect of rubber applications in the 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 wet asphalt process in which recycled tire rubber partially reacts with asphalt

binder was first 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 the 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 cold region in China and Scandinavia (Cheng et al., 2011).

As a result, this method significantly 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 the 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. Severe 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 for full-scale reevaluation and

improvement of asphalt concrete pavement in the entire region (SCDERA, 1999; CATRA 2006).

1.4. Recycled Tire Rubber Waste Impacts on The Engineering Properties and

Applications of Asphalt

There is a global demand for durable, sustainable and cost-effective road infrastructures

(Error! Reference source not found.) to fulfill the future requirement 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

the 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 lifespan of pavement (Error! Reference source not found.). Also, these factors

increase the need of rehabilitation 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 in 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,

aging and water damage. Also, 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 excellent 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 needed 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 most substantial 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 (Error! Reference source not found.) 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 streets. Also, wet weathering causes weaken 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 techniques for enhancing 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 useful 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 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 (Error! Reference source not found.)

is processed through either by wet process or dry process (Error! Reference source not found.):

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 (Error! Reference source not found.) 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 a well-

recognized method of its high consumption of tire rubber waste which is environmentally

beneficial. However, one of the reasons prevent this methodology from the 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, the wet process is the only process approved by the Department of

Transportation (Caltrans) according to the 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 before adding bituminous asphalt.

This technique is well known of its low consumption of bitumen but highly consumes recycled

tire rubber waste. The dry process improves the resistance properties to deformation at high

temperature and cracking at low temperature. (Cao, 2007). Furthermore, the mixture in the 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 specially graded aggregate 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 very well thought of, and

there is not enough finding in the industry on this aspect. However, it is said that the natural

reactivity of rubber-bitumen is not chemical (Heitzman,1992). The enhancement of rubber in

asphalt mixture (Error! Reference source not found.) leads to partial digestion and adsorption into

bitumen. Besides, 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 a result, forms a gel structure that enhances the

viscosity of the 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 more massive particle (Oikonomou, and Mavridou,

2009). Thus, the serviceability of rubberized asphalt mixture is better than a usual mix (Tortum et

al., 2005). However, the low bulk density and modulus of elasticity of tire rubber waste increase

the compaction difficulty of the asphalt mixture. Hence, the interaction of rubber, bitumen, and

aggregate becomes tough (Khalid and Artamendi, 2002). The mixing and transporting of the

asphalt mixture result in promising properties as rubber highly reacts with bitumen, leading to a

better bond and rigidity than in usual 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 the large particle size of recycled tire rubber

in the mix 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 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 extremely 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 (Error! Reference source not found.) , 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 the temperature is

below 13 ºC (55.4 ºF) to avoid poor compaction and early raveling. HMA must be well mixed

before opening the roads to traffic to prevent early surface failures. This HMA can replace existing

pavement as a deterioration treatment caused by severing, weathering, oxidation, and raveling.

Also, HMA has been commonly used on concrete sidewalks, bridge foundations, and can provide

cost-effective feature as it can be generated with lower thickness layer than in conventional hot

mixture. Chip seals, on the other hand, has been extensively applied on an existing pavement as a

surface seal or as an interlayer rehabilitation to an asphalt concrete leveling course. HMA layer

can be covered with up to 50 mm (2 in) of chip seals and chip seals can be used as an interlayer in

2 to 3 coats to prevent cracking, improve skid resistance and durability of pavement. Studies

recommend that chip seals should be kept 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 recognized of

its impermeable feature which ascribed to its subtle 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 initiated as a stone matrix asphalt (SMA) forming a gap graded

HMA (Error! Reference source not found.). 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 fiber to create an asphaltic

structure with tall 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 the bituminous binder are profoundly impacted on the final performance of the

pavement. The addition of recycled tire rubber waste as a modifier proportion profoundly affects

the overall asphalt rubber binder properties (Error! Reference source not found.), 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 state, 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 significant 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 level of rubber in the asphalt binder, the asphalt mixture exhibits stiffer behavior than

conventional asphalt (Agrawal, 2014). Studies highlighted the unusual 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 live mixing between the aggregate

and asphalt proportions. Studies also illustrate the right blend and transporting as another reason,

as the rubber particles react with asphalt causing primary interaction and changing in the

properties, shape, and rigidity of the utilized rubber. The performance of the rubber asphalt

modifier varies 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). Besides, 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 (Error! Reference source

not found.) (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 thickness (Cong et al., 2013). Hence, rubber asphalt

modified blend exhibits higher viscoelastic and viscosity than conventional asphalt, resulting in a

better deformation resistance (Liang et al., 2015). Further, large particle surface areas ease the

absorption process in the binder and enhance 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 stiffness of this

rubberized pavement develops a non-adhesive surface between the rubber modified asphalt

pavement and the ice layer causing multiple breakdowns 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 profoundly

put into consideration. In general, the design of asphalt pavement should highlight several essential

components includes the quality of materials, construction status, traffic loading, road geometry,

and environment. The investigation of these factors before paving the roads reduces the chance of

new 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 temperature 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. Also, 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 the condition and stress loadings. Therefore, improving the properties

of binder materials integrated into 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 modified to extend and support the

pavement structural capacity, while the hot mix asphalt in the pavement works as a stress

distributor and as a protector of sublayers. Hence, to construct eco-efficient roadway, 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 the asphalt

mixture. Accordingly, this has been an important research topic in the construction of highly

modified asphalt to overcome this plastic deformation failure and to develop pavement with elastic

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 important safety, functional and economic 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 roadway, bituminous asphalt works

as the holder of aggregates and a 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 the roadway. Concerning 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 this pavement. This escalates the importance of designing

such an exceptional asphalt rubber concrete pavement (Error! Reference source not found.) 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 are needed to obtain

these factors and to maintain the pavement function to grip compressive, tensile and shear stresses

are required. Consequently, these layers develop high resistance and such promising flexible

concrete pavement structure (Hamed, 2010).

This efficient design leads to high resistance against fatigue cracking which is generated in the

bottom of the asphalt layer (Shaw,1980). Hence, researchers explain that the rheological properties

of the regular asphalt need to be intensively modified, to resist the crack behavior initiated from

the bottom layers of the 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, fewer voids, and segregation, besides resistance to high

temperature and deformation (Hamed, 2010; Mahrez, 2008). Regarding rubber asphalt modified

mixture properties, both crumb or ground rubber and bitumen have 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 tensile 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 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, features 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 moving vehicle loads and thermal

contraction associated with temperature fluctuations. First, vehicle loading results in different

upsets 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 distress, the vertically 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

Some 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 in 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 categorized into thermal cracking and load -associated fatigue cracking.

Thermal cracking occurs as a result of a combination of thermal tensile stress alongside the tension

applied by the passing traffic. Weight associated fatigue (Error! Reference source not found.) arises

when repeated or fluctuated stresses which cause pavement to flex, and the base of bituminous

layer reaches its maximum tensile strain, which results in various pavement surface fractures and

fatigues. Therefore, the fatigue cracking resistance depends on the tensile strength and elastic

properties of bituminous asphalt mixture (Mahrez, 2008).

Before failure or rupture, asphalt pavement layer experiences two crack generations which are

crack initiation and crack propagation. Crack initiation is multiple microcracks, which initiates as

a result of macrocrack formations caused by the tensile strains. This crack behavior results in

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 initiated 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 the road is freshly paved and when the temperature soars too high degrees

which soften the newly placed asphalt. However, hardening and aging of asphalt reduce 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 (Error! Reference source not found.) ascribes to the shear load

occurs close to the surface of pavement which is the contact point between the tire and the

sidewalk. This shear load is the primary 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 essential factor in absorbing and resisting the

rutting deformations which affect the serviceability lifespan 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 dramatically helps to achieve environmental and economic targets.

2- By reducing the environmental 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 in 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 to protect 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 severe weathering deteriorate the asphalt

pavement of roads and elevate the possibility of pavement structures to fail rapidly, and thus, more

investigations on building tough and green roads are still required.

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