International Journal of Civil Engineering and Technology (IJCIET) Volume 10, Issue 05, May 2019, pp. 678-694, Article ID: IJCIET_10_05_070 Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJCIET&VType=10&IType=5 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 © IAEME Publication UTILIZATION OF SYNTHETIC REINFORCED FIBER IN ASPHALT CONCRETE – A REVIEW N. F. A. A. Musa Faculty of Civil and Environmental Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia M. Y. Aman Faculty of Civil and Environmental Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia Z. Shahadan Politeknik MeTRo, Tasek Gelugor, 13300 Tasek Gelugor, Pulau Pinang, Malaysia M. N. M. Taher Faculty of Civil and Environmental Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia Z. Noranai Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia ABSTRACT Asphalt concrete pavement which consists of aggregates and asphalt binder is widely employed in pavement construction worldwide. These materials have commonly been used for constructing the first layer of flexible road pavements. However, flexible pavements have little or even insignificant flexural strength, and their structural actions is fairly flexible under high traffic volume and load which may contribute to the tensile stresses and strain at the bottom of the bituminous layers as a result of continues flexing from to the load acting on the pavement. The strain magnitude is depends on the overall stiffness of the pavement. In recent years, a dramatic increase in traffic volume and load have contributed to road congestion and subsequently effect the pavement performance. As the world continues to urbanize, the construction of transportation roadways constantly requires quality pavement, particularly on strength, durability and driving comfort. Due to these demands, transportation experts and engineers are focusing on improving the performance and life span of asphalt concrete pavements. For the last few decades, highway materials researchers have tried different methods and additives in improving asphalt pavements performance and one of the most effective way is to reinforce asphalt mixtures by incorporating fibers. Different types of fibers are known to be used in this application and these include synthetic and natural fibers. The main function of fibers http://www.iaeme.com/IJCIET/index.asp 678 editor@iaeme.com Utilization of Synthetic Reinforced Fiber in Asphalt Concrete – A Review incorporated into asphalt mixture is to enhance the mechanical performance namely tensile strength, rutting resistance, and fatigue cracking. This paper reviewed the synthetic fiber modified asphalt concrete particularly discuss fundamental problems incorporating fiber in asphalt concrete mixture, mixing process and effects of different fibers on asphalt concrete. It is found that synthetic fiber modified asphalt concrete has significantly improved in performance compared to conventional asphalt concrete. Key words: Fiber Reinforced Asphalt Concrete, Synthetic Fiber, Mixing Process, Fiber’s Properties. Cite this Article: N. F. A. A. Musa, M. Y. Aman, Z. Shahadan, M. N. M. Taher, Z. Noranai, Utilization of Synthetic Reinforced Fiber in Asphalt Concrete – A Review, International Journal of Civil Engineering and Technology 10(5), 2019, pp. 678-694. http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=10&IType=5 1. INTRODUCTION Asphalt concrete is a material which mainly consists of aggregates and asphalt binder, and it is widely employed in pavement construction [1-5]. They have commonly been used as a material for constructing the first layer in flexible road pavements [6] because of the strong adhesion for bonding aggregates and binders [6-7] which provides excellent stability, improved mechanical properties [8] as well as superior service performance in providing driving comfort, durability and water resistance [9-10]. Nowadays, hot mix asphalt (HMA) is used as one of the main components in the construction of flexible pavement systems [9]. In Malaysia, 80% of the roads are paved, and most of the paved roads are flexible pavement constructed with hot mix asphalt (HMA) application as HMA is one of the most economical materials available and it is very suitable for Malaysia’s climate [11]. However, flexible pavements have low or negligible flexural strength, and their structural actions are fairly flexible under high traffic volume and load [12]. Recent years, the highway construction industry is swiftly developing all over the sphere due to a dramatic increase in traffic loads [13]. The increase in traffic volume creates congestion on the road pavement and induces the pavement performance. The constant loading caused by traffic flow will lead to the rise of tensile and shear stresses in the asphalt concrete which causes the loss of integrity in its structure. As a result, development of fatigue cracks will occur as the traffic induced tensile and shear stresses approach the strength of the material [14], hence, affects the longterm performance of asphalt concrete, degrade the asphalt materials [15] and slowly reduce the strength of the pavement structure [16]. Instead of traffic volume, deterioration of asphalt concrete is also caused by environmental factors as mentioned by [17-19] as well as its coating layer which demonstrates severe temperature susceptibility in terms of hightemperature rutting, medium temperature fatigue and low temperature cracking damage [7]. As the world continues to urbanize, the construction of transportation roadways constantly requires quality pavement. Due to these demands, transportation experts and engineers focused on improving the performance and life of pavements [20]. Many studies and research searching for better materials or modifications that could improve the characteristics of the asphalt mix and reduce or even eliminate the development of asphalt pavement deteriorations [17]. It should be noted that the main drawback of asphalt paving material is its weakness in tension [21]. Therefore, the application of reinforcement in asphalt concrete is one of the techniques applied to enhance their tensile strength and engineering properties, particularly when the traditional mixes do not function in accordance to the traffic, environment and the requirement of pavement structure as mentioned by Bonica et al., [5]. http://www.iaeme.com/IJCIET/index.asp 679 editor@iaeme.com N. F. A. A. Musa, M. Y. Aman, Z. Shahadan, M. N. M. Taher, Z. Noranai Utilization of fiber as a reinforcing agent is believed as one of the ways to address the drawback of asphalt pavement in term of tensile strength improvement. 2. FIBER IN ASPHALT CONCRETE Fibers have been used to reinforce paving materials since decades ago in most parts of the world. The reinforcement method of using fibers is executed through random distribution within the materials or by applying oriented fibrous materials [22]. Different types of fibers, including nylon, polyester, polypropylene and carbon have been used for reinforcing asphaltic mixtures as per reviewed by Abtahi et al. [23]. Fibers are normally used to prevent binder drain-down from aggregate particles particularly in stone matrix asphalt and porous or opengraded mixtures. However, utilization of fiber to reduce rutting and improve resistance to cracking in dense-graded mixtures are fewer [24]. Nevertheless, incorporating fiber in asphalt mixture exhibit a small increment in the optimum binder content compared to the neat asphalt mixture. Thus, it can be inferred that the addition of fiber is similar with adding a very fine aggregate into the asphalt mixture [25]. Recently, the improvement of asphalt pavement with different technologies subjected to its performance has gaining more and more popular among pavement researchers. The incorporation of fiber as a reinforcement material in asphalt concrete mixtures is one of such technologies that was invented from the cement concrete fiber reinforcement [26]. However, the application of fibers in asphalt mixtures is not a new technology. The invention of fiber can be traced back to 4000 years old arch in China built up by mixing earth clay with fibers or the Great Wall constructed 2000 years ago [27]. In the early 1900s, Warren Brothers Company of Boston, MA, patented their use of asbestos fibers in sheet asphalts and bridge pavements for the purpose of bleeding prevention of asphalt mix during humid weather service [28]. Asbestos fibers were then further used in cold-laid asphalt pavements to prevent segregation of aggregate during the placement process [28]. Asbestos has been a standard component of asphalt bridge planks, bituminous joint filling compounds, seal-coating compounds, asphalt curbing, and pavement for years [28]. Kietzman [28] reported that asbestos fibers may significantly increase the plastic strength of asphalt mixes. However, the use of asbestos fiber in asphalt concrete was continued until the 1960s [29] and no longer available due to health hazard and environmental concerns [28-30]. In 1954, Williams [31] used wire mesh reinforcement into asphaltic concrete pavement overlays to assess its effectiveness in preventing reflection cracking of bituminous concrete overlying cement concrete pavements and lateral displacement of bituminous concrete pavements when it is subjected to accelerating and decelerating traffic. However, the researcher observed some difficulty in buckling and deforming of the wire mesh during paving. In 1961, Deen and Florence [32] further reported on the same project conducted by Williams [31] on field performance of test sections. They revealed that wire mesh significantly prevent reflection of cracking of joints and replacement patches of bituminous overlay on cement concrete pavements. Nevertheless, no comments or conclusions with regard to the use of wire mesh in preventing lateral displacement of the bituminous overlay when it is subjected to accelerating and decelerating traffic. In the 1970s, due to health and environmental concerns associated with the use of asbestos fibers, researchers start to practice other types of fiber in asphalt concrete which are synthetic fibers such as polyester, polypropylene, and mineral fibers like slag wool and rock wool [30]. In 1980s, more researches has been done on the use of synthetic fibers in HMA pavement in an attempt to prevent or at least retard the occurrence of pavement cracking on pavement [30]. Hence, the use of synthetic fibers were then explored for the purpose of http://www.iaeme.com/IJCIET/index.asp 680 editor@iaeme.com Utilization of Synthetic Reinforced Fiber in Asphalt Concrete – A Review reinforcement due to its superior performance in term of tensile strengths and durability in asphalt concrete [30]. 3. FUNDAMENTAL PROBLEMS INCORPORATING FIBRE IN ASPHALT CONCRETE MIXTURE Utilization of fiber to enhance material properties have a scientific background in recent years in civil engineering. Basically, the use of fiber as reinforcing materials is mainly for the purpose in providing extra tensile strength in the asphalt mix that may increase the amount of strain energy which can be absorbed throughout the processes of fatigue and fracture of the mixture [33]. Theoretically, stresses can be transmitted to the strong fibers, thereby reducing the stresses on the relatively weak asphalt mix. The existence of good adhesion between fiber and asphalt binder helps to efficiently transfer the stresses and a larger surface area on the fiber can support this adhesion [24]. However, use of fibers to make high performance reinforced asphalt mixes need to be improved due to lack of understanding on reinforcing mechanisms and ways of optimizing fiber properties [34]. Too long fiber can create the balling problem where some of the fiber may lump together and cannot achieve a suitable blend in the asphalt concrete while too short fiber cannot provide a suitable reinforcing effect in the mix [34]. Furthermore, fiber needs to be homogeneously distributed in the mixture to prevent stress concentrations [29]. Too low fiber content may increase the probability of creating a weak cross section for cracks propagation in the surface while too high fiber content may reduce the cohesion between aggregates and shrink all fibers in one place [35]. Therefore, it is essential to select an appropriate amount of fiber and optimize the fiber characteristic in the asphalt mixture. In bitumen-fiber mastics, bitumen is called as the matrix material, the characteristics of which are changed by using fibers in the matrix as the stabilizing additives. Fibers are usually added for preventing the binder from draining out when the asphalt mixture is hot. Mastic that consist of fibers and bitumen can be considered as the medium that binds the aggregate together, thus becoming an essential part of hot-mix asphalt concrete. The mechanism of fiber that affects the bitumen is complex, and the effects on pavement performance is intense. The use of fibers in the mixture with bitumen may increase the stiffness of the binder, which can cause brittleness in the asphalt mixture. Pavement distress will occur when there is too much stiffening and it involves the disintegration and fracture under the influence of climate and traffic loading. Hence, the understanding of the bitumen-fiber mastics properties is essential in order to have better control in the performance of asphalt pavements as it is poorly classified scientifically [36]. 4. MIXING METHOD OF FIBRE IN ASPHALT CONCRETE MIXTURE Generally, there are two mixing methods used to disperse the fiber in asphalt concrete mixture, namely dry process and wet process [23,37-39]. Figure 1(a) shows the dry process, which mixes the fibers with aggregates that functions as the binder in the mixture. While in the wet process as displayed in Figure 1(b), depending to the type of additive and its nature, the additive is mixed with aggregates before adding binder [23,38,40] or added after mixing the binder and aggregates as a part of solid materials [40]. Normally, the dry process is preferred over the wet process. Furthermore, the field work done on fiber reinforced asphalt mixture has commonly utilized the dry process, probably because of the production problems that introduces fibers directly into the asphalt [23]. The advantages and disadvantages of these two methods are summarized in Table 1. http://www.iaeme.com/IJCIET/index.asp 681 editor@iaeme.com N. F. A. A. Musa, M. Y. Aman, Z. Shahadan, M. N. M. Taher, Z. Noranai (a) Dry Process [43] (b) Wet Process [44] Figure 1 Mixing method of fiber in asphalt mix However, some of the researchers modified the mixing methods to achieve better dispersion of fiber. For instance, instead of mixing the waste plastic bottle (PET) by the dry process, Ahmadinia et al., [41] added the PET after adding the bitumen and blended with the aggregate into the mixture called modified dry process. It is hypothesized that the modified dry process will result in slight changes in the shape and properties of PET during mixing. Therefore, it is essential to make a comparison in the performance of asphalt mixes prepared with PET by both dry and modified dry processes in order to determine the viability of each process [37]. Alidadi and Khabiri, [35] visually comparing both approaches to find the most efficient method of Polypropylene (PP) fiber. They recognized that dry method was suitable for PP fiber due to it homogenous dispersion and fiber placement through the mix. However, in another study conducted by Zahedi et al. [42], the researchers did a trial blend of Polypropylene fiber (PP) by the wet and dry method to observe the homogeneity of fiber in the asphalt mixture. The observation of blending fiber by wet process shows that the fibers were shirked and there was no mixing between fibers and other materials. Meanwhile, observation from the dry process indicates that, balling happened due to absorbing bitumen by fibers resulted in unsuitable mixing of PP fibers with aggregates. Hence, they claimed that both wet and dry method was not appropriate methods for mixing PP fibers in the asphalt mixture. Since there were no homogenous mixtures in these methods, they tried complex method by mixing the aggregates and bitumen for 5 to 10 seconds by mixer before gradually added segregated fiber into the mixture. From this method, the fibers were mixed uniformly with the mixture. Thus, it is reported that complex method is an ideal method for constructing and performing experiments for their research. The mixing process for the different type of fibers is summarized in Table 2. Table 1 Advantages and disadvantages of fiber’s mixing methods Dry Process Wet Process Advantages Better dispersion and placement of fiber through the mix [23,35]. Easier to carry out and normally used in fieldwork [23]. Reduces major issues of clumping or balling of fibers in the mixture [23]. Appropriately applied in plastics such as low-density polyethylene (LDPE), highdensity polyethylene (HDPE) and polypropylene (PP) with the melting points under 160°C [37] http://www.iaeme.com/IJCIET/index.asp 682 Disadvantages Compromise the adhesion between aggregate and binder because some portion of fiber like PET may melt when added to the hot aggregates [37]. Not melt in the asphalt [45]. Fiber will stick to each other. Unfeasible for PET due to its high melting point which is between 250°C and 300°C that makes it hard to attain a homogenous mixture and its tendency to segregate from binder [40, 46]. editor@iaeme.com Utilization of Synthetic Reinforced Fiber in Asphalt Concrete – A Review Table 2 Mixing Process of Fibre Researchers Hamedi et la. [47]; Moubark et al. [48]; Ramadevi et al. [49]; Zahedi et al. [42]; Qadir, [44]; Tapkın et al. [50]; Al-Hadidy and Yi-Qiu [51]; Kim et al. [52]; Sheng et al. [53]; Ye and Wu [54]; Wu et al. [7] Shanbara et al. [1]; Fakhri and Husseini, [55]; Alidadi and Khabiri [35]; Mahreh and Karim [56] Button and Hunter [57] Kim et al. [52]; Alidadi and Khabiri [35] Klinsky et al. [17]; Jaskuła et al. [26]; Takaikaew et al. [58]; Aliha et al. [59]; Muniandy and Aburkaba [60]; Mondschein et al. [61] Deghan and Modarres [46]; Usman et al. [62]; Modarres and Hamedi [40]; Moghaddam et al. [63]; Soltani et al. [64] Type of Fiber Polypropylene Mixing Process Wet Polyester Wet Glass Dry Aramid Carbon Forta-Fi Dry Dry Dry Polyethylene Terephthalate Dry 5. SYNTHETIC FIBRE REINFORCED ASPHALT CONCRETE 5.1. Polypropylene Fiber Polypropylene fibers (PP) are widely utilized as a type of reinforcing agent in concrete [65] and one of the most widely used polymers in the world because of the widespread availability, low manufacturing cost, [66] low density, high softening point and good mechanical properties [42]. The three-dimensional reinforcement offered by PP helps the concrete to become more tough and durable [67]. Table 3 displayed the engineering indices of PP fiber. Table 3 Physicochemical Indices of Polypropylene fibers [68] Indices Colour Density (g/cm3) Length (mm) Diameter (µm) Melt point (◦C) Flash point (◦C) Tensile strength (MPa) Elastic modulus (MPa) Thermal and electrical conductivities Corrosion resistance to acid and alkali Data Natural White 0.91 12–19 Around 100 160–170 590 560–770 3500 Very low Very strong Figure 2 Polypropylene fibers [44] http://www.iaeme.com/IJCIET/index.asp 683 editor@iaeme.com N. F. A. A. Musa, M. Y. Aman, Z. Shahadan, M. N. M. Taher, Z. Noranai Utilization of PP (Figure 2) is not only limited to the concrete industry as it is also can be utilized in highway construction. Tapkin [67] reported that the addition of polypropylene fibers offers a positive impact on the performance of asphalt pavements. The increase of PP contents exhibited higher stability index, 58% for the fabricated reinforced specimen incorporating 1% of PP fibers and extends the fatigue life by 27% [67]. Kim et al., [52] noted that PP fibers enhanced the Marshall stability, indirect tensile strength, and moisture susceptibility, at a volume fraction of 0.5%. Abtahi et al. [69] discovered that PP modified asphalt concrete contribute to the higher performance of asphalt concrete mixture. The results show Marshall stability and percent of air void increase while flow property decreases. AlHadidy and Yi-qiu [51] had inferred that the PP-modified asphalt mixtures performed better in comparison to traditional mixtures in term of Marshall, indirect tensile strength and compressive strength. On the other hand, the temperature susceptibility was also decreased after adding PP in the asphalt mixture. Habib et al. [70] performed dry and wet methods to evaluate the effect of both the mixing processes on asphalt mixture. The result shows that 3% PP modified wet bituminous mixture exhibited good performance in terms density, stability, and stiffness compared to 1% and 2% wet bituminous mixture. Meanwhile, the dry bituminous mixture containing 1% of PP displayed better than 2% and 3% PP dry bituminous mixture in term of stability, flow, density, and stiffness. 5.2. Polyethylene Terephthalate Fiber Polyethylene Terephthalate (PET) is a thermoplastic polymer resin of the polyester [40] produced by polymerization of ethylene glycol and terephthalic acid and is broadly used to produce plastic bottles [40,46]. The engineering properties of PET is shown in Table 4. Most of the PET production in the world is for synthetic fibers with bottle production [37,71] accounting for about 30% of the global demand [71]. The life span of PET is longer due to high resistance to biodegradation and as a result, large quantities of PET waste are accumulated [72] causing a serious environmental challenge [37,73]. With the increasing concern of keeping the environment clean, highway industry recycles the PET waste by adding it as an additive in asphalt concrete or as a substitution of fine aggregate. PET can be added either by the dry and wet process. However most of the researchers adopted dry process by adding PET into asphalt mixture as a part of solid materials due to its high melting rate which is between the temperature of 250oC up to 300oC as it impracticable to mix by wet process because of non-homogeneity dispersion, where the temperature of binder during the mixing time is substantially less than its melting point, therefore it is not usually possible to attain a homogeneous distribution of PET using the wet process [37]. Table 4 Engineering properties of PET [64] Property Specific gravity, g/cm3 Water absorption, % Tensile strength, Psi Tensile Modulus, Psi Elongation at break, % Flexural strength, Psi Flexural modulus, Psi Approx. glass transition temperature, oC Approx. melting temperature, oC http://www.iaeme.com/IJCIET/index.asp 684 Data 1.35 0.11 11,500 4x105 70 15,000 4x105 75 250 editor@iaeme.com Utilization of Synthetic Reinforced Fiber in Asphalt Concrete – A Review Choudhary et al. [37] evaluate the effect of PET Size ranging between 2.36–1.18 mm and 0.30–0.15 mm, PET contents of 2.5%, 5.0%, and 7.5% by weight of binder, and both dry and wet mixing process on the properties of PET modified asphalt mixes. The results showed that the modified dry process increased the Marshall stability in comparison to the dry process for every PET contents and sizes. However, the stability of PET modified mixes by using both dry and modified dry process depicted a significantly higher than the control mix for the PET content up to 5%. The PET size had also influenced the volumetric properties of the mix where the increased of PET in asphalt mix contributed to the increase of bulk density, lower air voids, WMA and VFB. Besides that, better resistance to moisture damage was discovered for the mix fabricated by modified dry process where the tensile strength ratio (TSR) was significantly higher than the mix produced by dry process and up to 5% of PET size reflected to the higher TSR value. Therefore, the researchers claimed that PET modified mixes that was produced through a modified dry process with coarser PET size had shown comparatively greater performance in terms of volumetric, Marshall parameters, and resistance against moisture induced damage. In another study by Moghaddam et al. [63], a response surface methodology (RSM) was performed to optimize the asphalt content and polyethylene terephthalate (PET) in asphalt mixtures with concentration of PET and binder content varies from 0% to 1% and 5 to 7% by weight of aggregate particles respectively. The experimental results indicated that the amount of 5.88% of asphalt content and 0.18% of PET were determined as the optimal values to satisfy the requirements of the Marshall mix design. Previous studies also reported the potential of PET to be reused as an additive in asphalt concrete. Results showed that, the addition of PET in asphalt mix enhanced the resistance against permanent deformation and rutting [39,74]. Meanwhile, Deghan and Modarres [46] reported that PET modified mixture had reduced the flexural stiffness of asphalt through the 4-point beam bending test. 5.3. Polyester Fiber Polyester fibers act as a good additive for asphalt mixes and have been broadly used in asphalt pavements in recent years [75]. The physical properties of polyester fiber are shown in Table 5. According to Anurag et al. [76], polyester is a type of synthetic fiber that have been used in pavements to reduce the reflective cracking. Shunzhi et al. [77] evaluated the effects of fibers in reinforcing asphalt binder under low temperature. They had reported that the addition of polyester fiber can produce notable improvement in the tensile properties of the fiber reinforced asphalt particularly in the aspect of failure tensile strain. Sheng et al., [53] conducted a comparative study on the SMA mixture with four different fibers; flocculent lignin fiber, mineral fiber, polyester fiber, and blended fiber to investigate the effects of fibers on the percent voids in mineral aggregate (VMA) in asphalt concrete. It is seen that polyester fiber and natural fiber had significantly influenced the volumetric properties, and, therefore, displayed better VMA compared to traditional SMA blend with lignin fiber. Table 5 Physical properties of polyester fibre [53] Property Length, mm Diameter, µm Relative density Melting points, oC Tensile strength, MPa Oil absorption rate, times Moisture absorption rate, % http://www.iaeme.com/IJCIET/index.asp 685 Value 6 20 1.317 260 750 4.1 2.43 editor@iaeme.com N. F. A. A. Musa, M. Y. Aman, Z. Shahadan, M. N. M. Taher, Z. Noranai Anurag et al. [76] discovered that, the addition of the polyester fiber had improved the wet tensile strength and tensile strength ratio (TSR) of the modified mixture, increase the toughness value in both dry and wet conditions as well as increase the void content, asphalt content, unit weight, and Marshall stability. Wu et al. [7] reported that, the fatigue property of asphalt mixture is improved with fibers addition, especially at lower stress levels in comparison to the mixture without fiber. Zahedi et al. [78] revealed that specimen incorporating 0.5% polyester fibers depict about 21% higher strength than the base specimen and it is suitable for moderate weather and less traffic volume. 5.4. Forta-Fi Fiber Forta-Fi® fiber is one of the synthetic fiber mainly composed of aramide Kevlar 29, polyolephin fibres and other materials manufactured by Forta Corporation in the USA. FortaFi® is a high tensile strength synthetic fiber blend that is formulated for the purpose of reinforcing the asphalt mixes in both new construction or rehab projects. Kevlar 29 aramid fibers have high tensile strength and are considered as three-dimensional asphalt reinforcement that can help to increase the resistance of asphalt mixture. Aramid fibers are known for their strength and durability in both high and low temperatures and will not melt in the asphalt mix. Polyolefin in the fibers will melt in the temperature range of asphalt mixture and works as a modifier of bitumen [79]. The engineering properties of Forta- Fi fiber is depicted in Table 6. Forta Corporation recommends adding Forta-Fi (Figure 3) fiber at a rate of 0.5 kg per ton of asphalt mixture. Table 6. Physical properties of Forta-Fi fibre [80] Material Form Specific gravity Tensile strength, Mpa Length, mm Acid/Alkali resistance Decomposition temperature, oC Polypropylene Twisted fibrillated 0.91 483 19.05 Inert 157 Aramid Monofilament 1.45 3000 19.05 Inert .>450 Figure 3 Forta-Fi® The potential improvement of asphalt mix incorporating Forta-Fi fiber has attracted pavement researchers to explore the benefits brought by this fiber either in field site or laboratory simulation. In order to improve the performance of asphalt concrete pavement in Thailand, Takaikaew et al. [58] conducted a laboratory study to evaluate the performance characteristics of the modified asphalt mixture with various asphalt binders. It is seen that, http://www.iaeme.com/IJCIET/index.asp 686 editor@iaeme.com Utilization of Synthetic Reinforced Fiber in Asphalt Concrete – A Review adding Forta-Fi fiber at 0.05% by mass of total mix had significantly improved rutting resistance, fatigue life, and resilient modulus of the asphalt mixture. They also revealed that, fiber reinforced mixes experienced higher recoverable deformation and tensile strength against control mix which contributes to a better resistance in permanent deformation and crack propagation. They claimed that incorporating fiber in asphalt pavement helps road surface pavement perform better and last longer over traditional asphalt concrete pavement. However, Jaskula et al. [26] discovered that the permanent deformation of asphalt mixture with Forta-Fi fiber at high temperature was not improved over the traditional mixtures. This can be seen when adding 0.05% of Forta-Fi fiber by weight of mixture, dynamic modulus of asphalt mix for binder course (35/50) was slightly increased for most frequencies if compared to the control mixture whereas binder course containing polymer modified bitumen (25/55-60) depicts no significant changes between the mixtures with and without fibers. On the other hand, wearing course incorporating fibers confirmed their ability to enhance properties at low temperatures. Bending beam test and fracture mechanisms theory were used to assess the low temperature cracking. The result from these methods shows that the addition of Forta-Fi fibers in asphalt mixtures performed better in term of low temperature cracking. The mixtures with fiber increase the flexural strength, critical strain and reduce the flexural stiffness modulus in -20°C and also higher fracture energy than asphalt mixture with no fibers. In another study reported by Aliha et al. [59] who assess the influence fibers on lowtemperature behavior of warm mix asphalt (WMA) materials. The comparison has been made between Jute fiber and Aramid-Polyolefin fiber. Semi-circular bending test was conducted on both fiber to obtain the result of the fracture toughness of WMA. It is determined that both fibers are able to intensify the fracture resistance of WMA mixtures in comparison with the unmodified mixture. However, the use of synthetic fiber will result in greater crack growth resistance of asphalt mixture at the test temperatures of 0oC, -10oC, and -20oC. Thus, they have claimed that Forta-Fi fiber may contribute a better resistance characteristics for crack growth more than the Jute fiber for the WMA mixture. A study also has been conducted at Arizona State University by Kaloush et al. [80] to evaluate the performance of FORTA Fiber-Reinforced Asphalt Mixtures placed on Evergreen Street in Tempe, Arizona with the overall length of pavement section equal to 211 feet. A comparison has been made on asphalt mixture with zero fiber, a mixture that contained 1-lb (0.45kg) and 2-lb (0.91kg) of fiber per ton of asphalt mixture. The samples were brought back to Arizona State University laboratory for testing. The mohr coulomb envelope was developed for all type of mixtures. The results revealed that a mixture containing 2-lb fiber experienced higher cohesion, c in comparison to other mixture which indicates that the 2-lb fiber mixture has higher resistance to shearing stress. However, its internal fraction, ϕ depicts the lowest value where 1-lb shows improvement in term of an internal fraction, ϕ over mixture with zero and 2-lb fibers which mean that 1-lb fiber mixture contributed to the increase in strength and reduce the potential of permanent deformation. Besides that, the flexural strength test shows the improvement of flexural strength for 1-lb fiber/ton mix while 2-lb fiber/ton mix decreased the flexural strength, and this may be due to the excessive fiber content in the mix. Thus, they claimed that 1-lb fiber mixture yields the best performance of fiber asphalt mix as per suggested by the manufacturer where 0.5 kilograms per ton asphalt mix is the optimum weight to be added in the asphalt mixture. They also revealed that high variability was observed which mainly due to the variance of the fiber distribution and orientation within the samples. Further information on this study can refer to Kaloush et al. [80]. http://www.iaeme.com/IJCIET/index.asp 687 editor@iaeme.com N. F. A. A. Musa, M. Y. Aman, Z. Shahadan, M. N. M. Taher, Z. Noranai 5.5. Glass Fiber Glass fiber offers interesting properties as a reinforcing material due to its strength and flexibility since it is thermally and chemically stable at bituminous mix temperatures of 200°C [81]. Glass fiber (Figure 4) is an inorganic fiber with high tensile strength and has been used to alter asphalt mixture effectively in order to enhance the deformation [82]. The utilization of glass fiber reinforced bituminous mixes may rise the construction cost but then minimize the cost of maintenance due to its advantages [56]. It is broadly used due to mechanical properties and affordable price compared to different carbon fibers, aramid and basalt [35]. The properties of glass fiber are shown in Table 7. Mahreh and Karim [56] had evaluated the fatigue characteristics of stone mastic asphalt mix reinforced with different amount of fiber glass. It is seen that, the additional of glass fiber had lessened the stability but it had increased the void in the mixture. Additionally, asphalt concrete mixture with more than 0.2% fiber content had resulted lower resistance to permanent deformation. They also revealed that fiberglass has the ability in resisting the structural distress that occurs in road pavement due to the increased of traffic loads. Thus, it decreases fatigue life by improving the resistance level against cracking and permanent deformation particularly at greater stress level. In another study, Shukla Tiwari and Sitaramanjaneyulu [83] conducted a study to evaluate fatigue life, skid resistance and rutting resistance of asphalt mix prepared with glass fiber. The results indicated that glass fiber modified asphalt mixes increased flexural stiffness and resilient modulus, enhanced resistance to permanent deformation and displayed higher fatigue life cycles in comparison to conventional asphalt mix. Morea and Zerbino [84] reported that glass macro-fibers enhanced the fracture resistance of the asphalt concretes. Fiberglass had a greater effect on increasing rutting resistance and increase the percentage of glass fiber in the mix tends to increase the ratio of Marshall leading to rutting reduction as discovered by Khabiri and Alidadi [35]. Table 7 Properties of glass fibre [55] Indices Glass type Specific gravity, g/cm3 Length, mm Tensile strength, MPa Softening point, oC Filament diameter, µm Length/diameter ratio Moisture content, % Loss of ignition, % Data E-glass 2.58 12 3100 – 3400 840 13 923 0.03 0.57 Figure 4 Glass fibers [55] http://www.iaeme.com/IJCIET/index.asp 688 editor@iaeme.com Utilization of Synthetic Reinforced Fiber in Asphalt Concrete – A Review 6. CONCLUSION This paper reviewed the potential of utilization synthetic fibers in flexible pavements as reinforcement in asphalt concrete. Utilization of synthetic fiber has strongly improved the performance of asphalt mixture such as rutting and fatigue cracking as per discussed in respective fiber type in this paper. In addition, there are two potential methods to introduce fiber in asphalt concrete; the wet and dry processes. The method for dispersing the fiber in asphalt mix should be done carefully to obtain a homogenous distribution within asphalt mix because different fiber has its respective properties and the mixing process depends on its properties. Finally, it is recommended that the detailed investigation should be done on the fiber like reinforcing mechanisms as well as optimum fiber content in asphalt concrete. Furthermore, the performance of fiber in asphalt mixture is inconsistent, therefore it is a need to evaluate the fiber distribution and orientation within asphalt mixture with the aid of scanning electron microscopy or x-ray computed tomography scan. 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