Fiber-reinforced asphalt-concrete – A review

Construction and Building Materials 24 (2010) 871–877

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Review

Fiber-reinforced asphalt-concrete – A review Sayyed Mahdi Abtahi a,1, Mohammad Sheikhzadeh b,2, Sayyed Mahdi Hejazi b,* a b

Department of Civil Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Department of Textile Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

a r t i c l e

i n f o

Article history: Received 18 May 2009 Received in revised form 11 November 2009 Accepted 18 November 2009 Available online 22 December 2009 Keywords: Asphalt concrete Fiber Reinforcement Bituminous mixtures

a b s t r a c t Asphalt concrete (AC), a mixture of bitumen and aggregates, is a sensitive material compared to other configurations used in civil engineering. Therefore scientists and engineers are constantly trying to improve the performance of asphalt pavements. Modification of the asphalt binder is one approach taken to improve pavement performance. Nowadays, there are different materials that have been employed to reinforce asphalt concrete. Furthermore, fibers and polymers are two important examples used for this purpose. However, it has been claimed that among various modifiers for asphalt, fibers have gotten much attention for their improving effects. Different researchers reported the results of the addition of a large variety of fibers to asphalt concrete as fiber-reinforced asphalt-concrete (FRAC). Basically, fiber reinforcement is considered as a coin with two sides. One side includes the randomly direct inclusion of fibers into the matrix, i.e. asphalt concrete and/or Portland Cement Concrete slabs. Another side comprises oriented fibrous materials, e.g. Geo-synthetics family. It is emphasized that the former concept is not as wellknown as the second. As a result, this paper is going to focus on the first side of the coin and to investigate FRAC materials modified by random fiber inclusion. Also, the effect of different fibers, mixing procedures and executive problems on asphalt concrete will be inspected. In this way, different literature reviews illustrated that the use of fibers in AC material has been involved with three dissimilar targets: mechanical improvement, preparation of electrically conductive mixtures, and creation of a new market to manage the waste fibers. Ó 2009 Elsevier Ltd. All rights reserved.

Contents 1.

2. 3. 4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Essentialness of bitumen reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Flexible pavements and fiber reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FRAC: methods of sample preparation and executive problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The history of fiber reinforcement in pavement engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FRAC: different fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Polypropylene fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Polyester fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Asbestos (mineral) fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Cellulose fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Carbon fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Glass fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7. Nylon fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* Corresponding author. Tel.: +98 913 316 7242; fax: +98 311 387 3049. E-mail addresses: [email protected] (S.M. Abtahi), [email protected] (M. Sheikhzadeh), [email protected] (S.M. Hejazi). 1 Tel.: +98 913 110 4626; fax: +98 311 391 2700. 2 Tel.: +98 311 391 5013; fax: +98 311 391 2524. 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.11.009

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1. Introduction 1.1. Essentialness of bitumen reinforcement Asphalt concrete (AC), a mixture of bitumen and aggregates, is a sensitive material compared to other materials used in civil engineering. Moisture beneath pavement softens subgrade soil and weakens base materials to destroy the structural capacity of the pavement [1–4]. On the other hand, traffic load induces daily damages such as fatigue cracks and pavement failures [4–6]. Therefore, scientists and engineers are constantly trying to improve the performance of asphalt mixtures and pavements. Modification of the asphalt binder is one approach taken to improve pavement performance [7]. Fundamentally, asphalt production in most refineries is a secondary process that cannot compete with fuel and other products in revenue generation. Therefore, production of better-performing asphalts is not a common strategy in petroleum refining. When the produced asphalt does not meet the climatic, traffic, and pavement structure requirements, modification has been used as one of the attractive alternatives to improve asphalt properties [8]. Generally, Fibers and Polymers are two important cases used in this way, [8–10] but the most popular bitumen modification technique is polymer modification [11,12]. However, it has been claimed that among various modifiers for asphalt, fibers have gotten much attention due to their improving effects [10].

1.2. Flexible pavements and fiber reinforcement Reinforcement consists of incorporating certain materials with some desired properties within other material which lack those properties [13]. Consequently, fiber reinforcement is considered as a coin with two sides. One side includes the randomly direct inclusion of fibers into the matrix, i.e. asphalt concrete and/or Portland Cement Concrete slabs. Another side comprises the oriented fibrous materials, e.g. Geo-synthetics family. It is emphasized that the former concept is not as well-known as the second, not only in optimizing fiber properties, fiber diameter, length, surface texture etc., but also in reinforcing mechanism [14]. Obviously, if the fibers are too long, it may create the so called ‘‘balling” problem, i.e. some of the fibers may lump together, and the fibers may not blend well with the asphalt. In the same way, too short fibers may not provide any reinforcing effect. They may just serve as expensive filler in the mix. Fundamentally, the principal functions of fiber as reinforcing materials are to provide additional tensile strength in the resulting composite; this may increase the amount of strain energy that can be absorbed during the fatigue and fracture process of the mix [15]. Some fibers have high tensile strength relative to asphalt mixtures, thus it was found that fibers have the potential to improve the cohesive and tensile strength of bituminous mixes. They are believed to impart physical changes to asphalt mixtures [16]. Research and experience have shown that fibers tend to perform better than polymers in reducing draindown of AC mixtures, that’s why fibers are mostly recommended [17]. It is thought that adding fibers to asphalt enhances material strength and fatigue characteristics while at the same time increasing ductility because of the inherent compatibility of fibers with asphalt and excellent mechanical properties [7]. Fiber reinforcement is used as a crack barrier rather than as a reinforcing element whose function is to carry the tensile loads as well as to prevent the formation and propagation of cracks [13]. Finely divided fibers also provide a high surface area per unit weight and behave much like filler materials. Fibers also tend to bulk the asphalt, so it will not run off the aggregates during construction. In terms of efficiency, mixtures with fiber showed a slight increase in the optimum binder content

compared to the control mix. In this way, adding fibers to asphalt is very similar to the addition of very fine aggregates to it. Thus, fiber can stabilize asphalts to prevent asphalt leakage [18]. That is why fibers are used in stone matrix asphalt (SMA) and opengraded friction-course (OGFC). It is important to know that the appropriate quantity of asphalt needed to coat the fibers depends on the absorption rate and the surface area of the fibers and is, therefore, affected not only by different concentrations of fibers but also by different kinds of fibers [19]. Fundamentally, fiber changes the viscoelasticity of the modified asphalt [20], increases dynamic modulus [21], moisture susceptibility [22], creep compliance, rutting resistance [23] and freeze– thaw resistance [24], while reducing the reflective cracking of asphalt mixtures and pavements [24–26]. Mohammad and Martin [27] found that the use of polymer modifiers nearly doubled both the strength and permeability of porous asphalt, but the introduction of fiber modifiers just produced a significant loss in permeability. On the other hand, researches show that fiber-reinforced asphalt materials (FRAM) develop good resistance to aging, fatigue cracking, moisture damage, bleeding and reflection cracking [28]. It has been stated that due to the lack of understanding on the reinforcing mechanisms as well as ways of optimizing fiber properties, e.g. fiber diameter, length, surface texture, etc., performance enhancement of asphalted mixtures reinforced by these fibers was found to be marginal [14]. However, several efforts have led to modeling of FRAC materials. For instance, the performance of different fibers has been predicted by ‘‘Slippage Theory” derived from the composite science. Thus, on the base of intrinsic properties of each fiber, an index k can be obtained through the following formula:

k ¼ ðdf  Ef  f Þ=ð2  s  Lf Þ

ð1Þ

Where df, Ef, ef are diameter, Young’s Modulus and strain at failure of fiber, respectively. s is an interfacial shear stress between fiber and matrix, i.e. asphalt concrete mixture [29,30]. As k increases, the corporation between fiber and the matrix will decrease. Also, through the use of ‘‘Equal Cross-Section” theory, another indicator was obtained. This indicator, which depends on certain parameters including the ratio of Young’s Modulus of fiber and matrix, the total number of reinforcing fibers and the fiber cross-section and/or fiber finesse, can be easily calculated. The more the existence of the mentioned parameters, the higher the FRAC mechanical performance [30]. Some researchers believe that fiber reinforcement of asphalt concrete is a type of polymer modification [31], but a view derived from composite science indicates that fibers would play different and distinguished roles in AC mixtures compared with polymers (Compare the Refs. [29] and [30] with [31]). 2. FRAC: methods of sample preparation and executive problems Modified binders are often used mainly for high stress, high traffic volume, and/or extreme climate conditions [32]. Consequently, FRAC materials are preferred to conventional mixes for overlays (maintenance), bridge membranes, and composite and multi-course asphalt applications [24]. However, currently there is no standard test protocol to quantify the performance characteristics of modified binders. It is important to know that unlike unmodified (neat) binders, which are Newtonian fluids, modified binders typically represent a phenomenon known as pseudo-plasticity, i.e. the viscosity values depend on the shear rate. Therefore, the Superpave binder test methods may not provide suitable guidance for the use of modified binders [32]. It seems that no research

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program has been conducted on this field and there is a wide exploring field for research in this area. Accordingly, the process of fiber introduction into the mixture has not been specified and matched by ASTM and this is of great importance. There are two potential methods for the introduction of the fibers: the wet process and the dry process. The wet process blends the fibers with the asphalt cement prior to incorporating the binder into the mixture. The dry process mixes the fiber with the aggregate before adding asphalt. Generally, the dry one is preferred over the other due to some reasons. Experimentally, the dry process is the easiest to perform and allows for the best fiber distribution in the mixture. Meanwhile since the fibers used do not melt in the asphalt there are no apparent special benefits to the wet process. In addition, the field work done on fiber-reinforced asphalt mixtures has generally used the dry process [24,33,34], possibly due to the production problems of introducing the fibers directly into the asphalt. Another reason for using the dry process is that it minimizes the major problem of clumping or balling of fibers in the mixture [35]. Abtahi et al. [36] reported that there is no difference between the wet and dry procedures, in Marshal Properties (Stability and Flow) when the Nylon 6.6 fibers of 12 mm were used in FRAC mixtures. However, widespread acceptance of FRAC technology is plagued by inherent mix problems that raise production costs, thereby reducing usage. Therefore, Echols described a new mix method which used a pre-separation blower. It was indicated that forced induction was preferred to and better than manual operation [24]. Other executive details associated with FRAC implementations are available in Refs. [37–42]. Fundamentally, fibers are largely used in SMA mixtures to act as a stabilizer preventing the draining down of the asphalt binder [43].

3. The history of fiber reinforcement in pavement engineering Hongou and Philips believed that the idea of using fibers to improve the behavior of materials is an old suggestion. The use of fibers can be traced back to a 4000 year old arch in China constructed with a clay earth mixed with fibers or the Great Wall built 2000 years ago [44]. However, the modern developments of fiber reinforcement started in the early 1960s [15]. Zube [45] published the earliest known study on the reinforcement of asphalt mixtures. This study evaluated various types of wire mesh placed under an asphalt overlay in an attempt to prevent reflection cracking. The study concluded that all types of wire reinforcement prevented or greatly delayed the formation of longitudinal cracks. Zube suggests that the use of wire reinforcement would allow the thickness of overlays to be decreased while still achieving the same performance. No problem was observed with steel/AC mixture compatibility. However, over 60 years ago in South Carolina, coarsely-woven cotton layers were spread between coats of asphalt to strengthen the road surface and comfort the ride [46,47]. The cotton served both as a binder for the asphalt and waterproof blanket to restrain water from seeping through cracks and eroding the road base. In 1976, a test site in New Jersey showed good results after one year time that led to spreading this paving practice to Georgia, Louisiana and Texas [48]. More recently, Serfass and Samanos [49] examined the effects of fiber-modified asphalt on asphalt mixtures utilizing asbestos, rock wool, glass wool and cellulose fibers. The tests conducted included resilient modulus, low-temperature direct tension, rutting resistance and fatigue resistance. Three studies were performed on a test track in Nantes, France. The first showed that fiber modified mixtures maintained the highest percentage of voids while a 13 metric ton axle load entered for 1.1 million times compared

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with unmodified asphalt and two elastomer modified mixtures. The authors concluded that this corresponds to better drainage and therefore a decreased susceptibility to moisture related distress in the porous mixtures tested. In the second study, two million load applications were applied to a fiber-modified asphalt which was used as an overlay mixture on pavements with signs of fatigue distress. After the load applications, the pavement surface was noted to have ‘‘a well maintained macrostructure, and practically no cracking whatsoever, even on the flexible structural pavement” [49]. The authors concluded that this verified the performance of fiber-modified asphalt concrete as an overlay mixture. The macrostructure integrity relates to maintained skid resistance over time, and the lack of fatigue cracks implies that the fatigue life of the fiber modified overlay is greater than the fatigued, unmodified pavement underneath. Fiber modified overlays were also constructed over fatigued pavements in the third study reported by Serfass and Samanos [49]. After 1.2 million load applications it was observed that all of the fiber modified overlays showed no sign of fatigue related distresses or rutting compared to the unmodified samples which did show signs of distress. This corresponded to the findings of the second study saying that the fatigue life of the fiber modified pavement is improved over unmodified mixtures. Fiber modification also allowed for an increase in film thickness, resulting in less aging and improved binder characteristics. Addition of fibers also resulted in the reduction of temperature susceptibility of asphalt mixtures. ‘‘Adding fibers enables the development of mixtures rich in bitumen [asphalt binder], and therefore displaying high resistance to moisture, aging, fatigue and cracking”, Serfass and Samanos reported. In a separate study, a fracture mechanics approach was used to evaluate the effects of fiber reinforcement on crack resistance. Therefore Jenq et al. [50] used Polyester and Polypropylene fibers to modify mixtures that were then tested for modulus of elasticity, fracture energy and tensile strength. Fracture energy in modified samples increased by 50–100%, implying increased toughness but elasticity and tensile strength results were not significantly affected by these fibers. Simpson et al. [51] conducted a research on modified bituminous mixtures in Somerset, Kentucky. Polypropylene, Polyester fibers and polymers were used to modify the asphalt binder. Two proprietary blends of modified binder were also evaluated. An unmodified mixture was used as a control sample. The testing procedures included Marshall Stability, indirect tensile strength (IDT), moisture damage susceptibility, freeze/thaw susceptibility, resilient modulus and repeated load deformation. Mixtures containing polypropylene fibers were found to have higher tensile strengths and resistance to cracking. None of the fiber modified mixtures showed resistance to moisture induced, freeze/thaw damage. Fiber Modified mixtures showed no improvement in stripping potential. IDT results predict that the control and Polypropylene mixtures will not have problems with thermal cracking whereas the mixtures made with Polyester fibers and polymers may. Mid-range temperature resilient modulus tests show that Polypropylene fiber modified mixtures were stiffest, while high temperature resilient modulus testing measured increased stiffness for all mixtures over the control. Rutting potential as measured by repeated load deformation testing was found to decrease only in polypropylene modified samples. Uniform distribution of fibers in a composite mixture is a key to mixture performance. In a study of carbon fiber modified Portland cement concrete, Chen et al. [52] concluded that an even distribution of fibers was essential for the improved performance of mixtures. Three dispersing agents were used and the concrete mixtures were tested for flexural strength at different fiber contents. All mixtures containing dispersing agents resulted in significantly higher flexural strength

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and toughness than the control mixtures. It is noted that the fibers used in this study had an average length of 5.1 mm. Putman and Amirkhanian [22] used waste tire and carpet fibers in SMA, comparing mixtures made with cellulose and polyester fibers. The outcomes showed that adding wastes increases the toughness of SMA mixtures, without making any significant difference in permanent deformation. Until now the researches in this field are continuing. Some of them are discussed in the following.

4. FRAC: different fibers 4.1. Polypropylene fiber Polypropylene fibers are widely used as reinforcing agents in concrete [53–57]. The polypropylene fibers provide the threedimensional reinforcement of the concrete. In this way, concrete becomes more tough and durable [58,59]. Polypropylene fibers are vital components of high-performance concrete [60,61]. Polypropylene fibers were also used as modifiers in asphalt concrete in the United States. Ohio State Department of Transportation (ODOT) has published a standard for the use of polypropylene fibers in high-performance asphalt concrete [62]. Polypropylene fibers were used in a 1993 study by Yi and Mc Daniel [63] in an attempt to reduce reflection cracking in an asphalt overlay. Although crack intensities were less on the fiber modified overlay sections, no reduction or delay in reflection cracking was observed. Sections in which the pavement had been cracked and seated before the overlay were found to have less reflection cracking when fibers were used in either the base or binder layers. [63]. Huang and White [20] conducted a research on asphalt overlays modified with polypropylene fiber. These mixtures together with others that had no fiber were sampled by coring and taken to the laboratory for further analysis. It was concluded from the laboratory testing that the fiber modified mixtures were slightly stiffer and showed improved fatigue life. The biggest problem encountered with polypropylene fibers was the inherent incompatibility with hot asphalt binder due to the low melting point of the fiber. Huang and White also stated that further research was needed to understand the viscoelastic properties of fiber-modified asphalt mixtures. Based on the extensive research carried out by Tapkin [25], asphalt concrete specimens with polypropylene fibers were manufactured at the optimum bitumen content. It was observed in fiber-reinforced specimens that the Marshall Stability values increased and flow values decreased in a noticeable manner. The fatigue life of these specimens was also increased. The fiberreinforced asphalt mixture exhibits good resistance to rutting, prolonged fatigue life and less reflection cracking. A comparative study carried out by Abtahi et al. [64] showed that the performance of Poly Propylene (PP) fibers, 0.125% by the total weight of mix and 12 mm length was more statistically desirable than Styrene–Butadiene-Styrene (SBS). The tests included: Marshall and Resilient Modulus. Another research program illustrated that the performance of PP fibers are more advantageous and this is because of their low melting point, i.e. 162 °C. Consequently, a property called ‘‘tackiness” glues complementarily the fiber to the matrix. This achievement has been verified by an Artificial Neural Network (ANN) and has been viewed through experiments [65]. Recently, Tapkin et al. [31,66] concluded that the PP fiber of 3 mm long by the total dosage of 0.3% modifying bitumen, using the wet procedure, represents the best FRAC samples in Marshall Specifications and Static Creep Properties. The mixing rate, time

and temperature were 500 rpm, 2 h and 163 °C, respectively. So, in this optimized condition the Marshall stability increased by 20% and the FRAC specimens under repeated creep loading at different loading patterns improved by 5–12 times compared with control specimens. Another exploring paper suggests the use of wet procedure for PP fibers in FRAC sample preparation [67]. 4.2. Polyester fibers Polyester is the polymerized product of components from crude oil of which asphalt is also a component. Shiuh and Kuei [68] reported that polyester fibers would be used if the strong and durable reinforcement of bitumen-fiber mastics was needed at higher temperatures. BoniFibers™ is the trade name for polyester fibers manufactured and supplied to blend with asphalt. A report is available which implies the successful application of BoniFibers™ of 6 mm length in the city of Tacoma, United States of America. Based on this report, in order to assure uniform distribution of BoniFibers™ throughout the asphalt concrete, they must be added to the aggregate at the beginning of the dry-mix cycle which lasts at least 15 and preferably 30 s [69]. Maurer and Malasheskie [26] investigated the influence of fibers in overlay mixtures. Polyester was chosen over polypropylene because of its higher melting point. It was announced that the construction of the mixture was done without difficulty or extra equipment. The polyester fiber modified mixture was compared to several types of fiber reinforced segments and a control section, i.e. without any reinforcement. Test sections were rated for ease of construction, cost and resistance to reflection cracking. Shopeng et al. [70] investigated the effects of polyester fibers on the rheological characteristics and fatigue properties of asphalt. The results indicated that the viscosity of asphalt binder is increased with increasing polyester fiber contents, especially at lower temperature. They confirmed that the fatigue property of asphalt mixture can be improved by adding fiber, especially at lower stress levels. 4.3. Asbestos (mineral) fiber Asbestos is the only mineral substance used as a textile fiber. The substance is found in fibrous reins of serpentine or amphibole rock [87]. At first, it was tried to use non-synthetic fibers in pavements; therefore, cotton fibers and asbestos fibers were used, but these were degradable and were not suitable as the long term reinforcements [71]. Asbestos was also used until it was recognised as a health hazard [72,73]. In 1990 Huet et al. [74] published the results of a study in which they had compared changes in void contents and hydraulic properties of plain and modified asphalt mixtures placed on the Nantes fatigue test track in France. A polymer modifier (SBS) was used in two of the mixtures and a mineral fiber (asbestos) was used in the third one to modify the base mixture. Plain and SBS modified mixtures showed similar decreases in void content and hydraulic properties after 1,100,000 load cycles. In contrast, Huet concluded that the mixtures modified with fibers ‘‘had undergone no reduction in void content; its drainage properties were practically unchanged and rutting was minimal” after the same loading. 4.4. Cellulose fiber Decoene [75] investigated the effects of cellulose fibers on bleeding, void content reduction, abrasion, and drainage in porous asphalt in this studies. Cellulose fibers in the mixture allowed asphalt contents to be increased while drastically decreasing bleeding of the binder. No changes were observed in either void

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content or abrasion after adding cellulose fibers. Full-scale test sections on Belgian roads were monitored for drainage over a sixmonth period. Those sections containing fibers retained the same drainage quality over six months, while the drainage time doubled in sections without fibers [76]. Stuart et al. [77] investigated loose cellulose fibers, a pelletized cellulose fiber and two polymers. The mixtures were evaluated for binder drain-down and resistance to rutting, low temperature cracking, aging and moisture damage. Drain-down tests illustrated that all mixtures with fiber drained significantly less than those with polymers or the control. Fiber modified mixtures were the only ones to meet test specifications for draindown. The control samples were found to have excellent resistance to rutting and no significant difference was observed between the control and mixtures with modified binder. Low temperature and moisture damaging results were inconclusive. Polymer modified mixtures were found to have better resistance to aging. Partl et al. [78] used various contents of cellulose fibers in a study of SMA. Mixtures were evaluated using thermal stress restrained specimen tests and indirect tensile tests. Problems with fiber clumping occurred in the mixing process. Distribution of fibers was improved by increasing mixing temperature and duration, but some clumps were still present. The study concluded that SMA with cellulose fiber did not significantly improve the mix based on the two tests conducted. The authors believe that the poor distribution of fibers may have caused the limited improvement. They suggested further research to confirm this theory. Cellulose fibers were used in another study of SMA by Selim et al. [79]. Testing included binder drain-down, moisture susceptibility (reported as tensile strength ratio), and static creep modulus and recovery efficiency. Fibers were added to mixtures which contained standard and polymer modified binders. Binder drain-down results show dramatic improvement in all mixtures containing the cellulose fibers. Mixtures with plain asphalt binder and fibers presented the highest indirect tensile strength. The tensile strength ratio induced damage of all the mixtures tested after conditioning compared to polymer modified mixtures that contained fibers that showed the lowest tensile strength and resistance to moisture, however, variable, statistical analysis proved that creep modulus and recovery efficiency were better in mixtures containing fibers and plain binder rather than with fibers and polymer modifier. A cellulosed fiber material was added to Recycled asphalt concretes (RAC) as a supplement. The results indicated that adding fiber improves the basic performances of RAC in terms of resistance to rutting, moisture susceptibility and the cracking as well as its durability. It was concluded that RAC with modified asphalt binder at the recycling rate of 70% is recommended as a balanced result [80]. The dynamic characteristics of fiber-modified asphalt mixture were investigated by Shaopeng et al. [81] Cellulose fiber, polyester fiber and mineral fiber were used as additives to asphalt mixture. Experimental results show that all fiber-modified asphalt mixtures have higher dynamic modulus compared with control mixture. 4.5. Carbon fibers Carbon fibers are thought to offer more advantages than other fiber types for the modification of asphalt binder. Since the fibers are composed of carbon and asphalt is a hydrocarbon, they are thought to be inherently compatible. Since carbon fibers are produced at extremely high temperatures (over 1000 °C), fiber melting is not an important issue due to high mixing temperatures. The high tensile strength of carbon fibers should increase the tensile strength and related properties of AC mixtures, including resistance to thermal cracking. The stiffening effect showed by adding other fibers should also occur in carbon fiber modified mixtures,

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increasing the fatigue life of pavements. Therefore it was hypothesized that carbon fibers should be the most compatible, best performing fiber type available for modification of asphalt binder. Carbon fibers are produced from either poly acrylonitrile (PAN) or pitch precursors [82]. Carbon fiber showed consistency in results, and as such it was observed that adding fiber does affect the properties of asphalt mixtures, i.e. an increase in its stability and a decrease in the flow value as well as an increase in voids in the mixture. The results indicated that fibers have the potential to resist structural distress in pavement, in the wake of growing traffic loads, and thus improve fatigue by increasing resistance to cracks or permanent deformation. Therefore, it was concluded based on the results that adding carbon fiber to asphalt mixture will improve some of the mechanical properties of the mixture like fatigue and deformation in the flexible pavement [83]. It is interesting to know that the use of Carbon fibers in AC mixtures also improves the electrical conductivity of the pavement and its performance is better than graphite. Therefore, removing snow and ice may be possible by thermo-electrical techniques on highways in winter [84,85]. 4.6. Glass fibers The historical origin of glass and glass fibers is uncertain. The fiber-forming substance is glass. Glass fiber has high strength and its elongation is only 3–4%, but its elastic recovery is 100 percent. Fibers of glass will not burn. However they soften at about 815 °C and their strength begins to decline at temperatures above 315 °C [87]. It is thought that adding glass fibers to asphalt mixtures enhances material strength and fatigue characteristics while increasing ductility. Due to their excellent mechanical properties, glass fibers might offer an excellent potential for asphalt modification. With new developments in producing glass fiber, reinforced bituminous mixtures can be more cost competitive and cost effective as compared to modified binders. The use of glass fiber-reinforced asphalt mixtures may increase the construction cost, however this may reduce and save the maintenance cost [15]. The critical stress intensity factor or fracture toughness for glass FRAC is higher than that for plain asphalt concrete which indicates stronger resistance to crack propagation. Glass fiber-reinforced asphalt concrete can improve the stability and the deformability of the asphalt concrete with no increasing bitumen content of Hot Mix Asphalt (HMA) which will be beneficial to prevent rutting and bleeding in high temperature degrees during the hot season [86]. 4.7. Nylon fibers The term ‘‘Nylon” was derived from no-run. The name originally considered by its invertors to emphasize durability of ladies hosiery manufactured from it [88]. Nylon, a popular facing yarn of carpets, is used for the actual recycled carpet fibers in asphalt pavement. Joon et al. [89] investigated the influence of Nylon fibers on the fatigue cracking resistance of asphalt concrete using fracture energy. The experimental program was designed with two phases: the single fiber pull-out test and the indirect tension strength test. Through pull-out tests of 15-denier single nylon fibers, the critical fiber embedded length was determined to be 9.2 mm. As for indirect tension strength tests, samples of asphalt concrete mixed with nylon fibers of two lengths, i.e. 6 and 12 mm, were prepared and tested based on the results of the pull-out tests (critical embedded length) and three volume fractions of 0.25%, 0.5% and 1%. The use of asphalt concrete samples fabricated with fibers of 1% volume and the length of 12 mm re-

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sults in 85% higher fracture energy than non-reinforced specimens, showing improved fatigue cracking resistance [89].

5. Conclusions This paper investigated the advantages of using random-inclusion fibers in flexible pavements as fiber-reinforced asphalt-concrete, FRAC, materials. Thus it was indicated that the use of fibers in AC mixtures has been involved with three dissimilar targets: mechanical improvement, preparation of electrically conductive mixtures and creating a new market to manage the waste textile fibers. Generally, fibers change the viscoelasticity of mixture; improve dynamic modulus, moisture susceptibility, creep compliance, rutting resistance and freeze–thaw resistance; while they reduce the reflective cracking of asphalt mixtures and pavements. These properties were separately discussed for different kinds of fibers including: Polypropylene, Polyester, Asbestos, Cellulose, Carbon, Glass and Nylon. In addition, methods of sample preparation and executive problems were conversed. Therefore it was found that there are two potential methods for the introduction of the fibers: the wet process and the dry process. Generally, the dry one is preferred over the other due to some reasons discussed in the paper. On other hand, modeling of mechanical properties of FRAC mixtures using composite science principles can be considered as a new research field. Finally, it is recommended that the orientation of fibers through the FRAC specimen can be examined with the aid of optical and/or scanning electron microscopy and it seems that this is an unoccupied research area.

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