II fracture strength of asphalt mixtures

Construction and Building Materials 293 (2020) 117850

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Influence of natural fibers (kenaf and goat wool) on mixed mode I/II fracture strength of asphalt mixtures S. Pirmohammad ⇑, Y. Majd Shokorlou, B. Amani Department of Mechanical Engineering, Faculty of Engineering, University of Mohaghegh Ardabili, Daneshgah Street, Ardabil 56199-11367, Iran

h i g h l i g h t s
Mixed mode I/II fracture behavior of asphalt mixtures containing two natural fibers is evaluated.
Three different lengths and percentages of fibers are used to prepare mixtures.
Mixtures containing 0.3% kenaf fibers with 8 mm in length showed the best results compared to those reinforced by kenaf fibers.
Mixtures reinforced by 0.3% goat wool fibers with 4 mm in length exhibited the best results compared to those reinforced by goat wool fibers.
The kanaf and goat wool fibers are found to be more suitable for the dominant mode I and dominant mode II loadings, respectively.

a r t i c l e

i n f o

Article history: Received 26 May 2019 Received in revised form 6 December 2019 Accepted 10 December 2019

Keywords: Asphalt mixture Mode mixity Kenaf and goat wool fibers Fracture strength

a b s t r a c t Asphalt mixtures are one of the most prevalently used materials in road structures. The aim of this paper is to investigate the influence of natural fibers (i.e., kenaf and goat wool) on fracture strength of asphalt mixtures. Both fibers with three different lengths (i.e., 4, 8 and 12 mm) and contents (i.e., 0.1, 0.2 and 0.3% by weight of total asphalt mixture) are used to prepare asphalt mixtures. Fracture experiments are conducted under various mode mixities (i.e., pure mode I, pure mode II and two mixed modes of I/II) using SCB (semi-circular bend) specimen at a low temperature of 15 °C. The results reveal that both the kenaf and goat wool fibers improve the fracture strength of asphalt mixtures significantly. Findings of this research indicate that the kenaf and goat wool fibers are promising materials for manufacturing asphalt pavements. However, the asphalt mixtures reinforced by 0.3% kenaf fibers with 8 mm length and 0.3% goat wool fibers with 4 mm length demonstrate the best results compared to other mixtures studied in this research. Furthermore, the kanaf and goat wool fibers are found to be suitable for the dominant mode I and dominant mode II loadings, respectively. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Cracking at low temperatures is one of the main types of asphalt pavement deteriorations [1]. This phenomenon accelerates the deterioration of asphalt overlays which need maintenance sooner than anticipated. In the past years, many theoretical and experimental studies have been performed on the cracking of asphalt mixtures at low temperatures to explore the failure mechanism for choosing suitable materials with improved fracture resistance [2]. For example, Aliha et al. [3] and Pirmohammad and Ayatollahi [4] studied the influence of binder type and amount of air void on crack growth resistance of asphalt concretes. According to their results, the amount of air void accelerated the crack propagation,

⇑ Corresponding author. E-mail address: [email protected] (S. Pirmohammad). https://doi.org/10.1016/j.conbuildmat.2019.117850 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

and the soft binder improved the fracture resistance of asphalt concrete. The results of the experiments conducted by Fakhri et al. [5] on various asphalt mixtures showed that lime aggregates had better fracture resistance than silica aggregates at low temperatures. Based on investigations performed by Pirmohammad and Kiani [6,7], asphalt mixtures exposed to fluctuating temperature conditions showed less fracture resistance compared to when they were exposed to constant temperature conditions. It is also noticed that the fracture resistance of asphalt mixtures initially increases and then reduces by decreasing the ambient temperature [8–12]. In another study, Behbahani et al. [13] investigated the influence of various additives like SBS, ASA and crumb-rubber. Their results indicated that the mentioned additives increased the strength of asphalt mixtures against fracture. In addition, Fakhri et al. [14] studied the influence of different parameters (i.e., aggregate gradation, rubber and cement contents) on strength of the asphalt mixtures against fracture.

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A number of studies have been also carried out on the influence of nanomaterials on crack growth behavior of asphalt mixtures. For example, Ameri et al. [15] added carbon Nano-tubes to asphalt mixture. Their results exhibited that nanomaterials can affect the fatigue and fracture behavior of asphalt mixtures significantly. In another study, Kordi and Shafabakhsh [16] investigated the influence of Nano Fe2O3 on the mechanical properties of asphalt mixture. Pirmohammad et al. [17,18] studied the crack growth behavior of asphalt mixtures modified by nanomaterials. Their results demonstrated that all the carbon Nano-tubes, Nanoclay, Fe2O3 and Al2O3 particles increased the strength of asphalt mixtures against crack growth. Fibers are one of the most popular materials for reinforcing cement concretes and asphalt mixtures [19]. Researchers utilized different types of fibers as additives to improve the performance of asphalt mixture. Generally, there are two main types of fibers: synthetic (i.e., human-made fibers) and natural fibers (mainly plant fibers and animal fibers) [20]. Many investigations can be found in the literature concerning the application of various synthetic fibers such as polypropylene [21], polyester [22], polyethylene terephthalate [23], steel [24], carbon [25], glass [26], etc. for improving the properties of asphalt mixture. For example, Klinsky et al. [27] used a mixture of aramid and polypropylene fibers as an additive to improve the performance of asphalt mixture. Apostolidis et al. [28] employed two types of synthetic fibers (i.e., aramid and polyolefin) to evaluate the fracture performance of asphalt mortar. They concluded that the synthetic fibers improved the mechanical characteristics of asphalt mortar. On the other hand, researchers have also employed natural fibers as an additive in asphalt mixtures and concretes. Natural fibers are environmentally friendly due to having no or very low content of CO2 in their production process. Therefore, natural fibers are also called green fibers, particularly in the case of plant fibers. Meanwhile, the plant fibers like jute, kenaf, sisal, coconut, etc. absorb CO2 from the atmosphere. Previous investigations on the concrete revealed that natural fibers led to improvement in concrete properties such as its durability, toughness, cracking and fatigue resistances [29]. Elsaid et al. [30] characterized the mechanical properties of kenaf fiber reinforced concrete. According to their results, the kenaf fiber reinforced concrete showed more distributed cracking and greater toughness than plain concrete. According to Sani et al. [31] and Mughal et al. [32], the stability increased by the addition of coir and kenaf fibers to asphalt mixture. Mansourian et al. [33] and Aliha et al. [20] added jute fibers to WMA (warm mix asphalt) concrete to improve its fracture properties. Based on an investigation performed by Morova [34], the stability of asphalt mixtures enhanced by utilizing basalt fibers. Kenaf fiber is known to increase the strength of concrete, but its effect on asphalt mixture is to some extent unknown [35]. It is also pointed out that goat wool fibers have been used for removing the cracks in a kind of concrete called Sarooj [36]. As mentioned above, there are very few investigations carried out on the performance of kenaf fiber reinforced asphalt mixtures. Particularly, no investigation has been reported so far on the fracture strength of asphalt mixtures reinforced by kenaf and goat wool fibers. Therefore, this paper investigates fracture behavior of asphalt mixtures reinforced by kenaf and goat wool fibers under different fracture modes. To measure the fracture strength of such materials, four steps must be performed: 1) selecting a suitable test specimen, 2) performing finite element simulations on the selected test specimen to calculate geometry factors, 3) manufacturing the test specimens, and 4) performing experiments on the test specimens prepared from asphalt mixtures to calculate the fracture strength. These steps are explained in detail in the following sections.

2. Materials Asphalt mixtures are composite materials constituting from three main components including binder, aggregate and air void [37]. In this study, binder with a penetration grade of 60/70 is used for preparation of asphalt mixtures. The full specifications of this binder are given in Table 1. An aggregate gradation presented in Table 2 is used for all the mixtures considered in this research. Also, an air void content of 4% is selected for all the mixtures. In order to study the influence of kenaf and goat wool fibers on fracture behavior of asphalt mixtures, both the kenaf and goat wool fibers with three different lengths (i.e., 4 mm, 8 mm and 12 mm) and contents (i.e., 0.1%, 0.2% and 0.3% by weight of total asphalt mixture) are mixed with the binder and aggregates described above. Meanwhile, another asphalt mixture called control asphalt mixture (which contains no fiber) is also prepared to highlight the influence of fibers. Fig. 1 shows these fibers chopped by a cutter. Properties of the kenaf and goat wool fibers are given in Table 3. The standard Marshal mix design is used for designing the mixtures. In order to ensure the dryness of the chopped fibers, they are put into an oven to be heated for one hour at 160 °C, as recommended by Arabani and Shabani [38]. Generally, there are three methods of wet, dry and combination of them to add fibers to asphalt mixtures. In the wet method, fibers are firstly added to binder by using a high shear mixer, and the mixture of them is then blended with aggregates to produce fiber reinforced asphalt mixtures [25,38]. Conversely, in the dry method, fibers are initially mixed with aggregates, and the mixture of them is then blended with the binder. Researchers indicated that the dry method is preferable to the wet one because the dry method is simple with no requirement of high shear mixer. In the meantime, agglomeration of fibers (which causes creation of weak zones within asphalt mixture) in the dry method is much less than the wet method [23,39,40]. In the third method, aggregates and binder are firstly mixed together, and fibers are blended with them at the end [41]. In this research, all the three methods were evaluated, and the third method was finally selected because of having the least agglomeration of fibers in the mixture. For further clarifying the third method used in this research, it is pointed out that aggregates and binder are mixed together for about three minutes to ensure that all the aggregates are coated by binder. Afterwards, the fibers are gradually added to the mixture, and blended for five

Table 1 Specifications of the binder used in this research. Property

ASTM Standard

Value

Specific gravity at 25 °C (g/cm3) Flash point (°C) Penetration at 25 °C (0.1 mm) Ductility at 25 °C (cm) Softening point (°C) Kinematic viscosity at 135 °C (mm2/s)

D70 D99 D5 D113 D36 D2170

1.017 280 65 100 50 420

Table 2 Aggregate gradation used in this research. Sieve size (mm)

Gradation limits

Passing percentage (%)

12.5 9.5 4.75 2.36 0.3 0.075

100 90–100 55–85 32–67 7–23 2–10

100 95 70 49 15 6

S. Pirmohammad et al. / Construction and Building Materials 293 (2020) 117850

3

Fig. 1. a) kenaf fibers, b) goat wool fibers.

preparing the asphalt mixtures in this research, the fibers are not chemically treated anymore.

Table 3 Properties of the kenaf and goat wool fibers. Fiber type Kenaf Goat wool

Elastic modulus (GPa)

Tensile strength (MPa)

Diameter (lm)

Density (g/cm3)

3. Methodology

38 19

701 1580

70–75 60–75

0.94 1.28

3.1. Geometry of test specimen

minutes to achieve a uniform mixture with a minimum agglomeration of fibers in the mixture. In the mixing process, the temperature is set at 145 °C. It is noticed that at the initial step of this research, we added fibers with five lengths (i.e., 4 mm, 8 mm, 12 mm, 16 mm and 20 mm) and five contents (i.e., 0.1%, 0.2%, 0.3%, 0.4% and 0.5%) to asphalt mixture. But, the problem of agglomeration was observed for the long fibers (i.e., 16 mm and 20 mm) and the high fiber contents (i.e., 0.4% and 0.5%). Hence, these choices of the fiber lengths and contents were not selected for further investigation. It is worth mentioning that the researchers conduct chemical pretreatment on fibers in preparation of cement concretes to ensure that the fibers are in a suitable condition [30,42]. Chemical pretreatments are used to increase the bond between the cement mixtures and fibers. In the first step of the current task, the alkaline treatment was conducted on the fibers. Some mixtures were prepared with and without alkaline treatment, but no discrepancy was observed in the fracture strength of asphalt mixtures. This may be imputed to the fact that the chemical material coated on the fibers is vaporized by receiving the oven’s heat during the preparation process of asphalt mixtures. Hence, both the treated and untreated fibers showed the same results. Consequently, for

Looking at the literature reveals that researchers have employed three main specimens (i.e., SENB (single edge notched beam) [43–49], DC-T (disc-shaped compact tension) [50,51] and SCB [3,11,47,52,53]) to conduct fracture experiments on asphalt mixtures. It is worth mentioning that the DC-T specimen is only employed for performing tests under pure mode I loading, and the SENB specimen can produce only pure mode I and limited mixed mode I/II loadings; while, all mode mixities (i.e., pure mode I, pure mode II and mixed mode I/II) can be simulated by the SCB specimen. This specimen is also popular for asphalt engineering community because of producing cylindrical samples of asphalt mixtures using gyratory compactor, marshal compactor, etc. Fig. 2a displays geometry of the SCB specimen containing a vertical edge crack. The SCB specimen is put upon bottom supports (located at S1 and S2) and loaded at its top surface by applying a vertical load of P. As mentioned earlier, all mode mixities including pure mode I, pure mode II and mixed mode I/II can be simulated by changing the position of the crack (i.e., L). 3.2. Compaction and preparation of SCB specimen The asphalt mixtures described in Section 2 are poured into cylindrical molds, with 100 mm in diameter and 60 mm in height,

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Fig. 2. a) Geometry of SCB specimen, b) meshing of SCB specimen.

and are then compacted. In the next step, the cylindrical specimens are cut into discs with 25 mm in thickness using a masonry sawing machine. These discs are then cut into two pieces of semi-circle. A crack with 17 mm in length is finally carved within the semicircles at the locations presented in Table 4 using a cutting machine with a very thin blade (i.e. 0.3 mm). It is worth mentioning that the geometrical dimensions of the SCB specimen used in this research is selected based on an investigation performed by Molenaar et al. [52]. They used the SCB specimens with three different diameters (i.e., 100, 150 and 220 mm) and thicknesses (i.e., 25, 50 and 75 mm) for calculating the fracture toughness (i.e., KIc) at different temperatures (i.e., 10, 0, 15 and 25 °C). According to their results, the fracture toughness was independent of the SCB diameter ranging from 100 mm to 220 mm and independent of the SCB thickness ranging from 25 mm to 75 mm at a temperature of 0 °C or lower; whereas, its value was dependent on the SCB diameter and thickness at high temperatures (i.e., 15 °C and higher). Thus, since the experiments are performed at a low

temperature of 15 °C, the SCB specimen with 100 mm in diameter and 25 mm in thickness is used in the current research. 3.3. Numerical modeling of SCB specimen Finite element simulations are carried out on the SCB specimen using ABAQUS by considering the parameters as: SCB thickness t = 25 mm, SCB radius R = 50 mm, crack length a = 17 mm, Young’s modulus E = 12.5 GPa and Poisson’s ratio t = 0.35. About 70,000 three-dimensional solid elements (i.e., C3D20R) are regarded in the finite element simulations to find the crack parameters with high accuracy. Meanwhile, the first ring around the crack front is selected as collapsed elements to consider the singularity of stresses at the crack front of the SCB specimens. The size of elements at the regions close to the crack front is considered as small as possible to increase the accuracy of the results. Fig. 2b shows meshing of the SCB specimen with three-dimensional elements. Similar to an investigation performed by Alkilicgil [54], in the finite element

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S. Pirmohammad et al. / Construction and Building Materials 293 (2020) 117850 Table 4 Results of finite element analyses on the SCB specimen for different mode mixities. Mode mixity

Geometry & Loading

KI pffiffiffiffiffi (MPa m)

KII pffiffiffiffiffi (MPa m)

YI

YII

Me = 1

0.39

0.00

4.29

0.00

Me = 0.66

0.15

0.08

1.63

0.95

Me = 0.32

0.08

0.15

0.89

1.62

Me = 0

0.00

0.21

0.00

2.26

models (see Fig. 2b), a load of P = 1000 N is vertically applied on the top of the SCB along its thickness by using a disc. This disc is similar to the top fixture used in the fracture tests. Bottom roller supports are fixed in all directions, signifying that the contact points of the bottom boundary of the SCB specimen model are fixed in vertical y-direction. Meanwhile, support materials are modeled as analytical rigid in the finite element models. Mode I and mode II geometry factors (i.e., YI and YII) are calculated from the following equations. It is noticed that these factors are required for calculating fracture toughness of materials.

K I 2Rt Y I ¼ pffiffiffiffiffiffi pa P

ð1Þ

K II 2Rt Y II ¼ pffiffiffiffiffiffi pa P

ð2Þ

where, KI and KII are respectively the mode I and mode II stress intensity factors, which are directly extracted from the finite element analyses. In addition, the values of R, t, P and a are those mentioned above. The values of YI and YII together with the corresponding values of L (i.e., location of the crack) for different mode mixities are given in Table 4. The parameter Me, mentioned in this Table, represents the relative amount of mode I and mode II at the crack front, which is defined as follows:

Me ¼

2

p

 tan1

KI K II

 ð3Þ

According to Eq. (3), the value of Me for the pure mode II and pure mode I loadings is 0 and 1, respectively, and any value between 1 and 0 refers to a mixed mode I/II loading. To summary the abovementioned explanations, it can be said that there are two reasons for performing numerical simulations:

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Pure mode I

Mixed mode of Me=0.32

Mixed mode of Me=0.32

Pure mode II Fig. 3. Distribution of von Mises stress in the SCB specimens subjected to pure mode I, mixed mode of Me = 0.66, mixed mode of Me = 0.32 and pure mode II.

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S. Pirmohammad et al. / Construction and Building Materials 293 (2020) 117850

5

Load (kN)

4 3 2 1 0 0

0.2

0.4

0.6

0.8

LLD (mm)

Fig. 4. Variations of KI against number of elements used in the SCB specimen.

Fig. 6. Sample of load-LLD curves for the asphalt mixtures studied in this research. Table 5 The effects of load P and Young’s modulus on the geometry factor YI. P (N)

E (GPa)

KI (MPa. m0.5)

YI

1000 1000 1000 1000 1000 1000 1000 1000 100 500 1500 2000 2500 3000 5000 7000

2.5 5.5 8.5 12.5 16.5 30 60 90 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5

0.397465 0.397465 0.397465 0.397465 0.397465 0.397465 0.397465 0.397465 0.039746 0.198732 0.596197 0.794929 0.993193 1.19239 1.98732 2.78225

4.29 4.29 4.29 4.29 4.29 4.29 4.29 4.29 4.29 4.29 4.29 4.29 4.29 4.29 4.29 4.29

1) to find the position of crack (i.e. L) that produces an especial value of mode mixity (i.e. Me). 2) to calculate the values of geometry factors (i.e., YI and YII) that are essential for obtaining the fracture toughness of any material. Hence, these values would be used later to

calculate the fracture toughness of asphalt mixtures studied in this research. Fig. 3 displays distribution of von Mises stress in the SCB specimens subjected to pure mode I, mixed mode of Me = 0.66, mixed mode of Me = 0.32 and pure mode II loadings. In the pure mode I, the stress contour is symmetric relative to the crack plane; whereas, in the mixed mode I/II and pure mode II, the contour plots are asymmetric with respect to the crack plane. Sensitivity of the finite element results on mesh density is also evaluated in this research, as shown in Fig. 4. According to this figure, as the number of elements increases, the value of KI firstly decreases sharply, and then remains constant. Indeed, KI converges to a constant value for the element numbers higher than about 40000. While, about 70,000 elements are used in this research in the finite element analyses performed on the SCB specimen. Hence, it can be concluded that the results of finite element analyses shown in Table 4 are independent of the mesh density. It is worth mentioning that Young’s modulus of asphalt mixture depends on the temperature and composition of the mixture [55]. According to Timm et al. [55], as the temperature decreases, the modulus of asphalt concrete increases. On the other hand, decrease in the temperature results in the enhancement of fracture tough-

Fig. 5. Loading of SCB specimen using three-point bend fixture.

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S. Pirmohammad et al. / Construction and Building Materials 293 (2020) 117850

ness [56]; hence, a direct relation exists between the modulus and fracture toughness of asphalt concrete. Asphalt concretes have a Young’s modulus E between 10 GPa and 20 GPa at a temperature below 0 °C [55]. Since fracture behavior of different asphalt concretes at a low temperature of 15 °C is studied in this research; hence, applicability of the geometry factors given in Table 4 for different asphalt mixtures and temperatures should be checked. Table 5 shows the effect of Young’s modulus on the mode I geometry factor (as an example). The geometry factor YI is found to be independent of Young’s modulus in the range between 2.5 GPa and 90 GPa. The effect of the load applied upon the SCB specimen (i.e. P) on the geometry factor is also investigated. According to Table 5, although the value of stress

intensity factor KI changes by varying the value of P, but the geometry factor YI is independent of the load P in the range between 100 N and 7000 N. Consequently, the geometry factors presented in Table 4 are only dependent on the geometrical dimensions of SCB specimen, and can be therefore used for any material with different mechanical properties. 3.4. Experimental testing In the current research study, 304 SCB specimens made of the aforementioned asphalt mixtures are totally prepared and tested (i.e. 16, 144 and 144 SCB specimens for the control asphalt concrete, kenaf and goat wool reinforced asphalt concretes, respec-

Fig. 7. Fracture surface of SCB specimens.

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S. Pirmohammad et al. / Construction and Building Materials 293 (2020) 117850 Table 6 Fracture load Pcr (kN) obtained from the experiments for the control and natural fiber reinforced asphalt mixtures with the length of a) 4 mm, b) 8 mm and c) 12 mm. (a) Me

1

0.66

0.32

0

Test No.

Control asphalt

1 2 3 4 Average 1 2 3 4 Average 1 2 3 4 Average 1 2 3 4 Average

1.87 2.00 1.93 2.11 1.98 4.06 4.22 3.92 4.11 4.08 4.00 – 3.67 3.75 3.81 3.51 3.47 – 3.58 3.52

Test No.

Control asphalt

Kenaf fiber

Goat wool fiber

0.1%

0.2%

0.3%

0.1%

0.2%

0.3%

2.38 2.22 2.02 2.39 2.25 4.29 4.09 4.57 4.49 4.36 4.01 3.84 4.10 4.20 4.04 3.60 3.78 3.57 3.66 3.65

2.45 2.28 2.19 2.36 2.32 4.72 4.48 4.55 4.53 4.57 3.86 4.18 4.01 4.49 4.14 3.62 3.61 – 3.92 3.72

2.32 2.47 2.27 2.52 2.40 4.72 4.80 4.64 4.91 4.77 4.40 4.43 4.04 3.93 4.20 3.69 3.56 3.94 3.84 3.76

2.17 2.15 2.31 2.26 2.22 4.64 4.46 4.34 4.28 4.43 4.09 4.56 4.34 4.42 4.35 3.74 4.14 4.08 3.77 3.93

2.28 2.22 2.35 2.34 2.30 4.99 4.65 4.64 4.55 4.71 4.16 4.36 4.56 4.68 4.44 4.21 4.56 3.92 3.77 4.12

2.26 2.40 2.60 2.48 2.44 4.69 4.99 5.10 4.59 4.84 4.57 4.29 4.81 4.74 4.60 3.96 4.08 4.33 4.57 4.24

0.1%

0.2%

0.3%

0.1%

0.2%

0.3%

2.43 2.30 2.15 2.28 2.29 4.58 4.55 4.20 4.66 4.50 4.09 3.97 4.02 4.23 4.08 3.83 3.62 3.55 3.72 3.68

2.32 2.46 2.25 2.47 2.38 4.62 4.79 4.62 4.87 4.73 4.00 4.30 4.17 4.20 4.17 3.85 3.71 3.64 3.80 3.75

2.43 2.70 2.38 2.48 2.50 4.67 4.70 5.14 4.95 4.87 4.53 4.04 4.29 4.24 4.28 3.70 3.94 3.68 3.87 3.80

2.31 2.18 2.15 2.17 2.20 4.20 4.49 4.47 4.36 4.38 4.32 4.45 4.52 4.02 4.33 4.14 3.69 3.58 4.11 3.88

2.27 2.14 2.39 2.11 2.23 4.22 4.5 4.35 4.73 4.45 4.61 4.17 4.51 4.35 4.41 3.97 3.73 4.02 4.05 3.94

2.53 2.22 2.35 2.21 2.33 – 4.59 5.00 4.79 4.79 4.49 4.81 4.10 4.70 4.53 3.96 3.94 4.62 4.12 4.16

0.1%

0.2%

0.3%

0.1%

0.2%

0.3%

2.32 2.05 2.13 2.25 2.19 4.17 4.44 4.53 4.20 4.34 3.70 4.14 4.18 3.87 3.97 3.66 3.51 3.63 3.53 3.58

2.27 2.29 2.33 2.25 2.29 4.56 4.27 – 4.34 4.39 4.26 – 3.99 3.94 4.06 3.74 3.66 3.71 3.64 3.69

2.27 2.43 2.24 2.27 2.30 4.46 4.49 4.37 4.59 4.48 3.75 4.01 3.69 3.98 3.86 3.65 3.46 3.59 – 3.57

2.24 1.97 2.06 1.99 2.07 4.41 4.26 4.13 4.58 4.35 4.05 3.78 4.30 3.96 4.02 3.56 3.80 – 3.54 3.63

2.33 2.20 – 2.23 2.25 4.60 4.97 4.62 4.48 4.67 4.05 3.98 4.47 4.25 4.19 3.61 – 3.65 3.98 3.75

2.44 2.16 2.22 2.12 2.24 4.59 4.39 4.65 4.44 4.52 4.31 3.89 4.03 4.00 4.06 3.54 3.57 3.80 3.77 3.67

(b) Me

1

0.66

0.32

0

1 2 3 4 Average 1 2 3 4 Average 1 2 3 4 Average 1 2 3 4 Average

1.87 2.00 1.93 2.11 1.98 4.06 4.22 3.92 4.11 4.08 4.00 – 3.67 3.75 3.81 3.51 3.47 – 3.58 3.52

Test No.

Control asphalt

Kenaf fiber

Goat wool fiber

(c) Me

1

0.66

0.32

0

1 2 3 4 Average 1 2 3 4 Average 1 2 3 4 Average 1 2 3 4 Average

1.87 2.00 1.93 2.11 1.98 4.06 4.22 3.92 4.11 4.08 4.00 – 3.67 3.75 3.81 3.51 3.47 – 3.58 3.52

Kenaf fiber

Goat wool fiber

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S. Pirmohammad et al. / Construction and Building Materials 293 (2020) 117850

tively). The procedure of the experiments is such that the SCB specimens are initially cooled in a freezer set at 15 °C for 6 h, ensuring that the temperature gradient within the specimens is zero. Then, each SCB specimen is immediately placed upon the bottom supports of the UTM (universal test machine) shown in Fig. 5, and the top fixture goes downwards at a displacement rate of 3 mm/min to load the specimen. It is pointed out that the bottom supports of the UTM are fixed at the locations of S1 = 33 mm and S2 = 33 mm for the case of pure mode I loading. While, they are adjusted at S1 = 33 mm and S2 = 13 mm for the cases of mixed mode I/II and pure mode II loadings, as calculated from the finite element simulations (see Table 4). Sample of the load-LLD (load line displacement) curves recorded from the tests is presented in Fig. 6. As shown in this figure, the relationship between the load and LLD is linear, indicating that the fracture takes place in accordance with the LEFM (linear elastic fracture mechanics) approach. In other words, the brittle fracture is observed during the experiments. Another evidence for the brittle fracture of the specimens is breakage of the aggregates at the crack growth path, as shown in Fig. 7. Critical SIFs (stress intensity factors) are used in the LEFM approach to characterize fracture strength. These parameters are calculated from the following equations:

K If ¼ Y I

Pcr pffiffiffiffiffiffi pa 2Rt

K IIf ¼ Y II K eff ¼

ð4Þ

P cr pffiffiffiffiffiffi pa 2Rt

ð5Þ

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi K 2If þ K 2IIf

ð6Þ

where, the fracture load Pcr is the maximum load available in the load-LLD curve, and the value of which for the experiments is given

in Table 6. YI and YII refer to the mode I and mode II geometry factors, which are given in Table 4. It is noticed that KIf and KIIf correspond to the mode I and mode II critical SIFs, respectively. Furthermore, the effective critical SIF (i.e., Keff) represents the fracture strength of materials. 4. Results and discussions As mentioned above, the fracture experiments on the kenaf and goat wool reinforced asphalt mixtures as well as the control asphalt mixtures are carried out under different mode mixities. Fracture strength of these mixtures calculated using the Eqs. (4)– (6) and the averaged values of Pcr (given in Table 6) are presented in Table 7. Influences of the natural fibers on the fracture properties of asphalt mixtures are discussed below. 4.1. Influence of kenaf fibers Fig. 8 exhibits fracture strength of the kenaf reinforced asphalt mixtures under different mode mixities. According to Fig. 8, fracture strength of the kenaf reinforced asphalt mixtures is greater than that of the control asphalt mixture, indicating that the presence of the kenaf fibers within the mixture improves the fracture strength. It is noticed that other researchers have also indicated that fibers can improve the fracture strength of asphalt mixture. For example, according to Mansourian et al. [33], jute fiber has a significant effect on increasing the fracture toughness of WMA mixture. Indeed, the fibers used in the mixture provide additional tensile strength, and therefore increase the resistance of asphalt mixture against the crack propagation. In addition, based on the SEM (scanning electron micrographs) image shown in Fig. 9a, the surface of

Table 7 pffiffiffiffiffi Critical SIFs (MPa m) calculated for the control and natural fiber reinforced asphalt mixtures with the length of a) 4 mm, b) 8 mm, and c) 12 mm. (a) Asphalt type

Content (%)

Me = 1 KIf

KIIf

Keff

KIf

KIIf

Keff

KIf

KIIf

Keff

KIf

KIIf

Keff

Control Kenaf

– 0.1 0.2 0.3 0.1 0.2 0.3

0.79 0.89 0.92 0.95 0.88 0.91 0.94

0 0 0 0 0 0 0

0.79 0.89 0.92 0.95 0.88 0.91 0.94

0.62 0.67 0.69 0.72 0.67 0.71 0.73

0.36 0.39 0.40 0.42 0.39 0.41 0.43

0.72 0.78 0.80 0.83 0.78 0.82 0.85

0.32 0.34 0.34 0.35 0.36 0.37 0.38

0.57 0.61 0.62 0.63 0.65 0.67 0.69

0.65 0.70 0.71 0.72 0.74 0.77 0.79

0 0 0 0 0 0 0

0.74 0.76 0.78 0.79 0.82 0.86 0.90

0.74 0.76 0.78 0.79 0.82 0.86 0.90

Asphalt type

Content (%)

Me = 1 KIf

KIIf

Keff

KIf

KIIf

Keff

KIf

KIIf

Keff

KIf

KIIf

Keff

Control Kenaf

– 0.1 0.2 0.3 0.1 0.2 0.3

0.79 0.91 0.94 0.99 0.87 0.89 0.93

0 0 0 0 0 0 0

0.79 0.91 0.94 0.99 0.87 0.89 0.93

0.62 0.68 0.71 0.74 0.66 0.69 0.72

0.36 0.40 0.42 0.43 0.39 0.40 0.42

0.72 0.79 0.82 0.86 0.77 0.80 0.83

0.32 0.34 0.35 0.36 0.36 0.36 0.38

0.57 0.61 0.63 0.64 0.64 0.65 0.68

0.65 0.70 0.72 0.73 0.73 0.74 0.78

0 0 0 0 0 0 0

0.74 0.77 0.78 0.79 0.81 0.82 0.87

0.74 0.77 0.78 0.79 0.81 0.82 0.87

Content (%)

Me = 1

Goat wool

Me = 0.66

Me = 0.32

Me = 0

(b)

Goat wool

Me = 0.66

Me = 0.32

Me = 0

(c) Asphalt type

Control Kenaf

Goat wool

– 0.1 0.2 0.3 0.1 0.2 0.3

Me = 0.66

Me = 0.32

Me = 0

KIf

KIIf

Keff

KIf

KIIf

Keff

KIf

KIIf

Keff

KIf

KIIf

Keff

0.79 0.87 0.91 0.91 0.82 0.87 0.87

0 0 0 0 0 0 0

0.79 0.87 0.91 0.91 0.82 0.87 0.87

0.62 0.66 0.66 0.68 0.66 0.69 0.67

0.36 0.38 0.39 0.39 0.38 0.40 0.39

0.72 0.76 0.77 0.78 0.76 0.80 0.77

0.32 0.33 0.34 0.32 0.33 0.35 0.34

0.57 0.60 0.61 0.58 0.60 0.63 0.61

0.65 0.68 0.70 0.66 0.68 0.72 0.70

0.74 0.75 0.77 0.75 0.78 0.80 0.82

0 0 0 0 0 0 0

0.74 0.75 0.77 0.75 0.78 0.80 0.82

S. Pirmohammad et al. / Construction and Building Materials 293 (2020) 117850

1

11

Me= 1

0.5

)

0.9

KIf (MPa. m

0.8

0.7

0.6

0.5 Length = 4mm

Control

Length = 8mm

%0.1 Kenaf

Length = 12mm

%0.2 Kenaf

1

%0.3 Kenaf

Me= 0.66

0.9

Keff (MPa. m

0.5

)

0.8

0.7

0.6

0.5 Length = 4mm

Control

Length = 8mm

%0.1 Kenaf

Length = 12mm

%0.2 Kenaf

%0.3 Kenaf

1

Me= 0.32

Keff (MPa. m

0.5

)

0.9

0.8

0.7

0.6

0.5 Length = 4mm

Control

Length = 8mm

%0.1 Kenaf

1

Length = 12mm

%0.2 Kenaf

%0.3 Kenaf

Me= 0

KIIf (MPa. m

0.5

)

0.9 0.8 0.7 0.6 0.5 Length = 4mm

Control

Length = 8mm

%0.1 Kenaf

0.2% Kenaf

Length = 12mm

0.3% Kenaf

Fig. 8. Fracture strength of the kenaf reinforced asphalt mixtures under different mode mixities.

the kenaf fibers is not smooth, and appears to have longitudinal ridges which can interlock with the binder matrix, and therefore

improve the cracking resistance of asphalt mixture. For the kenaf lengths of 4 mm and 8 mm, the pure mode I fracture strength

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Fig. 9. Surface of a) the kenaf fibers at a magnifications of 600 and b) the goat wool fibers at a magnification of 600.

increases as the content of kenaf fibers in the mixture enhances. For the kenaf length of 12 mm, as the content of kenaf fibers increases, the fracture strength initially increases and then remains nearly unchanged (for the cases of Me = 1 and Me = 0.66) or decreases (for the cases of Me = 0.32 and Me = 0). Fig. 8 also shows that by increasing the kenaf length, the fracture strength firstly enhances and then decreases; hence, the kenaf fibers with the length of 8 mm exhibit the best results of the fracture strength compared to those with the lengths of 4 mm and 12 mm. Fig. 10 shows the amount of improvement in fracture strength for the kenaf reinforced asphalt mixtures. According to Fig. 10a, the kenaf fibers with 8 mm in length demonstrate the highest positive effect on the fracture strength of asphalt mixtures reinforced by 0.1% kenaf fibers, while those with 4 mm and 12 mm in lengths are placed in the next ranks. In addition, as the proportion of mode II increases, the improvement effect of the kenaf fibers declines. In

this respect, the case of pure mode I shows the highest amount of improvement in fracture strength (i.e., 15%, 13% and 10% for the kenaf lengths of 8 mm, 4 mm and 12 mm, respectively); while, improvement of the fracture strength is not remarkable when the asphalt mixtures are loaded under pure mode II (i.e., that is below 4% for any kenaf lengths). According to Fig. 10b and c, for the cases that 0.2% and 0.3% of kenaf fibers are used in asphalt mixtures, the trend of the results is similar to that of the 0.1% kenaf fibers, as mentioned above. However, the asphalt mixture reinforced by 0.3% kenaf fibers with 8 mm in length demonstrates the best results compared to other mixtures reinforced by kenaf fibers. This mixture has 25% (for pure mode I), 19% (for mixed mode of Me = 0.66), 12% (for mixed mode of Me = 0.32), and 7% (for pure mode II) higher fracture strength than control asphalt mixture. Looking at the results given in Fig. 10 indicates that the fiber specifications (i.e., the fiber content and length) influence

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Fig. 10. Ratio of Keff for the fiber reinforced asphalt mixture to that for the control asphalt mixture with different fiber contents of a) 0.1%, b) 0.2% and c) 0.3%

mostly the mode I dominant (i.e., pure mode I and mixed mode of Me = 0.66) fracture strengths of asphalt mixtures, and the mode II fracture strengths are not much improved. Similar observations have been reported by Aliha et al. [20]. According to their results, both the jute and FORTA fibers improved the mode I fracture toughness of warm mix asphalt concretes more than the mode II one. In other words, for the cases that the amount of mode II was dominant, none of the fibers had considerable influence on the fracture strength.

The influence of the mode mixity on the fracture strength of kenaf reinforced asphalt mixtures is displayed in Fig. 11. The figure shows that by increasing the amount of mode II (i.e., by decreasing Me), the fracture strength initially decreases and then starts to increase after reaching its minimum value at the mode mixity of Me = 0.32. This result implies that the asphalt mixtures reinforced by kenaf fibers are subjected to the critical condition of fracture under a mixed mode loading with mode mixity of Me = 0.32.

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Fig. 11. Variations of Keff versus the mode mixity for the asphalt mixtures reinforced with a) 0.1% kenaf, b) 0.2% kenaf and c) 0.3% kenaf.

4.2. Influence of goat wool fibers Fig. 12 shows the influence of the goat wool fibers on the fracture strength of asphalt mixture. Similar to the results observed for the kenaf fibers, as discussed above, the goat wool fibers also increase the fracture strength of asphalt mixture. For the goat wool fibers with 4 mm and 8 mm in lengths, the results exhibit that the fracture strength increases as the content of goat wool fibers enhances. Generally, for those with 12 mm in length, the fracture strength has its maximum value when the content of goat wool fibers is 0.2%. Similar to the surface of the kenaf fibers, the surface

of the goat wool fibers is not smooth, as shown in Fig. 9b. Hence, the uneven surface can interlock with the binder matrix, and therefore improves the cracking resistance of asphalt mixtures. According to Fig. 10, the positive effect of the goat wool fibers decreases as the fiber length increases i.e. the amount of improvement in fracture strength has its maximum as the length of fibers is 4 mm. Furthermore, the improvement effect of the goat wool fibers on the fracture strength of asphalt mixture is nearly identical for all mode mixities. This improvement is on average about 11% (for the fiber length of 4 mm), 9% (for the fiber length of 8 mm) and 5% (for the fiber length of 12 mm) as 0.1% goat wool fibers are used in the

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Fig. 12. Fracture strength of the goat wool reinforced asphalt mixtures under different mode mixities.

mixture. For the mixtures prepared by 0.2% and 0.3% goat wool fibers, these values are respectively 16% and 21% (for the fiber length of 4 mm), 12% and 18% (for the fiber length of 8 mm), 10%

and 9% (for the fiber length of 12 mm). Accordingly, the use of 0.3% goat wool fibers with 4 mm in length provides the best results compared to other mixtures reinforced by goat wool fibers.

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Fig. 13. a) Crack bridging by goat wool fibers and b) breakage of the kenaf fibers during the crack propagation in the SCB specimens.

It is worth mentioning that the identical improvement in fracture strength of the goat wool reinforced asphalt mixtures subjected to any mode mixity can be attributed to the fact that the presence of goat wool fibers helps to bridge the crack, as shown in Fig. 13a, which leads to increase the fracture strength as the specimens are loaded under shear modes (i.e., mode II). In contrast, no crack bridging is observed in the fracture mechanism of the kenaf reinforced asphalt mixtures, as seen in Fig. 13b. In other words, unlike the goat wool fibers, the kenaf fibers are broken during the crack propagation process. The breakage of kenaf fibers signifies that they become brittle during the preparation process of asphalt mixtures, while the goat wool fibers are still soft after mixing with asphalt mixtures. Hence, the kenaf fibers differently improve the fracture strength under different mode mixities, as displayed in Fig. 10. Another important point from Fig. 10 is that the kenaf fibers increase the crack growth resistance of asphalt mixtures loaded under dominant mode I (i.e., pure mode I and mixed mode of Me = 0.66) more than that of asphalt mixtures loaded under dominant mode II (i.e., mixed mode of Me = 0.32 and pure mode II). On the other hand, the goat wool fibers have better performance than the kenaf fibers as the mixture is subjected to dominant mode II loadings. Consequently, the kanaf and goat wool fibers are more suitable for the dominant mode I and dominant mode II loadings, respectively. Fig. 14 shows the influence of the mode mixity on fracture strength of the goat wool reinforced asphalt mixtures. Similar to the results observed for the kenaf reinforced asphalt mixtures given in Fig. 11, the fracture strength initially declines

and then increases as the proportion of mode II enhances. Hence, the fracture strength has the lowest value at Me = 0.32, signifying that the asphalt mixtures reinforced by goat wool fibers are more vulnerable to mixed mode loadings than pure modes of I and II. 5. Conclusions In the current research study, two natural fibers including kenaf and goat wool fibers with three different lengths (i.e., 4, 8 and 12 mm) and contents (i.e., 0.1, 0.2 and 0.3%) are added to asphalt mixture to investigate their influence on fracture behavior of asphalt mixtures. Fracture experiments under various mode mixities (i.e., pure mode I, pure mode II and two mixed modes of I/II) are performed on the SCB specimens at 15 °C, and their fracture strengths are then measured. It is pointed out that to measure the fracture strength (i.e., Keff) of asphalt mixtures, the calculation of geometry factors (i.e., YI and YII) is necessary, so these parameters are computed from finite element simulations performed on the SCB specimens. The important results of this research are presented below:
Both the kenaf and goat wool fibers improve the fracture strength of asphalt mixture. The amount of improvement is dependent on the length and content of the fibers used in the mixture.
By increasing the proportion of mode II relative to mode I, the improvement effect of the kenaf fiber declines, such that the case of pure mode I shows the highest amount of improvement

S. Pirmohammad et al. / Construction and Building Materials 293 (2020) 117850

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Fig. 14. Variations of Keff versus the mode mixity for the asphalt mixtures reinforced with a) 0.1% goat wool, b) 0.2% goat wool and c) 0.3% goat wool.

in fracture strength; while, a negligible improvement in fracture strength is observed as the asphalt mixture is loaded under pure mode II.
The asphalt mixture reinforced by 0.3% kenaf fibers with 8 mm in length demonstrates the best results compared to other mixtures reinforced by kenaf fibers.
The improvement effect of the goat wool fibers on the fracture strength of asphalt mixtures is nearly identical for all mode mixities.


Application of 0.3% goat wool fibers with 4 mm in length provides the best results of fracture strength compared to other mixtures reinforced by goat wool fibers.
According to the results, the kenaf fibers more improve the fracture resistance of asphalt mixtures loaded under dominant mode I (i.e., pure mode I and mixed mode of Me = 0.66) than that of asphalt mixtures loaded under dominant mode II (i.e., mixed mode of Me = 0.32 and pure mode II). On the other hand, the goat wool fibers perform better than the kenaf fibers as the mix-

18

S. Pirmohammad et al. / Construction and Building Materials 293 (2020) 117850

ture is subjected to dominant mode II loadings. Consequently, the kanaf and goat wool fibers are more suitable for the dominant mode I and dominant mode II loadings, respectively.
By increasing the amount of mode II (i.e., by decreasing Me), the fracture strength initially decreases and after reaching its minimum value at the mode mixity of Me = 0.32, it increases, implying that the asphalt mixtures reinforced by both the kenaf and goat wool fibers are subjected to the critical condition of fracture under a mixed mode loading with mode mixity of Me = 0.32. CRediT authorship contribution statement S. Pirmohammad: Conceptualization, Methodology, Investigation, Writing - review & editing, Supervision, Project administration. Y. Majd Shokorlou: Software, Validation, Formal analysis, Resources, Writing - original draft, Visualization. B. Amani: Software, Data curation, Methodology. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The present research was carried out under a research grant of Project No.: 982 from University of Mohaghegh Ardabili. This support is gratefully acknowledged. References [1] X. Li, M. Marasteanu, Evaluation of the low temperature fracture resistance of asphalt mixtures using the semi circular bend test (with discussion), J. Assoc. Asphalt Paving Technol. 73 (2004). [2] X. Li, M. Marasteanu, The fracture process zone in asphalt mixture at low temperature, Eng. Fract. Mech. 77 (7) (2010) 1175–1190. [3] M.M. Aliha, H. Behbahani, H. Fazaeli, M. Rezaifar, Study of characteristic specification on mixed mode fracture toughness of asphalt mixtures, Constr. Build. Mater. 54 (2014) 623–635. [4] S. Pirmohammad, M. Ayatollahi, Asphalt concrete resistance against fracture at low temperatures under different modes of loading, Cold Reg. Sci. Technol. 110 (2015) 149–159. [5] M. Fakhri, E.H. Kharrazi, M. Aliha, Mixed mode tensile–in plane shear fracture energy determination for hot mix asphalt mixtures under intermediate temperature conditions, Eng. Fract. Mech. 192 (2018) 98–113. [6] S. Pirmohammad, A. Kiani, Impact of temperature cycling on fracture resistance of asphalt concretes, Comput. Concr. 17 (4) (2016) 541–551. [7] S. Pirmohammad, A. Kiani, Effect of temperature variations on fracture resistance of HMA mixtures under different loading modes, Mater. Struct. 49 (9) (2016) 3773–3784. [8] H. Ozer, I.L. Al-Qadi, P. Singhvi, T. Khan, J. Rivera-Perez, A. El-Khatib, Fracture characterization of asphalt mixtures with high recycled content using Illinois semicircular bending test method and flexibility index, Transp. Res. Rec.: J. Transp. Res. Board 2575 (2016) 130–137. [9] R. Dongre, M. Sharma, D. Anderson, Development of fracture criterion for asphalt mixes at low temperatures, Transp. Res. Rec. 1228 (1989) 94–105. [10] M. Hossain, S. Swartz, E. Hoque, Fracture and tensile characteristics of asphaltrubber concrete, J. Mater. Civ. Eng. 11 (4) (1999) 287–294. [11] M.R. Ayatollahi, S. Pirmohammad, Temperature effects on brittle fracture in cracked asphalt concretes, Struct. Eng. Mech. 45 (1) (2013) 19–32. [12] S. Pirmohammad, M. Ayatollahi, Fracture resistance of asphalt concrete under different loading modes and temperature conditions, Constr. Build. Mater. 53 (2014) 235–242. [13] H. Behbahani, M. Aliha, M. Reza, H. Fazaeli, S. Aghajani, Experimental fracture toughness study for some modified asphalt mixtures, Adv. Mater. Res., Trans Tech Publ (2013) 337–344. [14] M. Fakhri, E. Amoosoltani, M. Aliha, Crack behavior analysis of roller compacted concrete mixtures containing reclaimed asphalt pavement and crumb rubber, Eng. Fract. Mech. 180 (2017) 43–59. [15] M. Ameri, S. Nowbakht, M. Molayem, M. Aliha, Investigation of fatigue and fracture properties of asphalt mixtures modified with carbon nanotubes, Fatigue Fract. Eng. Mater. Struct. (2016). [16] Z. Kordi, G. Shafabakhsh, Evaluating mechanical properties of stone mastic asphalt modified with Nano Fe2O3, Constr. Build. Mater. 134 (2017) 530–539.

[17] S. Pirmohammad, Y. Majd-Shokorlou, B. Amani, Experimental investigation of fracture properties of asphalt mixtures modified with Nano Fe2O3 and carbon nanotubes, Road Mater. Pavement Des. (2020) 1–23. [18] S. Pirmohammad, Y. Majd-Shokorlou, B. Amani, Fracture resistance of HMA mixtures modified with nanoclay and Nano-Al2O3, J. Test. Eval. 47 (5) (2019). [19] P. Park, S. El-Tawil, S.-Y. Park, A.E. Naaman, Cracking resistance of fiber reinforced asphalt concrete at 20° C, Constr. Build. Mater. 81 (2015) 47– 57. [20] M. Aliha, A. Razmi, A. Mansourian, The influence of natural and synthetic fibers on low temperature mixed mode I+ II fracture behavior of warm mix asphalt (WMA) materials, Eng. Fract. Mech. 182 (2017) 322–336. [21] S. Tapkın, Ü. Usßar, A. Tuncan, M. Tuncan, Repeated creep behavior of polypropylene fiber-reinforced bituminous mixtures, J. Transp. Eng. 135 (4) (2009) 240–249. [22] S. Wu, Q. Ye, N. Li, Investigation of rheological and fatigue properties of asphalt mixtures containing polyester fibers, Constr. Build. Mater. 22 (10) (2008) 2111–2115. [23] Z. Dehghan, A. Modarres, Evaluating the fatigue properties of hot mix asphalt reinforced by recycled PET fibers using 4-point bending test, Constr. Build. Mater. 139 (2017) 384–393. [24] S. Serin, N. Morova, M. Saltan, S. Terzi, Investigation of usability of steel fibers in asphalt concrete mixtures, Constr. Build. Mater. 36 (2012) 238–244. [25] F. Moghadas Nejad, M. Vadood, S. Baeetabar, Investigating the mechanical properties of carbon fibre-reinforced asphalt concrete, Road Mater. Pavement Des. 15 (2) (2014) 465–475. [26] X. Wang, R. Wu, L. Zhang, Development and performance evaluation of epoxy asphalt concrete modified with glass fibre, Road Mater. Pavement Des. 20 (3) (2019) 715–726. [27] L. Klinsky, K. Kaloush, V. Faria, V. Bardini, Performance characteristics of fiber modified hot mix asphalt, Constr. Build. Mater. 176 (2018) 747–752. [28] P. Apostolidis, X. Liu, G.C. Daniel, S. Erkens, T. Scarpas, Effect of synthetic fibres on fracture performance of asphalt mortar, Road Mater. Pavement Des. (2019) 1–14. [29] H. Weerasinghe, Fundamental study on the use of coir fibre boards as a roofing material, Asian Inst. Technol. (1977). [30] A. Elsaid, M. Dawood, R. Seracino, C. Bobko, Mechanical properties of kenaf fiber reinforced concrete, Constr. Build. Mater. 25 (4) (2011) 1991–2001. [31] M.A. Mughal, M.R. Hainin, R.P. Jaya, M.F. Yunus, M. Rahman, M.K. Idham, M.N. M. Warid, Stabilizing asphalt concrete using kenaf fibers, Adv. Sci. Lett. 24 (6) (2018) 3963–3967. [32] M.A. Sani, A.A. Latib, C.P. Ng, M.A. Yusof, N. Ahmad, M.A.M. Rani, Properties of coir fibre and kenaf fibre modified asphalt mixes, Proceedings of the Eastern Asia Society for Transportation Studies The 9th International Conference of Eastern Asia Society for Transportation Studies, 2011, Eastern Asia Society for Transportation Studies, 2011, pp. 258–258. [33] A. Mansourian, A. Razmi, M. Razavi, Evaluation of fracture resistance of warm mix asphalt containing jute fibers, Constr. Build. Mater. 117 (2016) 37–46. [34] N. Morova, Investigation of usability of basalt fibers in hot mix asphalt concrete, Constr. Build. Mater. 47 (2013) 175–180. [35] M. Hainin, M. Idham, N. Yaro, S. Hussein, M. Warid, A. Mohamed, S. Naqibah, P. Ramadhansyah, Performance of hot mix asphalt mixture incorporating kenaf fibre, IOP Conf. Ser.: Earth Environ. Sci., IOP Publ. (2018) 012092. [36] G.B. Gholikandi, M. Sadrzadeh, S. Jamshidi, M. Ebrahimi, Water resource management in ancient Iran with emphasis on technological approaches: a cultural heritage, Water Sci. Technol. Water Supply 13 (3) (2013) 582– 589. [37] S. Pirmohammad, H. Shabani, Mixed mode I/II fracture strength of modified HMA concretes subjected to different temperature conditions, J. Test. Eval. 47 (5) (2019). [38] M. Arabani, A. Shabani, Evaluation of the ceramic fiber modified asphalt binder, Constr. Build. Mater. 205 (2019) 377–386. [39] S.M. Abtahi, M. Sheikhzadeh, S.M. Hejazi, Fiber-reinforced asphalt-concrete–a review, Constr. Build. Mater. 24 (6) (2010) 871–877. [40] H. Chen, Q. Xu, S. Chen, Z. Zhang, Evaluation and design of fiber-reinforced asphalt mixtures, Mater. Des. 30 (7) (2009) 2595–2603. [41] M. Lavasani, M.L. Namin, H. Fartash, Experimental investigation on mineral and organic fibers effect on resilient modulus and dynamic creep of stone matrix asphalt and continuous graded mixtures in three temperature levels, Constr. Build. Mater. 95 (2015) 232–242. [42] R. Mahjoub, J.M. Yatim, A.R.M. Sam, S.H. Hashemi, Tensile properties of kenaf fiber due to various conditions of chemical fiber surface modifications, Constr. Build. Mater. 55 (2014) 103–113. [43] B. Mobasher, M.S. Mamlouk, H.M. Lin, Evaluation of crack propagation properties of asphalt mixtures, J. Transp. Eng. 123 (5) (1997) 405–413. [44] K.W. Kim, Y.S. Doh, S. Lim, Mode I reflection cracking resistance of strengthened asphalt concretes, Constr. Build. Mater. 13 (5) (1999) 243–251. [45] K.W. Kim, M. El Hussein, Variation of fracture toughness of asphalt concrete under low temperatures, Constr. Build. Mater. 11 (7–8) (1997) 403–411. [46] K.W. Kim, S.J. Kweon, Y.S. Doh, T.-S. Park, Fracture toughness of polymermodified asphalt concrete at low temperatures, Can. J. Civ. Eng. 30 (2) (2003) 406–413. [47] I. Artamendi, H.A. Khalid, A comparison between beam and semi-circular bending fracture tests for asphalt, Road Mater. Pavement Des. 7 (sup1) (2006) 163–180. [48] M. Mamlouk, B. Mobasher, Cracking resistance of asphalt rubber mix versus hot-mix asphalt, Road Mater. Pavement Des. 5 (4) (2004) 435–451.

S. Pirmohammad et al. / Construction and Building Materials 293 (2020) 117850 [49] M.P. Wagoner, W.G. Buttlar, G.H. Paulino, Development of a single-edge notched beam test for asphalt concrete mixtures, J. Test. Eval. 33 (6) (2005) 452–460. [50] M. Wagoner, W.G. Buttlar, G. Paulino, Disk-shaped compact tension test for asphalt concrete fracture, Exp. Mech. 45 (3) (2005) 270–277. [51] M. Wagoner, W. Buttlar, G. Paulino, P. Blankenship, Investigation of the fracture resistance of hot-mix asphalt concrete using a disk-shaped compact tension test, Transp. Res. Rec.: J. Transp. Res. Board 2005 (1929) 183–192. [52] J. Molenaar, X. Liu, A. Molenaar, Resistance to crack-growth and fracture of asphalt mixture, 6th International RILEM Symposium, Zurich, Switzeland, 14– 16 April, 2003.

19

[53] S. Pirmohammad, H. Khoramishad, M. Ayatollahi, Effects of asphalt concrete characteristics on cohesive zone model parameters of hot mix asphalt mixtures, Can. J. Civ. Eng. 43 (3) (2015) 226–232. [54] C. Alkilicgil, Development of a New Method for Mode I Fracture Toughness Test on Disc Type Rock Specimens, MS Thesis, METU, Ankara, 2006. [55] D. Timm, B. Birgisson, D. Newcomb, Development of mechanistic-empirical pavement design in Minnesota, Transp. Res. Rec.: J. Transp. Res. Board (1629 1998) 181–188. [56] J. Molenaar, A. Molenaar, Fracture toughness of asphalt in the semi-circular bend test, 2nd Eurasphalt and Eurobitume Congress, Barcelona, Spain, 20–22 September, 2000.