AbstractFiber-matrix interface bonding is an important area of concern in fiber-reinforced composites because it is directly related to the mechanical behavior of composites. In this study, interfacial bond properties of four types of fiber at different curing ages were investigated by analyzing fiber pull-out responses. Special attention was given to the fiber–matrix bonding behavior, including fiber tensile strength, average bond strength, equivalent bond strength, and average pull-out energy. Pull-out tests were conducted at 7, 14, 28, and 56 days of curing. Different failure modes in steel fiber, synthetic macrofiber, polyacrylonitrile (PAN)-based carbon fiber, and pitch-based carbon fiber were investigated. According to the pull-out force versus slip curves, different failure patterns were recorded based on the fiber type. Carbon fibers experienced a sudden drop after reaching the peak load, whereas the load decrease in steel and synthetic fiber was not as abrupt. Results also confirmed that steel fibers exhibited the highest pull-out load and energy absorption capacity followed by lower values for synthetic and carbon fibers. While monofilament of steel fiber was able to absorb 1,050 N·mm, monofilament of synthetic fiber and twisted bundles of carbon fibers could absorb 277 and 55 N·mm, respectively. However, the bond strength of straight carbon fibers was comparable to that of synthetic fiber and still lower than steel fiber. It was also derived from the experimental data that an increase in cement matrix age correlates to an improvement in fiber maximum pull-out load, bond strength, and tensile strength. These parameters were identified and compared in all fiber types.IntroductionFibers in modern cement-based composites play an essential role by improving concrete’s inherent weakness in resisting tension. Fibers primarily contribute to post cracking behavior of a composite by transmitting forces via fibers that bridge cracks (Lin and Lai 1997). The aforementioned transmission of forces is achieved through interfacial bonds defined as the shearing stress at the interface between the fiber and the surrounding matrix (Naaman et al. 1991). However, if the interfacial bond between the fiber and the surrounding cement matrix is weak, the fiber will slip within the surrounding matrix and cannot help to prevent crack propagation. Conversely, if the bond is too strong, fibers may rupture before they can help improve the postcrack strength of the matrix (Jewell et al. 2015). Due to the effects of interfacial bond strength on postcrack behavior as well as the significant influence of the fiber–matrix interfacial transition zone (ITZ) on the mechanical and long-term behavior of fiber-reinforced cement composites (FRCC) (Bentur and Alexander 2000), bond behavior in fiber-reinforced concrete (FRC) is recognized as a major factor in composite action (Jewell et al. 2015). However, understanding bond characterization in FRCC is complex because of the simultaneous actions of several bond components, including (1) physical and chemical adhesion between fiber and matrix; (2) the mechanical component of bond such as in deformed, crimped, and hooked fibers; (3) fiber-to-fiber interlock; and (4) friction (Naaman et al. 1991).There are different pull-out techniques suggested by researchers to determine interfacial strength parameters in FRCC. One method involves single fibers that are pulled out of a cylindrical specimen in which the specimen was directly glued to a bottom plate (Markovich et al. 2001). Another method (Naaman et al. 1991) includes embedment of a single fiber in a cement matrix block. The top end of the fiber is held by a grip, and the bottom end is attached to a linear variable differential transducer (LVDT) connected to the bench of the machine. A method proposed by Singh et al. (2004) consists of pouring wet mortar in a small cylindrical mold (2-mm in diameter by 50-mm long) and putting the fiber in the matrix from the top and holding it in a slot. Another method suggested by Ratu (2016) consists of putting each fiber in a dog bone–shaped mold. This involves putting 25 mm embedded length on each side of a plastic separator located in the middle of the mold.Despite a variety of pull-out techniques, calculation of interfacial bond strength between fiber and matrix and fiber pull-out energy is still one of the common approaches to determine the performance of fiber in FRCC. In recent years, considerable progress has been made in practical aspects of pull-out experiments as well as in the methods of data acquisition. However, an analysis of the force–displacement curves obtained from pull-out tests still yields useful information that characterizes fiber–matrix interfacial bond properties more adequately.Numerous investigations have been conducted to determine fiber–matrix interfacial bond properties. Steel fibers (SF) are one of the common types of fibers used to reinforce concrete and as a result many research studies focused on bonding strength of SF in cementitious materials. Steel fibers are available in different shapes to be used in concrete. Naaman et al. (1991) studied three types of steel fibers (smooth, deformed, and hooked) embedded in low, medium, and high strength concrete. They noticed that deformed fibers resisted pull-out in an oscillatory fashion, while hooked fiber resistance consistently decreased as the hook straightened as it traveled through the tunnel. Abu-Lebdeh et al. (2011) reported that hooked end steel fibers showed a 88%–160% and a 14%–137% increase in the peak load and pull-out energy, respectively, when they were compared to smooth steel fibers in concrete. It is also reported that hooked end steel fibers with smaller diameter show better fiber-bond behavior (Qi et al. 2018). Other important parameters in the pull-out behavior of steel fibers such as embedment length (Kim et al. 2017), embedment inclination (Tai and El-Tawil 2017), loading rate (Kim et al. 2008), fiber distance (Kim and Yoo 2019), fiber coating (Pi et al. 2019), and matrix strength (Abu-Lebdeh et al. 2011) effects have also been studied in recent years.There is an increasing demand for other types of fibers such as polypropylene macrofibers (SI) and carbon fibers (CF) to be used in FRCC such as ground slabs and precast members (Babafemi et al. 2017; Donnini et al. 2018). Several studies on the synthetic fiber pull-out behavior in cement-based composites have been conducted. Straight SI were reported to show poor bonding behavior in matrix (Won et al. 2006) and crimped-shaped SI were reported to be the optimum shape when used in concrete (Oh et al. 2007). The effects of embedment length and loading rate were investigated by Babafemi and Boshoff (2017), and the results showed that the pull-out strength of synthetic macrofiber is dependent on loading rate. Specifically, a higher loading rate resulted in higher pull-out strength at all the embedment lengths.Cement-based composites with short CF are attractive due to their high flexural strength, high toughness, and low drying shrinkage in addition to their strain sensing ability (Chen and Chung 1996; Azhari and Banthia 2017; Chen and Liu 2008; Donnini et al. 2018; Sassani et al. 2017). CF when compared to other types of fibers are generally more expensive. However, using carbon fiber–reinforced composites as multifunctional structural elements can reduce costs and increase the durability of the composite once placed in service conditions (Donnini et al. 2018). This multifunctional composite shows superior structural functions such as enhanced mechanical strength (Ivorra et al. 2010; Almuhanna 2019) as well as nonstructural functions such as its self-sensing ability (Galao et al. 2016; Micheli et al. 2017; Yıldırım et al. 2018) attributed to the excellent mechanical properties and light weight of CF. Commercially available carbon fibers usually come in bundles; however, studies in which CF are dispersed uniformly have rarely been conducted (Wang et al. 2021). In a study conducted by Lu et al. (2018), the effect of nano-SiO2 surface coating was evaluated using single carbon fiber pull-out test in which the interfacial strength of the coated carbon fiber in cement matrix is reported to be significantly higher in comparison to that of plain carbon fiber. However, to the authors’ knowledge, no work has been published to assess the interfacial transition zone between the cement matrix and CF bundle and as a result, further study is needed to evaluate the bond behavior of CF bundles in the cement matrix.Although a large number of studies have been developed to investigate the effect of curing age on the pull-out behavior of steel fibers, it is still difficult to find information related to the curing age effect on CF and SI interfacial bonding. Jewell et al. (2015) recently investigated the effect of aging (1 to 56 days of curing) on bond strength of steel and polypropylene fibers (PP), and they reported that low-modulus PP are best suited to resist pull-out forces at early ages of curing (<7  days), while SF have the highest bonding strength at 28 days of curing. In another study by Le et al. (2018) bond strength of steel fibers in different ages and storage conditions was investigated until 120 days. The results showed that bond strength of steel fibers, cured in water for 28 days and then stored in air until 120 days increased, while the bond strength of steel fibers cured in water for 120 days decreased by time.While there is still a perceived lack of information on pull-out behavior of CF-reinforced cementitious composites and the effect of matrix aging, an experimental procedure was developed in this research to investigate and compare the effects of fiber type and curing age simultaneously. In this paper, a series of pull-out tests were performed at different ages using the pull-out method suggested by Ratu (2016) in which the load and the displacement were accurately measured simultaneously. Pull-out behavior of four different fiber types [steel fiber (S or SF), synthetic macrofiber, polyacrylonitrile (PAN)-based carbon fiber (PA), and pitch-based carbon fiber (PI)] were investigated. The pull-out loads versus displacement, peak loads, dissipated bond energy, and equivalent bond strength were evaluated. The main objective of this research is to experimentally investigate and compare the bond mechanisms in fiber-reinforced cementitious mortar. The results of this experimental investigation are important to better understand the effects of fiber types as well as matrix age on interfacial bond characteristics and to further improve the tensile and toughness properties of FRC.Experimental InvestigationMaterials and Cementitious MortarNatural river sand meeting ASTM C778 (ASTM 2017) and Quickrete portland cement conforming to ASTM C150 Type I (ASTM 2020b) were used. Cement mortar used in this experiment had a total binder content of 811  kg/m3. It was produced with a water-binder ratio of 0.4, and the mixing process was operated in accordance with ASTM C305 (ASTM 2020a). The compression strength (on 5×5×5  cm cubes) and tensile strength (on dog-bone samples) of plain cement mortar at 28 days of curing was 55.8 and 4.3 MPa, respectively. To investigate the pull-out response of fiber in cement matrix, hooked-end SF, STRUX BT50 SI and two types of PAN- and pitch-based CF from Mitsubishi chemicals were used (Fig. 1). The properties of each fiber as received from the respective suppliers and the mix design of mortar are presented in Tables 1 and 2, respectively.Table 1. Physical and mechanical properties of fibersTable 1. Physical and mechanical properties of fibersFiber typeSpecific gravityModulus of elasticity (GPa)Tensile strength (MPa)Length (mm)Nominal diameter (mm)Steel7.85200>1,100  MPa501Synthetic0.917550500.48×0.61 PAN1.82344,830500.007Pitch21862,340500.011Table 2. Mixture proportion of mortar mixTable 2. Mixture proportion of mortar mixMaterial typekg/m3Cement811Sand1,351Water324Specimen PreparationDog-bone shaped specimens were fabricated to measure the pull-out behavior of four types of fiber when embedded within the cementitious matrix. The fiber embedment lengths were 25 mm on both halves of the specimen as suggested by Ratu (2016). For obtaining reliable test data, a minimum of three specimens for every mix was prepared for each test. Carbon fiber bundles were twisted in three different bundle diameters (0.5, 0.8, and 1 mm). The reason to use bundles instead of single fiber was (1) embedding one single carbon fiber (diameter 7–11 μm) in cement mortar was impossible due to a single fiber’s brittleness making it prone to break when hit, and (2) the importance of bundle behavior evaluation. Although different dispersants were introduced to ensure suitable dispersion of carbon fibers in FRCC (Cao and Chung 2001; Chen et al. 1997; Fu et al. 1998; Chuang et al. 2017), the cost of these admixtures is relatively high (Chung 2005), and the homogenous cement paste structure is also reported to be affected by dispersant (Muhua and Roy 1987). In recent years, several alternative approaches were introduced to disperse CF (Gao et al. 2017; Wang et al. 2017), and still a large number of studies report the presence of CF bundles in the matrix (Gao et al. 2017; Bhosale et al. 2019; Deng and Li 2006).The reason to choose the aforementioned diameters was (1) these values were the smallest diameter with an acceptable accuracy that we could get from the optical microscope secure of fiber breakage, and (2) these dimensions were comparable to SF and SI, which had similar dimensions. As shown in Fig. 2, an optical microscope was used to find the accurate diameter of the twisted fibers. Measurements were done on at least three points on the fiber to validate the correct diameter. Because bundles were twisted, it was assumed that cement paste could not penetrate inside the twisted fibers, and they were considered as a single fiber with an equivalent diameter equal to 0.5, 0.8, and 1 mm. Fibers were also glued together at each end of bundle, so that they did not disperse when put in the mold. During the preparation procedure, a single fiber or fiber bundle (for CF) was positioned inside a small hole in the middle of a thin plastic plate with 1 mm thickness [Figs. 3(a and b)]. Then, half the depth of the molds was filled with mortar and the plastic plate was placed inside the matrix [Fig. 3(c)] such that the length of the fiber was oriented along the loading direction. The last step involved filling the rest of the mold with mortar [Fig. 3(d)]. A summary of sample preparation steps is shown in Fig. 3. Test specimens were cured at room temperature for the initial 24±2  h. Then, the specimens were demolded and cured in a water bath at 23°C ± 3°C until they were tested.Test Setup and ProcedurePASCO materials testing machine (MTM) with a load cell capacity of 7,100 N (Fig. 4) and a 1  mm/min crosshead loading rate was used to carry out the pull-out test. The fixture consists of identical upper and lower grips which were fixed to the test machine. During testing, the upward movement of the actuator applies a pull-out force on the upper half of the specimen. The machine is equipped with an optical encoder to measure the displacement of the sample. Force data from the load cell and displacement data from the encoder module can be recorded by a PASCO interface with PASCO data acquisition software (PASCO capstone). PASCO capstone is a data collecting software which can automatically collect and record data with different adjustable frequencies. In this experiment, data values were recorded at a frequency of 25 Hz. The optical encoder in the test machine was used to ensure no preload was applied to the specimen and to minimize the initial seating-based errors while sample gripping.Experimental Results and DiscussionThe pull-out behavior of fiber is based on different factors including the physio-chemical adhesion, friction, and mechanical resistance (Naaman and Najm 1991). In this test, pull-out behavior of fiber is evaluated by analyzing force-displacement curves. Similar to previous studies (Wu et al. 2018; Bhutta et al. 2017), pull-out force versus displacement curves are used to reflect the amount of load and total displacement of fibers being pulled out; the effect of fiber type and matrix age on the pull-out mechanism is evaluated by comparing pull-out results. All types of fibers are pulled out at 7, 14, 28, and 56 days.According to Le et al. (2018), a typical pull-out load versus slip curve for smooth steel fiber consists of the elastic stage, the partial debonding stage and the frictional stage. There is a linear elastic fiber-matrix bond in the elastic stage that is followed by a gradual separation in the adhesion between fiber and matrix, which is called partial debonding phase and finally results in complete debonding. In the final stage and after debonding of the fiber, the pull-out resistance is only governed by friction (Le et al. 2018). Analyzing the three mentioned stages will help to get a broad insight into pull-out behavior of different types of fibers.Different parameters can evaluate the pull-out behavior and resistance of fibers. Maximum fiber stress (σf,max) is a vital parameter to assess the performance of fibers in concrete. If the maximum fiber stress exceeds its tensile strength, the fiber will be fractured before complete pull-out. The maximum fiber stress can be calculated using Eq. (1) (1) where σf,max = maximum fiber stress; Pmax = maximum pull-out force; and Af = cross-sectional area of fibers (in samples with CF, diameter of CF bundle is used to calculate the cross-sectional area).Average bond strength (τav) between the fiber and the matrix is calculated using Eq. (2) (Kanda and Li 1998) (2) where τav = average bond strength; df = fiber diameter; and Le = initial embedded length of the fiber.Fiber pull-out energy (Wp) is the area under the pull-out force versus displacement curve up to the point where the pull-out load becomes zero; it can be calculated using Eq. (3). Interface toughness of fibers is one of the most critical factors that evaluates performance of fiber and enhances the ductility of FRC. Interface toughness is defined as the mechanical energy consumed during fiber pull-out and is determined by integrating the area under the pull-out force-displacement curve (3) where Wp = fiber pull-out work; and P(s) = pull-out force at a certain slip.Interfacial bond strength at the interface between the fiber and matrix can be evaluated by calculating equivalent bond strength (τeq) using Eq. (4) (assuming shear strength is evenly distributed over the length of fiber) (Kim et al. 2007) (4) The results of maximum pull-out load (Pmax), standard deviation of pull-out loads (SD), displacement at maximum pull-out load (Δpmax), fiber pull-out energy (Wp), maximum fiber stress (σf,max), average bond strength (τav), and equivalent bond strength (τeq) are given in Table 3 in which numbers after PA and PI represent the diameter of the twisted bundle of fiber in millimeters. For example, PA0.5 represents mortar mix with PAN-based CF and 0.5 mm diameter.Table 3. Summary of pull-out test results of fibers at different curing agesTable 3. Summary of pull-out test results of fibers at different curing agesSpecimenAge (day)Pmax (N)SDΔpmax (mm)Wp (N·mm)σf,max (MPa)τav (MPa)τeq (MPa)S73089.13.1925392.43.90.943S1438212.13.7915486.64.90.932S283580.73.51,050456.14.61.070S5634671.13.61,053440.84.41.073SI710629.13.6255353.31.90.374SI1411538.53.1245383.32.10.360SI2812225.43.5277406.72.20.407SI56140154.18453466.72.50.665PA0.573112.10.816158.00.80.033PA0.5144321.90.814219.21.10.029PA0.528———————PA0.55676—0.715387.41.90.031PA0.8765——————PA0.8147333.230.736145.41.20.046PA0.828135—0.736268.92.10.046PA0.856143—0.838284.92.30.048PA1713916.91.136177.11.80.037PA114144—0.844183.41.80.045PA128155—0.847197.52.00.048PA156160—0.954203.82.00.055PI0.578110.20.627412.82.10.055PI0.514100—0.748509.72.50.098PI0.528111—0.750565.72.80.102PI0.556———————PI0.879835.30.736195.21.60.046PI0.814110—0.742219.11.80.054PI0.828———————PI0.856127—0.855253.02.00.070PI17100—0.728127.41.30.029PI114104—0.736132.51.30.037PI128156108.50.555198.72.00.056PI156165220.762210.22.10.063Effect of Fiber Type on Fiber Pull-Out BehaviorPull-out behavior of four types of fibers was investigated. Fig. 5 shows optical microscopy images of fibers after being pulled out. According to the image, after the pull-out test, cement paste was still attached to the surface of steel fiber in some areas of the surface, which is proposed to be an indication of strong fiber-matrix bonding. In hooked end SF, the failure was a combination of matrix spalling and fiber-matrix debonding. However, in CF and SI, fiber-matrix debonding was the only and predominant failure in CF and SI.Different failure modes were observed during pull-out test. In SF and SI samples, the failure mode was complete pull-out, while in CF, as it is shown in Fig. 6, different failure patterns were observed including fiber fracture and fiber pull-out. The complete pull-out failure mode in SF and SI indicates that that bonding between fiber and matrix is weaker compared to the strength of the matrix or fiber (Singh et al. 2004; Richardson 2005). Complete pull-out failure mode is a desirable failure mode to prevent sudden brittle failure in structures. However, the fracture failure mode reveals that the interfacial bonding is stronger than fiber tensile strength.Fig. 7 is presented to show the failure pattern of different fibers. Note that each curve in the figure is a representative curve indicating a typical failure behavior of that specific fiber while being pulled out from the matrix. According to the results, the failure pattern in different fibers can be summarized as follows. In CF, a sudden drop in the pull-out load occurred after reaching the peak load. Considering the hydrophobic nature of CF, as well as the straight geometry of the fiber, the sudden drop at around 1 mm of CF bundle slip is attributed to the poor interfacial bonding between the bundle and the matrix. After deboning, friction is the predominant mechanism in pull-out behavior (Le et al. 2018) and as the slip increases, the pull-out load decreases due to the reduced friction area. However, because of the geometry of CF, the friction is much lower in CF bundles compared to SF and SI, which results in lower postcrack energy absorption. In SF, the decrease in the post peak load was not as sudden as for CF similar to what was found by Nieuwoudt and Boshoff (2017). After reaching the peak load, the increase in slip was followed by a strain softening behavior with a smooth decrease in pull-out load until anchorage of the fiber was straightened, which occurred at slip of approximately 5 mm. In SI, the slope of the curve before peak load was even smoother than SF, and after reaching the peak load, a slip-hardening behavior was observed leading to zero load at around 4.5 mm of fiber slip. The reason for slip-hardening behavior is the friction bonding, which is mainly dependent on the properties of fiber rather than concrete properties (Nanni et al. 1995). Hence, fiber geometry is one of the important factors in fiber bonding behavior, and by improving the geometrical shape of fiber, the energy absorption of fibers can be enhanced significantly. For example, crimping fibers is an effective way of improving the postcrack behavior of SI resulting in slip-hardening behavior after the maximum pull-out force is reached.The detailed results of the pull-out test are presented in Table 3. During the preparation and testing of CF, some samples were damaged or broken due to the high vulnerability of fibers to any kind of impacts (missing values are presented in Table 3). These impacts were produced with sample handling, pretest preparation, testing, and also due to the matrix-to-matrix adherence of the two parts of the dog-bone sample. Although attempts were made to install the plastic plates in such a way that fibers were the only holders of the two parts of the briquette, still some small portion of the matrix attached to each other and needed to be broken. The load needed to break the cement matrix by the MTM was very low, but it could damage or break the bundle. Another reason for brittle behavior of CF is the high loading rate of MTM (1  mm/min). The future scope of the carbon fiber pull-out test will be (1) increase in number of samples to make sure enough results will be provided in case of fiber damage; (2) decrease in loading rate to avoid brittle fiber breakage; and (3) fix the divider plate inside the mold in order to make sure there is no matrix-to-matrix adherence.According to the bond strength results, SF had the highest bond strength and pull-out energy, followed by SI and then CF. According to the literature, the superior bond behavior will result in better fracture behavior (Bhosale et al. 2019; Deng and Li 2006). At 28 days of curing, steel fibers were able to carry an average of 358 N force, while this amount was 122, 155, and 156 N for SI, PA1, and PI1 at 28 days, respectively. This means steel fiber could carry more than twice the amount of force compared to other fibers, and the other three fiber types showed comparable results in terms of maximum pull-out force. However, for both S and SI samples, the maximum load was at 3.5 mm of slip, while for PA1 and PI1, the displacement at maximum load was 0.8 and 0.5, respectively.Another factor to assess the performance of fiber is the amount of energy that can be absorbed by fibers. As shown in Table 3, the average fiber pull-out energy obtained was 1,050, 277, 15, 36, 47, 50, 50, and 55 N·mm for S, SI, PA0.5, PA0.8, PA1, PI0.5, PI0.8, and PI1 at 28 days, respectively. The results showed the superior behavior of SF to absorb energy as well as a suitable behavior of SI and thus, in order to provide an equal reinforcement effect to a cement-based composite, a much greater amount of carbon fiber will be needed. The reason is mainly due to the fact that hooked-end steel fibers anchor at the ends and crimped fibers anchor along the length while in straight filament fibers the frictional shear stress at the interface determines the energy absorption capacity of fibers.Fig. 8 shows the tensile strength of fibers generated by force. According to the results, PI0.5 had the highest tensile strength; notably it was higher that SF and SI and thicker bundles. The increase in tensile strength with the decrease in diameter of the bundle approves the importance of fiber dispersion in order to maximize the capacity of CF. However, by comparing fiber tensile strength and energy absorption values, fibers with better tensile strength did not result in better energy absorption while fibers with better shear or bonding strength had improved energy absorption and after crack behavior. Although the tensile strength generated by force in all carbon fiber samples is lower than the fiber tensile strength, a few carbon fibers were fractured before complete pull-out. This is mainly due to the initiation of fracture of single fibers around the bundle, which are adhered to cement matrix and then the gradual breakage of inner fibers until all of fibers are fractured.Effect of Matrix Age on Fiber Pull-Out BehaviorFig. 9 shows maximum pull-out force versus matrix age of fibers. In case of the effect of matrix age, an increase in specimens curing age led to an improvement in fiber tensile strength in all fiber types except SF. The reason why SF didn’t show the same pattern as in CF and SI is because of an important additional major contributor to pull-out resistance, that is, mechanical anchorage (Tai and El-Tawil 2017; Le et al. 2018). This means when the hooked end fiber is pulled out, a large amount of energy is needed for the fiber to become straightened, and hence the interlock between the anchorage and the matrix is one of the determining factors in pull-out behavior of fiber.The improvement in interfacial bond properties with an increase in curing age is mainly due to the production of hydration products especially calcium silicate hydrate and development of microstructure of interfacial transition zone (ITZ) as the degree of hydration increases over time. For carbon fibers, after curing for 7 days, all tests ended with the fibers being pulled out from the mortar, which indicated the weak bonding between fiber and matrix at early ages. As the curing age increased to 28 and 56 days, a few fiber samples were fractured while being pulled out, which indicates improvement in fiber bonding at later ages.Interfacial StrengthThe fiber pull-out test is important since it provides information about interfacial properties of carbon fiber in the cement matrix. The pull-out force versus slip curves of various specimens is shown in Fig. 10. Although the test machine was equipped with an optical encoder to minimize the gripping error, a few samples still show some preloading values in the pull-out force versus slip curve. These errors, which can be seen in Fig. 10(a), were removed during the analysis.The pull-out curves revealed the characteristics of the bonding properties of each type of fiber. In CF samples, after failure of fiber (after Pmax), the pull-out force decreased with increase in slip and no slip-hardening or slip-softening was observed, which is in conformity with previous studies (Lu et al. 2018). In synthetic macrofibers, the initial incline of the force-slip curve is linear to the point of failure and then follows a smooth decrease in the amount of force until fiber failure. In steel fibers due to the anchorage at the end of the fiber, the cement matrix will be damaged while fibers are being pulled out from the matrix and as a result several drops in load prior to final fiber failure are seen in the force-slip curve (Fig. 10). Having a smooth surface is the main reason for a sudden drop in the pull-out force of carbon fibers.Although steel and synthetic fibers showed a much better energy absorption capacity, the bonding behavior of carbon fibers is comparable with other fibers. Average bond strength (τav) is considered as an important factor to evaluate the ITZ quality between fiber and matrix. According to Table 3, after steel fibers, most carbon fibers showed a comparable bond strength compared with synthetic fibers. 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