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Effects of fiber shape and distance on the pullout behavior of steel fibers embedded in ultra-high-performance concrete
Cement and Concrete Composites 103 (2019) 213–223
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Effects of fiber shape and distance on the pullout behavior of steel fibers embedded in ultra-high-performance concrete
T
Jae-Jin Kim, Doo-Yeol Yoo∗ Department of Architectural Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul, 04763, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultra-high-performance concrete Steel fibers Pullout resistance Fiber distance Fiber bundling effect
This study examines the implications of the fiber type and the distance between fibers on the pullout behavior of steel fibers embedded in ultra-high-performance concrete (UHPC). For this, three different types of steel fibers, i.e., straight, hooked, and twisted, and four different distances between fibers, corresponding to fiber volume fractions of 1%, 2%, and 7% and a fiber bundle, were considered. To evaluate the effect of the distance between fibers, four individual fibers were included in a single dog-bone specimen, and a single fiber specimen was also fabricated and tested as a control specimen. Test results indicate that the twisted steel fiber exhibited the greatest pullout resistance, followed by the hooked and straight steel fibers. Approximately 30% lower bond strengths were obtained for the specimens with multiple fibers as compared to those with a single fiber, regardless of the fiber type and distance between fibers. The average bond strengths of the hooked and twisted steel fibers were improved by decreasing the distance between fibers up to 1 mm, corresponding to a volume fraction of 7%, while the bundled fiber specimens provided the poorest pullout resistance in terms of bond strength and energy absorption capacity for all types of fibers. The reduction rate of pullout resistance was the most significant for the straight fiber, relative to the hooked and twisted fibers. Minor matrix damage was obtained for the straight fiber specimen, and its pullout performance was not influenced by the surrounding fibers. In contrast, severe matrix damage was observed for the hooked and twisted fibers, and they were overlapped, causing a larger spalling area with a closer fiber distance.
1. Introduction Ultra-high-performance concrete (UHPC) is a special type of concrete having very high strength, and it is composed of fine particles with diameters of less than 0.5 mm without coarse aggregate. This material has been developed due to the demand for high-rise buildings, long-span bridges, and specially designed buildings with thin cross sections [1]. Research on reactive powder concrete was started as a type of UHPC in the mid-1990s, and several other studies [2–4] have focused on practical applications of UHPC for buildings and civil infrastructures. It is generally known that UHPC has high ductility and durability [5,6], but concrete of high-strength such as UHPC characterized by a compressive strength of more than 150 MPa [7,8] has extremely brittle properties than ordinary concrete due to an abrupt dissipation of large energies stored when it is failed [9]. In order to overcome the high brittleness of UHPC, a high volume of discontinuous steel fibers is incorporated to reinforce it and to enhance its energy absorption capacity [10–12]. In this way, UHPC containing fibers is called ultra-high-
∗
performance fiber-reinforced concrete (UHPFRC). The addition of fibers to UHPC improves the resistance to crack formation and propagation in the cement matrix, enhancing the post-cracking tensile performance in terms of strength and ductility [13,14]. Multiple discontinuous steel fibers serve as bridges in cracked sections of concrete to improve the stress transfer, contributing to the enhancement of post-cracking tensile behavior [15,16]. The stress transfer at a cracked section is affected by the bond properties of the fiber and matrix, and the failure behavior of the matrix varies depending on the stress distribution transferred from the fibers [17–19]. If the bond is weak, the fiber is pulled out from the matrix, whereas if the bond is too strong, the fiber will be fractured before complete pullout. Therefore, bond properties between the fiber and cementitious matrix are some of the important parameters for the tensile behavior of UHPFRC [20–22]. The bond properties of fibers are divided into physicochemical and mechanical bond properties. The physicochemical bond is influenced by adhesion and friction at the interface, while the mechanical bond has an anchor effect at the fiber end or along the fiber, as well as adhesion and friction [23–25]. In order to understand the tensile behaviors of
Corresponding author. E-mail address: [email protected] (D.-Y. Yoo).
https://doi.org/10.1016/j.cemconcomp.2019.05.006 Received 14 March 2018; Received in revised form 30 April 2019; Accepted 7 May 2019 Available online 09 May 2019 0958-9465/ © 2019 Elsevier Ltd. All rights reserved.
Cement and Concrete Composites 103 (2019) 213–223
J.-J. Kim and D.-Y. Yoo
study investigating the effect of adjacent fibers according to distance between the fibers and fiber geometry on the pullout behaviors of various steel fibers embedded in UHPC matrix, and this is the world first research. Exploring the effects of the distance between fibers and fiber type on the pullout behavior will help create a better understanding of the post-cracking tensile behaviors of UHPFRC with various steel fibers. Accordingly, this study first investigates the implications of the distance between fibers on the pullout behaviors of straight, hooked, and twisted steel fibers embedded in UHPC. The detailed objectives are as follows: 1) to compare the bond properties of a single fiber and multiple fibers, 2) to examine the group effect of bundled steel fibers, and 3) to evaluate the effect of the distance between fibers, corresponding to fiber volume fractions of 1%, 2%, and 7%, on the bond properties of multiple fibers in UHPC.
UHPFRC, numerous studies [19,24–30] have been performed on the bond properties from a fiber pullout test, and several major factors of the bond properties, i.e., fiber geometry, orientation, embedded length, surface, and matrix strength, were examined [31]. Using pre-deformed fiber reinforcement has attracted attention from many researchers because deformed steel fibers exhibit much higher bond strength than that of straight steel fibers. Almost all of the fibers used for reinforcement are mechanically deformed, among which hooked-end steel fibers are the most used fiber type [32]. Abdallah et al. [24] have examined the effects of fiber geometry, embedded length, and water-to-binder (W/B) ratio on the pullout behaviors of hooked-end steel fibers embedded in UHPC. They [24] reported several useful findings: (1) a decrease of the W/B ratio from 0.2 to 0.11 significantly enhanced the bond strength, (2) the pullout behavior of 5D hooked fibers was not influenced by the embedment length, and (3) the 5D hooked fiber was more effective at improving the pullout work and strength than the ordinary 3D and 4D hooked fibers. Wille and Naaman [25] evaluated the pullout performance of smooth and deformed (i.e., hooked and twisted) steel fibers in UHPC and denoted that using the hooked and twisted steel fibers provided four to five times greater equivalent bond strength than that of smooth steel fibers. They [25,26] also reported that by increasing the particle packing of the surrounding matrix, the bond characteristics of steel fibers can be significantly enhanced based on the optimization of UHPC mixtures. Stengel [27] proved that the bond strength of steel fibers in UHPC can be improved by modifying the fiber surface with fiber surface roughening based on abrasion or using sandpaper. Wu et al. [30] also reported that the inclusion of suitable nano calcium carbonate (CaCO3) is able to significantly improve the interfacial bond strength between the straight steel fiber and UHPC matrix up to the critical amount of 3.2%. The bond strength and pullout energy of straight fiber in UHPC improved as much as 45% and 200%, respectively, due to the nanoCaCO3 added. Lee et al. [19] have performed a number of pullout tests to examine the effect of the inclined angle of smooth steel fibers in UHPC and reported that the greatest bond strengths are obtained when the fibers are inclined as much as 30° and 45°. In contrast, the slip capacity, which indicates the slip corresponding to the peak strength, continuously increased with increasing the inclination angle. Tai and El-Tawil [28] evaluated the implications of fiber type, such as smooth, hooked, and twisted fibers, inclined angle, and loading rate, ranging from 0.018 mm/s (quasi-static) to 1800 mm/s (impact), on the pullout behaviors of steel fibers in UHPC. Based on their test results, the pullout resistance and energy dissipation capacity of smooth steel fibers in UHPC were enhanced by increasing the loading rate and inclination angle up to 45°, consistent with the findings of Lee et al. [19]. Additionally, the smooth steel fiber provided the highest sensitivity to the loading rate, causing a dynamic increase factor that was as high as 2.32, compared to the twisted and hooked steel fibers. In addition, the pullout performance of single steel fibers embedded in UHPC have been investigated by several researchers [14,24–28,31], and research on the effects of the steel fiber volume fraction on the compressive and tensile behaviors of UHPFRC also has been published [33]. However, in previous studies [34], the pullout behavior of singly embedded steel fibers in UHPC did not exhibit similar behavior with those in the UHPFRC composites. Since the correlation between the pullout behavior of singly aligned deformed steel fibers in UHPC and the flexural behavior of UHPFRC is fairly low due to several influential parameters, it is difficult to predict the composite's behavior through only the single fiber pullout test results. There are more important variables that affect fiber pullout behavior, and the effect of variables needs to be confirmed. Therefore, in this study, we conducted numbers of experimental tests with embedding multiple steel fibers in UHPC matrix considering the fiber spacing according to certain fiber volume fractions ranging from 1% to 7%. Actually, only few studies [19] have conducted pullout tests considering the fiber group effect using multiple fibers. To the best of the authors' knowledge, there is no published
2. Experimental program 2.1. Mixture proportion of UHPC The mixture proportion and the average strength of the ultra-highperformance concrete (UHPC) used in this experiment is given in Table 1. The cementitious materials consisted of Type I Portland cement (cement strength class of 42.5 MPa) and silica fume (SF), and their chemical compositions and physical properties are summarized in the Table 2. Silica sand with a diameter of 0.2–0.3 mm was used as fine aggregate, and it has a density of 2.62 g/cm3. Silica powder with an average grain size of 4.2 μm, containing approximately 98% SiO2, was used as filler. Since coarse aggregate has not been generally used for making UHPC [1,32], it was also excluded in this study. Due to a low water-to-binder (W/B) ratio and high amount of fine powder for UHPC, its fluidity was significantly reduced. Thus, to increase the fluidity, polycarboxylic acid-based superplasticizer (SP) was added. 2.2. Fabrication of the test specimen To investigate the effects of the geometry and volume fraction of steel fibers on their pullout behavior in UHPC, three different types of steel fibers, i.e., straight, hooked, and twisted steel fibers, and four different distances were considered. For the straight and twisted steel fibers, the distances were 2.7 mm, 1.9 mm, 1.0 mm, and 0 mm, while for the hooked steel fiber, the distances were 3.3 mm, 2.3 mm, 1.2 mm, and 0 mm, respectively. The fiber distance was calculated based on the assumption of a perfectly homogeneous distribution of fibers, and the corresponding volume fractions. For instance, the fiber distances of 2.7 mm, 1.9 mm, and 1.0 mm indicate the fiber volume fractions of 1%, 2%, and 7%, respectively. Sizes used for the straight, hooked-end, and twisted fibers are as follows: S30 with an aspect ratio (lf / df ) of 30/ 0.3 mm/mm = 100, H30 with lf / df of 30/0.375 mm/mm = 80, and T30 with lf / df of 30/0.3 mm/mm = 100. Herein, lf is the fiber length and df is the fiber diameter. The twisted steel fiber has a triangular shape, and thus, the normalized diameter of 0.3 mm was calculated by Table 1 Mixture proportion and compressive strength of UHPC. W/Bb
0.2
Unit weight [kg/m3] Water
Cement
Silica fume
Silica sand
Silica powder
SPa
Compressive strength, fc’ [MPa]
160.3
788.5
197.1
867.4
236.6
52.6
190.2
[Note] W/B = water-to-binder ratio, and SP = superplasticizer. a Superplasticizer includes 30% solid (= 15.8 kg/m3) and 70% water (= 36.8 kg/m3). b W/B is calculated by dividing total water content (160.3 kg/m3 + 36.8 kg/ m3) by total amount of binder (788.5 kg/m3 + 197.1 kg/m3). 214
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fraction of the fibers is directly related to the distance between fibers, the distance between fibers is calculated based on the volume fraction. Additionally, for comparative analysis of the group effect of the fibers, a bundle type with attached fibers (zero distance between fibers) and single fiber specimens were also fabricated. The detailed information regarding fiber distance and corresponding volume fraction is summarized in Table 4. The specimens are dog-bone shaped, and their height, width, thickness, and cross-sectional area were 75 mm, 40 mm, 25 mm, and 25 × 25 mm2, respectively. The detailed geometry of the dog-bone samples is shown in Fig. 1. To fabricate the UHPC mortar, a special mixing process was applied, as follows. First, cementitious materials, such as cement and SF, silica sand, and silica powder were premixed for 10 min using a Hobart-type mixer. Water premixed with SP was added to the dry ingredients and mixed for an additional 10 min to achieve a flowable mixture. Two separate pouring procedures were used to fix the fibers to the dog-bone-shaped mold. The fibers were first fixed at the center of the mold. Then, UHPC mortar was poured into only one side of the mold and cured at room temperature for 48 h. After the fiber fixture was removed, the mortar was poured into the remaining mold and cured for 48 h at identical room temperature conditions. The specimens were then removed from the mold and steam cured, heating at 90 °C ± 2 °C for 3 days to achieve full strength development. Specimens were removed from the steam curing machine and stored in the laboratory at room temperature until the pullout test. A schematic description on the manufacturing process specimen for the pullout test is shown in Fig. 2.
Table 2 Chemical compositions and physical properties of cement and silica fume. Composition [%] (mass)
Type I Portland cement
Silica fume
CaO Al2O3 SiO2 Fe2O3 MgO SO3 Specific surface area [cm2/g] Density [g/cm3]
61.33 6.40 21.01 3.12 3.02 2.30 3,413 3.15
0.38 0.25 96.00 0.12 0.10 200,000 2.10
Table 3 Geometrical and physical properties of steel fibers.
S30 H30 T30
df [mm]
lf [mm]
Aspect ratio (lf/ df)
Density [g/ cm3]
fft [MPa]
Ef [GPa]
0.300 0.375 0.300
30.0 30.0 30.0
100.0 80.0 100.0
7.9 7.9 7.9
2580 2900 2428
200 200 200
[Note] S30 = straight steel fiber with a length of 30 mm, H30 = hooked steel fiber with a length of 30 mm, T30 = twisted steel fiber with a length of 30 mm, df = fiber diameter, lf = fiber length, fft = tensile strength of fiber, and Ef = elastic modulus of fiber. Table 4 Test variables. Specimens
Fiber type
Volume fraction [%]
Distance between fibersa [mm]
S30 (1.0%) S30 (2.0%) S30 (7.0%) S30 (bundle) S30 (single) H30 (1.0%) H30 (2.0%) H30 (7.0%) H30 (bundle) H30 (single) T30 (1.0%) T30 (2.0%) T30 (7.0%) T30 (bundle) T30 (single)
Straight
1.0 2.0 7.0 1.0 2.0 7.0 1.0 2.0 7.0 -
2.7 1.9 1.0 0.0 3.3 2.3 1.2 0.0 2.7 1.9 1.0 0.0 -
a
Hooked
Twisted
2.3. Test setup and procedure Dog-bone-shaped pullout specimens were fabricated and tested to investigate the pullout behavior of three types of steel fibers (i.e., straight, hooked, and twisted fibers). For obtaining reliable test data, five samples were tested to obtain an average value. The length of all steel fibers was 30 mm, and an embedment length of 10 mm was applied at one side to induce fiber pullout. Fracture is more likely to occur in the case of twisted fibers because they have a higher bond strength but slightly lower tensile strength than the other fibers. Therefore, exceptionally, additional specimens were fabricated to have a 5-mm embedment length for the case of the twisted fibers. A uniaxial tensile
Distance between fibers is corresponding to the fiber volume fraction.
Fig. 1. Schematic description of dog-bone specimens for fiber pullout tests: (a) dimensions of specimen, (b) details of fiber location.
changing the triangle to a circle with an identical cross sectional area. The detailed geometric and mechanical properties of the fibers used are shown in Table 3. In order to examine the effect of fiber distance on the pullout behavior, four fibers were embedded in a 2 × 2 array in the center of the cross-sectional area of UHPC mortar. Since the volume
Fig. 2. Schematic description of manufacturing process for pullout tests. 215
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τav =
Pmax πdf LE
(2)
where τav is the average bond strength and LE is the initial embedded length of the fiber. The fiber pullout energy (Wp) is equal to the area under the pullout load versus slip curves up to the point where the pullout load becomes zero. Therefore, it can be evaluated based on integral calculus, as follows: s = LE
WP =
∫
P (s ) ds
s=0
where WP is the fiber pullout work and represents the total energy until the fiber is completely pulled out (or failed) and P(s) is the pullout load at a certain slip, s. The fiber pullout energy at the unit embedded surface area is also calculated by dividing Eq. (3) by the embedded area and is given as Wpb. Assuming that the shear stress is evenly distributed over the length of the embedded fiber, the equivalent bond strength (τeq) can also be calculated based on the fiber pullout work (WP) [35], as follows:
Fig. 3. Test setup for fiber pullout tests.
load was applied through a universal testing machine (UTM) with a maximum load capacity of 3 kN. In order to apply a quasi-static loading rate and obtain sufficient data points up to the peak strength, a loading rate of 0.018 mm/s was adopted, similar to a previous study [28]. After placing a full dog-bone specimen in the grip jig, the upper grip was raised at a quasi-static loading rate, and the pullout load and slip begin to be measured at the moment when the load was actually created. Based on an assumption that elastic deformations of both the fibers, matrix, and steel grip jig are sufficiently small, the stroke recorded was used as the fiber slip as usual. However, since multiple, four, steel fibers were embedded in UHPC, the measured slip capacities were slightly higher than those in literature using single steel fiber. Up to the maximum pullout load, the straight steel fiber is not pulled out but rather partially debonded, while the deformed steel fibers are only slightly pulled out. Thus, the slip capacity is related to both the fiber slip and elastic deformations of fiber and matrix. Since multiple steel fibers were used in this study, the maximum pullout load was greatly higher than that of single fiber case, leading to the higher elastic deformations reflected in the slip capacity. The detailed test setup is shown in Fig. 3.
τeq =
(4)
3.2. Effects of fiber geometry on bond stress versus normalized end-slip (τ-s/ LE) behaviors To investigate the effect of the steel fiber type on the average bond stress versus normalized slip (τ-s/LE) behaviors, single fiber specimens were fabricated and tested. In the case of twisted steel fiber specimens, the fiber was embedded only at 5 mm in order to prevent breakage before complete pullout, while the straight and hooked steel fibers were embedded at 10 mm. Thus, in order to reasonably compare their pullout behavior, the average bond stress and normalized slip parameters were used by assuming that they were not influenced by the embedment length. As shown in Fig. 4, the τ-s/LE curves showed noticeable differences in the bond stress in accordance with the fiber type: the twisted and hooked fibers exhibited much higher bond strengths than the straight fiber due to their additional mechanical anchorage effects. This is consistent with the findings from Yoo et al. [34] and Wille and Naaman [25]. The straight steel fiber is initially chemically adhered with the adjacent cement matrix up to the critical pullout load, and once the maximum bond stress at the interface exceeds the bond strength, a partial debonding process is activated up to the peak pullout load along with the generation of a frictional bond [36]. However, for the deformed steel fibers, such as the twisted and hooked-end fibers,
3.1. Evaluation parameters To evaluate the pullout behavior, the calculation formulae for strength and energy absorption capacity are given as follows. For reliable test results, five dog-bone-shaped fiber pullout test specimens were prepared for each variable and average values were used. Specimens damaged during the fabrication due to their insufficient matrix strength developed before steam curing were not included in the calculations. First, the maximum fiber stress (σf,max) is a very important factor for judging the suitability of fibers in concrete, because if the maximum fiber stress exceeds its tensile strength, the fiber will be fractured before complete pullout. The maximum fiber stress can be calculated based on the maximum pullout load and the cross-sectional area of the fiber, as follows:
Pmax Af
2Wp πdf LE2
where τeq is the equivalent bond strength.
3. Experimental results and discussion
σf ,max =
(3)
(1)
where σf,max is the maximum tensile stress of the fiber, Pmax is the maximum pullout load, Af is the fiber cross-sectional area (=πdf2/4), and df is the fiber diameter. The average bond strength (τav) between the fiber and the matrix is calculated using the maximum pullout load, diameter of the fiber, and initial embedment length, as follows:
Fig. 4. Average bond stress versus normalized slip responses of single steel fibers. 216
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calculated based on the peak pullout load, so that even with fibers of different shapes, the bond strength can be easily evaluated. The shape of the straight and twisted fibers is comparatively uniform throughout the entire embedment length, so their equivalent bond strength can be compared. The hooked fiber has a special end-hook, so the bond stress is not constant along the embedded fiber length. However, in order to compare the equivalent bond strength according to the fiber type, the parameter τeq was intentionally calculated for the case of the hooked fiber. The maximum fiber tensile stress is an important parameter for determining the failure mode (pullout or fracture) of the fibers [37]. In Fig. 5c, the 10-mm embedded twisted fibers were all fractured before complete pullout, since the maximum fiber tensile stresses generated by the external pullout force exceeded their tensile strength. In contrast, when we reduced the embedment length of the twisted steel fiber from 10 mm to 5 mm (Fig. 5d), they were pulled out without fracture, and the pullout energy could be calculated.
mechanical anchorage at the end hook for the hooked fiber and throughout the entire embedment length by twisting for the twisted fiber is additionally generated, causing the higher bond strength. As a result, the highest bond strength was obtained for the twisted fiber, followed by the hooked fiber and the straight fiber. The post-peak bond stresses of the twisted and straight fibers decreased roughly and uniformly in proportion to the amount of slip, while that of the hooked fiber was not constant. The bond stress reduction rate was larger for the case of the hooked steel fiber after reaching the peak point, because its hooked shape greatly enhanced the bond strength by mechanical anchorage. However, since the end-hook is stretched during the pullout process, the mechanical anchorage effect by the hook greatly decreased after stretching. The much steeper decrease of the post-peak bond stress of the hooked fiber than those of the twisted and straight fibers was also reported by previous studies [25,34]. The highest maximum bond strength of 34.0 MPa was found for the twisted fiber, approximately 482% and 36% higher than those of the straight and hooked fibers, respectively.
3.3.1. Straight steel fibers To investigate the effect of the interference of adjacent fibers on the pullout behavior, the dog-bone specimens were fabricated by including four fibers in each specimen. Therefore, it is assumed that the pullout force acts on each fiber equally in order to compare them easily with specimens including only a single fiber. As shown in Fig. 6a, the maximum fiber tensile stress of a single straight fiber was found to be 1159.8 MPa, leading to a fiber strength ratio of 45%. Identical average and equivalent bond strengths of 8.8 MPa were obtained. The specimens with four fibers with distances equal to 1%, 2%, and 7% (by volume) steel fibers showed similar values for the maximum fiber tensile strength, with decreases of approximately 15% compared to the single fiber specimen, while the bundled fiber specimen provided a maximum fiber tensile stress of only 52% of the single fiber specimen. The average bond strengths of the multiple fiber specimens decreased by 22%–30% compared to the single fiber specimen, and the bundled fiber specimens decreased by 52%, which is much higher than the
3.3. Effects of fiber geometry and distance on the pullout load versus slip behaviors The average pullout load versus slip curves are shown in Fig. 5. We adopted two different embedment lengths of 10 mm and 20 mm to induce the four embedded fibers to be pulled out from one side of matrix. The several pullout parameters, i.e., pullout work (Wp), maximum fiber pullout load (Pmax), maximum fiber tensile stress (σf,max), average bond strength (τav), and equivalent bond strength (τeq), are also summarized in Table 5. As mentioned earlier, the pullout work is the energy required until the fiber is completely pulled out. Assuming that the bond stress is constant throughout the embedment length, the pullout work can be used to calculate the equivalent bond strength, which is related with the cracking behavior of high-performance fiber-reinforced cement composite (HPFRCC) [35]. The average bond strength can be
Fig. 5. Average pullout load versus slip curves: (a) straight fiber, (b) hooked fiber, (c) twisted fiber, and (d) twisted fiber with a short embedment length of 5 mm. 217
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Table 5 Summary of pullout test results of different fiber types and distances. Fiber type S30
H30
T30a
Parameters −3
Wp [ × 10 J] Wpb [ × 10−3 J/mm2] Pmax [N] σf,max [MPa] τav [MPa] τeq [MPa] Wp [ × 10−3 J] Wpb [ × 10−3 J/mm2] Pmax [N] σf,max [MPa] τav [MPa] τeq [MPa] Wp [ × 10−3 J] Wpb [ × 10−3 J/mm2] Pmax [N] σf,max [MPa] τav [MPa] τeq [MPa]
1%
2%
7%
Bundle
Single
1935.3 (3.98) 53.4 (11.42) 277.1 (3.98) 979.90 (137.88) 6.19 (0.53) 8.99 (1.55) 4640.3 (1212.71) 98.7 (25.73) 937.9 (128.73) 2098.4 (291.39) 19.67 (2.73) 19.69 (5.15) 1259.5 (94.95) 88.6 (9.08) 494.4 (21.00) 1748.7 (74.28) 26.23 (2.59) 26.73 (4.80)
1693.2 (5.10) 45.0 (7.82) 279.4 (5.10) 988.03 (18.03) 6.91 (0.78) 8.35 (1.46) 4752.2 (471.53) 100.8 (9.62) 956.6 (150.54) 2165.3 (86.68) 20.30 (0.81) 20.17 (1.92) 1271.1 (107.58) 76.4 (6.47) 494.0 (44.24) 1747.2 (99.41) 26.21 (3.82) 26.97 (2.73)
1886.1 (64.41) 47.7 (9.77) 280.0 (64.41) 990.38 (227.82) 6.81 (2.40) 8.81 (3.24) 3940.5 (921.45) 83.6 (19.55) c 1081.3 (123.31) 2447.5 (279.12) 22.95 (2.62) c 16.72 (3.91) c 1039.1 (185.94) 67.4 (6.17) 476.1 (32.67) 1683.9 (115.54) 28.08 (0.28) 27.09 (0.61)
1300.3 (17.34) 35.3 (7.39) 170.5 (17.34) 603.05 (61.33) 4.26 (0.91) 6.70 (2.08) 3974.1 (733.01) 84.3 (15.56) 884.1 (188.49) 2001.3 (426.66) 18.76 (4.00) 16.87 (3.11) 1103.1 (62.92) 61.0 (11.23) 454.6 (86.20) 1607.7 (15.73) 24.43 (3.16) 24.65 (5.24)
400.9 (13.42) 43.3 (12.65) 82.0 (13.42) 1159.8 (189.92) 8.82 (1.65) 8.82 (2.91) 1480.5 (270.42) 125.7 (22.95) 320.3 (53.29) 2899.7 (482.46) 27.18 (4.52) 25.13 (4.59) 386.6 (58.59) 59.9 (0.03) 165.8 (7.78) 2345.6 (110.04) 38.32 (4.04) 38.75 (5.90)
c
c c
[Note] Wp = pullout work, Pmax = maximum pullout load, σf,max = maximum fiber tensile stress, τav = average bond strength, τeq = equivalent bond strength, and ( ) = standard deviation. a T30 specimens with an embedment length of 5 mm. b Pullout energy per unit embedded surface area. c Specimen with fiber fracture.
multiple fiber cases. It was expected that the straight fiber may have a similar average bond strength (τav) regardless of the number of fibers. This is because it behaves individually without affecting the surrounding area during the pullout process, caused by the fact that the straight fiber resists the pullout force only through the frictional bond. There were different slip capacities, which is the slip at the point of the maximum pullout load, even though identical straight steel fibers were embedded, as shown in Fig. 7. This is due to heterogeneity of matrix which is one of the characteristics of cementitious materials, and the eccentric effect during pullout of the fibers. Fig. 7 shows the fiber comparative pullout load versus slip curves of four dog-bone specimens with a single fiber. The summation of all load-slip curves is also given in Fig. 7, denoted as S30 (sum). The summation of the maximum pullout load of the four specimens with a single fiber was found to be 350.7 N, which was approximately 7.4% greater than that of the specimen S30 (sum). This is caused by the fact that, since the slip capacities (at the peak point) of the four specimens were different from each other, the maximum pullout load of the specimen S30 (sum) became smaller than the sum of the maximum values of the respective fiber specimens. This phenomenon is equally applicable to the hooked and twisted fiber specimens. As shown in Fig. 6, the equivalent bond strength (τeq), calculated based on the pullout work (Wp), of multiple fiber specimens was
Fig. 7. Pullout load versus slip curves of straight fibers embedded in UHPC matrix.
similar to that of a single fiber specimen with a difference of only up to 5%. In contrast, the difference became quite severe for the bundled fiber specimen: an approximately 32% higher value of τeq was obtained for the single fiber specimen relative to the bundled fiber specimen.
Fig. 6. Summary of bond parameters of straight steel fibers in UHPC: (a) maximum fiber stress, (b) average bond strength, (c) equivalent bond strength. 218
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Fig. 8. SEM images on the surface of pulled out straight steel fibers from UHPC matrix: (a) fiber distance of 2.7 mm, (b) fiber distance of 0 mm (bundled fiber).
fibers were approximately 2.9–3.1 times higher than those of the straight fibers. As was noted previously, due to the different slip capacities of each fiber in multiple fiber specimen, the maximum fiber stresses in the multiple fiber specimens decreased by approximately 16%–28%, compared to that of the single fiber specimen. The bundled fiber specimen provided a greater decrease of 31%. Given the fiber bundling effect, the straight steel fiber exhibited a greater decrease of the bond strength (52%) than the hooked steel fiber (31%). For the case of straight fibers, since the pullout load resistance is generated mostly by the friction shear stress, which is closely related to the bonding area, the bundled specimen showed a lower pullout resistance due to the decreased total fiber surface area. However, for the case of hooked fibers, the resistance to the pullout force is activated by both a frictional bond and mechanical anchorage at the end hook, and the effect of the total fiber surface area reduction on the pullout resistance was offset. Therefore, the reduction rate of the bond strength by fiber bundling was relatively minor for the hooked fiber specimen compared to the straight fiber specimen. It is interesting to note that the average bond strength increased by decreasing the distance between fibers up to 1.2 mm, which is equal to 7% fibers (by volume) by assuming that all fibers are perfectively distributed. For example, when we decreased the distance between fibers from 3.3 mm to 1.2 mm (1 vol% to 7 vol%), the average bond strength increased as much as 17%, as shown in Fig. 9b. This is because the expansive pressure generated by the end-hook under the pullout load pressed the adjacent fibers, causing a higher resistance to the pullout force. In other words, expansive pressure is activated when we pull out the hooked steel fiber from the UHPC matrix, which sometimes causes splitting cracks in the matrix [39]. When the fibers are closer, the fiber pullout resistance can be enhanced due to the expansive pressure additionally generated by the surrounding fibers; therefore, a higher pullout resistance for the hooked steel fibers was obtained with a
Since the straight steel fiber resists the pullout force mainly by the frictional shear stress at the interface, the bonding area between the fiber and the matrix is the most important factor. Fig. 8 shows scanning electron microscopy (SEM) images for the fiber surface after complete pullout for the specimens with bundled and multiple fibers. As shown in Fig. 8a, scratches at the surface of the fiber were uniformly formed along the fiber length for the multiple fiber specimens having separated fibers, while the degree of surface scratches for the bundled fiber specimen was different according to the location, as shown in Fig. 8b. A portion of the fiber that is in contact with a surrounding fiber has much fewer surface scratches than other portions that are in contact with the cement matrix. This observation could verify a fiber bundling effect [38], indicating that the bond strength of bundled fibers is reduced as compared with that of a single fiber or separated fibers due to the reduced total fiber surface area bonding to the matrix. Therefore, this decrease in the surface contact area between the fiber and matrix is considered to be the main cause of the deteriorated pullout resistance of the bundled fibers. 3.3.2. Hooked steel fibers As summarized in Fig. 9, the single hooked fiber exhibited the maximum fiber tensile stress of 2899.7 MPa and the fiber strength ratio of 99.9% in the UHPC matrix, and hooked fiber showed approximately 2.5 times higher maximum fiber stress than that of the straight fiber in single fiber pullout test. However, the fibers in the UHPFRC composites are randomly oriented and the ones in the inclined state produce greater maximum pullout loads and stresses during pullout in general, which can cause fiber fracture with high probability. It is therefore difficult to be interpreted as the hooked fiber is more effective than the straight fiber. The average and equivalent bond strengths were 27.2 MPa and 25.1 MPa, respectively, which were also much higher than those of the straight steel fibers. The bond strengths of the hooked
Fig. 9. Summary of bond parameters of hooked steel fibers in UHPC: (a) maximum fiber stress, (b) average bond strength, (c) equivalent bond strength. 219
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the former test configuration have ruptured. As shown in Fig. 7, since the four fibers embedded did not reach the maximum load at the same value of slip, the measured maximum pullout load applied for fibers was reduced in the multiple fiber specimens than the counterpart. Due to the inhomogeneity and eccentricity of the specimen, some of the fibers in the multiple fiber specimen are already fractured with additional pressure by adjacent fibers before the other fibers reach their maximum pullout load. Thus, the actual pullout stress was localized at a portion of multiple fibers, leading to the smaller maximum fiber stress at the point of fiber rupture since the maximum pullout load was divided into four fibers based on an assumption that the pullout stress was evenly distributed to all of the fibers. These are the reasons why the lower maximum fiber stress was obtained in the series H30 (7%) than the series H30 (single). The equivalent bond strength also decreased as much as 20%–22% by embedding multiple fibers for the cases of 1% and 2% fibers, compared to that of a single fiber specimen, while it significantly decreased for the bundled fiber specimen by as much as 31%. Since the specimen with the closest distance between fibers of 1.2 mm (7%) exhibited fiber fracture before the completion of pullout, a much smaller energy absorption capacity (pullout work) was obtained, as summarized in Table 5, causing the much smaller equivalent bond strength. It can be concluded that using the hooked steel fiber needs to be carefully at such a high fiber volume fraction to prevent fiber fracture in UHPC, which leads to a sufficient decrease of the energy absorption capacity.
Fig. 10. Schematic descriptions on (a) dimension of dog-bone specimen and (b) details of fiber location for strain mapping.
smaller fiber distance (or higher fiber volume fraction). To verify this explanation, a strain map of the dog-bone specimen with multiple hooked steel fibers was recorded based on digital image correlation (DIC) images. In order to capture strain variations at the surface of a dog-bone specimen by pulling out a hooked fiber, additional specimens were fabricated. Four hooked fibers were embedded and placed close to the surface of the specimen to easily observe strain changes on the surrounding matrix. The schematic descriptions of the tested specimens are shown in Fig. 10and the strain distributions on the specimen surface are shown in Fig. 11 according to the applied pullout load. The stress was generated in the surrounding matrix locally near the end-hook with an increase of the pullout load. The size of the stress area and its magnitude were larger with increasing the pullout load up to the peak point (B), while these values were reduced by decreasing the pullout load in the descending zone (point (C)). This indicates that in the ascending branch, the confined pressure, additionally applied to the fiber at the end-hook by adjacent fibers, increased with increasing the pullout load, and it is maximized at the peak load. It was also obvious that, due to the expansive pressure of the hooked steel fiber, stresses are localized near the end-hook, and the local stresses additionally press the adjacent hooked fiber to resist it being pulled out from the matrix, causing the higher pullout resistance and bond strength. Although the maximum fiber stress of the 7 vol% specimen was smaller than that of a single fiber specimen, the hooked steel fibers in
3.3.3. Twisted steel fibers The data of the 5-mm embedded specimens was used to examine the pullout behavior of the twisted steel fibers, since, when the twisted fibers were embedded as much as 10 mm, similar to other specimens, they were all fractured (Fig. 5c). As shown in Fig. 12, the single fiber specimen with an embedment length of 5 mm showed the maximum fiber tensile stress of 2345.6 MPa and the fiber strength ratio of 96.6%. The average and equivalent bond strengths were also found to be 38.3 MPa and 38.8 MPa, respectively. The twisted fiber exhibited the highest pullout resistance compared to the straight and hooked fibers, which is consistent with the findings from Wille and Naaman [25]. For instance, the average bond strength of the twisted fiber was approximately 3.4 times higher than that of the straight fiber and 1.4 times higher than that of the hooked fiber. Its equivalent bond strength was
Fig. 11. (a) Pullout load versus slip curve of hooked fiber specimen, (b), (c), and (d) strain maps of specimens at the points of (A), (B), and (C). 220
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Fig. 12. Summary of bond parameters of twisted steel fibers in UHPC: (a) maximum fiber stress, (b) average bond strength, (c) equivalent bond strength.
3.3.4. Matrix spalling effect For the hooked and twisted fiber specimens, the equivalent bond strength was rather reduced with decreasing the fiber distance, inconsistent with the test data of the maximum fiber tensile stress and average bond strength. To verify these observations and provide a reasonable explanation, a cross section of the dog-bone specimens after fiber pullout was investigated, as shown in Fig. 13. For the case of straight steel fibers, only minor damage in the matrix was obtained after fiber pullout regardless of the distance between fibers, as shown in Fig. 13. In addition, the matrix damage was not overlapped even for the specimen with the closest distance between fibers of 1.0 mm (7%), meaning that they did not interfere with each other, even at a very close distance. However, the spalling failures of the matrix near the pulled out fiber were obviously observed for the hooked and twisted fiber specimens, as illustrated in Fig. 13. This is because when the twisted and hooked fibers were pulled out, splitting cracks were created in the surrounding matrix due to their mechanical anchorage effect, and when the fiber embedding distance decreased, the spalling overlapped and appeared to create a larger spalling area. Thus, it could be verified that the pullout work and equivalent bond strength of the hooked and twisted fibers embedded in the UHPC matrix were not increased as much as the average bond strength, or were even reduced, due to the overlapped matrix damage formed by splitting cracks.
also approximately 4.4 and 1.5 times higher than those of the straight and hooked fibers, respectively. When we compared the pullout behaviors of the multiple and bundled fiber specimens with the single fiber specimen, the average bond strengths were reduced by as much as 27%–32% for the multiple fiber specimens (1%, 2%, and 7%) and 36% for the bundled fiber specimen. The reduction rate (36%) of the bond strength of the twisted fiber by fiber bundling was smaller than that of the straight fiber (51%), due to the additional mechanical anchorage effect as explained above. The equivalent bond strengths were also decreased by as much as 30%–31% for the multiple fiber specimens (1%, 2%, and 7%) and 36% for the bundled fiber specimen. Such a decrease of average and equivalent bond strengths of the multiple and bundled fiber specimens was caused by the different slip capacities of each fiber included. The multiple fiber specimens with distances between fibers of 2.7 mm and 1.9 mm (1% and 2%, respectively) exhibited the similar average bond strength of approximately 26.2 MPa, whereas the specimen with a distance between fibers of 1.0 mm (7%) provided a noticeably higher bond strength of 28.1 MPa, which is approximately 7% higher. This is consistent with the trend of the hooked fiber test results, showing that the pullout resistance increased with decreasing the distance between fibers due to the additional confined pressure applied, which is generated by the expansive pressure of the adjacent fibers. Due to an untwisting process of quadrangular twisted steel fiber, expansive pressure is applied to the cement matrix, and thus, several splitting cracks are formed in the matrix under the pullout force. This was also observed by a previous study [25]. The increase rate of the equivalent bond strength of the twisted fibers in terms of the decrease in the distance between fibers was smaller than that of the average bond strength. This might be caused by the fact that, since the twisted fiber led to the formation of cracks in the matrix significantly when it is untwisted and stretched under the pullout load, the cracks more greatly affected the post-peak pullout resistance when the fibers were placed closer together. The cracks normally lead to a steeper decrease of the post-peak pullout load, which causes the reduced pullout work and corresponding equivalent bond strength. Due to this, the equivalent bond strength of the twisted steel fiber specimen with fibers placed the closest together (7%) was approximately 4% smaller than the average bond strength, although other specimens, i.e., the single fiber and multiple fibers with distances between fibers of 2.7 and 1.9 mm (1% and 2%, respectively), showed equivalent bond strengths slightly greater than the average bond strength, as shown in Fig. 12. The bundled fiber specimen gave the lowest pullout resistance in terms of the maximum fiber tensile stress and average and equivalent bond strengths due to the decreased total fiber surface area and increased deteriorating effect of matrix cracks on the adjacent fibers.
4. Conclusion This study investigated the effects of the fiber type and distance between fibers on the pullout behaviors of steel fibers embedded in an UHPC matrix. For this, three different types of steel fibers, i.e., straight, hooked, and twisted, and three different distances corresponding to fiber volume fractions of 1%, 2%, and 7% were tested. To examine the fiber bundling effect, bundled fiber specimens were also considered along with a single fiber specimen as a control specimen. SEM images and strain maps were analyzed to reasonably verify and explain the test results. Based on the test results obtained in this study, the following conclusions can be drawn: 1) The highest bond strength of 34.0 MPa was obtained for the twisted fiber in the UHPC matrix, approximately 482% and 36% greater than those of the straight and hooked fibers, respectively. 2) The multiple fiber specimens provided smaller bond strengths than the single fiber specimen regardless of the fiber type due to the different slip capacities of each fiber. The decrease rate of bond strength by the multiple fibers was similar for all the fiber types (i.e., straight, hooked, and twisted), approximately 30%. 3) The bundled fiber specimen exhibited the poorest pullout resistance in terms of bond strength and energy absorption capacity for all the 221
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Fig. 13. Comparative matrix spalling failures at the exit: (a) fiber distance corresponding to volume fraction of 1%, (b) fiber distance corresponding to volume fraction of 7%.
fiber types. The deterioration rate of the pullout resistance by fiber bundling was the most significant for the straight fibers as compared to the hooked and twisted fibers. 4) The average bond strengths of the hooked and twisted steel fibers were improved by decreasing the fiber distance up to 1.0 mm, corresponding to the fiber volume fraction of 7% based on an assumption of perfect fiber distribution. 5) For the case of straight steel fibers, minor matrix damage was observed, and it did not interfere with the pullout performance of the adjacent fibers. However, severe matrix damage was formed in the matrix by splitting cracks for the hooked and twisted steel fibers, and they were overlapped to create a larger spalling area as the fiber distances decreased.
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Acknowledgements
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This work was supported by the National Research Foundation of Korea grant funded by the Korea government (MSIT) (No. 2017R1C1B2007589).
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Effects of fiber shape and distance on the pullout behavior of steel fibers embedded in ultra-high-performance concrete
T
Jae-Jin Kim, Doo-Yeol Yoo∗ Department of Architectural Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul, 04763, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultra-high-performance concrete Steel fibers Pullout resistance Fiber distance Fiber bundling effect
This study examines the implications of the fiber type and the distance between fibers on the pullout behavior of steel fibers embedded in ultra-high-performance concrete (UHPC). For this, three different types of steel fibers, i.e., straight, hooked, and twisted, and four different distances between fibers, corresponding to fiber volume fractions of 1%, 2%, and 7% and a fiber bundle, were considered. To evaluate the effect of the distance between fibers, four individual fibers were included in a single dog-bone specimen, and a single fiber specimen was also fabricated and tested as a control specimen. Test results indicate that the twisted steel fiber exhibited the greatest pullout resistance, followed by the hooked and straight steel fibers. Approximately 30% lower bond strengths were obtained for the specimens with multiple fibers as compared to those with a single fiber, regardless of the fiber type and distance between fibers. The average bond strengths of the hooked and twisted steel fibers were improved by decreasing the distance between fibers up to 1 mm, corresponding to a volume fraction of 7%, while the bundled fiber specimens provided the poorest pullout resistance in terms of bond strength and energy absorption capacity for all types of fibers. The reduction rate of pullout resistance was the most significant for the straight fiber, relative to the hooked and twisted fibers. Minor matrix damage was obtained for the straight fiber specimen, and its pullout performance was not influenced by the surrounding fibers. In contrast, severe matrix damage was observed for the hooked and twisted fibers, and they were overlapped, causing a larger spalling area with a closer fiber distance.
1. Introduction Ultra-high-performance concrete (UHPC) is a special type of concrete having very high strength, and it is composed of fine particles with diameters of less than 0.5 mm without coarse aggregate. This material has been developed due to the demand for high-rise buildings, long-span bridges, and specially designed buildings with thin cross sections [1]. Research on reactive powder concrete was started as a type of UHPC in the mid-1990s, and several other studies [2–4] have focused on practical applications of UHPC for buildings and civil infrastructures. It is generally known that UHPC has high ductility and durability [5,6], but concrete of high-strength such as UHPC characterized by a compressive strength of more than 150 MPa [7,8] has extremely brittle properties than ordinary concrete due to an abrupt dissipation of large energies stored when it is failed [9]. In order to overcome the high brittleness of UHPC, a high volume of discontinuous steel fibers is incorporated to reinforce it and to enhance its energy absorption capacity [10–12]. In this way, UHPC containing fibers is called ultra-high-
∗
performance fiber-reinforced concrete (UHPFRC). The addition of fibers to UHPC improves the resistance to crack formation and propagation in the cement matrix, enhancing the post-cracking tensile performance in terms of strength and ductility [13,14]. Multiple discontinuous steel fibers serve as bridges in cracked sections of concrete to improve the stress transfer, contributing to the enhancement of post-cracking tensile behavior [15,16]. The stress transfer at a cracked section is affected by the bond properties of the fiber and matrix, and the failure behavior of the matrix varies depending on the stress distribution transferred from the fibers [17–19]. If the bond is weak, the fiber is pulled out from the matrix, whereas if the bond is too strong, the fiber will be fractured before complete pullout. Therefore, bond properties between the fiber and cementitious matrix are some of the important parameters for the tensile behavior of UHPFRC [20–22]. The bond properties of fibers are divided into physicochemical and mechanical bond properties. The physicochemical bond is influenced by adhesion and friction at the interface, while the mechanical bond has an anchor effect at the fiber end or along the fiber, as well as adhesion and friction [23–25]. In order to understand the tensile behaviors of
Corresponding author. E-mail address: [email protected] (D.-Y. Yoo).
https://doi.org/10.1016/j.cemconcomp.2019.05.006 Received 14 March 2018; Received in revised form 30 April 2019; Accepted 7 May 2019 Available online 09 May 2019 0958-9465/ © 2019 Elsevier Ltd. All rights reserved.
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study investigating the effect of adjacent fibers according to distance between the fibers and fiber geometry on the pullout behaviors of various steel fibers embedded in UHPC matrix, and this is the world first research. Exploring the effects of the distance between fibers and fiber type on the pullout behavior will help create a better understanding of the post-cracking tensile behaviors of UHPFRC with various steel fibers. Accordingly, this study first investigates the implications of the distance between fibers on the pullout behaviors of straight, hooked, and twisted steel fibers embedded in UHPC. The detailed objectives are as follows: 1) to compare the bond properties of a single fiber and multiple fibers, 2) to examine the group effect of bundled steel fibers, and 3) to evaluate the effect of the distance between fibers, corresponding to fiber volume fractions of 1%, 2%, and 7%, on the bond properties of multiple fibers in UHPC.
UHPFRC, numerous studies [19,24–30] have been performed on the bond properties from a fiber pullout test, and several major factors of the bond properties, i.e., fiber geometry, orientation, embedded length, surface, and matrix strength, were examined [31]. Using pre-deformed fiber reinforcement has attracted attention from many researchers because deformed steel fibers exhibit much higher bond strength than that of straight steel fibers. Almost all of the fibers used for reinforcement are mechanically deformed, among which hooked-end steel fibers are the most used fiber type [32]. Abdallah et al. [24] have examined the effects of fiber geometry, embedded length, and water-to-binder (W/B) ratio on the pullout behaviors of hooked-end steel fibers embedded in UHPC. They [24] reported several useful findings: (1) a decrease of the W/B ratio from 0.2 to 0.11 significantly enhanced the bond strength, (2) the pullout behavior of 5D hooked fibers was not influenced by the embedment length, and (3) the 5D hooked fiber was more effective at improving the pullout work and strength than the ordinary 3D and 4D hooked fibers. Wille and Naaman [25] evaluated the pullout performance of smooth and deformed (i.e., hooked and twisted) steel fibers in UHPC and denoted that using the hooked and twisted steel fibers provided four to five times greater equivalent bond strength than that of smooth steel fibers. They [25,26] also reported that by increasing the particle packing of the surrounding matrix, the bond characteristics of steel fibers can be significantly enhanced based on the optimization of UHPC mixtures. Stengel [27] proved that the bond strength of steel fibers in UHPC can be improved by modifying the fiber surface with fiber surface roughening based on abrasion or using sandpaper. Wu et al. [30] also reported that the inclusion of suitable nano calcium carbonate (CaCO3) is able to significantly improve the interfacial bond strength between the straight steel fiber and UHPC matrix up to the critical amount of 3.2%. The bond strength and pullout energy of straight fiber in UHPC improved as much as 45% and 200%, respectively, due to the nanoCaCO3 added. Lee et al. [19] have performed a number of pullout tests to examine the effect of the inclined angle of smooth steel fibers in UHPC and reported that the greatest bond strengths are obtained when the fibers are inclined as much as 30° and 45°. In contrast, the slip capacity, which indicates the slip corresponding to the peak strength, continuously increased with increasing the inclination angle. Tai and El-Tawil [28] evaluated the implications of fiber type, such as smooth, hooked, and twisted fibers, inclined angle, and loading rate, ranging from 0.018 mm/s (quasi-static) to 1800 mm/s (impact), on the pullout behaviors of steel fibers in UHPC. Based on their test results, the pullout resistance and energy dissipation capacity of smooth steel fibers in UHPC were enhanced by increasing the loading rate and inclination angle up to 45°, consistent with the findings of Lee et al. [19]. Additionally, the smooth steel fiber provided the highest sensitivity to the loading rate, causing a dynamic increase factor that was as high as 2.32, compared to the twisted and hooked steel fibers. In addition, the pullout performance of single steel fibers embedded in UHPC have been investigated by several researchers [14,24–28,31], and research on the effects of the steel fiber volume fraction on the compressive and tensile behaviors of UHPFRC also has been published [33]. However, in previous studies [34], the pullout behavior of singly embedded steel fibers in UHPC did not exhibit similar behavior with those in the UHPFRC composites. Since the correlation between the pullout behavior of singly aligned deformed steel fibers in UHPC and the flexural behavior of UHPFRC is fairly low due to several influential parameters, it is difficult to predict the composite's behavior through only the single fiber pullout test results. There are more important variables that affect fiber pullout behavior, and the effect of variables needs to be confirmed. Therefore, in this study, we conducted numbers of experimental tests with embedding multiple steel fibers in UHPC matrix considering the fiber spacing according to certain fiber volume fractions ranging from 1% to 7%. Actually, only few studies [19] have conducted pullout tests considering the fiber group effect using multiple fibers. To the best of the authors' knowledge, there is no published
2. Experimental program 2.1. Mixture proportion of UHPC The mixture proportion and the average strength of the ultra-highperformance concrete (UHPC) used in this experiment is given in Table 1. The cementitious materials consisted of Type I Portland cement (cement strength class of 42.5 MPa) and silica fume (SF), and their chemical compositions and physical properties are summarized in the Table 2. Silica sand with a diameter of 0.2–0.3 mm was used as fine aggregate, and it has a density of 2.62 g/cm3. Silica powder with an average grain size of 4.2 μm, containing approximately 98% SiO2, was used as filler. Since coarse aggregate has not been generally used for making UHPC [1,32], it was also excluded in this study. Due to a low water-to-binder (W/B) ratio and high amount of fine powder for UHPC, its fluidity was significantly reduced. Thus, to increase the fluidity, polycarboxylic acid-based superplasticizer (SP) was added. 2.2. Fabrication of the test specimen To investigate the effects of the geometry and volume fraction of steel fibers on their pullout behavior in UHPC, three different types of steel fibers, i.e., straight, hooked, and twisted steel fibers, and four different distances were considered. For the straight and twisted steel fibers, the distances were 2.7 mm, 1.9 mm, 1.0 mm, and 0 mm, while for the hooked steel fiber, the distances were 3.3 mm, 2.3 mm, 1.2 mm, and 0 mm, respectively. The fiber distance was calculated based on the assumption of a perfectly homogeneous distribution of fibers, and the corresponding volume fractions. For instance, the fiber distances of 2.7 mm, 1.9 mm, and 1.0 mm indicate the fiber volume fractions of 1%, 2%, and 7%, respectively. Sizes used for the straight, hooked-end, and twisted fibers are as follows: S30 with an aspect ratio (lf / df ) of 30/ 0.3 mm/mm = 100, H30 with lf / df of 30/0.375 mm/mm = 80, and T30 with lf / df of 30/0.3 mm/mm = 100. Herein, lf is the fiber length and df is the fiber diameter. The twisted steel fiber has a triangular shape, and thus, the normalized diameter of 0.3 mm was calculated by Table 1 Mixture proportion and compressive strength of UHPC. W/Bb
0.2
Unit weight [kg/m3] Water
Cement
Silica fume
Silica sand
Silica powder
SPa
Compressive strength, fc’ [MPa]
160.3
788.5
197.1
867.4
236.6
52.6
190.2
[Note] W/B = water-to-binder ratio, and SP = superplasticizer. a Superplasticizer includes 30% solid (= 15.8 kg/m3) and 70% water (= 36.8 kg/m3). b W/B is calculated by dividing total water content (160.3 kg/m3 + 36.8 kg/ m3) by total amount of binder (788.5 kg/m3 + 197.1 kg/m3). 214
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fraction of the fibers is directly related to the distance between fibers, the distance between fibers is calculated based on the volume fraction. Additionally, for comparative analysis of the group effect of the fibers, a bundle type with attached fibers (zero distance between fibers) and single fiber specimens were also fabricated. The detailed information regarding fiber distance and corresponding volume fraction is summarized in Table 4. The specimens are dog-bone shaped, and their height, width, thickness, and cross-sectional area were 75 mm, 40 mm, 25 mm, and 25 × 25 mm2, respectively. The detailed geometry of the dog-bone samples is shown in Fig. 1. To fabricate the UHPC mortar, a special mixing process was applied, as follows. First, cementitious materials, such as cement and SF, silica sand, and silica powder were premixed for 10 min using a Hobart-type mixer. Water premixed with SP was added to the dry ingredients and mixed for an additional 10 min to achieve a flowable mixture. Two separate pouring procedures were used to fix the fibers to the dog-bone-shaped mold. The fibers were first fixed at the center of the mold. Then, UHPC mortar was poured into only one side of the mold and cured at room temperature for 48 h. After the fiber fixture was removed, the mortar was poured into the remaining mold and cured for 48 h at identical room temperature conditions. The specimens were then removed from the mold and steam cured, heating at 90 °C ± 2 °C for 3 days to achieve full strength development. Specimens were removed from the steam curing machine and stored in the laboratory at room temperature until the pullout test. A schematic description on the manufacturing process specimen for the pullout test is shown in Fig. 2.
Table 2 Chemical compositions and physical properties of cement and silica fume. Composition [%] (mass)
Type I Portland cement
Silica fume
CaO Al2O3 SiO2 Fe2O3 MgO SO3 Specific surface area [cm2/g] Density [g/cm3]
61.33 6.40 21.01 3.12 3.02 2.30 3,413 3.15
0.38 0.25 96.00 0.12 0.10 200,000 2.10
Table 3 Geometrical and physical properties of steel fibers.
S30 H30 T30
df [mm]
lf [mm]
Aspect ratio (lf/ df)
Density [g/ cm3]
fft [MPa]
Ef [GPa]
0.300 0.375 0.300
30.0 30.0 30.0
100.0 80.0 100.0
7.9 7.9 7.9
2580 2900 2428
200 200 200
[Note] S30 = straight steel fiber with a length of 30 mm, H30 = hooked steel fiber with a length of 30 mm, T30 = twisted steel fiber with a length of 30 mm, df = fiber diameter, lf = fiber length, fft = tensile strength of fiber, and Ef = elastic modulus of fiber. Table 4 Test variables. Specimens
Fiber type
Volume fraction [%]
Distance between fibersa [mm]
S30 (1.0%) S30 (2.0%) S30 (7.0%) S30 (bundle) S30 (single) H30 (1.0%) H30 (2.0%) H30 (7.0%) H30 (bundle) H30 (single) T30 (1.0%) T30 (2.0%) T30 (7.0%) T30 (bundle) T30 (single)
Straight
1.0 2.0 7.0 1.0 2.0 7.0 1.0 2.0 7.0 -
2.7 1.9 1.0 0.0 3.3 2.3 1.2 0.0 2.7 1.9 1.0 0.0 -
a
Hooked
Twisted
2.3. Test setup and procedure Dog-bone-shaped pullout specimens were fabricated and tested to investigate the pullout behavior of three types of steel fibers (i.e., straight, hooked, and twisted fibers). For obtaining reliable test data, five samples were tested to obtain an average value. The length of all steel fibers was 30 mm, and an embedment length of 10 mm was applied at one side to induce fiber pullout. Fracture is more likely to occur in the case of twisted fibers because they have a higher bond strength but slightly lower tensile strength than the other fibers. Therefore, exceptionally, additional specimens were fabricated to have a 5-mm embedment length for the case of the twisted fibers. A uniaxial tensile
Distance between fibers is corresponding to the fiber volume fraction.
Fig. 1. Schematic description of dog-bone specimens for fiber pullout tests: (a) dimensions of specimen, (b) details of fiber location.
changing the triangle to a circle with an identical cross sectional area. The detailed geometric and mechanical properties of the fibers used are shown in Table 3. In order to examine the effect of fiber distance on the pullout behavior, four fibers were embedded in a 2 × 2 array in the center of the cross-sectional area of UHPC mortar. Since the volume
Fig. 2. Schematic description of manufacturing process for pullout tests. 215
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τav =
Pmax πdf LE
(2)
where τav is the average bond strength and LE is the initial embedded length of the fiber. The fiber pullout energy (Wp) is equal to the area under the pullout load versus slip curves up to the point where the pullout load becomes zero. Therefore, it can be evaluated based on integral calculus, as follows: s = LE
WP =
∫
P (s ) ds
s=0
where WP is the fiber pullout work and represents the total energy until the fiber is completely pulled out (or failed) and P(s) is the pullout load at a certain slip, s. The fiber pullout energy at the unit embedded surface area is also calculated by dividing Eq. (3) by the embedded area and is given as Wpb. Assuming that the shear stress is evenly distributed over the length of the embedded fiber, the equivalent bond strength (τeq) can also be calculated based on the fiber pullout work (WP) [35], as follows:
Fig. 3. Test setup for fiber pullout tests.
load was applied through a universal testing machine (UTM) with a maximum load capacity of 3 kN. In order to apply a quasi-static loading rate and obtain sufficient data points up to the peak strength, a loading rate of 0.018 mm/s was adopted, similar to a previous study [28]. After placing a full dog-bone specimen in the grip jig, the upper grip was raised at a quasi-static loading rate, and the pullout load and slip begin to be measured at the moment when the load was actually created. Based on an assumption that elastic deformations of both the fibers, matrix, and steel grip jig are sufficiently small, the stroke recorded was used as the fiber slip as usual. However, since multiple, four, steel fibers were embedded in UHPC, the measured slip capacities were slightly higher than those in literature using single steel fiber. Up to the maximum pullout load, the straight steel fiber is not pulled out but rather partially debonded, while the deformed steel fibers are only slightly pulled out. Thus, the slip capacity is related to both the fiber slip and elastic deformations of fiber and matrix. Since multiple steel fibers were used in this study, the maximum pullout load was greatly higher than that of single fiber case, leading to the higher elastic deformations reflected in the slip capacity. The detailed test setup is shown in Fig. 3.
τeq =
(4)
3.2. Effects of fiber geometry on bond stress versus normalized end-slip (τ-s/ LE) behaviors To investigate the effect of the steel fiber type on the average bond stress versus normalized slip (τ-s/LE) behaviors, single fiber specimens were fabricated and tested. In the case of twisted steel fiber specimens, the fiber was embedded only at 5 mm in order to prevent breakage before complete pullout, while the straight and hooked steel fibers were embedded at 10 mm. Thus, in order to reasonably compare their pullout behavior, the average bond stress and normalized slip parameters were used by assuming that they were not influenced by the embedment length. As shown in Fig. 4, the τ-s/LE curves showed noticeable differences in the bond stress in accordance with the fiber type: the twisted and hooked fibers exhibited much higher bond strengths than the straight fiber due to their additional mechanical anchorage effects. This is consistent with the findings from Yoo et al. [34] and Wille and Naaman [25]. The straight steel fiber is initially chemically adhered with the adjacent cement matrix up to the critical pullout load, and once the maximum bond stress at the interface exceeds the bond strength, a partial debonding process is activated up to the peak pullout load along with the generation of a frictional bond [36]. However, for the deformed steel fibers, such as the twisted and hooked-end fibers,
3.1. Evaluation parameters To evaluate the pullout behavior, the calculation formulae for strength and energy absorption capacity are given as follows. For reliable test results, five dog-bone-shaped fiber pullout test specimens were prepared for each variable and average values were used. Specimens damaged during the fabrication due to their insufficient matrix strength developed before steam curing were not included in the calculations. First, the maximum fiber stress (σf,max) is a very important factor for judging the suitability of fibers in concrete, because if the maximum fiber stress exceeds its tensile strength, the fiber will be fractured before complete pullout. The maximum fiber stress can be calculated based on the maximum pullout load and the cross-sectional area of the fiber, as follows:
Pmax Af
2Wp πdf LE2
where τeq is the equivalent bond strength.
3. Experimental results and discussion
σf ,max =
(3)
(1)
where σf,max is the maximum tensile stress of the fiber, Pmax is the maximum pullout load, Af is the fiber cross-sectional area (=πdf2/4), and df is the fiber diameter. The average bond strength (τav) between the fiber and the matrix is calculated using the maximum pullout load, diameter of the fiber, and initial embedment length, as follows:
Fig. 4. Average bond stress versus normalized slip responses of single steel fibers. 216
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calculated based on the peak pullout load, so that even with fibers of different shapes, the bond strength can be easily evaluated. The shape of the straight and twisted fibers is comparatively uniform throughout the entire embedment length, so their equivalent bond strength can be compared. The hooked fiber has a special end-hook, so the bond stress is not constant along the embedded fiber length. However, in order to compare the equivalent bond strength according to the fiber type, the parameter τeq was intentionally calculated for the case of the hooked fiber. The maximum fiber tensile stress is an important parameter for determining the failure mode (pullout or fracture) of the fibers [37]. In Fig. 5c, the 10-mm embedded twisted fibers were all fractured before complete pullout, since the maximum fiber tensile stresses generated by the external pullout force exceeded their tensile strength. In contrast, when we reduced the embedment length of the twisted steel fiber from 10 mm to 5 mm (Fig. 5d), they were pulled out without fracture, and the pullout energy could be calculated.
mechanical anchorage at the end hook for the hooked fiber and throughout the entire embedment length by twisting for the twisted fiber is additionally generated, causing the higher bond strength. As a result, the highest bond strength was obtained for the twisted fiber, followed by the hooked fiber and the straight fiber. The post-peak bond stresses of the twisted and straight fibers decreased roughly and uniformly in proportion to the amount of slip, while that of the hooked fiber was not constant. The bond stress reduction rate was larger for the case of the hooked steel fiber after reaching the peak point, because its hooked shape greatly enhanced the bond strength by mechanical anchorage. However, since the end-hook is stretched during the pullout process, the mechanical anchorage effect by the hook greatly decreased after stretching. The much steeper decrease of the post-peak bond stress of the hooked fiber than those of the twisted and straight fibers was also reported by previous studies [25,34]. The highest maximum bond strength of 34.0 MPa was found for the twisted fiber, approximately 482% and 36% higher than those of the straight and hooked fibers, respectively.
3.3.1. Straight steel fibers To investigate the effect of the interference of adjacent fibers on the pullout behavior, the dog-bone specimens were fabricated by including four fibers in each specimen. Therefore, it is assumed that the pullout force acts on each fiber equally in order to compare them easily with specimens including only a single fiber. As shown in Fig. 6a, the maximum fiber tensile stress of a single straight fiber was found to be 1159.8 MPa, leading to a fiber strength ratio of 45%. Identical average and equivalent bond strengths of 8.8 MPa were obtained. The specimens with four fibers with distances equal to 1%, 2%, and 7% (by volume) steel fibers showed similar values for the maximum fiber tensile strength, with decreases of approximately 15% compared to the single fiber specimen, while the bundled fiber specimen provided a maximum fiber tensile stress of only 52% of the single fiber specimen. The average bond strengths of the multiple fiber specimens decreased by 22%–30% compared to the single fiber specimen, and the bundled fiber specimens decreased by 52%, which is much higher than the
3.3. Effects of fiber geometry and distance on the pullout load versus slip behaviors The average pullout load versus slip curves are shown in Fig. 5. We adopted two different embedment lengths of 10 mm and 20 mm to induce the four embedded fibers to be pulled out from one side of matrix. The several pullout parameters, i.e., pullout work (Wp), maximum fiber pullout load (Pmax), maximum fiber tensile stress (σf,max), average bond strength (τav), and equivalent bond strength (τeq), are also summarized in Table 5. As mentioned earlier, the pullout work is the energy required until the fiber is completely pulled out. Assuming that the bond stress is constant throughout the embedment length, the pullout work can be used to calculate the equivalent bond strength, which is related with the cracking behavior of high-performance fiber-reinforced cement composite (HPFRCC) [35]. The average bond strength can be
Fig. 5. Average pullout load versus slip curves: (a) straight fiber, (b) hooked fiber, (c) twisted fiber, and (d) twisted fiber with a short embedment length of 5 mm. 217
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Table 5 Summary of pullout test results of different fiber types and distances. Fiber type S30
H30
T30a
Parameters −3
Wp [ × 10 J] Wpb [ × 10−3 J/mm2] Pmax [N] σf,max [MPa] τav [MPa] τeq [MPa] Wp [ × 10−3 J] Wpb [ × 10−3 J/mm2] Pmax [N] σf,max [MPa] τav [MPa] τeq [MPa] Wp [ × 10−3 J] Wpb [ × 10−3 J/mm2] Pmax [N] σf,max [MPa] τav [MPa] τeq [MPa]
1%
2%
7%
Bundle
Single
1935.3 (3.98) 53.4 (11.42) 277.1 (3.98) 979.90 (137.88) 6.19 (0.53) 8.99 (1.55) 4640.3 (1212.71) 98.7 (25.73) 937.9 (128.73) 2098.4 (291.39) 19.67 (2.73) 19.69 (5.15) 1259.5 (94.95) 88.6 (9.08) 494.4 (21.00) 1748.7 (74.28) 26.23 (2.59) 26.73 (4.80)
1693.2 (5.10) 45.0 (7.82) 279.4 (5.10) 988.03 (18.03) 6.91 (0.78) 8.35 (1.46) 4752.2 (471.53) 100.8 (9.62) 956.6 (150.54) 2165.3 (86.68) 20.30 (0.81) 20.17 (1.92) 1271.1 (107.58) 76.4 (6.47) 494.0 (44.24) 1747.2 (99.41) 26.21 (3.82) 26.97 (2.73)
1886.1 (64.41) 47.7 (9.77) 280.0 (64.41) 990.38 (227.82) 6.81 (2.40) 8.81 (3.24) 3940.5 (921.45) 83.6 (19.55) c 1081.3 (123.31) 2447.5 (279.12) 22.95 (2.62) c 16.72 (3.91) c 1039.1 (185.94) 67.4 (6.17) 476.1 (32.67) 1683.9 (115.54) 28.08 (0.28) 27.09 (0.61)
1300.3 (17.34) 35.3 (7.39) 170.5 (17.34) 603.05 (61.33) 4.26 (0.91) 6.70 (2.08) 3974.1 (733.01) 84.3 (15.56) 884.1 (188.49) 2001.3 (426.66) 18.76 (4.00) 16.87 (3.11) 1103.1 (62.92) 61.0 (11.23) 454.6 (86.20) 1607.7 (15.73) 24.43 (3.16) 24.65 (5.24)
400.9 (13.42) 43.3 (12.65) 82.0 (13.42) 1159.8 (189.92) 8.82 (1.65) 8.82 (2.91) 1480.5 (270.42) 125.7 (22.95) 320.3 (53.29) 2899.7 (482.46) 27.18 (4.52) 25.13 (4.59) 386.6 (58.59) 59.9 (0.03) 165.8 (7.78) 2345.6 (110.04) 38.32 (4.04) 38.75 (5.90)
c
c c
[Note] Wp = pullout work, Pmax = maximum pullout load, σf,max = maximum fiber tensile stress, τav = average bond strength, τeq = equivalent bond strength, and ( ) = standard deviation. a T30 specimens with an embedment length of 5 mm. b Pullout energy per unit embedded surface area. c Specimen with fiber fracture.
multiple fiber cases. It was expected that the straight fiber may have a similar average bond strength (τav) regardless of the number of fibers. This is because it behaves individually without affecting the surrounding area during the pullout process, caused by the fact that the straight fiber resists the pullout force only through the frictional bond. There were different slip capacities, which is the slip at the point of the maximum pullout load, even though identical straight steel fibers were embedded, as shown in Fig. 7. This is due to heterogeneity of matrix which is one of the characteristics of cementitious materials, and the eccentric effect during pullout of the fibers. Fig. 7 shows the fiber comparative pullout load versus slip curves of four dog-bone specimens with a single fiber. The summation of all load-slip curves is also given in Fig. 7, denoted as S30 (sum). The summation of the maximum pullout load of the four specimens with a single fiber was found to be 350.7 N, which was approximately 7.4% greater than that of the specimen S30 (sum). This is caused by the fact that, since the slip capacities (at the peak point) of the four specimens were different from each other, the maximum pullout load of the specimen S30 (sum) became smaller than the sum of the maximum values of the respective fiber specimens. This phenomenon is equally applicable to the hooked and twisted fiber specimens. As shown in Fig. 6, the equivalent bond strength (τeq), calculated based on the pullout work (Wp), of multiple fiber specimens was
Fig. 7. Pullout load versus slip curves of straight fibers embedded in UHPC matrix.
similar to that of a single fiber specimen with a difference of only up to 5%. In contrast, the difference became quite severe for the bundled fiber specimen: an approximately 32% higher value of τeq was obtained for the single fiber specimen relative to the bundled fiber specimen.
Fig. 6. Summary of bond parameters of straight steel fibers in UHPC: (a) maximum fiber stress, (b) average bond strength, (c) equivalent bond strength. 218
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Fig. 8. SEM images on the surface of pulled out straight steel fibers from UHPC matrix: (a) fiber distance of 2.7 mm, (b) fiber distance of 0 mm (bundled fiber).
fibers were approximately 2.9–3.1 times higher than those of the straight fibers. As was noted previously, due to the different slip capacities of each fiber in multiple fiber specimen, the maximum fiber stresses in the multiple fiber specimens decreased by approximately 16%–28%, compared to that of the single fiber specimen. The bundled fiber specimen provided a greater decrease of 31%. Given the fiber bundling effect, the straight steel fiber exhibited a greater decrease of the bond strength (52%) than the hooked steel fiber (31%). For the case of straight fibers, since the pullout load resistance is generated mostly by the friction shear stress, which is closely related to the bonding area, the bundled specimen showed a lower pullout resistance due to the decreased total fiber surface area. However, for the case of hooked fibers, the resistance to the pullout force is activated by both a frictional bond and mechanical anchorage at the end hook, and the effect of the total fiber surface area reduction on the pullout resistance was offset. Therefore, the reduction rate of the bond strength by fiber bundling was relatively minor for the hooked fiber specimen compared to the straight fiber specimen. It is interesting to note that the average bond strength increased by decreasing the distance between fibers up to 1.2 mm, which is equal to 7% fibers (by volume) by assuming that all fibers are perfectively distributed. For example, when we decreased the distance between fibers from 3.3 mm to 1.2 mm (1 vol% to 7 vol%), the average bond strength increased as much as 17%, as shown in Fig. 9b. This is because the expansive pressure generated by the end-hook under the pullout load pressed the adjacent fibers, causing a higher resistance to the pullout force. In other words, expansive pressure is activated when we pull out the hooked steel fiber from the UHPC matrix, which sometimes causes splitting cracks in the matrix [39]. When the fibers are closer, the fiber pullout resistance can be enhanced due to the expansive pressure additionally generated by the surrounding fibers; therefore, a higher pullout resistance for the hooked steel fibers was obtained with a
Since the straight steel fiber resists the pullout force mainly by the frictional shear stress at the interface, the bonding area between the fiber and the matrix is the most important factor. Fig. 8 shows scanning electron microscopy (SEM) images for the fiber surface after complete pullout for the specimens with bundled and multiple fibers. As shown in Fig. 8a, scratches at the surface of the fiber were uniformly formed along the fiber length for the multiple fiber specimens having separated fibers, while the degree of surface scratches for the bundled fiber specimen was different according to the location, as shown in Fig. 8b. A portion of the fiber that is in contact with a surrounding fiber has much fewer surface scratches than other portions that are in contact with the cement matrix. This observation could verify a fiber bundling effect [38], indicating that the bond strength of bundled fibers is reduced as compared with that of a single fiber or separated fibers due to the reduced total fiber surface area bonding to the matrix. Therefore, this decrease in the surface contact area between the fiber and matrix is considered to be the main cause of the deteriorated pullout resistance of the bundled fibers. 3.3.2. Hooked steel fibers As summarized in Fig. 9, the single hooked fiber exhibited the maximum fiber tensile stress of 2899.7 MPa and the fiber strength ratio of 99.9% in the UHPC matrix, and hooked fiber showed approximately 2.5 times higher maximum fiber stress than that of the straight fiber in single fiber pullout test. However, the fibers in the UHPFRC composites are randomly oriented and the ones in the inclined state produce greater maximum pullout loads and stresses during pullout in general, which can cause fiber fracture with high probability. It is therefore difficult to be interpreted as the hooked fiber is more effective than the straight fiber. The average and equivalent bond strengths were 27.2 MPa and 25.1 MPa, respectively, which were also much higher than those of the straight steel fibers. The bond strengths of the hooked
Fig. 9. Summary of bond parameters of hooked steel fibers in UHPC: (a) maximum fiber stress, (b) average bond strength, (c) equivalent bond strength. 219
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the former test configuration have ruptured. As shown in Fig. 7, since the four fibers embedded did not reach the maximum load at the same value of slip, the measured maximum pullout load applied for fibers was reduced in the multiple fiber specimens than the counterpart. Due to the inhomogeneity and eccentricity of the specimen, some of the fibers in the multiple fiber specimen are already fractured with additional pressure by adjacent fibers before the other fibers reach their maximum pullout load. Thus, the actual pullout stress was localized at a portion of multiple fibers, leading to the smaller maximum fiber stress at the point of fiber rupture since the maximum pullout load was divided into four fibers based on an assumption that the pullout stress was evenly distributed to all of the fibers. These are the reasons why the lower maximum fiber stress was obtained in the series H30 (7%) than the series H30 (single). The equivalent bond strength also decreased as much as 20%–22% by embedding multiple fibers for the cases of 1% and 2% fibers, compared to that of a single fiber specimen, while it significantly decreased for the bundled fiber specimen by as much as 31%. Since the specimen with the closest distance between fibers of 1.2 mm (7%) exhibited fiber fracture before the completion of pullout, a much smaller energy absorption capacity (pullout work) was obtained, as summarized in Table 5, causing the much smaller equivalent bond strength. It can be concluded that using the hooked steel fiber needs to be carefully at such a high fiber volume fraction to prevent fiber fracture in UHPC, which leads to a sufficient decrease of the energy absorption capacity.
Fig. 10. Schematic descriptions on (a) dimension of dog-bone specimen and (b) details of fiber location for strain mapping.
smaller fiber distance (or higher fiber volume fraction). To verify this explanation, a strain map of the dog-bone specimen with multiple hooked steel fibers was recorded based on digital image correlation (DIC) images. In order to capture strain variations at the surface of a dog-bone specimen by pulling out a hooked fiber, additional specimens were fabricated. Four hooked fibers were embedded and placed close to the surface of the specimen to easily observe strain changes on the surrounding matrix. The schematic descriptions of the tested specimens are shown in Fig. 10and the strain distributions on the specimen surface are shown in Fig. 11 according to the applied pullout load. The stress was generated in the surrounding matrix locally near the end-hook with an increase of the pullout load. The size of the stress area and its magnitude were larger with increasing the pullout load up to the peak point (B), while these values were reduced by decreasing the pullout load in the descending zone (point (C)). This indicates that in the ascending branch, the confined pressure, additionally applied to the fiber at the end-hook by adjacent fibers, increased with increasing the pullout load, and it is maximized at the peak load. It was also obvious that, due to the expansive pressure of the hooked steel fiber, stresses are localized near the end-hook, and the local stresses additionally press the adjacent hooked fiber to resist it being pulled out from the matrix, causing the higher pullout resistance and bond strength. Although the maximum fiber stress of the 7 vol% specimen was smaller than that of a single fiber specimen, the hooked steel fibers in
3.3.3. Twisted steel fibers The data of the 5-mm embedded specimens was used to examine the pullout behavior of the twisted steel fibers, since, when the twisted fibers were embedded as much as 10 mm, similar to other specimens, they were all fractured (Fig. 5c). As shown in Fig. 12, the single fiber specimen with an embedment length of 5 mm showed the maximum fiber tensile stress of 2345.6 MPa and the fiber strength ratio of 96.6%. The average and equivalent bond strengths were also found to be 38.3 MPa and 38.8 MPa, respectively. The twisted fiber exhibited the highest pullout resistance compared to the straight and hooked fibers, which is consistent with the findings from Wille and Naaman [25]. For instance, the average bond strength of the twisted fiber was approximately 3.4 times higher than that of the straight fiber and 1.4 times higher than that of the hooked fiber. Its equivalent bond strength was
Fig. 11. (a) Pullout load versus slip curve of hooked fiber specimen, (b), (c), and (d) strain maps of specimens at the points of (A), (B), and (C). 220
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Fig. 12. Summary of bond parameters of twisted steel fibers in UHPC: (a) maximum fiber stress, (b) average bond strength, (c) equivalent bond strength.
3.3.4. Matrix spalling effect For the hooked and twisted fiber specimens, the equivalent bond strength was rather reduced with decreasing the fiber distance, inconsistent with the test data of the maximum fiber tensile stress and average bond strength. To verify these observations and provide a reasonable explanation, a cross section of the dog-bone specimens after fiber pullout was investigated, as shown in Fig. 13. For the case of straight steel fibers, only minor damage in the matrix was obtained after fiber pullout regardless of the distance between fibers, as shown in Fig. 13. In addition, the matrix damage was not overlapped even for the specimen with the closest distance between fibers of 1.0 mm (7%), meaning that they did not interfere with each other, even at a very close distance. However, the spalling failures of the matrix near the pulled out fiber were obviously observed for the hooked and twisted fiber specimens, as illustrated in Fig. 13. This is because when the twisted and hooked fibers were pulled out, splitting cracks were created in the surrounding matrix due to their mechanical anchorage effect, and when the fiber embedding distance decreased, the spalling overlapped and appeared to create a larger spalling area. Thus, it could be verified that the pullout work and equivalent bond strength of the hooked and twisted fibers embedded in the UHPC matrix were not increased as much as the average bond strength, or were even reduced, due to the overlapped matrix damage formed by splitting cracks.
also approximately 4.4 and 1.5 times higher than those of the straight and hooked fibers, respectively. When we compared the pullout behaviors of the multiple and bundled fiber specimens with the single fiber specimen, the average bond strengths were reduced by as much as 27%–32% for the multiple fiber specimens (1%, 2%, and 7%) and 36% for the bundled fiber specimen. The reduction rate (36%) of the bond strength of the twisted fiber by fiber bundling was smaller than that of the straight fiber (51%), due to the additional mechanical anchorage effect as explained above. The equivalent bond strengths were also decreased by as much as 30%–31% for the multiple fiber specimens (1%, 2%, and 7%) and 36% for the bundled fiber specimen. Such a decrease of average and equivalent bond strengths of the multiple and bundled fiber specimens was caused by the different slip capacities of each fiber included. The multiple fiber specimens with distances between fibers of 2.7 mm and 1.9 mm (1% and 2%, respectively) exhibited the similar average bond strength of approximately 26.2 MPa, whereas the specimen with a distance between fibers of 1.0 mm (7%) provided a noticeably higher bond strength of 28.1 MPa, which is approximately 7% higher. This is consistent with the trend of the hooked fiber test results, showing that the pullout resistance increased with decreasing the distance between fibers due to the additional confined pressure applied, which is generated by the expansive pressure of the adjacent fibers. Due to an untwisting process of quadrangular twisted steel fiber, expansive pressure is applied to the cement matrix, and thus, several splitting cracks are formed in the matrix under the pullout force. This was also observed by a previous study [25]. The increase rate of the equivalent bond strength of the twisted fibers in terms of the decrease in the distance between fibers was smaller than that of the average bond strength. This might be caused by the fact that, since the twisted fiber led to the formation of cracks in the matrix significantly when it is untwisted and stretched under the pullout load, the cracks more greatly affected the post-peak pullout resistance when the fibers were placed closer together. The cracks normally lead to a steeper decrease of the post-peak pullout load, which causes the reduced pullout work and corresponding equivalent bond strength. Due to this, the equivalent bond strength of the twisted steel fiber specimen with fibers placed the closest together (7%) was approximately 4% smaller than the average bond strength, although other specimens, i.e., the single fiber and multiple fibers with distances between fibers of 2.7 and 1.9 mm (1% and 2%, respectively), showed equivalent bond strengths slightly greater than the average bond strength, as shown in Fig. 12. The bundled fiber specimen gave the lowest pullout resistance in terms of the maximum fiber tensile stress and average and equivalent bond strengths due to the decreased total fiber surface area and increased deteriorating effect of matrix cracks on the adjacent fibers.
4. Conclusion This study investigated the effects of the fiber type and distance between fibers on the pullout behaviors of steel fibers embedded in an UHPC matrix. For this, three different types of steel fibers, i.e., straight, hooked, and twisted, and three different distances corresponding to fiber volume fractions of 1%, 2%, and 7% were tested. To examine the fiber bundling effect, bundled fiber specimens were also considered along with a single fiber specimen as a control specimen. SEM images and strain maps were analyzed to reasonably verify and explain the test results. Based on the test results obtained in this study, the following conclusions can be drawn: 1) The highest bond strength of 34.0 MPa was obtained for the twisted fiber in the UHPC matrix, approximately 482% and 36% greater than those of the straight and hooked fibers, respectively. 2) The multiple fiber specimens provided smaller bond strengths than the single fiber specimen regardless of the fiber type due to the different slip capacities of each fiber. The decrease rate of bond strength by the multiple fibers was similar for all the fiber types (i.e., straight, hooked, and twisted), approximately 30%. 3) The bundled fiber specimen exhibited the poorest pullout resistance in terms of bond strength and energy absorption capacity for all the 221
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Fig. 13. Comparative matrix spalling failures at the exit: (a) fiber distance corresponding to volume fraction of 1%, (b) fiber distance corresponding to volume fraction of 7%.
fiber types. The deterioration rate of the pullout resistance by fiber bundling was the most significant for the straight fibers as compared to the hooked and twisted fibers. 4) The average bond strengths of the hooked and twisted steel fibers were improved by decreasing the fiber distance up to 1.0 mm, corresponding to the fiber volume fraction of 7% based on an assumption of perfect fiber distribution. 5) For the case of straight steel fibers, minor matrix damage was observed, and it did not interfere with the pullout performance of the adjacent fibers. However, severe matrix damage was formed in the matrix by splitting cracks for the hooked and twisted steel fibers, and they were overlapped to create a larger spalling area as the fiber distances decreased.
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This work was supported by the National Research Foundation of Korea grant funded by the Korea government (MSIT) (No. 2017R1C1B2007589).
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