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Effects of strain rate on the tensile behavior of cementitious composites made with amorphous metallic fiber
Journal Pre-proof Effects of Strain Rate on the Tensile Behavior of Cementitious Composites made with Amorphous Metallic Fiber Hongseop Kim, Gyuyong Kim, Sangkyu Lee, Gyeongcheol Choe, Jeongsoo Nam, Takafumi Noguchi, Viktor Mechtcherine PII:
S0958-9465(20)30010-X
DOI:
https://doi.org/10.1016/j.cemconcomp.2020.103519
Reference:
CECO 103519
To appear in:
Cement and Concrete Composites
Received Date: 28 March 2019 Revised Date:
26 November 2019
Accepted Date: 8 January 2020
Please cite this article as: H. Kim, G. Kim, S. Lee, G. Choe, J. Nam, T. Noguchi, V. Mechtcherine, Effects of Strain Rate on the Tensile Behavior of Cementitious Composites made with Amorphous Metallic Fiber, Cement and Concrete Composites, https://doi.org/10.1016/j.cemconcomp.2020.103519. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Effects of Strain Rate on the Tensile Behavior of Cementitious Composites made with Amorphous Metallic Fiber
Hongseop Kima, Gyuyong Kimb, *, Sangkyu Leeb, Gyeongcheol Choeb, Jeongsoo Namb, Takafumi Noguchic, Viktor Mechtcherined
a
Building Safety Research Center, Department of Living and Built Environment Research, Korea
Institute of Civil engineering and Building Technology, 283, Goyang-daero, Ilsanseo-gu, Goyang-si, Gyeonggi-do, 10223, Korea b
Department of Architectural Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu,
Daejeon 34134, Korea c
Department of Architecture, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan d
Institute for Construction Materials, TU Dresden, Germany
* Corresponding author.
E-Mail: [email protected] (G. Kim)
Abstract Amorphous metallic fiber has higher tensile strength as well as corrosion and wear resistance than common, crystalline steel fibers. Its utilization as reinforcement improves the crack resistance and flexural and tensile performance of concrete. In the study at hand, the tensile behavior of thin plate amorphous metallic fiber-reinforced cementitious composites (AFRCC) is compared with that of hooked steel fiber-reinforced cementitious composites (HSFRCC) for both quasi-static and dynamic loading regimes. AFRCC exhibites a high stress distribution effect and higher tensile strength, strain capacity, and peak toughness than HSFRCC, but lower tensile toughness and lower dynamic increase factor values for tensile strength, strain capacity, and toughness.
Keywords: fiber reinforcement; amorphous metallic fiber; hooked steel fiber; tensile behavior; strain rate effect; dynamic increase factor
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1. Introduction Concrete materials are widely used in construction because they are economical and have high compressive strength and durability. However, they exhibit brittle fracture characteristics, their flexural and tensile strengths are low, their resistance to cracking, especially subject to impact loading, is weak. In fiber-reinforced cementitious composites (FRCC), short fibers are distributed discontinuously inside the matrix. Crack occurrence and propagation are suppressed through the bonding between fibers and matrix as well as the bridging effect exhibited by fibers; consequently, the flexural and tensile performances are significantly improved. FRCC has been intensively investigated since the 1960s and applied to various structures since the 1970s [1-6]. For FRCC, various kinds of short fibers made of steel, organic polymers, carbon and other materials have been developed and applied. Previous studies showed that steel fiber-reinforced concrete has higher flexural and tensile strengths as well as higher impact resistance and fracture energy in comparison to ordinary concrete, and that such improvement in the tensile performance is significantly affected by the fiber type, fiber dimensions, fiber tensile strength, volume fraction, fibermatrix bond and pull-out behavior [7-13]. Consequently, various types of steel fibers have been developed for reinforcement of concrete, and numerous studies on the enhancement of the flexural and tensile performances of concrete and concrete elements have been conducted. However, steel fibers are not always economical, while the amount of rebound of shotcrete and the weight of associated structures are high, owing to the high density of steel fibers. Moreover, they are corrosive in presence of moisture and chemical agents, which causes problems with respect to the durability of structures in which they are used. Amorphous metallic fiber is fabricated by the melt spinning method, in which liquid metal is 5
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quenched at a rate of 10 -10 ℃/s, as shown in Fig. 1(a). This speedy cooling results in fiber having an amorphous structure and exhibiting higher tensile strength, corrosion resistance, and wear resistance than steel fibers with crystalline structures. In addition, amorphous metallic fiber has a high aspect ratio and a large specific surface area because it is fabricated in a thin plate shape, as shown in Fig. 1(b). Furthermore, it has excellent bonding performance with the matrix because of its rough surface. Finally, owing to its ultra-lightweight characteristic compared to the high density of steel, for the given fiber geometry and dosage by weight, the number of amorphous fibers in mixture is higher than that of common steel fibers. Thus, the use of amorphous metallic fiber in concrete can improve its crack resistance, its flexural and tensile performance and solve the durability deterioration problem caused by the corrosion of common steel fibers. Hameed et al. [14] studied flexural properties of fiber-reinforced concretes made with amorphous metallic fiber, with hooked steel fiber, and with a combination of both types. They stated that amorphous metallic fiber is effective for controlling micro-cracks owing to its high bonding strength with the matrix, while hooked steel fiber is effective for controlling macro-cracks. Moreover, they reported that hybrid reinforcement consisting of amorphous metallic fiber and hooked steel fiber is 2
effective for controlling cracks and improving flexural strength as well as toughness. Won et al. [15, 16] evaluated the bonding properties of amorphous metallic fiber and hooked steel fiber as well as the flexural properties of concrete reinforced with each fiber. They found that amorphous metallic fiber exhibited high bonding load with the matrix because of its rough fiber surface and large specific surface area. The flexural strength of amorphous metallic fiber-reinforced concrete was found to be higher than that of hooked steel fiber-reinforced concrete because amorphous metallic fiber has high bonding strength with the matrix and larger number of fibers for the same portion by weight. However, since the thin plate-shaped fibers have a low shear resistance, they fracture without being pulled out, thereby causing a rapid decrease in stress in the post-peak phase. Choi et al. [17] evaluated the corrosion resistance and shrinkage control performance of amorphous metallic fiber and hooked steel fiber in various aggressive environments. Amorphous metallic fiber was found to exhibit over 96% residual tensile strength even after 90 days of exposure. In contrast, hooked steel fiber exhibited degradation by corrosion on the fiber surface, while its residual tensile strength was 3.1 to 10.3% lower than that of amorphous metallic fiber. Moreover, it was confirmed that amorphous metallic fiber has excellent plastic shrinkage control performance compared to hooked steel fiber as well as PP and PVA fiber. Park et al. [18, 19] examined the crack behavior and tension-stiffening effect of reinforced concrete tension members reinforced with hooked steel fiber or amorphous metallic fiber, and stated that the amorphous metallic fiber-reinforced concrete had better control over the occurrence and progress of splitting cracks than the hooked steel fiber-reinforced concrete, thereby exhibiting more pronounced tension-stiffening effect. Ku et al. [20] confirmed both higher flexural strength amorphous metallic fiber-reinforced concrete in comparison to hooked steel fiber-reinforced concrete and a rapid decrease in residual strength of AMFRC in post-peak phase. This can be traced back to fact that amorphous metallic fiber fails without being pulled out, thus before its high bonding strength is reached, whereas hooked steel fiber has high energy dissipation capacity due to its pronounced pullout behavior. Also Yoon et al. [21] observed that amorphous metallic fiber fails because it is vulnerable to shear force and that hooked steel fiber is pulled out from the matrix. They confirmed that amorphous metallic fiber-reinforced concrete has a higher flexural strength whereas hooked steel fiber-reinforced concrete has higher ductility and deflection capacity after the peak, being also more effective in improving the flexural performance of high-strength concrete. Yoo et al. [22] found that the addition of amorphous metallic fiber improved compressive strength, flexural strength, and ductility of concrete. They also showed that the surface crack occurrence resistance of concrete pavement against repeated vehicle wheel. Moreover, the flexural performance improved with increases in length, volume fraction, and strength of amorphous metallic fiber. Amorphous metallic fiber-reinforced shotcrete has higher flexural strength than hooked steel fiber-reinforced shotcrete but lower residual flexural strength and toughness [23]. Yi et al. [24] evaluated the pull-out properties according to matrix strength and the embedded length of amorphous metallic fiber. They found that amorphous metallic fiber pulled out undamaged when the embedded length was 5 mm or lower or when the matrix compressive strength was 18 MPa or lower, whereas it fractured without being pulled out when the matrix strength was 30 MPa or higher or when the embedded length was 8 or 12 mm. In contrast, 3
hooked steel fiber was pulled out from the matrix, and the pull-out energy increased with matrix strength and the embedded length. These previous studies confirm that amorphous metallic fiber is effective in controlling plastic shrinkage and fine cracks as well as improving the flexural and tensile strengths when used as a reinforcement for concrete, and that it has excellent performance in terms of long-term durability because it is nearly free from corrosion. Previous studies on the dynamic properties of FRCC were predominantly conducted under quasistatic loading conditions. However, in recent years, studies on dynamic applications of FRCC have been conducted to secure the safety performance of structures against extreme loads, such as earthquakes, impacts, and explosions. It was observed that fracture mechanisms, such as the debonding and fiber pull-out, vary depending on the strain rate. Thus, in-depth examination and understanding of the fiber-matrix interface properties and pull-out behavior, tensile strength of FRCC as well as its strain capacity and toughness are required under various strain rate conditions. For this purpose researchers have developed various types of testing equipment capable of implementing high strain rates in various forms, and the dynamic properties of FRCC have been investigated using such equipment [25-30]. Moreover, the effect of the strain rate on the tensile behavior of FRCC, which are reinforced with various fibers, including smooth steel fiber, hooked steel fiber, twisted steel fiber, and organic fiber, has been studied by various researchers [31-33]. However, to the best of our knowledge, no study has been conducted on the effect of strain rate on the tensile behavior of AFRCC. Therefore, a study on the tensile behavior of AFRCC according to strain rate is required. In this investigation, FRCC reinforced with thin plate-shaped amorphous metallic fiber or hooked -6
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steel fiber, and tensile behaviors were evaluated at strain rates of 10 /s (quasi-static) and 10 /s (high strain rate). To examine the effects of fiber-matrix bonding and pull-out according to the fiber type as well as the fracture properties of fiber on the tensile behavior of FRCC, tensile stress-strain curves were derived for both strain rates. In addition, the effects of strain rate, fiber type, and fiber volume fraction on the tensile strength, strain capacity, and toughness of FRCC were analyzed.
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2. Experimental program 2.1 Materials and mixture proportions Table 1 shows the properties of the amorphous metallic fiber and hooked steel fiber used in this study. The amorphous metallic fiber exhibits high bonding performance with the matrix because of its large specific surface area and high friction of its rough surface. As the amorphous metallic fiber is the thin plate type, the number of mixed fibers is approximately 16 times larger than that of hooked steel fiber under the same fiber volume fraction. The steel fiber has hooks at both ends. The hooked steel fiber exhibit mechanical bonding to concrete matrix by the hooked ends. Fig. 2 shows the shape of the hooked steel fiber. Table 2 shows the experimental series used to investigate the tensile behavior of AFRCC and HSFRCC according to the strain rate; the static compressive strengths of the mixtures at 28 days are also provided. The fiber content in FRCC specimens varied between 1.0, 1.5, and 2.0 vol.% both for amorphous metallic fiber and hooked steel fiber. In the case of AFRCC, the compressive strength was decreased as the fiber volume fraction increased due to the large number of fibers being mixed. The mix proportions of the FRCC under investigation are given in Table 3. W/B was set to 0.4. Type 1 ordinary portland cement and fly-ash were used as binder. Type 7 silica sand was used as aggregate. To satisfy the target flow of 170±20 mm, polycarboxylic acid-based high-performance water-reducing agent was added. Table 4 gives the properties of the raw materials for the finegrained concrete matrix.
2.2 Preparation of specimens The twin-shaft mixer was used for the mix of fiber-reinforced cementitious composites. For the mixing process of FRCC, cement, fly-ash and silica sand were first mixed in dry condition for 1 minute. And then a mortar base was fabricated by mixing and adding water and admixture for 1 minute. Subsequently, fiber was added and evenly distributed and sufficient mixing was performed to meet a predetermined fluidity for about 2~3 minutes. The evaluation of fluidity of fiber-reinforced cementitious composite was performed in accordance with KS L 5105. The mixed FRCC was then poured into a mold and cured in an environmental chamber at a temperature of 23 ± 2 °C at a relative humidity of 60 ± 5% for one day. Subsequently, all specimens were de-molded and cured at 20 °C of water for 28 days. For the uniaxial tension tests, dumbbell-shaped specimens were used, see Fig. 3. The dimensions were: length = 400 mm, width = 100 mm, and thickness = 25 mm. The central cross-section had a width of 50 mm and a thickness of 25 mm. To prevent the failure at the sections where the narrow part of the specimen begins (stress concentration due to flat notch) and to guarantee that cracks occur within the gauge measurement range, both ends of specimen were reinforced by wire mesh. 5
Ten specimens were fabricated for each test series, thus, five tension tests were performed at each -6
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strain rates of 10 /s and 10 /s.
2.3 Evaluation methodology
The fiber pull-out test was done by embedding half of the fiber (15 mm) into the center of the specimen, which had a cross section of 25 mm × 25 mm, see Fig. 4. The loading velocity of fiber pullout test was set to 1 mm/min.[34]. The quasi-static tension test was conducted at a displacement rate of 1 mm/min using the direct tensile test set-up with a capacity of 250 kN, see Fig. 5. The displacement of the specimen was measured using displacement meters (LVDTs) installed on the opposite sides of the specimen. Fig. 6 shows the high-velocity loading test equipment enabling a 5 m/s loading velocity which 1
corresponds to a 10 /s average strain rate for the specimens under investigation. The load is applied by rapidly discharging hydraulic pressure with an accumulator after filling the high-pressure tank with hydraulic fluid. The compressive loads generated by the accumulator at the top and the high-pressure oil tank are transferred to the tensile jig. The compressive load is transmitted along the frame of the tensile jig. Then, the specimen connected to the load cell of the tensile jig is transferred to the tensile load. Load cells installed at the top of the direct tensile jig measure load, the stress is then calculated from the load. Free-core type LVDTs(LP-30FP) installed on the specimen measure tensile strain. The stress and strain values of FRCC at high-velocity loading were collected at a sampling rate of 30,000 Hz using a high-speed data logger(DEWE43). The FIR filter process was performed to mitigate noise of collected stress and strain data. FIR filtering can reduce noise phenomena of measurement data. The strain rate was calculated as the average slope from the time-strain curve obtained by direct tensile testing until failure of the specimen. Based on the collected stress and strain values, a tensile stress-strain curve was derived as shown in Fig. 7. From this curve, the tensile strength (maximum stress point, the maximum stress,
), strain capacity (strain at
), and tensile toughness (area of the stress-strain curve) were evaluated. The
peak toughness was obtained as the area within OAB, which is the strain-hardening region, and the tensile toughness was obtained as the area within OAC. The tensile properties measured at the strain 1
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rate of 10 /s were divided by the corresponding values measured at the quasi-static strain rate of 10 /s to calculate the dynamic increase factor (DIF).
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3. Results and Discussion 3.1 Pull-out force-displacement curve and fracture shape of single fiber Fig. 8 shows the pull-out force-displacement curve of a single fiber under the static loading condition. Although the amorphous metallic fiber had excellent bonding performance with the matrix, because of its rough surface and large specific surface area, it was subjected to a fracture without being pulled out from the matrix. Obviously, its thin plate shape make the fiber vulnerable to shear force. In contrast, in the case of the hooked steel fiber, the force slowly decreased as the fiber was pulled out from the matrix after the maximum bonding load. Moreover, a stepwise force reduction occurred owing to the mechanical interlock of the hooked ends, followed by the straightening of the hooked ends [35].
3.2 Tensile stress-strain curve and fracture shape of fiber by strain rate Figs. 9 and 10 show the tensile stress-strain curves of AFRCC and HSFRCC according to the strain rate and crack patterns of the specimens. Under all test conditions, strain-hardening behavior accompanied by multiple cracking was observed. Under the quasi-static loading condition, AFRCC clearly exhibited strain-hardening behavior accompanied by multiple cracking, which can be tracked back to its excellent fiber-matrix bonding and the pronounced stress distribution effect owing to the large number of AF fibers. Moreover, AFRCC showed higher tensile strength and strain capacity than HSFRCC. However, in the strain-softening region, after the peak stress, the amorphous metallic fiber was not pulled out from the matrix because of its strong bond to the matrix resulting froom the rough fiber surface and its large specific surface area. The fiber failed, see Fig. 11(b), leading to a sharp decrease in stress. Moreover, it was observed that the cement matrix was still attached to the surface of the fiber. It is noteworthy that the amount of the matrix attached to the fibers decreased as the fiber volume fraction increased, see Fig. 12(a) and Table 5. Therefore, the fiber-matrix bonding efficiency decreased with increasing fiber content. It was observed that AFRCC1.5 and AFRCC2.0 had a smaller number of fibers subjected to a fracture than AFRCC1.0 and that the number of fibers pulled out from the matrix increased. This change in fracture behavior could be one reason for some decrease in the strain capacity as the fiber volume fraction increased. Moreover, in the case of the pulled-out amorphous metallic fiber, scratches that occurred during the pull-out process were found on the fiber surface, as shown in Fig. 11(c). For HSFRCC, after the formation of initial cracks, strain-hardening occurred owing to the form interlocking between the fibers and the matrix (bridging effect). In the strain-softening region, the fibers were slowly pulled out from the matrix as described in Section 3.1, resulting in a slow reduction in stress. Moreover, the hooked ends of the fiber were straightened in the process, as shown in Fig. 13(a), and it was found that scratches were generated on the fiber surface. 7
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At a strain rate of 10 /s, the strain-hardening behavior was more pronounced under all test conditions, and both the tensile strength and strain capacity increased. In the case of AFRCC, the number of pulled-out fibers decreased and the number of fibers subjected to a fracture increased compared to the quasi-static loading condition, obviously because the fiber-matrix bond strength increased more pronouncedly with with higher strain rate as the tensile strength of the fiber, see Fig. 12(b) and Table 5. In the strain-softening region, the reduction in stress due to the fracture of the amorphous metallic fiber was accelerated by the increase in the strain rate. In contrast, at a strain rate 1
of 10 /s, the fracture behavior in which the hooked ends of the fibers were maintained and withdrawn as the matrix was fractured without the fibers being pulled out from the matrix occurred in HSFRCC because the fiber-matrix bonding strength increased. Moreover, unlike the quasi-static loading condition, there was no scratch on the fiber surface, while some matrix attached to the fiber surface was observed, see Fig. 13(b). Under the tensile loading condition, the strain-hardening behavior was observed for both AFRCC and HSFRCC. It was found that the increase in the strain rate improved the tensile strength and strain capacity because it enhances the strength of the matrix, the tensile strength of the fibers, and the fiber-matrix bonding strength. In the strain-softening region after the peak stress, however, a faster decrease in stress occurred for both AFRCC and HSFRCC compared to the static loading condition as the strain rate increased. This appears to be because, while the pull-out or fracture of the fibers gradually occurred between crack openings under the static loading condition, high loading velocity was maintained even after the peak stress under the high strain rate condition, and thus the time to reach the final fracture was short. The above results for the tensile strain-hardening and strain-softening behavior of AFRCC and HSFRCC according to the strain rate are summarized in Fig. 14. Due to the increase in the strain rate, the strain-hardening behavior became more pronounced, and both the tensile strength and strain capacity increased, while stress decreased more rapidly in the strain-softening region. The fracture mechanisms according to the reinforcing fiber type can be summarized as follows. ■ Amorphous metallic fiber-reinforced cementitious composite (AFRCC) ① Strain-hardening region: - Strain-hardening occurs along with multi-cracks because the fiber-matrix bonding performance is excellent and the number of fibers is large. - The tensile strength and strain capacity are higher compared to HSFRCC. ② Strain-softening region: - A rapid decrease in stress occurs because the fibers fractured without being pulled out from the matrix regardless of the strain rate. ■ Hooked steel fiber-reinforced cementitious composite (HSFRCC) ① Strain-hardening region: 8
- Strain-hardening occurs because of the bridging effect between the fibers and the matrix as well as the mechanical interlock of the hooked ends. ② Strain-softening region: - A slow decrease in loading occurs as the fibers are pulled out from the matrix under the static loading condition (straightened pull-out). - The matrix is fractured while the hooked ends of the fibers are maintained under the high strain rate condition because the fiber-matrix bond is improved (Non-straightened pull-out).
3.3 Tensile properties by strain rate Table 6 shows the measured tensile properties, namely tensile strength, strain capacity, peak toughness, and tensile toughness, of AFRCC and HSFRCC according to the strain rate and the DIF for each material parameter. Moreover, Fig. 15 shows the tensile properties as functions of the fiber type, fiber volume fraction, and strain rate. All specimens exhibited increased tensile strength as the fiber volume fraction and strain rate increased. This could be expected since the increase in the strain rate improves the strengths of the matrix, the fibers and the matrix-fiber bond. The peak toughness is closely related to the tensile strength and strain capacity because it is calculated from the area of the stress-strain curve up to the peak stress. As AFRCC has higher tensile strength and strain capacity than HSFRCC, its peak toughness was also found to be higher. In the case of the tensile toughness, AFRCC showed a higher value than HSFRC under the static loading condition, but HSFRCC exhibited a higher value than AFRCC under the high strain rate. Under the static loading condition and high strain rate, AFRCC exhibited a rapid decrease in stress because the fibers were subjected to a fracture after the peak stress. On the other hand, HSFRCC exhibited a slower decrease in stress than AFRCC because the fibers were pulled out from the matrix or the fibers were withdrawn because of the cracks in the matrix. These phenomena were more obvious under the high strain rate. As a result of these phenomena, the tensile toughness of AFRCC appears to be lower than that of HSFRCC under the high strain rate.
3.4 Dynamic increase factor (DIF) by strain rate Fig. 16 shows the DIF of tensile properties according to the fiber type and strain rate. HSFRCC exhibits higher values than AFRCC for all evaluation factors. The increase in the strain rate significantly affects the matrix strength, fiber strength, and fiber-matrix bonding strength. In the case of AFRCC, the amount of the matrix surrounding a single fiber is relatively smaller because its number of mixed fibers is approximately 16 times higher than that of HSFRCC. As the bonding efficiency of the fiber-matrix interface, which is affected by the strain rate, is relatively lower, the DIF values appear to be lower for AFRCC. Moreover, the results appear to be due to the fact while the fracture behavior involving the pull-out of the fibers changed to the fracture behavior involving the 9
matrix fracture due to the increase in the strain rate for HSFRCC, most of the fibers were subjected to a fracture regardless of the strain rate for AFRCC.
4. Conclusion From the comparison of the strain rate effect on the compressive and tensile behavior of amorphous metallic fiber-reinforced cementitious composite (AFRCC) relative to those of hooked steel fiberreinforced cementitious composite (HSFRCC), the following conclusions can be drawn. (1) It was confirmed that amorphous metallic fiber has excellent bonding performance with the matrix because of its rough fiber surface and large specific surface area, but it fractured without being pulled out from the matrix under static and high strain rate because its thin plate shape is vulnerable to shear force. (2) Amorphous metallic fiber exhibited excellent bonding performance with the matrix because of its rough surface and large specific surface area. It also exhibited clear strain-hardening behavior accompanied by multi-cracks because of its large stress distribution effect inside the matrix. However, as its thin plate-shaped geometry is vulnerable to shear force, it fractured without being pulled out from the matrix, resulting in a rapid decrease in stress in the strainsoftening region. (3) In the case of HSFRCC, fibers were pulled out from the matrix and the hooked ends of the fibers were straightened was observed under the static loading condition. As the strain rate increased, however, the fiber-matrix bonding strength and the mechanical bonding force of the hooked ends increased, resulting in a fracture behavior in which the number of fibers pulled out from the matrix decreased and the hooked ends were maintained and pulled out owing to the matrix fracture. (4) AFRCC exhibited a large stress distribution effect because its fiber-matrix bonding performance is excellent and the number of mixed fibers is larger than HSFRCC at the same fiber volume fraction. It also exhibited higher tensile strength, strain capacity, and peak toughness than HSFRCC because more multi-cracks occurred. Its tensile toughness, however, was lower than that of HSFRCC because a rapid decrease in stress occurred in the strain-softening region owing to the fracture of amorphous metallic fiber under the high strain rate. (5) Moreover, it was confirmed that AFRCC has lower dynamic increase factor (DIF) values for the tensile strength, strain capacity and toughness than HSFRCC because amorphous metallic fiber has a higher number of mixed fibers than hooked steel fiber and thus the amount of the matrix surrounding a single fiber is relatively smaller.
Acknowledgment Funding: This work was supported by the Technology Innovation Program (20002166, Development 10
of standard systems on carbon composite-based reinforcement materials for repair of road pavements and building structures) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea)
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Table 1 Properties of the fiber under investigation Fiber
Properties
Amorphous metallic fiber (AF)
Length:30 mm, Width: 1.6 mm, Thickness: 29 µm Density: 7.2 g/cm3, Tensile strength: 1,400 MPa
Hooked steel fiber (HSF)
Length: 30 mm, Diameter: 0.5 mm, Aspect ratio: 60 Density: 7.85 g/cm3, Tensile strength: 1,140 MPa
Table 2 Series of specimens under investigation Series ID
Fiber type
AFRCC1.0 AFRCC1.5 AFRCC2.0 HSFRCC1.0 HSFRCC1.5 HSFRCC2.0
Amorphous metallic fiber
Hooked steel fiber
Volume fraction (vol.%)
Compressive strength (MPa)
1.0
59.30
1.5 2.0 1.0 1.5 2.0
54.72 48.27 59.72 66.37 64.50
AFRCC: Amorphous metallic fiber-reinforced cementitious composite HSFRCC: Hooked steel fiber-reinforced cementitious composite
Table 3 Mix proportions by weight ratio W/B
C/B
FA/B
S/B
0.4
0.85
0.15
0.35
W: Water, B(=C+FA): Binder, C: Cement, FA: Fly-ash, S: Silica sand AF: Amorphous metallic fiber, HSF: Hooked steel fiber
Table 4 Properties of the raw materials for concrete matrix Materials
Properties
Cement (C)
Ordinary portland cement, Density: 3.15 g/cm3, Fineness: 3,200 cm2/g
Fly-ash (FA)
Density: 2.20 g/cm3, Fineness: 3,000 cm2/g
Silica sand (S)
Type 7, Density: 2.64 g/cm3, Absorptance: 0.38 %
Super plasticizer
Polycarboxylic acid type
Table 5 Percentage of pull-out and fracture of amorphous metallic fiber Fiber volume fraction (%)
Strain rate 10-6 /s
Strain rate : 101 /s
Pull-out
Fracture
Pull-out
Fracture
1.0
15%
85%
14%
86%
1.5
55%
45%
44%
56%
2.0
68%
32%
31%
69%
Table 6 Test data for tensile properties and DIF ID.
Strain rate Quasistatic
AFRCC1.0
Quasistatic
AFRCC1.5
Quasistatic
AFRCC2.0
Quasistatic
HSFRCC1.0
Quasistatic
HSFRCC1.5
Quasistatic
HSFRCC2.0
Tensile strength
Strain capacity
Peak toughness
Tesile toughness
MPa
DIF
%
DIF
kN·mm
DIF
kN·mm
DIF
Test No.
0.000001
Ave.
4.70
-
0.659
-
2.96
-
8.00
-
4.204
No.1
6.05
1.29
0.673
1.02
4.45
1.51
7.91
0.99
5.200
No.2
7.96
1.69
0.565
0.86
4.47
1.51
8.31
1.04
3.539
No.3
7.05
1.50
0.537
0.82
3.74
1.27
5.31
0.66
4.565
No.4
7.45
1.59
0.525
0.80
4.23
1.43
5.72
0.71
13.856
No.5
7.83
1.67
0.865
1.31
7.90
2.67
10.49
1.31
6.274
Ave.
7.27
1.55
0.633
0.96
4.96
1.68
7.55
0.94
0.000001
Ave.
7.10
-
0.505
-
3.26
-
13.33
-
7.951
No.1
10.90
1.53
0.635
1.26
8.08
2.24
11.86
0.89
5.757
No.2
10.40
1.46
0.726
1.44
8.52
2.69
11.26
0.84
9.818
No.3
9.44
1.33
0.686
1.36
7.52
2.47
11.40
0.86
6.641
No.4
9.42
1.33
0.675
1.34
6.93
2.21
9.69
0.73
4.116
No.5
11.40
1.60
0.685
1.36
8.51
2.56
10.93
0.82
6.863
Ave.
10.31
1.45
0.682
1.35
7.91
2.43
11.03
0.83
0.000001
Ave.
8.05
-
0.504
-
3.91
-
14.35
-
10.009
No.1
13.50
1.68
1.141
2.26
15.78
4.04
21.37
1.49
9.865
No.2
12.12
1.50
1.064
2.11
13.07
3.35
18.64
1.30
7.499
No.3
12.08
1.50
0.451
0.90
6.21
1.59
15.73
1.10
10.742
No.4
12.50
1.55
0.937
1.86
12.49
3.20
16.21
1.13
12.260
No.5
14.23
1.77
0.551
1.09
6.90
1.77
15.93
1.11
10.085
Ave.
12.89
1.60
0.829
1.65
10.89
2.79
17.58
1.23
0.000001
Ave.
2.47
-
0.203
-
0.47
-
4.01
1.93
9.97
No.1
6.86
2.77
0.223
1.10
1.52
3.26
7.75
12.38
No.2
6.63
2.68
0.587
2.89
4.40
9.44
10.74
2.68
12.52
No.3
5.99
2.42
0.573
2.82
2.46
5.28
5.52
1.38
10.67
No.4
5.22
2.11
0.426
2.09
1.09
2.34
8.65
2.16
11.12
No.5
6.10
2.47
0.343
1.69
1.26
2.70
6.86
1.71
11.336
Ave.
6.16
2.49
0.430
2.11
2.15
4.61
7.90
1.97
0.000001
Ave.
3.81
-
0.198
-
0.88
-
8.21
-
15.18
No.1
9.50
2.49
0.476
2.40
4.21
4.78
15.38
1.87
13.58
No.2
9.20
2.41
0.421
2.12
3.81
4.33
16.02
1.95
8.73
No.3
10.05
2.63
0.536
2.70
5.01
5.69
12.46
1.52
12.52
No.4
10.93
2.87
0.448
2.26
4.11
4.67
13.19
1.61
12.69
No.5
8.90
2.33
0.599
3.02
5.33
6.06
13.52
1.65
12.540
Ave.
9.72
2.55
0.496
2.50
4.49
5.11
14.11
1.72
0.000001
Ave.
4.88
-
0.256
-
1.30
-
9.23
-
13.75
No.1
13.45
2.76
0.397
1.55
5.42
4.18
21.30
2.31
13.13
No.2
10.89
2.23
0.954
3.72
6.47
4.99
12.57
1.36
16.44
No.3
12.18
2.50
0.938
3.66
11.90
9.18
21.18
2.30
15.25
No.4
11.58
2.37
0.747
2.91
8.05
6.21
17.35
1.88
15.74
No.5
12.17
2.49
0.719
2.80
7.77
6.00
22.80
2.47
14.861
Ave.
12.05
2.47
0.751
2.93
7.92
6.11
19.04
2.06
(a) Manufacturing process
(b) Shape of amorphous metallic fiber
Fig. 1 Manufacturing process of amorphous metallic fiber (AF)
Fig. 2 Hooked steel fiber (HSF)
Fig. 3 Schematic view of tension test specimen
Fig. 4 Schematic view of fiber pull-out test set up
Fig. 5 Quasi-static tension test set-up
Fig. 6 Schematic view of the high-velocity loading test equipment (Strain rate : 100 to 101/s, Loading velocity : 5m/s)
Fig. 7 Tensile stress-strain curve
Fig. 8 Pull-out force-displacement curve of amorphous metallic fiber and hooked steel fiber
(a) AFRCC1.0
(b) HSFRCC1.0
(c) AFRCC1.5
(d) HSFRCC1.5
(e) AFRCC2.0
(f) HSFRCC2.0
Fig. 9 Tensile stress-strain curves of FRCC under tensile quasi-static and dynamic loading
10-6/s
6.27/s
(a) AFRCC1.0
10-6/s
11.34/s
(d) HSFRCC1.0
10-6/s
6.86/s
(b) AFRCC1.5
10-6/s
12.54/s
(e) HSFRCC1.5 Fig. 10 Crack pattern according to strain rate
10-6/s
10.09/s
(c) AFRCC2.0
10-6/s
14.86/s
(f) AFRCC2.0
[One side]
[The other side] (a) Plain
(b) Fracture
(c) Pull-out Fig. 11 Surface of amorphous metallic fiber depending on the fracture conditions
[Vf = 1.0%]
[Vf = 1.5%]
[Vf = 2.0%] -6
(a) Strain rate : 10 /s
[Vf = 1.0%]
[Vf = 1.5%]
[Vf = 2.0%] 1
(b) Strain rate : 10 /s *Red circle : Fracture, Yellow circle : Pull-out Fig. 12 Fracture and pull-out pattern of amorphous metallic fibers
(a) Strain rate : 10-6/s
(b) Strain rate : 101/s
Fig. 13 Fracture and pull-out pattern of hooked steel fibers
(a) Strain rate : 10-6/s
(b) Strain rate : 101/s Fig. 14 Strain-hardening and strain-softening behaviors of fiber-reinforced cementitious composites related to the fracture mechanisms of amorphous metallic fiber and hooked steel fiber (Fiber volume fraction = 1.5%)
(a) Tensile strength
(b) Strain capacity
(c) Peak toughness
(d) Fracture toughness
Fig. 15 Tensile properties according to fiber type and strain rate
(a) Tensile strength
(b) Strain capacity
(c) Peak toughness
(d) Fracture toughness
Fig. 16 DIF of tensile properties according to fiber type and strain rate
S0958-9465(20)30010-X
DOI:
https://doi.org/10.1016/j.cemconcomp.2020.103519
Reference:
CECO 103519
To appear in:
Cement and Concrete Composites
Received Date: 28 March 2019 Revised Date:
26 November 2019
Accepted Date: 8 January 2020
Please cite this article as: H. Kim, G. Kim, S. Lee, G. Choe, J. Nam, T. Noguchi, V. Mechtcherine, Effects of Strain Rate on the Tensile Behavior of Cementitious Composites made with Amorphous Metallic Fiber, Cement and Concrete Composites, https://doi.org/10.1016/j.cemconcomp.2020.103519. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Effects of Strain Rate on the Tensile Behavior of Cementitious Composites made with Amorphous Metallic Fiber
Hongseop Kima, Gyuyong Kimb, *, Sangkyu Leeb, Gyeongcheol Choeb, Jeongsoo Namb, Takafumi Noguchic, Viktor Mechtcherined
a
Building Safety Research Center, Department of Living and Built Environment Research, Korea
Institute of Civil engineering and Building Technology, 283, Goyang-daero, Ilsanseo-gu, Goyang-si, Gyeonggi-do, 10223, Korea b
Department of Architectural Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu,
Daejeon 34134, Korea c
Department of Architecture, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan d
Institute for Construction Materials, TU Dresden, Germany
* Corresponding author.
E-Mail: [email protected] (G. Kim)
Abstract Amorphous metallic fiber has higher tensile strength as well as corrosion and wear resistance than common, crystalline steel fibers. Its utilization as reinforcement improves the crack resistance and flexural and tensile performance of concrete. In the study at hand, the tensile behavior of thin plate amorphous metallic fiber-reinforced cementitious composites (AFRCC) is compared with that of hooked steel fiber-reinforced cementitious composites (HSFRCC) for both quasi-static and dynamic loading regimes. AFRCC exhibites a high stress distribution effect and higher tensile strength, strain capacity, and peak toughness than HSFRCC, but lower tensile toughness and lower dynamic increase factor values for tensile strength, strain capacity, and toughness.
Keywords: fiber reinforcement; amorphous metallic fiber; hooked steel fiber; tensile behavior; strain rate effect; dynamic increase factor
1
1. Introduction Concrete materials are widely used in construction because they are economical and have high compressive strength and durability. However, they exhibit brittle fracture characteristics, their flexural and tensile strengths are low, their resistance to cracking, especially subject to impact loading, is weak. In fiber-reinforced cementitious composites (FRCC), short fibers are distributed discontinuously inside the matrix. Crack occurrence and propagation are suppressed through the bonding between fibers and matrix as well as the bridging effect exhibited by fibers; consequently, the flexural and tensile performances are significantly improved. FRCC has been intensively investigated since the 1960s and applied to various structures since the 1970s [1-6]. For FRCC, various kinds of short fibers made of steel, organic polymers, carbon and other materials have been developed and applied. Previous studies showed that steel fiber-reinforced concrete has higher flexural and tensile strengths as well as higher impact resistance and fracture energy in comparison to ordinary concrete, and that such improvement in the tensile performance is significantly affected by the fiber type, fiber dimensions, fiber tensile strength, volume fraction, fibermatrix bond and pull-out behavior [7-13]. Consequently, various types of steel fibers have been developed for reinforcement of concrete, and numerous studies on the enhancement of the flexural and tensile performances of concrete and concrete elements have been conducted. However, steel fibers are not always economical, while the amount of rebound of shotcrete and the weight of associated structures are high, owing to the high density of steel fibers. Moreover, they are corrosive in presence of moisture and chemical agents, which causes problems with respect to the durability of structures in which they are used. Amorphous metallic fiber is fabricated by the melt spinning method, in which liquid metal is 5
6
quenched at a rate of 10 -10 ℃/s, as shown in Fig. 1(a). This speedy cooling results in fiber having an amorphous structure and exhibiting higher tensile strength, corrosion resistance, and wear resistance than steel fibers with crystalline structures. In addition, amorphous metallic fiber has a high aspect ratio and a large specific surface area because it is fabricated in a thin plate shape, as shown in Fig. 1(b). Furthermore, it has excellent bonding performance with the matrix because of its rough surface. Finally, owing to its ultra-lightweight characteristic compared to the high density of steel, for the given fiber geometry and dosage by weight, the number of amorphous fibers in mixture is higher than that of common steel fibers. Thus, the use of amorphous metallic fiber in concrete can improve its crack resistance, its flexural and tensile performance and solve the durability deterioration problem caused by the corrosion of common steel fibers. Hameed et al. [14] studied flexural properties of fiber-reinforced concretes made with amorphous metallic fiber, with hooked steel fiber, and with a combination of both types. They stated that amorphous metallic fiber is effective for controlling micro-cracks owing to its high bonding strength with the matrix, while hooked steel fiber is effective for controlling macro-cracks. Moreover, they reported that hybrid reinforcement consisting of amorphous metallic fiber and hooked steel fiber is 2
effective for controlling cracks and improving flexural strength as well as toughness. Won et al. [15, 16] evaluated the bonding properties of amorphous metallic fiber and hooked steel fiber as well as the flexural properties of concrete reinforced with each fiber. They found that amorphous metallic fiber exhibited high bonding load with the matrix because of its rough fiber surface and large specific surface area. The flexural strength of amorphous metallic fiber-reinforced concrete was found to be higher than that of hooked steel fiber-reinforced concrete because amorphous metallic fiber has high bonding strength with the matrix and larger number of fibers for the same portion by weight. However, since the thin plate-shaped fibers have a low shear resistance, they fracture without being pulled out, thereby causing a rapid decrease in stress in the post-peak phase. Choi et al. [17] evaluated the corrosion resistance and shrinkage control performance of amorphous metallic fiber and hooked steel fiber in various aggressive environments. Amorphous metallic fiber was found to exhibit over 96% residual tensile strength even after 90 days of exposure. In contrast, hooked steel fiber exhibited degradation by corrosion on the fiber surface, while its residual tensile strength was 3.1 to 10.3% lower than that of amorphous metallic fiber. Moreover, it was confirmed that amorphous metallic fiber has excellent plastic shrinkage control performance compared to hooked steel fiber as well as PP and PVA fiber. Park et al. [18, 19] examined the crack behavior and tension-stiffening effect of reinforced concrete tension members reinforced with hooked steel fiber or amorphous metallic fiber, and stated that the amorphous metallic fiber-reinforced concrete had better control over the occurrence and progress of splitting cracks than the hooked steel fiber-reinforced concrete, thereby exhibiting more pronounced tension-stiffening effect. Ku et al. [20] confirmed both higher flexural strength amorphous metallic fiber-reinforced concrete in comparison to hooked steel fiber-reinforced concrete and a rapid decrease in residual strength of AMFRC in post-peak phase. This can be traced back to fact that amorphous metallic fiber fails without being pulled out, thus before its high bonding strength is reached, whereas hooked steel fiber has high energy dissipation capacity due to its pronounced pullout behavior. Also Yoon et al. [21] observed that amorphous metallic fiber fails because it is vulnerable to shear force and that hooked steel fiber is pulled out from the matrix. They confirmed that amorphous metallic fiber-reinforced concrete has a higher flexural strength whereas hooked steel fiber-reinforced concrete has higher ductility and deflection capacity after the peak, being also more effective in improving the flexural performance of high-strength concrete. Yoo et al. [22] found that the addition of amorphous metallic fiber improved compressive strength, flexural strength, and ductility of concrete. They also showed that the surface crack occurrence resistance of concrete pavement against repeated vehicle wheel. Moreover, the flexural performance improved with increases in length, volume fraction, and strength of amorphous metallic fiber. Amorphous metallic fiber-reinforced shotcrete has higher flexural strength than hooked steel fiber-reinforced shotcrete but lower residual flexural strength and toughness [23]. Yi et al. [24] evaluated the pull-out properties according to matrix strength and the embedded length of amorphous metallic fiber. They found that amorphous metallic fiber pulled out undamaged when the embedded length was 5 mm or lower or when the matrix compressive strength was 18 MPa or lower, whereas it fractured without being pulled out when the matrix strength was 30 MPa or higher or when the embedded length was 8 or 12 mm. In contrast, 3
hooked steel fiber was pulled out from the matrix, and the pull-out energy increased with matrix strength and the embedded length. These previous studies confirm that amorphous metallic fiber is effective in controlling plastic shrinkage and fine cracks as well as improving the flexural and tensile strengths when used as a reinforcement for concrete, and that it has excellent performance in terms of long-term durability because it is nearly free from corrosion. Previous studies on the dynamic properties of FRCC were predominantly conducted under quasistatic loading conditions. However, in recent years, studies on dynamic applications of FRCC have been conducted to secure the safety performance of structures against extreme loads, such as earthquakes, impacts, and explosions. It was observed that fracture mechanisms, such as the debonding and fiber pull-out, vary depending on the strain rate. Thus, in-depth examination and understanding of the fiber-matrix interface properties and pull-out behavior, tensile strength of FRCC as well as its strain capacity and toughness are required under various strain rate conditions. For this purpose researchers have developed various types of testing equipment capable of implementing high strain rates in various forms, and the dynamic properties of FRCC have been investigated using such equipment [25-30]. Moreover, the effect of the strain rate on the tensile behavior of FRCC, which are reinforced with various fibers, including smooth steel fiber, hooked steel fiber, twisted steel fiber, and organic fiber, has been studied by various researchers [31-33]. However, to the best of our knowledge, no study has been conducted on the effect of strain rate on the tensile behavior of AFRCC. Therefore, a study on the tensile behavior of AFRCC according to strain rate is required. In this investigation, FRCC reinforced with thin plate-shaped amorphous metallic fiber or hooked -6
1
steel fiber, and tensile behaviors were evaluated at strain rates of 10 /s (quasi-static) and 10 /s (high strain rate). To examine the effects of fiber-matrix bonding and pull-out according to the fiber type as well as the fracture properties of fiber on the tensile behavior of FRCC, tensile stress-strain curves were derived for both strain rates. In addition, the effects of strain rate, fiber type, and fiber volume fraction on the tensile strength, strain capacity, and toughness of FRCC were analyzed.
4
2. Experimental program 2.1 Materials and mixture proportions Table 1 shows the properties of the amorphous metallic fiber and hooked steel fiber used in this study. The amorphous metallic fiber exhibits high bonding performance with the matrix because of its large specific surface area and high friction of its rough surface. As the amorphous metallic fiber is the thin plate type, the number of mixed fibers is approximately 16 times larger than that of hooked steel fiber under the same fiber volume fraction. The steel fiber has hooks at both ends. The hooked steel fiber exhibit mechanical bonding to concrete matrix by the hooked ends. Fig. 2 shows the shape of the hooked steel fiber. Table 2 shows the experimental series used to investigate the tensile behavior of AFRCC and HSFRCC according to the strain rate; the static compressive strengths of the mixtures at 28 days are also provided. The fiber content in FRCC specimens varied between 1.0, 1.5, and 2.0 vol.% both for amorphous metallic fiber and hooked steel fiber. In the case of AFRCC, the compressive strength was decreased as the fiber volume fraction increased due to the large number of fibers being mixed. The mix proportions of the FRCC under investigation are given in Table 3. W/B was set to 0.4. Type 1 ordinary portland cement and fly-ash were used as binder. Type 7 silica sand was used as aggregate. To satisfy the target flow of 170±20 mm, polycarboxylic acid-based high-performance water-reducing agent was added. Table 4 gives the properties of the raw materials for the finegrained concrete matrix.
2.2 Preparation of specimens The twin-shaft mixer was used for the mix of fiber-reinforced cementitious composites. For the mixing process of FRCC, cement, fly-ash and silica sand were first mixed in dry condition for 1 minute. And then a mortar base was fabricated by mixing and adding water and admixture for 1 minute. Subsequently, fiber was added and evenly distributed and sufficient mixing was performed to meet a predetermined fluidity for about 2~3 minutes. The evaluation of fluidity of fiber-reinforced cementitious composite was performed in accordance with KS L 5105. The mixed FRCC was then poured into a mold and cured in an environmental chamber at a temperature of 23 ± 2 °C at a relative humidity of 60 ± 5% for one day. Subsequently, all specimens were de-molded and cured at 20 °C of water for 28 days. For the uniaxial tension tests, dumbbell-shaped specimens were used, see Fig. 3. The dimensions were: length = 400 mm, width = 100 mm, and thickness = 25 mm. The central cross-section had a width of 50 mm and a thickness of 25 mm. To prevent the failure at the sections where the narrow part of the specimen begins (stress concentration due to flat notch) and to guarantee that cracks occur within the gauge measurement range, both ends of specimen were reinforced by wire mesh. 5
Ten specimens were fabricated for each test series, thus, five tension tests were performed at each -6
1
strain rates of 10 /s and 10 /s.
2.3 Evaluation methodology
The fiber pull-out test was done by embedding half of the fiber (15 mm) into the center of the specimen, which had a cross section of 25 mm × 25 mm, see Fig. 4. The loading velocity of fiber pullout test was set to 1 mm/min.[34]. The quasi-static tension test was conducted at a displacement rate of 1 mm/min using the direct tensile test set-up with a capacity of 250 kN, see Fig. 5. The displacement of the specimen was measured using displacement meters (LVDTs) installed on the opposite sides of the specimen. Fig. 6 shows the high-velocity loading test equipment enabling a 5 m/s loading velocity which 1
corresponds to a 10 /s average strain rate for the specimens under investigation. The load is applied by rapidly discharging hydraulic pressure with an accumulator after filling the high-pressure tank with hydraulic fluid. The compressive loads generated by the accumulator at the top and the high-pressure oil tank are transferred to the tensile jig. The compressive load is transmitted along the frame of the tensile jig. Then, the specimen connected to the load cell of the tensile jig is transferred to the tensile load. Load cells installed at the top of the direct tensile jig measure load, the stress is then calculated from the load. Free-core type LVDTs(LP-30FP) installed on the specimen measure tensile strain. The stress and strain values of FRCC at high-velocity loading were collected at a sampling rate of 30,000 Hz using a high-speed data logger(DEWE43). The FIR filter process was performed to mitigate noise of collected stress and strain data. FIR filtering can reduce noise phenomena of measurement data. The strain rate was calculated as the average slope from the time-strain curve obtained by direct tensile testing until failure of the specimen. Based on the collected stress and strain values, a tensile stress-strain curve was derived as shown in Fig. 7. From this curve, the tensile strength (maximum stress point, the maximum stress,
), strain capacity (strain at
), and tensile toughness (area of the stress-strain curve) were evaluated. The
peak toughness was obtained as the area within OAB, which is the strain-hardening region, and the tensile toughness was obtained as the area within OAC. The tensile properties measured at the strain 1
-6
rate of 10 /s were divided by the corresponding values measured at the quasi-static strain rate of 10 /s to calculate the dynamic increase factor (DIF).
6
3. Results and Discussion 3.1 Pull-out force-displacement curve and fracture shape of single fiber Fig. 8 shows the pull-out force-displacement curve of a single fiber under the static loading condition. Although the amorphous metallic fiber had excellent bonding performance with the matrix, because of its rough surface and large specific surface area, it was subjected to a fracture without being pulled out from the matrix. Obviously, its thin plate shape make the fiber vulnerable to shear force. In contrast, in the case of the hooked steel fiber, the force slowly decreased as the fiber was pulled out from the matrix after the maximum bonding load. Moreover, a stepwise force reduction occurred owing to the mechanical interlock of the hooked ends, followed by the straightening of the hooked ends [35].
3.2 Tensile stress-strain curve and fracture shape of fiber by strain rate Figs. 9 and 10 show the tensile stress-strain curves of AFRCC and HSFRCC according to the strain rate and crack patterns of the specimens. Under all test conditions, strain-hardening behavior accompanied by multiple cracking was observed. Under the quasi-static loading condition, AFRCC clearly exhibited strain-hardening behavior accompanied by multiple cracking, which can be tracked back to its excellent fiber-matrix bonding and the pronounced stress distribution effect owing to the large number of AF fibers. Moreover, AFRCC showed higher tensile strength and strain capacity than HSFRCC. However, in the strain-softening region, after the peak stress, the amorphous metallic fiber was not pulled out from the matrix because of its strong bond to the matrix resulting froom the rough fiber surface and its large specific surface area. The fiber failed, see Fig. 11(b), leading to a sharp decrease in stress. Moreover, it was observed that the cement matrix was still attached to the surface of the fiber. It is noteworthy that the amount of the matrix attached to the fibers decreased as the fiber volume fraction increased, see Fig. 12(a) and Table 5. Therefore, the fiber-matrix bonding efficiency decreased with increasing fiber content. It was observed that AFRCC1.5 and AFRCC2.0 had a smaller number of fibers subjected to a fracture than AFRCC1.0 and that the number of fibers pulled out from the matrix increased. This change in fracture behavior could be one reason for some decrease in the strain capacity as the fiber volume fraction increased. Moreover, in the case of the pulled-out amorphous metallic fiber, scratches that occurred during the pull-out process were found on the fiber surface, as shown in Fig. 11(c). For HSFRCC, after the formation of initial cracks, strain-hardening occurred owing to the form interlocking between the fibers and the matrix (bridging effect). In the strain-softening region, the fibers were slowly pulled out from the matrix as described in Section 3.1, resulting in a slow reduction in stress. Moreover, the hooked ends of the fiber were straightened in the process, as shown in Fig. 13(a), and it was found that scratches were generated on the fiber surface. 7
1
At a strain rate of 10 /s, the strain-hardening behavior was more pronounced under all test conditions, and both the tensile strength and strain capacity increased. In the case of AFRCC, the number of pulled-out fibers decreased and the number of fibers subjected to a fracture increased compared to the quasi-static loading condition, obviously because the fiber-matrix bond strength increased more pronouncedly with with higher strain rate as the tensile strength of the fiber, see Fig. 12(b) and Table 5. In the strain-softening region, the reduction in stress due to the fracture of the amorphous metallic fiber was accelerated by the increase in the strain rate. In contrast, at a strain rate 1
of 10 /s, the fracture behavior in which the hooked ends of the fibers were maintained and withdrawn as the matrix was fractured without the fibers being pulled out from the matrix occurred in HSFRCC because the fiber-matrix bonding strength increased. Moreover, unlike the quasi-static loading condition, there was no scratch on the fiber surface, while some matrix attached to the fiber surface was observed, see Fig. 13(b). Under the tensile loading condition, the strain-hardening behavior was observed for both AFRCC and HSFRCC. It was found that the increase in the strain rate improved the tensile strength and strain capacity because it enhances the strength of the matrix, the tensile strength of the fibers, and the fiber-matrix bonding strength. In the strain-softening region after the peak stress, however, a faster decrease in stress occurred for both AFRCC and HSFRCC compared to the static loading condition as the strain rate increased. This appears to be because, while the pull-out or fracture of the fibers gradually occurred between crack openings under the static loading condition, high loading velocity was maintained even after the peak stress under the high strain rate condition, and thus the time to reach the final fracture was short. The above results for the tensile strain-hardening and strain-softening behavior of AFRCC and HSFRCC according to the strain rate are summarized in Fig. 14. Due to the increase in the strain rate, the strain-hardening behavior became more pronounced, and both the tensile strength and strain capacity increased, while stress decreased more rapidly in the strain-softening region. The fracture mechanisms according to the reinforcing fiber type can be summarized as follows. ■ Amorphous metallic fiber-reinforced cementitious composite (AFRCC) ① Strain-hardening region: - Strain-hardening occurs along with multi-cracks because the fiber-matrix bonding performance is excellent and the number of fibers is large. - The tensile strength and strain capacity are higher compared to HSFRCC. ② Strain-softening region: - A rapid decrease in stress occurs because the fibers fractured without being pulled out from the matrix regardless of the strain rate. ■ Hooked steel fiber-reinforced cementitious composite (HSFRCC) ① Strain-hardening region: 8
- Strain-hardening occurs because of the bridging effect between the fibers and the matrix as well as the mechanical interlock of the hooked ends. ② Strain-softening region: - A slow decrease in loading occurs as the fibers are pulled out from the matrix under the static loading condition (straightened pull-out). - The matrix is fractured while the hooked ends of the fibers are maintained under the high strain rate condition because the fiber-matrix bond is improved (Non-straightened pull-out).
3.3 Tensile properties by strain rate Table 6 shows the measured tensile properties, namely tensile strength, strain capacity, peak toughness, and tensile toughness, of AFRCC and HSFRCC according to the strain rate and the DIF for each material parameter. Moreover, Fig. 15 shows the tensile properties as functions of the fiber type, fiber volume fraction, and strain rate. All specimens exhibited increased tensile strength as the fiber volume fraction and strain rate increased. This could be expected since the increase in the strain rate improves the strengths of the matrix, the fibers and the matrix-fiber bond. The peak toughness is closely related to the tensile strength and strain capacity because it is calculated from the area of the stress-strain curve up to the peak stress. As AFRCC has higher tensile strength and strain capacity than HSFRCC, its peak toughness was also found to be higher. In the case of the tensile toughness, AFRCC showed a higher value than HSFRC under the static loading condition, but HSFRCC exhibited a higher value than AFRCC under the high strain rate. Under the static loading condition and high strain rate, AFRCC exhibited a rapid decrease in stress because the fibers were subjected to a fracture after the peak stress. On the other hand, HSFRCC exhibited a slower decrease in stress than AFRCC because the fibers were pulled out from the matrix or the fibers were withdrawn because of the cracks in the matrix. These phenomena were more obvious under the high strain rate. As a result of these phenomena, the tensile toughness of AFRCC appears to be lower than that of HSFRCC under the high strain rate.
3.4 Dynamic increase factor (DIF) by strain rate Fig. 16 shows the DIF of tensile properties according to the fiber type and strain rate. HSFRCC exhibits higher values than AFRCC for all evaluation factors. The increase in the strain rate significantly affects the matrix strength, fiber strength, and fiber-matrix bonding strength. In the case of AFRCC, the amount of the matrix surrounding a single fiber is relatively smaller because its number of mixed fibers is approximately 16 times higher than that of HSFRCC. As the bonding efficiency of the fiber-matrix interface, which is affected by the strain rate, is relatively lower, the DIF values appear to be lower for AFRCC. Moreover, the results appear to be due to the fact while the fracture behavior involving the pull-out of the fibers changed to the fracture behavior involving the 9
matrix fracture due to the increase in the strain rate for HSFRCC, most of the fibers were subjected to a fracture regardless of the strain rate for AFRCC.
4. Conclusion From the comparison of the strain rate effect on the compressive and tensile behavior of amorphous metallic fiber-reinforced cementitious composite (AFRCC) relative to those of hooked steel fiberreinforced cementitious composite (HSFRCC), the following conclusions can be drawn. (1) It was confirmed that amorphous metallic fiber has excellent bonding performance with the matrix because of its rough fiber surface and large specific surface area, but it fractured without being pulled out from the matrix under static and high strain rate because its thin plate shape is vulnerable to shear force. (2) Amorphous metallic fiber exhibited excellent bonding performance with the matrix because of its rough surface and large specific surface area. It also exhibited clear strain-hardening behavior accompanied by multi-cracks because of its large stress distribution effect inside the matrix. However, as its thin plate-shaped geometry is vulnerable to shear force, it fractured without being pulled out from the matrix, resulting in a rapid decrease in stress in the strainsoftening region. (3) In the case of HSFRCC, fibers were pulled out from the matrix and the hooked ends of the fibers were straightened was observed under the static loading condition. As the strain rate increased, however, the fiber-matrix bonding strength and the mechanical bonding force of the hooked ends increased, resulting in a fracture behavior in which the number of fibers pulled out from the matrix decreased and the hooked ends were maintained and pulled out owing to the matrix fracture. (4) AFRCC exhibited a large stress distribution effect because its fiber-matrix bonding performance is excellent and the number of mixed fibers is larger than HSFRCC at the same fiber volume fraction. It also exhibited higher tensile strength, strain capacity, and peak toughness than HSFRCC because more multi-cracks occurred. Its tensile toughness, however, was lower than that of HSFRCC because a rapid decrease in stress occurred in the strain-softening region owing to the fracture of amorphous metallic fiber under the high strain rate. (5) Moreover, it was confirmed that AFRCC has lower dynamic increase factor (DIF) values for the tensile strength, strain capacity and toughness than HSFRCC because amorphous metallic fiber has a higher number of mixed fibers than hooked steel fiber and thus the amount of the matrix surrounding a single fiber is relatively smaller.
Acknowledgment Funding: This work was supported by the Technology Innovation Program (20002166, Development 10
of standard systems on carbon composite-based reinforcement materials for repair of road pavements and building structures) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea)
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13
Table 1 Properties of the fiber under investigation Fiber
Properties
Amorphous metallic fiber (AF)
Length:30 mm, Width: 1.6 mm, Thickness: 29 µm Density: 7.2 g/cm3, Tensile strength: 1,400 MPa
Hooked steel fiber (HSF)
Length: 30 mm, Diameter: 0.5 mm, Aspect ratio: 60 Density: 7.85 g/cm3, Tensile strength: 1,140 MPa
Table 2 Series of specimens under investigation Series ID
Fiber type
AFRCC1.0 AFRCC1.5 AFRCC2.0 HSFRCC1.0 HSFRCC1.5 HSFRCC2.0
Amorphous metallic fiber
Hooked steel fiber
Volume fraction (vol.%)
Compressive strength (MPa)
1.0
59.30
1.5 2.0 1.0 1.5 2.0
54.72 48.27 59.72 66.37 64.50
AFRCC: Amorphous metallic fiber-reinforced cementitious composite HSFRCC: Hooked steel fiber-reinforced cementitious composite
Table 3 Mix proportions by weight ratio W/B
C/B
FA/B
S/B
0.4
0.85
0.15
0.35
W: Water, B(=C+FA): Binder, C: Cement, FA: Fly-ash, S: Silica sand AF: Amorphous metallic fiber, HSF: Hooked steel fiber
Table 4 Properties of the raw materials for concrete matrix Materials
Properties
Cement (C)
Ordinary portland cement, Density: 3.15 g/cm3, Fineness: 3,200 cm2/g
Fly-ash (FA)
Density: 2.20 g/cm3, Fineness: 3,000 cm2/g
Silica sand (S)
Type 7, Density: 2.64 g/cm3, Absorptance: 0.38 %
Super plasticizer
Polycarboxylic acid type
Table 5 Percentage of pull-out and fracture of amorphous metallic fiber Fiber volume fraction (%)
Strain rate 10-6 /s
Strain rate : 101 /s
Pull-out
Fracture
Pull-out
Fracture
1.0
15%
85%
14%
86%
1.5
55%
45%
44%
56%
2.0
68%
32%
31%
69%
Table 6 Test data for tensile properties and DIF ID.
Strain rate Quasistatic
AFRCC1.0
Quasistatic
AFRCC1.5
Quasistatic
AFRCC2.0
Quasistatic
HSFRCC1.0
Quasistatic
HSFRCC1.5
Quasistatic
HSFRCC2.0
Tensile strength
Strain capacity
Peak toughness
Tesile toughness
MPa
DIF
%
DIF
kN·mm
DIF
kN·mm
DIF
Test No.
0.000001
Ave.
4.70
-
0.659
-
2.96
-
8.00
-
4.204
No.1
6.05
1.29
0.673
1.02
4.45
1.51
7.91
0.99
5.200
No.2
7.96
1.69
0.565
0.86
4.47
1.51
8.31
1.04
3.539
No.3
7.05
1.50
0.537
0.82
3.74
1.27
5.31
0.66
4.565
No.4
7.45
1.59
0.525
0.80
4.23
1.43
5.72
0.71
13.856
No.5
7.83
1.67
0.865
1.31
7.90
2.67
10.49
1.31
6.274
Ave.
7.27
1.55
0.633
0.96
4.96
1.68
7.55
0.94
0.000001
Ave.
7.10
-
0.505
-
3.26
-
13.33
-
7.951
No.1
10.90
1.53
0.635
1.26
8.08
2.24
11.86
0.89
5.757
No.2
10.40
1.46
0.726
1.44
8.52
2.69
11.26
0.84
9.818
No.3
9.44
1.33
0.686
1.36
7.52
2.47
11.40
0.86
6.641
No.4
9.42
1.33
0.675
1.34
6.93
2.21
9.69
0.73
4.116
No.5
11.40
1.60
0.685
1.36
8.51
2.56
10.93
0.82
6.863
Ave.
10.31
1.45
0.682
1.35
7.91
2.43
11.03
0.83
0.000001
Ave.
8.05
-
0.504
-
3.91
-
14.35
-
10.009
No.1
13.50
1.68
1.141
2.26
15.78
4.04
21.37
1.49
9.865
No.2
12.12
1.50
1.064
2.11
13.07
3.35
18.64
1.30
7.499
No.3
12.08
1.50
0.451
0.90
6.21
1.59
15.73
1.10
10.742
No.4
12.50
1.55
0.937
1.86
12.49
3.20
16.21
1.13
12.260
No.5
14.23
1.77
0.551
1.09
6.90
1.77
15.93
1.11
10.085
Ave.
12.89
1.60
0.829
1.65
10.89
2.79
17.58
1.23
0.000001
Ave.
2.47
-
0.203
-
0.47
-
4.01
1.93
9.97
No.1
6.86
2.77
0.223
1.10
1.52
3.26
7.75
12.38
No.2
6.63
2.68
0.587
2.89
4.40
9.44
10.74
2.68
12.52
No.3
5.99
2.42
0.573
2.82
2.46
5.28
5.52
1.38
10.67
No.4
5.22
2.11
0.426
2.09
1.09
2.34
8.65
2.16
11.12
No.5
6.10
2.47
0.343
1.69
1.26
2.70
6.86
1.71
11.336
Ave.
6.16
2.49
0.430
2.11
2.15
4.61
7.90
1.97
0.000001
Ave.
3.81
-
0.198
-
0.88
-
8.21
-
15.18
No.1
9.50
2.49
0.476
2.40
4.21
4.78
15.38
1.87
13.58
No.2
9.20
2.41
0.421
2.12
3.81
4.33
16.02
1.95
8.73
No.3
10.05
2.63
0.536
2.70
5.01
5.69
12.46
1.52
12.52
No.4
10.93
2.87
0.448
2.26
4.11
4.67
13.19
1.61
12.69
No.5
8.90
2.33
0.599
3.02
5.33
6.06
13.52
1.65
12.540
Ave.
9.72
2.55
0.496
2.50
4.49
5.11
14.11
1.72
0.000001
Ave.
4.88
-
0.256
-
1.30
-
9.23
-
13.75
No.1
13.45
2.76
0.397
1.55
5.42
4.18
21.30
2.31
13.13
No.2
10.89
2.23
0.954
3.72
6.47
4.99
12.57
1.36
16.44
No.3
12.18
2.50
0.938
3.66
11.90
9.18
21.18
2.30
15.25
No.4
11.58
2.37
0.747
2.91
8.05
6.21
17.35
1.88
15.74
No.5
12.17
2.49
0.719
2.80
7.77
6.00
22.80
2.47
14.861
Ave.
12.05
2.47
0.751
2.93
7.92
6.11
19.04
2.06
(a) Manufacturing process
(b) Shape of amorphous metallic fiber
Fig. 1 Manufacturing process of amorphous metallic fiber (AF)
Fig. 2 Hooked steel fiber (HSF)
Fig. 3 Schematic view of tension test specimen
Fig. 4 Schematic view of fiber pull-out test set up
Fig. 5 Quasi-static tension test set-up
Fig. 6 Schematic view of the high-velocity loading test equipment (Strain rate : 100 to 101/s, Loading velocity : 5m/s)
Fig. 7 Tensile stress-strain curve
Fig. 8 Pull-out force-displacement curve of amorphous metallic fiber and hooked steel fiber
(a) AFRCC1.0
(b) HSFRCC1.0
(c) AFRCC1.5
(d) HSFRCC1.5
(e) AFRCC2.0
(f) HSFRCC2.0
Fig. 9 Tensile stress-strain curves of FRCC under tensile quasi-static and dynamic loading
10-6/s
6.27/s
(a) AFRCC1.0
10-6/s
11.34/s
(d) HSFRCC1.0
10-6/s
6.86/s
(b) AFRCC1.5
10-6/s
12.54/s
(e) HSFRCC1.5 Fig. 10 Crack pattern according to strain rate
10-6/s
10.09/s
(c) AFRCC2.0
10-6/s
14.86/s
(f) AFRCC2.0
[One side]
[The other side] (a) Plain
(b) Fracture
(c) Pull-out Fig. 11 Surface of amorphous metallic fiber depending on the fracture conditions
[Vf = 1.0%]
[Vf = 1.5%]
[Vf = 2.0%] -6
(a) Strain rate : 10 /s
[Vf = 1.0%]
[Vf = 1.5%]
[Vf = 2.0%] 1
(b) Strain rate : 10 /s *Red circle : Fracture, Yellow circle : Pull-out Fig. 12 Fracture and pull-out pattern of amorphous metallic fibers
(a) Strain rate : 10-6/s
(b) Strain rate : 101/s
Fig. 13 Fracture and pull-out pattern of hooked steel fibers
(a) Strain rate : 10-6/s
(b) Strain rate : 101/s Fig. 14 Strain-hardening and strain-softening behaviors of fiber-reinforced cementitious composites related to the fracture mechanisms of amorphous metallic fiber and hooked steel fiber (Fiber volume fraction = 1.5%)
(a) Tensile strength
(b) Strain capacity
(c) Peak toughness
(d) Fracture toughness
Fig. 15 Tensile properties according to fiber type and strain rate
(a) Tensile strength
(b) Strain capacity
(c) Peak toughness
(d) Fracture toughness
Fig. 16 DIF of tensile properties according to fiber type and strain rate