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Direct tensile behavior of amorphous metallic fiber-reinforced cementitious composites: Effect of fiber length, fiber volume fraction, and strain rate
Composites Part B 177 (2019) 107430
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Direct tensile behavior of amorphous metallic fiber-reinforced cementitious composites: Effect of fiber length, fiber volume fraction, and strain rate Hongseop Kim a, Gyuyong Kim b, Sangkyu Lee b, Gyeongcheol Choe b, Takafumi Noguchi c, Jeongsoo Nam b, * a
Building Safety Research Center, Department of Living and Built Environment Research, Korea Institute of Civil engineering and Building Technology, 283, Goyangdaero, Ilsanseo-gu, Goyang-si, Gyeonggi-do, 10223, Republic of Korea Department of Architectural Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon, 34134, Republic of Korea c Department of Architecture, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Amorphous metallic fiber Direct tensile behavior Fiber length Fiber volume fraction Strain rate Dynamic increase factor
In this study, the effects of the fiber length, fiber volume fraction, and strain rate on amorphous metallic fiberreinforced cementitious composites (AFRCCs) were investigated. The experimental results showed that the amorphous metallic fibers with a 30 mm length had excellent bonding performance with the matrix because of the rough fiber surface, large specific surface area, and high aspect ratio under both static and high strain rate conditions. The fibers, however, were not pulled out from the matrix and were subjected to fracture because the thin-plate shape of the fibers was vulnerable to shear force. On the other hand, the amorphous metallic fibers with a 15 mm length exhibited decreased bonding efficiency with the matrix because of the low aspect ratio and the increased number of mixed fibers, and the fibers were pulled out from the matrix. As the fiber length increased, the tensile strength, strain capacity, and tensile toughness increased because the stress dispersion effect increased alongside the increase in the internal binding force and the crosslinking reaction range inside the matrix. As for the dynamic increase factor (DIF), the amorphous metallic fibers with a 30 mm length exhibited fracture without being pulled out from the matrix. The amorphous metallic fibers with a 15 mm length, however, were pulled out from the matrix, thereby increasing the bonding efficiency of the fiber-matrix interface that is affected by the strain rate. Therefore, it was found that AFRCC-L15 had a higher DIF for the tensile strength, strain capacity, and tensile toughness.
1. Introduction Cementitious materials have been widely used for social in frastructures, such as skyscrapers, bridges, and plant facilities, because they are economical and have high compressive strength and durability. However, they have weak resistance to cracks and are subject to brittle fracture under flexural and tensile loads. Fiber-reinforced cementitious composites are materials that suppress the crack occurrence and prop agation using the bonding and crosslinking reaction between the fibers and the matrix and significantly improve the flexural and tensile per formance. Such fiber-reinforced cementitious composites have been studied since the 1960s and have been applied to various structures since the 1970s [1–6]. Steel fiber-reinforced cementitious composites are materials with excellent flexural/tensile strengths and fracture en ergy, and have been widely used as repair/reinforcement materials for
construction structures. The flexural and tensile behavior of the steel fiber-reinforced cementitious composites are significantly affected by the fiber length and diameter, aspect ratio, tensile strength, volume fraction, bonding and pull out properties of the fiber-matrix interface, and the type and strength of the matrix [7–13]. Therefore, steel fibers with different shapes are being developed to improve the bonding per formance between the cement matrix and steel fibers. In addition, ex periments on the flexural and tensile performance of the cementitious composites reinforced by steel fibers with various shapes and studies on their application to construction structures have been conducted. However, steel fibers are not economical and they increase the weight of structures due to their high density. They also lower the durability of concrete materials due to the corrosion of the fibers. Amorphous metallic fibers produced by quenching molten metal at 105 � C/s have amorphous structures. Thus, they have higher tensile
* Corresponding author. E-mail address: [email protected] (J. Nam). https://doi.org/10.1016/j.compositesb.2019.107430 Received 16 April 2019; Received in revised form 13 July 2019; Accepted 10 September 2019 Available online 12 September 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Shape of the used amorphous metallic fibers. Table 1 Mechanical properties of the amorphous metallic fibers. Length (mm)
Width (mm)
Thickness (μm)
Equivalent diameter (mm)
Length/Diameter ratio
Number of fibers (/kg)
Specific surface (m2/ kg)
Density (g/ cm3)
Tensile Strength (MPa)
30 15
1.6 1.0
29 24
0.25 0.18
120 83
100,000 385,000
10
7.2
1400
Table 2 Details of specimen. ID. a
Fiber length (mm)
Volume fraction (vol %)
Compressive strength b (MPa)
AFRCC-L30V1.0 AFRCC-L30V1.5 AFRCC-L30V2.0 AFRCC-L15V1.0 AFRCC-L15V1.5 AFRCC-L15V2.0
30
1.0
59.30
1.5
54.72
2.0
48.27
1.0
51.81
1.5
51.18
2.0
49.52
15
a AFRCC-L-V: Amorphous metallic fiber-reinforced cementitious composite Fiber length - Fiber volume fraction. b Average value of compressive strength measured by 3 specimens at the age of 28 days.
strength, corrosion resistance, and wear resistance than ordinary steel fibers. In addition, they have higher residual tensile strength than hooked steel fibers, which are vulnerable to corrosion, because they are not corroded in various degradation environments, and they can improve the decrease in the durability of concrete caused by the corrosion of the fibers [14]. Amorphous metallic fibers have a high aspect ratio and a large specific surface area because they have a thin-plate shape and an excellent bonding performance with the matrix due to the rough fiber surface. Moreover, they are ultra-lightweight and the number of mixed fibers is larger compared to ordinary steel fibers under the same volume fraction. Therefore, amorphous metallic fiber-reinforced concrete has better flexural and tensile performance than hooked steel fiber-reinforced concrete. However, due to the brit tleness of the amorphous metal and the vulnerability of the thin-plate shape fibers to shear force, the fibers are subjected to fracture without
Fig. 2. Schematic diagram of dumbbell shape direct tensile test specimen.
being pulled out after the peak load [15–21]. Meanwhile, in a study on the bonding and pull out of amorphous metallic fibers with a 30 mm length, it was confirmed that the pull out and fracture behavior of the fibers varied depending on the embedded length of the fibers and the strength of the matrix [22]. Fiber-reinforced cementitious composites with high tensile strength and energy absorption capacity are attracting attention as construction materials for improving the safety performance of structures against 2
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Fig. 5. Tensile stress-strain curve and tensile parameters.
Fig. 3. Static direct tensile test equipment.
Fig. 6. Definition and calculation method of DIF.
The tensile behavior of fiber-reinforced cementitious composites is significantly affected by the bonding and pull out properties of the fibermatrix interface and the fracture properties of the fibers. Moreover, the fracture mechanism varies depending on the strain rate as with the bonding and pull out of the fiber-matrix interface. Therefore, it is necessary to evaluate the bonding and pull out behavior between the fibers and the matrix as well as the tensile strength, strain capacity, and toughness of fiber-reinforced cementitious composites under various strain rate conditions to fully understand the tensile behavior of various fiber-reinforced cementitious composites [36–39]. In the authors’ previous study, the static mechanical properties and impact resistance performance of amorphous metallic fiber-reinforced cementitious composites were evaluated. The results revealed that, under static and impact loading conditions, the fracture behaviors of the fiber and matrix interface were different [40]. In addition, the need for further study on the dynamic properties of amorphous metallic fiber-reinforced cementitious composites according to the strain rate was presented. Based on this, this study evaluates the strain rate effect on the tensile behavior of amorphous metallic fiber-reinforced cemen titious composites. Additionally, the pull-out and fracture behaviors of the fibers according to the fiber length were analyzed. In this regard, this study is novel and more advanced than the previous studies. In this study, the effects of the fiber length, fiber volume fraction, and strain rate on the direct tensile behavior of thin-plate shape amorphous
Fig. 4. Dynamic direct tensile test equipment (Strain rate: 100–101/s, Loading velocity: 5 m/s).
extreme loads, such as earthquakes, impacts, and explosions. In addi tion, many studies are being conducted to apply them as construction materials for improving the safety performance of social infrastructures. To this end, many researchers have developed special test devices, and have conducted various studies using material conditions, such as fiber mixing conditions (e.g., fiber type, fiber strength, and fiber volume fraction) matrix strength, and loading conditions, such as the strain rate and impact load, as variables. Representative studies include the anal ysis of the fracture mechanism by the impact of projectiles, analysis of collisions using finite element analysis software, and studies on tensile behavior under high strain rate conditions [23–35]. 3
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Fig. 7. Tensile stress-strain curve and crack pattern at static load.
metallic fiber-reinforced cementitious composites were investigated. Fiber-reinforced cementitious composites were fabricated using thinplate shape amorphous metallic fibers with different lengths and aspect ratios, and their direct tensile behavior was evaluated at the strain rates of 10 6/s (static) and 101/s (high strain rate). In addition, the effects of the bonding and pull out between the fibers and the matrix and the fracture properties of the fibers on the direct tensile behavior of the amorphous metallic fiber-reinforced cementitious composites (AFRCCs) were analyzed according to the fiber length, fiber volume fraction, and strain rate.
385,000, which was approximately 3.85 times larger than that of the amorphous metallic fibers with a 30 mm length under the same mixing amount. Table 2 shows the experimental conditions for evaluating the direct tensile behavior of the AFRCCS according to the fiber length and the static compressive strength of the age of 28 days. The amorphous metallic fibers with 15 and 30 mm lengths were mixed at the volume fractions of 1.0, 1.5, and 2.0 vol%. In the compressive strength test, three cylindrical specimens with a 100 mm diameter and a 200 mm height were used for each condition. All the specimens met the target strength at the age of 28 days [41]. The mix proportion of AFRCC and mechanical properties of the used materials are the same as those in the authors’ previous papers [27–29,40].
2. Experimental design and method 2.1. Materials and mixture proportions
2.2. Preparation of specimens
Fig. 1 and Table 1 show the shapes and mechanical properties of the amorphous metallic fibers used in this study. The amorphous metallic fibers used were produced by Saint Gobain SEVA in France. The amor phous metallic fibers had a 7.2 g/cm3 density, a 1400 MPa tensile strength, and the thin-plate shape. The amorphous metallic fibers with a 30 mm length had a 1.6 mm width, 29 μm thickness, and 120 aspect ratio. The number of fibers per kg was 100,000. On the other hand, the amorphous metallic fibers with a 15 mm length had a 1.0 mm width, a 24 μm thickness, and an 83 aspect ratio. The number of fibers per kg was
For the mixing of fiber-reinforced cementitious composites, the binders (cement þ fly ash) and silica sand were dry mixed and then water and admixture (plasticizer and AE agent) were added to fabricate the mortar base. Later, the fibers were added and sufficient mixing was performed so that the cementitious composites could meet the pre scribed fluidity in the uncured state. The uncured fiber-reinforced cementitious composites were poured into a mold and cured in a con stant temperature and humidity chamber with a 23 � 2 � C temperature 4
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Fig. 8. Fracture shape of amorphous metallic fiber according to fiber length.
and a 60 � 5% humidity for one day. All the specimens were then demolded and subjected to water curing until the age of 28 days. Fig. 2 shows the schematic diagram of the dumbbell shape direct tensile test specimen. The specimen had a 400 mm length, 100 mm width, and 25 mm thickness. Its center section has a 50 mm width and 25 mm thickness. To control the occurrence of cracks in the center section of the specimen with reduced width due to the stress concen tration and to induce the occurrence of cracks within the deformation measurement range, wire mesh was applied to both ends of the specimen to produce a direct tensile test specimen. Ten specimens were fabricated for each test level, and direct tensile tests were conducted using five
specimens at each of the strain rates of 10
6
/s and 101/s.
2.3. Test methodology Fig. 3 shows the static direct tensile test equipment with a 250 kN capacity. The loading rate of the static direct tensile test was set to 1 mm/min, and the tensile deformation of each specimen was measured by installing LVDT displacement meters on the left and right sides of the specimen. Ball bearings were installed on the upper and lower sections of the tensile jig to control the eccentricity of the test specimen during the direct tensile test. Additionally, the test specimen was also secured to 5
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Fig. 9. Tensile stress-strain curve and crack pattern at high strain rate.
a jig to prevent it from being moved during the test. Fig. 4 shows the dynamic direct tensile test equipment with a 101/s average strain rate, a 5 m/sec. loading rate, and a 3000 kN maximum loading capacity. As this equipment is typically used for conducting uniaxial compression tests, direct tensile jig was installed to conduct direct tensile tests. The compressive load was transferred to the specimen as a tensile load through the direct tensile jig. The direct tensile stress was measured using a 500 kN-capacity load cell installed at the top of the direct tensile jig, and the tensile deformation was measured by installing LVDT displacement meters on the specimen. The stress and strain data of the fiber-reinforced cementitious composites in the dynamic direct tensile test were collected at a sampling rate of 30,000 Hz using a high-speed data logger. Fig. 5 shows the stress-strain curve and direct tensile properties. As for the direct tensile properties, the tensile strength (maximum stress point, fp ), strain capacity (strain at the peak tensile strength, δp ), and tensile toughness (area of the stress-strain curve) were evaluated. The tensile toughness was divided into the peak toughness and fracture toughness. The peak toughness was obtained through the area of OAB while the fracture toughness was obtained through the area of OAC. The dynamic increase factor (DIF) was obtained by dividing the tensile property value measured at the high strain rate of 101/s by the tensile property value measured at the static strain rate (10 6/s), as shown in Fig. 6. The DIF obtained in this study was compared with that of the
proposed equation in the CEB-FIP model code 2010 [42]. 3. Results and discussion 3.1. Static direct tensile stress-strain curve and fracture shape (strain rate 10 6/s) Fig. 7 shows the tensile stress-strain curves and crack patterns of the AFRCCs according to the fiber length and fiber volume fraction under the static loading condition. Under all test conditions, strain-hardening behavior was observed along with multiple cracks after the initial cracking. For AFRCC-L30, the bonding performance with the matrix was excellent and the stress dispersion effect was large due to the high aspect ratio of the fibers and high specific surface area, and thus, the strainhardening behavior with the strain capacity higher than 0.5% was observed. In addition, the tensile strength and strain capacity were higher and the number of multiple cracks was larger compared to AFRCC-L15. However, the strain capacity and the number of multiple cracks showed a tendency to slightly decrease as the fiber volume fraction increased, even though the tensile strength increased. For AFRCC-L15, on the other hand, both the tensile strength and strain ca pacity increased as the fiber volume fraction increased. Fig. 8 shows the fracture shapes of amorphous metallic fibers in the tensile fracture section according to the fiber length. In the case of the 6
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Table 3 Test data of direct tensile properties and DIF. ID.
Strain rate
AFRCC-L30-V1.0
Static
AFRCC-L30-V1.5
Static
AFRCC-L30-V2.0
Static
AFRCC-L15-V1.0
Static
AFRCC-L15-V1.5
Static
AFRCC-L15-V2.0
Static
0.000001 4.204 5.200 3.539 4.565 13.856 6.274 0.000001 7.951 5.757 9.818 6.641 4.116 6.863 0.000001 10.009 9.865 7.499 10.742 12.260 10.085 0.000001 3.191 7.068 9.676 4.029 3.897 5.571 0.000001 6.321 5.437 5.802 7.811 8.906 6.867 0.000001 10.724 8.216 11.259 8.040 9.798 9.617
Test No.
Tensile strength
Strain capacity
Peak toughness
MPa
DIF
%
DIF
kN⋅mm
DIF
kN⋅mm
DIF
Ave. No.1 No.2 No.3 No.4 No.5 Ave. Ave. No.1 No.2 No.3 No.4 No.5 Ave. Ave. No.1 No.2 No.3 No.4 No.5 Ave. Ave. No.1 No.2 No.3 No.4 No.5 Ave. Ave. No.1 No.2 No.3 No.4 No.5 Ave. Ave. No.1 No.2 No.3 No.4 No.5 Ave.
4.70 6.05 7.96 7.05 7.45 7.83 7.27 7.10 10.90 10.40 9.44 9.42 11.40 10.31 8.05 13.50 12.12 12.08 12.50 14.23 12.89 4.62 7.35 6.92 6.62 7.67 6.64 7.04 6.84 9.60 10.58 10.06 9.77 9.51 9.90 7.07 11.27 10.72 11.92 10.81 12.67 11.48
1.29 1.69 1.50 1.59 1.67 1.55 1.53 1.46 1.33 1.33 1.60 1.45 1.68 1.50 1.50 1.55 1.77 1.60 1.59 1.50 1.43 1.66 1.44 1.52 1.40 1.55 1.47 1.43 1.39 1.45 1.59 1.52 1.69 1.53 1.79 1.62
0.659 0.673 0.565 0.537 0.525 0.865 0.633 0.505 0.635 0.726 0.686 0.675 0.685 0.682 0.504 1.141 1.064 0.451 0.937 0.551 0.829 0.188 0.368 0.374 0.186 0.323 0.286 0.307 0.284 0.328 0.433 0.307 0.501 0.420 0.398 0.361 0.812 0.981 0.613 0.821 0.705 0.786
1.02 0.86 0.82 0.80 1.31 0.96 1.26 1.44 1.36 1.34 1.36 1.35 2.26 2.11 0.90 1.86 1.09 1.65 1.96 1.99 0.99 1.72 1.52 1.64 1.15 1.52 1.08 1.76 1.48 1.40 2.25 2.72 1.70 2.28 1.95 2.18
2.96 4.45 4.47 3.74 4.23 7.90 4.96 3.26 8.08 8.52 7.52 6.93 8.51 7.91 3.91 15.78 13.07 6.21 12.49 6.90 10.89 0.94 2.80 2.83 1.48 2.67 2.16 2.39 2.07 4.73 7.45 5.26 5.34 6.49 5.85 2.65 9.31 11.12 7.94 9.41 9.32 9.42
1.51 1.51 1.27 1.43 2.67 1.68 2.24 2.69 2.47 2.21 2.56 2.43 4.04 3.35 1.59 3.20 1.77 2.79 2.97 3.01 1.57 2.84 2.29 2.54 2.29 3.61 2.55 2.58 3.14 2.83 3.51 4.19 2.99 3.55 3.51 3.55
8.00 7.91 8.31 5.31 5.72 10.49 7.55 13.33 11.86 11.26 11.40 9.69 10.93 11.03 14.35 21.37 18.64 15.73 16.21 15.93 17.58 5.13 4.21 6.80 5.05 5.29 7.23 5.72 8.54 12.00 11.28 10.93 9.20 13.16 11.31 12.63 16.08 18.88 14.87 16.59 17.30 16.74
0.99 1.04 0.66 0.71 1.31 0.94 0.89 0.84 0.86 0.73 0.82 0.83 1.49 1.30 1.10 1.13 1.11 1.23 0.82 1.33 0.98 1.03 1.41 1.11 1.40 1.32 1.28 1.08 1.54 1.32 1.27 1.50 1.18 1.31 1.37 1.33
amorphous metallic fibers with a 30 mm length under the static loading condition, the bonding performance with the matrix was excellent because of the rough fiber surface, large specific surface area, and high aspect ratio. The fibers, however, were subjected to fracture without being pulled out from the matrix because the thin-plate shape of the fibers was vulnerable to shear force. As the fiber volume fraction increased, however, the bonding efficiency of the fiber-matrix interface decreased because the amount of the matrix surrounding a single fiber decreased. Therefore, the number of fibers subjected to fracture decreased and pulled out fibers were observed. In the case of the amorphous metallic fibers with a 15 mm length, on the other hand, the aspect ratio of the fibers was smaller and the number of mixed fibers was larger compared to the amorphous metallic fibers with a 30 mm length. Therefore, the bonding efficiency with the matrix decreased and the behavior where the fibers were pulled out from the matrix was observed.
Fracture toughness
capacity increased. On the other hand, the stress reduction in the strainsoftening region after the peak stress occurred more sharply in all the specimens compared to the static loading condition. It appears that the time required to reach the final fracture of the specimen was shorter under the high strain rate because high loading rate was maintained while the fracture of the fibers or the pull out of the fibers slowly occurred between the crack openings after the peak stress under the static loading condition. Even under the high strain rate condition of 101/s, the amorphous metallic fibers with a 30 mm length were subjected to fracture without being pulled out from the matrix as with the static loading condition, and the amorphous metallic fibers with a 15 mm length exhibited the behavior where the fibers were pulled out from the matrix. As the strain rate increased, however, the number of pulled-out fibers decreased and the number of the fibers subjected to fracture increased because the bonding performance between the fibers and the matrix was improved.
3.2. Dynamic direct tensile stress-strain curve and fracture shape (strain rate 101/s)
3.3. Direct tensile properties according to fiber length, volume fraction and strain rate
Fig. 9 shows the average values of the tensile stress-strain curves and crack patterns of the AFRCCs measured using five specimens under the high strain rate condition. As the strain rate increased, the strainhardening behavior became more obvious and the tensile strength increased because the bonding performance of the fiber-matrix interface was improved. In addition, the number of multiple cracks and the strain
Table 3 summarizes the measured tensile strength, strain capacity, and tensile toughness data of the AFRCCs according to the fiber length, fiber volume fraction, and strain rate as well as DIF for each tensile property. In addition, Fig. 10 shows the tensile properties according to the fiber length, fiber volume fraction, and strain rate. Under the static 7
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Fig. 10. Tensile properties according to fiber length and strain rate.
loading condition, the tensile strengths of AFRCC-L30-V1.0, AFRCCL30-V1.5, and AFRCC-L30-V2.0 were 4.70, 7.10, and 8.05 MPa, while those of AFRCC-L15-V1.0, AFRCC-L15-V1.5, and AFRCC-L15-V2.0 were 4.62, 6.84, and 7.07 MPa, respectively. On the other hand, under the high strain rate condition, the tensile strengths of AFRCC-L30-V1.0, AFRCC-L30-V1.5, and AFRCC-L30-V2.0 were 7.27, 10.31, and 12.89 MPa, while those of AFRCC-L15-V1.0, AFRCC-L15-V1.5, and AFRCC-L15-V2.0 were 7.04, 9.90, and 11.48 MPa, respectively. Under the same fiber volume fraction and strain rate conditions, as the aspect ratio of the fibers increased, the tensile strength of AFRCC-L30 was higher than that of AFRCC-L15 because the crosslinking reaction range and the internal binding force of the fibers increased. Meanwhile, the tensile strength increase ratio for the 0.5% increase in the fiber volume fraction was 151% for the increase from AFRCC-L30V1.0 to AFRCC-L30-V1.5 and 113% for the increase from AFRCC-L30V1.5 to AFRCC-L30-V2.0, 148% for the increase from AFRCC-L15V1.0 to AFRCC-L15-V1.5, and 103% for the increase from AFRCC-L15V1.5 to AFRCC-L15-V2.0 under the static loading condition. Under the high strain rate condition, the tensile strength increased by 142% for the increase from AFRCC-L30-V1.0 to AFRCC-L30-V1.5, 125% for the in crease from AFRCC-L30-V1.5 to AFRCC-L30-V2.0, 141% for the increase from AFRCC-L15-V1.0 to AFRCC-L15-V1.5, and 116% for the increase from AFRCC-L15-V1.5 to AFRCC-L15-V2.0. As the fiber volume fraction increased, the tensile strength showed a tendency to increase. However,
the tensile strength increase ratio for the fiber volume fraction increase from 1.0% to 1.5% was higher than that for the fiber volume fraction increase from 1.5% to 2.0%. It appears that when the fiber volume fraction is higher than 1.5%, the tensile strength increase ratio decreases alongside the decrease in the bonding efficiency of the fiber-matrix interface because the amount of the matrix surrounding a single fiber decreases as the number of the mixed fibers increases. On the other hand, the increase in the strain rate improved the strengths of the matrix and the fibers as well as the bonding performance between the matrix and the fibers, thereby increasing the tensile strength under all test conditions. The tensile strength increase ratio according to the strain rate is explained in detail through the DIF comparison. In the case of the strain capacity according to the fiber length, AFRCC-L30 showed higher strain capacity values than AFRCC-L15 under both static and high strain rate conditions as shown in Fig. 10 (b). AFRCC-L30 has higher internal binding force and a larger cross linking reaction range inside the matrix because the aspect ratio of the fibers is higher. This appears to increase the stress dispersion effect, strain-hardening behavior, and strain capacity. In addition, the fibers were subjected to fracture without being pulled out from the matrix. It appears that the strain capacity was high because cracks were first induced in the weak part of the matrix and a large number of multiple cracks were generated before the fibers between the crack openings were subjected to fracture in the course of crack occurrence and 8
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Fig. 11. DIF of tensile properties according to fiber length.
propagation in the matrix. In the case of AFRCC-L15, however, it ap pears that the number of multiple cracks and the strain capacity were smaller compared to AFRCC-L30 because the crosslinking reaction range was small in the crack openings of the matrix due to the low aspect ratio of the fibers and the fibers were pulled out from the matrix occurred. Under the static loading condition, AFRCC-L30 exhibited a decrease in the number of the fibers subjected to fracture and an increase in the number of the pulled out fibers as the fiber volume fraction increased because the amount of the matrix surrounding a single fiber decreased; thus, the bonding efficiency between the fibers and the matrix decreased. Owing to this change in the fracture mechanism of the fibers, the number of multiple cracks and strain capacity showed a tendency to decrease slightly. Under the high strain rate, on the other hand, the bonding performance between the fibers and the matrix was improved by the increase in the strain rate. Therefore, the number of the fibers subjected to fracture increased, and the strain capacity also showed a tendency to increase as the fiber volume fraction increased. In the case of AFRCC-L15, the strain capacity increased as the fiber volume fraction increased under both the static and high strain rate conditions. The peak toughness and fracture toughness are closely related to the tensile strength and strain capacity because they are calculated using the areas of the stress-strain curve. Therefore, as the tensile strength and strain capacity of AFRCC-L30 are higher than those of AFRCC-L15, the
tensile toughness was also higher. In addition, the tensile toughness also showed a tendency to increase as the fiber volume fraction increased. For the amorphous metallic fibers, it was found that the fracture of the fibers by securing sufficient bonding performance between the fibers and the matrix had a larger effect on the improvement of the tensile strength, strain capacity, and tensile toughness than the pull out of the fibers from the matrix. Therefore, the optimization of the tensile strength, strain capacity, and tensile toughness of amorphous metallic fiber-reinforced cementitious composites can be achieved by deriving the optimum mixing ratio considering the length and volume fraction of the amorphous metallic fibers and the amount of the matrix. 3.4. Dynamic increase factor according to fiber length Fig. 11 shows the DIF of tensile properties according to the fiber length. The DIF of AFRCC-L15 showed a tendency to be higher than that of AFRCC-L30 for all evaluation factors. Previous studies on the tensile behavior of fiber-reinforced cementitious composites according to the strain rate have reported that the increase in the strain rate significantly affects the bonding of the fiber-matrix interface as well as the pull out behavior of the fibers [26,27,30–32]. The major tensile fracture behavior of AFRCC-L30 is affected more by the fiber strength than by the bonding of the fiber-matrix interface because the fibers are subjected to 9
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Fig. 11. (continued).
fracture without being pulled out from the matrix. For AFRCC-L15, on the other hand, it appears that the bonding of the fiber-matrix interface and the pull out behavior of the fibers have a significant effect on the tensile behavior of fiber-reinforced cementitious composites because the fibers show the behavior where they are pulled out from the matrix without being subjected to fracture. Therefore, it is judged that the effect of the strain rate increase on the improvement of tensile properties is significant for AFRCC-L15 because the bonding efficiency of the fiber-matrix interface, which is affected by the strain rate, is relatively higher than that of AFRCC-L30.
shape of the fibers was vulnerable to shear force. On the other hand, the amorphous metallic fibers with a 15 mm length and a 83 aspect ratio showed the behavior where the fibers were pulled out from the matrix because the aspect ratio of the fibers was low and the number of mixed fibers was large. (2) As the aspect ratio of the fibers increased, the tensile strength of AFRCC-L30 was higher than that of AFRCC-L15 because of the higher internal binding force and larger crosslinking reaction range inside the matrix as well as the higher stress dispersion effect. In addition, it appears that the strain capacity of AFRCCL30 was high because cracks were induced in the vulnerable part of the matrix, resulting in the generation of a large number of multiple cracks and the increase in the strain-hardening behavior before the fibers between the crack openings were subjected to fracture. In the case of AFRCC-L15, on the other hand, it appears that the number of multiple cracks and the strain capacity were small because the crosslinking reaction range was small in the crack openings of the matrix due to the low aspect ratio of the fibers and the fracture behavior occurred where the fibers were pulled out from the matrix. (3) AFRCC-L30 was affected more by the fiber strength than by the bonding between the fibers and the matrix because the fibers exhibited fracture behavior without being pulled out from the matrix. For AFRCC-L15, on the other hand, the bonding
4. Conclusions In this study, the effects of the fiber length, fiber volume fraction, and strain rate on the direct tensile behavior of amorphous metallic fiberreinforced cementitious composites (AFRCCs) were evaluated. The re sults were as follows. (1) Under the static and high strain rate conditions, the amorphous metallic fibers with a 30 mm length and a 120 aspect ratio exhibited excellent bonding performance with the matrix because of their rough fiber surface, large specific surface area, and high aspect ratio. The fibers, however, were subjected to fracture without being pulled out from the matrix because the thin-plate 10
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performance of the fiber-matrix interface was significantly affected by the strain rate because the fracture behavior occurred where the fibers were pulled out from the matrix without being subjected to fracture. Therefore, it is judged that the effect of the strain rate increase on the improvement of tensile properties is significant for AFRCC-L15 because its bonding efficiency of the fiber-matrix interface, which is affected by the strain rate, is relatively high.
[15] Park KW, Lee JS, Kim W, Kim DJ, Lee GY. Cracking behavior of RC tension members reinforced with amorphous steel fibers. J Korea Concr Inst 2014;26(4): 475–82. [16] Park KW, Lee JS, Kim W, Kim DJ, Lee GY. Tension stiffening effect of RC tension members reinforced with amorphous steel fibers. J Korea Concr Inst 2014;26(5): 581–9. [17] Ku DO, Kim SD, Kim HS, Choi KK. Flexural performance characteristics of amorphous steel fiber-reinforced concrete. J Korea Concr Inst 2014;26(4):483–9. [18] Yoo DY, Banthia N, Yang JM, Yoon YS. Size effect in normal- and high-strength amorphous metallic and steel fiber reinforced concrete beams. Constr Build Mater 2016;121:676–85. [19] Yoo DY, Banthia N. Experimental and numerical analysis of the flexural response of amorphous metallic fiber reinforced concrete. Mater Struct 2017;50:64. [20] Yang JM, Kim JK, Yoo DY. Effects of amorphous metallic fibers on the properties of asphalt Concrete. Constr Build Mater 2016;128:176–84. [21] Yang JM, Kim JK, Yoo DY. Performance of shotcrete containing amorphous fibers for tunnel applications. Tunn Undergr Space Technol 2017;64:85–97. [22] Kim BJ, Yi CK, Ahn YR. Effect of embedment length on pullout behavior of amorphous steel fiber in portland cement composites. Constr Build Mater 2017; 143:83–91. [23] Habel K, Gauvreau P. Response of ultra-high performance fiber reinforced concrete (UHPFRC) to impact and static loading. Cement Concr Compos 2008;30:938–46. [24] Fujikake K, Shinozaki Y, Ohno T, Mizuno J, Suzuki A. Post-peak and strainsoftening of concrete materials in compression under rapid loadingvol. 627. Japan Society of Civil Engineers; 1999. p. 37–54. [25] Fujikake K, Uebayachi K, Ohno T, Emori K. Study on dynamic tensile softening characteristic of concrete material under high strain-ratevol. 669. Japan Society of Civil Engineers; 2001. p. 125–34. [26] Mechtcherine V, Andrade Silva F, Butler M, Zhu D, Mobasher B, Gao SL, M€ ader E. Behaviour of strain-hardening cement-based composites under high strain rates. J Adv Concr Technol 2011;9:51–62. [27] Kim H, Kim G, Lee S, Son M, Choe G, Nam J. Strain rate effects on the compressive and tensile behavior of bundle-type polyamide fiber-reinforced cementitious composite. Compos B Eng 2019;160:50–65. [28] Nam J, Shinohara Y, Atou T, Kim H, Kim G. Comparative assessment of failure characteristics on fiber-reinforced cementitious composite panels under highvelocity impact. Compos B Eng 2016;99:84–97. [29] Nam J, Kim H, Kim G. Experimental investigation on the blast resistance of fiberreinforced cementitious composite panels subjected to contact explosions. Int J Concr Struct Mater 2017;11(1):29–43. [30] Tran TK, Kim DJ. Investigating direct tensile behavior of high performance fiber reinforced cementitious composites at high strain rates. Cement Concr Res 2013; 50:62–73. [31] Kim JJ, Park GJ, Kim DJ, Moon JH, Lee JH. High-rate tensile behavior of steel fiber-reinforced concrete for nuclear power plants. Nucl Eng Des 2014;266:43–54. [32] Tran TK, Kim DJ. High strain rate effects on direct tensile behavior of high performance fiber reinforced cementitious composites. Cement Concr Compos 2014;45:186–200. [33] Choi JG, Lee GC, Koh KT. The effect of mixture rate and aspect ratio of steel fiber on mechanical properties of ultra high performance concrete. J Recycl Constr Resour 2017;5(1):14–20. [34] Jeong GY, Jang SJ, Kim YC, Yun HD. Effect of steel fiber strength and aspect ratio on mechanical properties of high-strength concrete. J Korea Concr Inst 2018;30(2): 197–205. [35] Park GJ, Park JJ, Kim SW, Lee JH. Evaluation of flexural performance of high performance fiber reinforced cementitious composites according to fiber shape, aspect ratio and volume fraction. J Korea Acad Ind 2017;18(12):697–704. [36] Zhang X, Shi Y, Li ZX. Experimental study on the tensile behavior of unidirectional and plain weave CFRP laminates under different strain rates. Composites Part B 2019;164:524–36. [37] Sassi S, Tarfaoui M, Nachtane M, Yahia HB. Strain rate effects on the dynamic compressive response and the failure behavior of polyester matrix. Composites Part B 2019;174:107040. [38] Fan J, Wang C. Dynamic compressive response of a developed polymer composite at different strain rates. Composites Part B 2018;152:96–101. [39] Yuan C, Chen W, Pham TM, Hao H, Jian C, Shi Y. Strain rate effect on interfacial bond behaviour between BFRP sheets and steel fibre reinforced concrete. Composites Part B 2019;174:107032. [40] Kim H, Kim G, Nam J, Kim J, Han S, Lee S. Static mechanical properties and impact resistance of amorphous metallic fiber-reinforced concrete. Compos Struct 2015; 134:831–44. [41] ASTM C39/C39M standard test method for compressive strength of cylindrical concrete specimens. [42] CEB-FIP model code 2010 first complete draft.
For the amorphous metallic fibers, it was found that the fracture of the fibers by securing sufficient bonding performance between the fibers and the matrix had a larger impact on the improvement of the tensile strength, strain capacity, and tensile toughness than the pull out of the fibers from the matrix. On the other hand, for the sensitivity of the strain rate, the pull out behavior, which is the bonding at the fiber-matrix interface, had a larger impact than the fracture of the fibers. There fore, it is important to consider the length and volume fraction of amorphous metallic fibers as well as the amount of the matrix to derive the optimum mixing ratio in consideration of the direct tensile proper ties of AFRCCs, such as the tensile strength, strain capacity, and tensile toughness, as well as DIF according to the strain rate. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(2017R1D1A1B03034776). References [1] Kanda T, Li VC. Interface property and apparent strength of high strength hydrophilic fiber in cement matrix. J Mater Civ Eng 1998;10:5–13. [2] Bencardino F, Rizzuti L, Spadea G, Swamy RN. Implications of test methodology on post-cracking and fracture behaviour of steel fibre reinforced concrete. Compos B Eng 2013;46:31–8. € The effects of macro synthetic fiber [3] Bolat H, S¸ims¸ek O, Çullu M, Durmus¸ G. Can O. reinforcement use on physical and mechanical properties of concrete. Compos B Eng 2014;61:191–8. [4] Soroushian P, Tlili A, Alhozaimy A, Khan A. Development and characterization of hybrid polyethylene fiber reinforced cement composites. ACI Mater J 1993;90: 182–90. [5] Maalej M, Li VC. Introduction of strain hardening engineered cementitious composites in the design of reinforced concrete flexural members for improved durability. Am Concr Inst Struct J 1995;92:167–76. [6] Naaman AE, Shah SP. Pull-out mechanism in steel fiber-reinforced concrete. J Struct Div 1976;102(8):1537–48. [7] Betterman LR, Ouyang C, Shah SP. Fiber-matrix interaction in micro fiber reinforced mortar. Adv Cem Base Mater 1995;2(2):53–61. [8] Rossi P, Harrouche N. Mix design and mechanical behaviour of some steel fibrereinforced concretes used in reinforced concrete structures. Mater Struct 1990;23 (4):256–66. [9] Chan YW, Chu SH. Effect of silica fume on steel fiber bond characteristics in reactive powder concrete. Cement Concr Res 2004;34(7):1167–72. [10] Li VC, Wang Y, Backer S. Effect of inclining angle, bundling and surface treatment on synthetic fibre pull-out from a cement matrix. Composites 1990;21(2):132–40. [11] Hameed R, Turatsinze A, Duprat F, Sellier A. Study on the flexural properties of metallic-hybrid-fiber reinforced concrete. Maejo Int J Sci Technol 2010;4(02): 169–84. [12] Won JP, Hong BT, Choi TJ, Lee SJ, Kang JW. Flexural behaviour of amorphous micro-steel fibre-reinforced cement composites. Compos Struct 2012;94:1443–9. [13] Won JP, Hong BT, Lee SJ, Choi SJ. Bonding properties of amorphous micro-steel fibre-reinforced cementitious composites. Compos Struct 2013;102:101–9. [14] Choi SJ, Hong BT, Lee SJ, Won JP. Shrinkage and corrosion resistance of amorphous metallic-fiber-reinforced cement composites. Compos Struct 2014;107: 537–43.
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Direct tensile behavior of amorphous metallic fiber-reinforced cementitious composites: Effect of fiber length, fiber volume fraction, and strain rate Hongseop Kim a, Gyuyong Kim b, Sangkyu Lee b, Gyeongcheol Choe b, Takafumi Noguchi c, Jeongsoo Nam b, * a
Building Safety Research Center, Department of Living and Built Environment Research, Korea Institute of Civil engineering and Building Technology, 283, Goyangdaero, Ilsanseo-gu, Goyang-si, Gyeonggi-do, 10223, Republic of Korea Department of Architectural Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon, 34134, Republic of Korea c Department of Architecture, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Amorphous metallic fiber Direct tensile behavior Fiber length Fiber volume fraction Strain rate Dynamic increase factor
In this study, the effects of the fiber length, fiber volume fraction, and strain rate on amorphous metallic fiberreinforced cementitious composites (AFRCCs) were investigated. The experimental results showed that the amorphous metallic fibers with a 30 mm length had excellent bonding performance with the matrix because of the rough fiber surface, large specific surface area, and high aspect ratio under both static and high strain rate conditions. The fibers, however, were not pulled out from the matrix and were subjected to fracture because the thin-plate shape of the fibers was vulnerable to shear force. On the other hand, the amorphous metallic fibers with a 15 mm length exhibited decreased bonding efficiency with the matrix because of the low aspect ratio and the increased number of mixed fibers, and the fibers were pulled out from the matrix. As the fiber length increased, the tensile strength, strain capacity, and tensile toughness increased because the stress dispersion effect increased alongside the increase in the internal binding force and the crosslinking reaction range inside the matrix. As for the dynamic increase factor (DIF), the amorphous metallic fibers with a 30 mm length exhibited fracture without being pulled out from the matrix. The amorphous metallic fibers with a 15 mm length, however, were pulled out from the matrix, thereby increasing the bonding efficiency of the fiber-matrix interface that is affected by the strain rate. Therefore, it was found that AFRCC-L15 had a higher DIF for the tensile strength, strain capacity, and tensile toughness.
1. Introduction Cementitious materials have been widely used for social in frastructures, such as skyscrapers, bridges, and plant facilities, because they are economical and have high compressive strength and durability. However, they have weak resistance to cracks and are subject to brittle fracture under flexural and tensile loads. Fiber-reinforced cementitious composites are materials that suppress the crack occurrence and prop agation using the bonding and crosslinking reaction between the fibers and the matrix and significantly improve the flexural and tensile per formance. Such fiber-reinforced cementitious composites have been studied since the 1960s and have been applied to various structures since the 1970s [1–6]. Steel fiber-reinforced cementitious composites are materials with excellent flexural/tensile strengths and fracture en ergy, and have been widely used as repair/reinforcement materials for
construction structures. The flexural and tensile behavior of the steel fiber-reinforced cementitious composites are significantly affected by the fiber length and diameter, aspect ratio, tensile strength, volume fraction, bonding and pull out properties of the fiber-matrix interface, and the type and strength of the matrix [7–13]. Therefore, steel fibers with different shapes are being developed to improve the bonding per formance between the cement matrix and steel fibers. In addition, ex periments on the flexural and tensile performance of the cementitious composites reinforced by steel fibers with various shapes and studies on their application to construction structures have been conducted. However, steel fibers are not economical and they increase the weight of structures due to their high density. They also lower the durability of concrete materials due to the corrosion of the fibers. Amorphous metallic fibers produced by quenching molten metal at 105 � C/s have amorphous structures. Thus, they have higher tensile
* Corresponding author. E-mail address: [email protected] (J. Nam). https://doi.org/10.1016/j.compositesb.2019.107430 Received 16 April 2019; Received in revised form 13 July 2019; Accepted 10 September 2019 Available online 12 September 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Shape of the used amorphous metallic fibers. Table 1 Mechanical properties of the amorphous metallic fibers. Length (mm)
Width (mm)
Thickness (μm)
Equivalent diameter (mm)
Length/Diameter ratio
Number of fibers (/kg)
Specific surface (m2/ kg)
Density (g/ cm3)
Tensile Strength (MPa)
30 15
1.6 1.0
29 24
0.25 0.18
120 83
100,000 385,000
10
7.2
1400
Table 2 Details of specimen. ID. a
Fiber length (mm)
Volume fraction (vol %)
Compressive strength b (MPa)
AFRCC-L30V1.0 AFRCC-L30V1.5 AFRCC-L30V2.0 AFRCC-L15V1.0 AFRCC-L15V1.5 AFRCC-L15V2.0
30
1.0
59.30
1.5
54.72
2.0
48.27
1.0
51.81
1.5
51.18
2.0
49.52
15
a AFRCC-L-V: Amorphous metallic fiber-reinforced cementitious composite Fiber length - Fiber volume fraction. b Average value of compressive strength measured by 3 specimens at the age of 28 days.
strength, corrosion resistance, and wear resistance than ordinary steel fibers. In addition, they have higher residual tensile strength than hooked steel fibers, which are vulnerable to corrosion, because they are not corroded in various degradation environments, and they can improve the decrease in the durability of concrete caused by the corrosion of the fibers [14]. Amorphous metallic fibers have a high aspect ratio and a large specific surface area because they have a thin-plate shape and an excellent bonding performance with the matrix due to the rough fiber surface. Moreover, they are ultra-lightweight and the number of mixed fibers is larger compared to ordinary steel fibers under the same volume fraction. Therefore, amorphous metallic fiber-reinforced concrete has better flexural and tensile performance than hooked steel fiber-reinforced concrete. However, due to the brit tleness of the amorphous metal and the vulnerability of the thin-plate shape fibers to shear force, the fibers are subjected to fracture without
Fig. 2. Schematic diagram of dumbbell shape direct tensile test specimen.
being pulled out after the peak load [15–21]. Meanwhile, in a study on the bonding and pull out of amorphous metallic fibers with a 30 mm length, it was confirmed that the pull out and fracture behavior of the fibers varied depending on the embedded length of the fibers and the strength of the matrix [22]. Fiber-reinforced cementitious composites with high tensile strength and energy absorption capacity are attracting attention as construction materials for improving the safety performance of structures against 2
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Fig. 5. Tensile stress-strain curve and tensile parameters.
Fig. 3. Static direct tensile test equipment.
Fig. 6. Definition and calculation method of DIF.
The tensile behavior of fiber-reinforced cementitious composites is significantly affected by the bonding and pull out properties of the fibermatrix interface and the fracture properties of the fibers. Moreover, the fracture mechanism varies depending on the strain rate as with the bonding and pull out of the fiber-matrix interface. Therefore, it is necessary to evaluate the bonding and pull out behavior between the fibers and the matrix as well as the tensile strength, strain capacity, and toughness of fiber-reinforced cementitious composites under various strain rate conditions to fully understand the tensile behavior of various fiber-reinforced cementitious composites [36–39]. In the authors’ previous study, the static mechanical properties and impact resistance performance of amorphous metallic fiber-reinforced cementitious composites were evaluated. The results revealed that, under static and impact loading conditions, the fracture behaviors of the fiber and matrix interface were different [40]. In addition, the need for further study on the dynamic properties of amorphous metallic fiber-reinforced cementitious composites according to the strain rate was presented. Based on this, this study evaluates the strain rate effect on the tensile behavior of amorphous metallic fiber-reinforced cemen titious composites. Additionally, the pull-out and fracture behaviors of the fibers according to the fiber length were analyzed. In this regard, this study is novel and more advanced than the previous studies. In this study, the effects of the fiber length, fiber volume fraction, and strain rate on the direct tensile behavior of thin-plate shape amorphous
Fig. 4. Dynamic direct tensile test equipment (Strain rate: 100–101/s, Loading velocity: 5 m/s).
extreme loads, such as earthquakes, impacts, and explosions. In addi tion, many studies are being conducted to apply them as construction materials for improving the safety performance of social infrastructures. To this end, many researchers have developed special test devices, and have conducted various studies using material conditions, such as fiber mixing conditions (e.g., fiber type, fiber strength, and fiber volume fraction) matrix strength, and loading conditions, such as the strain rate and impact load, as variables. Representative studies include the anal ysis of the fracture mechanism by the impact of projectiles, analysis of collisions using finite element analysis software, and studies on tensile behavior under high strain rate conditions [23–35]. 3
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Fig. 7. Tensile stress-strain curve and crack pattern at static load.
metallic fiber-reinforced cementitious composites were investigated. Fiber-reinforced cementitious composites were fabricated using thinplate shape amorphous metallic fibers with different lengths and aspect ratios, and their direct tensile behavior was evaluated at the strain rates of 10 6/s (static) and 101/s (high strain rate). In addition, the effects of the bonding and pull out between the fibers and the matrix and the fracture properties of the fibers on the direct tensile behavior of the amorphous metallic fiber-reinforced cementitious composites (AFRCCs) were analyzed according to the fiber length, fiber volume fraction, and strain rate.
385,000, which was approximately 3.85 times larger than that of the amorphous metallic fibers with a 30 mm length under the same mixing amount. Table 2 shows the experimental conditions for evaluating the direct tensile behavior of the AFRCCS according to the fiber length and the static compressive strength of the age of 28 days. The amorphous metallic fibers with 15 and 30 mm lengths were mixed at the volume fractions of 1.0, 1.5, and 2.0 vol%. In the compressive strength test, three cylindrical specimens with a 100 mm diameter and a 200 mm height were used for each condition. All the specimens met the target strength at the age of 28 days [41]. The mix proportion of AFRCC and mechanical properties of the used materials are the same as those in the authors’ previous papers [27–29,40].
2. Experimental design and method 2.1. Materials and mixture proportions
2.2. Preparation of specimens
Fig. 1 and Table 1 show the shapes and mechanical properties of the amorphous metallic fibers used in this study. The amorphous metallic fibers used were produced by Saint Gobain SEVA in France. The amor phous metallic fibers had a 7.2 g/cm3 density, a 1400 MPa tensile strength, and the thin-plate shape. The amorphous metallic fibers with a 30 mm length had a 1.6 mm width, 29 μm thickness, and 120 aspect ratio. The number of fibers per kg was 100,000. On the other hand, the amorphous metallic fibers with a 15 mm length had a 1.0 mm width, a 24 μm thickness, and an 83 aspect ratio. The number of fibers per kg was
For the mixing of fiber-reinforced cementitious composites, the binders (cement þ fly ash) and silica sand were dry mixed and then water and admixture (plasticizer and AE agent) were added to fabricate the mortar base. Later, the fibers were added and sufficient mixing was performed so that the cementitious composites could meet the pre scribed fluidity in the uncured state. The uncured fiber-reinforced cementitious composites were poured into a mold and cured in a con stant temperature and humidity chamber with a 23 � 2 � C temperature 4
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Fig. 8. Fracture shape of amorphous metallic fiber according to fiber length.
and a 60 � 5% humidity for one day. All the specimens were then demolded and subjected to water curing until the age of 28 days. Fig. 2 shows the schematic diagram of the dumbbell shape direct tensile test specimen. The specimen had a 400 mm length, 100 mm width, and 25 mm thickness. Its center section has a 50 mm width and 25 mm thickness. To control the occurrence of cracks in the center section of the specimen with reduced width due to the stress concen tration and to induce the occurrence of cracks within the deformation measurement range, wire mesh was applied to both ends of the specimen to produce a direct tensile test specimen. Ten specimens were fabricated for each test level, and direct tensile tests were conducted using five
specimens at each of the strain rates of 10
6
/s and 101/s.
2.3. Test methodology Fig. 3 shows the static direct tensile test equipment with a 250 kN capacity. The loading rate of the static direct tensile test was set to 1 mm/min, and the tensile deformation of each specimen was measured by installing LVDT displacement meters on the left and right sides of the specimen. Ball bearings were installed on the upper and lower sections of the tensile jig to control the eccentricity of the test specimen during the direct tensile test. Additionally, the test specimen was also secured to 5
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Fig. 9. Tensile stress-strain curve and crack pattern at high strain rate.
a jig to prevent it from being moved during the test. Fig. 4 shows the dynamic direct tensile test equipment with a 101/s average strain rate, a 5 m/sec. loading rate, and a 3000 kN maximum loading capacity. As this equipment is typically used for conducting uniaxial compression tests, direct tensile jig was installed to conduct direct tensile tests. The compressive load was transferred to the specimen as a tensile load through the direct tensile jig. The direct tensile stress was measured using a 500 kN-capacity load cell installed at the top of the direct tensile jig, and the tensile deformation was measured by installing LVDT displacement meters on the specimen. The stress and strain data of the fiber-reinforced cementitious composites in the dynamic direct tensile test were collected at a sampling rate of 30,000 Hz using a high-speed data logger. Fig. 5 shows the stress-strain curve and direct tensile properties. As for the direct tensile properties, the tensile strength (maximum stress point, fp ), strain capacity (strain at the peak tensile strength, δp ), and tensile toughness (area of the stress-strain curve) were evaluated. The tensile toughness was divided into the peak toughness and fracture toughness. The peak toughness was obtained through the area of OAB while the fracture toughness was obtained through the area of OAC. The dynamic increase factor (DIF) was obtained by dividing the tensile property value measured at the high strain rate of 101/s by the tensile property value measured at the static strain rate (10 6/s), as shown in Fig. 6. The DIF obtained in this study was compared with that of the
proposed equation in the CEB-FIP model code 2010 [42]. 3. Results and discussion 3.1. Static direct tensile stress-strain curve and fracture shape (strain rate 10 6/s) Fig. 7 shows the tensile stress-strain curves and crack patterns of the AFRCCs according to the fiber length and fiber volume fraction under the static loading condition. Under all test conditions, strain-hardening behavior was observed along with multiple cracks after the initial cracking. For AFRCC-L30, the bonding performance with the matrix was excellent and the stress dispersion effect was large due to the high aspect ratio of the fibers and high specific surface area, and thus, the strainhardening behavior with the strain capacity higher than 0.5% was observed. In addition, the tensile strength and strain capacity were higher and the number of multiple cracks was larger compared to AFRCC-L15. However, the strain capacity and the number of multiple cracks showed a tendency to slightly decrease as the fiber volume fraction increased, even though the tensile strength increased. For AFRCC-L15, on the other hand, both the tensile strength and strain ca pacity increased as the fiber volume fraction increased. Fig. 8 shows the fracture shapes of amorphous metallic fibers in the tensile fracture section according to the fiber length. In the case of the 6
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Table 3 Test data of direct tensile properties and DIF. ID.
Strain rate
AFRCC-L30-V1.0
Static
AFRCC-L30-V1.5
Static
AFRCC-L30-V2.0
Static
AFRCC-L15-V1.0
Static
AFRCC-L15-V1.5
Static
AFRCC-L15-V2.0
Static
0.000001 4.204 5.200 3.539 4.565 13.856 6.274 0.000001 7.951 5.757 9.818 6.641 4.116 6.863 0.000001 10.009 9.865 7.499 10.742 12.260 10.085 0.000001 3.191 7.068 9.676 4.029 3.897 5.571 0.000001 6.321 5.437 5.802 7.811 8.906 6.867 0.000001 10.724 8.216 11.259 8.040 9.798 9.617
Test No.
Tensile strength
Strain capacity
Peak toughness
MPa
DIF
%
DIF
kN⋅mm
DIF
kN⋅mm
DIF
Ave. No.1 No.2 No.3 No.4 No.5 Ave. Ave. No.1 No.2 No.3 No.4 No.5 Ave. Ave. No.1 No.2 No.3 No.4 No.5 Ave. Ave. No.1 No.2 No.3 No.4 No.5 Ave. Ave. No.1 No.2 No.3 No.4 No.5 Ave. Ave. No.1 No.2 No.3 No.4 No.5 Ave.
4.70 6.05 7.96 7.05 7.45 7.83 7.27 7.10 10.90 10.40 9.44 9.42 11.40 10.31 8.05 13.50 12.12 12.08 12.50 14.23 12.89 4.62 7.35 6.92 6.62 7.67 6.64 7.04 6.84 9.60 10.58 10.06 9.77 9.51 9.90 7.07 11.27 10.72 11.92 10.81 12.67 11.48
1.29 1.69 1.50 1.59 1.67 1.55 1.53 1.46 1.33 1.33 1.60 1.45 1.68 1.50 1.50 1.55 1.77 1.60 1.59 1.50 1.43 1.66 1.44 1.52 1.40 1.55 1.47 1.43 1.39 1.45 1.59 1.52 1.69 1.53 1.79 1.62
0.659 0.673 0.565 0.537 0.525 0.865 0.633 0.505 0.635 0.726 0.686 0.675 0.685 0.682 0.504 1.141 1.064 0.451 0.937 0.551 0.829 0.188 0.368 0.374 0.186 0.323 0.286 0.307 0.284 0.328 0.433 0.307 0.501 0.420 0.398 0.361 0.812 0.981 0.613 0.821 0.705 0.786
1.02 0.86 0.82 0.80 1.31 0.96 1.26 1.44 1.36 1.34 1.36 1.35 2.26 2.11 0.90 1.86 1.09 1.65 1.96 1.99 0.99 1.72 1.52 1.64 1.15 1.52 1.08 1.76 1.48 1.40 2.25 2.72 1.70 2.28 1.95 2.18
2.96 4.45 4.47 3.74 4.23 7.90 4.96 3.26 8.08 8.52 7.52 6.93 8.51 7.91 3.91 15.78 13.07 6.21 12.49 6.90 10.89 0.94 2.80 2.83 1.48 2.67 2.16 2.39 2.07 4.73 7.45 5.26 5.34 6.49 5.85 2.65 9.31 11.12 7.94 9.41 9.32 9.42
1.51 1.51 1.27 1.43 2.67 1.68 2.24 2.69 2.47 2.21 2.56 2.43 4.04 3.35 1.59 3.20 1.77 2.79 2.97 3.01 1.57 2.84 2.29 2.54 2.29 3.61 2.55 2.58 3.14 2.83 3.51 4.19 2.99 3.55 3.51 3.55
8.00 7.91 8.31 5.31 5.72 10.49 7.55 13.33 11.86 11.26 11.40 9.69 10.93 11.03 14.35 21.37 18.64 15.73 16.21 15.93 17.58 5.13 4.21 6.80 5.05 5.29 7.23 5.72 8.54 12.00 11.28 10.93 9.20 13.16 11.31 12.63 16.08 18.88 14.87 16.59 17.30 16.74
0.99 1.04 0.66 0.71 1.31 0.94 0.89 0.84 0.86 0.73 0.82 0.83 1.49 1.30 1.10 1.13 1.11 1.23 0.82 1.33 0.98 1.03 1.41 1.11 1.40 1.32 1.28 1.08 1.54 1.32 1.27 1.50 1.18 1.31 1.37 1.33
amorphous metallic fibers with a 30 mm length under the static loading condition, the bonding performance with the matrix was excellent because of the rough fiber surface, large specific surface area, and high aspect ratio. The fibers, however, were subjected to fracture without being pulled out from the matrix because the thin-plate shape of the fibers was vulnerable to shear force. As the fiber volume fraction increased, however, the bonding efficiency of the fiber-matrix interface decreased because the amount of the matrix surrounding a single fiber decreased. Therefore, the number of fibers subjected to fracture decreased and pulled out fibers were observed. In the case of the amorphous metallic fibers with a 15 mm length, on the other hand, the aspect ratio of the fibers was smaller and the number of mixed fibers was larger compared to the amorphous metallic fibers with a 30 mm length. Therefore, the bonding efficiency with the matrix decreased and the behavior where the fibers were pulled out from the matrix was observed.
Fracture toughness
capacity increased. On the other hand, the stress reduction in the strainsoftening region after the peak stress occurred more sharply in all the specimens compared to the static loading condition. It appears that the time required to reach the final fracture of the specimen was shorter under the high strain rate because high loading rate was maintained while the fracture of the fibers or the pull out of the fibers slowly occurred between the crack openings after the peak stress under the static loading condition. Even under the high strain rate condition of 101/s, the amorphous metallic fibers with a 30 mm length were subjected to fracture without being pulled out from the matrix as with the static loading condition, and the amorphous metallic fibers with a 15 mm length exhibited the behavior where the fibers were pulled out from the matrix. As the strain rate increased, however, the number of pulled-out fibers decreased and the number of the fibers subjected to fracture increased because the bonding performance between the fibers and the matrix was improved.
3.2. Dynamic direct tensile stress-strain curve and fracture shape (strain rate 101/s)
3.3. Direct tensile properties according to fiber length, volume fraction and strain rate
Fig. 9 shows the average values of the tensile stress-strain curves and crack patterns of the AFRCCs measured using five specimens under the high strain rate condition. As the strain rate increased, the strainhardening behavior became more obvious and the tensile strength increased because the bonding performance of the fiber-matrix interface was improved. In addition, the number of multiple cracks and the strain
Table 3 summarizes the measured tensile strength, strain capacity, and tensile toughness data of the AFRCCs according to the fiber length, fiber volume fraction, and strain rate as well as DIF for each tensile property. In addition, Fig. 10 shows the tensile properties according to the fiber length, fiber volume fraction, and strain rate. Under the static 7
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Fig. 10. Tensile properties according to fiber length and strain rate.
loading condition, the tensile strengths of AFRCC-L30-V1.0, AFRCCL30-V1.5, and AFRCC-L30-V2.0 were 4.70, 7.10, and 8.05 MPa, while those of AFRCC-L15-V1.0, AFRCC-L15-V1.5, and AFRCC-L15-V2.0 were 4.62, 6.84, and 7.07 MPa, respectively. On the other hand, under the high strain rate condition, the tensile strengths of AFRCC-L30-V1.0, AFRCC-L30-V1.5, and AFRCC-L30-V2.0 were 7.27, 10.31, and 12.89 MPa, while those of AFRCC-L15-V1.0, AFRCC-L15-V1.5, and AFRCC-L15-V2.0 were 7.04, 9.90, and 11.48 MPa, respectively. Under the same fiber volume fraction and strain rate conditions, as the aspect ratio of the fibers increased, the tensile strength of AFRCC-L30 was higher than that of AFRCC-L15 because the crosslinking reaction range and the internal binding force of the fibers increased. Meanwhile, the tensile strength increase ratio for the 0.5% increase in the fiber volume fraction was 151% for the increase from AFRCC-L30V1.0 to AFRCC-L30-V1.5 and 113% for the increase from AFRCC-L30V1.5 to AFRCC-L30-V2.0, 148% for the increase from AFRCC-L15V1.0 to AFRCC-L15-V1.5, and 103% for the increase from AFRCC-L15V1.5 to AFRCC-L15-V2.0 under the static loading condition. Under the high strain rate condition, the tensile strength increased by 142% for the increase from AFRCC-L30-V1.0 to AFRCC-L30-V1.5, 125% for the in crease from AFRCC-L30-V1.5 to AFRCC-L30-V2.0, 141% for the increase from AFRCC-L15-V1.0 to AFRCC-L15-V1.5, and 116% for the increase from AFRCC-L15-V1.5 to AFRCC-L15-V2.0. As the fiber volume fraction increased, the tensile strength showed a tendency to increase. However,
the tensile strength increase ratio for the fiber volume fraction increase from 1.0% to 1.5% was higher than that for the fiber volume fraction increase from 1.5% to 2.0%. It appears that when the fiber volume fraction is higher than 1.5%, the tensile strength increase ratio decreases alongside the decrease in the bonding efficiency of the fiber-matrix interface because the amount of the matrix surrounding a single fiber decreases as the number of the mixed fibers increases. On the other hand, the increase in the strain rate improved the strengths of the matrix and the fibers as well as the bonding performance between the matrix and the fibers, thereby increasing the tensile strength under all test conditions. The tensile strength increase ratio according to the strain rate is explained in detail through the DIF comparison. In the case of the strain capacity according to the fiber length, AFRCC-L30 showed higher strain capacity values than AFRCC-L15 under both static and high strain rate conditions as shown in Fig. 10 (b). AFRCC-L30 has higher internal binding force and a larger cross linking reaction range inside the matrix because the aspect ratio of the fibers is higher. This appears to increase the stress dispersion effect, strain-hardening behavior, and strain capacity. In addition, the fibers were subjected to fracture without being pulled out from the matrix. It appears that the strain capacity was high because cracks were first induced in the weak part of the matrix and a large number of multiple cracks were generated before the fibers between the crack openings were subjected to fracture in the course of crack occurrence and 8
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Fig. 11. DIF of tensile properties according to fiber length.
propagation in the matrix. In the case of AFRCC-L15, however, it ap pears that the number of multiple cracks and the strain capacity were smaller compared to AFRCC-L30 because the crosslinking reaction range was small in the crack openings of the matrix due to the low aspect ratio of the fibers and the fibers were pulled out from the matrix occurred. Under the static loading condition, AFRCC-L30 exhibited a decrease in the number of the fibers subjected to fracture and an increase in the number of the pulled out fibers as the fiber volume fraction increased because the amount of the matrix surrounding a single fiber decreased; thus, the bonding efficiency between the fibers and the matrix decreased. Owing to this change in the fracture mechanism of the fibers, the number of multiple cracks and strain capacity showed a tendency to decrease slightly. Under the high strain rate, on the other hand, the bonding performance between the fibers and the matrix was improved by the increase in the strain rate. Therefore, the number of the fibers subjected to fracture increased, and the strain capacity also showed a tendency to increase as the fiber volume fraction increased. In the case of AFRCC-L15, the strain capacity increased as the fiber volume fraction increased under both the static and high strain rate conditions. The peak toughness and fracture toughness are closely related to the tensile strength and strain capacity because they are calculated using the areas of the stress-strain curve. Therefore, as the tensile strength and strain capacity of AFRCC-L30 are higher than those of AFRCC-L15, the
tensile toughness was also higher. In addition, the tensile toughness also showed a tendency to increase as the fiber volume fraction increased. For the amorphous metallic fibers, it was found that the fracture of the fibers by securing sufficient bonding performance between the fibers and the matrix had a larger effect on the improvement of the tensile strength, strain capacity, and tensile toughness than the pull out of the fibers from the matrix. Therefore, the optimization of the tensile strength, strain capacity, and tensile toughness of amorphous metallic fiber-reinforced cementitious composites can be achieved by deriving the optimum mixing ratio considering the length and volume fraction of the amorphous metallic fibers and the amount of the matrix. 3.4. Dynamic increase factor according to fiber length Fig. 11 shows the DIF of tensile properties according to the fiber length. The DIF of AFRCC-L15 showed a tendency to be higher than that of AFRCC-L30 for all evaluation factors. Previous studies on the tensile behavior of fiber-reinforced cementitious composites according to the strain rate have reported that the increase in the strain rate significantly affects the bonding of the fiber-matrix interface as well as the pull out behavior of the fibers [26,27,30–32]. The major tensile fracture behavior of AFRCC-L30 is affected more by the fiber strength than by the bonding of the fiber-matrix interface because the fibers are subjected to 9
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Fig. 11. (continued).
fracture without being pulled out from the matrix. For AFRCC-L15, on the other hand, it appears that the bonding of the fiber-matrix interface and the pull out behavior of the fibers have a significant effect on the tensile behavior of fiber-reinforced cementitious composites because the fibers show the behavior where they are pulled out from the matrix without being subjected to fracture. Therefore, it is judged that the effect of the strain rate increase on the improvement of tensile properties is significant for AFRCC-L15 because the bonding efficiency of the fiber-matrix interface, which is affected by the strain rate, is relatively higher than that of AFRCC-L30.
shape of the fibers was vulnerable to shear force. On the other hand, the amorphous metallic fibers with a 15 mm length and a 83 aspect ratio showed the behavior where the fibers were pulled out from the matrix because the aspect ratio of the fibers was low and the number of mixed fibers was large. (2) As the aspect ratio of the fibers increased, the tensile strength of AFRCC-L30 was higher than that of AFRCC-L15 because of the higher internal binding force and larger crosslinking reaction range inside the matrix as well as the higher stress dispersion effect. In addition, it appears that the strain capacity of AFRCCL30 was high because cracks were induced in the vulnerable part of the matrix, resulting in the generation of a large number of multiple cracks and the increase in the strain-hardening behavior before the fibers between the crack openings were subjected to fracture. In the case of AFRCC-L15, on the other hand, it appears that the number of multiple cracks and the strain capacity were small because the crosslinking reaction range was small in the crack openings of the matrix due to the low aspect ratio of the fibers and the fracture behavior occurred where the fibers were pulled out from the matrix. (3) AFRCC-L30 was affected more by the fiber strength than by the bonding between the fibers and the matrix because the fibers exhibited fracture behavior without being pulled out from the matrix. For AFRCC-L15, on the other hand, the bonding
4. Conclusions In this study, the effects of the fiber length, fiber volume fraction, and strain rate on the direct tensile behavior of amorphous metallic fiberreinforced cementitious composites (AFRCCs) were evaluated. The re sults were as follows. (1) Under the static and high strain rate conditions, the amorphous metallic fibers with a 30 mm length and a 120 aspect ratio exhibited excellent bonding performance with the matrix because of their rough fiber surface, large specific surface area, and high aspect ratio. The fibers, however, were subjected to fracture without being pulled out from the matrix because the thin-plate 10
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performance of the fiber-matrix interface was significantly affected by the strain rate because the fracture behavior occurred where the fibers were pulled out from the matrix without being subjected to fracture. Therefore, it is judged that the effect of the strain rate increase on the improvement of tensile properties is significant for AFRCC-L15 because its bonding efficiency of the fiber-matrix interface, which is affected by the strain rate, is relatively high.
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For the amorphous metallic fibers, it was found that the fracture of the fibers by securing sufficient bonding performance between the fibers and the matrix had a larger impact on the improvement of the tensile strength, strain capacity, and tensile toughness than the pull out of the fibers from the matrix. On the other hand, for the sensitivity of the strain rate, the pull out behavior, which is the bonding at the fiber-matrix interface, had a larger impact than the fracture of the fibers. There fore, it is important to consider the length and volume fraction of amorphous metallic fibers as well as the amount of the matrix to derive the optimum mixing ratio in consideration of the direct tensile proper ties of AFRCCs, such as the tensile strength, strain capacity, and tensile toughness, as well as DIF according to the strain rate. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(2017R1D1A1B03034776). References [1] Kanda T, Li VC. Interface property and apparent strength of high strength hydrophilic fiber in cement matrix. J Mater Civ Eng 1998;10:5–13. [2] Bencardino F, Rizzuti L, Spadea G, Swamy RN. Implications of test methodology on post-cracking and fracture behaviour of steel fibre reinforced concrete. Compos B Eng 2013;46:31–8. € The effects of macro synthetic fiber [3] Bolat H, S¸ims¸ek O, Çullu M, Durmus¸ G. Can O. reinforcement use on physical and mechanical properties of concrete. Compos B Eng 2014;61:191–8. [4] Soroushian P, Tlili A, Alhozaimy A, Khan A. Development and characterization of hybrid polyethylene fiber reinforced cement composites. ACI Mater J 1993;90: 182–90. [5] Maalej M, Li VC. Introduction of strain hardening engineered cementitious composites in the design of reinforced concrete flexural members for improved durability. Am Concr Inst Struct J 1995;92:167–76. [6] Naaman AE, Shah SP. Pull-out mechanism in steel fiber-reinforced concrete. J Struct Div 1976;102(8):1537–48. [7] Betterman LR, Ouyang C, Shah SP. Fiber-matrix interaction in micro fiber reinforced mortar. Adv Cem Base Mater 1995;2(2):53–61. [8] Rossi P, Harrouche N. Mix design and mechanical behaviour of some steel fibrereinforced concretes used in reinforced concrete structures. Mater Struct 1990;23 (4):256–66. [9] Chan YW, Chu SH. Effect of silica fume on steel fiber bond characteristics in reactive powder concrete. Cement Concr Res 2004;34(7):1167–72. [10] Li VC, Wang Y, Backer S. Effect of inclining angle, bundling and surface treatment on synthetic fibre pull-out from a cement matrix. Composites 1990;21(2):132–40. [11] Hameed R, Turatsinze A, Duprat F, Sellier A. Study on the flexural properties of metallic-hybrid-fiber reinforced concrete. Maejo Int J Sci Technol 2010;4(02): 169–84. [12] Won JP, Hong BT, Choi TJ, Lee SJ, Kang JW. Flexural behaviour of amorphous micro-steel fibre-reinforced cement composites. Compos Struct 2012;94:1443–9. [13] Won JP, Hong BT, Lee SJ, Choi SJ. Bonding properties of amorphous micro-steel fibre-reinforced cementitious composites. Compos Struct 2013;102:101–9. [14] Choi SJ, Hong BT, Lee SJ, Won JP. Shrinkage and corrosion resistance of amorphous metallic-fiber-reinforced cement composites. Compos Struct 2014;107: 537–43.
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