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Effect of different cross-section types on mechanical properties of carbon fibers-reinforced cement composites
Materials Science and Engineering A366 (2004) 348–355
Effect of different cross-section types on mechanical properties of carbon fibers-reinforced cement composites Soo-Jin Park a,∗ , Min-Kang Seo a , Hwan-Boh Shim a , Kyong-Yop Rhee b a
Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-600, South Korea b School of Mechanical and Industrial System Engineering, Kyunghee University, Yongin 449-701, South Korea Received 29 March 2003; received in revised form 22 August 2003
Abstract To study the effect of shape factor, the mechanical properties of three shapes of carbon fiber, i.e., round, H-shape, and C-shape randomly oriented carbon fibers-reinforced cement composites (CFRCCs) were investigated. As a result, C-shape CFRCC showed higher tensile and flexural strength than any other shape carbon fibers-reinforced composites. It was noted that C-shape CFRCC presented stronger fiber-matrix interfacial adhesive forces, due to the mechanical anchorage into the matrix. However, the compressive strength of the CFRCC was decreased with increasing the aspect ratio and fiber volume fraction. This was probably due to the fact that the amounts of entrained air contents were increased during the mixing of each fiber. Also, both the tensile and flexural strength of the CFRCC were significantly increased by the additions of fumed silica to the composites. © 2003 Elsevier B.V. All rights reserved. Keywords: Shape factor; Mechanical properties; Carbon fibers-reinforced cement composites; Aspect ratio; Fiber volume fraction
1. Introduction Fibers-reinforced cement-matrix composites are structural materials that are gaining in importance rapidly due to the increasing the demand of superior structural and functional properties. Especially, the carbon fibers are one of the most common for reinforcing cementitious materials because of their excellent mechanical properties [1,2]. When carbon fibers are incorporated into the cementitious matrix, some properties of carbon fibers-reinforced cement-matrix composites (CFRCCs) are improved, including the tensile strength, flexural strength, compressive strength, toughness, and drying shrinkage, etc. [3,4]. Therefore, the carbon fibers are generally used as a reinforcement to enhance the mechanical properties of a brittle matrix, such as, cement paste and especially non-round shaped carbon fibers are more effective for strengthening than round shaped one [5–7]. The mechanical properties of CFRCC are influenced by carbon fiber loading, fiber length, fiber shape, and fiber thickness, etc. [8–10]. Wen and Chung [11] reported that ∗ Corresponding author. Tel.: +82-42-861-4151; fax: +82-42-861-4151. E-mail address: [email protected] (S.-J. Park).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.08.123
the mechanical properties of pitch-based carbon fibersreinforced cement composites are improved by the preparation of non-circular shape carbon fibers, such as, C-shape and Y-shape. Fortin et al. [12] also reported that the mechanical properties of carbon fibers-reinforced cement composites are largely depended on the transverse alignment of graphene layers within slit type carbon fibers. Generally, CFRCC composites contain short carbon fibers, typically 5 mm in length, as the short fibers can be used as an admixture in concrete and short fibers are less expensive than continuous fibers. However, due to the their weak bond between carbon fibers and cement matrix, continuous fibers are much more effective than short fibers in reinforcing concrete in spite of the complex added to the concrete mixtures [13–16]. And the admixtures, such as, latex, methylcellulose, and silica fume also help the bond of elements in the composites. Although the addition of carbon fibers to the concrete may increase in air void content, resulting in lowering the thermal conductivity, the mechanical strengths of the concrete are largely increased with increasing the fiber volume fraction [17]. And, the effective use of carbon fibers in concrete requires the dispersion of the fibers in the mixture and also the good dispersion (a fine particulate) of the silica fume is essential to enhance the
S.-J. Park et al. / Materials Science and Engineering A366 (2004) 348–355
mechanical properties of the composites as an admixture [18,19]. In common, C and H-shape carbon fibers have higher specific surface area than that of conventional round-shape carbon fibers. This result can probably affect the mechanical properties of the composites. In the present study, we have investigated the composites made with different shape carbon fibers. The aim of the study is to investigate the relationship between the fiber shapes and the mechanical properties in the carbon fibers-reinforced cement-matrix composites with experimental different-shape carbon fibers as reinforcements. Therefore, three different shapes of carbon fibers, such as, C, H, and round shapes, are used to reinforce a cementitious matrix containing lightweight aggregates with and without silica fume in order to enhance the mechanical bonds. And the flexural, tensile, and compressive properties of the cement composites reinforced with randomly oriented shapes of carbon fibers are studied in order to investigate the effect of the fiber shape, fiber volume fraction, and aspect ratio on mechanical strengths and microstructures of the composites.
349
L
d2 D
d2
L
d
2. Experimental The isotropic round, hollow, and C-shape carbon fibers were manufactured by the melt-spun of isotropic pitch made by heat treatment for 3 h at 390 ◦ C and 3 h at 350 ◦ C from Naphta cracking bottom oil in a pilot-scale. Schematic diagrams of C-shape and round-shape carbon fibers were shown in Fig. 1 [13]. The lightweight cementitious composites were prepared with ordinary Portland cement (OPC, Ssangyong Cement of Korea), lightweight fine aggregates (Sirasu Balloon, MSB301 Japan, and Micro cell, SL150 Austria, bulk density 0.35–0.4, fineness maximum 180 m), and silica fume
Fig. 1. Schematic diagram of the round and C-shape carbon fiber used.
(Blaine specific surface area from BET’s equation 20 m2 /g [20]). The typical formulation of cementitious matrix of CFRCC was shown in Table 1. The Carbon fibers were incorporated into the above lightweight cementitious composites. Table 2 showed the properties of the carbon fibers used. And the particle size distribution of cement and fine fillers (aggregates) was plotted in Fig. 2.
Table 1 Mixing ratios of cement matrix for CFRCC (unit: wt.%) Ordinary cement
100 a
Fine aggregates
Admixtures (C, wt.%)
Sirsu ballon
Micro cell
Fumed silica
BMC
A-803
M-150
15
5
16
0.25
0.5
3.0
Carbon fibers
W/C
1 – 3 Vf %, 3 – 25 mm, C, H, Ra
0.465
C: C-shape carbon fiber; H: H-shape carbon fiber; R: round-shape carbon fiber.
Table 2 Properties of the carbon fibers used in this work Fiber shapes
Tensile strength (MPa)
Elastic modulus (GPa)
Elongation (%)
Diameter (m)
Cross-sectional area (m2 ) 120◦ )
Specific gravity
C
920
102
0.9
40.2 (Do 22.8 (Di )b
594 (open θ =
H
830
118
0.7
36.1 (Do )a 23.8 (Di )b
590
1.78
860
72
1.2
26.8
564
1.74
R a b
Do : outer diameter. Di : inner diameter.
)a
1.76
350
S.-J. Park et al. / Materials Science and Engineering A366 (2004) 348–355
Sirasu Balloon Micro cell Portland cement
Passing quantity (%)
100
80
60
40
20
0 1
10
100
1000
Particle size (µm) Fig. 2. Pore size distribution of the cement and fine fillers used.
Specimens were prepared according to the following process: firstly, carbon fibers and dry materials were mixed for 5 min, water was added to the dry mixtures, and they were mixed for 5 min in an Omni mixer. Secondly, the wet mixtures were cast for experimental demand size. Finally, the cast samples were pre-cured at 20 ◦ C with 80% RH for 2 days, autoclave-cured at 180 ◦ C with 7 MPa for 4 h to improve the packing density of the composites at the same water/binder ratio, and then dried at room temperature/70% for 14 days.
The three-point loading flexural tests were carried out using Instron model 1125 tester according to the requirements of ASTM D-790-92. The span-to-depth ratio was 16:1 scale and cross-head speed was 0.5 mm/min. The dimension of the specimens was 50 mm (length) × 20 mm (width) × 2 mm (thickness). Tensile tests were carried out on an Instron model 1125 tester, installed with hydraulic grips, following ASTM standard D3039-93 (standard test method for tensile properties of polymer matrix composite materials) at a cross-head speed of
Flexural strength (MPa)
12
9
6 R-shape C-shape H-shape R-shape+fumed silica C-shape+fumed silica H-shape+fumed silica
3
no fibers 0 0
1
2
Fiber volume fraction (%) Fig. 3. Flexural strength of CFRCC as a function of fiber volume fraction.
3
S.-J. Park et al. / Materials Science and Engineering A366 (2004) 348–355
2 mm/min. The dimension of each tensile test specimen was 200 mm (length) × 25 mm (width) × 2 mm (thickness). The compressive strength was measured by using Instron model 1125 tester. Compression experiments were performed at a crosshead speed of 1 mm/min in an Instron test machine (Instron model 1125 tester), using samples of dimensions 40 mm (length) × 40 mm (width) × 40 mm (thickness) according to the ASTM D-695-58. Six specimens were tested for all the experiments and the average value was taken in the present work studied. The surface sections and fracture surfaces of the composites were examined by scanning electron microscope (SEM).
3. Results and discussion Fig. 3 shows the flexural strength of CFRCC as a function of fiber volume fraction. The flexural strength of CFRCC is increased with increasing the fiber volume fraction and the CFRCC containing fumed silica shows higher flexural strength than that of the CFRCC without fumed silica because of increasing the interfacial areas between fibers and matrix, resulting from the densification of microstructure [21,22]. Also, the C-CFRCC presents higher flexural strengths than those of R- or H-CFRCC, regardless of the presence of fumed silica, as shown in Fig. 3. Especially, the C-CFRCC containing fumed silica shows the highest flexural strength, about 40% higher than R- and H-CFRCC at a fiber volume fraction of 3%. This is probably due to the fact that the inside space of C-shape carbon fibers is compacted by the matrix and the fibers, thus, act as a mechanical anchor, resulting from the C-shape carbon fibers having a large interface between fibers and matrix, as shown in Fig. 5. And, H-CFRCC without fumed silica shows higher flexural
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strength than R-CFRCC, while H- and R-CFRCC containing fumed silica show almost the same flexural strength. Fig. 4 shows the flexural strength of 2% fibers loaded CFRCC as a function of aspect ratio. The C-CFRCC presents high flexural strength and an optimum aspect ratio, i.e., 150 and 250 for with and without fumed silica, respectively. The reason is that the fibers are easily agglomerated together during the mixing process when they are too long, resulting in decreasing the dispersability and flowability, and increasing the air entrapped in the matrix [23]. The aspect ratio of R-CFRCC is less significant for the flexural strength than that of C-CFRCC. And the flexural strength of H-CFRCC is decreased with increasing the aspect ratio because it is difficult for the matrix to penetrate the hole of fiber by capillary force when the fibers are too long. Consequently, the fumed silica can greatly influence the improving the flexural strength of CFRCC, as do fibers loading in Fig. 3. Fig. 5 shows SEM micrographs for the different carbon fiber shapes incorporated into CFRCC containing fumed silica. The shape of fibers significantly influences the interfacial area between fibers and matrix. Especially, the C-shape carbon fibers mechanically anchor in the matrix and increase the interfacial contact area, resulting in improving the mechanical properties of C-CFRCC. And the H-shape of carbon fibers also shows the compaction of the hole of fibers by the matrix, which is acted as a mechanical anchor when the fiber is short enough for the matrix to penetrate. Fig. 6 shows the tensile strength of CFRCC as a function of fiber volume fraction. The tensile strength of CFRCC is proportionally increased with increasing the fiber loading and shown a similar trend compared to the flexural strength, as shown in Fig. 3. The fumed silica is also a significant factor for increasing the tensile strength of CFRCC. This fumed silica, added to the cement matrix, probably can penetrate
Flexural strength (MPa)
12
9
6
R-shape C-shape H-shape R-shape+fumed silica C-shape+fumed silica H-shape+fumed silica
3
0 0
200
400
600
Aspect ratio (L/D) Fig. 4. Flexural strength of 2% fibers loaded CFRCC as a function of aspect ratio.
800
352
S.-J. Park et al. / Materials Science and Engineering A366 (2004) 348–355
Fig. 5. SEM micrographs of surface section for the different shapes of CFRCC.
Tensile strength (MPa)
8
6
4 R-shape C-shape H-shape R-shape+fumed silica C-shape+fumed silica H-shape+fumed silica
2
no fibers 0 0
1
2
Fiber volume fraction (%) Fig. 6. Tensile strength of CFRCC as a function of fiber volume fraction.
3
S.-J. Park et al. / Materials Science and Engineering A366 (2004) 348–355
353
Tensile strength (MPa)
8
R-shape C-shape H-shape R-shape+fumed silica C-shape+fumed silica H-shape+fumed silica 6
4
0 0
200
400
600
800
Aspect ratio (L/D) Fig. 7. Tensile strength of 2% fibers loaded CFRCC as a function of aspect ratio.
well into a hole or surface groove of fibers and densify the matrix, resulting in improving the fiber-matrix interfacial bonding zone [24–26]. And C-CFRCC presents the highest tensile strength at fiber loading volume of 3% with fumed silica. Especially, in case of the H-shape fiber, the tensile strength is more strongly anchored in the matrix when tensile stress is applied to the composites. Therefore, the increasing ratio of tensile strength is increased with increasing the fiber loading volume compared to that of flexural strength in CFRCC [27]. And R-CFRCC has the lowest tensile strength.
Fig. 7 shows the tensile strengths of 2% fibers loaded CFRCC as a function of aspect ratio. C-CFRCC shows the highest tensile strength at 250 aspect ratio, regardless of the presence of fumed silica, and also all CFRCC present the highest value near at 250 aspect ratio. However, the tensile strength of CFRCC is decreased at a high aspect ratio greater than 250 values because of the formation of bulk fibers and increasing the porosity in the matrix [28]. R-CFRCC presents higher tensile strength than H-CFRCC at high aspect ratio, due to the difficult in penetrating
Compressive strength (MPa)
60
50
40
R-shape C-shape H-shape R-shape+fumed silica C-shape+fumed silica H-shape+fumed silica
30
no fibers
0 0
1
2
3
Fiber volume fraction (%) Fig. 8. Compressive strength of CFRCC as a function of fiber volume fraction.
354
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R-shape C-shape H-shape R-shape+fumed silica C-shape+fumed silica H-shape+fumed silica
Compressive strength (MPa)
60
50
40
30
0 0
200
400
600
800
Aspect ratio (L/D) Fig. 9. Compressive strength of 2% fibers loaded CFRCC as a function of aspect ratio.
Fig. 10. SEM micrographs for carbon fibers distributed in the cement matrix.
S.-J. Park et al. / Materials Science and Engineering A366 (2004) 348–355
of matrix into a hole of fiber when the fibers are too long. Consequently, the tensile strength of C-CFRCC is higher than that of R- and H-CFRCC. This is probably resulted from the structural differences. That is, it can be explained that the superior mechanical properties of C-CFRCC are resulted from the well-aligned transverse texture, which can be easily converted to the graphitic crystallinity during heat treatment, resulting in improving the resistance to failure. Fig. 8 shows the relationship between fiber volume fraction and compressive strength. In regard to all shapes of the carbon fibers, it is expected that the compressive strength of CFRCC is decreased a little as the fiber volume fraction increases. Also, Fig. 9 presents the compressive strength of CFRCC as a function of aspect ratio. As a result, the same tendency is obtained in the compressive strength. These results are probably due to the increasing the amount of air content and porosity for the mixing of each fiber and decreasing the packing density, resulting in decreasing the interfacial binding force of the composites, as shown in Fig. 5 [29]. Also, it is known that the decreasing ratio of H- and C-CFRCC is higher than that of R-CFRCC, resulting from the retained pore in the composites where C- and H-shape carbon fibers are not filled enough to cement matrix and filler. SEM micrographs for C-shape carbon fiber distributed in the cement matrix are shown in Fig. 10. The fibers are distributed homogeneously in the cement matrix and well contact with each other within an individual cluster, although the clusters are separate. 4. Conclusions We investigate the effect of different cross-sectional shapes of carbon fibers on mechanical properties of CFRCC. As a result, C-CFRCC shows the highest flexural and tensile strength than any other CFRCC. Because C-shape carbon fibers act as an anchor in the matrix, resulting in increasing the interfacial area between fibers and matrix. The 3% fibers loaded C-CFRCC containing fumed silica exhibits the improvement in flexural and tensile strength compared to those of H- and R-CFRCC because of the mechanical anchorage of H- and R-shape carbon fibers having less than
355
to C-shape carbon fibers. However, in case of the compressive strength in CFRCC, the value is decreased a little with increasing the fiber volume fraction and fiber length, and R-shape carbon fibers loaded composites have the highest value in strength. Also, we can find that the fumed silica is a greatly significant factor for improving the mechanical properties of CFRCC.
References [1] E. Fitzer, M. Heine, in: A.R. Bunsell (Ed.), Fibre Reinforced Composite Materials, vol. 2, Elsevier, Amsterdam, 1988, pp. 73–148. [2] P. Tsotra, K. Friedrich, Compos. Part A 34 (2003) 75. [3] Y. Xu, D.D.L. Chung, Carbon 39 (2001) 1995. [4] T.O. Mason, M.A. Campo, A.D. Hixson, L.Y. Woo, Cem. Concr. Compos. 24 (2002) 457. [5] H.A. Toutanji, T. El-Korchi, R.N. Katz, Cem. Concr. Res. 23 (1993) 618. [6] T. Fukushima, K. Shiratama, K. Hitotsuya, T. Maruyama, Fiber Reinforced Cem. Concr. RILEM A 18 (1992) 965. [7] S.J. Park, J.B. Donnet, J. Colloid Interface Sci. 206 (1998) 29. [8] M.G. Harwell, D.E. Hirt, D.D. Edie, N. Popovska, G. Emig, Carbon 38 (2000) 1111. [9] A. Badanoiu, J. Holmgren, Cem. Concr. Compos. 25 (2003) 387. [10] S.J. Park, M. Bremdle, J. Colloid Interface Sci. 188 (1997) 336. [11] S. Wen, D.D.L. Chung, Cem. Concr. Res. 29 (1999) 445. [12] F. Fortin, S.H. Yoon, I. Mochia, Carbon 32 (1996) 1119. [13] S.J. Park, M.K. Seo, H.B. Shim, Mater. Sci. Eng. A 352 (2003) 34. [14] S.J. Park, M.H. Kim, J. Mater. Sci. 35 (2000) 1901. [15] A. Khalifa, A. Nanni, Cem. Concr. Compos. 22 (2000) 165. [16] X. Fu, D.D.L. Chung, Cem. Concr. Res. 27 (1997) 1313. [17] L. Bagel, Cem. Concr. Res. 28 (1999) 1819. [18] Y.I. Yue, G.Z. Li, X.S. Xu, Z.J. Zhao, Cem. Concr. Res. 30 (2000) 1983. [19] X. Fu, W. Lu, D.D.L. Chung, Carbon 36 (1998) 1337. [20] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309. [21] H.N. Garden, L.C. Hollaway, Compos. Part B 29 (1998) 411. [22] S.J. Park, M.S. Cho, Carbon 38 (2000) 1053. [23] H. Kolsch, J. Compos. Construct 2 (1998) 25. [24] P.C. Mishra, V.K. Singh, K.K. Narang, N.K. Singh, Mater. Sci. Eng. A 357 (2003) 13. [25] A. Katz, A. Bentur, Cem. Concr. Res. 24 (1994) 214. [26] P.W. Chen, D.D.L. Chung, Composites 24 (1993) 33. [27] A. Katz, V.C. Li, andA. Kazmer, J. Mater. Civ. Eng. 5 (1995) 125. [28] J.C. Maso, Interfaces in Cementitious Composites, Wiley, New York, 1992. [29] P.R. Blankenhorn, B.D. Blankenhorn, M.R. Silsbee, M. DiCola, Cem. Concr. Res. 31 (2001) 1049.
Effect of different cross-section types on mechanical properties of carbon fibers-reinforced cement composites Soo-Jin Park a,∗ , Min-Kang Seo a , Hwan-Boh Shim a , Kyong-Yop Rhee b a
Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-600, South Korea b School of Mechanical and Industrial System Engineering, Kyunghee University, Yongin 449-701, South Korea Received 29 March 2003; received in revised form 22 August 2003
Abstract To study the effect of shape factor, the mechanical properties of three shapes of carbon fiber, i.e., round, H-shape, and C-shape randomly oriented carbon fibers-reinforced cement composites (CFRCCs) were investigated. As a result, C-shape CFRCC showed higher tensile and flexural strength than any other shape carbon fibers-reinforced composites. It was noted that C-shape CFRCC presented stronger fiber-matrix interfacial adhesive forces, due to the mechanical anchorage into the matrix. However, the compressive strength of the CFRCC was decreased with increasing the aspect ratio and fiber volume fraction. This was probably due to the fact that the amounts of entrained air contents were increased during the mixing of each fiber. Also, both the tensile and flexural strength of the CFRCC were significantly increased by the additions of fumed silica to the composites. © 2003 Elsevier B.V. All rights reserved. Keywords: Shape factor; Mechanical properties; Carbon fibers-reinforced cement composites; Aspect ratio; Fiber volume fraction
1. Introduction Fibers-reinforced cement-matrix composites are structural materials that are gaining in importance rapidly due to the increasing the demand of superior structural and functional properties. Especially, the carbon fibers are one of the most common for reinforcing cementitious materials because of their excellent mechanical properties [1,2]. When carbon fibers are incorporated into the cementitious matrix, some properties of carbon fibers-reinforced cement-matrix composites (CFRCCs) are improved, including the tensile strength, flexural strength, compressive strength, toughness, and drying shrinkage, etc. [3,4]. Therefore, the carbon fibers are generally used as a reinforcement to enhance the mechanical properties of a brittle matrix, such as, cement paste and especially non-round shaped carbon fibers are more effective for strengthening than round shaped one [5–7]. The mechanical properties of CFRCC are influenced by carbon fiber loading, fiber length, fiber shape, and fiber thickness, etc. [8–10]. Wen and Chung [11] reported that ∗ Corresponding author. Tel.: +82-42-861-4151; fax: +82-42-861-4151. E-mail address: [email protected] (S.-J. Park).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.08.123
the mechanical properties of pitch-based carbon fibersreinforced cement composites are improved by the preparation of non-circular shape carbon fibers, such as, C-shape and Y-shape. Fortin et al. [12] also reported that the mechanical properties of carbon fibers-reinforced cement composites are largely depended on the transverse alignment of graphene layers within slit type carbon fibers. Generally, CFRCC composites contain short carbon fibers, typically 5 mm in length, as the short fibers can be used as an admixture in concrete and short fibers are less expensive than continuous fibers. However, due to the their weak bond between carbon fibers and cement matrix, continuous fibers are much more effective than short fibers in reinforcing concrete in spite of the complex added to the concrete mixtures [13–16]. And the admixtures, such as, latex, methylcellulose, and silica fume also help the bond of elements in the composites. Although the addition of carbon fibers to the concrete may increase in air void content, resulting in lowering the thermal conductivity, the mechanical strengths of the concrete are largely increased with increasing the fiber volume fraction [17]. And, the effective use of carbon fibers in concrete requires the dispersion of the fibers in the mixture and also the good dispersion (a fine particulate) of the silica fume is essential to enhance the
S.-J. Park et al. / Materials Science and Engineering A366 (2004) 348–355
mechanical properties of the composites as an admixture [18,19]. In common, C and H-shape carbon fibers have higher specific surface area than that of conventional round-shape carbon fibers. This result can probably affect the mechanical properties of the composites. In the present study, we have investigated the composites made with different shape carbon fibers. The aim of the study is to investigate the relationship between the fiber shapes and the mechanical properties in the carbon fibers-reinforced cement-matrix composites with experimental different-shape carbon fibers as reinforcements. Therefore, three different shapes of carbon fibers, such as, C, H, and round shapes, are used to reinforce a cementitious matrix containing lightweight aggregates with and without silica fume in order to enhance the mechanical bonds. And the flexural, tensile, and compressive properties of the cement composites reinforced with randomly oriented shapes of carbon fibers are studied in order to investigate the effect of the fiber shape, fiber volume fraction, and aspect ratio on mechanical strengths and microstructures of the composites.
349
L
d2 D
d2
L
d
2. Experimental The isotropic round, hollow, and C-shape carbon fibers were manufactured by the melt-spun of isotropic pitch made by heat treatment for 3 h at 390 ◦ C and 3 h at 350 ◦ C from Naphta cracking bottom oil in a pilot-scale. Schematic diagrams of C-shape and round-shape carbon fibers were shown in Fig. 1 [13]. The lightweight cementitious composites were prepared with ordinary Portland cement (OPC, Ssangyong Cement of Korea), lightweight fine aggregates (Sirasu Balloon, MSB301 Japan, and Micro cell, SL150 Austria, bulk density 0.35–0.4, fineness maximum 180 m), and silica fume
Fig. 1. Schematic diagram of the round and C-shape carbon fiber used.
(Blaine specific surface area from BET’s equation 20 m2 /g [20]). The typical formulation of cementitious matrix of CFRCC was shown in Table 1. The Carbon fibers were incorporated into the above lightweight cementitious composites. Table 2 showed the properties of the carbon fibers used. And the particle size distribution of cement and fine fillers (aggregates) was plotted in Fig. 2.
Table 1 Mixing ratios of cement matrix for CFRCC (unit: wt.%) Ordinary cement
100 a
Fine aggregates
Admixtures (C, wt.%)
Sirsu ballon
Micro cell
Fumed silica
BMC
A-803
M-150
15
5
16
0.25
0.5
3.0
Carbon fibers
W/C
1 – 3 Vf %, 3 – 25 mm, C, H, Ra
0.465
C: C-shape carbon fiber; H: H-shape carbon fiber; R: round-shape carbon fiber.
Table 2 Properties of the carbon fibers used in this work Fiber shapes
Tensile strength (MPa)
Elastic modulus (GPa)
Elongation (%)
Diameter (m)
Cross-sectional area (m2 ) 120◦ )
Specific gravity
C
920
102
0.9
40.2 (Do 22.8 (Di )b
594 (open θ =
H
830
118
0.7
36.1 (Do )a 23.8 (Di )b
590
1.78
860
72
1.2
26.8
564
1.74
R a b
Do : outer diameter. Di : inner diameter.
)a
1.76
350
S.-J. Park et al. / Materials Science and Engineering A366 (2004) 348–355
Sirasu Balloon Micro cell Portland cement
Passing quantity (%)
100
80
60
40
20
0 1
10
100
1000
Particle size (µm) Fig. 2. Pore size distribution of the cement and fine fillers used.
Specimens were prepared according to the following process: firstly, carbon fibers and dry materials were mixed for 5 min, water was added to the dry mixtures, and they were mixed for 5 min in an Omni mixer. Secondly, the wet mixtures were cast for experimental demand size. Finally, the cast samples were pre-cured at 20 ◦ C with 80% RH for 2 days, autoclave-cured at 180 ◦ C with 7 MPa for 4 h to improve the packing density of the composites at the same water/binder ratio, and then dried at room temperature/70% for 14 days.
The three-point loading flexural tests were carried out using Instron model 1125 tester according to the requirements of ASTM D-790-92. The span-to-depth ratio was 16:1 scale and cross-head speed was 0.5 mm/min. The dimension of the specimens was 50 mm (length) × 20 mm (width) × 2 mm (thickness). Tensile tests were carried out on an Instron model 1125 tester, installed with hydraulic grips, following ASTM standard D3039-93 (standard test method for tensile properties of polymer matrix composite materials) at a cross-head speed of
Flexural strength (MPa)
12
9
6 R-shape C-shape H-shape R-shape+fumed silica C-shape+fumed silica H-shape+fumed silica
3
no fibers 0 0
1
2
Fiber volume fraction (%) Fig. 3. Flexural strength of CFRCC as a function of fiber volume fraction.
3
S.-J. Park et al. / Materials Science and Engineering A366 (2004) 348–355
2 mm/min. The dimension of each tensile test specimen was 200 mm (length) × 25 mm (width) × 2 mm (thickness). The compressive strength was measured by using Instron model 1125 tester. Compression experiments were performed at a crosshead speed of 1 mm/min in an Instron test machine (Instron model 1125 tester), using samples of dimensions 40 mm (length) × 40 mm (width) × 40 mm (thickness) according to the ASTM D-695-58. Six specimens were tested for all the experiments and the average value was taken in the present work studied. The surface sections and fracture surfaces of the composites were examined by scanning electron microscope (SEM).
3. Results and discussion Fig. 3 shows the flexural strength of CFRCC as a function of fiber volume fraction. The flexural strength of CFRCC is increased with increasing the fiber volume fraction and the CFRCC containing fumed silica shows higher flexural strength than that of the CFRCC without fumed silica because of increasing the interfacial areas between fibers and matrix, resulting from the densification of microstructure [21,22]. Also, the C-CFRCC presents higher flexural strengths than those of R- or H-CFRCC, regardless of the presence of fumed silica, as shown in Fig. 3. Especially, the C-CFRCC containing fumed silica shows the highest flexural strength, about 40% higher than R- and H-CFRCC at a fiber volume fraction of 3%. This is probably due to the fact that the inside space of C-shape carbon fibers is compacted by the matrix and the fibers, thus, act as a mechanical anchor, resulting from the C-shape carbon fibers having a large interface between fibers and matrix, as shown in Fig. 5. And, H-CFRCC without fumed silica shows higher flexural
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strength than R-CFRCC, while H- and R-CFRCC containing fumed silica show almost the same flexural strength. Fig. 4 shows the flexural strength of 2% fibers loaded CFRCC as a function of aspect ratio. The C-CFRCC presents high flexural strength and an optimum aspect ratio, i.e., 150 and 250 for with and without fumed silica, respectively. The reason is that the fibers are easily agglomerated together during the mixing process when they are too long, resulting in decreasing the dispersability and flowability, and increasing the air entrapped in the matrix [23]. The aspect ratio of R-CFRCC is less significant for the flexural strength than that of C-CFRCC. And the flexural strength of H-CFRCC is decreased with increasing the aspect ratio because it is difficult for the matrix to penetrate the hole of fiber by capillary force when the fibers are too long. Consequently, the fumed silica can greatly influence the improving the flexural strength of CFRCC, as do fibers loading in Fig. 3. Fig. 5 shows SEM micrographs for the different carbon fiber shapes incorporated into CFRCC containing fumed silica. The shape of fibers significantly influences the interfacial area between fibers and matrix. Especially, the C-shape carbon fibers mechanically anchor in the matrix and increase the interfacial contact area, resulting in improving the mechanical properties of C-CFRCC. And the H-shape of carbon fibers also shows the compaction of the hole of fibers by the matrix, which is acted as a mechanical anchor when the fiber is short enough for the matrix to penetrate. Fig. 6 shows the tensile strength of CFRCC as a function of fiber volume fraction. The tensile strength of CFRCC is proportionally increased with increasing the fiber loading and shown a similar trend compared to the flexural strength, as shown in Fig. 3. The fumed silica is also a significant factor for increasing the tensile strength of CFRCC. This fumed silica, added to the cement matrix, probably can penetrate
Flexural strength (MPa)
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Aspect ratio (L/D) Fig. 4. Flexural strength of 2% fibers loaded CFRCC as a function of aspect ratio.
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Fig. 5. SEM micrographs of surface section for the different shapes of CFRCC.
Tensile strength (MPa)
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4 R-shape C-shape H-shape R-shape+fumed silica C-shape+fumed silica H-shape+fumed silica
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no fibers 0 0
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Fiber volume fraction (%) Fig. 6. Tensile strength of CFRCC as a function of fiber volume fraction.
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R-shape C-shape H-shape R-shape+fumed silica C-shape+fumed silica H-shape+fumed silica 6
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Aspect ratio (L/D) Fig. 7. Tensile strength of 2% fibers loaded CFRCC as a function of aspect ratio.
well into a hole or surface groove of fibers and densify the matrix, resulting in improving the fiber-matrix interfacial bonding zone [24–26]. And C-CFRCC presents the highest tensile strength at fiber loading volume of 3% with fumed silica. Especially, in case of the H-shape fiber, the tensile strength is more strongly anchored in the matrix when tensile stress is applied to the composites. Therefore, the increasing ratio of tensile strength is increased with increasing the fiber loading volume compared to that of flexural strength in CFRCC [27]. And R-CFRCC has the lowest tensile strength.
Fig. 7 shows the tensile strengths of 2% fibers loaded CFRCC as a function of aspect ratio. C-CFRCC shows the highest tensile strength at 250 aspect ratio, regardless of the presence of fumed silica, and also all CFRCC present the highest value near at 250 aspect ratio. However, the tensile strength of CFRCC is decreased at a high aspect ratio greater than 250 values because of the formation of bulk fibers and increasing the porosity in the matrix [28]. R-CFRCC presents higher tensile strength than H-CFRCC at high aspect ratio, due to the difficult in penetrating
Compressive strength (MPa)
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50
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R-shape C-shape H-shape R-shape+fumed silica C-shape+fumed silica H-shape+fumed silica
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no fibers
0 0
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Fiber volume fraction (%) Fig. 8. Compressive strength of CFRCC as a function of fiber volume fraction.
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R-shape C-shape H-shape R-shape+fumed silica C-shape+fumed silica H-shape+fumed silica
Compressive strength (MPa)
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40
30
0 0
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Aspect ratio (L/D) Fig. 9. Compressive strength of 2% fibers loaded CFRCC as a function of aspect ratio.
Fig. 10. SEM micrographs for carbon fibers distributed in the cement matrix.
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of matrix into a hole of fiber when the fibers are too long. Consequently, the tensile strength of C-CFRCC is higher than that of R- and H-CFRCC. This is probably resulted from the structural differences. That is, it can be explained that the superior mechanical properties of C-CFRCC are resulted from the well-aligned transverse texture, which can be easily converted to the graphitic crystallinity during heat treatment, resulting in improving the resistance to failure. Fig. 8 shows the relationship between fiber volume fraction and compressive strength. In regard to all shapes of the carbon fibers, it is expected that the compressive strength of CFRCC is decreased a little as the fiber volume fraction increases. Also, Fig. 9 presents the compressive strength of CFRCC as a function of aspect ratio. As a result, the same tendency is obtained in the compressive strength. These results are probably due to the increasing the amount of air content and porosity for the mixing of each fiber and decreasing the packing density, resulting in decreasing the interfacial binding force of the composites, as shown in Fig. 5 [29]. Also, it is known that the decreasing ratio of H- and C-CFRCC is higher than that of R-CFRCC, resulting from the retained pore in the composites where C- and H-shape carbon fibers are not filled enough to cement matrix and filler. SEM micrographs for C-shape carbon fiber distributed in the cement matrix are shown in Fig. 10. The fibers are distributed homogeneously in the cement matrix and well contact with each other within an individual cluster, although the clusters are separate. 4. Conclusions We investigate the effect of different cross-sectional shapes of carbon fibers on mechanical properties of CFRCC. As a result, C-CFRCC shows the highest flexural and tensile strength than any other CFRCC. Because C-shape carbon fibers act as an anchor in the matrix, resulting in increasing the interfacial area between fibers and matrix. The 3% fibers loaded C-CFRCC containing fumed silica exhibits the improvement in flexural and tensile strength compared to those of H- and R-CFRCC because of the mechanical anchorage of H- and R-shape carbon fibers having less than
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to C-shape carbon fibers. However, in case of the compressive strength in CFRCC, the value is decreased a little with increasing the fiber volume fraction and fiber length, and R-shape carbon fibers loaded composites have the highest value in strength. Also, we can find that the fumed silica is a greatly significant factor for improving the mechanical properties of CFRCC.
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