- Email: [email protected]
Carbon-cement composites
ooo8.6223189 163.00+ .oo 0 1989 Pergamon Press plc
Carbon Vol. 27. No. 5. pp. 729-737. 1989 Printed in Great Britain.
CARBON-CEMENT
COMPOSITES
YOSHIHIKO OHAMA College of Engineering,Nihon Unive~ity, Koriyama, Fukushima-ken,
963 Japan
ABSTRAm-The carbon-cement composites treated in this review include carbon-fiber-reinforced cement and concrete (CFRC). A broad literature survey is made of carbon fibers for cement reinforcements, the fabrication processes, mechanical properties, dimensional stability, durability and applications of the carbon-~ber-reinforced cement and concrete, and the results are reported. Some research on the carbon-fiber-reinforced cement and concrete in the 1970s were performed by using expensive PAN (polyacrylonitrile)-based carbon fibers, and were not extended to the practical applications. However, in Japan, the carbon-fiber-reinforced cement and concrete were developed by using low-cost pitchbased carbon fibers in the 198Os, and are increasingly employed in various practical applications in the constNction industry at present. In this case, the short carbon fibers are randomly dispersed in the cement and concrete during mixing. Particularly, various practical applications of the carbon-fiberreinforced cement and concrete using the pitch-based short carbon fibers in Japan are introduced in this review. Kev Words-Carbon fleiural behavior.
fibers. reinforced cement and concrete, cements, aggregates, tensile behavior.
searches were performed using expensive PAN (polyac~lonit~le)-bard continuous carbon fibers, and were not extended to the practical applications, although CFRC fabrication processes such as hand lay-up and filament winding were developed. In Japan, CFRC was actively developed using low-cost pitch-based short carbon fibers by Ohama et uZ.[5], Akihama et a[.[61 and Tsuji et a/.[71 in the 198Os, and is increasingly employed in various practical applications in the const~ction industry at present. The present article reviews the current state and trend of research and development of CFRC.
1. YNTRODUCTION Recently, much attention has been directed towards improving mechanical properties such as tensile strength, extensibility, and impact resistance of cement and concrete, popular construction materials, by fibrous reinforcements. Various types of fibers including steel fibers, alkali-resistant glass fibers, vinylon fibers, polypropylene fibers, aramid fibers, and carbon fibers have been developed for this purpose, and are commercially available in the construction industry at the present time. Of such fibrous reinforcements, the carbon fibers are an attractive choice to reinforce a cement-based brittle matrix in spite of their high cost because of their high specific strength, elastic modulus and chemical inertness in the highly alkaline environment. They cause no rust stain problems on concrete surfaces like the steel fibers. Another merit led by their chemical inertness is the possibility of accelerated curing, for example, autoclaving, for the cement composites reinforced with them at elevated temperature without degrading the fibers. In the 198Os, inexpensive low-modulus carbon fibers made from coal and petroleum pitches have been developed in Japan, and become promising as reinforcements for the cement-based matrix. The carbon fibers are mainly used as reinforcements for cement paste (without fine aggregate, finer than 5 mm) and cement-fine aggregate composite like the glass fibers because of their fiber characteristics. In general, the cement paste and cement-tine aggregate composite reinforced with the carbon fibers are called carbon-fiber-reinforced cement and concrete (CFRC). The first study on CFRC was published by Ali el al. in 1972[1]. After this publication, some studies on CFRC were presented by Wallerf21, Sakar and Bailey[3], and Briggs et a1.[4]. Wowever, these recm 2115-f
2. MATJZRXALSFOR CFRC 2.1 Carbon fibers Carbon fibers used for CFRC are chemically classified into two categories: pitch- and PAN-based, and done in shape into two groups, short and long (or continuous) fibers. Table 1 lists the typical prop erties of the pitch- and PAN-based carbon fibers for CFRC commercially available in Japan. The diameter of the carbon fibers is very small, 7 to 20 pm, and the fibers are not stiff. In the use of the short carbon fibers, their length is normally 10 mm or less, for example, 3, 6 or 10 mm, and their aspect ratio is 200 to 700. This may obstruct the uniform dispersion of the short carbon fibers in cement matrix. For the purpose of improving the bond between the carbon fibers and the cement matrix, their surfaces may be treated by oxidation processes to give irregularities and form functional groups such as carboxyl and phenolic hydroxyl groups on the surfaces[8]. 2.2 Cements To ensure the uniform dispersion of carbon fibers in cement matrix and a good bond between the car729
730
Y. OHAMA Table 1. Properties of commercial carbon fibers for CFRC in Japan
Type of fibers
Diameter (Pm)
Specific gravity
Tensile strength (kgf/mm2)
Young’s modulus (l@ kgf/mm2)
cost (yen/kg)
Pitch-based PAN-based
14-15 7-a
1.6-1.7 1.7-1.8
80-120 300-400
4-10 25-40
1500-3000 8000-10000
bon fibers and cement matrix, the cement particles must be infiltrated between the carbon fibers. Accordingly, the use of the cements with the smaller particle size of less than 45 pm is generally recommended for CFRC[9]. Such cements are finely ground ordinary portland cement, high-e~ly-stren~h portland cement and ultra high-early~strength portland cement. In the use of coarser cements like ordinary portland cement and alumina cement, very fine inorganic admixtures such as silica fume[lO] and ground blast furnace slag[7] are mixed into the cements because of the effective dispersion of the carbon fibers and the increased bond between the fibers and the matrix due to filling the matrix voids around the fibers with the admixtures. This is seen in Fig. 1. The particle size of the cements for CFRC is given in Table 2. 2.3 Fine ffggreg~fes In the production of CFRC, fine aggregates are used for CFRC with short carbon fibers, but are not done for CFRC with long carbon fibers. A proper fine aggregate size at which their uniform dispersion is achieved in CFRC ranges from 50 to 200 pm[ll]. The fine aggregates recommended for CFRC are ground silica sand, fly ash, and Shirasu balloons. The Shirasu balloons are low-cost lightweight aggregates used in Japan, which are produced by burning volcanic ash at about 1ooo”C. 2.4 Admixtures Generally, the production of CFRC with long carbon fibers does not need the addition of any admixtures. However, in the fabrication processes of CFRC with short carbon fibers, the use of any admixtures is indispensable for the uniform dispersion of very fine, unstiff carbon fibers, the good bond between the fibers and cement matrix, and the im-
Table 2. Particle size of cements for CFRC
Type of cement Ordinary portland cement High-early-strength portland cement Ultra high-early-strength portland cement Alumina cement
proved consistency. The admixtures recommended for CFRC are high-range water-reducing agents (superplasticizers), air-entraining agents, latex-type cement modifiers, methyl cellulose, and very fine inorganic admixtures such as silica fume (specific surface area, ca. 280000 cm2/g) and ground blast furnace slag (specific surface area, ca. 8~ cm*lg). In particular, the methyl cellulose and very fine inorganic admixtures act as dispersants for the carbon fibers. These admixtures cause an increase in the cement matrix viscosity to hold and disperse the carbon fibers in the matrix.
3. FABRICATION PROCESSES FOR
CFRC
Fig. 2 represents the classification of fabrication processes for CFRC. The fabrication processes for CFRC with long carbon fibers and the mats and cloths made of the long fibers are hand lay-up and filament winding methods, which are almost the same methods as used for FRP (fiber-reinforced plastics). However, such processes have not been put to practical use yet. Briggs’ review may serve as a good reference on the fabrication processes for CFRC with the long carbon fibersf91. The fabrication processes for CFRC with short carbon fibers are the spray-up method in two-dimensional orientation, and casting, pressing, and extrusion molding methods in three-dimensional orientation after uniform mixing. Of these molding processes, the casting method is most widely applied. In the casting method, first the short carbon fibers are randomly dispersed in three dimensions inside the cement matrix by using a proper mixer, and then the mixed fresh CFRC is cast in various forms. The effectiveness of the reinforcement due to the three-dimensional random orientation is generally lower than that due to oneor two-dimensional random orientation. However, if the three-dimensional random orientation of the carbon fibers is easily obtained by using an effective mixer and the casting in the forms is possible like conventional concrete, the lower mechanical properties of CFRC are found to be compensated by a reduction in the production cost due to the ease of such molding process[6].
Average particle size
Specific surface area
(r.M
(cm?/g~
16-17
3200-33Do
13-14
4300-4400
4. TYPICAL PROPERTIES OF CFRC
7-8
5850-5860
As CFRC with long carbon fibers is hardly employed in the practical applications at present, the properties of CFRC which is produced in three-di-
19-21
3~-4~
Carbon-cement
731
composites
Fig. 1. Fracture surface of CFRC showing contact zone between carbon fibers and cement matrix.
mensional random orientation by using short carbon fibers are mainly described in this review. Most short carbon fibers used are low-modulus pitch-based type developed for cement reinforcement in Japan. The properties of CFRC with the long fibers are referred to in the above-mentioned Briggs’ review[9]. In general, the properties of CFRC are greatly influenced by the type and nature of the carbon fibers, fiber content, fiber orientation and cement matrix properties. Symbols in figures below mean as follows: W/ C, water-cement ratio (wt%); A/C, fine aggregatecement ratio (by weight); and Vf, fiber content (vol%).
fiber content. Fig. 4 shows the tensile stress-strain curves for the same lightweight CFRC under cyclic loading[6]. The envelope of the stress-strain curve under cyciic loading is similar to that under ordinary, direct tensile loading, and the restitutive force characteristic is good. Fig. 5 exhibits the effect of fiber content on the tensile strength of CFRC using silica fume[5]. The tensile strength of CFRC increases with an increase in the fiber content. Recently, Zheng and Chung have reported that CFRC with pitchbased short carbon fibers provides a 113 to 164% increase in the tensile strength at a fiber content 0.28 ~01% as compared to unreinforced cement[l2].
4.1 Tensile behavior Fig. 3 illustrates the tensile stress-strain curves for lightweight CFRC with a bulk specific gravity of 1.0[6]. The tensile stress-strain curves for the lightweight CFRC become bilinear, like steel, at a fiber content of about 2 vol%, and the bilinear form stabilizes at fiber contents of 3 ~01% or more. The tensile strength and elongation increase with raising
4.2 Flexural behavior Fig. 6 indicates the flexural stress deflection curves for CFRC using silica fume[5]. The flexural strength and deformation behavior of CFRC are markedly improved with an increase in fiber content. Generally, CFRC with a fiber length of 3 mm shows a higher flexural strength or deformability (or toughness) than one with a fiber length of 10 mm. Fig. 7
Type
of Fibers
Long Fibers
Orientation of Fibers -
Molding Process
One-dimensional Orientation
Hand Lay-up Filament-winding
Fiber Mats and ClOthS
Two-dimensional Random Orientation
Hand
Lay-up
Short Fibers
Two-dimensional Random Orientation
Spray-up
Three-dimensional Random Orientation
Casting Pressing EXtnvliOII
Fig. 2. Fabrication processes for CFRC.
732
Y. OHAMA 113 % 0.71 Autoclaving
W/C A/C CUIV?
I 8ooo
4ooo
I 12ooo
I
16ooo
TensileStrain (xlo-6)
Fig. 3. Tensile stress-strain curves for lightweight CFRC.
W/C
A/C Vf Cure
113% 0.71 4.05 vol% Autoclaving
Number Of 15 Cycles Repeat
Tensile Strain (x~O-~)
Fig. 4. Tensile stress-strain curves for lightweight CFRC under cyclic loading.
iliustrates the comparison of the flexural strengths of pitch- and PAN-based CFRCs made by the same method[ 131. The flexural strength of the PAN-based CFRC is about twice that of the pitch-based CFRC. Irrespective of the types of the carbon fibers, the development of the high strength may be explained to be due to the effects of silica fume and waterreducing agent. The flexural strength of CFRC is normally affected to a great extent by factors such as cement matrix strength, fiber content, the Young’s modulus and aspect ratio of the carbon fibers. The flexural strength can be generaily expressed as the function of these factors by the following equation:
150
t
lOOFiber Length
I
0
I
I
I
1 2 3 4 Fiber Content (vol %)
1
af = 333 log[ao(l
- Vf) + m
+Vf] - 585
5
Fig. 5. Carbon fiber content vs. tensile strength of CFRC using silica fume.
where uf and cro are the flexural strengths of CFRC and unreinforced cement respectively. Vf is the voiume fraction of the carbon fibers, and E and lid are
Carbon-cement
composites
733
Fiber Length : 3 mm
Fiber Length : 10 mm
Deflection (mm)
Deflection (mm)
Fig. 6. Flexurai stress deflection curves for CFRC
the Young’s modulus
and aspect ratio of the carbon This relationship empirically obtained is shown in Fig. 8[ 131. Consequently, the prediction of the flexural strength of CFRC is found to be possible by applying the empirical equation. Fig. 9 shows the effects of fiber content and polymer modification by SBR (styrene-butadiene rubber) latex on the flexural behavior of lightweight CFRC with a large amount of fly ash[14]. From Fig. 9, remarkable improvements in its flexural strength and toughness by the polymer modification are recognized. Ohama er al. developed high-performance fibers, respectively.
using
siiica
fume.
ferrocements with pitch-based CFRC as a matrix, and showed that the flexural behavior of the ferrocements can be remarkably improved by use of CFRC instead of ordinary mortar as a matrix]E]. 4.3 Dimensional stability Fig. 10 represents the relation between the drying period and drying shrinkage of CFRC using silica fume after autoclaving[5]. The drying shrinkage of CFRC increases with additional drying period, and becomes nearly constant at 14 days. The drying shrinkage is considerably reduced with an increase
"E 600-
1
PANBased
PitchBased
: M 5 soo& pj-400z fc 300v1 !! at
l
200-
loo: (/:Coefflclent
O1
5 Fiber Content (vol%)
Fig. 7. Comparison of flexural strengths of pitch- and PAN-based CFRCs using silica fume.
p
0
of
Pitch-Based
Correletlon)
1000
2000
4,(l-vf)+/GiE~v~ Fig. 8. Prediction of flexural strength of CFRC using silica tume
Y.
134
OHAMA
Table 3. Precast and cast-in-place applications of CFRC in Japan
___ Unmodified - SBR-Modified
Application Precast products
Cast-m-place applications
Example Curtain walls Free-access floor panels Permanent forms I- and L-shaped beams Conductive floor panels’*~” Wave absorbersn-‘x Ferrercement@ Mortars for external walls Mortars for repairing work1Y.2” Conductive concrete for lightning arresVh
75°C for 5 months[6].
Deflection (mm) Fig. 9. Flexural stress deflection curves for lightweight CFRC using fly ash.
in fiber content. Fig. 11 presents the drying shrinkage of lightweight CFRC after various curing[6]. The l-year drying shrinkage of the lightweight CFRC after autoclaving is less than one-fifth or one-sixth of that of CFRC continuously stored at 20°C and 65% R.H. or in 20°C water after demolding. This means that the autoclaved CFRC is much more dimensionally stable than CFRC cured by other methods. 4.4 Durability and other properties Fig. 12 shows the flexural stress deflection curves for lightweight CFRC after hot water immersion at
From this figure, it is clear that the flexural strength and toughness are not reduced by hot water. The freeze-thaw durability of the same CFRC is indicated in Fig. 13[6]. This CFRC has an excellent freeze-thaw durability even after 300 cycles. The effect of fiber content on the impact resistance of CFRC using silica fume is presented in Fig. 14[5]. Regardiess of the fiber length, the impact resistance of CFRC is markedly improved with raising fiber content. Unreinforced cement is fractured at one blow of a steel ball, but CFRC with a fiber content of 5 ~01% is not broken at 3000 blows or more. The electric conductivity and wave-absorbing property of CFRC that are developed by the inclusion of electrically conductive carbon fibers by nature are studied for its application to conductive concretes and wave absorbers[l6,17.18].
5. APPLICATIONS OF CFRC
CFRC has advantages over conventional concrete and mortar in its mechanical properties such as tensile and flexural strengths, toughness and impact resistance, dimensional stability, electric conductivity, wave absorbing property, and durability. The de-
Drying Period (d) 10 ,
0
-
30 I
.
Y s.
20 I
0 3.0
Fiber Len&b
Fig. 10. Drying period vs. drying shrinkage of CFRC using silica fume.
Carbon-cement
135
composites
5I h
I
I A
LI -toredat 20°C and 65 % R.H. after Autoclaving
11:: w/c 130 % 0.71 A/C Fiber Length 3 mm 2 VSlX Vf
40-
50-
I lr
. .
Water Curing
I
I
I
4w
13w
6m
I IY
Drying Period (r:ueek.m:l(onth,y:Year)
Fig. Il. Drying shrinkage of lightweight CFRC.
Precure
: Autoclaving
150
mersed for 4 weeks
;
Y 6. YM - loo P g LII : b : Z
5o
0
2
1
3
Deflection (8~)
Fig. 12. Flexural stress deflection curves for lightweight CFRC immersed in water at 75°C.
0
50
loo
velopment of CFRC applications should be performed to make the best use of its high performance. Table 3 lists the examples of the precast and castin-place applications of CFRC in the construction industry in Japan, where the development of the practical applications progresses most quickly in the world. Of such practical applications, the first largescale application was the use of lightweight CFRC panels with tile cladding in the Al Shaheed Monument in Baghdad, Iraq, which was constructed by Kajima Corporation, Japan, in 1982[6]. The monument consists of two domes of the same shape with a height of 40 m and a base diameter of 45 m. The domes were covered with CFRC panels, 100 x 50 x 4 cm (total area, 10000 m?), which had been produced in Japan and shipped to Baghdad. Fig. 15 shows the construction work of the ARK Mori Buildings, high-rise office buildings using CFRC curtain walls, which were completed by the same company in the Akasaka-Roppongi area of Tokyo in 1986. The buildings were covered with the curtain walls, 1.47 x 3.76 m, and the total area was 32000 m’. Approximately 187 tons of pitch-based
150
200
250
300
Number of Cycles for Freezing and Thawing
Fig. 13. Change in dynamic modulus of elasticity of lightweight CFRC through freezing and thawing cycling.
Y. OHAMA
736 Fiber Length 3nml
N
10 mm
Fig. 16. Heat-resisting 0
1
3
CFRC cylinder head for motorcycle engine.
5
Fiber Content (~01%)
Fig. 14. Carbon fiber content vs. impact resistance CFRC using silica fume.
of
short carbon fibers were used for the production of the CFRC curtain walls[6]. Recently, CFRC has drawn much attention as a high-performance material in the electrical and mechanical industries as well as the construction industry in Japan. Fig. 16 represents an interesting
application of heat-resisting CFRC with alumina cement to a cylinder head for motorcycle engine. This application has experimentally been developed by Kanno et al. [21].
6.
CONCLUSION
Some research on CFRC using PAN-based carbon fibers in the 1970s were not extended to its practical applications because of the high cost of the carbon
Fig. 15. Construction work of ARK Mori buildings using CFRC curtain walls. (By Courtesy of Kajima Corporation.)
Carbon-cement composites fibers and some difficulty in its fabrication processes. In recent years, a great deal of attention has been oriented towards the development of CFRC which is made in the three-dimensional random orientation of low-cost pitch-based short carbon fibers. In particular, CFRC products fabricated by this process are increasingly employed in the construction industry in Japan. However, the data on the durability and long-term performance of CFRC which is a newcomer to the field of fibrous construction materials are likely to be rather scant yet, and the accumulation of such data is strongly required. In the near future, the developments of the inexpensive pitchbased carbon fibers with higher mechanical properties and the cheaper fabrication processes for CFRC are expected to expand the applications of CFRC.
of Japan, Tokyo (1987).
8. S. Ito, M. Deguchi and K. Suzuki, In Semento Gijutsu 9.
10.
Nempo 1986, pp. 479-482. The Cement Association of Japan, Tokyo (1986). A. Briggs, Journal of Materials Science l&384 (1977). Y. Ohama and M. Amano, In Proceedings ofthe Twentyseventh Japan Congress on Materials Research, pp. 18% 191. The Societv of Materials Science. Kvoto. Jaoan (1984). ’ K. Shirakawa and K. Nakagawa, In Proceedings of the JCI 4th Conference, pp. 153-156. Japan Concrete Institute, Tokyo (1982). Q. Zheng and D. D. L. Chung, Presented at the SymI<
11.
12.
IL
posium S: Advanced Cements and Chemically Bonded Ceramics, MRS International Meeting on Advanced Materials, Tokyo (1988). 13. Y. Ohama, Y. Sato and M. Endo, In Proceedings of the Asia-Pacific Concrete Technology Conference ‘86,
pp. 5.1-5.8. Institute for International Research, Singapore (1986). 14. Y. Ohama, K. Demura and Y. Sato, In Proceedings of the International Symposium on Fibre Reinforced Concrete (Edited by V. S. Parameswaran and T. S. Krish-
REFERENCES
1. M. A. Ah, A. J. Majumdar and D. L. Rayment, Cement and Concrete Research 2. 201 (1972). 2. J. A. Wailer, in Fiber Reinforced Concrete; Publication SP-44, pp. 143-161. American Concrete Institute, Detroit (1974). 3. S. Sarkar and M. B. Bailey, In Fiber Reinforced Cement and Concrete (Edited by A. Neville), pp. 361371. The Construction Press, Lancaster (1975). 4. A. Briggs, D. H. Bowen and J. Kollek, In Proceedings of the Second International Conference on Carbon Fibres, Their Place in Modern Technology, Paper No.
15.
16. 17.
18.
17, pp. 114-121. Unwin, Old Working (1974).
5. Y. Ohama, M. Amano and M. Endo, Concrete International 7(3), 58 (1985). 6. S. Akihama, T. Suenaga and H. Nakagawa, Concrete International, 10,40 (1988). 7. S. Furukawa, Y. Tsuji and M. Miyamoto, In Review of the 41st General Meeting/Technical
737
Review 1987), pp. 336-339. The Cement Association
Session (CAJ
19. 20. 21.
namoorthy), Vol. 1, pp. 3.23-3.31. Oxford & IBH Publishing, New Delhi (1987). Y. Ohama and A. Shirai, In Ocean Space Utilization ‘85 (Edited by W. Kato), Vol. 2, pp. 391-398. SpringerVerlag, Tokyo (1985). S. Takami and T. Higuchi, Kansai Denryoku K. K. Soken Hokoku, No. 29, pp. 250-252 (1982). T. Ito, M. Furukawa and-K. Yamamura, In Semento Giiutsu Nemoo 1987. DD. 467-470. The Cement Association of Japan, Tokyo (1987). Y. Shimizu, A. Nishikata, N. Maruyama and A. Sugiyama, Journal of the Institute of Television Engineers of Japan 40, 780 (1986). K. Nagata, The Bosui Journal lS(9), 115 (1984). H. Tanabe, Construction Finishing Techniques U( 134). 68 (1986). M. Kanno, Y. Ohama, H. Kawai and K. Demura, In Preprints of the 37th Annual Meeting, pp. 334-336. Society of Materials Science, Kyoto, Japan (1988).
Carbon Vol. 27. No. 5. pp. 729-737. 1989 Printed in Great Britain.
CARBON-CEMENT
COMPOSITES
YOSHIHIKO OHAMA College of Engineering,Nihon Unive~ity, Koriyama, Fukushima-ken,
963 Japan
ABSTRAm-The carbon-cement composites treated in this review include carbon-fiber-reinforced cement and concrete (CFRC). A broad literature survey is made of carbon fibers for cement reinforcements, the fabrication processes, mechanical properties, dimensional stability, durability and applications of the carbon-~ber-reinforced cement and concrete, and the results are reported. Some research on the carbon-fiber-reinforced cement and concrete in the 1970s were performed by using expensive PAN (polyacrylonitrile)-based carbon fibers, and were not extended to the practical applications. However, in Japan, the carbon-fiber-reinforced cement and concrete were developed by using low-cost pitchbased carbon fibers in the 198Os, and are increasingly employed in various practical applications in the constNction industry at present. In this case, the short carbon fibers are randomly dispersed in the cement and concrete during mixing. Particularly, various practical applications of the carbon-fiberreinforced cement and concrete using the pitch-based short carbon fibers in Japan are introduced in this review. Kev Words-Carbon fleiural behavior.
fibers. reinforced cement and concrete, cements, aggregates, tensile behavior.
searches were performed using expensive PAN (polyac~lonit~le)-bard continuous carbon fibers, and were not extended to the practical applications, although CFRC fabrication processes such as hand lay-up and filament winding were developed. In Japan, CFRC was actively developed using low-cost pitch-based short carbon fibers by Ohama et uZ.[5], Akihama et a[.[61 and Tsuji et a/.[71 in the 198Os, and is increasingly employed in various practical applications in the const~ction industry at present. The present article reviews the current state and trend of research and development of CFRC.
1. YNTRODUCTION Recently, much attention has been directed towards improving mechanical properties such as tensile strength, extensibility, and impact resistance of cement and concrete, popular construction materials, by fibrous reinforcements. Various types of fibers including steel fibers, alkali-resistant glass fibers, vinylon fibers, polypropylene fibers, aramid fibers, and carbon fibers have been developed for this purpose, and are commercially available in the construction industry at the present time. Of such fibrous reinforcements, the carbon fibers are an attractive choice to reinforce a cement-based brittle matrix in spite of their high cost because of their high specific strength, elastic modulus and chemical inertness in the highly alkaline environment. They cause no rust stain problems on concrete surfaces like the steel fibers. Another merit led by their chemical inertness is the possibility of accelerated curing, for example, autoclaving, for the cement composites reinforced with them at elevated temperature without degrading the fibers. In the 198Os, inexpensive low-modulus carbon fibers made from coal and petroleum pitches have been developed in Japan, and become promising as reinforcements for the cement-based matrix. The carbon fibers are mainly used as reinforcements for cement paste (without fine aggregate, finer than 5 mm) and cement-fine aggregate composite like the glass fibers because of their fiber characteristics. In general, the cement paste and cement-tine aggregate composite reinforced with the carbon fibers are called carbon-fiber-reinforced cement and concrete (CFRC). The first study on CFRC was published by Ali el al. in 1972[1]. After this publication, some studies on CFRC were presented by Wallerf21, Sakar and Bailey[3], and Briggs et a1.[4]. Wowever, these recm 2115-f
2. MATJZRXALSFOR CFRC 2.1 Carbon fibers Carbon fibers used for CFRC are chemically classified into two categories: pitch- and PAN-based, and done in shape into two groups, short and long (or continuous) fibers. Table 1 lists the typical prop erties of the pitch- and PAN-based carbon fibers for CFRC commercially available in Japan. The diameter of the carbon fibers is very small, 7 to 20 pm, and the fibers are not stiff. In the use of the short carbon fibers, their length is normally 10 mm or less, for example, 3, 6 or 10 mm, and their aspect ratio is 200 to 700. This may obstruct the uniform dispersion of the short carbon fibers in cement matrix. For the purpose of improving the bond between the carbon fibers and the cement matrix, their surfaces may be treated by oxidation processes to give irregularities and form functional groups such as carboxyl and phenolic hydroxyl groups on the surfaces[8]. 2.2 Cements To ensure the uniform dispersion of carbon fibers in cement matrix and a good bond between the car729
730
Y. OHAMA Table 1. Properties of commercial carbon fibers for CFRC in Japan
Type of fibers
Diameter (Pm)
Specific gravity
Tensile strength (kgf/mm2)
Young’s modulus (l@ kgf/mm2)
cost (yen/kg)
Pitch-based PAN-based
14-15 7-a
1.6-1.7 1.7-1.8
80-120 300-400
4-10 25-40
1500-3000 8000-10000
bon fibers and cement matrix, the cement particles must be infiltrated between the carbon fibers. Accordingly, the use of the cements with the smaller particle size of less than 45 pm is generally recommended for CFRC[9]. Such cements are finely ground ordinary portland cement, high-e~ly-stren~h portland cement and ultra high-early~strength portland cement. In the use of coarser cements like ordinary portland cement and alumina cement, very fine inorganic admixtures such as silica fume[lO] and ground blast furnace slag[7] are mixed into the cements because of the effective dispersion of the carbon fibers and the increased bond between the fibers and the matrix due to filling the matrix voids around the fibers with the admixtures. This is seen in Fig. 1. The particle size of the cements for CFRC is given in Table 2. 2.3 Fine ffggreg~fes In the production of CFRC, fine aggregates are used for CFRC with short carbon fibers, but are not done for CFRC with long carbon fibers. A proper fine aggregate size at which their uniform dispersion is achieved in CFRC ranges from 50 to 200 pm[ll]. The fine aggregates recommended for CFRC are ground silica sand, fly ash, and Shirasu balloons. The Shirasu balloons are low-cost lightweight aggregates used in Japan, which are produced by burning volcanic ash at about 1ooo”C. 2.4 Admixtures Generally, the production of CFRC with long carbon fibers does not need the addition of any admixtures. However, in the fabrication processes of CFRC with short carbon fibers, the use of any admixtures is indispensable for the uniform dispersion of very fine, unstiff carbon fibers, the good bond between the fibers and cement matrix, and the im-
Table 2. Particle size of cements for CFRC
Type of cement Ordinary portland cement High-early-strength portland cement Ultra high-early-strength portland cement Alumina cement
proved consistency. The admixtures recommended for CFRC are high-range water-reducing agents (superplasticizers), air-entraining agents, latex-type cement modifiers, methyl cellulose, and very fine inorganic admixtures such as silica fume (specific surface area, ca. 280000 cm2/g) and ground blast furnace slag (specific surface area, ca. 8~ cm*lg). In particular, the methyl cellulose and very fine inorganic admixtures act as dispersants for the carbon fibers. These admixtures cause an increase in the cement matrix viscosity to hold and disperse the carbon fibers in the matrix.
3. FABRICATION PROCESSES FOR
CFRC
Fig. 2 represents the classification of fabrication processes for CFRC. The fabrication processes for CFRC with long carbon fibers and the mats and cloths made of the long fibers are hand lay-up and filament winding methods, which are almost the same methods as used for FRP (fiber-reinforced plastics). However, such processes have not been put to practical use yet. Briggs’ review may serve as a good reference on the fabrication processes for CFRC with the long carbon fibersf91. The fabrication processes for CFRC with short carbon fibers are the spray-up method in two-dimensional orientation, and casting, pressing, and extrusion molding methods in three-dimensional orientation after uniform mixing. Of these molding processes, the casting method is most widely applied. In the casting method, first the short carbon fibers are randomly dispersed in three dimensions inside the cement matrix by using a proper mixer, and then the mixed fresh CFRC is cast in various forms. The effectiveness of the reinforcement due to the three-dimensional random orientation is generally lower than that due to oneor two-dimensional random orientation. However, if the three-dimensional random orientation of the carbon fibers is easily obtained by using an effective mixer and the casting in the forms is possible like conventional concrete, the lower mechanical properties of CFRC are found to be compensated by a reduction in the production cost due to the ease of such molding process[6].
Average particle size
Specific surface area
(r.M
(cm?/g~
16-17
3200-33Do
13-14
4300-4400
4. TYPICAL PROPERTIES OF CFRC
7-8
5850-5860
As CFRC with long carbon fibers is hardly employed in the practical applications at present, the properties of CFRC which is produced in three-di-
19-21
3~-4~
Carbon-cement
731
composites
Fig. 1. Fracture surface of CFRC showing contact zone between carbon fibers and cement matrix.
mensional random orientation by using short carbon fibers are mainly described in this review. Most short carbon fibers used are low-modulus pitch-based type developed for cement reinforcement in Japan. The properties of CFRC with the long fibers are referred to in the above-mentioned Briggs’ review[9]. In general, the properties of CFRC are greatly influenced by the type and nature of the carbon fibers, fiber content, fiber orientation and cement matrix properties. Symbols in figures below mean as follows: W/ C, water-cement ratio (wt%); A/C, fine aggregatecement ratio (by weight); and Vf, fiber content (vol%).
fiber content. Fig. 4 shows the tensile stress-strain curves for the same lightweight CFRC under cyclic loading[6]. The envelope of the stress-strain curve under cyciic loading is similar to that under ordinary, direct tensile loading, and the restitutive force characteristic is good. Fig. 5 exhibits the effect of fiber content on the tensile strength of CFRC using silica fume[5]. The tensile strength of CFRC increases with an increase in the fiber content. Recently, Zheng and Chung have reported that CFRC with pitchbased short carbon fibers provides a 113 to 164% increase in the tensile strength at a fiber content 0.28 ~01% as compared to unreinforced cement[l2].
4.1 Tensile behavior Fig. 3 illustrates the tensile stress-strain curves for lightweight CFRC with a bulk specific gravity of 1.0[6]. The tensile stress-strain curves for the lightweight CFRC become bilinear, like steel, at a fiber content of about 2 vol%, and the bilinear form stabilizes at fiber contents of 3 ~01% or more. The tensile strength and elongation increase with raising
4.2 Flexural behavior Fig. 6 indicates the flexural stress deflection curves for CFRC using silica fume[5]. The flexural strength and deformation behavior of CFRC are markedly improved with an increase in fiber content. Generally, CFRC with a fiber length of 3 mm shows a higher flexural strength or deformability (or toughness) than one with a fiber length of 10 mm. Fig. 7
Type
of Fibers
Long Fibers
Orientation of Fibers -
Molding Process
One-dimensional Orientation
Hand Lay-up Filament-winding
Fiber Mats and ClOthS
Two-dimensional Random Orientation
Hand
Lay-up
Short Fibers
Two-dimensional Random Orientation
Spray-up
Three-dimensional Random Orientation
Casting Pressing EXtnvliOII
Fig. 2. Fabrication processes for CFRC.
732
Y. OHAMA 113 % 0.71 Autoclaving
W/C A/C CUIV?
I 8ooo
4ooo
I 12ooo
I
16ooo
TensileStrain (xlo-6)
Fig. 3. Tensile stress-strain curves for lightweight CFRC.
W/C
A/C Vf Cure
113% 0.71 4.05 vol% Autoclaving
Number Of 15 Cycles Repeat
Tensile Strain (x~O-~)
Fig. 4. Tensile stress-strain curves for lightweight CFRC under cyclic loading.
iliustrates the comparison of the flexural strengths of pitch- and PAN-based CFRCs made by the same method[ 131. The flexural strength of the PAN-based CFRC is about twice that of the pitch-based CFRC. Irrespective of the types of the carbon fibers, the development of the high strength may be explained to be due to the effects of silica fume and waterreducing agent. The flexural strength of CFRC is normally affected to a great extent by factors such as cement matrix strength, fiber content, the Young’s modulus and aspect ratio of the carbon fibers. The flexural strength can be generaily expressed as the function of these factors by the following equation:
150
t
lOOFiber Length
I
0
I
I
I
1 2 3 4 Fiber Content (vol %)
1
af = 333 log[ao(l
- Vf) + m
+Vf] - 585
5
Fig. 5. Carbon fiber content vs. tensile strength of CFRC using silica fume.
where uf and cro are the flexural strengths of CFRC and unreinforced cement respectively. Vf is the voiume fraction of the carbon fibers, and E and lid are
Carbon-cement
composites
733
Fiber Length : 3 mm
Fiber Length : 10 mm
Deflection (mm)
Deflection (mm)
Fig. 6. Flexurai stress deflection curves for CFRC
the Young’s modulus
and aspect ratio of the carbon This relationship empirically obtained is shown in Fig. 8[ 131. Consequently, the prediction of the flexural strength of CFRC is found to be possible by applying the empirical equation. Fig. 9 shows the effects of fiber content and polymer modification by SBR (styrene-butadiene rubber) latex on the flexural behavior of lightweight CFRC with a large amount of fly ash[14]. From Fig. 9, remarkable improvements in its flexural strength and toughness by the polymer modification are recognized. Ohama er al. developed high-performance fibers, respectively.
using
siiica
fume.
ferrocements with pitch-based CFRC as a matrix, and showed that the flexural behavior of the ferrocements can be remarkably improved by use of CFRC instead of ordinary mortar as a matrix]E]. 4.3 Dimensional stability Fig. 10 represents the relation between the drying period and drying shrinkage of CFRC using silica fume after autoclaving[5]. The drying shrinkage of CFRC increases with additional drying period, and becomes nearly constant at 14 days. The drying shrinkage is considerably reduced with an increase
"E 600-
1
PANBased
PitchBased
: M 5 soo& pj-400z fc 300v1 !! at
l
200-
loo: (/:Coefflclent
O1
5 Fiber Content (vol%)
Fig. 7. Comparison of flexural strengths of pitch- and PAN-based CFRCs using silica fume.
p
0
of
Pitch-Based
Correletlon)
1000
2000
4,(l-vf)+/GiE~v~ Fig. 8. Prediction of flexural strength of CFRC using silica tume
Y.
134
OHAMA
Table 3. Precast and cast-in-place applications of CFRC in Japan
___ Unmodified - SBR-Modified
Application Precast products
Cast-m-place applications
Example Curtain walls Free-access floor panels Permanent forms I- and L-shaped beams Conductive floor panels’*~” Wave absorbersn-‘x Ferrercement@ Mortars for external walls Mortars for repairing work1Y.2” Conductive concrete for lightning arresVh
75°C for 5 months[6].
Deflection (mm) Fig. 9. Flexural stress deflection curves for lightweight CFRC using fly ash.
in fiber content. Fig. 11 presents the drying shrinkage of lightweight CFRC after various curing[6]. The l-year drying shrinkage of the lightweight CFRC after autoclaving is less than one-fifth or one-sixth of that of CFRC continuously stored at 20°C and 65% R.H. or in 20°C water after demolding. This means that the autoclaved CFRC is much more dimensionally stable than CFRC cured by other methods. 4.4 Durability and other properties Fig. 12 shows the flexural stress deflection curves for lightweight CFRC after hot water immersion at
From this figure, it is clear that the flexural strength and toughness are not reduced by hot water. The freeze-thaw durability of the same CFRC is indicated in Fig. 13[6]. This CFRC has an excellent freeze-thaw durability even after 300 cycles. The effect of fiber content on the impact resistance of CFRC using silica fume is presented in Fig. 14[5]. Regardiess of the fiber length, the impact resistance of CFRC is markedly improved with raising fiber content. Unreinforced cement is fractured at one blow of a steel ball, but CFRC with a fiber content of 5 ~01% is not broken at 3000 blows or more. The electric conductivity and wave-absorbing property of CFRC that are developed by the inclusion of electrically conductive carbon fibers by nature are studied for its application to conductive concretes and wave absorbers[l6,17.18].
5. APPLICATIONS OF CFRC
CFRC has advantages over conventional concrete and mortar in its mechanical properties such as tensile and flexural strengths, toughness and impact resistance, dimensional stability, electric conductivity, wave absorbing property, and durability. The de-
Drying Period (d) 10 ,
0
-
30 I
.
Y s.
20 I
0 3.0
Fiber Len&b
Fig. 10. Drying period vs. drying shrinkage of CFRC using silica fume.
Carbon-cement
135
composites
5I h
I
I A
LI -toredat 20°C and 65 % R.H. after Autoclaving
11:: w/c 130 % 0.71 A/C Fiber Length 3 mm 2 VSlX Vf
40-
50-
I lr
. .
Water Curing
I
I
I
4w
13w
6m
I IY
Drying Period (r:ueek.m:l(onth,y:Year)
Fig. Il. Drying shrinkage of lightweight CFRC.
Precure
: Autoclaving
150
mersed for 4 weeks
;
Y 6. YM - loo P g LII : b : Z
5o
0
2
1
3
Deflection (8~)
Fig. 12. Flexural stress deflection curves for lightweight CFRC immersed in water at 75°C.
0
50
loo
velopment of CFRC applications should be performed to make the best use of its high performance. Table 3 lists the examples of the precast and castin-place applications of CFRC in the construction industry in Japan, where the development of the practical applications progresses most quickly in the world. Of such practical applications, the first largescale application was the use of lightweight CFRC panels with tile cladding in the Al Shaheed Monument in Baghdad, Iraq, which was constructed by Kajima Corporation, Japan, in 1982[6]. The monument consists of two domes of the same shape with a height of 40 m and a base diameter of 45 m. The domes were covered with CFRC panels, 100 x 50 x 4 cm (total area, 10000 m?), which had been produced in Japan and shipped to Baghdad. Fig. 15 shows the construction work of the ARK Mori Buildings, high-rise office buildings using CFRC curtain walls, which were completed by the same company in the Akasaka-Roppongi area of Tokyo in 1986. The buildings were covered with the curtain walls, 1.47 x 3.76 m, and the total area was 32000 m’. Approximately 187 tons of pitch-based
150
200
250
300
Number of Cycles for Freezing and Thawing
Fig. 13. Change in dynamic modulus of elasticity of lightweight CFRC through freezing and thawing cycling.
Y. OHAMA
736 Fiber Length 3nml
N
10 mm
Fig. 16. Heat-resisting 0
1
3
CFRC cylinder head for motorcycle engine.
5
Fiber Content (~01%)
Fig. 14. Carbon fiber content vs. impact resistance CFRC using silica fume.
of
short carbon fibers were used for the production of the CFRC curtain walls[6]. Recently, CFRC has drawn much attention as a high-performance material in the electrical and mechanical industries as well as the construction industry in Japan. Fig. 16 represents an interesting
application of heat-resisting CFRC with alumina cement to a cylinder head for motorcycle engine. This application has experimentally been developed by Kanno et al. [21].
6.
CONCLUSION
Some research on CFRC using PAN-based carbon fibers in the 1970s were not extended to its practical applications because of the high cost of the carbon
Fig. 15. Construction work of ARK Mori buildings using CFRC curtain walls. (By Courtesy of Kajima Corporation.)
Carbon-cement composites fibers and some difficulty in its fabrication processes. In recent years, a great deal of attention has been oriented towards the development of CFRC which is made in the three-dimensional random orientation of low-cost pitch-based short carbon fibers. In particular, CFRC products fabricated by this process are increasingly employed in the construction industry in Japan. However, the data on the durability and long-term performance of CFRC which is a newcomer to the field of fibrous construction materials are likely to be rather scant yet, and the accumulation of such data is strongly required. In the near future, the developments of the inexpensive pitchbased carbon fibers with higher mechanical properties and the cheaper fabrication processes for CFRC are expected to expand the applications of CFRC.
of Japan, Tokyo (1987).
8. S. Ito, M. Deguchi and K. Suzuki, In Semento Gijutsu 9.
10.
Nempo 1986, pp. 479-482. The Cement Association of Japan, Tokyo (1986). A. Briggs, Journal of Materials Science l&384 (1977). Y. Ohama and M. Amano, In Proceedings ofthe Twentyseventh Japan Congress on Materials Research, pp. 18% 191. The Societv of Materials Science. Kvoto. Jaoan (1984). ’ K. Shirakawa and K. Nakagawa, In Proceedings of the JCI 4th Conference, pp. 153-156. Japan Concrete Institute, Tokyo (1982). Q. Zheng and D. D. L. Chung, Presented at the SymI<
11.
12.
IL
posium S: Advanced Cements and Chemically Bonded Ceramics, MRS International Meeting on Advanced Materials, Tokyo (1988). 13. Y. Ohama, Y. Sato and M. Endo, In Proceedings of the Asia-Pacific Concrete Technology Conference ‘86,
pp. 5.1-5.8. Institute for International Research, Singapore (1986). 14. Y. Ohama, K. Demura and Y. Sato, In Proceedings of the International Symposium on Fibre Reinforced Concrete (Edited by V. S. Parameswaran and T. S. Krish-
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1. M. A. Ah, A. J. Majumdar and D. L. Rayment, Cement and Concrete Research 2. 201 (1972). 2. J. A. Wailer, in Fiber Reinforced Concrete; Publication SP-44, pp. 143-161. American Concrete Institute, Detroit (1974). 3. S. Sarkar and M. B. Bailey, In Fiber Reinforced Cement and Concrete (Edited by A. Neville), pp. 361371. The Construction Press, Lancaster (1975). 4. A. Briggs, D. H. Bowen and J. Kollek, In Proceedings of the Second International Conference on Carbon Fibres, Their Place in Modern Technology, Paper No.
15.
16. 17.
18.
17, pp. 114-121. Unwin, Old Working (1974).
5. Y. Ohama, M. Amano and M. Endo, Concrete International 7(3), 58 (1985). 6. S. Akihama, T. Suenaga and H. Nakagawa, Concrete International, 10,40 (1988). 7. S. Furukawa, Y. Tsuji and M. Miyamoto, In Review of the 41st General Meeting/Technical
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Review 1987), pp. 336-339. The Cement Association
Session (CAJ
19. 20. 21.
namoorthy), Vol. 1, pp. 3.23-3.31. Oxford & IBH Publishing, New Delhi (1987). Y. Ohama and A. Shirai, In Ocean Space Utilization ‘85 (Edited by W. Kato), Vol. 2, pp. 391-398. SpringerVerlag, Tokyo (1985). S. Takami and T. Higuchi, Kansai Denryoku K. K. Soken Hokoku, No. 29, pp. 250-252 (1982). T. Ito, M. Furukawa and-K. Yamamura, In Semento Giiutsu Nemoo 1987. DD. 467-470. The Cement Association of Japan, Tokyo (1987). Y. Shimizu, A. Nishikata, N. Maruyama and A. Sugiyama, Journal of the Institute of Television Engineers of Japan 40, 780 (1986). K. Nagata, The Bosui Journal lS(9), 115 (1984). H. Tanabe, Construction Finishing Techniques U( 134). 68 (1986). M. Kanno, Y. Ohama, H. Kawai and K. Demura, In Preprints of the 37th Annual Meeting, pp. 334-336. Society of Materials Science, Kyoto, Japan (1988).