10 CARBON-CEMENT COMPOSITES D.D.L.Chung 1 Introduction Carbon-cement composites refer to cement-matrix composites that contain carbon (e.g.carbon fibers).Carbon in a discontinuous form is usually used,because this form can be added to the cement mix in the mixer (i.e.it can be used as an admixture).In contrast, carbon in a continuous form cannot be used as an admixture.Mixing is the most convenient way of incorporating any ingredient in a cement-based material.Not only is mixing inex- pensive,it can be done in the field.Another disadvantage of using continuous carbon fibers is the high cost of continuous fibers compared to discontinuous fibers.Low cost is essential for a concrete to be practical.Although there are many forms of discontinuous carbon,short carbon fibers are the only form that has been shown to be useful for improving the proper- ties of cement-based materials.Therefore,this chapter is focused on cement-matrix composites containing short carbon fibers. Carbon fiber cement-matrix composites are structural materials that are gaining in importance quite rapidly due to the decrease in carbon fiber cost(Newman,1987)and the increasing demand of superior structural and functional properties.These composites con- tain short carbon fibers,typically 5 mm in length.However,due to the weak bond between carbon fiber and the cement matrix,continuous fibers(Furukawa et al.,1987;Saito et al., 1989;Wen and Chung,1999a)are much more effective than short fibers in reinforcing con- crete.Surface treatment of carbon fiber (e.g.by heating(Sugama et al.,1989)or by using ozone (Fu et al.,1996,1998a),silane (Xu and Chung,1999a,2000),SiO,particles (Yamada et al.,1991)or hot NaOH solution(Sugama et al.,1988))is useful for improving the bond between fiber and matrix,thereby improving the properties of the composite.In the case of surface treatment by ozone or silane,the improved bond is due to the enhanced wettability by water.Admixtures such as latex (Fu et al.,1996;Larson et al.,1990),methylcellulose (Fu et al.,1996)and silica fume (Katz et al.,1995)also help the bond. The effect of carbon fiber addition on the properties of concrete increases with fiber volume fraction(Park and Lee,1993),unless the fiber volume fraction is so high that the air void content becomes excessively high (Chen et al.,1997).(The air void content increases with fiber content and air voids tend to have a negative effect on many properties,such as the compressive strength.)In addition,the workability of the mix decreases with fiber con- tent (Park and Lee,1993).Moreover,the cost increases with fiber content.Therefore,a rather low volume fraction of fibers is desirable.A fiber content as low as 0.2 vol%is effec- tive (Chen and Chung,1993a),although fiber contents exceeding 1 vol%are more common (Akihama et al.,1984;Brandt and Kucharska,1996).The required fiber content increases ©2003 Taylor&Francis
10 CARBON–CEMENT COMPOSITES D. D. L. Chung 1 Introduction Carbon–cement composites refer to cement–matrix composites that contain carbon (e.g. carbon fibers). Carbon in a discontinuous form is usually used, because this form can be added to the cement mix in the mixer (i.e. it can be used as an admixture). In contrast, carbon in a continuous form cannot be used as an admixture. Mixing is the most convenient way of incorporating any ingredient in a cement-based material. Not only is mixing inexpensive, it can be done in the field. Another disadvantage of using continuous carbon fibers is the high cost of continuous fibers compared to discontinuous fibers. Low cost is essential for a concrete to be practical. Although there are many forms of discontinuous carbon, short carbon fibers are the only form that has been shown to be useful for improving the properties of cement-based materials. Therefore, this chapter is focused on cement–matrix composites containing short carbon fibers. Carbon fiber cement–matrix composites are structural materials that are gaining in importance quite rapidly due to the decrease in carbon fiber cost (Newman, 1987) and the increasing demand of superior structural and functional properties. These composites contain short carbon fibers, typically 5 mm in length. However, due to the weak bond between carbon fiber and the cement matrix, continuous fibers (Furukawa et al., 1987; Saito et al., 1989; Wen and Chung, 1999a) are much more effective than short fibers in reinforcing concrete. Surface treatment of carbon fiber (e.g. by heating (Sugama et al., 1989) or by using ozone (Fu et al., 1996, 1998a), silane (Xu and Chung, 1999a, 2000), SiO2 particles (Yamada et al., 1991) or hot NaOH solution (Sugama et al., 1988)) is useful for improving the bond between fiber and matrix, thereby improving the properties of the composite. In the case of surface treatment by ozone or silane, the improved bond is due to the enhanced wettability by water. Admixtures such as latex (Fu et al., 1996; Larson et al., 1990), methylcellulose (Fu et al., 1996) and silica fume (Katz et al., 1995) also help the bond. The effect of carbon fiber addition on the properties of concrete increases with fiber volume fraction (Park and Lee, 1993), unless the fiber volume fraction is so high that the air void content becomes excessively high (Chen et al., 1997). (The air void content increases with fiber content and air voids tend to have a negative effect on many properties, such as the compressive strength.) In addition, the workability of the mix decreases with fiber content (Park and Lee, 1993). Moreover, the cost increases with fiber content. Therefore, a rather low volume fraction of fibers is desirable. A fiber content as low as 0.2 vol% is effective (Chen and Chung, 1993a), although fiber contents exceeding 1 vol% are more common (Akihama et al., 1984; Brandt and Kucharska, 1996). The required fiber content increases © 2003 Taylor & Francis
with the particle size of the aggregate,as the flexural strength decreases with increasing par- ticle size (Kamakura et al.,1983). Effective use of the carbon fibers in concrete requires dispersion of the fibers in the mix. The dispersion is enhanced by using silica fume (a fine particulate)as an admixture(Ohama and Amano,1983;Ohama et al.,1985;Katz and Bentur,1994;Chen et al.,1997).A typi- cal silica fume content is 15%by weight of cement (Chen et al.,1997).The silica fume is typically used along with a small amount(0.4%by weight of cement)of methylcellulose for helping the dispersion of the fibers and the workability of the mix (Chen et al.,1997).Latex (typically 15-20%by weight of cement)is much less effective than silica fume for helping the fiber dispersion,but it enhances the workability,flexural strength,flexural toughness, impact resistance,frost resistance and acid resistance (Soroushian et al.,1991;Zayat and Bayasi,1996;Chen et al.,1997).The ease of dispersion increases with decreasing fiber length (Ohama et al.,1985). The improved structural properties rendered by carbon fiber addition pertain to the increased tensile and flexible strengths,the increased tensile ductility and flexural tough- ness,the enhanced impact resistance,the reduced drying shrinkage and the improved freeze- thaw durability (Kamakura et al.,1983;Ohama and Amano,1983;Akihama et al.,1984; Ohama et al.,1985;Lal,1990;Park and Lee,1990;Soroushian,1990;Park et al.,1991; Soroushian et al.,1992a,b;Park and Lee,1993;Toutanji et al.,1993;Chen and Chung, 1993a;Katz and Bentur,1994;Banthia et al.,1994a,b,1998;Banthia and Sheng,1996; Pigeon et al.,1996;Zayat and Bayasi,1996;Chen et al.,1997).The tensile and flexural strengths decrease with increasing specimen size,such that the size effect becomes larger as the fiber length increases(Urano et al.,1996).The low drying shrinkage is valuable for large structures and for use in repair (Chen et al.,1995;Ali and Ambalavanan,1998)and in joining bricks in a brick structure(Zhu and Chung,1997;Zhu et al.,1997). The functional properties rendered by carbon fiber addition pertain to the strain sensing ability (Chen and Chung,1993b,1995a,1996a,b;Chung,1995;Zhao et al.,1995;Fu and Chung,1996,1997a;Mao et al.,1996a,b;Fu et al.,1997,1998a,b;Sun et al.,1998,2000; Shi and Chung,1999;Wen and Chung,2000a,2001a,b,2002a,b)(for smart structures),the temperature sensing ability (Sun et al.,1998a,b;Wen and Chung,1999b,2000b-d),the damage sensing ability (Chen and Chung,1993b,1996b;Lee and Batson,1996;Bontea et al.,2001;Wen and Chung,20001f),the thermoelectric behavior (Chen and Chung, 1993b,1996b;Sun et al.,1998a,b;Wen and Chung,1999c,2000b-d),the thermal insulation ability (Shinozaki,1990;Fu and Chung,1999;Xu and Chung,1999b)(to save energy for buildings),the electrical conduction ability(Clemena,1988;Banthia et al.,1992;Chen and Chung,1993c,1995b;Fu and Chung,1995;Shui et al.,1995;Xie et al.,1996;Brousseau and Pye,1997;Hou and Chung,1997;Wang et al.,1998;Wen and Chung,2001c-f) (to facilitate cathodic protection of embedded steel and to provide electrical grounding or connection),and the radio wave reflection/absorption ability(Shimizu et al.,1986;Fujiwara and Ujie,1987;Fu and Chung,1997b,1998a,b)(for electromagnetic interference or EMI shielding,for lateral guidance in automatic highways,and for television image transmission) In relation to the structural properties,carbon fibers compete with glass,polymer,and steel fibers (Lal,1990;Mobasher and Li,1994,1996;Banthia et al.,1994a,b,1998;Banthia and Sheng,1996;Pigeon et al.,1996;Chen and Chung,1996c).Carbon fibers (isotropic pitch based)(Chen and Chung,1996c;Newman,1987)are advantageous in their superior ability to increase the tensile strength of concrete,even though the tensile strength,modu- lus and ductility of the isotropic pitch based carbon fibers are low compared to most other ©2003 Taylor&Francis
with the particle size of the aggregate, as the flexural strength decreases with increasing particle size (Kamakura et al., 1983). Effective use of the carbon fibers in concrete requires dispersion of the fibers in the mix. The dispersion is enhanced by using silica fume (a fine particulate) as an admixture (Ohama and Amano, 1983; Ohama et al., 1985; Katz and Bentur, 1994; Chen et al., 1997). A typical silica fume content is 15% by weight of cement (Chen et al., 1997). The silica fume is typically used along with a small amount (0.4% by weight of cement) of methylcellulose for helping the dispersion of the fibers and the workability of the mix (Chen et al., 1997). Latex (typically 15–20% by weight of cement) is much less effective than silica fume for helping the fiber dispersion, but it enhances the workability, flexural strength, flexural toughness, impact resistance, frost resistance and acid resistance (Soroushian et al., 1991; Zayat and Bayasi, 1996; Chen et al., 1997). The ease of dispersion increases with decreasing fiber length (Ohama et al., 1985). The improved structural properties rendered by carbon fiber addition pertain to the increased tensile and flexible strengths, the increased tensile ductility and flexural toughness, the enhanced impact resistance, the reduced drying shrinkage and the improved freezethaw durability (Kamakura et al., 1983; Ohama and Amano, 1983; Akihama et al., 1984; Ohama et al., 1985; Lal, 1990; Park and Lee, 1990; Soroushian, 1990; Park et al., 1991; Soroushian et al., 1992a,b; Park and Lee, 1993; Toutanji et al., 1993; Chen and Chung, 1993a; Katz and Bentur, 1994; Banthia et al., 1994a,b, 1998; Banthia and Sheng, 1996; Pigeon et al., 1996; Zayat and Bayasi, 1996; Chen et al., 1997). The tensile and flexural strengths decrease with increasing specimen size, such that the size effect becomes larger as the fiber length increases (Urano et al., 1996). The low drying shrinkage is valuable for large structures and for use in repair (Chen et al., 1995; Ali and Ambalavanan, 1998) and in joining bricks in a brick structure (Zhu and Chung, 1997; Zhu et al., 1997). The functional properties rendered by carbon fiber addition pertain to the strain sensing ability (Chen and Chung, 1993b, 1995a, 1996a,b; Chung, 1995; Zhao et al., 1995; Fu and Chung, 1996, 1997a; Mao et al., 1996a,b; Fu et al., 1997, 1998a,b; Sun et al., 1998, 2000; Shi and Chung, 1999; Wen and Chung, 2000a, 2001a,b, 2002a,b) (for smart structures), the temperature sensing ability (Sun et al., 1998a,b; Wen and Chung, 1999b, 2000b–d), the damage sensing ability (Chen and Chung, 1993b, 1996b; Lee and Batson, 1996; Bontea et al., 2001; Wen and Chung, 20001f), the thermoelectric behavior (Chen and Chung, 1993b, 1996b; Sun et al., 1998a,b; Wen and Chung, 1999c, 2000b–d), the thermal insulation ability (Shinozaki, 1990; Fu and Chung, 1999; Xu and Chung, 1999b) (to save energy for buildings), the electrical conduction ability (Clemena, 1988; Banthia et al., 1992; Chen and Chung, 1993c, 1995b; Fu and Chung, 1995; Shui et al., 1995; Xie et al., 1996; Brousseau and Pye, 1997; Hou and Chung, 1997; Wang et al., 1998; Wen and Chung, 2001c–f) (to facilitate cathodic protection of embedded steel and to provide electrical grounding or connection), and the radio wave reflection/absorption ability (Shimizu et al., 1986; Fujiwara and Ujie, 1987; Fu and Chung, 1997b, 1998a,b) (for electromagnetic interference or EMI shielding, for lateral guidance in automatic highways, and for television image transmission). In relation to the structural properties, carbon fibers compete with glass, polymer, and steel fibers (Lal, 1990; Mobasher and Li, 1994, 1996; Banthia et al., 1994a,b, 1998; Banthia and Sheng, 1996; Pigeon et al., 1996; Chen and Chung, 1996c). Carbon fibers (isotropic pitch based) (Chen and Chung, 1996c; Newman, 1987) are advantageous in their superior ability to increase the tensile strength of concrete, even though the tensile strength, modulus and ductility of the isotropic pitch based carbon fibers are low compared to most other © 2003 Taylor & Francis
fibers.Carbon fibers are also advantageous in the relative chemical inertness(Uomoto and Katsuki,1994-5).PAN-based carbon fibers are also used(Ohama and Amano,1983;Katz and Bentur,1994;Toutanji et al.,1993,1994),although they are more commonly used as continuous fibers than short fibers.Carbon-coated glass fibers (Huang et al.,1996,1997) and submicron diameter carbon filaments(Shui et al.,1995;Xie et al.,1996;Fu and Chung, 1998a,b)are even less commonly used,although the former is attractive for the low cost of glass fibers and the latter is attractive for its high radio wave reflectivity(which results from the skin effect).C-shaped carbon fibers are more effective for strengthening than round car- bon fibers(Kim and Park,1998),but their relatively large diameter makes them less attrac- tive.Carbon fibers can be used in concrete together with steel fibers,as the addition of short carbon fibers to steel fiber reinforced mortar increases the fracture toughness of the inter- facial zone between steel fiber and the cement matrix (Igarashi and Kawamura,1994). Carbon fibers can also be used in concrete together with steel bars(Bayasi and Zeng,1997; Campione et al.,1999),or together with carbon fiber reinforced polymer rods (Yamada etal,1995). In relation to most functional properties,carbon fibers are exceptional compared to the other fiber types.Carbon fibers are electrically conducting,in contrast to glass and polymer fibers,which are not conducting.Steel fibers are conducting,but their typical diameter (60 um)is much larger than the diameter of a typical carbon fiber(15 um).The combi- nation of electrical conductivity and small diameter makes carbon fibers superior to the other fiber types in the area of strain sensing and electrical conduction.However,carbon fibers are inferior to steel fibers for providing thermoelectric composites,due to the high electron concentration in steel and the low hole concentration in carbon. Although carbon fibers are thermally conducting,addition of carbon fibers to concrete lowers the thermal conductivity (Fu and Chung,1999),thus allowing applications related to thermal insulation.This effect of carbon fiber addition is due to the increase in air void con- tent.The electrical conductivity of carbon fibers is higher than that of the cement matrix by about eight orders of magnitude,whereas the thermal conductivity of carbon fibers is higher than that of the cement matrix by only one or two orders of magnitude.As a result,the elec- trical conductivity is increased upon carbon fiber addition in spite of the increase in air void content,but the thermal conductivity is decreased upon fiber addition. The use of pressure after casting (Delvasto et al.,1986),and extrusion(Shao et al.,1995; Park,1998)can result in composites with superior microstructure and properties.Moreover, extrusion improves the shapability (Shao et al.,1995). This chapter is focused on short carbon fiber reinforced cement-matrix composites, including concrete(with fine and coarse aggregates),mortar(with fine aggregate and no coarse aggregate)and cement paste.Previous reviews are noted (Ohama,1989;Inagaki, Table 10.I Properties of isotropic-pitch-based carbon fibers Filament diameter 15±3μm Tensile strength 690 MPa Tensile modulus 48GPa Elongation at break 1.4% Electrical resistivity 3.0×10-30cm Specific gravity 1.6gcm-3 Carbon content 98wt% ©2003 Taylor&Francis
fibers. Carbon fibers are also advantageous in the relative chemical inertness (Uomoto and Katsuki, 1994–5). PAN-based carbon fibers are also used (Ohama and Amano, 1983; Katz and Bentur, 1994; Toutanji et al., 1993, 1994), although they are more commonly used as continuous fibers than short fibers. Carbon-coated glass fibers (Huang et al., 1996, 1997) and submicron diameter carbon filaments (Shui et al., 1995; Xie et al., 1996; Fu and Chung, 1998a,b) are even less commonly used, although the former is attractive for the low cost of glass fibers and the latter is attractive for its high radio wave reflectivity (which results from the skin effect). C-shaped carbon fibers are more effective for strengthening than round carbon fibers (Kim and Park, 1998), but their relatively large diameter makes them less attractive. Carbon fibers can be used in concrete together with steel fibers, as the addition of short carbon fibers to steel fiber reinforced mortar increases the fracture toughness of the interfacial zone between steel fiber and the cement matrix (Igarashi and Kawamura, 1994). Carbon fibers can also be used in concrete together with steel bars (Bayasi and Zeng, 1997; Campione et al., 1999), or together with carbon fiber reinforced polymer rods (Yamada et al., 1995). In relation to most functional properties, carbon fibers are exceptional compared to the other fiber types. Carbon fibers are electrically conducting, in contrast to glass and polymer fibers, which are not conducting. Steel fibers are conducting, but their typical diameter ( 60m) is much larger than the diameter of a typical carbon fiber (15m). The combination of electrical conductivity and small diameter makes carbon fibers superior to the other fiber types in the area of strain sensing and electrical conduction. However, carbon fibers are inferior to steel fibers for providing thermoelectric composites, due to the high electron concentration in steel and the low hole concentration in carbon. Although carbon fibers are thermally conducting, addition of carbon fibers to concrete lowers the thermal conductivity (Fu and Chung, 1999), thus allowing applications related to thermal insulation. This effect of carbon fiber addition is due to the increase in air void content. The electrical conductivity of carbon fibers is higher than that of the cement matrix by about eight orders of magnitude, whereas the thermal conductivity of carbon fibers is higher than that of the cement matrix by only one or two orders of magnitude. As a result, the electrical conductivity is increased upon carbon fiber addition in spite of the increase in air void content, but the thermal conductivity is decreased upon fiber addition. The use of pressure after casting (Delvasto et al., 1986), and extrusion (Shao et al., 1995; Park, 1998) can result in composites with superior microstructure and properties. Moreover, extrusion improves the shapability (Shao et al., 1995). This chapter is focused on short carbon fiber reinforced cement–matrix composites, including concrete (with fine and coarse aggregates), mortar (with fine aggregate and no coarse aggregate) and cement paste. Previous reviews are noted (Ohama, 1989; Inagaki, Table 10.1 Properties of isotropic-pitch-based carbon fibers Filament diameter 15 3m Tensile strength 690 MPa Tensile modulus 48 GPa Elongation at break 1.4% Electrical resistivity 3.0 103 cm Specific gravity 1.6 g cm3 Carbon content 98 wt% © 2003 Taylor & Francis
1991;Lin,1994;Zheng and Feldman,1995;Banthia,1996;Kucharska and Brandt,1997; Chung,1999,2000). Table 10.1 shows the properties of the isotropic-pitch-based carbon fibers(15 um in diameter,nominally 5mm long)used by the author in the cement-matrix composites described below for the purpose of illustration. 2 Structural behavior The properties relevant to the structural behavior of cement-matrix composites containing short carbon fibers are given in this section. Tables 10.2 and 10.3 show the tensile strength and modulus respectively of twelve types of cement pastes (Xu and Chung,1999a,2000).The strength is slightly increased by the addition of methylcellulose and defoamer,but the modulus is slightly decreased by the addi- tion of methylcellulose and defoamer.However,both strength and modulus are increased by the addition of fibers.The effectiveness of the fibers in increasing strength and modulus increases in the following order:as-received fibers,O3-treated fibers,dichromate-treated Table 10.2 Tensile strength (MPa)of cement pastes with and without fibers Formulation As-received Silane-treated silica fume silica fume A 1.53±0.06 2.04±0.06 A+ 1.66±0.07 2.25±0.09 A 2.00±0.09 2.50±0.11 A+0 2.25±0.07 2.67±0.09 AK 2.32±0.08 2.85±0.11 A+S 2.47±0.11 3.12±0.12 Notes A:cement water water reducing agent silica fume A+:A+methylcellulose +defoamer. A+F:A++as-received fibers. A*O:A++O3-treated fibers. A*K:A+dichromate-treated fibers. A*S:A++silane-treated fibers. Table 10.3 Tensile modulus (GPa)of cement pastes with and without fibers.Refer to Note in Table 10.2 Formulation As-received Silane-treated silica fume silica fume A 10.2±0.7 11.5±0.6 A+ 9.3±0.5 10.7±0.4 AF 10.9±0.3 12.9±0.7 AO 11.9±0.3 13.1±0.6 A+K 12.7±0.4 14.3±0.4 A+S 13.3±0.5 15.2±0.8 ©2003 Taylor&Francis
1991; Lin, 1994; Zheng and Feldman, 1995; Banthia, 1996; Kucharska and Brandt, 1997; Chung, 1999, 2000). Table 10.1 shows the properties of the isotropic-pitch-based carbon fibers (15m in diameter, nominally 5mm long) used by the author in the cement–matrix composites described below for the purpose of illustration. 2 Structural behavior The properties relevant to the structural behavior of cement–matrix composites containing short carbon fibers are given in this section. Tables 10.2 and 10.3 show the tensile strength and modulus respectively of twelve types of cement pastes (Xu and Chung, 1999a, 2000). The strength is slightly increased by the addition of methylcellulose and defoamer, but the modulus is slightly decreased by the addition of methylcellulose and defoamer. However, both strength and modulus are increased by the addition of fibers. The effectiveness of the fibers in increasing strength and modulus increases in the following order: as-received fibers, O3-treated fibers, dichromate-treated Table 10.2 Tensile strength (MPa) of cement pastes with and without fibers Formulation As-received Silane-treated silica fume silica fume A 1.53 0.06 2.04 0.06 A 1.66 0.07 2.25 0.09 AF 2.00 0.09 2.50 0.11 AO 2.25 0.07 2.67 0.09 AK 2.32 0.08 2.85 0.11 AS 2.47 0.11 3.12 0.12 Notes A: cement water water reducing agent silica fume A: A methylcellulose defoamer. AF: A as-received fibers. AO: A O3-treated fibers. AK: A dichromate-treated fibers. AS: A silane-treated fibers. Table 10.3 Tensile modulus (GPa) of cement pastes with and without fibers. Refer to Note in Table 10.2 Formulation As-received Silane-treated silica fume silica fume A 10.2 0.7 11.5 0.6 A 9.3 0.5 10.7 0.4 AF 10.9 0.3 12.9 0.7 AO 11.9 0.3 13.1 0.6 AK 12.7 0.4 14.3 0.4 AS 13.3 0.5 15.2 0.8 © 2003 Taylor & Francis
Table 10.4 Tensile ductility (%of cement pastes with and without fibers.Refer to Note in Table 10.2 Formulation As-received Silane-treated silica fume silica fume A 0.020±0.0004 0.020±0.0004 A+ 0.023±0.0004 0.021±0.0004 A+F 0.025±0.0003 0.024±0.0004 AO 0.026±0.0003 0.027±0.0004 AK 0.028±0.0003 0.030±0.0004 A+S 0.031±0.0004 0.034±0.0004 Table 10.5 Air void content(%,0.12)of cement pastes with and without fibers.Refer to Note in Table 10.2 Formulation As-received Silane-treated silica fume silica fume A 3.73 3.26 A+ 3.42 3.01 A+F 5.32 4.89 AO 5.07 4.65 A+K 5.01 4.49 A+S 4.85 4.16 fibers,and silane-treated fibers.This trend applies whether the silica fume is as-received or silane-treated.For any of the formulations,silane-treated silica fume gives substantially higher strength and modulus than as-received silica fume.The highest tensile strength and modulus are exhibited by cement paste with silane-treated silica fume and silane-treated fibers.Silane treatments of silica fume and of fibers are about equally valuable in providing strengthening. Table 10.4 shows the tensile ductility.It is slightly increased by the addition of methyl- cellulose and defoamer,and is further increased by the further addition of fibers.The effec- tiveness of the fibers in increasing the ductility also increases in the above order.This trend applies whether the silica fume is as-received or silane-treated.For any of the formulations involving surface treated fibers,silane-treated silica fume gives higher ductility than as- received silica fume.The highest ductility is exhibited by cement paste with silane-treated silica fume and silane-treated fibers. Table 10.5 shows the air void content.It is decreased by the addition of methylcellulose and defoamer,but is increased by the further addition of fibers,whether the fibers have been surface treated or not.Among the formulations with fibers,the air void content decreases in the following order:as-received fibers,O;-treated fibers,dichromate-treated fibers and silane-treated fibers.This trend applies whether the silica fume is as-received or silane- treated.For any of the formulations (including those without fibers),silane-treated silica fume gives lower air void content than as-received silica fume. Tables 10.6 and 10.7 give the dynamic flexural properties of twelve types of cement pastes.Six of the types have as-received silica fume;the other six have silane-treated silica fume.The loss tangent(Table 10.6)is increased slightly by the addition of methylcellulose. ©2003 Taylor&Francis
fibers, and silane-treated fibers. This trend applies whether the silica fume is as-received or silane-treated. For any of the formulations, silane-treated silica fume gives substantially higher strength and modulus than as-received silica fume. The highest tensile strength and modulus are exhibited by cement paste with silane-treated silica fume and silane-treated fibers. Silane treatments of silica fume and of fibers are about equally valuable in providing strengthening. Table 10.4 shows the tensile ductility. It is slightly increased by the addition of methylcellulose and defoamer, and is further increased by the further addition of fibers. The effectiveness of the fibers in increasing the ductility also increases in the above order. This trend applies whether the silica fume is as-received or silane-treated. For any of the formulations involving surface treated fibers, silane-treated silica fume gives higher ductility than asreceived silica fume. The highest ductility is exhibited by cement paste with silane-treated silica fume and silane-treated fibers. Table 10.5 shows the air void content. It is decreased by the addition of methylcellulose and defoamer, but is increased by the further addition of fibers, whether the fibers have been surface treated or not. Among the formulations with fibers, the air void content decreases in the following order: as-received fibers, O3-treated fibers, dichromate-treated fibers and silane-treated fibers. This trend applies whether the silica fume is as-received or silanetreated. For any of the formulations (including those without fibers), silane-treated silica fume gives lower air void content than as-received silica fume. Tables 10.6 and 10.7 give the dynamic flexural properties of twelve types of cement pastes. Six of the types have as-received silica fume; the other six have silane-treated silica fume. The loss tangent (Table 10.6) is increased slightly by the addition of methylcellulose. Table 10.4 Tensile ductility (%) of cement pastes with and without fibers. Refer to Note in Table 10.2 Formulation As-received Silane-treated silica fume silica fume A 0.020 0.0004 0.020 0.0004 A 0.023 0.0004 0.021 0.0004 AF 0.025 0.0003 0.024 0.0004 AO 0.026 0.0003 0.027 0.0004 AK 0.028 0.0003 0.030 0.0004 AS 0.031 0.0004 0.034 0.0004 Table 10.5 Air void content (%, 0.12) of cement pastes with and without fibers. Refer to Note in Table 10.2 Formulation As-received Silane-treated silica fume silica fume A 3.73 3.26 A 3.42 3.01 AF 5.32 4.89 AO 5.07 4.65 AK 5.01 4.49 AS 4.85 4.16 © 2003 Taylor & Francis
Table 10.6 Loss tangent (tan ,0.002)of cement pastes.Refer to Note in Table 10.2 Formulation With as-received silica fume (Hz) With silane-treated silica fume (Hz) 0.2 1.0 2.0 0.2 1.0 2.0 A 0.082 0.030 <10-4 0.087 0.032 <10-4 4+ 0.102 0.045 <10-4 0.093 0.040 <10-4 A 0.089 0.033 <10-4 0.084 0.034 <104 0.085 0.043 <10-4 0.084 0.032 <104 AK 0.079 0.039 <10-4 0.086 0.035 <10-4 AS 0.076 0.036 <10-4 0.083 0.033 <10-4 Table 10.7 Storage modulus(GPa,+0.03)of cement pastes.Refer to Note in Table 10.2 Formulation With as-received silica fume (Hz) With silane-treated silica fume (Hz) 0.2 1.0 2.0z 0.2 1.0 2.0 A 12.71 12.14 11.93 16.75 16.21 15.95 + 11.52 10.61 10.27 15.11 14.73 14.24 A+F 13.26 13.75 13.83 17.44 17.92 18.23 AO 14.14 14.46 14.72 18.92 19.36 19.57 AK 15.42 16.15 16.53 19.33 19.85 20.23 A+S 17.24 17.67 15.95 21.34 21.65 21.97 Table 10.8 Drying shrinkage strain(104,+0.015)different curing ages Formulation With as-received silica fume (days) With silane-treated silica fume(days) 1 4 8 19 4 8 19 B 1.128 3.021 3.722 4.365 1.013 2.879 3.623 4.146 BF 0.832 2.417 3.045 3.412 0.775 2.246 2.810 3.113 BO 0.825 2.355 3.022 3.373 0.764 2.235 2.793 3.014 BK 0.819 2.321 3.019 3.372 0.763 2.232 2.790 3.010 BS 0.812 2.316 2.976 3.220 0.752 2.118 2.724 2.954 Notes B:cement waterwater reducing agent silica fume methylcellulose defoamer. BF:B+as-received fibers BO:B+O:-treated fibers. BK:B+dichromate-treated fibers. BS:B+silane-treated fibers. Further addition of carbon fibers decreases the loss tangent.The loss tangent decreases in the following order:as-received fibers,ozone-treated fibers,dichromate-treated fibers and silane-treated fibers,at least for the case of as-received silica fume at 0.2 Hz.The storage modulus(Table 10.7)is decreased by the addition of methylcellulose.Further addi- tion of carbon fibers increases the storage modulus,such that the modulus increases in the ©2003 Taylor&Francis
Table 10.6 Loss tangent (tan , 0.002) of cement pastes. Refer to Note in Table 10.2 Formulation With as-received silica fume (Hz) With silane-treated silica fume (Hz) 0.2 1.0 2.0 0.2 1.0 2.0 A 0.082 0.030 104 0.087 0.032 104 A 0.102 0.045 104 0.093 0.040 104 AF 0.089 0.033 104 0.084 0.034 104 AO 0.085 0.043 104 0.084 0.032 104 AK 0.079 0.039 104 0.086 0.035 104 AS 0.076 0.036 104 0.083 0.033 104 Table 10.7 Storage modulus (GPa, 0.03) of cement pastes. Refer to Note in Table 10.2 Formulation With as-received silica fume (Hz) With silane-treated silica fume (Hz) 0.2 1.0 2.0 z 0.2 1.0 2.0 A 12.71 12.14 11.93 16.75 16.21 15.95 A 11.52 10.61 10.27 15.11 14.73 14.24 AF 13.26 13.75 13.83 17.44 17.92 18.23 AO 14.14 14.46 14.72 18.92 19.36 19.57 AK 15.42 16.15 16.53 19.33 19.85 20.23 AS 17.24 17.67 15.95 21.34 21.65 21.97 Table 10.8 Drying shrinkage strain (104 , 0.015) different curing ages Formulation With as-received silica fume (days) With silane-treated silica fume (days) 1 4 8 19 1 4 8 19 B 1.128 3.021 3.722 4.365 1.013 2.879 3.623 4.146 BF 0.832 2.417 3.045 3.412 0.775 2.246 2.810 3.113 BO 0.825 2.355 3.022 3.373 0.764 2.235 2.793 3.014 BK 0.819 2.321 3.019 3.372 0.763 2.232 2.790 3.010 BS 0.812 2.316 2.976 3.220 0.752 2.118 2.724 2.954 Notes B: cement water water reducing agent silica fume methylcellulose defoamer. BF: B as-received fibers. BO: B O3-treated fibers. BK: B dichromate-treated fibers. BS: B silane-treated fibers. Further addition of carbon fibers decreases the loss tangent. The loss tangent decreases in the following order: as-received fibers, ozone-treated fibers, dichromate-treated fibers and silane-treated fibers, at least for the case of as-received silica fume at 0.2 Hz. The storage modulus (Table 10.7) is decreased by the addition of methylcellulose. Further addition of carbon fibers increases the storage modulus, such that the modulus increases in the © 2003 Taylor & Francis
order:as-received fibers,ozone-treated fibers,dichromate-treated fibers and silane-treated fibers. Table 10.8 gives the drying shrinkage strain of ten types of cement paste as a function of curing age.The drying shrinkage is decreased by the addition of carbon fibers,such that it decreases in the following order:as-received fibers,ozone-treated fibers,dichromate-treated fibers,and silane-treated fibers.The drying shrinkage is decreased by the use of silane- treated silica fume in place of as-received silica fume,whether fibers are present or not. 3 Thermal behavior Table 10.9 shows the specific heat of cement pastes(Xu and Chung,1999b,2000).The spe- cific heat is significantly increased by the addition of silica fume.It is further increased by the further addition of methylcellulose and defoamer.It is still further increased by the still further addition of carbon fibers.The effectiveness of the fibers in increasing the specific heat increases in the following order:as-received fibers,O3-treated fibers,dichromate-treated fibers and silane-treated fibers.For any of the formulations,silane-treated silica fume gives higher specific heat than as-received silica fume.The highest specific heat is exhibited by the cement paste with silane-treated silica fume and silane-treated fibers.Silane treatment of fibers is more valuable than that of silica fume for increasing the specific heat Table 10.10 shows the thermal conductivity.It is significantly decreased by the addition of silica fume.The further addition of methylcellulose and defoamer or the still further Table 10.9 Specific heat (Jg-K-1,0.001)of cement pastes.The value for plain cement paste (with cement and water only)is 0.736Jg-K-.Refer to Note in Table 10.2 Formulation As-received Silane-treated silica fume silica fume A 0.782 0.788 A+ 0.793 0.803 A+F 0.804 0.807 AO 0.809 0.813 AK 0.812 0.816 A+S 0.819 0.823 Table 10.10 Thermal conductivity (Wm-K-,0.03) of cement pastes.The value for plain cement paste (with cement and water only)is 0.53 Wm-K-.Refer to Note in Table 10.2 Formulation As-received Silane-treated silica fume silica-fume A 0.35 0.33 A+ 0.34 0.30 A 0.35 0.34 AO 0.38 0.36 AK 0.39 0.37 AS 0.34 0.32 ©2003 Taylor&Francis
order: as-received fibers, ozone-treated fibers, dichromate-treated fibers and silane-treated fibers. Table 10.8 gives the drying shrinkage strain of ten types of cement paste as a function of curing age. The drying shrinkage is decreased by the addition of carbon fibers, such that it decreases in the following order: as-received fibers, ozone-treated fibers, dichromate-treated fibers, and silane-treated fibers. The drying shrinkage is decreased by the use of silanetreated silica fume in place of as-received silica fume, whether fibers are present or not. 3 Thermal behavior Table 10.9 shows the specific heat of cement pastes (Xu and Chung, 1999b, 2000). The specific heat is significantly increased by the addition of silica fume. It is further increased by the further addition of methylcellulose and defoamer. It is still further increased by the still further addition of carbon fibers. The effectiveness of the fibers in increasing the specific heat increases in the following order: as-received fibers, O3-treated fibers, dichromate-treated fibers and silane-treated fibers. For any of the formulations, silane-treated silica fume gives higher specific heat than as-received silica fume. The highest specific heat is exhibited by the cement paste with silane-treated silica fume and silane-treated fibers. Silane treatment of fibers is more valuable than that of silica fume for increasing the specific heat. Table 10.10 shows the thermal conductivity. It is significantly decreased by the addition of silica fume. The further addition of methylcellulose and defoamer or the still further Table 10.9 Specific heat (Jg1 K1 , 0.001) of cement pastes. The value for plain cement paste (with cement and water only) is 0.736 J g1 K1 . Refer to Note in Table 10.2 Formulation As-received Silane-treated silica fume silica fume A 0.782 0.788 A 0.793 0.803 AF 0.804 0.807 AO 0.809 0.813 AK 0.812 0.816 AS 0.819 0.823 Table 10.10 Thermal conductivity (Wm1 K1 , 0.03) of cement pastes.The value for plain cement paste (with cement and water only) is 0.53 Wm1 K1 . Refer to Note in Table 10.2 Formulation As-received Silane-treated silica fume silica-fume A 0.35 0.33 A 0.34 0.30 AF 0.35 0.34 AO 0.38 0.36 AK 0.39 0.37 AS 0.34 0.32 © 2003 Taylor & Francis
addition of fibers has little effect on the density.Surface treatment of the fibers by ozone or dichromate slightly increases the thermal conductivity,whereas surface treatment of the fibers by silane has negligible effect.For any of the formulations,silane-treated silica fume gives slightly lower(or essentially the same)thermal conductivity as as-received silica fume. Silane treatments of silica fume and of fibers contribute comparably to reducing the thermal conductivity. 4 Electrical behavior Figure 10.1 gives the volume electrical resistivity of composites at seven days of curing.The resistivity decreases much with increasing fiber volume fraction,whether a second filler (silica fume or sand)is present or not (Chen and Chung,1995b).When sand is absent,the addition of silica fume decreases the resistivity at all fiber volume fractions except the high- est volume fraction of 4.24%;the decrease is most significant at the lowest fiber volume fraction of 0.53%.When sand is present,the addition of silica fume similarly decreases the resistivity,such that the decrease is most significant at fiber volume fractions below 1%. When silica fume is absent,the addition of sand decreases the resistivity only when the fiber volume fraction is below about 0.5%;at high fiber volume fractions,the addition of sand even increases the resistivity due to the porosity induced by the sand.Thus,the addition of a second filler(silica fume or sand)that is essentially non-conducting decreases the resis- tivity of the composite only at low volume fractions of the carbon fibers and the maximum fiber volume fraction for the resistivity to decrease is larger when the particle size of the filler is smaller.The resistivity decrease is attributed to the improved fiber dispersion due to 1.000,000 100,000 g (wo 10,0001 g 1,000 11 (c) (d)i: 100 (a) 10 (b) 0 2 3 4 5 Vol.%fibers Figure 10.I Variation of the volume electrical resistivity with carbon fiber volume fraction(Chen and Chung,1995b).(a)Without sand,with methylcellulose,without silica fume; (b)Without sand,with methylcellulose,with silica fume;(c)With sand,with methyl- cellulose,without silica fume;(d)With sand,with methylcellulose,with silica fume. ©2003 Taylor&Francis
addition of fibers has little effect on the density. Surface treatment of the fibers by ozone or dichromate slightly increases the thermal conductivity, whereas surface treatment of the fibers by silane has negligible effect. For any of the formulations, silane-treated silica fume gives slightly lower (or essentially the same) thermal conductivity as as-received silica fume. Silane treatments of silica fume and of fibers contribute comparably to reducing the thermal conductivity. 4 Electrical behavior Figure 10.1 gives the volume electrical resistivity of composites at seven days of curing. The resistivity decreases much with increasing fiber volume fraction, whether a second filler (silica fume or sand) is present or not (Chen and Chung, 1995b). When sand is absent, the addition of silica fume decreases the resistivity at all fiber volume fractions except the highest volume fraction of 4.24%; the decrease is most significant at the lowest fiber volume fraction of 0.53%. When sand is present, the addition of silica fume similarly decreases the resistivity, such that the decrease is most significant at fiber volume fractions below 1%. When silica fume is absent, the addition of sand decreases the resistivity only when the fiber volume fraction is below about 0.5%; at high fiber volume fractions, the addition of sand even increases the resistivity due to the porosity induced by the sand. Thus, the addition of a second filler (silica fume or sand) that is essentially non-conducting decreases the resistivity of the composite only at low volume fractions of the carbon fibers and the maximum fiber volume fraction for the resistivity to decrease is larger when the particle size of the filler is smaller. The resistivity decrease is attributed to the improved fiber dispersion due to Figure 10.1 Variation of the volume electrical resistivity with carbon fiber volume fraction (Chen and Chung, 1995b). (a) Without sand, with methylcellulose, without silica fume; (b) Without sand, with methylcellulose, with silica fume; (c) With sand, with methylcellulose, without silica fume; (d) With sand, with methylcellulose, with silica fume. 1,000,000 100,000 10,000 1,000 100 Volume resistivity ( Ωcm), log scale 10 1 012 (b) Vol.% fibers (a) (d) (c) 345 © 2003 Taylor & Francis
the presence of the second filler.Consistent with the improved fiber dispersion is the increased flexural toughness and strength due to the presence of the second filler. The use of both silica fume and sand results in an electrical resistivity of 3.19 X 1030cm at a carbon fiber volume fraction of just 0.24 vol.%This is an outstandingly low resistivity value compared to those of polymer-matrix composites with discontinuous conducting fillers at similar volume fractions. Electrical conduction in cement reinforced by short carbon fibers below the percolation threshold is governed by carrier hopping across the fiber-matrix interface.The activation energy is decreased by increasing the fiber crystallinity,but is increased by using interca- lated fibers.The carbon fibers contribute to hole conduction,which is further enhanced by intercalation,thereby decreasing the absolute thermoelectric power and the resistivity (Wen and Chung,2001e). Electric polarization induces an increase of the measured electrical resistivity of carbon fiber reinforced cement paste during resistivity measurement.The effect is diminished by increasing the conductivity of the cement paste through the use of carbon fibers that are more crystalline,the increase of the fiber content,or the use of silica fume instead of latex as an admixture.Intercalation of crystalline fibers further increases the conductivity of the composite,but it increases the extent of polarization.Voltage polarity switching effects are dominated by the polarization of the sample itself when the four-probe method is used,but are dominated by the polarization at the contact-sample interface when the two-probe method is used.Polarization reversal is faster and more complete for the latter (Wen and Chung,2001d). 5 Radio wave reflectivity Due to the electrical conductivity of carbon fibers,the addition of carbon fibers to cement significantly increases the ability of the composite to reflect radio waves,thus allowing EMI shielding and lateral guidance in automatic highways.However,due to the skin effect(the phenomenon in which electromagnetic radiation at a high frequency,such as 1 GHz,pene- trates only the near surface region of a conductor),discontinuous carbon filaments of 0.1 um diameter,as made from carbonaceous gases by catalytic growth,are much more effective for radio wave reflection than conventional pitch-based carbon fibers of diameter 15 um (Fu and Chung,1997b,1998a,b).However,the 0.1 um diameter filaments are less effective than the 15 um diameter fibers as a reinforcement. The cement-matrix composites are more effective than corresponding polymer-matrix composites for radio wave reflection,due to the slight conductivity of the cement matrix and the insulating nature of the polymer matrix.The conductivity of the cement matrix allows some electrical connectivity of the filler units,even when the filler concentration is below the percolation threshold(Fu and Chung,1998b). 6 Cathodic protection of steel reinforcement in concrete Cathodic protection is one of the most common and effective methods for corrosion control of steel reinforced concrete.This method involves the application of a voltage so as to force electrons to go to the steel reinforcing bar(rebar),thereby making the steel a cathode.As the steel rebar is embedded in concrete,the electrons need to go through the concrete in order to reach the rebar.However,concrete is not electrically very conductive.The use of ©2003 Taylor&Francis
the presence of the second filler. Consistent with the improved fiber dispersion is the increased flexural toughness and strength due to the presence of the second filler. The use of both silica fume and sand results in an electrical resistivity of 3.19 103 cm at a carbon fiber volume fraction of just 0.24 vol. %. This is an outstandingly low resistivity value compared to those of polymer-matrix composites with discontinuous conducting fillers at similar volume fractions. Electrical conduction in cement reinforced by short carbon fibers below the percolation threshold is governed by carrier hopping across the fiber-matrix interface. The activation energy is decreased by increasing the fiber crystallinity, but is increased by using intercalated fibers. The carbon fibers contribute to hole conduction, which is further enhanced by intercalation, thereby decreasing the absolute thermoelectric power and the resistivity (Wen and Chung, 2001e). Electric polarization induces an increase of the measured electrical resistivity of carbon fiber reinforced cement paste during resistivity measurement. The effect is diminished by increasing the conductivity of the cement paste through the use of carbon fibers that are more crystalline, the increase of the fiber content, or the use of silica fume instead of latex as an admixture. Intercalation of crystalline fibers further increases the conductivity of the composite, but it increases the extent of polarization. Voltage polarity switching effects are dominated by the polarization of the sample itself when the four-probe method is used, but are dominated by the polarization at the contact-sample interface when the two-probe method is used. Polarization reversal is faster and more complete for the latter (Wen and Chung, 2001d). 5 Radio wave reflectivity Due to the electrical conductivity of carbon fibers, the addition of carbon fibers to cement significantly increases the ability of the composite to reflect radio waves, thus allowing EMI shielding and lateral guidance in automatic highways. However, due to the skin effect (the phenomenon in which electromagnetic radiation at a high frequency, such as 1 GHz, penetrates only the near surface region of a conductor), discontinuous carbon filaments of 0.1m diameter, as made from carbonaceous gases by catalytic growth, are much more effective for radio wave reflection than conventional pitch-based carbon fibers of diameter 15m (Fu and Chung, 1997b, 1998a,b). However, the 0.1m diameter filaments are less effective than the 15m diameter fibers as a reinforcement. The cement–matrix composites are more effective than corresponding polymer–matrix composites for radio wave reflection, due to the slight conductivity of the cement matrix and the insulating nature of the polymer matrix. The conductivity of the cement matrix allows some electrical connectivity of the filler units, even when the filler concentration is below the percolation threshold (Fu and Chung, 1998b). 6 Cathodic protection of steel reinforcement in concrete Cathodic protection is one of the most common and effective methods for corrosion control of steel reinforced concrete. This method involves the application of a voltage so as to force electrons to go to the steel reinforcing bar (rebar), thereby making the steel a cathode. As the steel rebar is embedded in concrete, the electrons need to go through the concrete in order to reach the rebar. However, concrete is not electrically very conductive. The use of © 2003 Taylor & Francis
carbon fiber reinforced concrete for embedding the rebar to be cathodically protected facilitates cathodic protection,as the short carbon fibers enhance the conductivity of the concrete.Although the increase in conductivity is not desirable for the corrosion resistance of the embedded rebar,the presence of either silica fume or latex along with the fibers compensates for this negative effect,because the silica fume or latex reduces the water absorptivity (Hou and Chung,2000). For directing electrons to the steel reinforced concrete to be cathodically protected,an electrical contact is needed on the concrete.The electrical contact is electrically connected to the voltage supply.One of the choices of an electrical contact material is zinc,which is a coating deposited on the concrete by thermal spraying.It has a very low volume resistivity (thus requiring no metal mesh embedment),but it suffers from poor wear and corrosion resistance,the tendency to oxidize,high thermal expansion coefficient,and high material and processing costs.Another choice is a conductor filled polymer(Pangrazzi et al.,1994), which can be applied as a coating without heating,but it suffers from poor wear resistance, high thermal expansion coefficient and high material cost.Yet another choice is a metal(e.g. titanium)strip or wire embedded at one end in cement mortar,which is in the form of a coat- ing on the steel reinforced concrete.The use of carbon fiber reinforced mortar for this coat- ing facilitates cathodic protection,as it is advantageous to enhance the conductivity of this coating. Due to the decrease in volume electrical resistivity associated with carbon fiber addition (0.35 vol%)to concrete (embedding steel rebar),concrete containing carbon fibers and silica fume reduces by 18%the driving voltage required for cathodic protection compared to plain concrete,and by 28%compared to concrete with silica fume.Due to the decrease in resistivity associated with carbon fiber addition(1.1 vol%)to mortar,overlay (embedding titanium wires for electrical contacts to steel reinforced concrete)in the form of mortar con- taining carbon fibers and latex reduces by 10%the driving voltage required for cathodic protection,compared to plain mortar overlay.In spite of the low resistivity of mortar over- lay with carbon fibers,cathodic protection requires multiple metal electrical contacts embedded in the mortar at a spacing of 11 cm or less. 7 Strain sensing Cement reinforced with short carbon fibers is capable of sensing its own strain due to the effect of strain on the volume electrical resistivity (a phenomenon known as piezoresistiv- ity)(Chen and Chung,1993b,1995a,1996a,b;Chung,1995;Zhao et al.,1995;Fu and Chung,1996,1997a;Fu et al.,1996,1997,1998b;Mao et al.,1996a,b;Sun et al.,1998;Shi and Chung,1999;Wen and Chung,2000a,2001a)and due to the effect of strain on the rel- ative dielectric constant (a phenomenon known as direct piezoelectricity)(Wen and Chung, 2002a,b). 7.1 Piezoresistivity Uniaxial tension of carbon fiber reinforced cement in the elastic regime causes reversible increases in the volume electrical resistivity in both longitudinal and transverse directions, such that the gage factor(fractional change in resistance per unit strain)is comparable in magnitude in the two directions(Wen and Chung,2000a).In contrast,uniaxial compression causes reversible decreases in the resistivity in both directions (Wen and Chung,2001a). ©2003 Taylor&Francis
carbon fiber reinforced concrete for embedding the rebar to be cathodically protected facilitates cathodic protection, as the short carbon fibers enhance the conductivity of the concrete. Although the increase in conductivity is not desirable for the corrosion resistance of the embedded rebar, the presence of either silica fume or latex along with the fibers compensates for this negative effect, because the silica fume or latex reduces the water absorptivity (Hou and Chung, 2000). For directing electrons to the steel reinforced concrete to be cathodically protected, an electrical contact is needed on the concrete. The electrical contact is electrically connected to the voltage supply. One of the choices of an electrical contact material is zinc, which is a coating deposited on the concrete by thermal spraying. It has a very low volume resistivity (thus requiring no metal mesh embedment), but it suffers from poor wear and corrosion resistance, the tendency to oxidize, high thermal expansion coefficient, and high material and processing costs. Another choice is a conductor filled polymer (Pangrazzi et al., 1994), which can be applied as a coating without heating, but it suffers from poor wear resistance, high thermal expansion coefficient and high material cost. Yet another choice is a metal (e.g. titanium) strip or wire embedded at one end in cement mortar, which is in the form of a coating on the steel reinforced concrete. The use of carbon fiber reinforced mortar for this coating facilitates cathodic protection, as it is advantageous to enhance the conductivity of this coating. Due to the decrease in volume electrical resistivity associated with carbon fiber addition (0.35 vol%) to concrete (embedding steel rebar), concrete containing carbon fibers and silica fume reduces by 18% the driving voltage required for cathodic protection compared to plain concrete, and by 28% compared to concrete with silica fume. Due to the decrease in resistivity associated with carbon fiber addition (1.1 vol%) to mortar, overlay (embedding titanium wires for electrical contacts to steel reinforced concrete) in the form of mortar containing carbon fibers and latex reduces by 10% the driving voltage required for cathodic protection, compared to plain mortar overlay. In spite of the low resistivity of mortar overlay with carbon fibers, cathodic protection requires multiple metal electrical contacts embedded in the mortar at a spacing of 11 cm or less. 7 Strain sensing Cement reinforced with short carbon fibers is capable of sensing its own strain due to the effect of strain on the volume electrical resistivity (a phenomenon known as piezoresistivity) (Chen and Chung, 1993b, 1995a, 1996a,b; Chung, 1995; Zhao et al., 1995; Fu and Chung, 1996, 1997a; Fu et al., 1996, 1997, 1998b; Mao et al., 1996a,b; Sun et al., 1998; Shi and Chung, 1999; Wen and Chung, 2000a, 2001a) and due to the effect of strain on the relative dielectric constant (a phenomenon known as direct piezoelectricity) (Wen and Chung, 2002a,b). 7.1 Piezoresistivity Uniaxial tension of carbon fiber reinforced cement in the elastic regime causes reversible increases in the volume electrical resistivity in both longitudinal and transverse directions, such that the gage factor (fractional change in resistance per unit strain) is comparable in magnitude in the two directions (Wen and Chung, 2000a). In contrast, uniaxial compression causes reversible decreases in the resistivity in both directions (Wen and Chung, 2001a). © 2003 Taylor & Francis