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