Carbon fiber cement composites

Carbon fiber cement composites have been studied [237-245]. 

Figure 20.46 Effect of fiber type on composite flexural behavior of a reinforced carbon fiber cement composite.

Carbon-Fiber, Cement Composites. Work carried out in Japan has shown the potential of carbon fibers as an effective reinforcement for... 

V.C. Li and K.H. Obla, Effect of fiber length variation on tensile properties of carbon fiber cement composites , int. J. of Composite Engineering. 4,1994, 947-964. 

Table 15.1 is a review by Toutanji and co-workers [40] of work undertaken on cfrc composites [43-48] and several workers [49 53] have detailed other work on carbon fiber reinforced mortar. Chung [54] describes work on carbon fiber cement matrix composites. 

Larson BK, Drzal LT, Sorousian P, Carbon-fiber-cement adhesion in carbon-fiber reinforced cement composites, Composites, 21(3), 205-215, 1990. 

Sihai Wen, Chung DDL, Piezoresistivity in continuous carbon fiber cement-matrix composite. Cement Concrete Res, 29(3), 445-449, 1999. 

Composites may be identified and classified many hundreds of ways. There are aggregate-cement matrix (concrete), aluminum film-plastic matrix, asbestos fiber-concrete matrix, carbon-carbon matrix, carbon fiber-carbon matrix, cellulose fiber-lignin/silicic matrix, ceramic fiber-matrix ceramic (CMC), ceramic fiber-metal matrix, ceramic-metal matrix (cermet), concrete-plastic matrix, fibrous-ceramic matrix, fibrous-metal matrix, fibrous-plastic matrix, flexible reinforced plastic, glass ceramic-amorphous glass matrix, laminar-layers of different metals, laminar-layer of glass-plastic (safety glass), laminar-layer of reinforced plastic, laminar-layers of unreinforced plastic. 

Cement Incorporating carbon fiber in a cement matrix was difficult due to the size of the cement particles, which tended to be filtered out by the fiber reinforcement, so a cement, Swiftcrete, with a very fine particle size (about 5 pm) was used. The fiber was spread as thinly as possible and sized with a water based compatible size such as sodium carboxymethylcellulose [155,156]. Composites with up to 10% F/-were obtained [157]. 

Concrete is extremely stiff and good in compression, but quite poor in tension. Steel rebar reinforcement is normally used to make a stronger structure, but because steel is subject to atmospheric corrosion, it has to be shielded with a relatively thick protective layer of concrete, which with time, is removed by weathering and erosion, eventually exposing the steel to attack. Carbon fiber has very good corrosion resistance and would not be affected by any alkalis in the cement. Hence it should be possible to utilize a carbon fiber composite such as carbon fiber/epoxy pultruded rods that would only require a relatively thin coating of concrete. Alternatively, carbon fiber can be incorporated directly into the concrete mix [9]. 

The bond strength in cfrc is improved by using silane-treated carbon fibers [30] and by surface treatment in conjunction with the addition of a polymer to the cement mix [31]. Larson et al [32] studied adhesion of cement to carbon fiber in cfrc composites. 

Figure 15.3 Effect of PAN based carbon fibers on the tensile strength of cementitious composites. Source Reprinted with permission from Toutanji HA, El-Korchi T, Katz RN, Leatherman GL, Cement Concrete Res, 23, 618-626, 1993. Copyright 1993, Elsevier.

Piezoresistivity [66] was observed in cement matrix composites with 2.6-7.4 vol% unidirectional continuous carbon fibers. The dc electrical resistance in the fiber direction increased upon tensile loading in the same direction, such that the effect was mostly reversible when the stress was below that required for the tensile modulus to decrease. The gage factor was up to 60. The resistance increase was due to the degradation of the interface of the fiber and matrix, which was mostly reversible. Above the stress at which the modulus started to decrease, the resistance abruptly increased with stress/strain, due to fiber breakage. The tensile strength and modulus of the composites were 88% and 84%, respectively, of the calculated values based on the rule of mixtures. 

Figure 15.4 Initial Young s modulus of continuous carbon fiber reinforced cement. Source Reprinted from Aveston J, Mercer RA, Sillwood JM, Conference Proceedings Composites Standards Testing and Design, NPL 1974, IPC Science Technoiogy Press, 93-103, 1974.

Qijun Zheng, Chung DDL, Carbon fiber reinforced cement composites improved by using chemical agents. Cement Concrete Res, 19, 25-41, 1989. 

Yang XM, Chung DDL, Latex-modified cement mortar reinforced by short carbon-fibers. Composites, 23(6), 453-460, 1992. 

Sakai H, Takahashi K, Mitsui Y, Ando T, Awata M, Hoshijima T, Flexural behavior of carbon fiber reinforced cement composite, American Concrete Institute Report, SP 142-7, 121-129. 

Chiou JM, Zheng Q, Chung DDL, Electromagnetic interference shielding by carbon fiber reinforced cement. Composites, 20(4), 379 381, 1989. 

Carbon fiber has been found to be an effective thermistor [192-194], such as a cement paste reinforced with chopped carbon fiber (about 5 mm long) with silica fume (15 wt% cement). Its electrical resistivity decreased reversibly with increasing temperature (1-45°C), with activation energy of electrical conduction (electron hopping) of 0.4 eV. This value is comparable to semiconductors (typical thermistor materials) and is higher than that of carbon fiber polymer matrix composites. The current-voltage characteristics of carbon fiber reinforced silica fume cement paste were linear up to 8 V at 20°C. 

The alkaline pH of cement matrix can also be reduced by reaction between carbon dioxide (CO ) and Ca(OH. This reaction is known as carbonation of cementitious matrix and occurs in three stages (i) Initially CO diffuses in matrix through pores and is dissolved (ii) subsequently CO reacts with sodium hydroxide (NaOH) of cement matrix, decreasing the pH which favors Ca(OH formation and (iii) finally a reaction occurs between COj and Ca(OH that forms Ca(HCQ ) and CaCO. Several authors have used this reaction to decrease pH of cement matrix and increase durability of cement composites reinforced with natural fibers [31, 37, 69]. To allow this reaction occurs, composite is cured in an environment rich in COj, which favors the formation of CaCQ and therefore decreases the pH in the matrix [32, 70-73]. However, cement... 

Toledo Filho et al. [32] conducted a comprehensive study to determine specific modifications of cement matrix and natural fibers that were effective increasing durability. Results indicated that carbonation of the specimens for 109 days at conditions of 26.5 C is a promising alternative for increasing the durabihty of cement composites reinforced with cellulosic fibers. 

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