Application of carbon fibers in cement and concrete (1)

Chung and co-workers have investigated piezoresistivity in carbon fiber composites and how the effect can be used to advantage. Piezoresistivity 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 the level of tensile modulus reduction. The increase in resistance was due to fiber-matrix interface degradation, 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.

Carbon fiber has been found to be an effective thermistor, such as a cement paste reinforced with chopped carbon fiber with silica fume. Its electrical resistivity decreased reversibly with increasing temperature, with activation energy of electrical conduction of 0.4 eV. This value is comparable to semiconductors 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 20C.

There are many cases where carbon fiber has been recorded as a smart material. Concrete containing short carbon fibers was found to be an intrinsically smart concrete that could sense elastic and inelastic deformation, as well as fracture. The signal provided is the change in electrical resistance, which is reversible for elastic deformation and irreversible for inelastic deformation and fracture. The presence of electrically conducting short fibers is necessary for the concrete to sense elastic or inelastic deformation, but the sensing of fracture does not require fibers. The fibers serve to bridge the cracks and provide a conduction path. The increase in resistance is due to conducting fiber pullout in the elastic regime, conducting fiber breakage in the inelastic regime and crack propagation at fracture.

Damage in carbon fiber and its polymer matrix and carbon matrix composites has been determined by electrical resistance measurement.

Carbon fibers in cement and concrete:

Carbon fiber reinforced mortar has used for composite slab construction. Sakai et al have used a cfrc for curtain walls, providing adequate wind resistance with higher fatigue strength than ordinary concrete.

Carbon fiber cement as a replacement for asbestos cement:

Carbon fiber has made significant advances in replacing asbestos in asbestos cement products in two basic applications- structural covering and flues.

In structural covering, the carbon fiber confers a waterproof effect and provides mechanical reinforcement, which is so effective that the amount of reinforcement previously used for asbestos can be reduced by an order of magnitude. To aid subsequent dispersion in an aqueous medium, 5% of glycerol size is applied to a large tow carbon fiber, which is then chopped and admixed with about 5% of other bulking fibers, selected fillers and an antifoam agent. The improvement in mechanical properties has been so good that longer sections of structural covering can be incorporated to provide the necessary resistance to withstand heat up to 450C with adequate mechanical reinforcement. Carbon fiber, unlike other types of fiber, can provide this temperature resistance, coupled with resistance to the action of alkalis (pH 12-13)



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