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&Franciscarbon 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