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&Francisthe 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 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 0.1m 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