-20 3 0 60 3 -80 -100 -120 -140 -160 0 50 100 150 200 250 Time (s) Figure 10.4 Variation of the fractional change in volume electrical resistivity with time and of the strain(negative for compressive strain)with time during dynamic compressive load- ing at increasing stress amplitudes within the elastic regime for latex cement paste at 7 days of curing (Wen and Chung,2001a). 0.2 0 -20 0.15 -40 -60 0.1 0 (9905 -100 0.05 -120 -140 -160 50 100150 200 250 Time(s) Figure 10.5 Variation of the fractional change in volume electrical resistivity with time and of the strain(negative for compressive strain)with time during dynamic compressive load- ing at increasing stress amplitudes within the elastic regime for latex cement paste at 28 days of curing(Wen and Chung,2001a). cement paste(Fig.10.2).In the presence of carbon fibers,the resistivity decreases reversibly upon loading;in the absence of fibers,the resistivity mainly increases upon unloading. Figures 10.2-10.5 show that the effect of carbon fibers on the variation of the resistivity with strain is more drastic at 28 days than at 7 days. 7.2 Direct piezoelectricity The direct piezoelectric effect was observed in cement pastes by voltage measurement(Sun et al.,2000)and by measurement of the relative dielectric constant (10kHz)(Wen and 2003 Taylor Franciscement paste (Fig. 10.2). In the presence of carbon fibers, the resistivity decreases reversibly upon loading; in the absence of fibers, the resistivity mainly increases upon unloading. Figures 10.2–10.5 show that the effect of carbon fibers on the variation of the resistivity with strain is more drastic at 28 days than at 7 days. 7.2 Direct piezoelectricity The direct piezoelectric effect was observed in cement pastes by voltage measurement (Sun et al., 2000) and by measurement of the relative dielectric constant (10 kHz) (Wen and Figure 10.4 Variation of the fractional change in volume electrical resistivity with time and of the strain (negative for compressive strain) with time during dynamic compressive load￾ing at increasing stress amplitudes within the elastic regime for latex cement paste at 7 days of curing (Wen and Chung, 2001a). 4 0 –40 –20 –60 –100 –80 –160 –140 –120 0 1 2 3 Fractional change in resistivity (%) Strain (10–6 ) Time (s) 0 50 100 150 200 250 Figure 10.5 Variation of the fractional change in volume electrical resistivity with time and of the strain (negative for compressive strain) with time during dynamic compressive load￾ing at increasing stress amplitudes within the elastic regime for latex cement paste at 28 days of curing (Wen and Chung, 2001a). Fractional change in resistivity (%) Strain (10–6 ) Time (s) 0 50 100 150 200 250 0 –40 –20 –60 –100 –80 –160 –140 –120 0.2 0.15 0.1 0.05 0 © 2003 Taylor & Francis