Table 10.1/Gage factor of carbon-fiber cement pastes under uniaxial compression and under uniaxial tension(Wen and Chung, 2000a,2001a) Carbon-fiber Carbon-fiber silica-fume latex cement cement paste paste Compression Longitudinal +350 +210 Transverse -390 -80 Tension Longitudinal +89 +51 Transverse -59 -36 0 -20 40 60 -80 -100 -2 -120 0 50 100150200 250 Time(s) Figure 10.2 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 carbon-fiber latex cement paste at 28 days of curing (Wen and Chung,2001a). Without the fibers,the resistivity changes are much smaller and less reversible.The resis- tivity increase is attributed to defect generation or aggravation under tension and defect healing under compression.The fractional change in resistance per unit strain(i.e.the gage factor)is higher in magnitude for carbon-fiber silica-fume cement paste than carbon-fiber latex cement paste,as shown in Table 10.11. Figure 10.2 shows the fractional change in resistivity along the stress axis as well as the strain during repeated compressive loading at an increasing stress amplitude for carbon- fiber latex cement paste at 28 days of curing.The strain varies linearly with the stress up to the highest stress amplitude.The strain returns to zero at the end of each cycle of loading. The resistivity decreases upon loading in every cycle(due to fiber push-in)and increases upon unloading in every cycle(due to fiber pull-out).The resistivity has a net increase after the first cycle,due to very minor damage.Little further damage occurs in subsequent cycles, as shown by the resistivity after unloading not increasing much after the first cycle.The greater the strain amplitude,the more is the resistivity decrease during loading,although the ©2003 Taylor&FrancisWithout the fibers, the resistivity changes are much smaller and less reversible. The resis￾tivity increase is attributed to defect generation or aggravation under tension and defect healing under compression. The fractional change in resistance per unit strain (i.e. the gage factor) is higher in magnitude for carbon-fiber silica-fume cement paste than carbon-fiber latex cement paste, as shown in Table 10.11. Figure 10.2 shows the fractional change in resistivity along the stress axis as well as the strain during repeated compressive loading at an increasing stress amplitude for carbon￾fiber latex cement paste at 28 days of curing. The strain varies linearly with the stress up to the highest stress amplitude. The strain returns to zero at the end of each cycle of loading. The resistivity decreases upon loading in every cycle (due to fiber push-in) and increases upon unloading in every cycle (due to fiber pull-out). The resistivity has a net increase after the first cycle, due to very minor damage. Little further damage occurs in subsequent cycles, as shown by the resistivity after unloading not increasing much after the first cycle. The greater the strain amplitude, the more is the resistivity decrease during loading, although the Table 10.11 Gage factor of carbon-fiber cement pastes under uniaxial compression and under uniaxial tension (Wen and Chung, 2000a, 2001a) Carbon-fiber Carbon-fiber silica-fume latex cement cement paste paste Compression Longitudinal 350 210 Transverse 390 80 Tension Longitudinal 89 51 Transverse 59 36 Figure 10.2 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 carbon-fiber latex cement paste at 28 days of curing (Wen and Chung, 2001a). 2 0 –20 –40 –60 –80 –100 –120 Fractional change in resistivity (%) Strain (10–6) 1 0 –1 –2 0 50 100 Time (s) 150 200 250 © 2003 Taylor & Francis