Piezoresistivity-based strain sensing in carbon fiber-reinforced cement

For piezoresistivity-based strain sensing using carbon fiber-reinforced cement (152 mm [6 in.] specimens under compression) in the elastic regime, the four-probe method of electrical resistance measurement is more effective than the two-probe method in that it provides gauge factor (fractional change in resistance per unit strain) that is higher and that varies less with the strain amplitude. The two-probe method also suffers from the resistance increasing irreversibly in the first few loading cycles due to minor degradation of the electrical contacts. The use of embedded stainless steel electrical contacts gives more effective strain sensing and slightly lower resistivity than the use of silver paint surface electrical contacts, whether the four-probe method or the two-probe method is used. In case of the four-probe method, the use of embedded steel contacts compared with the use of surface silver paint contacts results in greater linearity and lower noise in the variation of the resistance with strain. In case of the two-probe method, the use of embedded steel contacts compared with the use of surface silver paint contacts results in lower variability of the gauge factor and smaller fractional contribution of the contact resistance to the measured resistance.