Sihai Wen

Seebeck effect in carbon fiber-reinforced cement

The Seebeck effect in carbon fiber-reinforced cement paste was found to involve electrons from the cement matrix and holes from the fibers. The two contributions were equal at the percolation threshold, with a fiber content between 0.5 and 1.0% by mass of cement. The hole contribution increased monotonically with increasing fiber content below and above the percolation threshold. The fiber addition increased the linearity and reversibility of the Seebeck effect. Silica fume and latex as admixtures had minor influence on the Seebeck effect.

Carbon fiber-reinforced cement as a strain-sensing coating

Cement paste containing short carbon fibers was found to be an effective strain-sensing coating. The coating (with fibers) was on either the tension side or the compression side of a cement specimen (without fiber) under flexure. The resistance was measured with surface electrical contacts on either side. The resistance increased reversibly on the tension side upon loading and decreased reversibly on the compression side upon loading. The behavior was similar whether the strain-sensing coating contained silica fume or latex.

Uniaxial compression in carbon fiber-reinforced cement, sensed by electrical resistivity measurement in longitudinal and transverse directions

Uniaxial compression of carbon fiber-reinforced cement pastes in the elastic regime caused reversible decreases in both longitudinal and transverse electrical resistivities. In contrast, uniaxial tension had been previously reported to cause reversible increases in both resistivities. The fractional change in resistivity per unit strain is higher in magnitude for carbon fiber silica fume cement paste than carbon fiber latex cement paste.

Carbon fiber-reinforced cement as a thermistor

Carbon fiber ( z 5 mm long, 0.5% by mass of cement)-reinforced silica fume (15% by mass of cement) cement paste was found to be an effective thermistor. Its electrical resistivity decreased reversibly with increasing temperature (1‐45 8 C), with an activation energy of electrical conduction (electron hopping) of 0.4 eV. This is comparable to those of semiconductors (typical thermistor materials) and higher than that of carbon fiber polymer-matrix composites. Without carbon fibers, or with latex in place of silica fume, the activation energy was much lower and the resistivity was higher. The voltage range for a linear current-voltage characteristic was wider in the absence of fibers. The current-voltage characteristic of carbon fiber-reinforced silica fume cement paste was linear up to 8 V at 20 8 C. ©1999 Elsevier Science Ltd. All rights reserved.

Electric polarization in carbon fiber-reinforced cement

Abstract Electric polarization induced an increase of the measured electrical resistivity of carbon fiber-reinforced cement paste during resistivity measurement. The effect was diminished by increasing the conductivity of the cement paste through the use of carbon fibers that were 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 increased the conductivity of the composite, but it increased the extent of polarization. Voltage polarity switching effects were dominated by the polarization of the sample itself when the four-probe method was used, but were dominated by the polarization at the contact–sample interface when the two-probe method was used. Polarization reversal was faster and more complete for the latter.

Piezoresistivity in continuous carbon fiber cement-matrix composite

Abstract Piezoresistivity was observed in cement-matrix composites with 2.6–7.4 vol% unidirectional continuous carbon fibers. The direct-current electrical resistance in the fiber direction increased upon tensile loading in the same direction, such that the effect was mostly reversible when the stress was below that for the tensile modulus to decrease. The gage factor was up to 60. The resistance increase was due to fiber-matrix interface degradation, which was mostly reversible. Above the stress at which the modulus started to decrease, the resistance increased with stress/strain abruptly, due to fiber breakage. The tensile strength and modulus of the composites were 88% and 84%, respectively, of the calculated values based on the rule of mixtures.

Enhancing the seebeck effect in carbon fiber-reinforced cement by using intercalated carbon fibers

Abstract The absolute thermoelectric power of carbon fiber-reinforced cement paste was rendered as negative as −17 μV/°C by using bromine-intercalated carbon fibers, which had a high concentration of holes. The corresponding paste with pristine carbon fibers exhibited absolute thermoelectric power as negative as −0.8 μV/°C only.

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.

Damage monitoring of cement paste by electrical resistance measurement

Electrical resistance measurement is effective for monitoring damage (due to damage infliction and subsequent microcrack opening) and healing (due to microcrack closing) of cement pastes (plain, with silica-fume, and with latex) in real time during repeated compressive loading. Damage causes the resistance to increase; healing causes the resistance to decrease.

Strain-sensing characteristics of carbon fiber-reinforced cement

The ability of a structural material to sense its own strain (without attached or embedded sensors) is a positive attribute of smart structures. Specific applications include structural vibration control, traffic monitoring, weighting, room occupancy monitoring, and building security. This article reports on a study of strain sensing in carbon fiber reinforced cement, as enabled by piezoresistivity. This type of strain sensing is characterized by the gauge factor, which is defined as the fractional change in electrical resistance per unit strain. This study involved simultaneous measurement of the piezoresistive behavior in the longitudinal and transverse directions for each specimen. Results showed that, under uniaxial compression, the gauge factor in both longitudinal and transverse directions decrease in magnitude with increasing specimen size from 13 to 51 mm, due to a slight decrease in the degree of preferred orientation of the 5 mm-long fibers. The gauge factor in both directions also decreases in magnitude as the fiber content increases beyond the percolation threshold.