Effects of Environment and Fatigue on the Piezoresistivity of Carbon Nanotube–Cement Composite

The effects of environment and fatigue on the electrical resistance and piezoresistivity of the carbon nanotube/cement composite is investigated under a service environment using a four-probe method. The main influencing factors include temperature (20, 30, 40 ℃, and 50 ℃), water content (0%-6.4%), temperature and water content coupling, and number of fatigue cycles (20, 40, 60, 110, 160, and 240×10³). Test results showed that environment and fatigue considerably affected resistivity and piezoresistivity. The resistivity of the composite decreased as water content increased. When water content changed from 1.3% to 5.8%, resistivity was reduced 43%. By contrast, piezoresistivity and change in resistance initially increased and then decreased as water content increased. Temperature substantially affected resistivity and piezoresistivity. As temperature increased, resistivity decreased but piezoresistivity increased. Electrical resistance and change in resistivity exhibited a strong linear relationship with temperatures ranging from 20 ℃ to 50 ℃ with an R2 of over 0.99. As temperature increased from 20 ℃ to 50 ℃, resistivity decreased 35% and change in resistivity increased 84%, thereby indicating improved piezoresistivity. Piezoresistive sensitivity to temperature demonstrated a strong negative linear relationship with water content with an R2 of 0.915. Electrical resistance and piezoresistivity increased as the number of cycles increased. The major causes of changes in resistivity and piezoresistivity are a change in tunneling conduction and the development of micro-cracks.