Abstract
This paper presents a compressive test program designed to determine the mechanical and self-sensing properties of cement-matrix composites containing carbon fiber. Two kinds of mixes with 0.5% and 0.75% carbon fiber （CF） were prepared. The mechanical and piezoresistive properties of the cement-matrix composites were evaluated at 28, 90, 180, 270, and 360 d. The results show that the mechanical properties were enhanced in most cases. However, the compressive strength of carbon fiber reinforced cement-matrix composites decreased compared with the reference mix at the early curing ages, which agrees with the results mentioned in the literature. What is worth to mention, the 360 d compressive strength of carbon fiber reinforced cement-matrix composites reached the same level as that of the reference. Moreover, both negative and positive piezoresistivity were observed during the experiments. The amplitude of piezoresistivity was found to change with the variation of moisture content, and was not directly proportional to the magnitude of the stress. In the elastic stage with smaller stress amplitude, the piezoresistivity amplitude was larger. When the stress amplitude was multiplied, the piezoresistivity change was not synchronous. The gauge factor for the composite with 0.75% CF was higher than that of the composite with 0.5% CF and commercially available strain gauges.
Highlights
The cementitious composites containing CFs are porous materials, and capillary water exists in Recently, a new type of self-sensing material, carbon fiber reinforced cement-based composites with piezoresistivity, has shown promise for long-term health monitoring owing to the advantages of durability, low cost, and compatibility with a cement matrix [1,2,3,4,5,6]
The compressive strength of carbon-fiber-reinforced cement mortar (CFRCM) is lower than that of CM when the age is small. This may be due to the addition of carbon fibers, which increases the initial defects of cement mortar, so the compressive strength of CFRCM decreases at early age
Previous study showed that the addition of carbon filaments in cement matrix would decrease the mechanical properties of cement matrix composites
Summary
The cementitious composites containing CFs are porous materials, and capillary water exists in Recently, a new type of self-sensing material, carbon fiber reinforced cement-based composites with piezoresistivity, has shown promise for long-term health monitoring owing to the advantages of durability, low cost, and compatibility with a cement matrix [1,2,3,4,5,6]. With this property, the cementitious composites containing CF are able to sense strain through the variation of electric resistance. Zhao et al [14] studied the effect of MC on the piezoresistivity by drying, the results show that the relative change in resistance varies with the moisture content of the composites. This paper is focused on investigating the effect of moisture (free water due to water absorption after curing) on the mechanical properties and the piezoresistivity of carbon fiber-reinforced mortar. In the case of CM, cement, sand, water, and deformer were added and mixed in the rotary mixer for 5 min. Other groups were tested on 90 d, 180 d, 270 d, and 360 d During this time, the specimens were naturally cured in the room where the temperature and moisture were changing with the time.
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