UBC Theses and Dissertations
Carbon-fiber reinforced cement based sensors Chacko, Rose Mary
The addition of carbon fibers has proved to be one of the most effective ways of improving the electrical conductivity of ordinary cement pastes. Numerous studies have been conducted toward using this property for measurement of strain, temperature change and chloride penetration in concrete. In the present study, a small carbon fiber reinforced cement specimen with rectangular cross section was positioned in a 60 Hz, ± 10 V AC circuit with a data acquisition system in order to monitor the changes in its electrical resistivity under the influence of different parameters. The parameters studied included fiber content, water-cement ratio, moisture content, alkali concentration, temperature, compressive loading, and flexural loading. The influence of method of curing the specimen on its electrical resistivity over 28 days was also studied. It was seen that a high fiber fraction and low moisture content makes the specimen act like a pure resistor with negligible capacitance or inductance associated with it. Air curing was preferred over moist curing because the increase in resistivity over time was lesser for air-cured specimens. It was also observed that electronic conduction was dominant over electrolytic conduction in a mix proportion with high fiber volume fraction and low water to cement ratio. The influence of temperature on the electrical resistivity of pastes with large amount of fiber was barely significant. The resistivity was found to steadily decrease under compressive loading and then increase during the formation of macro-cracks. It has long been established that carbon fiber reinforced cement paste specimens have the ability to monitor its own state under various conditions by exhibiting a variation in its resistivity values. However no work has been carried out to show how such a specimen would behave if embedded in a concrete member. For this reason, the resistivity versus load behavior of such specimens embedded in concrete cylinders and beams under compressive and flexural loading were also studied. In flexure, resistivity of an embedded specimen showed a slight increase few minutes prior to the formation of the first crack in the beam. There was a steep increase in resistivity on further widening of the crack leading to believe that in flexural members this sensor may perform well as a crack sensor than as a strain sensor. It was observed that the sensor would more accurately mirror the strain in the concrete member it is attached to in its resistivity readings, if it is a resilient material with high yield strength and low modulus of elasticity.
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