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Effect of Admixtures, Chlorides, and Moisture on Dielectric Properties of Portland Cement Concrete in the Low Microwave Frequency Range
Pokkuluri, Kiran S.
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The use of electromagnetic waves as a nondestructive evaluation technique to evaluate Portland cement concrete (PCC) structures is based on the principle that a change in the structure, composition, or properties of PCC results in a change in its dielectric properties. The coaxial transmission line is one of the few devices that can measure the dielectric properties of PCC at a frequency range of 100-1000 MHz. A coaxial transmission line developed at Virginia Tech was used to study the effect of moisture, type of aggregate, water/cement ratio, curing period, admixture type (microsilica, superplasticizer, and shrinkage admixture), and chloride content on the dielectric properties of PCC. Measurements were conducted in the time domain and converted to the frequency domain using Fast Fourier Transform. The research found that an increase in the moisture content of PCC resulted in an increase in the dielectric constant. Mixes containing limestone aggregate had a greater dielectric constant than those containing granite. The dielectric constant decreased with curing period due to the reduction in free water availability. Mixes containing higher water/cement ratios exhibited a higher dielectric constant, especially in the initial curing period. The admixtures did not significantly affect the dielectric constant after one day of curing. After 28 days of curing, however, all three admixtures had an effect on the measured dielectric constant as compared to control mixes. Chloride content had a significant effect on the loss part of the dielectric constant especially during early curing. A relationship was also established between the chloride permeability (based on conductance measurements) of PCC and its dielectric constant after 75 days of moist curing.
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