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Concrete Duct Bank Thermal Resistivity and the Effect of Raw Material Sources

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Concrete Duct Bank Thermal Resistivity and the Effect of Raw Material Sources

Past experiments provided a method of determining concrete duct bank thermal resistivity using the constituents of the concrete mix. At that time material from only one source was used in the experiments. The question was raised whether the source of the sand or cement would affect the thermal resistivity of the concrete.

This paper describes an experiment designed to determine whether the source of the sand and cement would affect the thermal resistivity of a concrete mix. The experiment performed determined that the source of the raw material will affect the thermal resistivity of concrete. This paper describes the experiment and includes the analysis of the gathered data. The experiment showed that the source from which both sand and cement was procured does have an effect on the final resistivity of concrete, and will affect the resistivity values that are used when determining the ampacity of cables encased in concrete duct banks.

The concrete duct bank thermal resistivity surrounding cables is important in determining the ampacity of the encased cables. Past experimentation derived a set of equations that could be used in determining the thermal resistivity of concrete duct banks if the proportions of water, cement, and fly ash were known. Equation (1) was found to determine the thermal resistivity of a concrete-sand-water mix, if x is the water to cement ratio (w/c) and y is the thermal resistivity in cm˚C/W.

Equation (2) is then used to find the prediction interval for the mix, y0 is added and subtracted from y in Equation (1) to find the 95% prediction interval for any concrete mix with a w/c ratio of x.

For a cement-sand-fly ash-water mix, Equation (3) was found to predict the thermal resistivity.

The sand and cement used to derive these empirical equations was procured from a local source in Colorado. The question remained whether the source of the raw material used in the concrete mix would affect the thermal resistivity of the concrete.

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The Effect of Raw Material Source on Concrete Duct Bank Thermal Resistivity