Engineering the Thermal Resistivity of Concrete Duct Banks

Cables are often installed in underground conduits surrounded by concrete. To calculate the cable ampacity in these duct banks, the concrete thermal resistivity must be known. A set of experiments was performed to determine the effects that a concrete mixture has on the resulting thermal resistivity. Concrete flow-fill mixtures containing water-cement-sand and water-cement-fly-ash-sand were studied. Experiments showed that the water content of a mixture is not a factor in the final concrete thermal resistivity unless fly ash was included in the mixture; however, the water-to-cement ratio is significant for all mixtures. Empirical equations were derived to find the resistivity of concrete as a function of the constituents of the concrete mixture. These equations may be used to design a concrete mixture to produce the desired thermal resistivity or to calculate the thermal resistivity of a known concrete mixture.