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 were performed to determine the effects that the concrete mixture has on resulting thermal resistivity. Concrete flow fill mixtures containing water-cement-sand and water-cement-fly ash-sand were studied. Experiments showed that water content of the mixture is not a factor in 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 a desired thermal resistivity or calculate the thermal resistivity of a known concrete mixture.