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Engineering Thermal Resistivity of Concrete Duct Banks

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Engineering Thermal Resistivity of Concrete Duct Banks

Engineering thermal resistivity in underground conduits surrounded by concrete requires the cable ampacity in these duct banks to be calculated. 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 engineering thermal 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. An underground conduit is often surrounded by concrete in the form of a concrete duct bank. This is done to protect electrical conduits and their enclosed cables from damage due to excavation or traffic on the soil above the ducts.

The presence of the concrete has an additional benefit of improving the negative effects of any soil thermal instability that may be suspected at a particular location. The concrete improves the soil thermal stability since the concrete surrounding the electrical conduit will be relatively thermally stable compared with most soils since, unlike soil, little water migration is expected through hydrated concrete. Furthermore, the presence of the duct bank will result in a larger area through which surrounding soil moisture can return to the drying interface between the concrete duct bank and the soil. This interface will be heated by the enclosed cables and will be the area where drying is expected.

Using a concrete duct bank will thereby improve the thermal stability of an underground cable system, and the resulting stability will be largely determined by the overall diameter of the concrete surrounding the electrical ducts. When concrete surrounds electrical conduits, the concrete becomes part of the thermal circuit through which heat must be conducted to escape the vicinity of the cables [2]. To calculate the ampacity of the enclosed cables, the thermal resistivity of the concrete must be known. It may be desirable to have the capability of designing a concrete mix that will produce a certain target thermal resistivity. It is also useful to have the means of predicting the thermal resistivity that will result when the concrete of a known mixture is used.

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Engineering Thermal Resistivity of Concrete Duct Banks