Concrete strength vs Rho

[EEJaime]

Hello,

I am an electrical engineer. We often perform calculations on our multiple conduit duct banks to calculate the amount of heating that multiple runs of energized conductors impart to each other via their surrounding material. This tells us how much of the conductor's thermal capability is utilized in carrying current, and how much is sacrificed to surviving a hot environment.

One of the most critical variables in these calculations is the "Thermal Resistivity" of the material surrounding the conduit/cable. This is defined by the code as the "heat transfer capability through a substance by conduction." We call this "Rho" and it is expressed in Degrees C-cm/Watt. The typical value used in the Electrical Code, (California Electrical Code in this instance) is Rho=90 C-cm/Watt. The software we used when we substituted concrete encasement for earth backfill for our duct banks included a value for the concrete of Rho= 85 C-cm/Watt.

The question comes up: What concrete mix will give us that value? Since I am sure that water content, air content, etc..., will have a direct effect on the thermal resistivity- how does mix impact the Rho value? I have tried to google this in many different ways, but this factor does not seem to be a consideration in concrete design. There is one site that publishes the values for their specific mixes of "high strength-low rho" and low strength-low rho mixes, but they only publish the relationship between moisture content vs. rho.

After that long winded introduction I simply would like to ask: Does anyone know of any published data for this value as a function of concrete strength or mix? Any hint would be a help. We generally for the location of these duct banks would specify a 2500 psi@ 28 day strength.

Thank you and regards,
EEJaime

 

[frv]

I suspect that f'c would be a poor indicator, as f'c denotes only one of many properties of concrete; a property that can be achieved in many different ways.

But to answer your question, I'm not aware of any relation between these properties (rho means something completely different to a structural engineer). I can tell you, however, that 2500 psi is not very common. 3000 psi is about the lowest engineers will specify (usually).

 

[ishvaaag]

Look at this link for the range of variation of the thermal conductivity of concrete, right column in Tabla 2.8, for diverse kinds of concrete (Hormigón, in spanish)

http://www.knauf.es/knauf/ficheros/images/calidad/normativas/termico/CalculoTermico.pdf

There maybe some more info that I may have, and will post if I find.

 

[MiketheEngineer]

You might want to re-post under the "Heat Transfer" forum??

 

[ishvaaag]

Well, before such transfer to (heat) transfer, a scan of about the same properties of older book, in other units