×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Concrete strength vs Rho

Concrete strength vs Rho

Concrete strength vs Rho

(OP)
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
 

RE: Concrete strength vs Rho

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).

RE: Concrete strength vs Rho

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources