Concrete Resistivity

[cuky2000]

Does any one could advise the resitivity value of concrete for an indoor substation.

 

[Senselessticker]

Are you asking what the restivity value should be? Or asking what the resistivity value is?

 

[dpc]

This will depend on the moisture content and the amount of rebar. In general, reinforced concrete in contact with soil has fairly low resistivity.

http://www.dot.ca.gov/hq/esc/geotech/gg/geophysics2002/050chouteau_resistivity_tomography.pdf

 

[jghrist]

¶14.6 of IEEE Std 80-2000 says

Buried in soil, a concrete block behaves as a semiconducting medium with a resistivity of 30-90 ohm·m.

The floor slab of an indoor substation is not completely buried, and may be isolated by a vapor barrier. The steel mesh reinforcing will keep the floor slab at essentially the same potential. The steel mesh could be modelled as a buried metal structure within the concrete. It would be easier and more conservative to model the reinforcing as an isolated buried metal structure in the upper layer of soil. This will probably show that step and touch potentials within the building are acceptable.

 

[cuky2000]

CLARIFICATION:

This may be an unusual installation where there is uncertainty to assume a reasonable concrete resistivity value for an indoor Gas Insulated Substation (GIS) rated for 138kV/345kV.

The GIS building is located in a metropolitan area with external sidewalk and asphalted streets, virtually isolate from rain and other external moisture sources other than the indoor climate controlled air conditioning system and ground moisture traveling throughout a new 3 ft thick concrete floor build on top of existing concrete structure resting in rocky soil.

Under the above conditions, the moisture may not be able to migrate easily to the surface of the new concrete floor and may be considered dry. Therefore, the expected resistivity of the concrete surface is considered higher than usual.

Even though, the assumption appears to have merits, there is not agreement among the designer team to support a reasonable representative concrete resistivity value to determine the allowable step and touch potentials and modeling the concrete slab.

Thanks for any further comments.

 

[ePOWEReng]

I don't have a good reference off hand but there are concrete mixes avaliable that are specially made to control the resistivity. Try google or contactor in the area who does work for utilities.