Forum Message

Heading: resistivity
Can any one help me with some typical mix design for fluidised thermal backfill materials to be place around electrical conduits. Any text book or technical notes would be much appreciated.

Regards Dave Spasojevic
Sydney, Australia

David Spasojevic

Hi David

I've been involved with design and testing of HV cable backfill (fluid and ganular) for 15 years or so. I offer the following suggestions:
(a) if your cables are in conduits/ducts and low strength is not a requirement for the backfill then the best option is likely to be to use standard 10MPa concrete. You would need to measure it's thermal resistivity which will depend on the local aggregate type & quality. 10MPa concrete using New Zealand greywacke typically has TR of ~0.8-0.9 m.K/W.
(b) If you do require low strength please advise and I'll provide typical mix design.
(c) in either case quality control can be managed in a timely way by checking the dry density, with less frequent testing of the TR which will have a 2 - 3 week turnaround time (allowing for curing).

Tim Logan
Wellington, NZ
Tim Logan

As a start solution would be very practical to get the right project spec/required target TR value (m.K/W) in "dry state", more often specified, as a worst case scenario unless the calbes are at all time below the underground water level.
On same concrete mix the TR value measured in "wet state" could be very easy far far lower from the "dry state" TR value.The reason is the water is more thermal conductive than the air.

With all respect to Tim,
will say in NZ is very optimistic to state that with Greywacke Aggregate and concrete grade 10MPa( cement content say 150kg/m3) to achieve TR value in "dry state" 0.8m.K/W To achieve this you have to use some other available thermal conductive aggregate.
I agree with Tim's TR numbers but only under "wet state" condition.

Slave Tomov, BE
Auckland , NZ
Slave Tomov


Driven from the policy: " to try to learn every day". Would like to open Forum/ Peer thoughts exchange on the following subject:

Basalt vs. Greywacke aggregate used in structural concrete in Bridge Segmental Superstructure- Post Tension Structure.New Zealand

The average density for Basalt and Greywacke aggregates are 2800kg/m3 and 2700kg/m3 respectively. Difference in density is a trivial 3.7%

The questions to be addressed:

How critical is the aggregate density on the total self weight mass/dead load in regard to the structural analyses ?

Does the annual actual/realistic fluctuation in aggregate densities from the quarry exceed the average 100 kg difference in density?

Is the 3.7% mass difference in dead load critical in structural analysis, knowing that in many cases say we use load factors as 1.2G + 1.5Q ?

Does the structural dimensions tolerance provisions nullify the average aggregate/concrete density difference of less than 100kg per m3 concrete weight?

How critical is the different shrinkage values of the aggregates/concrete using Basalt or Greywacke on the quantities of design steel reinforcement/ post tension tendons, the degree of economy , etc?

I think it would be very interesting to explore the technical reasons/advantages why Basalt aggregate was used for Waiwera Viaduct, New Zealand ?

Open for discussion.


Slave Tomov New Zealand
Slave Tomov
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