Ufer Ground Pier Foundation

[hayeska]

Would like to discuss the validity of the following:

Reinforced concrete pier:

8-#4 bars (16 lf total) horizontal steel in 28"x28" footing
Footing is formed with plastic tube base that receives tube form. The tube base has holes in the top to help the concrete flow.
Tube form is a 12" diameter commercial grade form, coated cardboard. Form is typically stripped just below grade.
Vertical steel bars are 4-#5 bars with hooks. Approximately 16 lf of steel will be below grade, for a total of 32 lf.
Bare copper conductor 4 AWG attached to #4 bars horizontal and #5 bars vertical

In a discussion with an electrical contractor he thinks that the concrete is protected by the forms and will limit the amount of moisture that the concrete will absorb and therefore negate its conductivity.

All opinions welcome.

Thanks in advance.

 

[hayeska]

resqcapt19,
The bottom of the footing is in direct contact with the soil. The form is like a turtle shell, no plastic on bottom.
The vertical form is cardboard that will deteriorate over time.

 

[hayeska]

7anoter4,
Would the ground fault current also leave through the vertical rebar to the surrounding soil assuming the cardboard form disintegrates over time?

 

[7anoter4]

Sorry, I am only an electrical engineer and I don’t know this form card behavior. However, if the ground fault will occur tomorrow could be this form disintegrated already?

 

[EEJaime]

hayeska,

I think there are two problems with this detail. As has been noted by other posters, there appears to be 5" of concrete between the lowest horizontal layer of rebar and earth. This is the only steel that would even be in play. As waross pointed out, this would not comply with the Canadian code requirements. Even in the US, with the sides of this concrete insulated from earth by the form, I am not sure that I would accept that as being in direct contact with earth if I was doing the plan checking on this installation, (which I do on many others). I would require an additional auxiliary electrode, either plate or rod, connected to this system. You did not mention how this grounding electrode is connected to the electrical system. Is the copper conductor continued up through the footing to the ground bus at the service panel? Is all the steel at that level welded together to form a continuous length? The second problem is that you do not appear to have the requisite 20', (6M), of length IMHO.

EEJaime

 

[hayeska]

EEJaime,

There is 3" of cover for the horizontal steel. 2" of cover for the vertical steel.

The vertical steel bars are "L" shaped 12" horizontal. Therefore there is just over 20 lf in the footing.

The copper conductor continued up through the footing to the ground bus at the service panel and is clamped to the bars with UL rated clamps.

 

[EEJaime]

Hayeska,

Correct me if I'm wrong, but the 12" horizontal leg of the verticals is overlapped and wire-tied or cadwelded to the horizontal bars. Those lengths would not be counted as additional developed length, it is the same conductor as the horizontal members to which they are attached. The 2" of cover for the verticals does not come into play because those are 2" from a form, not from earth. I still don't see how this would be compliant.

EEJaime

 

[hayeska]

EEJaime,

All bars are wire tied.

250.52(A)(3) Concrete-Encased Electrode states

"One or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 13 mm (½ in.) in dia meter installed in one continuous 6.0 m (20 ft) length, or if in multiple pieces connected together by the usual steel tie wires, exothermic welding, welding, or other effective means to create a 6.0 m (20 ft) or greater length"