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.

 

[7anoter4]

It is not quite clear how the pier is built. In my opinion it is important to know the short-circuit current. See:
IEEE 142/2007 Table 4-7—Current capacity of building rebar
For #4 it will be 32 A/feet -for 1 sec. clearing time. So 16 ft. Isc=32*16=512 A
If the clearing time will be 5 cycles [no delay] will be 112 A/ft [1792 A total]
If ground-fault currents will be more elevated, the pier concrete may destructively explode concrete due to rapid drying out of the moisture in the concrete surrounding the rebar .

 

[waross]

From the Canadian Electrical code:

(3) A field-assembled grounding electrode shall consist of
(a) a bare copper conductor not less than 6 m in length, sized in accordance with Table 43 and encased
within the bottom 50 mm of a concrete foundation footing in direct contact with the earth at not
less than 600 mm below finished grade; or
(b) a bare copper conductor not less than 6 m in length, sized in accordance with Table 43 and directly
buried in earth at least 600 mm below finished grade.

This may be difficult to comply with for smaller piers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

NEC recommends both reinforcing bar or bare copper conductor. See:
Art. 250.52 Grounding Electrodes. (A) Electrodes Permitted for Grounding. (3) Concrete-Encased Electrode.
“A concrete-encased electrode shall consist of at least 6.0 m (20 ft) of either (1) or (2):
(1) One or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 13 mm (1⁄2 in.) in diameter, 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; or
(2) Bare copper conductor not smaller than 4 AWG.
Metallic components shall be encased by at least 50 mm (2 in.) of concrete and shall be located horizontally within that portion of a concrete foundation or footing that is in direct contact with the earth or within vertical foundations or structural components or members that are in direct contact with the earth.”

 

[hayeska]

7anoter4,

I meet the requirements for the concrete encased electrode.
The debate is centered on whether or not the formwork will limit the amount of moisture that the concrete will absorb and therefore negate its conductivity.
The construction details are described in the 1st post.

 

[7anoter4]

Sorry, the sketch is quite unclear. I think you have to send it as "attachment" -not included directly on post.

 

[7anoter4]

If only the below surface of the footing it is in direct contact with the earth then instead of 32’ will be only 16’ in direct contact [instead of minimum 20’as required by NEC].If you would know the grounding fault current it might be enough.

 

[hayeska]

Attached sketch.

Bottom of footing is 3'4" below grade.
Partial height of pier is below grade.
The question would be with respect to moisture and conductivity.

Thanks

 

[waross]

You may have trouble meeting the twenty foot minimum.
I would consider placing an approved grounding plate in the bottom of the excavation and pouring the pier on top of it.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hayeska]

Warcross,

I believe I have 20 lf; it is already installed. The discussion centers around the use of the forms.

Thanks

 

[jghrist]

If the footing form is plastic, it will insulate the concrete electrically from the soil. It is the electrical insulation, not the lack of moisture in the concrete that disqualifies the foundation from being an effective grounding electrode. The concrete must be in direct contact with the earth.

 

[7anoter4]

In my opinion, only the soil close to the contact “concrete plate” and the concrete around the rebar participate in grounding resistance-at least according to E.J.Fagan-and other- formula. The problem will be if-after a grounding fault-the humidity of the entire concrete column will stay still elevated in order to avoid cracks on the adjacent to vertical rebar concrete.

 

[hayeska]

jghrist,
Only the top of the bottom form is plastic.
It actually has small holes in the top; when considering migration/wicking of moisture.
The bottom of the form is exposed to earth.
The vertical steel is formed with a cardboard tube form.

7anoter4,
Not sure what your concern is regarding cracking of concrete?
The is enough steel present to control cracking from a design standpoint.

The detailing meets the NEC 250.52 requirements. The main question is whether the means of construction (forms), reduce the exposure to the moisture in the soil and therefore severely reduces its conductivity.

 

[7anoter4]

I don’t think you need a “concrete conductivity” since the current flows only in the rebar. The conductivity of the lower part of the footing-around the lower horizontal rebar-in contact with the soil is the single part where we need good conductivity.
In the case of a ground fault the current will flow from the -above the footing- source to the
lower part in contact with the soil through the vertical rebars. This current will heat the vertical rebar and the concrete around this rebar. This concrete will loose part of its moisture and –depending on how high this current will be , on duration and frequency of faults –the concrete around the vertical rebar will dry enough to present dangerous cracks.
So, usually-theoretically at least-the soil -around the footing- moisture could mitigate the concrete drying, in a way[my opinion].

 

[hayeska]

7anoter4,

The following is taken from an article regarding concrete as conductive:

"The principle of the Ufer ground is simple, it is very effective and inexpensive to install during new construction. The Ufer ground takes advantage of concrete’s properties to good advantage. Concrete absorbs moisture quickly and looses moisture very slowly. The mineral properties of concrete (lime and others) and their inherent pH means concrete has a supply of ions to conduct current. The soil around concrete becomes "doped" by the concrete, as a result, the pH of the soil rises and reduces what would normally be 1000 ohm meter soil conditions (hard to get a good ground). The moisture present, (concrete gives up moisture very slowly), in combination with the "doped" soil, make a good conductor for electrical energy or lightning currents."

 

[7anoter4]

Let's say your 20' #4 rebar system is o.k. for 112*20=2240 A ground-fault current.
[As per IEEE 142/2007 Table 4-7—Current capacity of building rebar- for 5 cycles [0.083 sec] 112 A/foot permissible Isc=20*112=2240 A].
Let's say this current will spread equally in 4 #5 vertical rebars in order to convey the current from the source to bottom rebar system. Isc/rebar=2240/4=560 A.
It seems that in this case no temperature rise in this rebar is expected. However, if the short-circuit will be 30 kA will produce 60oC [a dangerous temperature for concrete].

 

[waross]

Excerpt from the Canadian code:

encased within the bottom 50 mm of a concrete foundation footing in direct contact with the earth

Is there a similar requirement in the National code?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

NEC Art.250.52 requires:
"Metallic components shall be encased by AT LEAST 50 mm (2 in.) of concrete"
Maximum it is not indicated-as far as I know. However, in order to reduce the grounding resistance the maximum concrete thickness has not to be far from this. If the soil resistivity it is more than concrete resistivity then the concrete thickness around a rebar may be more.

 

[resqcapt19]

The following is the rule from the 2014 NEC for a concrete encased electrode.

"250.52(A)(3) Concrete-Encased Electrode. A concrete-encased electrode shall consist of at least 6.0 m (20 ft) of either (1) or (2):

(1) 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; or

(2) Bare copper conductor not smaller than 4 AWG

Metallic components shall be encased by at least 50 mm (2 in.) of concrete and shall be located horizontally within that portion of a concrete foundation or footing that is in direct contact with the earth or within vertical foundations or structural components or members that are in direct contact with the earth. If multiple concrete-encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system.

Informational Note: Concrete installed with insulation, vapor barriers, films or similar items separating the concrete from the earth is not considered to be in “direct contact” with the earth."

With the form left in place, I am not sure that the concrete is in "direct contact with the earth" as required by the NEC rule.

 

[jghrist]

Would the cardboard form prevent moisture from being absorbed into the concrete? Unless it is coated in some manner, I would think not. If it is coated, then it probably would also act as an electrical insulation and the pier could not be considered in direct contact with the earth.

The value of the ufer ground depends on the concrete being a lower resistivity than the soil. From IEEE Std 80-2013:

Concrete, being hygroscopic, attracts moisture. Buried in soil, a concrete block behaves as a semiconducting medium with a resistivity of 30 Ω-m to 200 Ω-m depending on the moisture level. This is of particular interest in medium and highly resistive soils because a wire or metallic rod encased in concrete has lower resistance than a similar electrode buried directly in the earth. This encasement reduces the resistivity of the most critical portion of material surrounding the metal element in much the same manner as a chemical treatment of soils.