Ufer Grounding

[Grounded1]

I am looking for an equation that can help me predict the resistance of a horizontal slab of concrete, on finished grade, with the rebar attached to some number of vertical ground rods in the earth. I have equations for ground grids, vertical rods, etc, but would like to calculate what kind of drop in resistance I would gain by using ufer grounds in a slab of concrete. Any help would be appreciated. I also have found one equation for a vertical concrete encased rod, but this is not the confuration I am looking for. Thanks in advance.

 

[Cavie]

I am looking for an equation that can help me predict the resistance of a horizontal slab of concrete, on finished grade, with the rebar attached to some number of vertical ground rods in the earth. I have equations for ground grids, vertical rods, etc, but would like to calculate what kind of drop in resistance I would gain by using ufer grounds in a slab of concrete. Any help would be appreciated. I also have found one equation for a vertical concrete encased rod, but this is not the confuration I am looking for. Thanks in advance.

Unless you can predic the weather on any given day and the moisture content of the earth on that same day you are wasting your time.

 

[Grounded1]

How then would the following equation exist?

resistance of a vertical rod encased in concrete.

RCErod = {rhoc?ln(Dc/d)+rho?[ln(8?L/Dc)-1]}/(2?pi?L) ohms

Where L is the rod length in m
Dc is the diameter of the concrete in m
d is the rod diameter in m
rhoc is the resistivity of concrete in ohm-m (between 30 and 90)
rho is the soil resistivity in ohm-m

 

[Grounded1]

Unless you can predic the weather on any given day and the moisture content of the earth on that same day you are wasting your time.

That is not true. I found an equation for a vertical concrete encased ground rod in another thread which does not account for soil moisture content:

There is an equation in IEEE Std 80, IEEE Guide for Safety in AC Substation Grounding for the resistance of a vertical rod encased in concrete.

RCErod = {rhoc?ln(Dc/d)+rho?[ln(8?L/Dc)-1]}/(2?pi?L) ohms

Where L is the rod length in m
Dc is the diameter of the concrete in m
d is the rod diameter in m
rhoc is the resistivity of concrete in ohm-m (between 30 and 90)
rho is the soil resistivity in ohm-m

But I am looking for the benefit of just a horizontal slab with either copper or rebar in it, attached to a few ground rods...

Anyone else think they can help?

 

[bphgravity]

I agree with Cavie. Ground resistance / impedance testing is only accurate at the moment of the test and will not necessarily indicate future conditions.

What is your purpose and reason for needing such a calculation?

 

[iwire]

That is not true. I found an equation for a vertical concrete encased ground rod in another thread which does not account for soil moisture content:

There is an equation in IEEE Std 80, IEEE Guide for Safety in AC Substation Grounding for the resistance of a vertical rod encased in concrete.

RCErod = {rhoc?ln(Dc/d)+rho?[ln(8?L/Dc)-1]}/(2?pi?L) ohms

Where L is the rod length in m
Dc is the diameter of the concrete in m
d is the rod diameter in m
rhoc is the resistivity of concrete in ohm-m (between 30 and 90)
rho is the soil resistivity in ohm-m

But I am looking for the benefit of just a horizontal slab with either copper or rebar in it, attached to a few ground rods...

Anyone else think they can help?

It sure looks like the equitation takes into account the moisture because the equitation depends on the soil resistivity in ohm-m which would change depending on the moisture content of the soil.

 

[Grounded1]

I agree with Cavie. Ground resistance / impedance testing is only accurate at the moment of the test and will not necessarily indicate future conditions.

What is your purpose and reason for needing such a calculation?

I would like to calculate an estimated ground resistance of what a ufer can attain because I most likely can't dig trenches for a ground ring/grid where I need to install a ground system. There are unknown fiber cables, power lines, etc, buried in the area. I am just moving a small comm building from under a tower to right next to the tower, on a new slab of concrete. I must follow military standards, e.g. MIL-HDBK-419A, if you are familiar. But there is not much mention of ufer grounds in this huge book. But I can estimate ground rods, rings, grids, etc, with different equations given in this handbook/theory. I don't really care too much about the water content. It can be an idealistic result that I can then work from.

Thanks!

 

[Grounded1]

It sure looks like the equitation takes into account the moisture because the equitation depends on the soil resistivity in ohm-m which would change depending on the moisture content of the soil.

I agree, so does anyone know of a similar equation for a slab of concrete?

 

[ohmhead]

Well can we just take that formula and say horizontal formula and and leave out the word vertical .


Sorry just a thought .

How about plate electrodes do they still make those .

 

[Open Neutral]

I must follow military standards, e.g. MIL-HDBK-419A, if you are familiar. But there is not much mention of ufer grounds in this huge book. But I can estimate ground rods, rings, grids, etc, with different equations given in this handbook/theory.

Side note: Ufer invented his namesake for the Army in the WWII era; to ground ammunition stores in the Southwest. And now they don't mention same? Sigh.. I find a reference to "IEEE Transactions paper # 63-1505."