Conducting concrete

[gfretwell]

Fine when they work and don't migrate to the ground water.

If your floor is off the ground, it will just be part of the debris field when the wind gets up under it.

[James Wilkinson Sword]

Perhaps, if the concrete is guaranteed to be wet, but then the ammo would get damp.

But I wasn't talking about ammo.

It simply isn't true, I tested it.

[gfretwell]

Try it and get back to me. That is 20meg at one small point. If you can find 2 pieces of rebar with 20' in contact with the concrete, check that to ground or another rebar in the same slab.

"Earth" is a pretty bad conductor in the first place so what you are really trying to create is an equipotential ground grid.

Cool, especially when the wind is blowing in off the North Sea in January. ;-)

[James Wilkinson Sword]

Not my problem.

If the walls are gone, why do you care about your floor?

[Dean Hoffman]

I guess one doesn't test earth grounding with a regular ohm meter. The instructor at one of my code refresher classes asked once if anyone had the correct type of tester. No one did out of at least 50 electricians. Earth grounding is primarily for damage from lightning. He also had a diagram showing how little current would flow through the earth if a well motor had a short to the frame. The resistance was so high that there was no way even a tiny fuse would blow if there was a short. This is from the meter maker Fluke:

[gfretwell]

The Ufer ground is not in the floor, it is in the foundation but your concrete floor still has enough conductivity to get you a nasty shock if you have sufficient contact.

[James Wilkinson Sword]

Rebar isn't common here, anyway you have to conduct the electricity down to the metal.

No it isn't. Earth tends to be damp.

Especially then. I run naked along the beach, which causes amusement.

[James Wilkinson Sword]

You guess?

"A good grounding resistance is 5 ohms or less" - well since I got >20 MOhms, I guess that isn't grounded.

[James Wilkinson Sword]

20 MOhms prevents shock.

[Dean Hoffman]

Some cut.

So did you use an actual ground tester or a volt/ohm meter? It sure seems odd that your results don't match Mr. Ufer's and bunches of people after him. An article in Electrical Contractor magazine says Ufer's grounding electrodes kept the resistance at 2-5 ohms over a 20 year period.

[James Wilkinson Sword]

Resistance is resistance. Concrete does NOT conduct. WATER conducts. Concrete is a porous material which may or may not contain water. The floor of your house should never contain water, or you have BIG problems, way above a possibility of shock.

[gfretwell]

Under a decent building code they will all stand or blow away together. The roof is continuously tied through the walls to the foundation In a concrete block building that will be continuous rebar from the foundation to the poured tie beam on the top of the wall, then strapped to the roof trusses. A typical 1800 sq/ft house will have almost 100,000 pounds of uplift protection at the roof.

[gfretwell]

Stop the presses, some naked scotsman has debunked 100 years of electrical science. Your Nobel prize is in the mail sir.

[James Wilkinson Sword]

I see. We don't have so much wind here, so my next question is, why are modern houses in the UK starting to have concrete foundations like yours? Old ones, you go under the floor, you can crawl around on dirt and fix the pipes and wires. New ones, you can't get down, it's just solid.

[James Wilkinson Sword]

I'm using common sense. You're completely misinterpreting what you read on the internet.

[gfretwell]

Maybe they think global warming is going to bring you tropical weather some day. Honestly most of the US does not really have an effective wind code either but as people start wondering why a summer squall or dust devil blows their houses to smithereens and a Cat 1 or even 2 here does minimal damage to newer houses here, they start to ponder why. Most of the hurricane damage pictures you see from Florida is 50 year old homes or, more likely trailers. I suppose Florida building codes may spread, if for no other reason that the insurance companies will push them. I know when I was building in Maryland, the main force people thought about was gravity with little more than a passing glance at wind pressure (maybe built to 60-70 MPH) and nothing at all about uplift. I am in the 150 MPH zone.

[clare]

Concrete without water in it isn't concrete. The lime in portland cement is hydrated. Don'rt believe me? What happens if you heat concrete quickly? It explodes from the steam poressure created by the entrained moisture in the concrete - the hydration. That's what refractory cement is made for, and the reason it exists.

from a concrete information site :

When water is added to cement, each of the compounds undergoes hydration and contributes to the final concrete product. Only the calcium silicates contribute to strength. Tricalcium silicate is responsible for most of the early strength (first 7 days). Dicalcium silicate, which reacts more slowly, contributes only to the strength at later times. Tricalcium silicate will be discussed in the greatest detail.

The equation for the hydration of tricalcium silicate is given by:

Tricalcium silicate + Water--->Calcium silicate hydrate+Calcium hydroxide + heat

2 Ca3SiO5 + 7 H2O ---> 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + 173.6kJ

Upon the addition of water, tricalcium silicate rapidly reacts to release calcium ions, hydroxide ions, and a large amount of heat. The pH quickly rises to over 12 because of the release of alkaline hydroxide (OH-) ions. This initial hydrolysis slows down quickly after it starts resulting in a decrease in heat evolved.

The reaction slowly continues producing calcium and hydroxide ions until the system becomes saturated. Once this occurs, the calcium hydroxide starts to crystallize. Simultaneously, calcium silicate hydrate begins to form. Ions precipitate out of solution accelerating the reaction of tricalcium silicate to calcium and hydroxide ions. (Le Chatlier's principle). The evolution of heat is then dramatically increased.

The formation of the calcium hydroxide and calcium silicate hydrate crystals provide "seeds" upon which more calcium silicate hydrate can form. The calcium silicate hydrate crystals grow thicker making it more difficult for water molecules to reach the unhydrated tricalcium silicate. The speed of the reaction is now controlled by the rate at which water molecules diffuse through the calcium silicate hydrate coating. This coating thickens over time causing the production of calcium silicate hydrate to become slower and slower.

[F Murtz]

Not that well, it is whatever salts and impurities in it that do.

[F Murtz]

My house is on a slab and I get five volt tingle from the taps when I am in the shower. I suspect that when it was built they did not bond the steel mesh to earth, which they are supposed to do now.

[Chris Green]

Not really (water that is), pure water is a very poor conductor. It's the impurities in water that make it a reasonable coductor but even tap water isn't that good.