Question About Concrete Base

[sws1]

Hi - New member here, but have been reading alot.

I just moved into a new home in which much of the back yard is already poured concrete slabs. I would like to move my hot tub from the old house. I can't tell if the concrete is reinforced, but my guess says no, since I'm not sure there would have been a need previously.

I can tell based on sticking my finger in a cracked area, not near where the tub is going, that the concrete is roughly 2.5 - 3" thick.

My hot tub dimensions are roughly 7' by 6.5'.

Where I'd like to place the tub is more or less towards the corner of a 7.5" x 11.5' slab. (The yard has lots of slabs, seperated by plastic dividers.) So, the 7' edge of the hot tub would go along the 7.5" edge of the slab. I would then move the tub 6" or so of the edge so the cover has room to fold. So in essence, it would run the entire length of a piece of slab, and about 1/2 way across it. On 2 sides of this piece of concrete (and therefore next to the tub) is dirt / mulching, etc.

To complicate things a bit, the town is going to require a bonding wire to be put around the tub. So the plan is to saw cut a groove in the concrete to insert some 8 gauge wire. 2 of the 4 sides of the loop would be under concrete, and the other 2 sides would be in the mulching. The cut through the concrete would then be patched.

SO, my questions are:

- Is this 2.5" concrete (without rebar) going to be strong enough for hot tub when filled?

- Does the fact that the hot tub is towards the corner of a big slab inherently introduce more stress? (About 6" of concrete)

- Does cutting and patching this groove that is needed for bond wire also cause structural issues?

I'm not really psyched about ripping up existing concrete, and given that it is winter, it's not really an option. Should I just give up?

[sws1]

Any thoughts?

[Pool Clown]

If it were me....

I would approach this with the mindset to be prepared to take the concrete out. But first, set the tub and see what happens. If the concrete fails (you begin to get cracks) It wont be catastrophic. The tub wont fall into a hole and be lost.

[JCE]

If the slab is 2-3" thick, it probably doesn't have rebar in it. It may have wire mesh.

If the tub is uniformly supported within the skirt you shouldn't have cracking under it. Cracking may occur around the perimeter because the slab under the tub may settle due to the local load it will receive, while the slab around it doesn't. The thing that causes cracking is differential settlement, not settlement itself. Think of walking in soft soil...the edge of your footprint would be like the edge of the spa and where the slab would crack. With a slab this lightly loaded it won't make a difference if you put it in the corner like you describe or in the middle so don't worry about that.

The problem you may have is on older spas where they were supported in the middle only, unlike todays spas. My parents had a spa about 20 years ago that was like this; a situation like this means the middle may settle more than the perimeter (if the slab cracks at the center) skirt that somewhat supports the outter shell. Then you have problems.

I agree with Pool Clown; if you are happy with the slab and don't feel compelled to replace it, put the spa on it and see what happens. That's what I would do.

Now, the bonding issue is something that is a little confusing to me and I am currently researching; I have a call into a electrical engineering coworker. This apparently is a new requirement in the code to ground the rebar within the foundation. I think I read somewhere recently about bonding the rebar to the tub, which makes zero sense to me. I am a civil/structural engineer that works on the industrial side of things, and this is something I am not clear on.

The problem you have is there is a good chance you have no steel in your slab. Then what? Maybe somebody can give me the lowdown on what the goal here is of the bonding, what the rebar is supposed to be connected to, etc. and we can go from there. I'm not sure this thin slab could be considered a "foundation" either, especially if it has no rebar in it so this requirement should go away. Sounds like a bunch of busy body code BS to me.....I'm not a fan of rehashing of codes every couple of years making up new requirements on engineering priciples that have been around many years.

[sws1]

Thanks.

This isn't a new tub. It's a 2008 (maybe 2007) Tiger River Bengal. It is supported fairly uniformly, and not just around the edges.

The bonding thing is a NEC 2008 requirement. There was some leaway in the 2005 code, but for the 2008 code, a hot tub requires that the surface within reach of where the tub is all be bonded together to prevent an issue where...you are getting out of the tub, and you touch both the water in the tub, and a piece of ground that may have some stray voltage. It's the same requirement for pools, although the dimensions are different. Technically, they want wire mesh 3' around the tub and underneath, to be tied together and bonded to the ground of the tub. HOWEVER, there is an exception that says you can create a loop of 8 gauge wire around the tub, 12-18" away from inside edge of tub, buried 4-8" deep.

Read paragraph 21 here: http://ecmweb.com/mag/electric_code_changes_13/

I plan to follow the "alternate means" which are described in that article.

[JCE]

Thanks.

This isn't a new tub. It's a 2008 (maybe 2007) Tiger River Bengal. It is supported fairly uniformly, and not just around the edges.

The bonding thing is a NEC 2008 requirement. There was some leaway in the 2005 code, but for the 2008 code, a hot tub requires that the surface within reach of where the tub is all be bonded together to prevent an issue where...you are getting out of the tub, and you touch both the water in the tub, and a piece of ground that may have some stray voltage. It's the same requirement for pools, although the dimensions are different. Technically, they want wire mesh 3' around the tub and underneath, to be tied together and bonded to the ground of the tub. HOWEVER, there is an exception that says you can create a loop of 8 gauge wire around the tub, 12-18" away from inside edge of tub, buried 4-8" deep.

Read paragraph 21 here: http://ecmweb.com/mag/electric_code_changes_13/

I plan to follow the "alternate means" which are described in that article.

OK, I follow....it's to equalize voltage differential between the ground and the spa. Haven't really heard of this but like I said I don't do residential design.

So what is the requirement if somebody puts a tub on a wooden deck?

[sws1]

Thanks.

This isn't a new tub. It's a 2008 (maybe 2007) Tiger River Bengal. It is supported fairly uniformly, and not just around the edges.

The bonding thing is a NEC 2008 requirement. There was some leaway in the 2005 code, but for the 2008 code, a hot tub requires that the surface within reach of where the tub is all be bonded together to prevent an issue where...you are getting out of the tub, and you touch both the water in the tub, and a piece of ground that may have some stray voltage. It's the same requirement for pools, although the dimensions are different. Technically, they want wire mesh 3' around the tub and underneath, to be tied together and bonded to the ground of the tub. HOWEVER, there is an exception that says you can create a loop of 8 gauge wire around the tub, 12-18" away from inside edge of tub, buried 4-8" deep.

Read paragraph 21 here: http://ecmweb.com/mag/electric_code_changes_13/

I plan to follow the "alternate means" which are described in that article.

OK, I follow....it's to equalize voltage differential between the ground and the spa. Haven't really heard of this but like I said I don't do residential design.

So what is the requirement if somebody puts a tub on a wooden deck?

Not sure, but it's not an option for me. I don't have room to put a deck around the spa. (Well, I do, but it'd become cramped.)

[JCE]

Not sure, but it's not an option for me. I don't have room to put a deck around the spa. (Well, I do, but it'd become cramped.)

After reading that link I don't think it applies to decks. I asked because I am building a deck right now before I buy my tub. It will be structurally adequate.

I've been reading up on this today. I don't know how many "the difference between grounding and bonding" articles I've read today, but it is alot. My initial reply was because I thought this was some type of grounding issue.....

Even after all the reading, all there in plane English, I still cannot understand why grounding the tub does not do what bonding does.

[hot_water]

Not sure, but it's not an option for me. I don't have room to put a deck around the spa. (Well, I do, but it'd become cramped.)

After reading that link I don't think it applies to decks. I asked because I am building a deck right now before I buy my tub. It will be structurally adequate.

I've been reading up on this today. I don't know how many "the difference between grounding and bonding" articles I've read today, but it is alot. My initial reply was because I thought this was some type of grounding issue.....

Even after all the reading, all there in plane English, I still cannot understand why grounding the tub does not do what bonding does.

Having your tub properly grounded is actually WHY you need the equipotential grid bonding. In a hot tub, the water is bonded to the electrical system equipment ground, often through the heater or the underwater light fixture. So the water is essentially at the same potential as the ground rod back at the main panel.

But the earth near the tub, which is often quite a distance away from the main panel, may be at some other potential - caused by power company transformer leakage, buried wires, or who-knows-what. If you're standing on ground at this "stray voltage" and touch the water... shock.

The Equipotential Grid requirement is is supposed to ensure that what your standing on, since it's bonded to the equipment, is at the same potential as the water. It's not a bad idea.... and easy enough to accomplish when installing a new slab. A perimeter wire, which the inspector seems to want, seems like an easy retrofit. Personally, I think it's nuts to require an exisiting slab to be removed and replaced to install a bonded rebar grid... tis has been suggested but I haven't actually heard of anyone being fored to do it. But again it's all up to the local authority. Fact is, there have been cases where stray voltages have caused shock - and also millions of tubs installed without an equipotential grid system.

My understanding is that if you're on wood (including composite decking) you are considered effectively insulated from stray voltage and thus don't need to install the equipotential grid. Where I live we are on NEC 2005 - so this may not be the case for NEC 2008. I looked at it a while ago but honestly can't remember the exact differences. Assuming wood is still an "out", I would lay down some Trex deckboards right on the slab in peference to pouring a new slab just because of the equipotential grid requirement.

As to the suitability of the 2.5" slab, it's really hard to tell if it will be okay since we don't know what was in the concrete mix, how much water was there (this affects the strength), proportion of aggregate, etc. But if the ground is pretty stable underneath (and this seems likely if you're not getting any cracks in the slab near where the tub will go) it seems a good bet to give it a try. As many others have mentioned, if you develop problems, you can fix it later.

-hot_water