All,
I need to construct a concrete pad for a spa, 91″ x91″ x4″. My questions are:
1) Should I use some type of reenforcing mesh/bar in the concrete?
2) Can I pour directly onto the ground or do I need to put down a layer of plastic then a layer of gravel prior to pouring?
Thanks,
dlb
.
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Replies
all depends on what you want, on grade or above, iether way I'd still remove the top soil, and compact a gravel base. The strength of the conc is determined by how long it takes to cure, so plastic to keep the earth from sucking it dry is good, but also keep the surface moist and covered with plastic for 2 or 3 days. Steel mesh is cheap, wouldn't hurt and will help keep it from cracking .. You may want the incorporate the electrical run in the conc pad so it'll be concealed and protected from the weather ..
Thanks for the reply. I'll Just Do It!
dlb
.
The undisciplined life is not worth examining.
Our spa is on a 4" slab and the conduit for the electricity runs underneath and comes up just inside the perimeter, I doubt they put any steel in it but it sure wouldn't hurt. Why they put the slab in the middle of the yard I have no idea.
ML
If you want to use plastic it goes directly under the concrete, not under the gravel. The plastic does 2 things: 1) keeps the water from the concrete from being sucked out by a dry base, which isn't as much of a concern if you wet the base before placing the concrete, and 2) acts as a vapor barrier to keep moisture from coming up through the slab - which in your application shouldn't be a a concern - it's outside anyway. Here, we only use plastic vapor barrier under interior concrete - basements, garage slabs, mono slab houses, etc.
The reason for using gravel is to 1) to aid in drainage - assuming you have somewhere for the water to drain to, and 2) provide uniform support for the concrete. You see in the best scenario the sub soil below a slab is all perfect and of the same compaction, consistency, type, etc. In reality that often doesn't happen. In a small area like you are talking about it may be easier to get a uniform base. The need for good drainage is partially dependant on what kind of frostline you have there. Here, we don't have a frostline, so it is not as much of a concern.
Wire is used to help hold the slab together in case of cracking. It keeps small cracks small. That small a piece of concrete, if properly done, shouldn't crack.
If it were me I'd place 5" of concrete on 4" of stone. If I wasn't entirely sure the sub base was great I'd put some rebar in it - like maybe 4 pc of #5 each way. Rebar prevents cracking and adds rigidity to the concrete. If you want a really good slab use rebar and wire (assuming you can get wire by the piece (not roll)) but it shouldn't be necessary - I'd guess your spa weight at around 1500#s and the weight should be fairly uniformly distributed.
Edited 4/21/2009 9:14 pm ET by Matt
Thanks for taking the time to explain the details. I will certainly follow your suggestions.
Thanks again,
dlb
.
The undisciplined life is not worth examining.
I'd guess your spa weight at around 1500#s and the weight should be fairly uniformly distributed.
You could probably get 1500#s of people in a spa. Now add 5-600 #s the spa weighs and about 4-500 gallons of water at 8 #s per.
Your advice about the concrete sounds dead on and a slab is the best support.
Tubs usually have load around the skirt and at the footwell, so these people with support around the outside and gravel, sand, paver stones in the middle will find their tubs will now teter-totter if put on a hard flat surface as the footwell has sunk.
Yea I messed that up....
Re >> You could probably get 1500#s of people in a spa. << You are gonna give me bad dreams.... I was thinking in terms of me (200#s) and three 120 #ers with water soluble bikinis... :-)
Edited 4/27/2009 9:51 pm ET by Matt
For what it's worth as far as weight distribution, our hot tub has been sitting for a decade or more on five treated 4x4s, resting directly on the ground. I failed to level the 4x4s perfectly, so it's out 1" over its width, but it works fine.
Concrete would be more comprehensive support.
Who says you need a pad?
Many spas come with a styrofoam 'pad' that is perfectly adequate, so long as you have a properly prepared sand bed.
The homeowner wants one. Can I argue?The undisciplined life is not worth examining.
If nothing else the slab will limit access to critters that love the warmth for nests. I have removed many smelly carcasses from inside tub surrounds. Do think about the power supply before pouring.
10-4 & thanks!
dlb
.
The undisciplined life is not worth examining.
My spa sits on a slab that is also 4" crete. The spa specs said that rebar was not necessary.
But I told the crete guy to put wire mesh in, just cuz.
My spa is sitting on 18" precast concrete pavers, and has been sitting there fine for 7 years. I don't think you need 4" concrete - the point load just isn't that high. You do need a pad that's well-leveled to put the pavers on.
I used four 4x4's to make a perimeter the same size as the hot tub, leveled it, filled it with gravel, tamped it down, screeded it off flat and put the hot tub on it.
The resulting gravel pad is well drained, easily removed (or moved) and seems to distribute the weight of the full hot tub evenly.
Karl
from experience.... if you are "sure" you want concrete make sure the concrete pad is at least 6" larger on all sides of the footprint, bigger is better, mainly for weight distribution, less chance of one side or corner "dipping" with the weight. I would also highly recommend using a product called "Fastform" , it allows the concrete to cure evenly and effectively. Have at least the top soil removed and a 4" to 6" packed sand base.
And VERY important! make sure there are NO rocks larger than a 1" within the ground 12 " below the concrete. And use at least 3/8" rebar on 12" centres.
In addition, making the pad larger on the side of the step and the side of the "spa pack" (pumps etc.) is also advantageous, especially when it come to maintenance and repair time.
Concrete is better, tubs do leak eventually, concrete sheds the water to the edge of the platform, less chance of settlement.
With that said, do not dismiss the idea of putting the tub on a packed gravel base, I recommend at least 3 to four inches of a good quality 'sand box" sand, wet packed then 8 to 12" of 1/2" or 1/4" minus on top, wet packed with a plate tamper every 2 to 3" on the filling. this does work well in places that has good draining soil.
Your choice should also take into consideration the soil you are building on a bog will be a lot different than a clay base, ask around, a local hot tub shop will provide you with "what" they have found best for your particular area.
This may sound stupid or you might think I bumped my head......I had a plumber at my place doing some work and he was telling me how he was doing the same thing, concrete pad for spa.
He was told by the BI, that if he places metal in the pad with the spa sitting on it, the metal, rerod/mesh, had to be grounded!!!! Seperate ground rod, wire, ectt.......
He never heard that. !!!!! This guy wasn't smoking houch when he told me that!!
A plumber, pouring concrete, that doesn't know the electrical code, as related by an artist?!?
Seems pretty far fetched to me.
Plumber there to do a little framing and drywall work?
Timmy, I didn't say that the plumber was pouring the concrete or doing the electrical work. He was having it done! And he related the story to be because he thought it was pretty far fetched too!!
Be carefull what you say about plumbers, I am not the plumber, but the plumbers son...........
"...and he was telling me how he was doing the same thing, concrete pad for spa."
Sorry, just read what you wrote. My bad. Frenchy version of English?
Plumbers are great as long as they stick with plumbing and the occasional concrete slab (supporting a plumbing fixture, such as a spa, you could argue, falls under their scope of work according to the Local...).
Now that we know where the plumbers place is here, I still wonder if anyone else has heard of grounding the rebar in the concrete???
My father was more than the basic plumber, he did commercial/industral work. More than once my father had to straighten out an engineer that thought the only thing to know in laying out a set of plans was, "sh*t runs down hill" I grew up working with him, I know.
No. I've had lots of tubs put on slabs, both inside and out, and it has never come up. Not with the tub suppliers or my electrician.
OK, back to the real question.
The answer is yes, grounding of "building steel" is very common and required in most jurisdictions and in (almost?) all commercial buildings. Rebar in the main structure always counts, as do equipment pads and the like and should be gounded/bonded. In a detatched slab with an electrcal feed/appliance (i.e. a spa or a light pole), I suppose the applicability of the Code is debatable, but certainly not clearly NA.
I didn't ground mine (I have a spa on a reinforced slab with liquidtight penetrating the slab) but is feed with a grounded and GFCI protected power supply.
>>I still wonder if anyone else has heard of grounding the rebar in the concrete???<<
In my area, this is required.
Technically, the metal in the concrete is "bonded" rather than grounded.
The nuts and bolts procedure is to connect, and provide access to, a ground wire to the metal mesh or rebar within the slab - this wire will be connected to the common bonding point within the spa once it is installed.
There is a common bonding point generally within the internal control panel area of the spa (typically a ground bar, just like in a SEP).
To the common bonding point there are a number of wires from / to various locations / items, including but not limited to:
- Metal frame of the unit
- Motor frames within the unit.
- Metal piping from within the unit.
- Metal within the pad.
-Any other metal within 5' of the spa - downspouts, metallic conduit, gas piping, water piping, handrails, etc.
- A ground wire going to the ground bar of the panel which serves to supply the power for the spa. Could be the SEP or a sub-panel.
No additional ground rods are required at the spa, at least in my area.
So, yes, I have heard of it and done it.
Jim
Never underestimate the value of a sharp pencil or good light.
Wow, never heard of it!
Tim & jtc1, thanks for the explation.