What Is The Ideal Concrete Slab For A Tub?

[pnwtub]

Hello,

I'm getting ready to put in an 8x10 6" thick concrete slab and wanted to hear what others have done before I give the green light to the contractor.

1. Is the 4" metal mesh good enough or is it best to use 1/2" rebars throughout the slab? Some concrete contractors say no mesh or rebar is necessary, others say mesh is enough, and others recommend the rebar which provides the most strength.

2. Every major spa manufacturer says that you should connect all the metal inside the slab to a bonding wire which is then supposed to attach to the motor or something else metal in the tub. Is this true? I live in WA where some electricians say they never ground the slab since the GFCI is protecting you already. While other electricians are recommending it since that is what the code says.

Thanks for sharing your site prep experiences.

pnwtub

[spa_repair_guy]

"1. Is the 4" metal mesh good enough or is it best to use 1/2" rebars throughout the slab? Some concrete contractors say no mesh or rebar is necessary, others say mesh is enough, and others recommend the rebar which provides the most strength. "

1/2" metal rebar requires a minimum of 3" of concrete on both sides. You will not be able to make that happen with certainty. Use the 4" mesh and/or fiber-mesh. Your slab of 5 5/8" to 6" should be flat and level under the spa.

2. Every major spa manufacturer says that you should connect all the metal inside the slab to a bonding wire which is then supposed to attach to the motor or something else metal in the tub. Is this true? I live in WA where some electricians say they never ground the slab since the GFCI is protecting you already. While other electricians are recommending it since that is what the code says.

Ground any metal objects that you can grab from the spa like handrails. Your spa should be GFCI protected with the approved cuttoff box. You MUST follow your local codes regardless of my opinions.

SRG

[Pool Owner]

Most likely, the reinforcing steel is not a big deal on an 8x10 slab. Generally I would go with the #4 rebar (1/2-inch diameter) since the difference is cost should be negligible and it provides better construction (there should not be any splices in the rebar on a slab this small). If you have expansive soil or a significant frost depth, #4 rebar at no more than 12 inches on center is a good choice. However, if the preparation and placement of concrete is done right there should not be a need for any reinforcing steel for this small slab. Since you will likely have a building permit for this construction (at least for the new electrical hookup) make sure you follow any local requirements.

All things being equal, I would provide a bonding wire (typically a 8 gauge bare wire attached to slab steel with a bonding clamp and sticking out of the slab near where you expect to place the pump (or where recommended by the spa manufacturer). Bonding is now required between all metal piping inside new building construction too. The whole idea about bonding is ensuring any object that that may become a “ground” is connected together to the same grounding source, which prevents the possibility of a person being shocked from a short as they complete the circuit to the “ground.” Spa Repair Guy’s comment is good except any metal parts should be bonded to the spa equipment or isolated from “ground.” If you elect not to provide reinforcing steel in your slab, you do not need to provide bonding.

That being said, I would expect that most spas placed on existing patio slabs are not bonded and the GFIC is likely to provide adequate safety protection. When possible, stick with non conducting material (e.g., plastic, fiberglass, wood, etc.) within reach (10 feet) of your spa so you do not need to worry about bonding.

[pnwtub]

Good stuff, thanks for the advice!

Can you help me define "good site preparation and placement of concrete that is done right". For example, some questions i have:

1. Should the ground be flat before the gravel is layed down? I would expect a 4" slab should be 4" all around. If the ground is not flat to begin with then part of the slab may be 3" and other parts may be 4".

2. How many inches of gravel should be used as a base?

3. Should the concrete surface be smooth like a garage floor or rough like a patio?

Any other things that make a slab good?

[Pool Owner]

Good stuff, thanks for the advice!

Can you help me define "good site preparation and placement of concrete that is done right". For example, some questions i have:

1. Should the ground be flat before the gravel is layed down? I would expect a 4" slab should be 4" all around. If the ground is not flat to begin with then part of the slab may be 3" and other parts may be 4".

Generally, grade should be flat (or sloped to match the top if needed) for a consistent thickness. 1/4-inch variance would not be a problem for a 4-inch slab. I would not worry about 1/2-inch variance with a 6- or 5.5-inch slab, which I think you said you will have.

2. How many inches of gravel should be used as a base?

If you have good draining soil, it would be okay to put the concrete directly on the soil. If not, one to two inches of sand or gavel should be fine. Make sure the base (soil, sand, and/or gravel) are compacted (hand is okay but power is better) before placing the concrete.

3. Should the concrete surface be smooth like a garage floor or rough like a patio?

If your spa will cover all of the concrete it probably does not matter. If some of the concrete will be a walking surface, have a rough finish to reduce the chance of slipping (unless it is part of a shuffleboard court – I have one).

Any other things that make a slab good?

I would ask for the concrete to be a 2,500 to 3,000 psi mix. “Fly ash” can sometimes make blotches on the surface, so it is better ask for a concrete mix without “fly ash” (yes – it can be done).

Any other things that make a slab good?

[pnwtub]

When pouring concrete, should the temperature be above a certain degree? I heard 50 degrees is the minimum, but wasn't sure how meaningful this is. The forecast is 30-39 degrees and the concrete is scheduled to be poured this week. If it's not going to cure properly, i'd rather postpone this work. Thanks in advance for any pointers.

[Pool Owner]

Good question. Concrete can be placed in cold weather but it is more difficult. Doing a Google search for “cold weather concrete” will provide a number of good resources. Basically if your concrete contractor/finisher is knowledgeable about the process of working in cold weather and is prepared to provide the supplies/manpower to properly cure the concrete, it should not be a problem. If you can wait and you expect a warm spell, it is safer to place concrete when the minimum air temperature remains above freezing (say 35 degrees F).

http://www.askthebuilder.com/104_Cold_Weat...tallation.shtml

http://www.deeconcrete.com/coldweatherconcrete.html

http://www.na.graceconstruction.com/custom...ads/TB_0106.pdf

The above websites may be helpful

If you go ahead, the procedure shown here should work for you (http://www.decorative-concrete.net/feature_11-04-02.php)

[GKL]

Good question. Concrete can be placed in cold weather but it is more difficult. Doing a Google search for “cold weather concrete” will provide a number of good resources. Basically if your concrete contractor/finisher is knowledgeable about the process of working in cold weather and is prepared to provide the supplies/manpower to properly cure the concrete, it should not be a problem. If you can wait and you expect a warm spell, it is safer to place concrete when the minimum air temperature remains above freezing (say 35 degrees F).

http://www.askthebuilder.com/104_Cold_Weat...tallation.shtml

http://www.deeconcrete.com/coldweatherconcrete.html

http://www.na.graceconstruction.com/custom...ads/TB_0106.pdf

The above websites may be helpful

If you go ahead, the procedure shown here should work for you (http://www.decorative-concrete.net/feature_11-04-02.php)

Is it realy necessary to go 6"?

I live in the Baltimore area and I beleive my last spa pad was 4".

I was planning my new pad to be the same BUT with 2-3" of gravel to allow drainage to day light because I am now in a very heavy clay soil.

Whats the word??

[pnwtub]

Thanks for the tips and fast reply Pool Owner. The links were very helpful.

[Pool Owner]

Generally I would expect a 4-inch slab to be adequate, especially with the base you described. And if there was an existing slab I would say “go for it.”

However, if your “heavy clay soil” is expansive, I would consider increasing the slab thickness to 6-inch and using at least #3 (maybe #4) rebar at 12 inches on center both ways (at least the section under your spa). It all comes down to construction cost vs. repair/reconstruction cost. It is not that hard to reconstruct a typical slab on grade but if your spa slab cracks for tilts significantly, it will require removing your spa during reconstruction. Check out the cost for a few more inches and better reinforcing steel under the spa. I’m from the “old school”…”when in doubt – make it stout!” If it is only a little more money up front, it is well worth it in the long run.

[GKL]

Generally I would expect a 4-inch slab to be adequate, especially with the base you described. And if there was an existing slab I would say “go for it.”

However, if your “heavy clay soil” is expansive, I would consider increasing the slab thickness to 6-inch and using at least #3 (maybe #4) rebar at 12 inches on center both ways (at least the section under your spa). It all comes down to construction cost vs. repair/reconstruction cost. It is not that hard to reconstruct a typical slab on grade but if your spa slab cracks for tilts significantly, it will require removing your spa during reconstruction. Check out the cost for a few more inches and better reinforcing steel under the spa. I’m from the “old school”…”when in doubt – make it stout!” If it is only a little more money up front, it is well worth it in the long run.

I'm now thinking about putting the spa on my 16'X35' stone/paver patio until the weather gets more stable. Baltimore weather isn't terriable but we do get cold snaps and that would probably be the few days before/during & after my slab pour. My power will be easilly accessable for the temporary locatio and the tub's final resting place. I will feel better not having the tub sit at the dealers shop waiting for me and better yet, I'll get to use it sooner.

Thanks,

GREG!

[Roger]

I'm now thinking about putting the spa on my 16'X35' stone/paver patio until the weather gets more stable. Baltimore weather isn't terriable but we do get cold snaps and that would probably be the few days before/during & after my slab pour. My power will be easilly accessable for the temporary locatio and the tub's final resting place. I will feel better not having the tub sit at the dealers shop waiting for me and better yet, I'll get to use it sooner.

Thanks,

GREG!

The only thing I would like to add is no matter how big you think the slab is, the bigger you make it the better. You will have plenty of room on all sides and a nice area for access if you make it bigger than it needs to be.3-4 feet on 3 sides and 6-8 on the approach if you have room will not hurt at all and after all is said and done you will enjoy the extra room.

With curing blankets and frost fighter heaters here in Northern Minnesota we pour concrete on all but the coldest days. If done correctly it is not an issue at all. The concrete factorys use a small amount of calcium chloride to prevent the water from freezing before curing, which does make for a dry powdery surface but it wears off quickly and your left with a perfect surface, broomed or otherwise. But I would broom it for traction.

[Stevie]

Personally, I have never been a fan of a concrete slab for a spa. Concrete can heave and crack and can be costly and time consuming to install and repair.

I prefer sidewalk blocks installed over 6" of road crush and sand. If it ever settles, it's easy to fix once the spa is empty by repacking the area, tipping the tub back down and away ya go! I would even consider a ground level deck instead of concrete or any of the other materials (eg. laminate) out there designed for spas now...

Steve

[Pool Owner]

Greg (GKL), it sounds like you have a good plan (get use of your spa sooner than later). If your existing pavers patio has been around for a few years and is in good shape, utilizing similar base construction should be adequate for a concrete slab (even 4-inch).

Roger confirmed that concrete can be placed during cold weather by knowledgeable contractor/finisher that is willing to do the job right.

[pnwtub]

Pool Owner and co.,

One of the bids i received for a concrete slab mentioned the use of Necon Fiber mesh for reinforcement (instead of metal mesh or rebar). Have you heard of this? Is this fiber mesh a good thing for hut tub slabs?

Another question I had was about the size of the slab. If the tub is listed as 7' x 7' in the specs, is it OK to have a slab that is 7' 1/2" x 7' 1/2"? If it was more than a few inches larger than the tub then I was thinking that the steps would not be flush up against the tub.

Thanks! Happy New Year!

[Pool Owner]

pnwtub, sorry for the delay in getting back to you. Fiber mesh has been used in concrete for at least two decades. It does not provide any structural strength to the concrete; therefore, I would not recommend it is you have an expansive soil concern. The fiber mesh is similar to 1- to 2- inch chopped pieces of “fiberglass” added and mixed into the wet concrete. The fibers help to hold the concrete together across their length, so it has proven to be an effective method for reducing “micro cracks” in concrete. Micro cracks can be penetrated by water and can increase the likelihood of concrete spalling during the freeze-thaw cycles in colder climates.

As to the size of the slab, generally this is personal preference assuming it is properly constructed. What you have described should work. I do not think there is any one absolute right or wrong method for installing an above ground spa’s support structure.

My recommendation is to pick a contractor you feel comfortable with and ask to see some examples of their work in a similar situation (e.g., soils, use, etc.). Also, unless you are comfortable with construction terms and methods, I would recommend that the contractor pull building permits to ensure the final installation meets all safety requirements (e.g., equipment bonding, etc.). Luckily the prefabricated above ground spas are sold with most of the safety concerns address in the manufacturing process and/or installation instructions.

Good luck.