Major mistake fixed. Is this safe?

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staycation

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Jul 16, 2008
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hello pool experts out there, I need your help.
my PB inadvertently made the pool 12 inches wider than city code allows.
he was forced to make the side wall 1 foot thicker with gunite. (I now have a two-foot thick wall with a one foot wide rebar)
what we have now is 12 inches of gunite beyond the original rebar, that runs the entire length (30 feet) of the side wall.
is this safe?
will i have problem later with the wall cracking etc, given that we now have a foot thick of extra gunnite wall? Or does it actually make the wall stronger?
thanks.
 
I didn't realize that there was a city enforced maximum thickness. I'm sure you passed the pregunite inspection. Did the inspector notice that the gunite was going to be two feet thick? I'm sure your inspector looked at the steel schedule versus the engineering drawings. Did your PB place a copy of the steel schedule in your permit envelope? If so, contact the engineering firm and ask their opinion. You may also consider going down to the city and talk to a city engineer.

Concrete has very good compressive strength and horrible tensile strength. This means you can push on a piece but can't pull on it. Adding steel makes the concrete strong in both tension as well as compression. When steel is added it is called reinforced concrete is is used to build things like bridges, overpasses parking structure etc.. If they added additional steel throughout the thickness of the extras thick areas you would for sure good as it would be stronger than normal. I'm not too sure what would happen if you only had steel in the edge of the two foot sections. I am a mechanical engineer and NOT a civil engineer. You need to talk to a license structural or civil engineer. These guys (or girls) have PE for a last name just like lawyers have Esq. for a last name.

My neighbor is one of these guys I will talk to him and see what his opinion is.

BTW - Who is your PB?

Let us know what happens.

Kevin
 
I am no civil Eng, but I would NOT take the PB's word for it. I would have it confirmed by someone that does not have money tied up in doing it wrong in the first place. I have been on many jobs with steel and concrete, and have never seen width added to a pour with out adding a wider rebar section.
 
Kevin:
I am getting nervous now, because all he added was an extra foot of concrete wall (now 2 feet thich) with the original rebar on the edge (meant for only one foot). Let me know what you neighbor thinks.
by the way, hope you are enjoying your pool.
thanks.
 
I looked outside several times today to see if my neighbor was outside but he wasn't. Tomorrow I should be able to catch up with him.

I'll let you know.

I haven't heard of K&R.

I would bet that the engineer who created the steel schedule for your pool is an independent company hired by your PB to do the drawings.
 
There are several problems that I can think of. The pool was not designed and engineered for the current weight distribution. The original design "safety factor" is now reduced. If it is reduced below 1.0, in any case, you will have serious problems. Ask the PB to tell you the new safety factor related to any affected design criteria.

Concrete has a density of about 150 lb. per cubic foot whereas water has a density of 62.4 lb./ft3. The extra weight of having concrete where water would be is (1ft. X 30ft. X 4.5ft est.) X (150-62.4) = 11,826 lb. = 5.91 tons.

I would be concerned about the extra weight causing a crack that would run the length of the pool where the wall meets the floor. If the concrete was not designed with a sufficient "safety factor", it will crack.

I would also be concerned about the pool settling unevenly. If the supporting soil and any additional structural elements, such as pillars, are insufficient, the pool will settle unevenly.

A few questions:

1) What is the thickness of the concrete?
2) What is the diameter of the rebar that was used?
3) How far apart are the rebars spaced?
4) What is the rated strength of the concrete?
5) What kind of soil do you have?
6) Did they hit water while digging the pool? How far down?
7) Were any support pillars used? Describe number, spacing, diameter, depth etc.
8) Was a soil engineering survey done and report issued?
9) Is the pool on flat land or is there any type of slope or hill? Describe rise/run.
10) If there is a slope to the land, was the extra concrete put on the high side or the lower side?
 

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As a Structural Engineer -- I'll offer my opinion.

Concrete (gunnite / shotcrete included) shrinks as it cures. The higher strength a mix -- with less water per pound of cement -- the less it will shrink. But all concrete shrinks. And as wisely stated earlier, since concrete has a very low tensile strength, all concrete cracks. To combat this shrinkage and cracking, reinforcing steel is needed. This steel serves to create a wall that can take the forces of the soil/water AND to combat shrinkage.

The code requires a certian amount of steel for the shrinkage that has to do with the volume of concrete. Since the volume has more than doubled, theoretically the steel should have also.

That having been said, the pool will be stronger with more concrete, but wil have the possability of more cracks. These cracks are NOT from soil movement and once cracked, should not get worse. The gunnite/shotcrete is not expected to be crack free -- the plaster is the waterproofing.

I would not worry at all about the extra concrete -- it will be stronger. I WOULD wait extra tome before plastering to let the concrete shrink/crack all it wants to. It will take longer for the exess moisture to come out of the concrete. In this case, you might wait 3-4 weeks before plastering to get the shell to the right point.

As for the added weight, as staed earlier, concrete weights 150 pcf, water 62.4 pcf. But remember the soil you dug out weighed about 110 pounds per cubic foot. The pool should still weigh less than the soil removed. The added weight is not an issue unless you are putting this pool on the second floor! :shock:

Steve
 
I have no idea if this is safe or not BUT if it were my pool I would not be happy. I say this because the PB should have known the limitations of the city code and built accordingly. Also I feel the rebar should be all the way through the concrete. I had a similar problem with my house and the footings. They made the footings too short and just wanted to pour more concrete. I made them add rebar. If it's built that way in the first place then it should be completed that way.

Just my 2 cents. I AM NOT and engineer and probably blowing smoke out my @ss anyway :)
 
If the soil does not have the structural strength to support the pool without settling then there will be problems. If the soil is backfill or other such weak soil or if the pool is on a slope there could be uneven settling. Even the slightest settling can cause stresses in the concrete that can cause structural cracks. The uneven weight distribution will cause any settling to be uneven.

It is my opinion that the initial structural shell should have the following minimum specifications to ensure a high safety factor:

1) 18 inches thick.
2) 5/8 inch rebar on 9 inch centers.
3) 7,000 psi rated strength.

It may be worthwhile to hire an independent soil engineer to do a core sample and advise you if they think that the soil is sufficient to handle the load. I agree that the "wall" will be stronger but the extra weight that the wall exerts on the floor could cause the concrete to crack along the wall to floor joint if the concrete and the soil are not strong enough. Quite possibly they are strong enough. If the pool builder assures you that they are, and that there's nothing to worry about, then you are probably OK. Ask them to include a warranty that specifically addresses this issue including what would constitute a warrantyable condition and what would be the specific remedy.

I know that many people will say that I am being unnecessarily cautious; but I have seen too many pools crack and settle unevenly to believe that the current level of design and engineering is sufficient. This is just my opinion based on my experience and my observations. I am not an engineer.

I think that the key is to determine how sure you want to be that there will not be any problems in the future and how much you're willing to spend to prevent those problems. Only the person paying for the job can determine what is worthwhile and what isn't.

One question: Did the contractor apply for a variance from the city so that they wouldn't have to reduce the width by 1 foot?
 
Thanks for your opinions.
I am told the soil here (Roseville, California) is compacted and I don't have to worry about shifting etc.
I think I'll be OK.
we never apply for a variance.
I'll probably delay the pebble sheen application as Stever suggested and ask for something in writing about warranty on this specific issue.
Again, thanks!
 
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