The current house I live in has a block foundation with a crawl space. When I lived in Ohio my neighbor had a block foundation, it cracked, leaked, and started to be pushed in.(house was OLD) To fix it they came in and placed rebar and sprayed shotcrete over it. Worked fine.
Is my engineer crazy??
- Thread starter mlawrence17
- Start date
Rebar or not, the problem with block is they are made for vertical loads. A pool will present a lateral load. The mortar that binds the blocks is not as strong. Ever notice how crack that form in foundations follow the mortar joints? Any lateral or settling and major issues arise. A liner might be able to have enough support until the next time it is changed but then the crack will get fixed. A plastered finish will require immediate attention.
Scott
Scott
The samples I've been seeing from IG vinyl builders down here are almost universally built with block both above and below grade and a few pics I saw most of one wall of a pool was above grade. Some of the pools only came up 2 or 3 blocks above ground. I know these pools passed city inspections. And please no one start bagging on on inspectors and permit reviews.
Get detailed plans with calculations and then take them to the city for review.
Get detailed plans with calculations and then take them to the city for review.
- May 3, 2007
- 16,841
- Pool Size
- 20000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Hayward Aqua Rite (T-15)
PoolGuyNJ said:
Blocks are used for basement walls which are below grade have have a lot more lateral load (dirt vs water) than a pool would so I really don't think wall strength is the issue here. I agree that with block foundations and only mortar, the strength is not that great and the mortar can separate but I don't think the engineer was suggesting to use just mortar or even mortar at all. If built properly with rebar AND concrete fill AND gunite coating, there is no logical reason that the wall shouldn't be just as strong as a gunite alone. The concrete fill and rebar will provide the structural strength and with the gunite coating will prevent joint separation and cracking.
When I first saw this thread, I thought it was interesting because of my experience with block walls. I had no idea it would attract so much interest but, because the discussion has remained very civil, it is a good opportunity to discuss pool structure and the resultant loads.
My primary point in posting again is that the strength of the wall is completely irrelevant based on the OP's previous post where, based on the illustration, the pool walls have virtually no need for lateral strength. Any lateral force has been carried out to the retaining wall and the pool wall becomes just like an IG pool and could be made constructed of ANY waterproof material and the lateral strength can be disregarded.
My secondary point is that, properly reinforced (rebar and filled with concrete), a block wall easily has enough lateral strength to hold back 6 feet of water based on my personal experience with my pool. I'm not saying that's the smartest, easiest or best way to build an unsupported pool wall......I'm just saying it will work.
BTW, where did the OP go??
My primary point in posting again is that the strength of the wall is completely irrelevant based on the OP's previous post where, based on the illustration, the pool walls have virtually no need for lateral strength. Any lateral force has been carried out to the retaining wall and the pool wall becomes just like an IG pool and could be made constructed of ANY waterproof material and the lateral strength can be disregarded.
My secondary point is that, properly reinforced (rebar and filled with concrete), a block wall easily has enough lateral strength to hold back 6 feet of water based on my personal experience with my pool. I'm not saying that's the smartest, easiest or best way to build an unsupported pool wall......I'm just saying it will work.
BTW, where did the OP go??
We scared them off? I hope not. It's this kind of discussion that makes this forum so great - it would never have stayed civil on some of the other boards I read (non pool related)
Due to the frequency this material is used in building, the strength of rebar reinforced hollow block construction with concrete filled voids has some very well documented strength characteristics. Given those documented characteristics, if the engineer actually sat down and did the calculations to make sure the lateral loads could be accommodated with an appropriate safety factor, then I would have no problem going with his plans.
In both concrete block and gunite construction, rebar is used to enhance the virtually nil tensile strength of concrete. Given the correct foundation construction and proper rebar reinforcement design, I would think the structure would be sound.
I would do the analysis both filled with water and unfilled, and also do an "unsupported wall" calculation to account for possible future excavation on site the would expose a wall by removing the backfill.
I have no doubt using back of the napkin calcs that it would work, but would want to consider the "unplanned for" things that could come back to bite you in the end. Like someone removing the backfill on one side for some project, or building a basement 5' away down the road after all this is done.
In both concrete block and gunite construction, rebar is used to enhance the virtually nil tensile strength of concrete. Given the correct foundation construction and proper rebar reinforcement design, I would think the structure would be sound.
I would do the analysis both filled with water and unfilled, and also do an "unsupported wall" calculation to account for possible future excavation on site the would expose a wall by removing the backfill.
I have no doubt using back of the napkin calcs that it would work, but would want to consider the "unplanned for" things that could come back to bite you in the end. Like someone removing the backfill on one side for some project, or building a basement 5' away down the road after all this is done.
Lershac said:
While thats true, buildings made of hollow concrete block do not have walls that are free standing on one side with a load on the other. Block walls, as they are being laid, have to have support to keep them standing until they can be tied together at the roof line.
What we're talking about here is building a wall out of block, putting a pool on one side, and using the other side as a free standing wall thats not back filled, at least thats what the drawing shows above. Now, I suppose if you filled the wall up with re-bar, poured concrete into the hollows of the block, and re-inforced it, it might work. However, if your going to do all that, you might as well just form up a concrete structure much like they pour for basements. I'm sure it would be much less cost than having a mason lay the block, run all that rebar, then call the concrete guys and fill the hollows.
FWIW, I considered a gunite pool at one point. I needed a back wall to hold up the pool. The engineer told me the design would use a concrete wall with the wall acting as one side of the pool. He suggested a concrete, rebar re-inenforced structure with gunite shot on the pool side, and veneer stone on the back side. At no time did he suggest a concrete block wall. It's just not the proper material without a ton of modifications. There are much cheaper and better alternatives that dont have the down side of block.
I interpreted that "model" differently. To me, it looks like the pool is only maybe 10-12 foot wide and the outside wall on the model is not the outside wall of the pool. It looks to me like the pool perimeter a good 6 feet or so inside the retaining wall........hence my post about lateral strength being unnecessary.
Well seeing as how the OP was specifying exactly that ( hollow block, rebar reinforced with a creamy concrete filling) I think that as long as the calculations to support the application are done, and meet the criteria, he will be fine.
Lershac said:
It might be fine. My point is that there are cheaper ways to do this and probably better material to use.
Dave, it's kind of hard to figure this out from the drawing. If it is as you suggest, I'm not sure what he's proposing to make the retaining wall out of. If the actual pool walls is made out of block, and the outside retaining wall is made out of another material, with backfill in between, then it might be ok. Now that I look at it, you are probably correct.
Now the question is what is the outside wall made of?
- May 3, 2007
- 16,841
- Pool Size
- 20000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Hayward Aqua Rite (T-15)
One thing that I think is being missed here is that although water may be very heavy, it really doesn't produce much of a load to the pool walls. At a depth of 5', the pressure is only 2 PSI which is the primary reason that an AG pool with very thin walls can support the water. So I really don't understand how anyone can think that a solid concrete block wall tied together with rebar can be any weaker than an AG pool wall.
Mas,
an AG pool relies on the tensile strength of the liner to resist tearing, when supported (to limit stretch of the liner) by the plastic or steel walls and the pvc rail support system. Some AG pools forgo even that and use a tougher material to contain the water with a reinforcing ring of inflated *even tougher* material at the top.
what the OP was detailing was essentially a rigid (and relatively brittle in relation to a liner) gunite coating over a block and rebar/concrete substrate. The concern in this case is the fracture of the substrate translating through to the waterproofing plaster coat which is not flexible in the same way as a liner (and so not forgiving). I agree that ANY failure in those modes for a rigid pool shell can be catastrophic, as the resulting water flow can undermine and destroy the surrounding area.
So the question is not so much one of catastrophic failure in normal usage, but all of the "special" cases that can occur during the lifetime of the pool... drainage (the inside of the pool wall unsupported), excavation on the outside of the pool for repairs or a project, or the construction of additional structures in close proximity of the pool exerting unplanned-for stresses on the pool wall, even tree root growth exerting tremendous pressures on pool shell. These are the modes a failure would likely occur in. The static stresses of just being a pool are relatively easy to deal with, and that is evident in the relatively inexpensive construction techniques and materials used on "seasonal" above ground pools, which in normal use can last many years!
The real rub here is a question of local economics (which design is cheaper) and owner "gut" (which design am I more comfortable with) which is what we all seem to be debating about.
Me, its pretty obvious which choice I made (gunite reinforced with rebar), but I recognize that other designs are workable, and in some circumstances may be more economical, or just preferred because of availability of materials, skilled labor or some other factor.
I considered steel reinforced concrete filled block for the pool shell, but decided against it for reasons not mentioned at all in this debate. I was worried about the time it would take me to construct the shell, and whether I could keep the structure intact during construction that would take a long time since I would be doing it myself. Since I could afford it, and was building the pool myself for personal pleasure and satisfaction at doing so rather than economic reasons, I chose to take the shortcut of having the shell shot in a monolithic fashion (an NOT SOON ENOUGH FOR ME!!!!) :blah: :blah:
an AG pool relies on the tensile strength of the liner to resist tearing, when supported (to limit stretch of the liner) by the plastic or steel walls and the pvc rail support system. Some AG pools forgo even that and use a tougher material to contain the water with a reinforcing ring of inflated *even tougher* material at the top.
what the OP was detailing was essentially a rigid (and relatively brittle in relation to a liner) gunite coating over a block and rebar/concrete substrate. The concern in this case is the fracture of the substrate translating through to the waterproofing plaster coat which is not flexible in the same way as a liner (and so not forgiving). I agree that ANY failure in those modes for a rigid pool shell can be catastrophic, as the resulting water flow can undermine and destroy the surrounding area.
So the question is not so much one of catastrophic failure in normal usage, but all of the "special" cases that can occur during the lifetime of the pool... drainage (the inside of the pool wall unsupported), excavation on the outside of the pool for repairs or a project, or the construction of additional structures in close proximity of the pool exerting unplanned-for stresses on the pool wall, even tree root growth exerting tremendous pressures on pool shell. These are the modes a failure would likely occur in. The static stresses of just being a pool are relatively easy to deal with, and that is evident in the relatively inexpensive construction techniques and materials used on "seasonal" above ground pools, which in normal use can last many years!
The real rub here is a question of local economics (which design is cheaper) and owner "gut" (which design am I more comfortable with) which is what we all seem to be debating about.
Me, its pretty obvious which choice I made (gunite reinforced with rebar), but I recognize that other designs are workable, and in some circumstances may be more economical, or just preferred because of availability of materials, skilled labor or some other factor.
I considered steel reinforced concrete filled block for the pool shell, but decided against it for reasons not mentioned at all in this debate. I was worried about the time it would take me to construct the shell, and whether I could keep the structure intact during construction that would take a long time since I would be doing it myself. Since I could afford it, and was building the pool myself for personal pleasure and satisfaction at doing so rather than economic reasons, I chose to take the shortcut of having the shell shot in a monolithic fashion (an NOT SOON ENOUGH FOR ME!!!!) :blah: :blah:
- May 3, 2007
- 16,841
- Pool Size
- 20000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Hayward Aqua Rite (T-15)
I understand your concern about cracking but many of the comments above had led me to believe that there were those who did not even believe that a concrete block wall could not support the forces of a water filled shell. My only point was that there is very little pressure on the wall and the greatest pressure is at the bottom of the wall where it would tie into the slab. So the lateral strength is strongest where it needs to be and the forces of the water should not compromise the wall in any way.
As for maintaining the integrity of the water proof plaster layer, I believe that it will although I can't prove it. As anyone with a gunite pool knows, gunite cracks as it is drying and as long as the cracks are not deep or wide, the plaster layer will maintain a water proof layer. So just as with gunite, the concrete block wall with a gunite layer can have a few minor cracks just not after the plaster layer is applied. So I guess the real question is if a concrete block wall is any more prone to cracking after applying the plaster layer than a gunite wall. I believe both should have similar performance.
As to the economics of the block approach, the cost of gunite has really gone up in the last few years so I understand why the engineer is proposing this design. But I would get a quote for both methods and then decide which is the right way to go for the op. The other factor that must be taken into consideration is the installers experience with this type of design but that is true for any design.
Update: I just found this reference regarding dry-stacked block walls: http://www.drystacked.com/standards.html
There is a standard for this type of construction and the claim at least is that because of the surface coating these types of walls have the same strength as a solid wall.
Testing: http://www.vobb.com/engineer-tests.htm
As for maintaining the integrity of the water proof plaster layer, I believe that it will although I can't prove it. As anyone with a gunite pool knows, gunite cracks as it is drying and as long as the cracks are not deep or wide, the plaster layer will maintain a water proof layer. So just as with gunite, the concrete block wall with a gunite layer can have a few minor cracks just not after the plaster layer is applied. So I guess the real question is if a concrete block wall is any more prone to cracking after applying the plaster layer than a gunite wall. I believe both should have similar performance.
As to the economics of the block approach, the cost of gunite has really gone up in the last few years so I understand why the engineer is proposing this design. But I would get a quote for both methods and then decide which is the right way to go for the op. The other factor that must be taken into consideration is the installers experience with this type of design but that is true for any design.
Update: I just found this reference regarding dry-stacked block walls: http://www.drystacked.com/standards.html
There is a standard for this type of construction and the claim at least is that because of the surface coating these types of walls have the same strength as a solid wall.
Testing: http://www.vobb.com/engineer-tests.htm
- May 3, 2007
- 16,841
- Pool Size
- 20000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Hayward Aqua Rite (T-15)
From the conversion of feet of head to PSI for water (0.433 lbs/ft-sqin), a column of water 5' deep has a pressure of 2.16 PSI at the bottom. However, at the water surface and near the top of the block wall were it would be the weakest due to of lack of support, there is no water pressure.
Wow! thanks for all the responses. That's what makes this site so awesome. And Duraleigh get's 15 bonus points for his investigative techniques : ) I'm not saying it can't be done, but I just don't feel comfortable building the pool that way, and that's what I've told my engineer. It looks like we'll be doing it the way I had envisioned.
bk406 said:
In Michigan, 90% of all foundations (basements) are made of cinder blocks. Without rebar or concrete filled to boot.
I know this is an old post but thought I'd comment in case someone stumbles upon it looking for answers. My pool is an in-ground concrete pool where the walls are filled concrete block reinforced with rebar with a concrete floor built in the mid 1970's. I think the walls were additionally coated in some sort of cementitious material to aid bonding. It is still in great shape with no evidence of failure or cracking. In fact, we are currently redoing it with a PebbleSheen finish in anticipation of it lasting another 20-30+ years.
This is straying from the OP's question. IMHO as a builder I would not build the wall from cinder block then shoot 2" of gunite over that and call it good. Bare minimum I would want at least 6" of gunite so I could put 1/2" rebar in and have it tied. Cinder blocks are good in some builds but personally I would want all the strength to come from the gunite shell and the cinder block as best to be a form wall