The effect of pvc-p waterstop with kicker to cantilever retaining wall shear capacity 2

[PeterJ1982]

Hi all.

As we know, pvc-p waterstops are good for watertightness. So we use them constantly with partial kickers so that the waterstop can be assembled. The problem is, PVC-P has a really much lower E than concrete. What really happens when adding the pcv-p inside concrete? It will create a hole to the section, and you can simulate it with an empty space inside the wall. Manufacturers tries to sell quite wide waterstops and I have heard some dimension instructions like this :" waterbar should be as wide as a wall". The quite ultimate case example is in one big waterbar company prochure here:

About 10 years ago I saw somebody added rebars both side to the waterstop. I thought that the reason was cold-joint shear dowelling, but now I think it might have been for the hole as well. Some discussion would be nice to have.

I am quite sure that inside stirrups help the situation because they limit the diagonal crack width and also, if shear wants to go to the kicker in tension side, inside stirrup keeps the concrete block under the main diagonal tension crack at place.

Also I think that we are somehow safe if and only if we have enough tension reinforcement so that it's tension is far under yielding stress in ULS. Also we are safer if there is compression reinforcement.

I would like to get better knowledge about several things:
- Does anybody have had real life issues with the problem I have described?
- I am completely sure that there is some limit how up that hole can rise without limiting the capacity at least without some extra bars. Is it safe enought to use a limit A<B in the picture?
- Of course ultimate solution would be using diagonal stirrups, but they are not so easy to assemble than vertical ones..
- I am also sure that the inside stirrup helps. Does those inside stirrups help a lot or only a little?
- What will be the effect of the amount of compression reinforcement to the capacity issue? I mean, basic truss analogy formulation gives us both compression and horizontal force in the base slab junction at the compression steel. Can we use compression steel dowel action if and only if there is quite much of a steel (because of backfill)?

 

[JAE]

Had a project that we were called on to investigate - similar details to yours above with a small upturned keyway with waterstop.

Cracks developed starting at the top of the waterstop end and progressing up the wall and veered out to the wall surface - again - similar to what you drew.

Cause was ultimately found to be a too-rich mix (9,000 psi) and we had autogenous shrinkage effects occur in the 14" thick walls.
Essentially the wall tried to shrink and the keyway served as a sort of prying effect and cracked the concrete internally.

Better detail would be a full width upturned key (matching the wall thickness) and controlling the mix design to avoid a too-rich mix.
Adding rebar might help keep cracks tight but it may still crack. Vertical wall reinforcement and associated dowels pretty much do the same thing.


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[JedClampett]

We design water retaining structures, a lot of them. We use starter walls. It's similar to your sketch, but full width. Contractors hate them worse than death. We get at least one complaint on every project. We've used on occasion, stay form keys and other partial width schemes. We're not crazy about them, but we'd rather have the contractor do a half a$$ job than a full one.
But we've never had shear issues on this. We've had rock pockets due to poor consolidation, vertical cracks due to shrinkage, but nothing I could track to shear issues. As far as your inside stirrup, it shouldn't be necessary. The wall is designed for the shear with a hefty factor of safety. For instance, we use a 1.7 factor times the water pressure for shear, even though ACI 318 allows a 1.4 factor. Now maybe your partial width key (a small chunk of concrete, with a good chance of being porous) complicates things, but if the concrete is well consolidated, the total section still resists shear. And the more bars that the concrete needs to be poured around, the more chance of the rock pockets.

 

[PeterJ1982]

JAE Your problem seems to be "serveability issue" not ultimate capacity issue, if those cracks were small and can be injected. In my country it is quite typical to make partial kickers and i have never heard of any problems.

I am trying to find out that:
1. Does the empty hole caused by waterstop make the wall weaker in shear (if compared to shear strength measured at d from support) and can we help it by adding middle stirrups?
2. How wide waterstop shall we make safely?

 

[PeterJ1982]

Jed Clampett, I am sure that 1000mm wide waterstop would affect shear strength in 500mm wall because it goes through the inclined compressible strut. There is some limit how wide plastic strip we can assemble. Okay i understand that half he width of the section should be fine.

Maybe we need Kootk or Hokie66 to verify this....

 

[JAE]

For strength - ΦVc for a wall might be looked at as if your depth, d, is 1/2 of the wall thickness less concrete cover....so as you suggest - a very reduced shear capacity.

However, for an economical concrete wall design, I would think that using a bit thicker wall to accommodate the perpendicular shear force would be much better than trying to add reinforcement.

I would like to get better knowledge about several things:
- Does anybody have had real life issues with the problem I have described?
[blue]yes - what I just posted above[/blue]
- I am completely sure that there is some limit how up that hole can rise without limiting the capacity at least without some extra bars. Is it safe enought to use a limit A<B in the picture?
[blue]not sure about adding bars (per my comment above) but your shear capacity is reduced to some degree[/blue]
- Of course ultimate solution would be using diagonal stirrups, but they are not so easy to assemble than vertical ones..
[blue]agree[/blue]
- I am also sure that the inside stirrup helps. Does those inside stirrups help a lot or only a little?
[blue]not a detail I would use - I'd thicken the wall to get the capacity I need.[/blue]


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[hokie66]

I am sure that JedClampett knows more about this than I do, but why use internal waterstops? External waterstops make more sense to me.

 

[dik]

Why not use a split 2x4 (or whatever) key going down into the footing with dowels coming through and a bentonite rod or PVC waterstop in the split 2xwhatever. I'll see if I have a standard detail.

Dik

 

[dik]

Attached detail:

Modify to suit... I almost never use an upstand... too difficult and costly to build.

Dik

 

[hokie66]

dik,
I couldn't open your file, but think I get the picture. If you want a waterproof joint, you need a starter wall. The only type key I ever consider is a setdown for the full wall width, but that interferes with the reinforcement.

 

[PeterJ1982]

Dik, the reason we are using upward kicker is the waterbar. It is impossible to put lower because it interferes tension steel in base slab. So if pvc waterbar is used, the key is upward. Whether is a partial or a full width, it depends on opinion. Some old engineer in my company told that partial is good because of the shear key, but according to this forum and also EC it is non necessary to make a key.

But there is some new products that can be assembled over tension reinfocement without a kicker wall, H-BAU PENTAFLEX KB. It gives 5 bar waterseal with only 30mm concrete cover. We have been using it a lot recently with no leakage.

http://www.h-bau.de/en/product/sealing/pentaflex/pentaflex-kb-1/

 

[dik]

I'm aware of the use of the starter wall/curb and don't use it and not had any issues; it is easier, faster and less costly to avoid them. I've had no problems using the PVC strips or bentonite rod (which I usually use) with the depressed keyway and have adjusted the reinforcing to suit. With the proximity of reinforcing, it is very easy to secure the PVC strip and not having it displace. I also use the PVC waterbar that is tacked to the formwork and serves as an 'external' type of waterbar. Normally the top reinforcing is not critical for the slab/wall interface and the area is generally in compression.

Dik

 

[dik]

Hokie:

Attached *.pdf.

I never use keyways full width (I guess just a difference in practice). often interferes with formwork and providing a seal... also almost never need strength of full shear area. Generally a keyway 50% to 70% of wall thickness is more than adequate. Concrete in full shear and not diagonal tension has a strength of several hundred psi, and the dowels keep it from wanting to lift out of the keyway.

Using the split 2x dimensioned lumber allows for 'stuff' between and the bevelled edges make removal of the strips very easy.

Dik

 

[dik]

Peter:

For joints I often use the flat type of PVC material similar as shown in the attached drawing. I also use it for horizontal applications and it is easy to 'weld' the horiz and vert PVC joint.

Dik