Concrete Tank Keyed Joint 3

[HeavyCivil]

In detailing a concrete tank with a 12" deep mat foundation I received mark ups instructing me to "key" the wall into the mat. I assume this is for shear transfer, and I have seen this done in the field (not in design, but I am new to design so go figure).

The problem I see is that if I key walls into mat, say, 3.5" which works well for obvious forming reasons, then the 12" waterstop I've been instructed to use goes into mat 9.5" and causes big time interference with my top layer of mat reinforcing. At that point the top bars might as well be cut because they will be at the same location as the bottom layer if they are bent to accommodate water stop.

They could be bend around a 9" waterstop I suppose but the same basic problem still exists if using a key.

If the mat is poured flat, with no key, and the joint area is roughened to a spec'd amplitude the top bar can be bent to accommodate waterstop much more easily. - Still, at that point, 6" bellow the surface I don't think it will be doing much for crack control and I'm hoping it wont need to do much for negative bending.

The question is this: are shear keys necessary for liquid retaining tanks (in this case the wall is only 8' tall but I'm also curious in general. I have checked shear based on base thickness and it is fine- but I'm afraid the basic shear calc does not assume there is a joint there).

The other question is why was I instructed to use a 12" waterstop when 9" seems like the standard...

The final question is, if a key is needed, the top bar perpendicular to the WS can be cut or does need to bend around it.

Thank you

 

[Gumpmaster]

-Don't bend the reinforcing to go under the waterstop. Depress the whole mat of reinforcing. To reduce the amount you'd depress the mat, you can either use a 6" water stop, or a starter wall.
-I don't think that a key gets you much in a concrete tank. Roughen the construction joint to 1/4" amplitude and use the shear friction method to check your shear. If you're using allowable shear stress, you should be good.
-A 12" waterstop seems like way overkill. Unless you have a HUGE head, or lots of movement you only need a 6" waterstop. You can get a large center bulb with a 9" waterstop that will give you extra flexibility in a movement joint. Check with the waterstop manufacturer. We use 6" ribbed center bulb waterstops in 95% of our joints, with few reports of leaks (and those being the result of poor consolidation).

 

[JedClampett]

You're asking for problems with a keyed joint. You can either use a localized key (1/3 the wall thickness), which has limited shear value, is hard to build and is likely to make the concrete consolidation around the waterstop poor, or key the whole wall into a depressed blockout, which thickens your slab to the point that someone will likely question it and make you reduce it back. Don't forget, you're going to have to pour the whole slab the thickness required at the edge.
As Gumpmaster said, the 12 inch waterstop is going to be a problem. Use 6 inch. And we are believers in starter walls. The contractors will complain, but it's the most economical solution. And plus, the contractors are going to complain about something anyhow, so this prepares you.

 

[HeavyCivil]

I had originally drawn the wall UP 3.5" from the mat so the joint was there. This got red ink from my supervisor (who is strictly civil, not structural, so mark ups are always a big battle for obvious reasons). I understand this would be tough to form though, but it keeps reinforcing straight.

I am going to argue for using 9" max water stops and either keeping original detail or at least eliminating the key. My one qualm with dropping top layer is that I would anticipate more S+T cracking.. If it was a floor slab I would certaintly not feel comfortable with bar 4.5" clear.

Thanks for the input. Its always nice to know there are people with more experience that agree with me so I can pick my battles.

 

[80smetalfm]

Hi mate,
I reccommend deleting the keyed joint if your tank is less than 4m tall. My experience is that the use of water proof membranes & admixtures allow ease of construction for small to medium water tanks & more economical. Builders normally dont like installing water stops as its quite fidly.

The proposed 12" sounds like an over kill.

 

[miecz]

Tell your supervisor that keyed joints are meant for unreinforced construction such as residential basements. Once you have reinforcing passing through the joint, the key is redundant, as the shear is taken by shear friction. Also, as pointed out by JedClampett,a key increases the conjestion at the joint, and increases the liklihood of leakage.

If you test your tank for leakage before it is backfilled, the top of the mat is in negative bending, and is carrying tension from the shear at the base of the wall. The top mat reinforcing is working hard at that point and can't be depressed that much. If you must use a key, detail it up into the wall, instead of down into the mat.

 

[HeavyCivil]

I found an article in ACI's 5/06 issue of concrete international. The article presents three (3) solutions:

1.Starter Wall

2.Deflect top bars

3.Lower top layer.

As stated, either of the first two options are going to reduce the base moment capacity. Bending bars down, I think, is a pretty labor intensive when you're dealing with #8's @ 16" o.c.

BUT, I cannot for the lift of me think of a good way to pout a started wall monolithically with a mat. I guess you'd have to cast dowels into the mat to support your form boards (2x4's probably).

I spoke with a gentlemen at JPSpecialties who said that they recoment clearance between rebar and waterstops of 1/5*diameter of largest aggregate. For the mix we had planned that is 1.125", further justifying raising the CJ and waterstop instead of dropping bar.

I know contractors will complain no matter what, but I've been in the field as a laborer and know how quickly engineering oversight can drive up man-hours required for otherwise simple jobs and try to avoid that. Anyone ever seen a starter wall formed and poured?

 

[HeavyCivil]

Sorry for those spelling mistakes.

Also, in speaking with JPSpecialties and looking at the head-ratings for various water stops, it is accurate that a 6" centerbulb-style ws is fine for this application. So if you neglect the clearance JPS wants between waterstops and reinforcing, you only need to drop top layer to 3" clear (not ideal but okay), if you heed their additional clearance you'd need to drop it to 4" clear- a somewhat substantial reduction in moment capacity and seams like a good invitation for wider shrinkage cracking.

 

[miecz]

I think you are overestimating the cost/work involved with bending the top bars. I designed a tank that way with no complaints. I should say, no complaints about that particular feature. You can move the waterstop towards the far face to reduce the degree of bend. I haven't used a starter wall.

 

[civilperson]

Keys at structural wall/slab joint are expensive and serve no purpose. The steel that crosses the junction should be sufficient for shear and the suggestion of roughened surface, (with laitance removal), will provide shear friction for the construction joint. Use of bentonite filled waterstops at the junction is an alternative to the plastic ribbed and bulb variety.

 

[JedClampett]

I've seen starter walls poured (not actually seen the pouring, but the results) many, many hundreds of times. I'm not exaggerating. It's the best solution to this challenge. If you don't insist on doing it right, the contractors will be rewarded for their intrangience (hope I spelled that right).
I've never had a contractor call me and thank me for eliminating the starter walls (I've done it a very few times). But you can be sure they charged for the additional four inches of concrete in the slab.

 

[weab]

If leakage is your issue and you can spend a few dollars extra, I'd recommend a victaulic clay. It is placed in the joint and expands if it gets wet and thereby closes any gap. You must be using rebar on both wall faces to use this.

Otherwise, use a rough joint to get some friction across the joint.