Upturned Key in Concrete Tank Construction?

[m63]

PCA publication 'Rectangular Concrete Tanks' calls for 'Upturned Key' between slab and wall as well as at construction joints. Can someone comment on reasoning for raised keys, could it be for better location of waterstop (higher above the plane of actual joint) or there are other reasons? Is it common to see this detail in practice or contractor would want to change it to depressed key?
Any comments on practicality of each would be appreciated.
Thanks.

 

[jike]

An upturned key is difficult to build. I believe it was developed so the waterstop does not interfere with the top layer of rebar in the mat.

A better option is pouring a curb monolithic with the mat and then place the waterstop in a normal downturned key at the top of the curb.

I have also seen engineers require that the top bars in the mat be shop bent (downward) to clear the waterstop in a normally detailed situation.

I have also seen a number of drawings that show the waterstop crossing the top layer of mat reinforcing. How is this possible? Obviously, they aren't thinking about how to build the detail.

 

[m63]

Makes good sense since even 3" or more of rebar cover would be lost mostly to depressed key itself and wouldn't allow for waterstop placement.
But, how do you build curb monolithically with slab?
I'm thinking that it would be the way to build raised key too.
Thanks for your input.

 

[jike]

I did not mean to imply this is a separate curb. It is really the mat with a step in it at the inside face of the wall. The contractor will either have to hang a form or support the form off of metal stakes.

The upturned key can be formed similarly, but I do not like the the fact that it is so narrow and then you are trying to squeeze a waterstop in it. Seems difficult at best.

As long as the curb or upturned key is poured monolithic with the mat, there should be no leakage problem. If they allow a cold joint to form, then that most likely will create a leak.

It is important to explain this detail to the contractor and your field inspector and let him know what your expectations are.