Starter wall and Upturn Key at Wall-to-Mat/footing - ACI 350 recommendations

[AK4S]

Looking for opinion regarding ACI350 recommendations for (1)Starter wall and (2)Upturn Key at Wall-to-Mat/footing interface in concrete water tanks.
Especially if someone is involved in the code committee or is aware of the upcoming revision to ACI350 which references this item.

I have been generally following a starter wall detail for this interface (see sample below):

Based on my discussions with other engineers in the past, I have generally avoided using an Upturn key detail at this interface (similar to the one shown below):

From what I remember the primary reasoning was that the upturn key has potential to cause diagonal crack to form from the key to the face of the wall opposite from the applied load. Something as shown in the sketch below:

Also, one could argue that the remaining portion of the wall (adjacent to the upturn key) that gets poured with the portion of wall above has a high chance of honeycombing /voids due to its location and size, thus compromising the quality of concrete at this interface.

Recently, I had a discussion with a fellow engineer who mentioned that the upcoming ACI350-20 revision is recommending against the use of starter walls and prohibiting the use of keys at this interface.
So the option available to accommodate the PVC waterstop required at this interface will be something like the rebar arrangement shown in the below sketch:

(1) Looking for reference/reasoning to confirm the issue with using an Upturn key detail at this interface. Few colleagues have specified this detail in the past and never seen/heard of cracks or faced any issue.
(2) Looking for reference/reasoning to confirm the issue with using a Starter wall detail at this interface. In my past projects dealing with concrete water tanks, I have never observed/heard of cracks or other issues when using starter walls (except that the contractor hates it since he has to hang forms for the starter wall).
(3)If both upturn key and starter wall detail are not recommended at this interface, then what is generally done to avoid waterstop/rebar interference? Would appreciate if any sketch/rebar details can be shared.

 

[HTURKAK]

Mr. AK4S (Structural),

I would like to share my past experience and opinions on the subject.

AWWA Standard D 110 also suggests similar detail with the 1st picture at your post. The excerpt from the same is attached below,

I never specified Upturn key and could agree if the wall has reasonable thickness for the keys.


I fully agree with you .. Starter walls ( kicker ) is the reasonable solution to prevent leakage from both sides.


IMO, the use of external type waterstop could be an option ( if the tank is buried and has exterior waterproof membrane )rather than stopping or bending the reinforcement.

In our zone , we follow the similar details provided at SIKA catalogue.

 

[bones206]

I once attended a lunch and learn seminar with a field engineer from some concrete industry group. He stressed that keyways are poor design and outdated details should be revised to get rid of them as much as possible. He presented several photos of various applications where keyed construction joints had failed due to the low shear capacity of the key. I have experienced myself one particular project with 24" thick walls with failed double keyway joints that required expensive leak repair.

I consider myself fully indoctrinated to into the "no key" school of thought after that presentation, although most of the standard details I show on my projects still have keyways since the older engineers are not comfortable with removing them. I think it's one of those things that looks intuitively mechanical on paper, but has more drawbacks than benefits in reality.

One thing I have done in the past is to simply increase the top clear cover to provide clearance for the waterstop. When I've used starter walls, I haven't experienced any issues or pushback from contractors.

 

[AK4S]

@HTURKAK:
Thanks for your thoughts. I have had good success using starter wall detail in the past to accommodate the Waterstop. I am trying to find more reasoning behind the proposed note in the upcoming ACI350 which recommends not to use them.

The upturn key used by others in the past is usually for wall thickness > 15", with key width 1/3rd of wall thickness.

Usually most tanks I have worked on are partially buried, so not sure if I can ONLY use the suggested "exterior waterproof membrane" instead of the Waterstops. I have used a combination of Preprufe and Bituthene 3000 membranes in the past for exterior application for below grade portion of the tank, but this was specified in addition to using Waterstop at construction joints in the tank.

 

[AK4S]

@bones206:
I agree. Using an upturn key needs better supervision to ensure the key is formed properly and the portion adjacent to it which is poured with the wall does not leave voids compromising the quality of concrete at this interface, leading to leaks.
Edit: << Do you remember for the failure cases observed, was the design of the wall purely relying on the shear key to transfer the load from the wall base to the supporting mat, instead of the wall reinforcing? >>

Adding additional 2" cover to clear the waterstop is an approach or one could stop the rebar and add splicing bars as shown in my last sketch (more suitable if intermediate walls are spaced out/top rebar is positioned to resist significant uplift forces).

However, I am still trying to understand the drawback of using a starter wall since it seems to provide a more economical detail (rather than increasing base mat thickness or changing rebar arrangement).

I could only find the attached reference which suggests that a system with a starter wall joint can have lower strength/cracking load when compared to a system without a starter wall.

 

[HTURKAK]

ONLY[/b] use the suggested "exterior waterproof membrane" instead of the Waterstops. I have used a combination of Preprufe and Bituthene 3000 membranes in the past for exterior application for below grade portion of the tank, but this was specified in addition to using Waterstop at construction joints in the tank.]

Dear AK4S,

I did not suggest only the use of exterior waterproof membrane instead of the internal Waterstops. But suggested ( an alternatife )external waterstops in combination with membrane system . The relevant picture from the document which i have posted .

 

[bones206]

Do you remember for the failure cases observed, was the design of the wall purely relying on the shear key to transfer the load from the wall base to the supporting mat, instead of the wall reinforcing?

For the most part, yes I believe that was the case for the examples presented in the lunch and learn. However, the case that I dealt with personally had substantial wall reinforcement and still had issues with the shear keys cracking.

 

[bones206]

Thanks for sharing that research paper AK4S. It's interesting that the upstand walls were found to be weaker than having a joint at the top of foundation level.

Just stumbled upon this detail from a DN Tanks brochure:

 

[strucbells]

I am relatively new at a company doing a lot of WWTP and WTP design. I have tended to drop the top mat of reinforcing in my ACI 350 designs to provide clearance to the usually 6" PVC waterstop based on advice from my reviewers. We have entirely moved away from keys. Some of the senior engineers still like the wall starter course, but we sometimes get pushback from the contractors regarding these.

It will be interesting to see what a new ACI 350 looks like. It is one of the older ACI docs that I frequently reference.

 

[AK4S]

@HTURKAK: Thank you for pointing out the relevant detail. Will look into these waterstops.
@bones206: Interesting joint detail used on these prestressed concrete tanks. I recall seeing similar (w/o waterstop) flexible connections on post-tensioned building walls.
@strucbells: Thank you for sharing your experience. Most of the time when we have seen pushback from the contractor to the use of starter wall, the alternate suggested by them is using upturn keys (which I do not recommend). I usually then share sample photos from other jobs where other contractors have used hanging forms to construct these starter walls and since the starter wall detail is on the bid drawings, there is no other argument on why they cannot construct it. Some contractors (especially on larger projects) come up with some interesting hanging forms for these starter walls: