PT Edge Stressing on Exposed Slab and Construction Joint 1

[BacBac]

Hi All,

Structural engineer from Australia here.
I have 2 questions regarding detailing a post-tensioned slab for its anchorage type (edge stressing vs pan stressing).

Q1. Does anyone know when edge stressing may be rejected by architect or builder?
Besides the obvious reasons such as edge of the slab is at the site boundary (i.e. no access/platform to do edge stressing) or the edge of the slab is not painted and exposed to view at architecturally sensitive areas since the grout patch will have different colour as per picture below.
If the edge of the slab is painted with white paint for example, would the edge stressing still be rejected by the architect? Not too sure if you're able to tell the difference once it's painted though.


Q2. When stressing through construction joint (pour joint or Temporary Movement Joint (TMJ) with proprietary dowel products). Would edge stressing be preferred here instead of pan stressing?
I note that with pan stressing you'd put additional trimmer reinforcement around the pan (pan reinforcement) that may lead to congestion. But I still see that sometimes pan stressing is done at these joints instead.

Any help would be appreciated.
Thanks.

 

[cliff234]

Please define "pan stressing". (I'm not familiar with that term.)

 

[Ingenuity]

Please define "pan stressing". (I'm not familiar with that term.)

It is a temporary partial-depth blockout/recess consisting of a (typically) plastic form pan to enable internal stressing without a full-depth slab penetration. Commonly used in BONDED PT slab systems. A curved stressing nose if often required to angle the stressing jack to clear the angled slab recess.

BacBac:

I avoid pans where possible. Elongations typically come out less than anticipated, chance of breaking wires/strands during stressing, and they need to be detailed correctly otherwise cracking occurs due to the PT force difference from one side to the other side of the internal anchorage.

Edge stressing, whilst preferred, are sometime problematic due to access or the finishes. If the edge pockets are to be blended in with the adjacent concrete, typically the slab edge would be 'cement washed' then painted, but under critical light sometime the PT pockets can still be seen. Need to work with the architect and builder.

 

[Tomfh]

Both are common. There are no rules stating when you may use pans. You need to work with the rest of the team.

Whether you can use edge stressing at joints depends on the access.

 

[Just Some Nerd]

If the edge of the slab is painted with white paint for example, would the edge stressing still be rejected by the architect? Not too sure if you're able to tell the difference once it's painted though.

Had this happen on a recent job, mid way through building we changed it all the edge stressing to pans because the builder/client was finding it unsightly on the lower levels. Finishing it over with render or similar before painting will often still be prone to cracking at the live ends.

When stressing through construction joint (pour joint or Temporary Movement Joint (TMJ) with proprietary dowel products). Would edge stressing be preferred here instead of pan stressing?

I assume this is a typo but in case it isn't, I would definitely not be stressing *through* a joint. Shift edge stressing to the far side, or change to pans if not possible. Builders won't be able to work on the second side of the pour until the first is fully stressed given you need access to stress live ends on the second pour, plus I think it's a horrible idea to have those live ends embedded inside concrete.

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Why yes, I do in fact have no idea what I'm talking about

 

[BacBac]

Thanks all for the insight. It's been very useful.

Ingenuity:
I agree with you. I'd avoid pans whenever possible due to the extra reinforcement, extra loss, potential congestion, possible clash with other reinforcement/pans, etc. which leads me to asking this question to know when architect/builder will push back against it.

Tomfh:
Agreed, the purpose of me asking this question is to get an insight on when I'd get a pushback from architect/builder when I nominate edge/pan stressing based on experience from the members here.

Just Some Nerd:
Good to know. I guess the really "safe" place to put edge stressing without any pushback from clients/architect is when the edge of the slab is covered by masonry or cladding then. The rest would depend on and need to be raised to the team.

Regarding stressing "at" joints, I've seen both where it's edge and pan stressing.
In engineering point of view, I'd have thought edge stressing would be better as it reduces congestion.
In the end, I think this needs to be communicated with the builder for them to consider in their sequences.
See snapshot below, blue dashed lines are the Construction Joints (CJ).

*edited "through" to "at"

 

[hokie66]

BacBac,

Live ends, dead ends at construction joint. What supports the side with the dead ends?

 

[Retrograde]

Regarding stressing through joints, I've seen both where it's edge and pan stressing.

I think what JSN was getting at is that the PT tendons stop and start each side of the CJ. The tendons do not pass "through" the joint.

 

[BacBac]

hokie66:
There's top and btm bars continuous through the construction joint to maintain moment continuity.
I believe the props would stay until both pour reaches its strength.

Retrograde:
Apologies for my bad wording. Not meant to have tendons pass through the CJ as it'll start-stop.
The question is more to the feasibility to stress the tendons at the CJ with edge stressing.

 

[Ingenuity]

There may be some regional difference on what is a ‘construction joint’.

My definition of a CJ is a temporary joint usually used at the convenience of the contractor as opposed to a permanent joint defined by the engineer of record on the structural drawings.

It would be very unusual to have top and bottom rebar continuous through a CJ for moment continuity withOUT also having the PT continuous through the joint. With continuous rebar through the joint why don’t you use intermediate coupling anchorages at the CJ, assuming this is not a permanent joint?

 

[BacBac]

Ingenuity
True, in that particular snapshot, the construction joint is a "pour joint" as the builder cannot pour the whole slab in 1 pour. Some people call these bars as "stitching bars"
Typical detail is as per snapshot below.

Did you mean intermediate coupling anchorages for the tendons?
I have been told to avoid these tendon couplers as there have been past safety issues with them not installed correctly or some sort.
I have not seen these couplers details used lately here.

 

[Ingenuity]

BacBac:

For a CJ that is part of a 'pour joint' we usually have the width of the joint to be about 4 feet (1200 mm) - or more - so you can accommodate slab edge stressing and avoid pan stressing.

I have not practiced in AU for many years, but back in the day we used coupled CJ anchors all the time. But that was when PT on site was conducted by experienced installers. I am going to guess that some swages at the coupled joint slipped and caused an accident that now prevents their use. Never had any problems with couplers when I was doing bonded PT.

I recently heard that double live-end stressing anchorages is NOT acceptable on many AU projects now.

Maybe PT companies in AU are now run by accountant or lawyers and not engineers.

 

[hokie66]

Regardless of whether you use couplers or stitching bars, I wouldn't position construction joints where shown. Along or close to a column centre line is no place for a construction joint. Provide them near centre span, or within the central half span. Ingenuity is probably right about the PT industry. I have seen couplers used many times. If you do use them, another reason not to locate CJ's on the column line (or right at centre span) is that additional clearance is required for the couplers.

 

[BacBac]

Ingenuity:
Thanks.
Yea, double live-end is not typically done anymore, and again it's due to safety issues.
We typically have 2 tendons stressed from each side and have it lapped.
Can't deny that we're not the best.

hokie66:
Thanks for the comments. The previous snapshot is from a different project that I was not involved it. I only took it as my reference.
But will keep your comments in mind.

 

[hokie66]

One more thing. A bunch of top bars through a construction joint is a constructability nightmare. Think of the formworkers, bar setters, and concreters. The stop ends have to come out in pieces, and when they are drilled for the bars, the debris is not easy to get out, so often can be seen on the slab soffit.

 

[Just Some Nerd]

I have not practiced in AU for many years, but back in the day we used coupled CJ anchors all the time. But that was when PT on site was conducted by experienced installers. I am going to guess that some swages at the coupled joint slipped and caused an accident that now prevents their use. Never had any problems with couplers when I was doing bonded PT.

I take it there's at least some emphasis on "many years" because I haven't encountered them at all here (Sydney, mostly work on resi towers). There's almost no PT installers I'd consider competent these days

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Why yes, I do in fact have no idea what I'm talking about

 

[Ingenuity]

I take it there's at least some emphasis on "many years" because I haven't encountered them at all here (Sydney, mostly work on resi towers). There's almost no PT installers I'd consider competent these days

Correct. Back in the day when there were mainly three international PT companies working on buildings: BBR (Structural Systems), FREYSSINET (Austress/PSC) and VSL. I am not sure if any of these 3 do PT to buildings in AU any more. Legacy lost, IMO.

 

[Trippelsewe]

Here in AU I've only done couplers on one job and that was a VSL job.

Nowadays it is always reo across the CJ.

I also prefer edge stressing but it is rejected a lot.

 

[BacBac]

Trippelsewe:
May I know what are the reasons edge stressing was rejected based on your experience?

 

[Trippelsewe]

@BacBac It's normally either safe access or if the slab edge is the finish (bare or painted). I think you've mentioned this is your experience as well.