Bursting of concrete after stressing 1

[struggle66]

Hi all
In one of my project, the concrete always bursts at the stressing ends of 200 mm thick slab tendon anchorages after stressing.

So I checked the things below

(1) Stressing jack - It was calibrated and used that jack for other projects and nothing happened.

(2) Bursting links - I used (bursting force = 0.23 Pre-stressed force)and provided more than enough links.

(3) Anchorages size - Our anchorages sizes big enough to transfer the force to concrete.

(4) Insufficient concrete strength - that is the plausible cause I can think of. The concrete grade is 35. Do we need to use an admixture for grade 35 concrete to control the heat released from cement. If it was due to concrete, I need some evidences to substantiate.

Can you think of any other causes?

 

[hokie66]

Ingenuity,
The way I interpreted his detail is that he is using a modification of Detail 1, with individual closed ties used rather than a spiral. If that is the case, I would be concerned about development of the ties. If the concrete or the anchorage reinforcement is suspect, 5-15.2 strands is a lot of force.

 

[struggle66]

hokie66,
Yes you are correct about the links detail.

Here with attached failure zone photos. In the last photo the anchorage just drew into the slab.

Thanks

 

[hokie66]

I could not open that type file.

 

[rapt]

Struggle,

I would not be blaming the rain.

Sounds and looks like low concrete strength to me and possibly bad compaction.

We would need to see how the ties are position at the anchorage to see if they are effective. The area supplied should easily be sufficient. Normal requirement would be 4 R10 ties, so about 1/2 of the area supplied and I think development would be ok as long as they are closed ties as you are only trying to develop about half of the strength of the bars anyway. But the positioning and the compaction are critical.

 

[struggle66]

Hokie66,
Uploading agian. My bursting links are not spiral so I think they cant resist the force together in a combined action. Is it still ok?

 

[struggle66]

Hi all
Kindly see links detail.

 

[rapt]

That should be sufficient reinforcement. Having sufficient reinforcement is not enough. Getting good compaction is possibly even more important.The questions are

Is it actually placed like that relative to the anchorage?

How can they achieve proper compaction around the anchorage. The ties are very close together. Are you end up with slurry run into the anchorage zone will very little aggregate and /or bad compaction?

Looking at the failed anchorage, the failure appears to be in the fines, not through the aggregate which would mean low concrete binder strength at the timer of stressing. Where were the cubes stored before testing for the timing of the stressing operations?

 

[struggle66]

Thank you all for your time and replies.

 

[slickdeals]

I had a similar issue on one of my projects and had a good discussion with RAPT on the issue. One of the items had to do with the oil on the strand, which reduced the bond strength. We added a little more bond length and ensured a more thorough QC process.

We also upped the amount of bursting steel.

Also, it might be better to get site cured samples to better match the strength gain versus cubes that are in a controlled environment.

And importantly, like others have said, there should be adequate compaction around the anchors.

 

[slickdeals]

And one other thing was to limit the strands at an anchor to a maximum of 3. This increased the number of anchors, but definitely helped with the bursting issue.

 

[hope9010]

struggle66 - Do you have a photo?

 

[Ingenuity]

Slickdeals,

Oil on the strands will affect bond, but this only plays into a dead-end anchorage zone during stressing, not a live-end like this instance.

Also, whilst a 3 strand tendon will much reduce the bursting and anchorage stresses, it will increase the PT cost substantially - PT supply and install is all about trying to maximize the number of stands per 5-strand anchorage, and maximize spacing and tendon length.

 

[rapt]

I agree with Ingenuity re 3 strand tendons. This is not solving the problem, it is finding a way around it.

5 strand tendons have been used successfully without problems for 30 years in countries where there is good control of concrete quality in all areas from mix design to compaction to strength determination and with well made anchorages using good quality cast iron (SG instead of Grey).

If you cannot get all of these right, you should not be doing prestressing at all.

 

[jgailla]

Sorry for the off topic, but why do you use cast iron instead of steel? I haven't been on a PT job in a while and have never designed one.

 

[Ingenuity]

SG (spheroidal graphite) cast iron is used for cast anchorages due to cost. Machining an anchorage would be expensive and time consuming.

However, for multi-strand bearing/anchor wedge plates, they are often CNC machined, so too are multi-strand coupling blocks.

 

[struggle66]

Hi all
I have to raise this thread again. In BS 8110, CP 65, at end block of stressing anchorage it does not mention to check the bearing capacity of concrete. But when I looked into ACI, the following simple equation needs to be satisfied.

0.6 x 0.85 x fc' x Root (A2/A1) > 1.15 Pj / A1

The capacity can be increased by 50 % if the concrete at end block is confined inside bursting links with a maximum recommended value 0.6 x 2.5 x fc'.

When I check concrete bearing capacity for this project, it was failed.

I believe that is what happened in my project.

Thanks all for your answer before.

 

[rapt]

struggle,

The actual bearing stress depends on the anchorage shape. If it is a typical slab type anchorage, the standard bearing rules do not apply because they are not a flat bearing plate. In cast anchorages Force is transferred over the length of the anchorage (sloping sides) and also by fins along the length, so all of the force is not being applied at the front face of the anchorage and thus the actual bearing stresses are smaller. Normally these anchorages are proven by testing as is allowed by the anchorage codes.

I would still be betting on low grade grey cast iron being used in the anchorages instead of SG Iron!

 

[ajk1]

I agree completely with what RAPt says. I doubt that any anchorage would work if checked by Code equation. The bearing capacity is normally determined by load testing, as RAPT has said.

Perhaps you have an oddball anchorage that is smaller than normal and has not been load tested, but much more likely causes are:

a) that the rebar links are not being fully developed, as there is insufficient development length available if the slab is only 8" thick (8"-1" cover top and bottom = 6". leaving only 3" development length available from the plane of maximum bursting stress!!; I would never rely upon rebar links to take bursting force in a member only 8" thick) a

and/or b) you have the anchors too close to each other. What is the spacing between anchors?

Have you tried turning the anchors so that their long dimension is horozontal rather than vertical?

My experience is that splitting cracks in the anchorage zone can worsen with time and are a very very serious issue.

I strongly recommend that you retain an engineer with specialist experinece in post-tensioned concrete to advise you. A few hours of his/her time will be more than worth it.

 

[struggle66]

Rapt,

There is only one fin in our anchorage and the slope is almost horizontal. Is it appropriate if I distribute the bearing stress depending on the area of the face behind anchorage and fin? And then I will check the bearing. I have also requested anchorage supplier for any proven test or example calculation.

ajk1,
(a) For the rebar links I utilized only 200 Mpa for 460 Mpa steel.
(b) Spacing around 700 mm. Longer dimension is horizontal.

Thanks

 

[rapt]

I do not think there is a way to use the bearing stress rules for these types of anchorages. There would be an overlap in the bearing effects from the face and the fin.

Test them yourself. The Australian code covering testing should be AS1314. I am sure Europe has an equivalent and presumably ACI does as well.