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Bursting of concrete after stressing

Bursting of concrete after stressing

Bursting of concrete after stressing

(OP)
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?

RE: Bursting of concrete after stressing

Have you taken test cubes/cylinders prior to stressing?

Kieran

RE: Bursting of concrete after stressing

(OP)
Yes we got the required strength of cubes 25 MPa.

RE: Bursting of concrete after stressing

1. Is this normal weight concrete, or semi-low density concrete?
2. What was the strength of the concrete slab at time of stressing?
3. Did you use Lok tests to determine the in-situ concrete strength before stressing? I always specify Lok tests for prestressed concrete; field cured cylinders placed on top of the slab are no good for this purpose (or for any other purpose).
4. Have you taken any cores to check the concrete strength?
5. Has the weather been cold and slowed the strength gain?
6. Do the links have proper development length?
7. Did anyone inspect the placement of the links before the concrete was placed?

A piece of advice: Don't do any more prestressing until you have determined the cause and addressed it. Bursting cracks can grow with time. About 30 years ago we were called in to investigate a beam that collapsed due to this issue. It collpapsed 9 months after the garage had been opened for use. In that case, there were no links provided, so that would have been worse than your case.

RE: Bursting of concrete after stressing

...and one more thought: What strength did you design the links for? If you used the yield strength, then you can expect cracking. I would use much less than that.

Sometimes the only way to get rid of cracking is to reduce the p.t. force. I find that it is much better to use less p.t. force than more i.e. use partial prestressing. What percentage of the dead load did you balance?

Also, you might try using the strut and tie model to determine the bursting force.

I think your problem is due to some combinaion of the factors that I listed in this post and my immediate previouse one.

Be sure that you find the right in-situ strength before stressing the tendons. Use either Lok tests or maturity meter method. I prefer the Lok tests.

RE: Bursting of concrete after stressing

What kind of anchorage? and do you require containment reinforcing? and what is the concrete strength at the time of stressing? and part of the mix design that might retard strength gain?

Dik

RE: Bursting of concrete after stressing

(OP)
ajk1,
1) It is normal weight concrete.
2) At time of stressing, the concrete strength is so strange. Within three days, it was around 55 Mpa.
3) Didnt carry out LOK test. Just cube test.
4) no
5) If it was raining, does it affect the concrete strength.
6) What do you mean by development length?
7) Yes, my site engineer inspected before concreting.

Used As required = bursting force/200 which is according to CP65.
Precompression is around 3Mpa.

dik,
It was 5 strands flat slab anchorage.

RE: Bursting of concrete after stressing

Why would a 35mPa mix break at 55mPa in 3 days? Something's wrong there. Are you sure you have the right cubes for the job?
Agree with ajk1....do insitu strength testing as that is more accurate for the stressing conditions than the lab condition...


ajk1....seeing your reference to LokTest brought back memories of John Bickley introducing me to that in the early 80's. It's not used down here, but got experience with it in my association with Trow. Good method.

RE: Bursting of concrete after stressing

Ron - yes, I first really learned of Lok tests, from Bickley whom I retained when we did a very large p.t. project at Carruthers & Wallace. It worked out quite well, although the contractor needed some convincing at the start.Where are you located now?

Struggle66 - I have a conference call in 10 minutes, but after that I will try to get back to you. The development length Ld is given in the various codes/stanards by formula (example: CSA Standard; ACI 318, etc.). If you don't have adequate Ld from the point of maximum stress of the link, to the end, the link may have some loss of bond to the concrete and open a crack. How thick is the slab that you are post-tensioning and what size (diameter)are the links? Did you design it, or someone else?

RE: Bursting of concrete after stressing

1. 5 strand slab anchorage, bonded tendons - are you sure the anchorage zone is getting compacted (vibrated)?

2. Is there a 25% initial stress prior to full stressing?

3. Any visually-observed boney/honeycomb concrete at anchorage zone/s?

4. Is the anchorage rebar a standard rectangular spiral, or 2 pairs of ties?

RE: Bursting of concrete after stressing

(OP)
I provided 5T13 @ 70 mm c/c just behind the anchorages.

According to CP 65 Ld is from 0.2Yo to 2Yo behind anchorages.
where Yo = half the side of loaded area.

I assumed the loaded area 200(slab thickness) x 350 ( minimum spacing between two flat anchorages).

So I used Yo = 200 as bursting is critical in Y direction.

How do you calculate Ld?

The concrete concrete was bursted so badly. Sometime without bursting the anchorages drew into the slab seem like concrete bearing failure.

One more thing our anchorages broke once the concrete failed. Is raining will be a reason?

One anchorage only bursted only after a few days. I am scared that it will happen again after the building has been opened like you experienced.

RE: Bursting of concrete after stressing

(OP)
Sorry 5 T 13 are closed links so I assumed 2 legs in one direction (x,y).

RE: Bursting of concrete after stressing

hope9010....Florida. Used to be affiliated with Trow, Ltd.

RE: Bursting of concrete after stressing

...learned about LokTest from John, too... about 30 years ago in Toronto. That dude sure got around... Had Trow involved with several of the parkades I designed...

Dik

RE: Bursting of concrete after stressing

struggle66 - sorry to take so long to get back to you, but I had to get something finished before leaving the office yesterday.

It is hard to say for sure without seeing the drawings and details, and perhaps I am misunderstanding the situation, but I fear that you have a design problem, not a concrete problem. If your slab is only 200 mm thick, then the links cannot be fully developed. I believe that the Code notes that stirrups cannot be developed in slabs less than 250 mm thick. Also, you should check the Code as to what stress you should design the links for. The PTI manual used to say to design for 30,000 psi, not for the 60,000 yield strength of the bar. Not sure what it says now, as I am not in the office. Studrails are usually used in slabs of this thickness. Another solution is have fewer tendons per anchorage and spread the anchorages farther apart.

Can you attach a sketch plan and section of the anchorage? It sounds like there is a very serious problem...sorry to be the bearer of bad news...this is one time that I hope that I am wrong. I think your best coaurse is to retain an engineer who has sepciualized in prestressed concrete design, to review your design.

Where are you located and to what Code are you designing?

RE: Bursting of concrete after stressing

As others have said, it would be handy to have more details about the actual stressing anchorage used. Whose casting is it? As ajk1 said, a sketch would help. What is the tendon spacing? Are the anchorages centrally located in the 200 slab depth? Which size are the strands? What was the general sequence of stressing?

There are two types of reinforcement around the casting which I am familiar with: 1) two bars top and bottom continuous along the edge, with ties around these bars, and two ties on each side quite close, and 2) a rectangular spiral around the casting. Which do you have?

ajk1, I don't know how the slab shear, studrails etc., comes into this. He is talking about bursting of the anchorage.

The concrete strength result sounds very strange indeed. 20 to 25 MPa is normally sufficient.

RE: Bursting of concrete after stressing

(OP)
ajk1,
Links yield stress is 460 MPa but I only used 200 MPa according to CP 65. In Singapore, they adopted BS as CP 65.

My company is PT specialist company in Singapore. We have been using the bursting links as per our company standards. Nothing was happened for many other projects.

Later I will attach sketch and detail for you. But now FYI,
It is an one way pt slab with seven (8.6 m) spans and with PT beams spanning in transverse direction. The concrete bursting locations are at the pour strip located at the one-third of the fourth span.The tendon spacing is around 1.2 m. Anchorage size is 90 x 300. 5 T 13 rectangular(300 x 120) closed links with 70 mm c/c were provided just behind the anchorages.

Thanks for your advice. Now we are using 5 strands tendon per anchorage but later I will design with 3 or 4 strands per anchorage for thin slab(<200) and will highlight my company to carry out LOK test before stressing.

Thanks

RE: Bursting of concrete after stressing

(OP)
Hokie66,
Our anchorage supplier is OVM from China.
Yes anchorage are centrally located at slab (C.G is 100).
It is 0.6 in dia strand stressed till 75 percent of breaking load.
First we stress 25 percent and then 100 percent.
We use rectangular closed links as mentioned above.

RE: Bursting of concrete after stressing

struggle66,

It appears you are using a variation of the attached DETAIL 2, prevalent in South East Asia, whilst the rectangular spiral (DETAIL 1) is more common in Australia. These details have been used in thousands of bonded PT slab systems without incident, other than the occasional failure primarily due to unconsolidated or low-strength concrete.

Without seeing a photograph of the failed PT anchorage zone/s, my guess is that the pour strip where the PT anchorage zone/s is/are failing is due to: 1) unconsolidated/vibrated concrete - is there evidence of boney/honeycombs at the anchorages? 2) the concrete batch placed at the subject pour strip was incorrectly supplied or understrength.

Is it possible to take 3 each 100 dia x 200mm deep field cores at the pour strip area and undertake compressive strength testing to determine probable cause?

To fix failed PT anchorage zones are a real pain, especially when there are multiple failures. The general contractor gets upset too - forms have to remain in place longer, and the construction schedule gets problematic.

If you have a photograph of the failed anchorage zone please upload it and share it with us.

RE: Bursting of concrete after stressing

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.

RE: Bursting of concrete after stressing

I could not open that type file.

RE: Bursting of concrete after stressing

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.

RE: Bursting of concrete after stressing

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?

RE: Bursting of concrete after stressing

(OP)
Thank you all for your time and replies.

RE: Bursting of concrete after stressing

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.

RE: Bursting of concrete after stressing

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.

RE: Bursting of concrete after stressing

struggle66 - Do you have a photo?

RE: Bursting of concrete after stressing

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.

RE: Bursting of concrete after stressing

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.

RE: Bursting of concrete after stressing

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.

RE: Bursting of concrete after stressing

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.

RE: Bursting of concrete after stressing

(OP)
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.

RE: Bursting of concrete after stressing

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!

RE: Bursting of concrete after stressing

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.

RE: Bursting of concrete after stressing

(OP)
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

RE: Bursting of concrete after stressing

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.

RE: Bursting of concrete after stressing

(OP)
Rapt,

I received test report from supplier. They carried out the test on 225mm thick concrete block sample with concrete strength 50 MPa. Now my slab is 200 mm and concrete strength at time of stressing is only 25 MPa.

Working without a mentor is not easy for me. Everything I have to do it on my own and your answers are always helpful for me.

Thanks

RE: Bursting of concrete after stressing

Testing an anchorage assembly in 50 MPa concrete is useless, probably intentionally misleading.

RE: Bursting of concrete after stressing

(OP)
Yes hokie66,

It was intentional.
I was surprised when I saw this test report. I dont know about other parts of the world. But in here the practice is to stress at 25 MPa.

RE: Bursting of concrete after stressing

Struggle,

You need to get a copy of the Anchorage design/test code relevant to your country and check that the tests are in accordance with nit. I assume the 25MPa is cube strength, is the 50MPa cube strength also?

It also depends on the concrete mix. You need to test the anchorages yourself under the conditions you use them, e.g. concrete mix etc.

The whole problem here is really in your last sentence. A PT company is supposed to be providing a high =technology sub-contracting service. It is supposed to make sure that the materials it uses are up to spec. Unfortunately the PT industry (I am now talking bonded outside USA as USA has its own problems) has developed to a stage where anybody thinks they can run PT comp[any. This has led much lower prices but also much lower standard of technology within the PT companies.

(start rant)When I started in PT many years ago (and the same for ingenuity, we designed and tested our own anchorages, wedges, blocks and even the duct and bar chairs. I might have started as a graduate engineer but I had several experienced engineers over me as well as a worldwide technology base within the company. But this costs money and requires much higher PT rates. These days, everything is purchased off the shelf, the senior engineer (if there is one) is a recent graduate and there is no technical knowledge within the company. And we end up with situations like yours where you are relying on questions to a free online forum to provide your companies technology backup. In the interests of the industry and engineering in general we should not be answering these questions!! (end rant)!

RE: Bursting of concrete after stressing

(OP)
Rapt,
50 Mpa is cube strength.

I am not trying to improve my company technologies and standards.Even if I suggest something(for example not to use average moment method)to my company, they will not accept it because it does not lead to lower prices. I just want to know what I am doing. I just at least want to work like an ENGINEER! unlike a business man who only care about money. You wont believe if I say there are so many designers in here who don't care or don't know about structure even in structural consultants firms.I will not give up trying to become a TRUE engineer. Hope my dream come true although it is not easy for a recent graduate with lower education background. :D
thanks all

RE: Bursting of concrete after stressing

struggle,

We do believe you. Sad, but true, and not just in your country.

RE: Bursting of concrete after stressing

Struggle,

I am not criticizing you, it is companies like yours and the "business men" who run them. What you have said about your company and others is exactly what I meant. You have been forced to be in the position that you are in and to rely on help from sites like this because of their attitude.

It is a sad direction for both construction and the engineering profession as well as the world in general!

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