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?

 

[kieran1]

Have you taken test cubes/cylinders prior to stressing?

Kieran

 

[struggle66]

Yes we got the required strength of cubes 25 MPa.

 

[ajk1]

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.

 

[ajk1]

...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.

 

[dik]

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

 

[struggle66]

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.

 

[Ron]

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.

 

[hope9010]

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?

 

[Ingenuity]

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?

 

[struggle66]

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.

 

[struggle66]

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

 

[Ron]

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

 

[dik]

...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

 

[ajk1]

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?

 

[hokie66]

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.

 

[struggle66]

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

 

[struggle66]

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.

 

[Ingenuity]

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.

 

[Ingenuity]

Sorry, the file upload did not seem to work. Trying again....