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?

 

[struggle66]

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

 

[hokie66]

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

 

[struggle66]

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.

 

[rapt]

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)!

 

[struggle66]

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.
thanks all

 

[hokie66]

struggle,

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

 

[rapt]

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!