loss of tension in post tensioned beam

[aladdin76]

Hi

What are the symptoms of tension loss of post tensioned beams.
I have several beams to be checked for tension loss. in one of them there is a horizontal crack in the web, is this one of the symptoms of tension loss.

Thanks

 

[rowingengineer]

I wouldn't expect horizontal cracks in the web of the beams from loss of tension, I would expect the failure to be similar to an over stressed beam, unless the shear strength failed first but I would then expect 45 deg cracks to form.

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[Ingenuity]

aladdin76,

Is the PT to the beams, UNbonded or bonded (grouted)?

What other characteristics are evident to the beams that necessitate "...beams to be checked for tension loss"?

I agree with rowingengineer, horizontal cracking to beam webs are not usually associated with loss of prestress, HOWEVER, if the tendons were incorrectly draped (sharp curvature over short lengths, for example) it is possible that horizontal cracking can occur during/shortly after stressing.

 

[Ron]

Determine bonded or unbounded. Check also for proper original placement of strands. You mention a web. Is there a chance the beam was installed upside down?

Crack description would lead me to suspect tendon placement issue.

 

[BAretired]

One of the symptoms of tension loss in a post tensioned beam could be beam deflection or loss of camber but unless elevations were measured prior to the tension loss, this is not likely to be easily recognized.

BA

 

[msquared48]

Ron:

A check on terminology here... An unbounded beam - is that one that was not bounced during placement?

Mike McCann
MMC Engineering

 

[graybeach]

Does the crack follow the path of the tendons? If so, and the tendon is grouted, the crack could be due to corrosion of the strands inside the duct. This causes expansion which leads to the crack. I have dealt with this on PT bridge girders. Once we opened up some test pits, there was no doubt. Interestingly, the outside of the duct and the adjacent rebar looked perfect. Inside the duct was a rusted mess. I believe the corrosion was caused by inadequate grouting leaving voids filled with bleed water.