PT duct material question

[Dinosaur]

A supplier told me that you can not use many plastic ducts because of the thermal coefficient of expansion. If the coefficient is too large, then the duct will swell when the warm concrete comes in contact, then after it has hardened and cured, the duct cools leaving behind a void slightly larger than the duct. Does anyone know about this here at this forum?

Also, how often should a PT duct be supported in the cage along its length to avoid excessive distortion when the concrete is placed in the girder. In our application, the girder web is from seven to nine inches thick.

 

[Zambo]

How many strand and duct diameter?

 

[Dinosaur]

I have been told the duct specified in the plans was to be 95 mm dia to accommodate something like an 18 or 19 strand tendon. I believe there is more than adequate room for this many strands. The question is still "Should polyethelyne duct be used or not?"

 

[dik]

Try to get more specifics from the supplier... generally friction losses with polyethylene ducting is next to zip...

Dik

 

[Dinosaur]

The supplier may be involved in a lawsuit over this. I doubt I could get any information from him. The friction losses are not the issue. If the material, because of its coefficient of thermal expansion, is inappropriate for use in PT girders, then I need to take steps to avoid it being used in the future.

 

[henri2]

PCI MNL 116 Manual for Quality Control for Plants and Production of Structural Precast Concrete products may have some useful information. Check Article 5.4

Is there any grouting involved?

Regarding use of high density polyethylene (HDPE) for duct material, Article 5.4.2 states " Ducts for post-tensioning tendons shall be constructed of flexible or semi-rigid metal or corrugated HDPE/polypropylene tubing installed within the member...". The commentary states that corrugated HDPE or polypropylene ducts used for grouted post-tensioned members have to meet appropriate Post-Tensioning Institute recommendations.

 

[Dinosaur]

The ducts are required to be grouted after the tendons have been stressed. I understand the grout spec is fairly comprehensive, so we should get good grout.

On another bridge in my area, the ducts deformed substantially during concrete placement. They actually fed a fiber optic camera down the duct to view the distortion. I was shocked to see how much the duct had crushed and shifted. Instead of being round inside, it was squished so that the short side of the oval was less than 1/4 the distance of the long side. They had to drill the concrete to create a space for the tendon before that could PT. This is why I was interested in knowing how much a duct needed to be supported so the pour didn't mess it up too much. Thx

 

[rapt]

Dinosaur,

That is probably why we always use metal duct. It does not crush like that during a pour.

Normal external suppoort for the duct should be at about 1m centres. More than that and you start to get more wobble losses than allowed for and less force in the tendons and lower extensions.

But this is not going to stop the duct from crushing, it is only a vertical support. You need stronger duct and the system should be fully tested before it is used commercially. The illconceived move in the USA to try to get "better" corrosion protection has led to the use of plastic ducts but obviously with unexpected consequences.

 

[dik]

Sorry... I didn't realize that the duct could collapse during construction... thought the concern was 'wobble' causing additional friction losses...

With vibration, are the forces sufficient to collapse the duct? If shrinkage is sufficient, the method might approach a non-bonded system. Is it possible that the ducting was 'kinked' during the installation and not a result of casting concrete?

Dik