greased tendons 3

[steelman123]

Do you always grease the tendons in pre-stress concrete projects, like bridges for example? If yes, what is the MAIN
REASON? Thanks Steelman123

 

[LittleWheels]

Hello Steelman,

Most countries I've worked in tend to grout tendons, both for corrosion resistance and to minise the effects of a broken tendon. I've never seen a job where tendons are greased but dimly recall that North America often does so. Could it be for corrosion resistance and to minimise friction losses along the duct?

Regards

Dave

 

[CottageGuy]

Steelman,

I don't have experience with prestressing, but with post-tension construction (in North America) the tendons can either be greased (more common), or grouted (less common).

With a greased system, the grease allows for easier installation and replacement, the tendon can be pulled out and reinstalled within the grease. It also limits the corrosion potential as the moisture will have to break down the grease before it can get at the tendon.

A greased system also allows for easy inspection of the tendons. You can open the tendon at one location (the low point) and determine if there are any broken, deteriorated, or destressed cables (along the full length of the tendon). In a grouted system, one opening will not tell you anything about the remainder of the cable.

I presume that these comments could be associated with prestressing as well and post-tensioning.

Hope this helps.

 

[Polecat]

Greasing post-tensioned tendons is a very common practice in the States, primarily in reinforcing floor slabs in high rise buildings or parking garages.

The purpose in doing so is twofold: corrosion protection and friction reduction while stressing. The tendon is commonly anchored with conical bearing plates and grippers at both extremities of the slab, but in some instances ---- such as in concrete poles, it can be terminated up in the member by bringing the wires out of the debonding sleeve for a distance of about 10 feet to engage in direct bonding with the surrounding concrete. In that case, the exposed strand must be free of all grease, or else the tensioning process will be for naught!

http://www.spiraleng.com

 

[Ibeam]

Steelman,

I'm wondering if this has to do with the bursting force (in the concrete) at the ends for prestressed concrete girders. I think that sometimes, some of the strand ends are debonded (partially debonded strands) for the purpose of relieving the high compressive forces at the ends (support point) of the beam.

 

[Polecat]

IBeam is on the right track.

Remember Poisson's Ratio? When a prestressing strand is tensioned, it reduces in diameter. Then, when the cable is detensioned for stress transfer to the concrete, it tries to swell back to its original position. Now, however, it is being restrained by the bond with the surrounding concrete so tremendous circumferential forces are developed at the ends of the member.

In highly prestressed areas, if some partial debonding is not done, it can literally burst the ends of a beam. Grease alone, however, is not enough to prevent this. It will certainly reduce the direct friction bond, but again, Poisson's law takes hold and you still have a mechanical swelling of the strand. Hence, at the ends you need to debond with a sleeve that completely surrounds the strand.

http://www.spiraleng.com