Studying for PE - bonded pre-stressed tendons? 7

[sponcyv]

Hello all - I'm studying for my PE and working through pre-stressed concrete design right now. I am looking for practical explanations of bonded and unbonded pre-stressed tendons with examples of how they are constructed in the field. The PCI manual doesn't give this and I'm having a hard time finding anything online. Does anyone know of a good website?

Thanks

 

[slickdeals]

The unbonded tendons are prestressing cables enclosed in a plastic sheathing. The aim of this is to prevent bonding against concrete during stressing operation. The downside to it is that if someone accidentally drills through the slab and hits the tendons, then you basically lost the cable.

The bonded tendons are fed through plastic sleeves which are embedded in concrete. The cables are stressed and then the sleeve is grouted. The bonding action is between grout, tendon and corrugations in the sleeve and concrete.

 

[sponcyv]

slickdeals - thanks for your response!

Is the aim for using unbonded tendons to more accurately calculate losses or to remove the losses that you experience with prestressing that is bonded to concrete?

 

[woodman88]

The aim of unbonded tendons is less cost. I have use them for all the post tension slabs on grade for the multi-story apartments I have done.

Garth Dreger PE
AZ Phoenix area

 

[Zambo]

Do they cost less? To me the grout around bonded post-tensioned tendons is an inexpensive method of corrosion protection (but has to be done well). How do you protect your unbonded tendons against corrosion?

 

[ImminentCollapse]

Lots of confusion here between prestressed and post-tensioned applications.

I have never heard of bonded prestressed tendons being placed in sleeves as slickdeals mentions. That sounds like post-tensioned to me. Bonded prestressed tendons are placed in a bed or form, stressed, then concrete is poured in direct contact with them, thus "bonded" tendons.

Debonded strands are in sleeves. They are used to reduce top stress in the end regions of a beam, not to reduce stress during the stressing operation. You debond strands when you need XX number of strands at mid-span to develop flexural capacity, but the same XX number of strands would cause excessive tensile stress in the tope flange at the ends of the girder. Therefore, you debond a certain number of the total strands for a few feet from the end of the girder. Debonding is done in lieu of draping (harping) strands.

IC

 

[BAretired]

ImminentCollapse,

Prestressing includes both pre-tensioning and post-tensioning. Prestressing means stressing before the live loads are introduced. Slickdeals is quite right in what he says.

In a precast plant, strands are stressed prior to casting the concrete. When the concrete has attained sufficient strength, the strands are released and bond to the concrete. If you don't want a portion to bond, you can enclose it in a plastic sheath or bond breaker.

Post-tensioning has two forms, bonded and unbonded. In the unbonded case, the strands are enclosed in plastic sheathes with lots of grease to reduce friction. In the bonded case, the strands are post-tensioned within metal or plastic sheathes without grease, then grouted to achieve bond with the sheath which in turn, bonds with the concrete.

BA

 

[dik]

BA... good explanation. Might add that the 'grease' is also to provide some corrosion resistance.

Zambo... the interstitial space between the strand/tendon is greater for post-tensioned grouted conduits to facilitate grouting. The savings is in the labout required to pressure grout the bonded system. You have special anchorages that permit grouting you need to hook up a grout supply and a means of pressurising it... and then you have the removal of the equipment.

Dik

 

[Zambo]

Dik,

yes it's agreed that there is a fair sized space to grout for bonded tendons. The only time I work with prestressing is post-tensioned bonded tendons.

But my question is what is being used for corrosion protection for prestressed unbonded tendons. Are the tendons greased, then PVC sheathed? Or are they galvanised then greased. Or are they not really corrosion protected at all?

 

[rapt]

Bonded tendons are now used in most countries for building work. Unbonded tends to be used when post-tensioning is first commenced in a country but most countries tend to convert to bonded over time, except North America (sound familiar, units etc).

In North America, the PT industry is set up based on a tendon manufacturing and supply industry. In most other countries, the PT companies offer a full PT service from design to supply, installation, stressing and grouting.

The discussion on costs is an interesting one. We looked at comparisons in the 1970's in Australia where the industry was already a bonded one and decided that overall cost was cheaper for bonded. Unbonded PT cost itself may be less per tonne, but overall cost iuncluding all site works, extra reinforceemnt required for unbonded etc changes things. The quality of the resulting building and expected liftime also matters. Once these are considered, bonded wins hands down to us.

Unbonded relies on the anchorages for the full life of the building. And that is where most degradation occurs. Bonded does not rely on this after grouting is completed. So long term performance is normally much better and repair costs much less. Bonded tendons also have greater capacity than unbonded tendons, requiring less reinforcement. And they are more flexible in that they are put together on site, so flat slabs can be laid out in a proper two way pattern rather than the restrictive one way pattern that is universally used for unbonded tendons, which has resulted in the design requireemnts in ACI code being very restrictive for flat slabs (see other current thread on reinforcement in tension zones of flat slabs. Thus full partial prestress design is possible with flat slabs with bonded tendons. And the bonded tendons contribute to the crack control reinforcement which is another negative for unbonded tendons.

 

[rapt]

I forgot to add, most bonded PT is still done with galvanized duct, not plastic. VSL tried to introduce a plastic duct 2 strand system into USA about 12-14 years ago. It was very expensive and a failure against the unbonded market, mostly because of the labor required for a 2 strand system compared to 4 and 5 strand systems used in most bonded PT works.

It also had to be designed to ACI so the cost benefits of bonded PT with partial prestressing were not available and rules written for an unbonded system dominate the ACI code, so the best cost and performance advantages could not be made from the bonded system relative to unbonded.

 

[TXStructural]

Post-tensioning is often used here to reduce deflection on relatively long, shallow members.

Even unbonded systems last longer when grouted rather than greased. Water typically pools at the low spot in the drape, but when pressure grouted, voids are filled and no space for water remains. The alkalinity protects the steel, and inside the duct the grout will not carbonate with the cover and deleterious agents cannot easily diffuse to the tendon.

 

[rapt]

TXStructural,

I agree with you, but Unbonded Building PT systems are never grouted, they use grease inside polythene of some sort (mostly). A lot of the older systems used paper wrapped grease. They were very prone to problems.

But the main problems for long term damage and repair still occur at the anchors if the anchorage recesses are not sealed properly. If the anchorage fails, the tendon is useless in an unbonded system.

 

[ImminentCollapse]

BA,
This from slickdeals is what did not make sense, it is combining prestressed and post-tensioned:

"The bonded tendons are fed through plastic sleeves which are embedded in concrete. The cables are stressed and then the sleeve is grouted. The bonding action is between grout, tendon and corrugations in the sleeve and concrete.

Bonded pre-stressed tendons are NEVER in sleeves (otherwise it would be unbonded). Bond is not transferred through the grout, then sleeve to the concrete. Bond is transferred directly form strand to concrete. Bonded strands can have a potion of their length debonded as I mentioned, but the sleeve is terminated allowing direct contact with the concrete from that point forward.

Grouted post-tension tendons in sleeves do not transfer force through grout to sleeve to concrete. The force is transfered through the anchorage in the end regions.

 

[BAretired]

ImminentCollapse,

Here are a few links to bonded post-tensioning which you may find interesting:

http://www.google.co.uk/search?q=bo...=en&sourceid=gd&rlz=1Q1IRFA_enCA356CA356&aq=t

BA

 

[slickdeals]

ImminentCollapse:

Bonded pre-stressed tendons are NEVER in sleeves

You are mistaken. Bonded post-tensioned tendons are always in sleeves (or more accurately DUCTS). The cables are stressed and then grouted.

 

[ImminentCollapse]

I believe we're mincing some words and I'm remiss for not using pretensioned and post-tensioned clearly.

For clarity:
Pretensioned tendons (prestressed)are cast in and bonded directly to the concrete, unless debonded as discussed above.

Post tensioned tendons, bonded or unbonded (still prestressed) are in ducts, sometimes grouted, sometimes greased.

I still disagree with slickdeals comment that grouted post-tensioned tendons transfer force through the grout to the sleeve to the concrete. What if they are greased, how does the force transfer? Post-tensioned applications transfer the force at the anchorages, not along the length of the tendons.

From BA's wiki link regarding bonded post-tensioned:
"The duct is then grouted to protect the tendons from corrosion."

The grout is not for transferring forces.


IC

 

[BAretired]

IC,

The grout is for protecting tendons against corrosion. It also bonds the strands to the concrete through the duct. Contrary to your statement, grout is for transferring forces.

If the strands are greased, the grease protects the strands from corrosion but the system is unbonded and the strands cannot transfer forces to the concrete.

That is why, in my opinion, bonded post tensioning is a better system than unbonded.

BA

 

[TXStructural]

The thing that makes it "bonded" is the bond between steel and concrete, in the case of "bonded, post-tensioning", the bond happens between the strand/tendon and concrete through the grout and duct.

Go to:

http://www.post-tensioning.org/technical.php

and scroll down to "Technical Notes".