Un-bonded P-T in bridges 2

[Ingenuity]

I am not aware of any new bridge projects in the USA today that use UN-bonded P-T in bridge decks - please correct me if I am wrong - if this is true, why? Does the AASHTO/DOT code/s prohibit its use?

Historically, has unbonded P-T been used in bridge decks in the USA or other parts of the world, and if so, does anyone have project references?

By UN-bonded P-T, I do NOT mean to include EXTERNAL P-T, rather greased and sheathed strand systems within concrete decks/elements etc.

Thanks in advance.

 

[RiBeneke]

I am not aware of any uses of unbonded PT tendons here in South Africa for bridges.
I think the reason for lack of use may be lower safety against corrosion failure anywhere along the tendon length ... a cement grouted tendon is more reliable in this respect.
There is a higher degree of redundancy in PT in slabs in buildings, plus often indoor conditions.
Also in our SA concrete code, crack width calculations are different for unbonded PT and probably give less favourable results.

 

[ishvaaag]

See the Colorado DOT limitations to unbondend strand use in the chapter 9 of it Bridge Design Manual

http://www.dot.state.co.us/DevelopProjects/DesignSupport/Bridge/DesignManual/dm_s09.pdf

 

[JohnGn]

I am involved in a project in Missouri where the entire 1.2 mile bridge is unbonded PT. In fact I was not aware that there were any restrictions on its use. But they don't tell me much...

The whole bridge is made up of hundreds of individual sections, each one unbonded PT, and once the segments are connected together the whole bridge is stressed. The bridge tendons may in fact not be unbonded.

The segments resemble the same design as many parking garages I've worked on. FYI the name of the project is the Creve Coeur Lake Memorial Bridge in St. Louis county. The contractor is Walter Const. It is a very interesting and impressive project. I would think that the MoDOT web site would have some info on it.

John

 

[Ingenuity]

thank you John,

I just want to clarify - since your bridge is segmental precast it is rare for them to use unbonded PT. They are mosrt often bonded multistrand tendons. Can you tell me if they are grouting the PT or not? Or, is the strand they are using just plain uncoated, or does it have a plastic sheath around each individual strand?
thanks

 

[JohnGn]

It is a segmental cast-in-place structure with unbonded PT sheathed in green plastic and well greased. It's not precast. The anchorages are all sealed off but the strands are not grouted, at least I don't think they are.

Is this the info you seek?

--John

 

[Ingenuity]

Thanks John,

I visited MoDOT but could not find any info on the subject bridge.

Is this a single or multi cell CIP box girder bridge?

Is the PT external (ie outide the concrete section) ? or is all the PT cast within the concrete section?

thanks

 

[JohnGn]

The bridge is cast in segments, and about every 5 segments there is a wall between the cells. The PT is totally internal. The segments have transverse cables and longitudinal cables, and the entire structure has cables that run the length of the bridge.

The contractor is Walter Construction and if you can find their web site you might find info on it there.

It's a very cool project as far as concrete is concerned. MoDOT used mass concrete in the columns and they were so afraid of differential thermal effects that they used 70% slag to keep the heat of hydration in check. 70%! It was blue when it came out of the forms. Way cool.

Let me know if I can provide any further info...

--John

 

[Ingenuity]

John,

Found Walter's web site and they have a photo of the project...see

http://www.walterna.com/sites.htm

Looks cold...lots of snow.

The walls (diaphragms) at every 5th segment - does the PT pass thru them or is the PT in the webs and flanges only?

Are the tendons stressed with a monostrand jack?

thanks...sorry for the million questions.

 

[JohnGn]

Glad to answer what I can!

It is my understanding that the PT runs only across the top of the box, not in the web walls. They stress within the first 24 hours or so. They need 3500# in-place to stress. Once the concrete reaches that they stress and move the travellers. The concrete must reach 6000# eventually.

I have never been on-site to watch stressing ops so I do not know what type of jack they are using, but since the strands travel in threes (I think) I would guess they are using a jack that pulls 3 at once.

I hesitate to even state the above because I am just not familiar with these aspects.

I would be happy to get the email of one of the engineers on the job who I am sure could answer all of your questions. You can send me an email with your email address at johngn@con-cure.com.

You can keep asking me questions, though, and I'll answer what I can.

By the way, this contractor is using our maturity testing system (referenced in another forum) to keep their project moving despite the cold. If they hadn't used it the state had threatened to shut the entire project down for 3 months out of fear that it would be too cold to pour concrete.

--John

 

[JohnGn]

Correction to info previously posted:

I was wrong. It is NOT unbonded but bonded grouted PT that is being used on the Creve Coeur Lake Bridge in Missouri.

The web walls are not PT--just the deck. The design actually did call for PT in the web walls but the contractor convinced everyone that it was such a royal pain that they told them to stop.

I'm very sorry for the misinformation!

--John

 

[Ingenuity]

JohnGn,

Thank you for your reply/ies and correction- i really appreciate it.

FYI - the reason i first posted this subject is because UNbonded PT is so rare in bridges (to my knowledge) but i wanted to see what others experience were.

Thank you John.

 

Waht is the main reason the tendons are greased in pre-stress concrete projects? Example like for bridges?
Is it for corrosion or so the cable will not adhere to the concrete in case of earthquakes? I'm puzzled.
Thank you.

 

[Zambo]

In the UK grouted post tensioned structures were very rare for several years. This was because of a poor QA/QC record with the grouting and corrosion of tendons had been found to be a problem. Other corrosion techniques such as greasing, sheaths, sealing of ducts were used which are more expensive but more easily inspected both during and post-construction. Now I understand grouting is again gaining prevelance but QA stepped-up.

Zambo