Camber Growth of Pretensioned Beams

[JohnRwals]

Hi!

Some people say precasters have problem when they erect two-week old piece adjacent to two-month old piece due to differential cambers.
I think camber does not grow much because these are generally steam-cured and f'c and Ec are not much different between two-week and two-month old pieces.
Can anybody share info/experience related with camber growth history of precast concrete double tees or bridge girders?

Thanks!

John.

 

[slickdeals]

Depending on where it is, "sun cambering" could be an issue. We used to have that problem on prestressed precast beams in South Florida.

 

[dik]

new term... not heard of it 'sun cambering'. I don't think it gets warm enough here... due to restraint? or desiccation of top surface?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JohnRwals]

Thermal bowing could be an issue...

 

[Settingsun]

It can be all over the shop for things cast a day apart. Here's a link with some measurements. I had similar issues (2:1 range) on another project, but the range was more like 40-80mm from memory.

https://www.google.com/url?sa=t&sou...FjAAegQIARAC&usg=AOvVaw3Fty1avtJpPDFu7wQUMcfk

 

[JohnRwals]

steveh49,

What did you try to show? I cannot see anything...

 

[rapt]

About 50% of Creep deflection could would occur in the first 2 months.

With the very high compression stresses in the bottom of pretensioned precast bridge beams before assembly, that would give very significant extra upward camber in that time.

 

[IDS]

Also shrinkage has a significant effect on camber.

If the top flange is in tension close to the cracking stress, combined shrinkage and differential temperature can have a very significant effect on deflections.

Differential temperature can have a significant effect in the summer just about anywhere.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

Shrinkage by itself would actually reduce pre-camber over time.

If the top flange is very thin compared to the remainder of the section, it will shrink faster. It is only if differential shrinkage between the top precast flange and the remainder of the precast member is sufficient to cause enough tension in the top to crack the top face that shrinkage would cause upward deflection.

On the other hand, creep could easily cause a 50% or more increase in precamber over 2 months depending on how highly compressed the compression face is.

 

[Settingsun]

JohnRwals, this paragraph and figure show the main point I was making. Identical or near-identical girders in terms of design, concrete mix and construction, but quite different hog that's not due to age difference.

 

[dik]

There used to be issues of using 8" HC for long spans... in the order of 32'; they are near max moment and over time, they generally moved upwards. Is this the same type of issue?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik