beam creep

[WOZnTR]

How many years or decades before a beam can creep significantly and big is the strain difference between original and decades later (let's say for a typical beam of 500mm) depth?

How would the rebar react to the creeping concrete. Would it debond or would rebar creep too along with the concrete?

 

[SLTA]

Have you read your concrete textbook? Or asked your professor? Try there first.

 

[rb1957]

would concrete (loaded in compression) creep ? isn't creep a tension phenomenon ?

Quando Omni Flunkus Moritati

 

[BAretired]

Tensile creep and compressive creep are approximately equal. A flexural member stressed primarily in compression will have an initial camber after initial stressing. The camber will grow as the member ages notwithstanding the loss in prestress force unless there is enough dead load to offset the camber. Sometimes top strands are placed in a simple span member in order to control camber.

BA

 

[asixth]

The creep curves given in the Australian code asymptote at 30 years and the prodominate creep zone occurs within the first year (around 80%).

 

[itez]

What's the strain of this creep?

A carbon fiber reinforced polymer has strain of 1.5% and 4000 MPA. If a beam 500mm in depth creeps. Would it be enough to make the CFRP ineffective and almost invisible 20 years later??

 

[asixth]

If it was applied to the compression face of a beam or on a prestressed section which is still to undergo some creep deformations than it would. I don't recall a frp product having a rupture stress of 4000MPa but there may be a new product on the market, I remember the rupture stress of frp being between 1000-1800MPa.

 

[hokie66]

rb1957,
Concrete does creep under compression. It cracks under tension. You are probably thinking about creep of metals. Steel doesn't creep due to load, but it creeps at high temperatures.

 

[IDS]

What's the strain of this creep?

A carbon fiber reinforced polymer has strain of 1.5% and 4000 MPA. If a beam 500mm in depth creeps. Would it be enough to make the CFRP ineffective and almost invisible 20 years later??

You need to do the numbers and check it. If you don't know how to do the numbers you need to find out. It's not something that you can make general statements about.

You should also be aware that creep projections over long periods of time (30+ years) in codes are often highly unconservative, so consider the consequences of failure, and where appropriate add your own degree of conservatism.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

Hokie66,

Slight correction, Steel does creep under long term high tensile stress. Relaxation in prestressing steel is tensile creep. Normal reinforcing bars cannot be used as "prestressed" steel because their creep under high long term tensile stress is far higher than that of prestressing steels.

 

[hokie66]

Thanks, rapt. Guess I was thinking just about structural steel and reinforcing bars where the stress is only in the 200 MPa range, if that.

 

[trilinga]

Rapt
I am not clear about your statement 'Relaxation in prestressing steel is tensile creep'.
Please correct me if I am wrong.
From my basic knowledge of Material science, Creep and relaxation are two different phenomena.
Creep is the continued deformation under sustained constant laoding and relaxation is the loss of stress under sustained constant deformation.

The loss of prestress in steel wires is not due to creep but due to relaxation.

 

[trilinga]

The two phenomena are explained through a spring-dashpot model with the spring and dashpot connected in series for one case and in parallel for the other.

 

[IDS]

"Creep" and "relaxation" are different names for the same general phenomenon, applied under slightly different conditions. The name applied to the general phenomenon is "creep".

In any case, in any real structure neither the stress nor the strain is constant. The creeping concrete has changing stress, and the relaxing strand has changing strain.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/