Camber/Creep Control of Prestressed Beam

[JohnRwals]

Hi!

Once I posted a question about precast concrete double tee camber adjustment.
This time I would like to concentrate on creep effect.

My goal is to make final finish surface as flat as I can like Fig-3.
In order to accomplish this goal, I realized that creep effect(camber growth) should be minimized
after prestressed precast concrete member is stripped.

My idea is to use heavy concrete block (P) to apply temporary load on the beam.
I guess this concrete block can depress/reduce camber growth TO SOME DEGREE.
But, I am not sure what will happen when this concrete block is removed.
Do you think creep effect will resume (UP TO 100%?) as soon as temporary load is removed until topping concrete is poured?
As suppressed creep returns fully, can future creep be strong enough to damage topping concrete?

Please share your ideas and experience to control/reduce creep effect.

Thanks!

JR

 

[JoshPlumSE]

JR -

I'm not a PT expert. My experience is purely theoretical from textbooks and college courses. That being said, I was always taught that you over camber a beam to account for long term creep. So, in that respect you have a beam that looks like Fig 1 when it first goes into service.... But, after a time it becomes flat like figure 3.

Hopefully, there are others who can discuss more practical methods of REDUCING long term creep effects via admixtures or the like. But, I highly doubt that you will be able to eliminate it completely.

 

[KootK]

1) Concrete creep is a pretty complex topic and I can point you to some good texts that cover it if you're interested in really getting into the meat of it.

2) The graph below should give you a flavor of the complexity and the time dependent effects that are involved. An applied load will result in creep over time. Similarly, removed load (essentially a negative load) will result in creep recovery over time. Unfortunately, creep recover is not complete.

3) There are a large number of approximately known parameters that go into the estimation of creep behavior. Mix design, curing practices, load history, ambient conditions, presence of reinforcement.... on and on.

But, I am not sure what will happen when this concrete block is removed.

4) Similar to the graph below, you will have an instantaneous recovery of the elastic deformation associated by the load block. Then, over time, you will also recover some but not all of the creep deformation associated with the load block.

As suppressed creep returns fully, can future creep be strong enough to damage topping concrete?

5) It can certainly result the in the tensile cracking of your topping. The degree of this is, obviously, a function of the parameters involved in the particular problem being studies.

 

[JohnRwals]

Let's assume camber grows to 3" unless any action is taken. (d1 is 3" in Fig-1.)
In order to reduce d1, I apply huge concrete block to make d2=5".
(Again, let's say d2=5" is elastic deformation happened immediately after concrete block is loaded.)
After two months, d2 becomes 6" (1" creep/shrinkage) and concrete block is removed.
Q1: Is there any reasonable way to decide d2 and temporary loading duration which will make beam close to the final position?
Let's say beam is returned to flat position like Fig-3 immediately.
Q2:And then, do you think camber change will continue further and be powerful enough to damage topping concrete?

I guess this is tough topic.
But, if you know any paper or info which helps to understand this problem,please let me know.

Thanks!

JR

 

[Settingsun]

KootK's item #2 is a simple method to estimate the effect: treat removal of the load as an upward load. Analyse for both the downward load of the concrete block's weight and the equal-opposite upward load of removal and superimpose the results. The two analyses use different values for Young's modulus and creep coefficient due to the greater age of the concrete at the time of removing the load. Ideally you will use parameters for your particular beams rather than code estimates.

The earlier you apply the load, the greater the effect.

I don't think the topping is at substantially greater risk if you remove the load before pouring the topping concrete. It was always going to be subject to this ongoing creep. The problem is if the magnitude of the creep is greater than assumed in design, in which case you re-run the calculation with the larger creep.

In the other topic, it was suggested that you might leave the weight on until the topping hardens. In that case, apply the upward load (concrete block removal) to the composite structure. This will model the 'locked-in' camber reduction and the tensile stress on the topping, and the effect on the neighbouring beams.