Increase of Cross Sectional Area of Existing Concrete Column 11

[BONILL]

I am trying to increase the cross sectional area of a concrete column by increasing its diameter from 0.90M. to 1.20M. The current column has a diameter of 0.90M. and a second pour of 0.30M. around the column is to be made. The additional longitudinal steel and stirrups are to be located in the second pour. The longitudinal steel is to be connected to the foundation. The slab above is not yet poured and hence the column is not subject to any superimposed loads at the moment. What could I use to ensure that the two pours work together as one.

 

[ishvaaag]

I would say that mechanical interlock through iintently roughening of the standing surface. Some kind of specialty treatment should identically work well since your ring if tight enough will ensure the effectivity of the interlock.

 

[JAE]

My first thought is you need more than just a rough surface - some kind of pattern of pins or dowels to interlock properly.

 

[enginerding]

I could imagine the roughened surface working. You could probably use a method similar to that of the ACI Shear Friction method. Instead of using the bar yield strength as the force providing the friction, you could use the hoop stress in the rings because these rings would have to stretch for the two pours to "slip" relative to one another.

Some caveats:
1. Unless this has actually been tested to confirm the theory used, you will have a hard time convincing the building official that this is proper - and you will have to use a pretty large factor of safety.
2. You will probably want to increase the number of rings to keep the spacing tight to ensure that the friction surface is properly tight for the full area
3. You will have to address what happens when the outer concrete shrinks and cracks - how does this affect the friction forces and the area used for friction?

These unknowns might make pins or dowels naturally move forward as a better choice.

 

[Teguci]

I am not sure what failure mechanism we need to prevent. With the added confining steel and longitudinal steel, the additional cover will provide the necessary confinement to the core of the column. Although shrinkage will suggest that the exterior overlay will not be as effective in taking axial loads, this is not significant in the long term since creep behavior of the concrete will transfer much of the axial load from the concrete to the longitudinal steel anyhow. The only significant concern is the transfer of stresses from the top and bottom of the column into the added steel. Since the overlay rebar will be tied into the foundation and the supporting structure has not been placed yet, this problem goes away.

I would still consider roughening the existing column, but more for a servicability concern than structural.

If someone else can explain what the failure mechanism will be I'd appreciate it.

 

[JAE]

Teguci, the concern I would have is that in column design philosophy you have the plain axial aspect - no problem here with just adding the donut around the existing - and you have the bending aspect of the column.

The bending is due to either applied end moments on the columns or on the column resisting buckling effects.

In the bending - you must have a contiguous section that behaves as a unit to be able to properly engage the reinforcement in the C = T methodology.

With a donut section outside of an interior section, you would get slippage, and thus the two portions would tend to act as two independent columns taking load only to the extent that their individual sections can resist.

My guess is that the OP is trying to create a new "composite" section and not two distinct columns - one wrapping around the other.

 

[Teguci]

OK, so we check VQ/I for bending on the donut section. We have some shear transfer if we go without roughening the surface and we get more shea transfer with the roughened surface for the full composite section. Since this is a column, bending is probably not the governing design but will need to be considered.

Can anyone else think of another failure mechanism that needs to be checked?

 

[hokie66]

I agree with you, Teguci. Much ado about nothing. I wouldn't even roughen the existing other than to make sure it is clean and moist when the donut is cast. I would double the usual number of ties.

 

[hokie66]

Look at it another way. If the "donut" is there first, how would you prepare the surface for the infill concrete? This happens with spun precast columns, and all you do is to do the best job possible of removing the laitance which settles to one side.

 

[FishTheStructure]

Perhaps you might consider wrapping the column with FRP instead of increasing the cross-sectional area of the concrete. By confining the concrete you could achieve around 3 times the original concrete strength. Obviously this would not help in bending but if you solely have an axial deficiency it may be the way to go.

Just a suggestion.

Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein

 

[JAE]

At least in the ACI, there is no shear transfer that you can count on without rebar crossing the plane of the interface.

Now practically I'd agree that there is some shear transfer, but you can't know for sure how much. I might agree with enginerding above with the suggestion that the hoop stress around the outer donut creates a compression of sorts on the interface. Not sure though.

Yes - perhaps much ado....and the thing is a column, not a beam. But if it is a slender column, or one with significant bending, not sure I'd count on a fully composite section there.

 

[ishvaaag]

Another aspect that we have not touched but is of interest is that now the additional concrete has something other than the formwork on which to bear against shrinkage, the preexistent column, and so the concrete must be moderate to low shrinkage mix. Extremely low shrinkage quite likely won't be recommendable since the addition still will have to accompany the remaining shrinkage of the core column.

 

[DST148]

@BONILL: Ref Structural Renovation of Buildings by Alexander Newman - He has dealt with strengthening of concrete columns by section enlargement.The composite action between the new and the existing concrete is achieved by a combination of bond and anchoring dowels using adhesive anchors. Bonding is achieved by roughening and cleaning the existing surfaces and applying bonding agent.
There is an example of increasing ultimate load capacity of reinforced concrete circular column with FRP wrap. (about 20% increase with 2 plies)

@ishvaag: Like you mentioned in your last post, Dr. Newman notes that for section-enlargement method to be successful, the new concrete must have an extremely low rate of drying shrinkage or be made with shrinkage-compensating concrete. He further adds that preplaced-aggregate concrete usually offers the lowest rates of drying shrinkage.

 

[wardak21]

Using dowels anchored (pasted) with epoxy for shear transfer will do work better if there is concern of huge bending moment, otherwise, wrapping the existing column with FRP would be a better choice.