Reinforcing unreinforced column

[patelam]

The concrete column, 32"x 32", approx 8ft tall is supporting mechanical equipment (condenser). The are four columns such as this, supporting equipment. Each of these columns are cracked at top 1ft, some of them has 4" deep (horizontally)concrete chunks spalled off without exposing any rebars. The rebars are supposed to be 3" deep as per drawings. On scanning the column faces, it is found out that top 1ft of all columns has no rebars. The forces acting on columns are vertically downwards, no horizontal forces as the base of equipment has bolt holes larger than bolt dia to allow for movements.

I am looking for various options to reinforce the top 1ft portion of unreinforced concerete of column:-
1. Wrapping FRP laminates. ACI 440.2R can be used to design the externally bonded FRP layers
2. Adding a concrete jacket around unreinforced concrete of column with vertical rebars and ties in it. I don't think so that the concrete inside the jacket will behave the same way as the reinforced concrete behaves. I think the concrete in the core will not have its strength increased from the jacket around it. The jacket will be effective only after the concrete core breaks further and exerts any force (latera) on the reinforcement in jacket. Is there a paper or standards that I can refer to design the concrete jacket?
3. Adding a steel jacket around unreinforced concrete, forming a steel tube with concrete inside. I have same questions as I have in the option of concrete jacket. Would steel jacket be effective before the original unreinforced concrete fails? Also, do we have paper or standards to design steel jacket?

Can you suggest any other option to reinforce the column?

 

[lexpatrie]

What about cutting the busted up part off roughening to 1/4" amplitude, removing laitance, and re-doing it with dowels and epoxy into the existing and re-doing the bearing plates with headed studs or something similar, (2) #3 ties in the top 5", etc.

As to #2 - I'm not aware of any publications on that subject, but it's not my main focus of study.
As to #3 - again, not aware of any publications.

 

[KootK]

If it's only the top 12" of the column that is unreinforced and you are confident that the only significant load is vertical, then I'd give some careful thought to whether or not this actually needs to be repaired.

I've seen lots of similar damage in heavily lorded precast columns that were reinforced all the way but had inadequate bearing pad designs. Some manner of Poisson ratio voodoo causes the perimeter to crack and spall. I would patch on some repair concrete but that was really just a cosmetic thing. In a sense, the spalling is just allowing the bearing surface to redistribute the bearing stresses in a stable manner.

Of course, it's a very different ball game if you've got significant lateral loads running through your anchor bolts.

 

[patelam]

Thanks for the response.

@lexpatrie - The equipment cant be taken off from the columns. With equipment on columns, it is exerting the vertical loads on it. The cracks and spalls in top 1ft of column, I believe, is the result unreinforced concrete not being able to take the bearing loads. Concrete being unreinforced, it is acting as a brittle material in compression. I doubt, that roughening or drilling dowels to the concrete will develop more cracks and spalls to the concrete, or even burst the concrete. This will settle the equipment which we dont want it.

@Kootk - If I dont do the repair, I am afraid that the load may further crack the column and may settle the equipment which is what we are trying to avoid.

 

[KootK]

I am afraid that the load may further crack the column and may settle the equipment which is what we are trying to avoid.

I don't believe that you need to fear settlement. Unless I misunderstand the extent of the damage, I suspect that the damage that has occurred will be all that will occur.

Have you done a plain concrete bearing check on the base plate? Concrete is pretty skookum stuff in compression, with or without the rebar.

 

[a_urbs]

I don't think the jacket philosophy is a bad idea. The spalling occurs because there's no confinement at the top of the column. If you could somewhat shore the condenser bearing point (somehow) and take some load off the column, you could go for a post-tensioned collar approach. It's quite an aggressive approach but I have seen it done.

 

[patelam]

@a_urbs - Can you elaborate little bit on post-tensioned collar approach? Is it the steel jacketing post-tensioned around the column?

 

[apsix]

As a_urbs noted the lack of confinement is allowing the spalling. The attached shows a method I've used to provide post-tensioned confinement. In your case, due to the size of the column, extra confinment could be provided with threaded rod through the column, located midway to each face.
Regarding horizontal load, unless there is a frictionless slip plane the oversize holes won't prevent the transfer of horizontal load.

 

[TLHS]

Key question for me is what the lateral loading condition is? I have more concerns if this is an area where there's significant seismic demand, as the lack of reinforcement could lead to significant loss of material if there's an appreciable shear load in this zone that hammers in cycles. Same question about the anchorage. Does it need to fully rely on transfer into stirrups/ties to make sure there's a lateral load path without breakout?

Without those two concerns, things are pretty relaxed and I'd mostly be trying to mechanically retain the concrete so it can't escape and would be less worried about replicating the load transfer requirements of rebar or tensioning it.

a_urbs, what's the concern about needing to reduce gravity load prior to installing that form of collar? Are you worried about overtensioning potentially cracking the column and a potential failure if it's under load? that feels like something you could work around especially since you likely need a pretty nominal tensioning for this specific case.

 

[a_urbs]

@TLHS: Just my thought was if you could partially unload the column, install the jacket nice and snug, then reload the column. I feel like you would get better load-sharing as now the additional load will engage the jacket as opposed to trying to tension it under the full loading to get full engagement.

 

[canwesteng]

In this position I've jacketed with a reinforced concrete per your option 2. Highly unlikely the concrete is working very hard, so not having a design guide to go by shouldn't matter. The lack of reinforcement can cause all kinds of problems with unrestrained shrinkage cracking - the cracking you see may not be caused by bearing but could have significantly weakened the column.