According to a couple of recent threads, just encase in epoxy! Wouldn't recommend that approach!!
First, determine cause of delamination. Is it rebar exfoliation/ expansion due to corrosion? Is it freeze-thaw? Is it reactive aggregate?
Once you've determined cause of issue, you can then decide repair method.
Assuming nothing insidious, chip all loose material, dowel into column, prep exposed rebar and gouge behind for keying, provide small rebar cage outside existing, form and place new concrete. Give plenty of clearance for proper placement and embedment. Make sure new concrete is dense and will reduce permeability to protect rebar.
column is not heavily loaded. i'm not sure but i guess it's the rebar/ties corrosion causing spalling or maybe because it's spiraled like baretired said. i have a few of these columns but this is the most severe. this is inside an old chemical plant.
the concrete appears to be in very poor condition, which might be explained by chemical exposure or poor consolidation or ASR? or just lots of water or humidity?
the column is very slender, bending or lateral force could cause the spalling. was there an earthquake? or did a forklift run into it?
That would be one idea, Mike...remove all concrete outside the spiral reinforcement, then wrap the thing with wire mesh and use shotcrete properly anchored to the concrete.
First thought is spalling due to porous concrete allowing corrosion of the steel. But it is hard to tell from the photos how much corrosion has occurred. The concrete looks very porous, sort of like concretes I have seen which had beach sand, shells, etc. as aggregate. I think this requires shoring, removal of cover concrete, and enclosing the column including new reinforcement.
Looks like corrosion... Matter of shoring and careful hydro demolition to remove the concrete for an inch or two beyond the rebar. A cementitious patch should be applied.
Looks like an aggregate size vs. spiral pitch issue during the original pour, the cover doesn't look like it has much aggregate. If this is the case, repair alone, is not going to cut it. I would look into additionally fiber wrapping this column in its entirety and any other columns that show similar signs of distress.
what does the concrete inside the cage look like? Is it cracked? Loose? if it is cracked it may give an indication of ASR, DEF, or something similar. If the concrte inside the cage is fine, rebar corrosion may be more likely.
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