retrofitting column with vertical hairline crack 3

[Ca_87t]

This round column measuring 12 inches diameter (0.3 meter) has 8 pcs of 20mm rebar with 10mm spiral ties spaced 8 inches (0.20 meter) apart.

There is a vertical hairline cracks measuring about 1 meter. I'd like to know the following:

1. Usually what caused the vertical hairline crack?

2. Can the rebar inside rust if nothing is done to them?

3. Should carbon fiber be wrapped around it?

 

[Agent666]

Get a crack width ruler as well, you cannot generally appreciate by eye if you have a 0.3mm or 0.5mm crack unless you have a ruler to compare against. The different between what might be acceptable from a durability standpoint and what might not is quite subtle (provided it just an isolated crack and nothing deeper going on once you hit the rebar).

As a quick reference point I've learnt/compared that the tape I carry has divisions on it that are about 0.3mm as a useful comparison when out in the field and I have not taken my crack width ruler with me. Depending on your tape this might vary but I've found it is a good reference point (just need to remember my tape more often).

 

[Ca_87t]

The hairline crack is only 0.05mm thickness (thickness of a paper).

Minimum thickness before one can inject epoxy resin must be at least 0.1mm

The column is only 1 foot across, I can't chip further without damaging it and decreasing the axial or bending capacity.

So if you can't inject epoxy resin, and it's just regular crack caused by shrinkage or movement. Can you let it remain that way?


Usually for corrosions of rebars.. how wide is the minimum crack gap caused by this? It would take much more chipping to reach the rebar and this would damage the column already.

 

[Agent666]

I doubt you are compromising the integrity of the column at all, all of the cover could fall off and you'd still in all liklihood be ok, also you intend to repair what concrete is removed with a proper structural repair in any case I bet.

Like Ingenuity noted, you need to find the root cause, at least confirm the reinforcement is intact/not rusted, etc. It look much wider than 0.05mm, but maybe the photos are not doing it justice. I think I'd stuggle to see a 0.05mm crack, but probably the paint and grime thats accumulated make it look wider in the exposed face.

For the last point you are thinking about it back the front, if the rebar is already corroding it doesn't matter what the crack width is, its already corroding. You cannot generalise "something corroded and produced a crack width of 'x' and is it still ok?". Clearly you have failed your durability limit state at this point if the rebar is being compromised. If you have a flexural/shrinkage crack without evidence of corrosion/spalling then check your code for acceptable crack widths for the environment to answer your last point. Generally up to 0.3mm is acceptable, though can be larger in certain conditions (maybe up to 0.5mm) without the code saying you have a durability issue for a 50 year design life. A 0.3mm crack is surprisingly large when you see it.

 

[Ca_87t]

Ok. I used a digital caliper meter to measure the gap. It's about 0.35mm crack gap.

If I chip into the rebar. This would result:

Imagine the column is bending to the right and there is a chipped area (in red), the concrete radius would be like 6" - 1.5" = 4.5 inches only instead of 6 instead. This can already affect the strength.

Is there other way to know without chipping it?

How about this. Since the gap is 0.35mm. I can inject epoxy resin. Then after that I'd put carbon fiber wrap around it. So even if there is corrosion inside, the carbon fiber can compensate?

 

[Ingenuity]

Is there other way to know without chipping it?

If it is corrosion, you could use half-cell corrosion cell and check the potential of corrosion (more than -350 mV has increased potential for corrosion) but you still need to attach to the rebar to complete the circuit, so chipping involved.

How about this. Since the gap is 0.35mm. I can inject epoxy resin. Then after that I'd put carbon fiber wrap around it. So even if there is corrosion instead, the carbon fiber can compensate?

Depends on how you place/orientate the CFRP. Vertical layers may enhance the flexural capacity (depending on how it is detailed), and circumferential wraps will provide confinement (and some enhancement of compression capacity), BUT if there is ongoing corrosion, and you do NOT encapsulate the column surface area such that oxygen/moisture can continue to enter, then corrosion will continue, and the expansive forces due to corrosion are significant.

Solely using epoxy injection to repair concrete cracks to corroding concrete elements is short sighted. Concrete will simply crack adjacent to those repaired cracks, and concrete spalling will result.

Best to determine if is or is not corrosion, then formulate a repair/strengthening plan.

It will take 15 minutes to continue controlled chipping of the column face and expose the longitudinal/transverse rebar rebar and determine if corrosion is the cause.

 

[BridgeSmith]

If your column is so close to its capacity under typical 'in service' loading that you can't chip out enough of the concrete to get a look at the rebar, the column is structurally inadequate and needs strengthening or replacement. I doubt that's the case, though, so unless you have a major seismic event or the biggest snowstorm ever while that little piece is chipped out, you shouldn't have an issue. As Agent666 noted, columns are typically designed to function adequately with no cover.

 

[bones206]

If corrosion is the cause and you really don't want to chip, Sika makes a surface-applied corrosion inhibitor called Ferroguard that gets absorbed into the concrete and slows down the corrosion of the rebar. I've never used it myself, so not sure how effective it would be. And you would probably have to inject the crack to seal it as well. A local Sika rep might be a good resource to see what technique would be most viable.

You could also chip off the cover concrete, treat the exposed rebar with corrosion inhibitor, then patch the cover back on with a repair mortar. But as others said, you should do some more chipping to determine the root cause first. Corrosion seems likely, probably exacerbated by the plaster coating, but I'm in agreement that your first step should be to expose the rebar to either confirm or rule out corrosion.

 

[Ca_87t]

Depends on how you place/orientate the CFRP. Vertical layers may enhance the flexural capacity (depending on how it is detailed), and circumferential wraps will provide confinement (and some enhancement of compression capacity), BUT if there is ongoing corrosion, and you do NOT encapsulate the column surface area such that oxygen/moisture can continue to enter, then corrosion will continue, and the expansive forces due to corrosion are significant.

Solely using epoxy injection to repair concrete cracks to corroding concrete elements is short sighted. Concrete will simply crack adjacent to those repaired cracks, and concrete spalling will result.

The carbon fiber wrap would be circumferential to serve as replacement for the spiral ties. When you guys mentioned the rebar corroding. Do you mean the longitudinal rebars or the spiral ties?

When you mentioned "BUT if there is ongoing corrosion, and you do NOT encapsulate the column surface". Do you mean chloride ions getting into the rebars? But the house is very far from the ocean. Or do you mean normal air or moisture can get into the concrete capillaries and still corrode the rebars? Why do I have to encapsulate the entire column surface area? Did you mean putting epoxy coating to the entire column surface?

Or is full epoxy resin injection enough to seal all cracks and no moisture would get in anymore through the concrete itself (provided no chloride salt)? If none. Then after full epoxy resin injection then will put full circumferential carbor fiber wrap from top to bottom. I guess this would solve it?

By the way. The column is constantly exposed to rain when it's raining and we commonly encounter magnitude 6 earthquakes.

 

[Ingenuity]

IF it is corrosion (if, this has not been established yet!!!), and you are far from coastal chlorides, and it rains significantly (like you stated) then the source of corrosion is constant wet/dry cycles permeating the pores of the cover concrete and given sufficient time, the steel rebar (both spirals and longitudinal) will no longer be passivated (protected by alkaline environment of the concrete) and corrosion begins.

It has been a long-held belief (especially by experienced concrete repair specialists) that to cover concrete elements with already corroded reinforcing steel with an exterior jacket of some kind and NOT repair the pre-existing corrosion damage, that corrosion may continue to occur. There has been some research where wet-layup CFRP or fiberglass jackets have been used to corroding elements withOUT repair and let time see what happens - results have been varied. There is some risk with encapsulating a non-repaired column if the corrosion damage is extensive and you have therefore masked the problem. Sacrificial anodes are often used (in combination of encapsulating jackets) so that if there is ongoing corrosion the anodes with sacrifice rather than the steel reinforcing (it will be the cathode).

In your case chip out a small 3" x 3" probe at the intersection of lateral hoops and long't rebar and see what is present, then formulate a fix, if required.

You could also do a phenolpthalein test of the probe to check for depth of carbonation.

 

[Agent666]

There are two mechanisms that lead to corrosion Carbonation of the concrete, and Chloride Ion ingress into the concrete. Look up both to get an idea of the differences.

If you don't suspect chloride ions due to the distance from the coast then the carbonation depth of the concrete is fairly easy to test for if you do some cores, (they put a chemical phenolpthalein on it and it turns pink to show the depth at which your concrete has in simple terms sufficient passivation effect left as mentioned by Ingenuity).

There are products in which you can apply to the surface which will provide additional protection from the environment if you note a carbonation problem but no reinforcement corrosion yet (silane based from memory, but I'm sure there are others?).

 

[Ca_87t]

But the ties are distant 8 inches apart. So how do I find where the ties would intersect the vertical crack? maybe with a metal detector? What kind of metal detector could pinpoint the ties so accurately?

And it would even be more difficult to find cracks that would intersect the longitudinal bar. And more hard to find where the longitudinal bar and ties would intersect! The house owner said the column has been exposed to rain for the past 20 years so passivity (rebar alkaline cover) would likely be affected or degraded.

 

[Ingenuity]

Phenolpthalein indicator test on a spall of concrete after a 'fresh' fracture:

We often test for carbonation with phenolpthalein indicator direct from the 'fresh' probes - saves having to do core samples and split them to face a 'fresh face'.

 

[Ingenuity]

But the ties are distant 8 inches apart. So how do I find where the ties would intersect the vertical crack? maybe with a metal detector? What kind of metal detector could pinpoint the ties so accurately?

And it would even be more difficult to find cracks that would intersect the longitudinal bar. And more hard to find where the longitudinal bar and ties would intersect! The house owner said the column has been exposed to rain for the past 20 years so passivity (rebar alkaline cover) would likely be affected or degraded.

Use a pachometer (like a metal detector, but more $) or ground penetrating radar (GPR) to accurately find rebar - both transverse and long't.

It is most probable that vertical cracks approx line up with the long't rebar, and transverse cracks approx line up with the hoops. Start there and see what you get.

 

[Ca_87t]

This is the original structural plan the owner gave me:

The column (blue arrow) is carrying a 5.4 meters long big beam carrying the house balcony and another beam perpendicular to it.

Just wondering. In your design. What do you do when the column is exterior and exposed to rain all the time. Do you just put elastomeric waterproof paint on them or do you avoid having columns exposed to rain at all?

 

[Ingenuity]

What do you do when the column is exterior and exposed to rain all the time. Do you just put elastomeric waterproof paint on them or do you avoid having columns exposed to rain at all?

Provide low w/c, high durability concrete, maybe add in corrosion inhibitor, increase concrete cover (sometimes that means making it larger, especially a 300 mm dia column), and provide a protective membrane, if necessary. The problem with protective membranes is that they have to be maintained/replaced every so often by the owner.

Some go with epoxy coated rebar, or MMFX, or stainless, or galvanized rebar, but not especially common, particularly in a residential setting.

 

[Ca_87t]

The problem with chipping it is how do you replace the small removed concrete with the same strength as the original and binding to the remaining or rest of the concrete?

Imagine there is huge moment to the right and the neutral axis is toward the right. Even if you plug cement or small concrete into it. The part put would just eject out during the bending. Unless you can bond the plug as strong as original? It will still be weaker than the original, right? How do you make it same strength as original?

It's very small column just 1 foot across so removing even 1.5" could affect the strength.

In case it is affected negatively. I need to be prepare to find ways to retrofit the entire column if the chipping damaged the column. What do you do when the column has big hole from chipping or other damage. How do you repair it in this case? Want to know before doing any chipping. Thanks very much.

 

[Agent666]

Talk to Sika or someone similar like a contractor who specialises in this type of repair in your area. Its done all the time and if done correctly the repair is probably better than the original concrete. Don't patch it with some sand and cement, or even high strength grout, instead use a micro concrete specific to this sort of repair.

If a fix is required, just do it correctly, the strength of the column will be same as before the repair for all intents and purposes. I hate to say it but you appear to be are overly worried about something that does not require worry. Its only a house, not a 120 storey building where load issues might be more of an issue if you remove a significant amount of the concrete.

When they repair it if a repair is required don't be there, as you may freak out over the amount of concrete they remove! They need to get behind the bars for two reasons, to remove any corrosion and treat the bars and it helps to ultimately retain the concrete repair. If its load critical, provide propping to maintain the service level loads.

 

[Ingenuity]

You can always do a check on M-N interaction diagram and see what effects a void has on section capacity.

Here is a conservative comparison of 1] NO void; 2] COMP void; and 3] TENSION void (assuming no rebar in void too):

Just do NOT do this unless you shore the floor framing:

 

[Ca_87t]

Has anyone actually tried to repair such column smashed down to the core. Lol.

If anyone does. How did you do it especially when it was the ground floor column of a 50 storey building? Just curious.

 

[RHTPE]

How did you do it especially when it was the ground floor column of a 50 storey building?

Like any other design problem - You evaluate the loads, determine what, if any, capacity exists in the remaining core concrete, then devise the means by which loads can be temporarily transferred around the affected column. This is what engineers do.


Ralph
Structures Consulting
Northeast USA