Problem of cracks in RC columns

hamza41 · May 1, 2017

Hello,
The structure is a space frame supported on 15 RC columns (3x5). Columns height is about 12 m, their diameter is 60 cm. 5 columns, on the same axis, present vertical cracks in their mid-height in addition of a visible deflection.
A site visit will be arranged in the comming days and rebound hammer and cover meter tests will be performed. We suspect a lack of concrete resistance or cover.
Your opinions and suggestions are welcome.
Regards.

hokie66 · May 1, 2017

Quite a spindly structure, including the footings, columns, and space frame.

Based on the condition of the space frame, I would say the environment is quite severe.

I would check for carbonation depth in the columns. Carbonation to a significant depth, leading to corrosion of the reinforcement, would be my guess.

The cover meter would be useful, but forget the rebound hammer. That won't give you any useful information.

Ingenuity · May 1, 2017

Agree with hokie66.

Chip off a cracked/spalled piece of concrete, break it in two to expose a 'fresh' surface and use phenolphthalein indicator solution to check for depth of carbonation.

If the structure is in a coastal environment you may also have increased chloride content, and it is easy to take samples and get them lab tested.

If it gets more 'complicated' you could take cores samples and do petrographic analysis/testing.

SlideRuleEra · May 1, 2017

During construction, the columns appear to have been jump formed. Notice in the marked up photo that cracks more or less stop at the residual form marks. Compare test results from all sampled areas on all five columns - there may have been problems with specific batches of concrete, contractor's means and methods on certain days, weather conditions, etc. during construction.

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hamza41 · May 1, 2017

Thank you for your answers.
Indeed, the structure is in a coastal environment is Saudi Arabia.
We thinked about coring samples and performing resistance, carbonation and chloride and sulphate content tests , but we were afraid this could cause damage to the columns since the cores are 100mm in diameter and up to 300mm long for only 600mm of diameter fo the column.
The problem is that the cracks are located only in the mid-length, between construction joints, for only 5 aligned columns, we think the quality of the concrete mix for these parts is pour. Is there any possibilities of repairs if carbonation or chloride content is proven ?
Thank you.

hamza41 · May 1, 2017

Indeed SlideRuleEra, we first suspected the concrete batch in these parts.

JoelTXCive · May 1, 2017

I took an FRP class and this seems like a perfect circumstance for confining the columns with an FRP wrap....But I don't have experience with the carbonation and chloride damage. Is that a chemical process that can be stopped, or once it starts; it just keeps going?

JedClampett · May 1, 2017

Or, rather than some high tech magic, you could shore the structure and repour the columns.
The space frame also looks to be in bad shape. I see peeling paint in your pictures and I suspect it is corroded. Maybe it's time to price a new structure and replace.

hamza41 · May 1, 2017

I forgot to tell you that the space frame will be replaced. The columns will have to withstand the reactions of the new one.

MrHershey · May 1, 2017

Tend to side with Jed on this one. May want to price out just replacing the columns if this is a widespread issue. It very well may be cheaper than tests plus repairs (assuming repairs actually address issue rather than just hide it), especially since you'll already be mobilized for the space frame replacement. And in the end you have brand new columns, not refurbished ones.

dik · May 1, 2017

How was it poured and formed? Are the joints shown 'cold joints'; cracking seems to indicate that.

Really spindally, what type of bracing? Looks to be too recent for carbonation to be an issue. Doesn't look like a rebar problem, but, if near coastal areas, you should have a couple of inches of cover and maybe HDG rebar.

Dik

Ingenuity · May 1, 2017

hamza41 said:
but we were afraid this could cause damage to the columns since the cores are 100mm in diameter and up to 300mm long for only 600mm of diameter fo the column.

Why do you need such large dia core samples?

2" dia cores for a 4" depth will usually be satisfactory for Cl^- testing and petrographic analysis. 2" cores can be sometime used for compression strength testing too after you factor in H/D ratio and use adequate test capping.

dik · May 1, 2017

I would think that 2" would be OK for some testing, but not compression testing... too small a sample to be meaningful. The columns are too small to take bigger cores, but could be patched. Rebar locator is essential.

None of this leads to 'why the cracks formed'; the procedure for construction has to be reviewed.

Dik

dik · May 1, 2017

I was just looking at the web members. They are very slender and thin, and are exhibiting substantial corrosion already. Whatever coating system appears to have failed. The columns may be the least of your problems.

Dik

hamza41 · May 2, 2017

Thank you all.
The structure was constructed in the 1980's, unfortunately, we don't have any information about its construction procedure.

Ron · May 2, 2017

Agree with hokie66....look at the concrete. A small core can tell a lot with proper evaluation, but can be misleading as well. I would shore the structure and do some destructive observation and sampling. Take as large a core as possible in multiple locations. It takes quite a bit of prepared surface area to do a proper voids assessment with petrography. As dik notes, a larger core is also better for strength evaluation. Chip away around rebar to assess condition directly.

dik · May 2, 2017

If the structure is that old, carbonation may be an issue that should be checked.

Dik

rapt · May 2, 2017

Agree with MrHershey, it would probably be better to replace the columns as well if you are replacing the spaceframe!

MacGruber22 · May 2, 2017

If that space frame was not being replaced, I would spend some effort testing with the hope to limit to selectively repairing. But, since that frame is being replaced, I wouldn't bother. Spending all that money to replace the frame and not spending the extra effort to provide a long-term solution for the columns sounds like a waste.

One in the hand is worth two in the bush.

hamza41 · May 3, 2017

You are right, the columns replacement may be the solution to adopt, I'm expecting it, at least for the defected columns, and may be from the level of the cracked concrete. But the owner seems to refuse this solution, we may have to make an effort to convince him.

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