What may have caused these cracks?

[NathanNZ]

Please see attached PDF with photos and existing drawings and a sketch.

The back wall of this single level large open span building has large cracks in it. The cracks have got larger over the last 5 years. The building is now 35 years old.

The wall is 6m high (19ft) 125mm thick precast panels (5 inches) and is designed to cantilever from concrete cast-in-situ piers/columns. There is a short a infill timber framed wall above.

The cracking only occurs on the one side of the piers which I find puzzling. the cracking pattern may follow either the pier links (300mm centres) or the wall reinforcing (450mm crs). There are 6 piers total along the wall length. The damage has mainly only occurred on the middle 2 piers.cracking is estimated between 1-2mm total.

I have thought of likely causes, but my best guess is combined issue where the construction tolerances, wind movements, and long term corrosion has combined and is becoming evident 30-35 years later.

Has anyone seen cracking like this before?

 

[civeng80]

Could the RC beam and the slab have been poured in two separate pours?

 

[Tomfh]

Shrinkage and weak laps.

 

[NathanNZ]

Civeng80, do you mean the foundations?

If by slab I assume you mean wall and beam you mean column, then yes they were poured in separate pours. Precast wall and cast in situ columns.

 

[NathanNZ]

Does shrinkage cracking in concrete still occur 30-35 years into a buildings life?

 

[Tomfh]

Shrinkage cracks normally appears more quickly, but certainly can worsen over time, as structure continues to shrink, as lap joints continue to be attacked by elements, and thermal and other loads continue to push and pull.

 

[NorthCivil]

I'm going to make a wild guess - these cracks are from corrosion of the reinforcing bar.

thats why the spalling is at regular centres - thats where our hooks are.

I recall learning that all reinforcing will corrode, deeper cover just kicks the corrosion can further down the road

 

[NathanNZ]

NorthCivil, I partly agree, but one thing I forgot to mention is that is looks like the wall is pulling away from the column along the whole vertical length. This could be shrinkage as mentioned...

Do you think it being a 6m high cantilever rather than being propped at the roof has made it worse?

 

[Tomfh]

agree that corrosion has likely compromised the lap joints. It’s a fairly flimsy connection.

 

[Agent666]

My guess is shrinkage. That level of horizontal reinforcing is pretty low, only just enough for the temp and shrinkage requirement in the NZ concrete code, and that assumes its free to shrink which it clearly isn't here in this configuration. So instead of getting distributed cracking intended by the code, the shrinkage is all concentrating/accumulating at the insitu joint to the column resulting in things trying to pull apart. The fact it only occurs on one side of the column is consistent with this view, this is the side that gave up first. The cracking could have happened quite some time ago, some of the painting seems to suggest its been painted over perhaps in the past when it wasn't quite so bad.

You can see how part of the column concrete that have spalled are still bonded to the panel face. Sometimes with quite a displacement laterally by the looks of it (10-20mm?). My concern would be if its essentially gotten a crack the entire depth of the wall thickness going partly though the columns (as evidenced by the external view suggesting a similar chunk of concrete has spalled but working into the column), and the uncertainty of how the wall horiz bars are retained by the column cage (or whether they even lap adequately with the adjacent horizontal bar from the next panel). The question to ask is if the horiz bars go outside the column cage as per your sketch, then by how much are they really hanging onto the unreinforced cover concrete of the column given the connection seems fairly compromised at least locally in a few locations?

It would seem prudent to suggest some steel angle brackets bolted right through the wall to at least ensure the wall panels are secured to the columns side face if a fix is warranted.

The other question to ask, given you are in NZ (as am I), is could it possibly be Earthquake damage from recent Christchurch or Kaikoura earthquakes (you didn't say where the building is located)?

Does the site have expansive clays? If there was some development next door that might have dried out the ground or similar, it could cause a larger seasonal shrink/swell cycle that could cause more movement for example?

 

[NathanNZ]

Agent666, thanks for your reply, very helpful.

It is in the Bay of Plenty on low lying silty soils. Possibly differential settlement could have caused this but the rest of the building is in reasonable condition and the shrinkage and corrosion does seem to match the issues.

Hopefully the wall reinforcing does actually come on the inside of the column main bars, although it makes things a lot tighter to squeeze in and achieve cover. I might get that investigated to help close the job out before going straight to steel angles as it may answer a few items.

Agree that it looks like it needs structural repair, rather than just a good sealant.

 

[Agent666]

Yeah you could easily get it scanned to see where you are positioned with the horizontal bars inside/outside the column cage.

I'd imagine even if you do a proper structural repair to get rid of all the spalled concrete chunks on the inside and outside that there is still a possibility of it simply occurring again if its some mechanism other than shrinkage. You can certainly put some crack monitoring gauges on the cracks to see if they are moving still.

One way to alleviate this occurring again might be to put some shallow vert saw cuts in at the column edge on the outside after repairing the spalling and fill them, so if it wants to crack it at least cracks there and still maintains the weather seal via the sealant you use to fill the sawcut.