Slipform concrete core disintegrating..

[MIStructE_IRE]

I’ve recently heard of a 10-15 storey tower, which thankfully I’m not involved with, but the cores were constructed using slipform concrete and now each horizontal joint in the core is effectively disintegrating!

I’ve heard that the concrete at each joint is turning to a chalk like material and can be easily raked out by finger! The contractor is now looking at raking it out and grouting it up. The vertical bars must be doing some serious work!!

It sounds to me as if its a chemical reaction between the retardant and whatever admixtures are in there to aid the slipform mix. But that’s just my guess.

Has anyone ever heard of this before? I’ve never understood this to be a typical issue with slipform core walls.

 

[Ron]

MIStructE IRE.....not common. Does the core have waterproofing issues by chance? Have them core through the area with as large a core as practicable and then do a petrographic examination of the core. If they can dry-core, it would be better, but if not, at least get the sample. Make sure the petrographer has a lot of concrete experience, not just rock experience.

Remove some of the soft residue and have chemical analysis done, preferable x-ray diffraction. Also have FT-IR done to check for organic influence.

 

[JAE]

Is this "slipform"?
Or "Jumpform"?
With slipform there are not joints as the forms are "slipped" upward as the concrete placement occurs.

 

[MIStructE_IRE]

I wish I could Ron. Its a problem with a project in my country that many in the industry are now aware of. I have no involvement however.

I’m just curious to see if anyone else has ever even heard of such a problem?

Its slipform. But I believe this has happened at each day-joint.

 

[msquared48]

Can you post any pictures?

How many horizontal joints are affected presently?

Sometimes metals of the same type, but of different casting lots can have dissimilar dielectric constants that encourage electrolysis, but that would not be very likely ... unless it is Chinese steel...?

Mike McCann, PE, SE (WA, HI)

 

[MIStructE_IRE]

I’d love to get some pics but haven’t seen any myself yet.

 

[oldestguy]

What is the schedule for vibration to the concrete,if any?

 

[hokie66]

Hard to say, but my suspicion is that when using slipforms, but stopping at the end of a day and after a substantial height of wall has been placed, the laitance was not removed. Laitance is soft and chalky. Probably not the concrete, but rather the construction procedures.

 

[JLNJ]

Maybe it rains every night and they don't cover the joint.