Miami Beach, Champlain Towers South apartment building collapse, Part 15 32

[SFCharlie]

A continuation of our discussion of this failure. Best to read the other threads first to avoid rehashing things already discussed.
Miami Beach, Champlain Towers South apartment building collapse
Miami Beach, Champlain Towers South apartment building collapse, Part 02
Miami Beach, Champlain Towers South apartment building collapse, Part 03
Miami Beach, Champlain Towers South apartment building collapse, Part 04
Miami Beach, Champlain Towers South apartment building collapse, Part 05
Miami Beach, Champlain Towers South apartment building collapse, Part 06
Miami Beach, Champlain Towers South apartment building collapse, Part 07
Miami Beach, Champlain Towers South apartment building collapse, Part 08
Miami Beach, Champlain Towers South apartment building collapse, Part 09
Miami Beach, Champlain Towers South apartment building collapse, Part 10
Miami Beach, Champlain Towers South apartment building collapse, Part 11
Miami Beach, Champlain Towers South apartment building collapse, Part 12
Miami Beach, Champlain Towers South apartment building collapse, Part 13
Miami Beach, Champlain Towers South apartment building collapse, Part 14

SF Charlie
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[zebraso]

And there is zero zippering in that photo. That steel didn't shear so much as crumble. and what accounts for the color of that concrete,err.... what contaminant? Let's conjecture.

Edit: That discoloration could be a long standing crack that had been infiltrated by materials.

 

[AusTony2046]

What size are those rebars supposed to be? If the pavers below are 4" wide, the broken rebar above them must be no more than .25" diameter.

 

[FJBnomics]

https://www.understanding-cement.com/sulfate.html

Interesting part of dlk's link to understanding-cement and sulfates and the internal and external attacks, is the following quote from the link: "Alteration of paste composition, with monosulfate phase converting to ettringite and, in later stages, gypsum formation."

The 'curious' concrete in the core samples and showing at the South wall for example sure looks like it has chemically changed and that describing the powdery white mix as gypsum formation sure seems to fit what I am seeing in the pictures.

The link goes on to say this results in loss of strength in concrete and salt water environments can result in this types reactions.

Definitely a good link to read about all concrete changing over time, thus resulting in loss of concrete strength with or without rebar corrosion.

 

[spsalso]

In the walk-through video mentioned above, I see:

a probably rusty thing hanging from the ceiling
several brownish hanging things
all of the above on a diagonal line, which is unlikely to be following the layout of rebar


In the picture with the five red circles, I see holes. Not rebar. I don't see any rusty discoloration around the holes.


Show me the rust-damaged rebar. Not rusty colored, by the way, but damaged enough to have been contributory to the collapse.

I am NOT saying there isn't such rebar. Odd, though, that evidence of it is so minimal.



Regarding the crumbly concrete: I don't doubt it. The explosives guy mentioned seeing it when he placed his charges. One could wonder how that came to be.


spsalso

 

[IanCA]

several brownish hanging things

calcium deposits leached from the concrete infused with rust from the rebar

all of the above on a diagonal line, which is unlikely to be following the layout of rebar

following the path of the documented cracks.

In the picture with the five red circles, I see holes. Not rebar. I don't see any rusty discoloration around the holes.

Please suggest the approximate tensile strength of them holes.

Show me the rust-damaged rebar. Not rusty colored, by the way, but damaged enough to have been contributory to the collapse.
I am NOT saying there isn't such rebar. Odd, though, that evidence of it is so minimal.

 

[spsalso]

Nope. Not good enough. You'll need to get a LOT closer shot of those 4 bars to show the rust damage.


I do like the graphic on the left in the photo. Gee, did someone "forget" some rebar?


spsalso

 

[zebraso]

So if spalling concrete due to suspected rebar corrosion does not show external stains or exposed rusty rebar then we don't believe there is corroded rebar. Question: does all corroded rebar cause spalling or can it be contained if it is deep enough?

 

[zebraso]

thus resulting in loss of concrete strength with or without rebar corrosion

How do we explain that efflorescence can damage structural concrete elements without rebar in the equation if that were not true?

 

[IanCA]

You'll need to get a LOT closer shot of those 4 bars to show the rust damage.

I will work on getting that for you. An important point to keep in mind here is that the bars within the slab are critical because if they fail in two different north-south locations (e.g. 4 feet and 9 feet from the wall, as indicated in my post on 3 Feb 22 06:07) that allows the deck to retract from the wall. But those locations were buried under debris and photographic evidence is not readily available, for several reasons.

 

[zebraso]

About the 5 red circles photo. Could the brownish (dare I say orange) stains be the outflux of iron oxide via a long standing crack that would also have left moisture access to steel? The iron oxide has to go somewhere.

 

[IanCA]

@dlk & @thermopyle2.1
Thanks for the link related to sulfates, that's interesting. I see it mentions seawater as a potential source.

 

[IanCA]

does all corroded rebar cause spalling

I can imagine two conditions where it may not. The first is when the corrosion is occurring on rebar exposed due to a crack in the concrete. Expansion of the rust into the crack may not cause spalling, but if the rebar is highly stressed at that location it would surely increase the chances of stress corrosion cracking. The second is when the rust becomes fluid and can migrate away from the corrosion site.

 

[IanCA]

What size are those rebars supposed to be? If the pavers below are 4" wide, the broken rebar above them must be no more than .25" diameter.

The 9.5" deck measures 97 pixels, the bar/hole measures 6 pixels, making the bar/hole 0.58". The bricks are 4" wide, because the tiles are 12", same as the Home Depot buckets seen in other shots. The bricks measure 35 pixels making the bar 0.69". The bar was intended to be #5 (0.625") so it's pretty close.

 

[zebraso]

I can imagine two conditions where it may not.

Thank you. By fluid I guess it means suspended on liquid. I guess part of the equation is determining whether the crack has the capacity to carry away the iron oxide and there is enough liquid flow to make that happen. Any reason to believe there was insufficient liquid on top of these decks? It's not that simple. I know.

 

[FJBnomics]

Looking at core samples we see where concrete appears to have delaminated with minimal looking rebar corrosion, and where outer layer of concrete looks like it has chemically changed to a white powdery substance, while the layer on inner side of delamination still looks more like concrete. From dlk’s links, it sounds like sulfates can migrate into concrete and creating layers as it progresses from external inward.

 

[IanCA]

These two posts are helpful in relation to the discussion on chloride ions and corrosion:

5 Jul 21 04:36
Reviewed a little history of the use of modern admixtures in concrete to reduce its permeability to chloride ions, and it seems that the modern use of super plasticizer was not common in the USA circa 1980 when this condo was built. Reducing the water/cement ratio to below 0.4 and adding fine pozzalans is used today to reduce the diffusion rate of chloride ions to the rebar, and the plasticizer aids in reducing the water to cement ratio. If the workability was not enhanced by using plasticizer, is it possible they added calcium chloride to improve workability but compromised rebar life?

5 Jul 21 04:56
I figured the w/c would be between 0.4 and 0.45. With RJC and the parking garage experience they had, we were typically using 4.5 to 5.0 ksi stuff back then for parkade slabs and 1-1/2" min cover for slabs and 2-1/2" for columns as well as 4" slump was common... substantially higher than most consultants at the time. This was not for a marine salt environment... put typical de-icer salts.

 

[IanCA]

12 Feb 22 02:49
This makes me question the latest concrete repair. IMHO, both the concrete strength and the concrete cover was way too low.

I completely agree, the coverage specified for CTS certainly appears less than adequate for the environment and doesn't align with current best practices. For example, Mirabato drawing .C2C-1.1 from 8777-collins-avenue---preliminary-review-plans-for-40-year-re-certification.pdf calls for only "3/4" MINIMUM COVER FOR SLABS"

Which is at odds with these documents:
Link recommends 2.5 inches of cover for marine exposure.
Link recommends 50mm for marine concrete.
ACI 318-14 table 20.6.1.3.1 requires 1 1/2" minimum cover for #5 or smaller bars exposed to the weather in all members.

Even ACI 318-14 seems inadequate to ensure that concrete used in marine environments has sufficient coverage. Corrosion Protection of reinforcement class C2 is defined as concrete exposed to moisture and an external source of chlorides from ... seawater or spray from these sources. Table 19.3.2.1 addresses the requirements for concrete by exposure class and specifies the maximum water to cementitious material ratio (w/cm) of 0.4, minimum compressive strength of 5000 psi, maximum chloride ion content %/wt 0.15 with the additional provision of concrete cover in accordance with 20.6. But section 20.6 does not contain any marine, salt, or chloride ion-related adjustments.
The commentary states: Conditions should be evaluated for structures exposed to chlorides, such as in parking structures where chlorides may be tracked in by vehicles, or in structures near seawater

The collapse of CTS gives us a very good reason to evaluate the conditions in marine environments and to consider increasing and clearly stating, the minimum coverage requirements under those conditions.

 

[dik]

They are two distinctly different problems. Chloride deterioration can occur in addition to sulphate attack. De-icer salts and a marine environment are similar, but two distinctly different environments. I've had very limited exposure to marine environments, but I'd know where to look...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

There's a process called crypto efflorescence, where soluble salt crystals expand and cause distress to the concrete. The expansion causes the concrete to fail. When this fails, it puts stress on the reinforcing. You should start Part 16, this thread is getting too long.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[zebraso]

I see now that the process where the salt is migrated into the pores from external sources is also called "secondary efflorescence".
I also see this interesting quote from the wikipedia article on efflorescence: "Virtual stalactites can be formed in some cases as a result of dissolved cement stone, hanging off cracks in concrete structures. Where this process has taken hold, the structural integrity of a concrete element is at risk. This is a common traffic infrastructure and building maintenance concern. Secondary efflorescence is akin to osteoporosis of the concrete" italics added by me. So what repair or remediation addresses osteoporosis of concrete? Anything short of replacement?

Also to add: were the cores from the parking deck supposed to be tested to destruction with a load cell?