This image reveals the aftermath of the Surfside condominium building collapse which occurred on June 24, 2021. It reveals the debris that arised from the collapse, and likewise plainly reveals the standing portion of the building. Credit: Miami-Dade Fire Rescue Department
Concrete buildings do not simply collapse out of the blue. Even earthquakes arent expected to bring them down. So why did the Champlain Towers South building in Surfside, Florida– a modern structure constructed in 1981– stop working?
Video Transcript:
Concrete buildings do not just collapse out of the blue. If youve ever seen a freshly put concrete pathway, and perhaps or perhaps not put a palm print into it, then youve seen what looks like concrete drying. One thing I will note, however, is grateful you said river and not in the ocean, since concretes perfectly delighted in the ocean for the a lot of part, um, its this enhancing steel that we use in structural concrete that does not like seawater.
If you make your concrete dense adequate and excellent enough, then you can actually safeguard the steel inside from access to seawater and secure it from rust, since the high pH of the concrete really passivates the steel and avoids it from um from rusting.
If seawater can get and permeate the concrete to the steel it can catalyze oxidation responses.
[NEWS BROADCAST] Were getting a makeover at the surfside apartment collapse site, engineers and investigators working–.
[GEORGE] How?
[NEWS BROADCAST]– would cause the Champlain Tower South to just collapse.
[GEORGE] I suggest, how?
Back in the spring we were dealing with a video about concrete chemistry, and then the surfside condominium collapse happened, and that altered things a bit. Concrete buildings dont simply fall down on their own, even earthquakes arent supposed to bring them down. How did a contemporary concrete building, built in 1981, simply fail?
Concrete is not seal. Cement is one active ingredient in concrete.
Concrete is made from rocks cement and water. Its the material that buildings, walkways, and all type of other stuff is made from.
And cement resembles the glue that holds concrete together, other than its the weirdest and most counter-intuitive glue out there.
What does that mean? If youve ever seen a newly poured concrete pathway, and possibly or perhaps not put a palm print into it, then youve seen what appears like concrete drying. As that damp walkway dries, the concrete hardens.
Turns out though, that is not whats happening. The concrete hardens through a procedure called “curing”, not from drying. In fact, its really much better for the concrete strength if it doesnt dry.
Whats in fact occurring? Well, cement is primarily calcium silicates and calcium aluminates. Include water to that mix and you get a chemical response that produces whats called calcium silicate hydrate.
Now I would like to reveal you the precise chain reaction here with all the reactants and the exact products you get out of it, however its various every time! Theres no set stoichiometry, theres no repaired crystal structure–.
The short variation of it however is that cement plus water kinds calcium silicate hydrate, which is comprised of calcium oxide, silicon dioxide, water, calcium hydroxide, each in some, you understand, varying amount.
Water needs to be available for the calcium silicate hydrate to form, thats what the word hydrate is doing there, it means contains water.
So as the concrete dries through evaporation youre actually losing among your reactants. And if that takes place too quickly the response will not have adequate time to completely complete.
What does that appear like? Lets go learn.
So Im blending a batch of cement– and splitting it up into 2 cement containers, likewise known as tupperwares in my bottom drawer.
Im covering one of them with plastic wrap to avoid the water from vaporizing and the other one Im putting right in front of a fan to encourage as much water to evaporate as possible.
These are the two concrete tupperwares side by side. From here they do not look that various however if you look very carefully you can see these sort of thin hairline fractures.
Okay lets compare the concrete cured under the fan to the concrete cured under plastic. Now, I looked at this very carefully and I could not see a single fracture essentially.
The other thing to look at is how strong each of these pieces is. Now in construction the concrete on website is routinely checked using all kinds of fancy devices.
I made a little sledgehammer test rig to try and smash my 2 test pieces of concrete with exactly the exact same force. Ill spare you the details and tell you that my certainly not at all DIY rig here was not accurate enough to determine any distinction in the strengths of these 2 pieces of concrete.
Luckily this is not how concrete is actually tested in the laboratory. In the lab there is a giant maker that presses down on cylinders of concrete, slowly increasing pressure up until the concrete fails.
And we understand from these tests that concrete treated in plenty of water has to do with two times as strong as concrete cured in open air after both have actually been treating for about a year.
You need water to stay in the concrete long enough for all that cement to produce calcium silicate hydrates. Often youll even see building and construction crews spraying water on concrete or covering it with tarpaulins to make certain it stays damp enough time to treat.
You can pour concrete under water and it can actually end up more powerful considering that theres more than enough water, so its never ever a restricting reactant.
So what does all of this pertain to the Surfside Condo Collapse?
Well, I was returning through this and asking myself that exact same question when I stumbled across a quote that leapt out at me. It was something a concrete specialist informed me back in the spring.
I was talking with Dr. Maria Juenger, who studies facilities materials engineering, and I asked her if the concrete in bridges over rivers was put straight in the water.
In some cases they are poured under water, in some cases theyre um, you know, pounded into place. One thing I will note, though, is pleased you stated river and not in the ocean, since concretes perfectly happy in the ocean for the most part, um, its this enhancing steel that we use in structural concrete that does not like seawater.
Concrete is truly strong if you compress it, but its tensile strength isnt great. If you stretch it or twist it, it tends to break.
Given that constructing structures have not only the force of their own weight to deal with, however likewise wind, storms, and even earthquakes, thats a problem.
So to offset that we strengthen structural concrete, usually with steel. However steel and sea water do not get along so you need to be actually cautious when building in or near the ocean.
[DR. JUENGER] If you make your concrete dense adequate and sufficient, then you can actually protect the steel inside from access to seawater and secure it from deterioration, due to the fact that the high pH of the concrete in fact passivates the steel and prevents it from um from rusting.
If seawater can permeate the concrete and get to the steel it can catalyze oxidation reactions. As more iron is pulled out of the steel support and transformed into rust, the steel loses strength.
And as increasingly more rust constructs up on the surface of those steel bars it presses versus the concrete and that can split the concrete and likewise make it lose strength.
Now, given that all this occurs inside the concrete, you might not discover the extent of the deterioration until its too late.
Is this what occurred at Surfside? The early indications are pointing to problems with the strengthening steel.
Over the previous couple of years inspectors and maintenance employees at surfside reported quote, significant mistakes, quote, in construction that allowed water to permeate in gather and remain in contact with the building, which could have worn away the reinforcing steel, weakening the entire structure.
