Concrete buildings don’t just collapse out of the blue. Even earthquakes aren’t supposed to bring them down. So why did the Champlain Towers South building in Surfside, Florida — a modern structure built in 1981 — fail?
[NEWS BROADCAST] We’re getting a new look at the surfside condo collapse site, investigators and engineers working—
[NEWS BROADCAST]—would cause the Champlain Tower South to just collapse.
[GEORGE] I mean, how?
Back in the spring we were working on a video about concrete chemistry, and then the surfside condo collapse happened, and that changed things a bit. Concrete buildings don’t just fall down on their own, even earthquakes aren’t supposed to bring them down. So how did a modern concrete building, built in 1981, just fail?
To answer that question we need to start with some concrete basics. First, concrete is not cement. People use those words interchangeably all the time, but they are two different things. Cement is one ingredient in concrete.
Concrete is made of rocks cement and water. It’s the material that buildings, sidewalks, and all kinds of other stuff is made from.
And cement is like the glue that holds concrete together, except it’s the weirdest and most counter-intuitive glue out there.
What does that mean? If you’ve ever seen a freshly poured concrete sidewalk, and maybe or maybe not put a palm print into it, then you’ve seen what looks like concrete drying. As that wet sidewalk dries out, the concrete hardens.
Turns out though, that is not at all what’s happening. The concrete hardens through a process called “curing”, not from drying out. In fact, it’s actually better for the concrete strength if it doesn’t dry out.
So what’s actually happening? Well, cement is mostly calcium silicates and calcium aluminates. Add water to that mixture and you get a chemical reaction that produces what’s called calcium silicate hydrate.
Now I would love to show you the exact chemical reaction here with all the reactants and the exact products you get out of it, but it’s different every time! There’s no fixed stoichiometry, there’s no fixed crystal structure—
The short version of it though is that cement plus water forms calcium silicate hydrate, which is made up of calcium oxide, silicon dioxide, water, calcium hydroxide, each in some, you know, varying amount.
Water has to be available for the calcium silicate hydrate to form, that’s what the word hydrate is doing there, it means contains water.
So as the concrete dries through evaporation you’re actually losing one of your reactants. And if that happens too quickly the reaction won’t have enough time to fully complete.
What does that look like? Let’s go find out.
So I’m mixing up a batch of cement— and splitting it up into two cement containers, also known as tupperwares in my bottom drawer.
I’m covering one of them with plastic wrap to prevent the water from evaporating and the other one I’m putting right in front of a fan to encourage as much water to evaporate as possible.
So these are the two concrete tupperwares side by side. This one was cured under the plastic wrap and this one under the fan. From here they don’t look that different but if you look super closely you can see these sort of thin hairline cracks. These cracks happen for the exact same reason that mudflats develop cracks as they dry out.
Okay let’s compare the concrete cured under the fan to the concrete cured under plastic. Now, I looked at this very closely and I could not see a single crack basically.
The other thing to look at is how strong each of these pieces is. Now in construction the concrete on site is regularly tested using all kinds of fancy equipment. I don’t have, um, any of that, but I do have these guns… and a sledgehammer. So, um, let’s go use those.
So I made a little sledgehammer test rig to try and smash my two test pieces of concrete with exactly the same force. I’ll spare you the details and tell you that my obviously not at all DIY rig here was not precise enough to measure any difference in the strengths of these two pieces of concrete.
Luckily this is not how concrete is actually tested in the lab. In the lab there is a giant machine that presses down on cylinders of concrete, slowly increasing pressure until the concrete fails.
And we know from these tests that concrete cured in plenty of water is about twice as strong as concrete cured in open air after both have been curing for about a year.
You need water to stay in the concrete long enough for all that cement to produce calcium silicate hydrates. Sometimes you’ll even see construction crews spraying water on concrete or covering it with tarps to make sure it stays wet long enough to cure.
In fact you can pour concrete under water and it can actually end up stronger since there’s more than enough water, so it’s never a limiting reactant.
So what does all of this have to do with the Surfside Condo Collapse?
Well, I was going back through this and asking myself that same question when I stumbled across a quote that jumped out at me. It was something a concrete expert told me back in the spring.
I was talking with Dr. Maria Juenger, who studies infrastructure materials engineering, and I asked her if the concrete in bridges over rivers was poured directly in the water.
[DR. JUENGER] Sometimes they are poured under water, sometimes they’re um, you know, pounded into place. One thing I will note, though, is glad you said river and not in the ocean, because concrete’s perfectly happy in the ocean for the most part, um, it’s this reinforcing steel that we use in structural concrete that doesn’t like seawater.
[GEORGE] Concrete is really strong if you compress it, but its tensile strength isn’t great. So if you stretch it or twist it, it tends to break.
Since building structures have not only the force of their own weight to deal with, but also wind, storms, and even earthquakes, that’s a problem.
So to make up for that we reinforce structural concrete, usually with steel. But steel and sea water don’t get along so you have to be really careful when building in or near the ocean.
[DR. JUENGER] If you make your concrete dense enough and good enough, then you can actually protect the steel inside from access to seawater and protect it from corrosion, because the high pH of the concrete actually passivates the steel and prevents it from um from rusting.
[GEORGE] If seawater can penetrate the concrete and get to the steel it can catalyze oxidation reactions. The iron in the steel reacts with oxygen or hydroxide ions, forming rust. As more iron is pulled out of the steel reinforcement and converted into rust, the steel loses strength.
And as more and more rust builds up on the surface of those steel bars it pushes against the concrete and that can crack the concrete and also make it lose strength.
Now, since all this happens inside the concrete, you might not notice the extent of the corrosion until it’s too late.
So is this what happened at Surfside? At this point we don’t know for sure. It’ll take years. But the early signs are pointing to problems with the reinforcing steel.
Over the past few years inspectors and maintenance workers at surfside reported quote, major errors, quote, in construction that allowed water to seep in collect and remain in contact with the building, which could have corroded the reinforcing steel, weakening the entire structure.