Surfside Champlain Towers South Condo Collapse & Science of Concrete [Video]

Surfside Champlain Towers South Condo Collapse & Science of Concrete [Video]

Video Transcript:


[GEORGE] I mean, how?

[NEWS BROADCAST]—would cause the Champlain Tower South to just collapse.

Story Highlights

  • 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—

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—

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.

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. 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.