It was something a concrete expert told me back in the spring. Well, I was going back through this and asking myself that same question when I stumbled across a quote that jumped out at me. So what does all of this have to do with the Surfside Condo Collapse? 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. 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. You need water to stay in the concrete long enough for all that cement to produce calcium silicate hydrates. In the lab there is a giant machine that presses down on cylinders of concrete, slowly increasing pressure until the concrete fails.Īnd 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. Luckily this is not how concrete is actually tested in the lab. 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. So I made a little sledgehammer test rig to try and smash my two test pieces of concrete with exactly the same force. I don’t have, um, any of that, but I do have these guns… and a sledgehammer. Now in construction the concrete on site is regularly tested using all kinds of fancy equipment. The other thing to look at is how strong each of these pieces is. Now, I looked at this very closely and I could not see a single crack basically. Okay let’s compare the concrete cured under the fan to the concrete cured under plastic. These cracks happen for the exact same reason that mudflats develop cracks as they dry out. From here they don’t look that different but if you look super closely you can see these sort of thin hairline cracks. This one was cured under the plastic wrap and this one under the fan. So these are the two concrete tupperwares side by side. 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 I’m mixing up a batch of cement- and splitting it up into two cement containers, also known as tupperwares in my bottom drawer. What does that look like? Let’s go find out. And if that happens too quickly the reaction won’t have enough time to fully complete. So as the concrete dries through evaporation you’re actually losing one of your reactants. 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. 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. Add water to that mixture and you get a chemical reaction that produces what’s called calcium silicate hydrate. ![]() ![]() So what’s actually happening? Well, cement is mostly calcium silicates and calcium aluminates. In fact, it’s actually better for the concrete strength if it doesn’t dry out. The concrete hardens through a process called “curing”, not from drying out. Turns out though, that is not at all what’s happening. As that wet sidewalk dries out, the concrete hardens. 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. It’s the material that buildings, sidewalks, and all kinds of other stuff is made from.Īnd cement is like the glue that holds concrete together, except it’s the weirdest and most counter-intuitive glue out there. Cement is one ingredient in concrete.Ĭoncrete is made of rocks cement and water. People use those words interchangeably all the time, but they are two different things. To answer that question we need to start with some concrete basics. ![]() So how did a modern concrete building, built in 1981, just fail? Concrete buildings don’t just fall down on their own, even earthquakes aren’t supposed to bring them down. ![]() would cause the Champlain Tower South to just collapse.īack 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. We’re getting a new look at the surfside condo collapse site, investigators and engineers working.
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