Author: American Chemical Society October 22, 2021

This image shows the aftermath of the collapse of the Surfside apartment building on June 24, 2021. It shows the rubble caused by the collapse and clearly shows the standing part of the building. Image source: Miami-Dade Fire and Rescue Department

The concrete building will not collapse suddenly. Even the earthquake shouldn't let them fall. So why did the Champlain Tower South in Surfside, Florida, a modern building built in 1981, fail?

[News Broadcast] We are re-examining the scene of the collapse of Surfing Apartment, investigators and engineers are working——

[News Broadcast]-Will cause the Champlain South Tower to collapse.

Back in the spring, we were making a video about the chemistry of concrete, and then the surf apartment collapsed, which changed a few things. Concrete buildings will not collapse on their own, and even an earthquake should not cause them to collapse. So, how did a modern concrete building built in 1981 fail?

To answer this question, we need to start with some specific basic knowledge. First of all, concrete is not cement. People always use these words interchangeably, but they are two different things. Cement is a component of concrete.

Concrete is made of rock cement and water. It is the material used to construct buildings, sidewalks, and various other things.

Cement is like glue that holds concrete together, but it is the strangest and most counterintuitive glue.

what does this mean? If you have ever seen a newly poured concrete sidewalk and may or may not have palm prints in it, you will see the concrete drying. As the wet sidewalk dries, the concrete hardens.

It turns out that this is not what is happening at all. The concrete hardens through a process called "curing" instead of drying. In fact, if it does not dry out, it actually has better strength for concrete.

So what actually happened? Well, cement is mainly calcium silicate and calcium aluminate. Add water to this mixture and you will have a chemical reaction, producing the so-called calcium silicate hydrate.

Now I would love to show you the exact chemical reactions of all the reactants and the exact products you get from them, but it is different every time! There is no fixed stoichiometry, no fixed crystal structure——

In short, adding water to cement will form calcium silicate hydrate, which is composed of calcium oxide, silicon dioxide, water, and calcium hydroxide, each of which has a different amount.

There must be water to form calcium silicate hydrate, which is what the word hydrate does there, it means containing water.

Therefore, when the concrete dries through evaporation, you are actually losing a reactant. If this happens too quickly, the reaction will not have enough time to complete completely.

What does that look like? Let's find out.

So I am mixing a batch of cement-and then dividing it into two cement containers, also called Tupperware in my bottom drawer.

I covered one of them with plastic film to prevent water from evaporating, and the other I placed in front of the fan to encourage as much water as possible to evaporate.

So these are two concrete Tupperware side by side. This is cured under the plastic wrap, and this is cured under the fan. From here, they don't look different, but if you look closely, you will see these thin hairline cracks. These cracks occur for exactly the same reasons as the cracks that appear when the mud flat dries up.

Okay, let's compare the concrete cured under the fan with the concrete cured under the plastic. Now, I have observed this very carefully, and I can hardly see a crack.

Another thing to look at is the strength of these fragments. Now in construction, various fancy equipment is used to regularly test the concrete on site. I don't have, uh, any of those, but I have these guns... and a sledgehammer. So, um, let's use those.

So I built a small sledgehammer test bench and tried to crush my two concrete specimens with exactly the same force. I will tell you the details and tell you that I am obviously not here because the DIY equipment is not accurate enough to measure the difference in strength between the two pieces of concrete.

Fortunately, this is not the way to actually test concrete in the laboratory. In the laboratory, a huge machine presses on a concrete cylinder and slowly increases the pressure until the concrete fails.

We learned from these tests that the strength of concrete cured in a large amount of water after curing for about a year is about twice that of concrete cured in the open air.

You need the water to stay in the concrete long enough for all the cement to produce calcium silicate hydrate. Sometimes you may even see construction workers spraying water on the concrete or covering it with a tarp to ensure that it stays moist long enough for curing.

In fact, you can pour concrete underwater, it can actually become stronger, because there is enough water, so it will never be a limiting reactant.

So what does all this have to do with the collapse of Surfside Condo?

Well, when I stumbled upon a compelling quote, I was reviewing this question and asking myself the same question. This is what a concrete expert told me in the spring.

I'm talking with Dr. Maria Juenger, who studies infrastructure materials engineering, and I asked her whether the concrete of the bridge over the river was poured directly into the water.

[PhD. JUENGER] Sometimes they are poured underwater, sometimes they are, you know, be beaten in place. However, one thing I want to pay attention to is glad you are talking about rivers and not the ocean, because concrete is very satisfying most of the time in the ocean, well, it’s what we use in structural concrete that doesn’t like sea water. This kind of steel.

[GEORGE] If you compress it, the concrete is really strong, but its tensile strength is not great. Therefore, if you stretch or twist it, it tends to break.

As the building structure not only has to deal with the force of its own weight, but also with wind, storms and even earthquakes, this is a problem.

Therefore, in order to compensate for this, we usually use steel to reinforce the structural concrete. But steel is incompatible with sea water, so you must be very careful when building in or near the ocean.

[PhD. JUENGER] If you make your concrete dense enough and good enough, then you can actually protect the steel inside from seawater and protect it from corrosion, because the high pH of the concrete actually passivates the steel and prevents it from growing rust.

[George] If sea water can penetrate concrete and reach steel, it will catalyze oxidation. Iron in steel reacts with oxygen or hydroxide ions to form rust. As more iron is pulled out of the rebar and converted into rust, the steel loses strength.

As more and more rust accumulates on the surface of these steel bars, it pushes the concrete, which can crack the concrete and make it lose its strength.

Now, since all of this happens inside the concrete, you may not notice the degree of corrosion until it's too late.

So is this what happened in Surfside? At this point, we are not sure. This will take several years. But early signs indicate that there is a problem with the rebar.

In the past few years, surfside inspectors and maintenance workers have reported quotations, major errors, and quotations, allowing water to penetrate and stay in contact with the building during construction, which may corrode the steel bars and weaken the entire structure.

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