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Explaining concrete while getting buried in it

Apr 16, 2023
I'm about to be


in cement. And


that happens, I'm going to explain everything you need to know about this substance. So the first thing I want to clarify is the difference between cement and


, because people often get the two confused, okay? Cement is like glue. It is the matrix of things. Oh there, sorry. Okay, that feels good. Now the


is cement plus aggregate, therefore more gravel and sand. And this is filling up pretty fast. Cement is the most important man-made substance on the planet. We use more of it than any other substance apart from water.
explaining concrete while getting buried in it
Every year, 500 kilograms of cement are created for every man, woman and child on earth. And with that amount of cement you can make two cubic meters of concrete, which is like two of these big fish tanks. This video is sponsored by Wren. I don't think people realize how important concrete is. So here's another way to think about it. Every year, we make a certain weight of copper products, plus aluminum, and then you have glass, asphalt, lime, iron is a big one for all the steel, and then there's ceramics and wood. But, by far, the solid product that we use the most is cementitious material, essentially cement.
explaining concrete while getting buried in it

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explaining concrete while getting buried in it...

We use as much as all the other materials combined. And it's easy to see why. Concrete is liquid rock. You can pour it in any way you want. It is resistant, durable and economical. And it's so easy to produce that people have been making a version for thousands of years. To make primitive cement, the key ingredient is limestone, which is basically calcium carbonate. If you heat it to around 1,000 degrees Celsius, you drive CO2 out of the rock, leaving calcium oxide, which is also known as quicklime. Now if you grind up calcium oxide and mix it with water, you get an exothermic reaction that creates calcium hydroxide, which you can pour into a mold.
explaining concrete while getting buried in it
And over time, it will absorb CO2 from the atmosphere, turning back into calcium carbonate as the water evaporates. Now, that's some pretty good primitive cement. It is the first way that people made cement, but it has several drawbacks. I mean, for one thing, you can't make very big molds because otherwise the CO2 can't penetrate all the material and harden it. Also, it will not work underwater where there is no CO2 to harden the mix. The Romans discovered the solution to these problems. They added volcanic ash called pozzolana to the crushed limestone before heating it. And they discovered that this cement was much stronger and more durable.
explaining concrete while getting buried in it
They used it to create the world's largest unreinforced concrete dome, the Pantheon, which has stood for 2,000 years. And they built concrete piers in the sea, which hardened under water. Some of which are still standing today. I didn't expect it to feel so heavy, like it really weighed down my feet and made me a little worried if I could get out. What I can tell you is that concrete is about three times as dense as water. - Are you able to lift one leg? I just want to make sure you're not going to get some suction. - Yeah, okay, I'm going to try to lift one leg just to see how bad it's going to get.
What I like is having the feet on the bottom and I'm not sure I can put them back there after lifting it up, but oh my! Well, that was like... I'm going to try to lift this leg very slowly and I was able to lift it up. - Good. - So, at least up to this point, I'm able to get out. My fear is that the concrete is so dense that when it rises up around my chest, it can put a lot of pressure on it and make it hard to breathe. Yes. - It's like being in a trench and it collapses on you.
Your lungs collapse so you can't breathe so once they contract and the weight hits your lungs. - That's going to be hard. - Have you thought about that? - Well, now that you mention it. But we have oxygen on hand, just in case. Earlier, I practiced how I can get out of the bowl if I'm having trouble. - Do you think it's okay? - That's not a problem. - Good, excellent. (truck signal blaring) So how did Roman concrete harden underwater? What about thick slabs that CO2 couldn't possibly penetrate? It is claimed to be superior to modern concrete.
So was it? For centuries, the Roman recipe was lost. It was only discovered in a book in a Swiss monastery in the 15th century. And since then, architects, scientists, and engineers have been experimenting with different cement recipes to try to achieve the best result. Now it turns out that the incredible strength, durability and setting capacity under water of the Roman cement, which came from the pozzolan, that volcanic ash that was added to it. And almost 2,000 years later, people discovered that adding clay or shale to limestone before crushing and heating it produced the same effect. And the reason is that all these materials contain silica and the silica totally changes the chemistry of the cement.
It means it doesn't need to dry to harden. In fact, the water becomes an integral part of that hardened concrete so that it reaches its maximum strength when it sits underwater. - So this is the compression cylinder curing room. We are required to maintain concrete samples at 100% humidity. So we have chosen to immerse them in water in a lime bath. - Whenever concrete suppliers pour concrete at a job site, they release sample cylinders of the material so they can later test it to make sure it has the required strength. - So we actually intercept a little bit of the concrete going into whatever structure they're pouring: a slab, a wall.
We'll intercept it on a wheelbarrow, take the wheelbarrow to our test station, and start molding these compression cylinders. Every day we are breaking these cylinders, testing their resistance. You can see these have broken today. - This resistance increases with time, so the samples are tested at 7, 14 and 28 days when the concrete is said to have reached its maximum resistance. - So what we do is list the date that will be placed on the compression machine on top of each cylinder. - Actually, it will continue to get stronger after that. The samples are placed in a hydraulic press and then pressure is increased on them until they fail. - Okay, ready? - Yeah, let's do it. - I'll start loading. - Sure. - We want to maintain around 30 psi per second. - And it's important to charge at that rate. - Yeah, you don't want to surprise him.
You don't want to shock him by applying too much charge too quickly. This middle number shows us our strength in pounds per square inch. And then the top number, the big number shows us the pound force that we're applying. - What are we going to get to, like 7,000 psi? - 10,000. - 10,000. So this is going to be very strong. - Very strong. - Well. - Yeah. - It's always fun when we get anything over 10,000 in a free program. Pop! - It's gonna rock this room. - And it has reinforced concrete walls. It's surprisingly like you can't see anything happen even under all that pressure. - It's still holding up. - I know right? - Oh, here goes. - It's starting to dry out. - Actually? - Here it goes. - 11,000. - Mm-hmm. - More than 11,000 now. (cement cylinder bursts) Oh yeah! - He still has a life.
Have it, not even- - Fun. (Derek laughs) - It's my favorite part. (cement cylinder bangs) - Yeah, that's great! - We'll see. - Yes I like that. - Like the strongest concrete in the world. - Well, the strongest concrete in the world will be like a laboratory thing. - In a competition it was above 100,000 psi. - Today, virtually all concrete is made using a cement formula discovered in the 1840s. It's known as Portland cement, but the name is really just a marketing term. You know, they claimed that the gray color of the cement would resemble these very desirable rocks, which were mined near the city of Portland, England.
But Portland cement was made by crushing limestone and then mixing it with a certain percentage of shale or clay to provide the silicates. And all of that was ground into a fine powder and put in a furnace and heated to very high temperatures. And what comes out are these very hard nodules. They are called clinker. Now, it is suspected that cement chemists in the past may have accidentally produced clinker when they overcooked their lime mixes. But since this clinker is so difficult to grind, they simply considered it waste. But if you grind it up, the cement it produces is vastly superior to basically any other chemistry we've discovered, which is why it's so commonly used today.
Now there are many compounds within Portland cement. The most common is tricalcium silicate. Now I feel incredibly upbeat, like I'm floating on concrete, which is pretty ridiculous because most of my body is out of the material. But because it's three times as dense as water, you can float up to your waist. This is totally unexpected. I didn't expect to be able to float on cement. My feet are off the ground. That's where my feet are. - My feet are here and my shoulders are up and I'm being pushed up to get out of this. Once you have the cement powder, the other things you need to make concrete are the aggregates, sand, and gravel. (quarry blasting) These are blasted from a quarry.
They are then ground to have particular sizes. There are very strict requirements on the sizes and shapes of the aggregate to be poured into the concrete because that will of course affect the strength of the resulting concrete. - Yes, this is just a well-grated concrete sand. The more complete material helps the contractor as to the finishing ability of it. - You know, when they're running that test on that slab, they want a rounded particle, not a jagged crushed particle. - Do you really want like a spherical sand? - Around it, yes, a spherical sand, like a river stone, like a river sand.
Designers on the strip want to pour raised concrete. They want to lighten the deck load. We will incorporate some light aggregate in place of this normal weight. (rock noise) 3/8 size. Typical concrete will weigh 150 pounds per cubic foot, normal weight. This gives us about 110 pounds per cubic foot. - The aggregates are transported to the concrete plant in large trucks. Alright, I'm going to drop the rock, (hydraulic hiss) where they're dumped (rock noise) and then they travel on big conveyor belts to the stockpiles. These are loaded into large hoppers and then weighed and dumped onto trucks. - Control operators can open this gate and drop material into this hopper, onto the mixer truck below.
This is where the lot operator controls the plant. Right here in front of our batch operator, William, is the actual recipe that he's going to dispense into the mixer. Each line represents a different component in the concrete. That one above is a 3/4 rock. We have a 3/8 intermediate rock below and there is your sand. Here, in this column, aim for, for example, 3/4 rock. You want to hit 13,000 pounds of material for that load. The computer will try to hit 13,042 pounds of 3/4 rock. - So everything is... - Everything is heavy... - Filling up? - Right now. - Now, if you go out of tolerance and pass a target, it will change the red, a red color.
Up here on this screen, we're actually showing the holding hopper. This material that you are weighing is directly on top of the mixer truck that just drove under the plant. - What about the water, William, being orange? - Yeah, we're a little below that. - Well. - But we can always add little water. - Yes. - We can't take it from you. Alright, I'm going to take a look inside the truck here. (groaning of spinning gears) That's what it looks like inside a cement truck. So I was interested to see what the difference in strength would be between straight cement, cement with sand, and cement with sand and gravel like the typical concrete mix they would make.
So I asked them to make these special cylinders and test them in the hydraulic press. You might think that since straight cement has the most glue, it would be the strongest. - Cement is magic, right? The glue. So if we decrease or decrease the amount of cement per unit volume, we will have less resistance. - But when we tested the pure cement cylinder, it fractured a lot with the application of the load. (the cement cylinder bursts) Is that all? - No. - Not officially. - Looks like he's moving on. It may take a little more pressure.
That? (cement cylinder bursts) - 8,000 psi, now failed. - Now, cement more sand. (cement cylinder strokes) 9,163 psi - Again. Edit the prediction. Oh, my prediction was based on the strength I had two weeks ago, which we have here. We have 14 days of 6,600. So, you know, the reason would be that it would probably break another 1000 psi on top of that, but it definitely gained a lot. - And finally, cement plus sand and gravel. This is the normal concrete mix. (the cement cylinder bursts) Is that all? No? - A part of her. (glass breaks) - Whoa! Ok, so that's it. - That's all. - We'll take it. - That's all. (cement cylinder hits) - So it failed at 8,300.
Well. I was surprised to see that all the cylinders ruptured under the same pressure. I think, to me, it's really interesting that you can haveall that cement, right? But you don't appreciably get... - Right. - Stronger than this or that. Cement is the most expensive part of concrete. So if you can cut it down to 30% in the mix and still get the same strength characteristics then you definitely should. The other interesting thing was that the neat cement seemed to flake and chip more as it was loaded. So it seemed like adding the aggregate actually helped the sample stick together and stay together even under all that load. (cement cylinder hits) So, was Roman concrete superior to modern concrete?
Well, the answer is no in short. I mean there were some amazing advantages that Roman concrete had. For example, it was actually worse mixed than modern concrete. So there were little blocks of undissolved calcium oxide or quicklime left inside the Roman concrete. And then what happened is when the concrete cracked and water got in, it would dissolve that calcium oxide, forming calcium hydroxide, and then you'd get the new growth of calcium carbonate. So Roman concrete was actually self-healing. That is a fascinating advantage. But I would say that generally when we look back at Roman structures, we only see the ones that have survived to this day.
So there is a survivorship bias. And finally, there is the issue of cost. We could make very strong concrete that could last an incredibly long time, but we choose not to because it's cheaper and we don't expect our buildings to last as long. Before the concrete goes on site, they need to make sure it is the right consistency for the customer; not too dry and not too runny. This can be adjusted with water, but you really don't want to because that can affect the strength as well. So here they use modern chemicals like superplasticizers. - Up here on the top left, you have our dosing units for all of our chemical additive blends. - And that's what they're adding now as a superplasticizer.
It makes concrete easier to work and spread without changing the water content much. To check that the consistency is correct, they perform something called a slump test or a scatter test. - We're going to fill the cone to the top and level it, and we're going to get that metal cone out. Measure the distance it travels on the board. - The concrete being poured for me must have a 27-inch spread. - Let's measure it on both sides. (crew member cheering) Right on a 27 on that one. - Yeah. - And go this way, right on a 27 at that one. - So why is it so important to get the concrete consistency right? - You don't want it too dry.
You want it to be wet enough to flow and fill whatever container you're trying to fill, in this case, the sphere you're in. - So the question you're probably asking yourself is, how long can I stay here before the concrete hardens? The usual response is about four hours without agitation. And that's why when you see concrete trucks driving down the highway, that drum has to be spinning to keep the concrete agitated and prevent it from settling. But what if the truck breaks down or there's a traffic jam or something breaks, then sometimes the concrete hardens inside the drum of these trucks and that's a terrible result.
But there is one thing you can do to slow down the setting of the concrete and that is to add some regular soda like Coca-Cola. The sugars within this coke actually prevent the setting process from happening and that can buy you a few hours. Apparently, these truckers drive with a few two-liter bottles of soda inside their cab and may dump it in their cargo if necessary to prevent it from settling. So I hope that means I'll be okay too. But how does concrete actually harden? Well you have the dry mix of gravel, sand and cement dust and then you add water.
The water begins to dissolve the powdered cement grains, so ions go into solution, and some of those ions are calcium hydroxide. That's what makes concrete a very basic solution. Do you see? - We are approaching 12, 11.8. - So the pH of concrete can go up to 12 or 13. The pH of this is 11.8 and that's incredibly basic, which means if it's on your skin, if it's on your body, it can dissolve your skin and your cells. Being


in cement is like jumping into a bleach bath. And that's why I'm actually wearing a dry suit and latex gloves. Yeah, not something you want to try at home.
So don't try to swim on concrete. So now the ions are dissolved in the solution. And remember, the most common compound in cement is tricalcium silicate. As it reacts with water, calcium silicate hydrate crystals begin to form, along with other hydrated minerals. And all these crystals grow and interlock, causing the concrete to harden. Please note that water is essential for the formation of these crystals. So the water does not evaporate, it does not dry out. In fact, it is becoming part of the solid concrete material. And that's why this chemistry is called cement hydration. This is also why freshly poured concrete should be kept in as humid an environment as possible.
Las Vegas is so dry that they often install sprinklers to spray new concrete to ensure the humidity is high enough. One of the things I realized


making this video is that limestone, the central component of cement and concrete, comes from ancient marine life. Limestone forms from the skeletons and shells of ancient marine organisms that, you know, died millions of years ago, and then all of that stuff got compressed. And now we use that to make huge skyscrapers and, you know, overpasses, basically every big piece of infrastructure, is made of concrete. And now, when you look at a beautiful city skyline, what you're really looking at is ancient marine life.
Skyscrapers are made of seashells. ♪ Skyscrapers are made of seashells ♪ ♪ Skyscrapers are made of seashells ♪ - Yeah, this is wild. This is very funny. I like it, I have no words. - I'm like trying to push myself down, but it's like pushing me up. Like he's pushing down on the concrete, but he's like bop! They're taking me out of this thing. I'm going to try to push myself down here. Oh! That's how it is. There's no... - You can use that. - You can't get buried in concrete. Well? I'm doing everything I can right now to like sinking and it's not happening.
But I'm going to use my arms to push myself. (Derek grunts) (Derek exclaims) Working against buoyancy. This is hard. We were worried about


me out, but I can't stay. I will try to enter. Here we go. That's all. (crew members laughing) Alright, here we go. One two three. Brilliant. (Derek exclaims) That feels so good. I, like, don't want to go out. This is so nice. Has anyone had a time check? These things are not going to settle on me, are they? As we have seen, concrete is one of the most important materials in the world.
It has made possible most of the large-scale infrastructure we depend on, but it also generates a lot of CO2, about 8% of the global total. That's more than the entire aviation sector. But together, we can do something about it. Personally I would like to offset a month of your carbon emissions. And I'll do it through this video sponsor, Wren. Wren is a website where you can calculate your carbon footprint, see which aspects of your lifestyle make the biggest contribution, and learn how to reduce your impact. And then, if you want, you can offset your carbon footprint by funding a diverse mix of carbon reduction projects, such as tree planting, mineral erosion, and rainforest protection.
For the first hundred people to sign up, I will personally offset their first month of issuance. Just click the link in the description. Now, I don't think we're going to solve climate change using just individual action. If you can change your light bulbs or install solar power, that's great. But what we really need is change on a systemic scale. And that's what I like about Wren's approach. They not only plant trees and protect rainforests, but also support policy groups that push for change, like the Clean Air Task Force, which advocates for new technologies and policies to get to a zero-emissions economy.
The way I see it, there are moneyed interests lobbying to keep things as they are. So we need to come together through organizations like Wren to push for change. And if you agree with me, I invite you to click the link in the description and join me in offsetting our carbon emissions and investing in large-scale systemic change. So I want to thank Wren for sponsoring Veritasium and I want to thank you for watching.

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