So today's lecture is gonna be a little bit different. Up until now, we've been talking about a single class of material. So today, we're talking about materials that are used for building and building materials can be broken down into many different classes. We talked about clay earlier on and clay obviously has been used for a very long period of time in buildings, but there are also other materials we use in buildings.

For example, early on, we used bitumen. Bitumen is an example of a polymer, we'll talk a little bit more about that and then there's also concrete. And of course, steel as used today. So these are examples that were developed thousands of years and are still used today. So if we move past clay bricks, we get into bitumen and bitumen is basically tar or asphalt.

It was discovered as a byproduct floating around on lakes like the Dead Sea and they would harvest it, cut it into bricks and then use it as a mortar between blocks of clay that had been fired or Sun-dried. So, it's similar to molasses. It's flammable, so you had some issues with naphtha, which is a natural form of gasoline, seeping out during the during the harvest process, but it is very waterproof and so it was good for joining these bricks.

Now polymers are of course, a long chain molecule. So we'll talk more about polymers in the future, but just so you know, it's a long chain of hydrocarbons that fit together and form almost what you might think of as a spaghetti noodle. And those spaghetti noodles tend to tangle and form a structure that can be quite strong and bitumen is an example of a natural polymer.

So now as I said, you can buy bitumen today. If you go to Home Depot or what not, it's used as a tar solution that you use on paving or on roofing and what not. And it typically has this characteristic, that it will soften around 300 degrees Fahrenheit.

So, it makes it easy to apply to surfaces and then it will harden up when it gets down to room temperature. Now when we get into the harder materials like concrete, the early forms of this stuff we uses were actually limestone and gypsum. The limestone is calcium carbonate, gypsum is calcium sulfate.

And the early Egyptians figured out that if they took limestone, which is this is an example of a limestone and they heated it. So, I'm gonna heat a piece of limestone to give you an idea of what they did. If we heat a piece of limestone, what would happen is this material is calcium carbonate.

But when I heat it up to very high temperatures, it would transform into calcium oxide. The CO2 leaves and you can't tell, but it becomes a whiter surface and that whiter surface is what we call lime and lime was very critical for making the early mortars. And so what they would do is they would take limestone and gypsum, heat it up, get it very hot, drive off the CO2 in the case of limestone and then they would mix it with a little bit of water and it would make a covering that you could put like a plaster on the surface of walls.

Gypsum was actually used as a mortar to align and slide the blocks for the pyramids and so this was an early form. Now this is where concrete got its start. However, if we stop and think about concrete, what are the properties we think about concrete? We don't think of a plaster that goes on a surface, we think of something that's very strong in compression, probably not terribly strong in tension, poor in tension, unless it's been reinforced with say steel bars.

It's insulating, it's not transparent, it's very hard and enduring. And by and large, very corrosion resistant. So in general, it was a very nice material to use for buildings. Now, it's been said that Roman concrete was the most significant material ever involved in the Roman Empire and the reason for that was that the Roman's came up with some interesting composition.

The Romans were the first to realize that you could actually mix limestone or lime after you've formed it with other materials to form a concrete. There had been attempts to make other forms of concrete, but the Romans really perfected this and one of the things that they did was they would create a composite.

Now concrete as you know it is not a single material, it is a composite. This is a jar of what you might know as concrete. This is before you add the water and you can tell right off the bat that it seems to be a mixture of a variety of different things, including a powder and there is some sand in there and there is even some gravel.

So how do you go about making concrete? Well, as I said, the first thing you're gonna do is you're gonna take your limestone and you're gonna heat it up and make lime. All right and now what you have to do is add other materials to the lime. So what they would typically do to begin with is they would add gypsum, volcanic ash or clay and heat it up to greater than 1250 degrees C, which is incredibly hot.

And then they would grind it up afterwards and that would produce what they call a clinker. Now once you add the clinker, the clinker is basically this grayish material the powder that's in the background, then you add sand and gravel or recycled bricks or whatever it was. And you made a composite and that is what you then finally call concrete.

Now we haven't talked about composites just yet, so I should mention a couple of things about composites. Number one, composites are a mixture of two different kinds of materials. And so, it could be a polymer mixed with a glass or it could be concrete in the case of a limestone or clinker mixed with gravel.

These are all forms of composites and composites have strengths that depend upon the orientation of the material that you're adding. You typically break it down into something that you would call your matrix and something else that would be your additive, that adds this enhanced properties to the composite.

So when we go into composites in general, we kind of break them down into three classes. We call it a particle-reinforced composite, a fiber-reinforced composite or a structural composite. And so you're familiar with fiber-reinforced composites, those are things like your tennis racket or a structural composite could be a laminate, like it might be plywood.

And then particle-reinforced composites can be either large particles like rocks that you add to concrete or they can be dispersion strength and by adding smaller particles and concrete is basically a form of a particle-reinforced composites. Now there are two kinds of cement or concrete that we work with.

The first is called a non-hydraulic and the second is called a hydraulic. A non-hydraulic is where I basically take limestone, form lime and then I add water to it. And when I add water to it, it forms a hydroxide gel and this hydroxide gel then reacts with CO2 in the air to eventually return back to a calcium carbonate state.

And that cannot dry or cure under water, so we call that a non-hydraulic cement. Hydraulic cements, however, can cure under water and they were very important to the Roman Empire. So the way a hydraulic cement works is that you do the normal reaction of forming lime, but then you added volcanic ash.

And volcanic ash that they got, they called pozzolana and the volcanic ash was composed of a lot of aluminosilicates and those aluminosilicates enabled the calcium oxide when mixed with it to form a gel that would then cure underwater. The Romans implemented a lot of different forms of concrete in their building structures.

For example, the dome at the Pantheon is an excellent example of being composed of three different kinds of concrete and the concrete got lighter as it went up towards the top of the dome. The dome itself is 142 feet in diameter and weighs 4,535 tons, which is amazing and it is still the largest free standing concrete structure in the world.

Now Roman concrete allowed the Roman's to do an awful lot of stuff, specifically they used it for building aqueducts and this enabled them to bring water from very large distances away from Rome into Rome. Now curiously enough as you'll find out in your readings, the Romans didn't use all this water primarily for bathing or for toilets.

They actually used more for their gardens, because they wanted to demonstrate to the other Romans that they could harness nature. And so, it was a status symbol to be able to use water frivolously like that. Now the key to they hydraulic cement was and making a aqueduct was that you had to ford a lot of rivers as you were trying to build this and so you needed to be able to build the piers that supported your aqueducts underwater.

Now the Romans developed this art of making concrete and perfected it. But somewhere around 400 AD with the fall of the Roman Empire, we basically lost their ability to make concrete and we didn't know how to do it for a long time. So for over a thousand years, it was forgotten.

And basically, what was going on in that thousand years is they were incompletely burning the clinker. They weren't getting that stuff hot enough, so it didn't completely convert all of the limestone to lime. And in doing so, it basically made a very weak form of concrete. Now in 1756, there was a British engineer named John Smeaton and he discovered a way to make cement and the challenge there was he was trying to make the base for a light house and he had to figure out a way to make concrete that would cure in 12 hours.

You might ask why does it have to cure in 12 hours? Well, that's because it had to cure between the tides, between when you're going from low tide to the next low tide. And so the ability to cure between the two high tides was critical. And so he discovered that if he went and got some clay and mixed it with the limestone, then he made a concrete that actually set up very rapidly underwater.

And of course, the clay had aluminosilicates in it, much like the volcanic ash did in ancient Rome. And so in 1818, there was a debut of this cement and the United States for the construction of the Erie Canal. And in 1824, a brick layer named Joseph Aspdin i nvented what we call Portland cement.

It was not named after Portland, Oregon, it was named after Portland, England, because it resembled the limestone that was in Portland, England. There were 91 different kinds of Portland cement by 1898. And in 1917, we'd establish, basically, a standard form of it. Now in modern concrete, we don't typically use concrete by itself.

This is just concrete. And if I mixed it with water, I would be able to pour it into a surface. But it tends to not have a lot of strength, so what we do is we reinforce it and the classic way we reinforce the concrete is with a steel rod.

So you'll see rebar or steel-reinforced concrete. You could also pre-stress the concrete. So in other words, you form it under a stressed condition that will actually counter the force that the weight of the structure may encounter later on. You can pre-cast the concrete, so you can actually cast it ahead of time and make pipes or beams ahead of time before you need to use them.

You can air-entrain the concrete. In other words, you can put little air bubbles in it and that allows the concrete to expand a lot and so it's used ostensibly in very cold environments where it freezes. You can have a early-strength concrete. Concrete that hardens very quickly and that's used in colder climates and you can also have a lightweight concrete.

So as far as reinforcements go, I mentioned steel is one and that's the most popular one. Now it is susceptible to corrosion and so that's a big challenge with a lot of the infrastructure in the United States right now is that the steel re-bar in these concrete structures is starting to corrode.

And we need ways of monitoring that corrosion, so that we can keep up with whether or not these structures are still viable of not and when they may fail. You could also use glass fiber. You could use plastic fiber, reinforced concrete. You could add chemical additives such as accelerators, which will speed up the hydration process, the absorption of water.

Or you can use retarders, which will slow down that setting process in case you have some complex structure that you're trying to get the concrete into before it hardens. You can add plasticizer that will reduce the amount of water that you have to add. You can actually use mineral additives and so this has become very popular now to try to add other things, for example, like fly ash.

And up to 60% of the cement can be replaced with fly ash, which is a byproduct of the coal burning process. The advantage to this is that you don't have to actually reduce the limestone to lime, because you already have the material and a form that can be used in the concrete.

Now a lot of people say, well, I'm drying my concrete. You don't actually dry concrete. Concrete actually cures and it can set in several hours, but it take up to four weeks to get up to even 90% of its strength. So it gains a lot further strength over the years, so concrete or something that continually gets harder and harder with time.

And that's because you're reversing that reaction, you're taking the CO2 out of the air, slowly incorporating it back into the concrete and making your limestone back again. All right and that just takes along time to do it. And that's one of the reasons why you place plastic, for example, over a sidewalk when you're curing it is that you wanna keep the moisture in there, because that hydration reaction is what's actually setting up that gel and allowing it to become harder.

So in terms of where do we get the concrete today, we actually make over 3,300 billion tons of concrete annually and about half of that is being used in China alone. India uses some and then the US uses some and then the rest of the nations don't use as much as those three countries combines.

So the challenge for this is if you think about it, 3,300 billion tons of concrete is the CO2 challenge. So when I take limestone. Limestone is calcium carbonate. If you think about it, what was I doing when I did that reaction? I was driving off the CO2, that's carbon dioxide.

That's a greenhouse gas and it goes into the atmosphere. And so when I make concrete, I actually have two sources CO2 coming off. The first is about 50% is because I'm just releasing the CO2 from the calcium carbonate. 40% of it comes from the fact that I have to heat it up and grind it and I'm heating it up to 1,250 degrees see and that takes a lot of energy.

And so when you combine those two and you take into account how much concrete we use, it accounts for 7% of the worldwide production of CO2 today. And so this is a tremendous challenge for us as to how do we continue to use concrete and use it in a form where we're not driving so many greenhouse gases into the atmosphere.

There are other challenges with concrete. For example, if you build plants, you wanna build them everywhere and the reason is obvious and that is that concrete is very heavy. And therefore, it's gonna cost a lot to transport it and so you want to have it close to where you're gonna be using it.

The other thing you deal with is of course, you've got surface runoff if you have lots of concrete. You have urban heat islands. These are regions that are so rich with concrete that they heat up naturally by themselves. You have silicosis, because you're dealing with very fine particles.

And of course, alkalinity. Cuz when I make concrete, this material, when I mix it with water is very, very basic, because of all those hydroxides that are reacting. So you have to make sure that whenever you're handling concrete, you wear gloves. So in the next video, we're gonna talk about new approaches to making smarter concrete.

For example, adding shape memory allows or embedding sensors and those will all be observed in your video and then we'll talk about that in the next lecture.