Placeholder Image

Subtitles section Play video

  • Concrete is as much a part of the urban landscape as trees are to a forest.

  • It's so ubiquitous that we rarely even give it any regard at all.

  • But, underneath that drab grey exterior is a hidden world of complexity.

  • Hey I'm Grady and this is Practical Engineering.

  • On today's episode - it's concrete 101.

  • This video is sponsored by Brilliant.

  • More on that later.

  • Concrete is one of the most versatile and widely-used construction materials on earth.

  • It's strong, durable, low maintenance, fire resistant, simple to use, and can be made

  • to fit any size or shape - from the unfathomably massive to the humble stepping stone.

  • However, none of those other advantages would matter without this: it's cheap.

  • Compared to other materials, concrete is a bargain.

  • And, it's easy to see why if we look at what it's made of.

  • Concrete has four primary ingredients: Water, sand (also called fine aggregate), gravel

  • (aka coarse aggregate), and cement.

  • A recipe that is not quite a paragon of sophistication.

  • One ingredient falls from the sky, and the rest come essentially straight out of the

  • ground.

  • But, from these humble beginnings are born essentially the basis of the entire world's

  • infrastructure.

  • Actually, of the 4, cement is the only ingredient in concrete with any complexity at all.

  • The most common type used in concrete is known as Portland cement.

  • It's made by putting quarried materials (mainly limestone) into a kiln, then grinding

  • them into a fine powder with a few extra herbs and spices.

  • Cement is a key constituent in a whole host of construction materials, including grout,

  • mortar, stucco, and of course, concrete.

  • A lot of people don't know this, but every time you say cement when you were actually

  • talking about concrete, a civil engineer's calculator runs out of batteries.

  • I'm just kidding of course, and you can hardly be blamed for not knowing the difference

  • if you've never mixed up a batch of concrete before.

  • Even if you have mixed some concrete, good chance it was in a ready-mixed bag where all

  • the ingredients were already portioned together.

  • But, each ingredient in concrete has a specific role to play, and cement's role is to turn

  • the concrete from a liquid to a solid.

  • Portland cement cures not through drying or evaporation of the water, but through a chemical

  • reaction called hydration.

  • The water actually becomes a part of cured concrete.

  • This is why you shouldn't let concrete dry out while it's curing.

  • Lack of water can prematurely stop the hydration process, preventing the concrete from reaching

  • its full strength.

  • In fact, as long as you avoid washing out the cement, concrete made with portland cement

  • can be placed and cured completely under water.

  • It will set and harden just as well (and maybe even better) as if it were placed in the dry.

  • But, you may be wondering, “If water plus cement equals hard, what's the need for

  • the aggregate?”

  • To answer that question, let's take a closer look by cutting this sample through with a

  • diamond blade.

  • Under a macro lense, it starts to become obvious how the individual constituents contribute

  • to the concrete.

  • Notice how the cement paste filled the gaps between the fine and coarse aggregate.

  • It serves as a binder, holding the other ingredients together.

  • You don't build structures from pure cement the same way you don't build furniture exclusively

  • out of wood glue.

  • Instead we use cheaper filler materials - gravel and sand - to make up the bulk of concrete's

  • volume.

  • This saves cost, but the aggregates also improve the structural properties of the concrete

  • by increasing the strength and reducing the amount of shrinkage as the concrete cures.

  • The reason that civil engineers and concrete professionals need to be pedantic about the

  • difference between cement and concrete is this: even though the fundamental recipe for

  • concrete is fairly simple with its four ingredients, there is a tremendous amount of complexity

  • involved in selecting the exact quantities and characteristics of those ingredients.

  • In fact, the process of developing a specific concrete formula is called mix design.

  • And I love that terminology because it communicates just how much effort can go into developing

  • a concrete formula that has the traits and characteristics needed for a specific application.

  • One of the most obvious knobs that you can turn on a mix design is how much water is

  • included.

  • Obviously, the more water you add to your concrete, the easier it flows into the forms.

  • This can make a big difference to the people who are placing it.

  • But, this added workability comes at a cost to the concrete's strength.

  • To demonstrate this balancing act, I'm mixing up some ready-mix concrete with different

  • amounts of water.

  • For the first sample, I'm using just enough water to wet the mix.

  • You can see it's extremely dry.

  • A mix like this is certainly not going to flow very easily into any forms, but you can

  • compact it into place.

  • In fact, dry concrete mixes like this are used in roller-compacted concrete which is

  • a common material in the construction of dams.

  • For the next three samples, I used increasing amounts of water up to what is pretty much

  • concrete soup.

  • After the concrete has had a week to cure, I cut the samples out of the molds.

  • It's time to see how strong they are.

  • This is actually more or less how concrete is tested for compressive strength in construction

  • projects.

  • Obviously I'm not running a testing lab here in my garage, but I think this will give

  • us good enough results to illustrate how water content affects concrete strength, plus these

  • cylinders look like they might attack at any time, and we need to deal with them.

  • I made three cylinders of each mix, and I'll break each one, watching how much pressure

  • the cylinder was applying at the moment of failure.

  • And this experiment was too cool not to invite my neighbors over to help.

  • We started with the samples that used the most water.

  • It was no surprise that it took almost no pressure at all to break them, on average

  • about 700 psi or 5 mPa.

  • You can see how crumbly the concrete is even after having a week to cure.

  • All that water just diluted the cement paste too much.

  • The next two samples used the range of water suggested on the premixed concrete bag.

  • These were much stronger, breaking at an average of 1600 psi and 2200 psi or 11 mPa and 15

  • mPa for the high and low end of the water content range.

  • And you can really see the difference in how the concrete breaks.

  • Finally we broke the samples with the least water added to the mix.

  • You can see how rough these samples were, because there wasn't enough water for the

  • concrete to flow smoothly into the molds.

  • But, despite looking the worst of the four, these were the strongest samples of all, breaking

  • at an average of around 3,000 psi or 20 mPa.

  • On this shot you can even see the crack propagating through the cylinder before it fails.

  • It just goes to show how important mix design can be to the properties of concrete.

  • Even varying the water content by a small amount can have a major impact on strength,

  • not to mention the workability, and even the finished appearance of the concrete.

  • It's impossible to state how much I am just scratching the surface here.

  • There is so much complexity to the topic of concrete partly because it has so many applications:

  • from skyscrapers to canoes and everything in between.

  • In fact, no matter where you are, you're rarely more than a few feet from concrete

  • - a fact that is inexplicably a source of great comfort to me.

  • But, I took less than 10 minutes to describe what is literally the foundation of our modern

  • society.

  • So I'm dedicating at least the next few videos to dive deeper into the topic of concrete.

  • The next video will be about its greatest weakness.

  • If you've got questions about concrete, put them down below in the comments and maybe

  • I can get them incorporated into the next videos.

  • Thank you for watching, and let me know what you think!

  • If you are watching my videos, you probably share my passion for understanding how things work

  • and wanting to apply that knowledge to your everyday life.

  • To do that requires not only learning core concepts, but developing intuition.

  • Brilliant is a problem solving website that teaches you how to think like an engineer.

  • Their courses are designed to help you gain a deep understanding of the topic.

  • I love this one on solar energy that goes into lots of detail on the intricacies of these systems.

  • To get started developing your intuition, go to brilliant.org/practicalengineering

  • or just click the link in the description and sign up for free.

  • The first 200 people will get 20% off the annual premium subscription.

  • Again, thank you for watching, and let me know what you think!

Concrete is as much a part of the urban landscape as trees are to a forest.

Subtitles and vocabulary

Operation of videos Adjust the video here to display the subtitles

B1 US concrete cement water mix psi aggregate

What is Concrete?

  • 8 0
    OolongCha posted on 2021/07/25
Video vocabulary