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!
Is this guy's channel pretty sound engineering? Thinking about subscribing since I'm in Construction Management and he has lots of videos about civil engineering.
Edit: Just surveyed a few of his vids and they seem solid. Subscribed.
I wonder how those low-water samples would react to fatigue. Seems the surface defects should allow for surface cracks to form and propagate pretty easily.
Note that aggregates do not generally make concrete stronger than just cement and water. My lab tests geotechnical grout used in soil nail walls and micro piles often (cement and water mixed to a flowable consistency, SG<2) and they regularly exceed 8,000 psi at 28 days.
Quite interesting. Looking forward to the next video. 👍
More on Roman concrete would be very interesting http://www.sciencemag.org/news/2017/07/why-modern-mortar-crumbles-roman-concrete-lasts-millennia
This guy almost makes civil engineering sound cool.
He seemed bang on in his videos about fluids, but I don't know anything about concrete to judge.