In the last video we talked about concrete
101, and why concrete is such a great construction material. But, I didn’t mention its greatest weakness. Hey I’m Grady and this is Practical Engineering. On Today’s episode, we’re continuing the
series on concrete with a discussion of reinforcement. This video is sponsored by Skillshare - more
on that later. To understand concrete’s greatest weakness,
first we need to know a little bit about mechanics of materials which is the fancy way of saying
“How Materials Behave Under Stress.” Stress, in this case, is not referring to
anxiety or existential dread but rather the internal forces of the material. There are three fundamental types of stress:
compression (pushing together), tension (pulling apart), and shear (sliding along a line or
plane). And, not all materials can resist each type
of stress equally. It turns out that concrete is very strong
in compression but very weak in tension. But, you don’t have to take my word for
it. Here’s a demonstration: These two concrete cylinders were cast from
the exact same batch, and we’ll see how much load they can withstand before failure. First, the compressive test. (Hand pump gag). Under compression the cylinder broke at a
load of about 1000 lb (that’s 450 kilo). For concrete, that’s pretty low because
I included a lot of water in this mix. The reason is my rig to test the tensile strength
isn’t quite as sophisticated. I cast some eye bolts into this sample, and
now I’m hanging it from the rafters in the shop. I filled up this bucket with gravel, but it
wasn’t quite enough weight to fail the sample. So, I added another dumbell to push it over
the edge. The weight of this bucket was only about 80
lbs or 36 kilos - that’s less than 10% of the compressive strength. All this to say, you shouldn’t make a rope
out of concrete. In fact, without some way to fix this weakness
to tensile stress, you shouldn’t make any kind of structural member out of concrete,
because rarely does a structural member experience just compression. In reality, almost all structures experience
a mixture of stresses. That’s no more apparent than in a classic
beam. This particular classic beam is homemade by
me out of pure concrete here in my garage. Applying a force on this beam causes internal
stresses to develop, and here’s what they look like: the top of the beam experiences
compressive stress. And the bottom of the beam experiences tensile
stress. You can probably guess where the failure is
going to occur on this concrete beam as I continue to increase the load. It happens almost instantly, but you can see
that the crack forms on the bottom of the beam, where tensile stress is highest, and
propagates upward until the beam fails. You see what I’m getting at here: concrete,
on its own, does not make a good structural material. There are just too many sources of tension
that it can’t resist by itself. So, in most situations, we add reinforcement
to improve its strength. Reinforcement within concrete creates a composite
material, with the concrete providing strength against compressive stress while the reinforcement
provides strength against tensile stress. And, the most common type of reinforcement
used in concrete is deformed steel, more commonly known as rebar. I made a new beam with a couple of steel threaded
rods cast into the lower portion of the concrete. These threads should act just like the deformed
ridges in normal rebar to create some grip between the concrete and steel. Under the press, the first thing you notice
is that this beam is much stronger than the previous one. We’re already well above the force that
failed the unreinforced sample. But the second thing you notice is that the
failure happens a little bit slower. You can easily see the crack forming and propagating
before the beam fails. This is actually a very important part of
reinforcing concrete with steel. It changes the type of failure from a brittle
mode, where there’s no warning that anything is wrong, to a ductile mode, where you see
the cracks forming before a complete loss of strength. This gives you a chance to recognize a potential
catastrophe and hopefully address it before it occurs. Rebar works great for most reinforcement situations. It’s relatively cheap, well-tested, and
understood. But it does have a few disadvantages, one
of major one being that it is a passive reinforcement. Steel lengthens with stress, so rebar can’t
start working to help resist tension until it’s had a chance to stretch out. Often that means that the concrete has to
crack before the rebar can take up any of the tensile stress of the member. Cracking of concrete isn’t necessarily bad
- after all, we’re only asking the concrete to resist compressive forces, which it can
do just fine with cracks. But there are some cases where you want to
avoid cracks or the excessive deflection that can come from passive rebar. For those cases, you might consider going
to an active reinforcement, also known as prestressed concrete. Prestressing means applying a stress to the
reinforcement before the concrete is placed into service. One way to do this is to put tension on the
steel reinforcement tendons as the concrete is cast. Once the concrete cures, the tension will
remain inside, transferring a compressive stress to the concrete through friction with
the reinforcement. Most concrete bridge beams are prestressed
in this way. Check out all that reinforcement in the bottom
of this beam. Another way to prestress reinforcement is
called post-tensioning. In this method, the stress in the reinforcement
is developed after the concrete has cured. For this next sample, I cast plastic sleeves
into the concrete. The steel rods can slide smoothly in these
sleeves. Once the beam cured, I tightened nuts onto
the rods to tension them. Under the press, this beam wasn’t any stronger
than the conventionally reinforced beam, but it did take more pressure before the cracks
formed. Also, this one wasn’t quite as dramatic
because instead of failing the actual steel rods, it was the threads on the nuts that
failed first. I hope these demonstrations helped show why
reinforcement is necessary in most applications of concrete - to add tensile strength and
to change the failure mode from brittle to ductile. Just like the last video, I’m just scratching
the surface of a very complicated and detailed topic. Many engineers spend their entire career studying
and designing reinforced concrete structures. But, I’m having some fun playing with concrete
and I hope you are finding it interesting. I’d love to continue this series on concrete,
so if you have questions on the topic, post them in the comments below. Maybe I can answer them in the next video. Thank you for watching, and let me know what
you think! Thanks to Skillshare for sponsoring this video. Just about every step of producing a video
for this channel is something I learned to do through online tutorials and videos. And we all know how varied the quality of
that content can be. Skillshare allows you to learn new skills
from experts in their fields producing high quality classes, like this one from world
famous burly graphic designer Aaron Draplin. I make a lot of technical illustrations on
Practical Engineering to communicate complex topics, so learning new tips and tricks from
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improve on an existing one, cut through the clutter of online tutorials and click on the
link in the description below to start learning with Skillshare. The first 1000 people to sign up will get
their 2 months free. Again, thank you for watching, and let me
know what you think!
Hey that's me ;) Thanks for posting this!
Excellent video.
I've learned more about concrete today than I ever have before.
You know those things you're not interested in until you watch a YouTube channel about it? This is that thing.
This guy seems like the nicest guy around!
I really love Grady's videos. They're always concise, easy to understand, and the productions are typically very pleasing. I wish there was a similar channel for electrical engineering. There's GreatScott, the EEVBlog, and a few others. I feel like GreatScott and ElectroBOOM come the closest to this, and there are a few Real Engineering videos that get close, but there's just something about Practical Engineering that seems... better?
Actually, now that I think about it, since ElectroBOOM has been doing the AC series, the quality is starting to get toward this. There's just typically a "watch me hurt myself" element.
I wonder what his thoughts are on non-commercial structures. Like this guy's structure on an island https://www.youtube.com/watch?v=CHdL_V43ydw&
Normally I see an 8 minute video get about 2 minutes in and am completely bored and move on. This one kept my attention and was fascinating all the way through.
The down side is I actually need to get stuff done at work today not spend all day on youtube, but here we are.
Would you be able to substitute reinforcement for encouragement?