Why Concrete Needs Reinforcement

Why Concrete Needs Reinforcement

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
someone like AJD is so valuable to me. If you’re trying to learn a new skill or
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!

About the Author: Michael Flood


  1. 2:00 You should at least mention #roman concrete that showed us how non-reinforced concrete is even better and avoid rods corrosion if you use a correct design. You only showed us horizontal beams, of course the forces are mixed, you completely avoid one of the most omnipresent intrinsic concept of the universe; Geometry! Talking #architecture which is also a big part of concrete use that you don't even talk about; #Arches are what allow to manage precisely the force, see the reverse chain methode.

  2. I have a question sir,
    If a beam of 27 feet is made to manage a weight of 100tones what should be its thickness and what would be the material to be used

  3. I have been making a search on how best to reinforce my concrete considering steel on one option FRP on the other and both FRP and still on another option. Can you help us test different concretes with these individual options so we can see which of them can withstand more stress? I know both steal and FRP rebars have their advantages and disadvantages and I want to know which one can best do the job or how to manage them or both to achieve the best results. I will be looking forward to those videos soon. Thank you.

  4. Hi
    I need to build bridges across some 3 ft deep x12 ft wide ditches on my property. The heaviest machine I have is about 14,000 lbs. I want to use 12" of 40lb I beam prestressed with 5/8" steel cables, cast in concrete. Any help would be great.

  5. Let's reinvent concrete and just skip rebard and just go for keflar.. and instead of too much sand in the mixture let's add some synthetic fiber.. see how well that does..

  6. use concrete having some specific zig saw puzzles piece, so the strength increased, if not understand discussed with me

  7. Hi, im a student from vietnam.
    I love your video.
    Would you mind if i sub all the video and re upload for the students in my country to understand more about this practical.
    Of course i will have to name your channel in my description.
    Thank you for reading this message.
    I hope you can answer my questions.

  8. thanks for the video I'm currently doing my degree in civil engineering but I have a difficult time understanding the pre-stress and post-tension concrete and your video help me a lot thanks

  9. I'm constructing a roof top of my house, and I want to make the roof of my house of concrete too. I have made all the reinforcement with steel rods. What psi should I used, and would it be ok to use regular cement instead of light way cement?

  10. Man you shouldn' watch innovation from Singapore – ConFlexPave. Flexible concrete. Search and you'll see… 😉

  11. I'm interested to learn about concrete because I'm going to build a house soon and in my country the philiipines,almost all houses now are made of concrete.unlike the house of my grandfather way back early 1900 and late 1800 ,there houses were made of solid wood.

  12. I used to work doing conc coring…i hope you know conc has a new chemical known as SILKIA its 100 times worse than asbestos..its a new bonding agent its in every aspect of concrete…do not inhale any dust particles..OSHA has new laws regarding this.. So that was a great video well done i loved it.. But be carefull my brother…. Look into it… Im in miami, florida👍👍❤❤👍👍

  13. Hello. We live in a concrete rebar reinforced home that was built in 1985. No problems so far. How long could it last? Thanks.

  14. After defining three (3) different types of stress, this video addresses only two (2) in explaining how steel reinforced concrete beams are made. How are shear stresses handled?

  15. Learning is a beautiful thing. Thx for the great vid ! Do more on concrete, how bridges are made and about structural engineering. ! Thx again

  16. I wonder if you make only a steel square tube the size of that concrete block what would be the thickness of its walls (and thus total steel material) to achieve the same compression strength as the concrete. (the image at 3:07 made me think do we even need concrete with this much reinforcement 🙂 … well ok, not as that rebar net structure, but appropriate types of beams etc…)


  18. Should have put up the numbers for the reinforced concrete so we could see just how much extra strength it added.

    And some of the crazy all-caps folks do have a point, you didn’t actually cure the concrete, and not sure your mixes were exactly the same every time.

    But I think the point of the video is still valid, just would have been nice to see all that done.

  19. Gud murning sir..your videos are very helpful for beginners like me..so can i ask u a question..whats the distance from support should the stirrups be more congested?

  20. I just learned that thin plastic fibers are added to steel reinforced concrete to improve fire resistance. Sounds counterintuitive at first as plastic decomposes at roughly 300 °C… the trick is that decomposing plastic leaves microscopic channels in concrete, dissipating steam pressure building up in concrete under extreme heat that would otherwise blast off pieces of concrete, exposing steel reinforcements to the fire and thus substantially weakening the entire structure as steel loses most of its strength at fire temperature. I'd like to see a video about this (but please don't set your garage on fire!)

  21. Isn’t every failure a Ductile failure then because no matter how quick the failure is you can always look at the failure in a slower speed hence showing you the problem a minuscule amount of time before final failure.

  22. Short answer; because
    Medium answer; because while its strong against pressure, it lack tensile strength. Rebar provide that.
    Long answer; watch the video.

  23. Just wanted to ask that for ductile failure, is the the fracture also ductile or the fracture for this case is brittle just like in brittle failure.

  24. What was your cure time on the first cylinders? DOT requires a 7, 14, and 28 day cylinder.  if you used the cheap Quickrete from lowes youll only have around a 1500 psi break.  the "high strength" Quickrete that advertises 5000 psi breaks at only around 3000 psi at 28 days.  do you have a video on curing time? there is a lot of factors with the concrete mix formula too that can affect the breaking time. I need to check out your channel and see what kind of videos you have.

  25. Your experiment with the cylinders of concrete had correct results, but the cylinder under tension would have a much higher tensile strength if the eye bolts weren’t secured in that manner. They eye bolts themselves are defects and therefore raise the stress at those points, leading to a much much lower measured tensile strength.

  26. Find a better material that flexes a little. There must be something they can mix in, that allows a little give, so it doesn't "snap" so easily. Kinda like trees that bend in a strong wind. Even tensioned concrete has issues: https://www.youtube.com/watch?v=hBjntrebxj8

  27. What about a 55 gallon steel drum filled with quikrete 4,000 psi yellow bags with a tractor draw bar going through the barrel? 11 1/2 bags 920 pounds without any rebar in it only the drawbar which is maybe 1" thick by 2 1/2" wide. I made a 3 point tractor connected counter weight and now wonder if it's going to break when I go to lift it with the tractor hitch? I'm thinking 1,000 pounds of weight isn't that much gravity force to cause it to break but what about bouncing over rough terrain with that weight? Hopefully the draw bar can handle that kind of weight too 😂
    Really hope it stays together and in the future I'll add rebar

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