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!

Comments (100)

  1. How about pouring concrete underwater for pillar?

  2. Have you experimented withAircrete ? I am just beginning to learn about it and wouldlike your insight.

  3. why the compressive strength of the material is greater than its tensile strength?

  4. What about mixing in a fibrous material such as straw? and would titanium rebar be any more or less effective than steel?

  5. Please make a video on BASALT reinforced concrete and compare it to Steel and Fibre-Glass. (Basalt Rebar, Basalt Fibre and Basalt Mesh)

  6. Very good movie! Explain let reader easy understanding

  7. Roof leaking problem solution

  8. concrete with rebar and traces of nylon brand spray paint

  9. super materials, thank you my freind

  10. 6:49 Always demonstrate proper technique in any informative video. Using pressurized water to mix concrete creates excessive silica dust which is a long term health concern. The dust stays airborne for 24 hours and is easily disturbed when settled. Really liked the video!

  11. If this would be true then my Minecraft houses would have been destroyed..

  12. Another fantastic video. Educational, interesting, and entertaining, thank you! 👏👏👏

  13. Are there materials which have less compressive strength than tension strength

  14. Thanks for video . Very helpful

  15. I started learning about the strengths and weaknesses of concrete when I was a kid. I could break thin pieces of concrete with my hands just by giving it some shear pressure, but push on a piece until my hands bled in an attempt to break it.

  16. Hi, i want you to test what happens if we equalise tensile strength to the strength against compression in a beam by introducing thinner bars of steel at the top side and thicker bars at the bottom side of a beam (of course calculations are involved regarding the thickness of steel bars). In my opinion we would get stronger beams if we are able to equalise the two strengths somehow.
    If you have already done so please share the experience.

  17. hey i was just wondering how glass reinforced concrete seems to have impossible tensile strength whilst glass fibres are so brittle?

  18. He talks so peacefully lol

  19. i am studying Chemical engineering
    Why the fuck am wasting some minutes of my life?

  20. Hey, I have just started watching your videos. I am a building supervisor and commercial diver and I have really found your videos informative and easy to watch.

    You have done such a good job of covering important aspects while not overloading with information given the relatively short time you have to explain a topic.

    Awesome job

  21. you sound like that guy who reviewed on hummer suv😁😁

  22. I was just doing cracks, stop spying on me youtube

  23. Rebar, dont you mean URBAN STICK

  24. Thanks for your complete and excellent explanation. Would you post a video about how to do a pull out test?
    I've searched lot's of sites but couldn't find any thing specific. Thanks

  25. A very promising concrete alternative that's getting a lot of attention is mycelium based construction material. It's more robust, has exceptional insulation properties, has a controlled failure path (cracks vice explodes under sudden, high pressure), floats, and do so on. BOLO for this tech; especially since the concrete industry is ripe for disruption. Enjoyed your video. Namaste, JaiChai

  26. I would like to see the reinforced bending test repeated with the ratio samples you ran in the last video. I'm wondering if the dryness/wetness result will be the same for tension under bending.

  27. Can you do a video comparing rebar reinforcements to concrete vs. fiber mesh added to the concrete as a reinforcement??

  28. Isn’t knowledge fascinating!

  29. I have heard adding grass or hay to your concrete can help with its pliability. Is this true?

  30. can we not reinforce concrete the same you reincforcex a sand block with layering so many sheets of carbon fiber?

  31. I love YouTube and am thankful for people like yourself that make these videos. I didn't realize how complicated concrete was.

  32. Can you make a video specific to the wtc buildings regarding the dynamics of the building failures? To disprove the conspiracy theorist about 9/11. Example, I know someone who believes that airplanes never hit the building's and the videos we see are cgi and the building collapse were controlled demo from underground nukes. Crazy, I know and I've hit him with all kinds of physics like how many Newtons the 767's impacted the buildings and what I'm suggesting is from an engineering point of view, what would it take to penetrate the wtc and is that consistent with the facts. I'm not the conspiracy theorist I've just argued with them. I'm a pilot with basic physics and engineering aptitude but I have above average understanding of flight. Thanks! Look forward to the video, if possible.

  33. What about trying Helix micro rebar? Or fiber?

  34. Rear cheap my ass. Try 4 to 8 dollars a piezo

  35. what would be the best and safest refractory castable formula be for a pizza oven and should the floor and dome be 2 different materials?

  36. i just opened home renovation business because of you many thanksss

  37. When make concrete, what happen if you change water with HOT water?

  38. Your images helps your explanation. Thanks and keep up the good work

  39. really good explanation , thank you very much

  40. Hi, wonder if you would see my comment. If we had two same reinforced beams as per your casting dimension. Will both have same ductile strength whilst either one has been cracking under pressure mode and repairing with epoxy resin and do nothing to the other one.

  41. How do you reinforce an eccentric footing?

  42. Can wood be used as a substitute to rebar?

    I've been watching your videos and some primitive technology videos where they create structure using limestone cement without any rebar

  43. Brilliant video. Thank u

  44. could you put the pressure figures?

  45. How demolishing of prestressed beams are done?

  46. Hi, thanks for the good work.
    Can you talk about concrete slabs reinforced with carbon fiber?

  47. How about using thin but strong bamboo as reinforcement?

  48. So how can you prestress rebar within a foundation slab of 900m2

  49. Bars are expensive, what about reinforcing the concrete with fibreglass or wool rope or line like the one used in ship, ? is that cheaper

  50. Do hydraulic press vs reinforced soil

  51. Thank you for sharing this awesome knowledge.. thank you..

  52. Thank you a lot its very helpfull, appreciate. I suggest all the fresher should watch this… Again thank you Mr..aaaaahhh. Will you explain abt "Structure analysi series" please??????

  53. "You shouldn't make a rope out of concrete"

  54. Please make a video on FERROCEMENT STRUCTURES ✌️
    I love watching ur videos 😍👍👍👍👍

  55. im very interested in use of plasticizers (i had to google it up to help me translate it). as i study building i would want to know how to use them and in which case i should definitely use them.

  56. Hey man did you know you need to vibrate the concrete to remove the air and the would strengthen it but don't over vibrate it you'll burn it

  57. Rock sand or robot sand compared to sand how strong it is as now days sand is banned considering environment.

  58. Not about what the church said you shit

  59. 字幕品質很好

  60. Please make a video on RCC calculations

  61. Here in my garaaaage

  62. How do 1st and 2nd floor concrete column stick to each other and how much tensile force can they sustain?

  63. Do we have concrete recipe against rats and underground crawlers?

  64. So, what about something like a wire mesh? Layers of it in the concrete.

  65. What is sfd and bmd , point of contraflexure…. And their uses

  66. "You should make nothing only out of concrete.". While that is FINANCIALLY true, it's not STRICTLY true because in practice, all the massive masonry buildings of the old (think of old castles, ancient massive buildings, even old village houses made out of rocks) worked, however, with huge sizing.
    In general, it would work but you need extremely thick walls and the roofing has to be something else, so, it's generally extremely impractical and expensive.

  67. A gigantic area regarding the subject is prevention of moisture. Prevention of deterioration of steel and concrete in general.

  68. great series thanks

  69. This has been in my watch later for over a year lol.

  70. Mix it with fiberglass

  71. Your videos are the best!!!
    Please upload more videos on concrete structures.
    Please please!!!

  72. Can u explain what’s the best concrete for super heavy things .. like trucks on a bridge..

  73. reinforce concrete with fiber-glass sheets

  74. Want civil engineering lectures subscribe to ST SOLUTIONS

  75. Interesting channel! I subscribed !

  76. Honestly idk why I'm here

  77. I'm in Accounting, yet here I am.

  78. It would be really cool if you did fire tests on concrete with temperatures ranging from 0 C to 5000 degrees Celsius.

  79. 0:58 what about torsion?

  80. i just watched a guy galk about crack

  81. Basically why tat one bridge collapsed

  82. Well u didn’t let it build up to its full strength

  83. 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.

  84. use a steel rebar having special design having zig zag zzzzzzzzzz it will give extra strength

  85. use a design like a screw so they are impossible to moving longitudinal direction

  86. You can do the same with ice by adding fibers like hair or thin rope

  87. The videO was juz waaaooo😍 aLmost made my confusions clear Thnks a Lot

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