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  • 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 sayingHow 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!

In the last video we talked about concrete 101, and why concrete is such a great construction

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B1 US concrete reinforcement beam stress steel tension

Why Concrete Needs Reinforcement

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    OolongCha posted on 2021/07/25
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