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How to strengthen concrete?

Concrete is strong, but not strong enough to make roads out of them. In this video, we will explore how we can use the concepts of elasticity to improve the strength of concrete. Created by Mahesh Shenoy.

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Video transcript

concrete is strong we love it because of its most unique property that it gets stronger and harder with time but it's not nearly as strong as it's needed to make roads and buildings out of it so then how do we make roads and buildings out of concrete if you take a slab of just concrete and support it at its ends then when you put a force on the top if you put a lot of force from the top then this force will cause the concrete to deform and as a result the concrete will end up bending and we've studied before that whenever materials deform they try to snap back by generating restoring forces and these restoring forces which we calculate as stress or here they depend upon the amount of deformation now if you look at this concrete bent concrete carefully then you see that the top part has become shorter because it's the inside of the curve and the bottom part has become longer as a result the top part which is become shorter is under compression so this is under compression so there's a lot of compressive stress generated on the top so there's compression on the top and at the bottom because it's the outer side which is longer side there's a lot of tension it has been stretched so there is a lot of tension so there's tension on the outside and we can visualize this by using an example imagine we had a beam like this with some sticks through that and if you bend this beam just like the concrete then notice the sticks at the top come close to each other just similar to this the molecules on the top of the concrete come close to each other and that's why this sticks over here we could say they're compressed and similarly notice the sticks at the bottom they've gone far away from each other and similarly the molecules at the bottom go far away and that's why they're under tension now concrete turns out to be quite brittle quite brittle it doesn't mean that it's weak it just means that if you put a lot of stress and you go beyond the elastic regions then very quickly it will and another property of concrete is that it has a great compressive strength compared to tensile strength in fact it turns out that the compressive strength of concrete compressive strength is roughly ten times its tensile strength which means we don't have to worry too much about concrete getting compression it can withstand compressive forces very nicely but it is very poor at withstanding tensile forces and as a result if this weight is too much then the concrete will end up cracking cracking at the bottom and that is very bad because first of all we can't see that and if the bottom part of the crack concrete cracks then the whole Road can break and this is very bad so how do we make our concrete stronger well we do that by reinforcing this bottom part of the concrete with steel wires so we use steel reinforcement this is steel now the good thing about steel is like unlike most materials which are good in compression but they are very bad in tension steel is unique in that it is having pretty much same compressive and tensile strength and the compressive and tensile strength of Steel is way more than that of concrete so reinforcing concrete with steel will actually make our entire concrete stronger and so what we do is we lay down with steel wires and then we pour concrete on top of that so we pour concrete on top of this steel we make sure the steel wires are pretty much at the bottom where we need the reinforcement and we allow this concrete to get set along with the steel in such a way that the concrete forms a bond with the steel and now if we put that same Lord as before we put the same weight as before again this concrete is going to sag a little bit not as much as before because now we have steel reinforce it but anyways it's going to sag however this time the tensile forces acting at the bottom of that bottom of the concrete is shared between concrete and steel and since steel is way stronger than most of the tensile forces are taken up by the steel this is very similar to how the bones in our body works whenever you carry any weight most of that weight is actually taken up by the bone only a small portion of that weight is taken up by the flesh or the muscles that are attached to the bone the concrete is pretty much like the flesh and this steel now literally acts like the backbone of this whole system but of course if you put too much weight on this as you start putting more and more weight on this the tensile forces start increasing more and more and even though steel is taking the majority of that force the concrete is taking a small portion of that force and as those force increases eventually the concrete will start developing cracks again again this is very similar to what happens in our body if you had to carry too much weight which your bones can withstand by the way but because the forces are being shared between the bones and the flesh there can be muscle fracture they can be there can be tears that your muscles might get pulled and so something similar is going to happen over here the steel is quite strong but since the forces are being shared if you put too much weight eventually this thing will crack and again this could be bad so this reinforced concrete is good it's strong but the big question is can we make it stronger I mean yeah of course we can put a lot more steel wires over there but there's a limit to how much you can put over that right besides that can we do something else to make it even stronger turns out that we can one way in which we make the concrete even stronger today is by changing this procedure of reinforcement just a little bit so we take these steel wires and we're gonna pour concrete on top of that but before we do that we stretch these steel wires by pulling them with huge forces so we are going to stretch these steel wires so let's say yeah we're gonna stretch them so we're gonna stretch them with huge horrendous forces because steel is extremely strong and because of these forces our steel is going to and up stretching oops it's going to end up stretching stretching like this so our steel wires now will be under tension and this is where it gets really interesting keeping these tensile forces we are going to now pour concrete on top of this all right so we're gonna pour concrete on top of this keeping the steel wires stretched and then we're going to once the concrete has set and it has bonded with this to you then we are going to remove these tensile forces question is what's going to happen well notice because there was because the steel is under tension right now the steel tries to snap back so there will be there will be restoring forces acting on the steel so steel tries to snap back by putting the restoring forces but the steel can't snap back because there's so much concrete over there and since the concrete is bonded to the steel now this restoring forces are transferred to concrete and as a result the concrete ends up being compressed you get that because the steel is trying to snap back over here and it's bonded to concrete the concrete ends up undergoing compression at the bottom he'll undergo compression and you might wonder like well why are we doing that whoa ask this bottom layer undergoes compression the bottom layer comes closer to each other so the bottom layer becomes smaller in size compared to the top layer but remember you are your concrete slab can't maintain shape like this so as a result of the compression at the bottom the concrete ends up buckling somewhat like this and so this concrete anyone without any load or even or any weight the concrete is already buckled it's already stressed out so the bottom layer now is under compression and of course the top layer is under a little bit of tension but we're going to make sure that the tensile force is not too much so not too way to the concrete won't crack this kind of concrete which already has a stress even before you know putting the Lord we give it a name we call that as pre-stressed concrete and the name itself tells you pre because even without learning it's already stressed out can you see where we are going with this can you see how this is beneficial to us well now when you start loading this if you now start putting weights on top of this so if you put weights on top of this because of the downward force the concrete now will snap back to its original shape in simple terms what we have done is since we know that putting weights is going to bend concrete this way to counter that we made sure initially itself the concrete rolls bent the other way so that when you put load the bending cancels out and the conquer ends up relaxing so even under this in under this influence of the weight the concrete now is in this relaxed in its natural state and has a result it can now extend even higher weights and this pre-stressed super strong concrete is used today in building constructing dams and and constructing roads and you know building new buildings and everything and so not is that understanding the elastic behavior of concrete and steel allowed us to enhance the properties of concrete and make it super super strong