Concrete: 8. Concrete Cover - Life Safety

This video is about cover and concrete. Based on the American Concrete Institute recommendations for minimum cover, walls and slabs that are usually thin, there's not a lot of thickness there, need to have a cover of at least three-quarter of an inch. Excuse me. While beams and columns on the interior need to have an inch and a half of cover surrounding the rebar. And if the columns or beams are on the perimeter, then they need two inches as a minimum cover. And anything in the foundation, rebar in the foundation in contact with the earth, needs to have a cover of three inches. And the cover is secured with these chairs right here, concrete chairs or bolsters. These are bolsters. And what they do is basically lift, they come in different heights. They lift the rebar the amount of the minimum cover required for that member. So the reasons to have this cover is basically to protect the rebar from fire to protect the rebar from any corrosion or exposure to weather. That's why on the perimeter, the minimum cover is 2 inches versus interior is 1.5 inches, because there is exposure to weather, moisture, etc. And the third reason is essentially to provide embedment for the rebar so that it doesn't pull out. If it's surrounded by concrete, a minimum cover, And then spacing of rebars also has to follow certain criteria by the ACI. So that concrete goes in between the rebars and totally surrounds and provides embedment. So the fourth reason is really the one I want to discuss in this video, and it's life safety. So let's recap very quickly some of the terminology in concrete. H, the dimension H of a beam, the overall depth, is basically the dimension of the formwork. B and H are the dimensions of the formwork, but the structural depth is D, and it's less than H, because there is cover. This is the amount of cover. This is the amount of cover. We need cover wherever there is rebar. We need to protect it. So there is cover, inch and a half, two inches, something like that, And then there is a stirrup. And to the center of the rebar, usually this is around 2 1⁄2 inches. So the difference between H and D is roughly 2 1⁄2 inches, but D is the structural depth. And this concrete, I'm going to use blue because this concrete is in tension. It's really not important structurally. It's there to protect the rebar from corrosion fire, etc. And this concrete is in compression and doing the work. And on the tension side, of course, the rebar is in charge of the tension. The concrete is in charge of the compression. Together, they make a couple between compression and tension, and they resist external moments. Very good. So, we have a structural depth. And furthermore, as far as concrete goes, we have a certain area of concrete, this much, structural concrete, B times D, not B times H. B times H is going to give me, overall depth is going to give me the dimensions of the formwork and the dead load of the concrete. But the structural calculations are based on D, not H. And we mixed the concrete in a certain way, and we got a cylinder test, and we know the strength of our concrete based on the water-cement ratio, the proportion of the aggregates, how many bags of cement per cubic yard. So these are the properties of the concrete. We have this much of it and of this strength. Versus the rebar. The rebar, of course, is in charge of tension. And there is a number of bars. I don't know. Are you using three bars, four bars? what diameter of rebar, and the strength of the rebar is usually a grade 60. That's the one with the surface deformations. So we have a strength of rebar, we have a certain area of rebars, for number 8 or for number 9 or whatever it is. So we have area of rebar, we have strength of rebar, we have area of concrete, we have strength of concrete. And at some point, there is something called a balanced concrete beam design. And the balanced concrete beam design is one in which the concrete and the rebar fail simultaneously. So this one, the rebar and the concrete fail simultaneously. And of course, this is a situation that is disastrous. Because the whole thing collapsed, everybody's dead. Rebar and concrete fail simultaneously. So, in any reinforced concrete design, task number one is find the balanced member design for this much rebar and this much concrete of this strength. Find the balanced condition and then make sure you avoid it. So, for any member, now only one of these two is safe. And that's the logic of reinforced concrete. Find the balance condition for the materials you're using, then avoid it for life safety. Then we'll talk sheer moment span, how much load we have to carry. That's secondary. First and foremost, life safety. So scenario B, another way of failure is the concrete fails before the rebar. Or the other scenario is the rebar fails before the concrete. So rebar fails before concrete. So only one of these is safe. So the answer lies in Hooke's Law. In Hooke's Law, which is essentially a plot of stress versus strain for different materials, Hooke's Law says, here's concrete. It's a brittle material. It reaches its ultimate strength. It breaks. versus steel. It does that. It's much more of a ductile material. Steel can do tension and or compression. Doesn't matter. A concrete does compression only, doesn't do tension. That's why it's a brittle material. So back to our question, find the balance condition, then avoid it. Which one is safe? If the concrete fails first or if the rebar fails first? So if the concrete fails first, that would be option B. If the concrete fails first, then it explodes. Here, it explodes. And everybody in the room is dead, sudden, sudden death. And the rebar is up there and strong, and you can hang your clothes on it, but you're already dead. So this one is not a good option. This one is called an over-strengthened beam. An over-strengthened beam. Over-reinforced beam. Sorry, over-reinforced beam. And we don't want an over-reinforced beam because it's not safe. If it's over-reinforced, then yes, it can span more, it can do more shear moment, all that good stuff, carry more load. but it's not safe because if it fails, the concrete goes first, and the rebar is stronger than the concrete, and we don't want that. So the correct answer is let the rebar, this is an under-reinforced, an under-reinforced beam is the better way to go. Let's find the balance condition, then make sure the rebar is weaker than the concrete because it will yield instead of explode. It'll yield and give us warning by breaking the cover. The cover is there to show a crack. That's the life safety issue. If the rebar is getting tired, it'll start stretching. It'll start yielding. You'll get a little crack at the bottom of the beam. Then you'll get some spalling, which is pieces of concrete that start falling, very small pieces. That's the cover. It's not part of our calculations, structural calculations. Then we see the bottom of the rebar. Well, you have another 5, 10 years to do something versus if the concrete goes, it's sudden death. So in summary, find the balance condition for the materials, the concrete and rebar you have, and then make sure the rebar is weaker than the concrete so that it yields and gives warning. So, please, let me illustrate this very quickly here and say the following. Do not think, please, do not think that this beam is safe because it's not. It's got too much rebar for that much concrete. If you're not cutting it in the shear and moment world and you need to span more, well, then you're going to need more beam. And then you can put that much rebar in this beam because it's a matter of proportion how much area of blue rebars to how much area of red concrete. That percentage is set by the code and it cannot be exceeded for life safety reasons. Hope this helps.