Concrete: 7. Prestressed Concrete Pretensioned And Posttensioned Comparison

In this video I'll talk about pre-stressed concrete and that includes pretension concrete and post-tension concrete. Pre-stressing is basically placing stresses in structural members to counteract predicted stresses when loads are applied. So we expect a certain load, we expect a certain deflection, we expect a certain bending. So we will counteract that by pre-stressing. So we'll do something to the wire, and then it'll transfer stress to the concrete. So the objective of pre-stressing is basically to send the wire into more tension and transfer that stress to concrete as compression. So put the wire in more tension, put the concrete in more compression. Very good. So in pre-tensioned concrete, a lot of our pre-stressed members are pre-tensioned. There's wire going in here. By the way, I'm sorry, I didn't explain. That's the bed, the formwork for a double T at a pre-caster. And there's wire in these stems. And then they pour. Sorry, they pull the wire. The wire is bare. It's not in a plastic sheath like with post-tension. In pre-tension, the wire bonds with the concrete. They pull it tight. They pour the concrete. Then they snip it. They release the tension. and it puts the concrete in more compression as the wire tries to relax back into its initial position. And it gives us a camber. Very good. With post-tensioned concrete, what happens is there is no bonding between the wire and the poured concrete. Whereas pre-tensioned concrete is in a manufacturer, in a plant, under quality control, Post-tensioned concrete is done on the site, and it's done after the concrete cures versus before it's poured, as is the case with pre-tensioned concrete. So with post-tensioning, I'd like to go over this image here. The idea of pre-stressed concrete, be it pre-tensioned or post-tensioned, is that it allows the member to carry either more load for the same area, Or if you want to carry a certain load, you will need a lot less area when you pre-stress concrete. Okay. A smaller area means less concrete, means less foundation under the concrete. Of course, that's more sustainable. So pre-stressing is a good thing in concrete as long as it's warranted economically. So it has to have enough floors in the case of post-tensioned concrete or enough repetition in the case of pre-tensioned concrete. Very good. So I'd like to go over this image here. Maybe I'll come back to it. Okay, let's come back to it later. But basically what happens here is as this wire is pulled, as this wire is pulled from both ends or fixed on one end, pulled on the other, this starts to go down, this starts to go up, so that we end up somewhere in the middle. And we're stressing the cable after the fact when the concrete is strong enough. We're pulling that wire, stressing the concrete, and pushing it down into more compression and reducing the amount of tension here and here by taking away the tension from the concrete and sending it into compression. Okay. Pretensioned concrete includes panels that can be used as walls that have insulation in them because they're poured flat and they are pretensioned. Also, hollow core planks are pretensioned. And, of course, double T's are pretensioned. Now, this image says it all because transportation is a very big deal because you've got to get it from the factory to the construction site or the highway or whatever it's being used for. And therefore, making a right turn becomes very difficult after a certain point and it becomes very expensive. So typically, up to 75 feet for pretensioned members is okay and licenses on the highway and whatnot are bearable. Above that, it gets expensive. You've got to put an orange flag and you've got to get a special permit and the follow-me truck, blah, blah, blah. Okay, so you don't have that problem with side-cast post-tension concrete. Very good. So here we're seeing the wire. It's braided and it's bare. It's not in a plastic sheath. And here it is. Here's the double T bed, and it's over here. And they put wire in here, as we said earlier. Over here, for example, it's not a double T, but it's a wall, a wall panel. And then they put some mesh here, and then they pour. And the concrete bonds with the wire, but the wire is pulled very tight. Then they snip it. And by the way, they add probably a little bit more tension in the wire in order to account for creep, which is deflection over time. So if you stress that wire a little bit more, then you can counter creep. And then you also get a concrete which is more resistant to shock and to vibration. So there's a lot of advantages. Okay, so that's the pretension concrete, and here we see the double T bed again. We see the prestress wire in there that is bare, and we see it here in this double T. It's right here, and the top flange is basically, look how thin it is. It's very thin, and it's basically, its function is to tie the two stems together. The stems have depth, they have section modulus, they have moment of inertia, they take care of bending and deflection, but to act together they need a flange. Very good. So here we are, and there is two stems, and they have pre-stress wire in there. They pour several of these at the same time, and there's a divider between them. And then they remove the divider and they snip the wire after the concrete is cured. This is a fast process. They're going to pour, they're going to fill the bed with however many double T's they can fit. They're going to pour the next day. So they're going to use high early strength concrete. Excellent. Versus post-tension concrete. Post-tension concrete is done on the site and it's in a plastic sheet and it's greased so that it can pull the wire or the post-tension tendon, as it's called, after the concrete cures. So it's anchored to the formwork here, or they can tie it inside the slab to rebar. Remember, it's not going to be pulled until the concrete is strong enough. And once it's pulled... Okay. So here, let's talk about this picture, where we have rebar here, and stirrups and traditional conventional concrete. But then in addition, we have that stuff, which is your post-tension cable. And I wish these were yellow like the previous slide. But if you realize what is, how it's happening, and maybe I'll go back to this slide now. it basically follows that moment diagram. Whatever the moment diagram does, the post-tension cable is very easy to drape. Based on these chairs, it's easy to drape it so that it does the moment diagram. And over here, over the girder, I have a negative moment. Here, I have a positive moment at mid-span. Then it comes up again, negative moment. Because what's happening here is when you have a continuous beam, which is what concrete is, what's going to happen here is something like that. And so negative moment needs rebar at the top. Positive moment needs it at the bottom. Either rebar or it's much easier with the post-tension cable to do it with the post-tension tendon. So I have a positive moment, then I have a negative moment, then I have a positive moment again, and then at the very end I have zero. So once the concrete is poured and it's strong enough, when you pull this cable with a hydraulic jack, as I will show in the next slide, this wants to go down, compressing the concrete. This, instead of sagging, now the concrete is being pulled up by this post-tension tendon, so it's going to experience less tension, which means more compression. The idea is to put the concrete in less tension and as much compression as you can. That's the role of pre-stressing, post-tensioning or pre-tensioning. Excellent. So here we see how this works. They'll put some chalk on the wire, this post-tensioning. They'll bring this hydraulic jack, and they know how much to pull it. once the concrete has cured. And we see here that the wire pulled a little bit. It's greased inside the sheath so it can move. Then they'll go back and they'll cut the wire and they will grout the holes so that no rust will leak on the concrete. That's the whole idea of this post-tensioning versus pre-stressing. Take the concrete out of tension, put it in more compression. Okay?