Concrete: 7b Prestressed Post Tensioned Concrete

So we talked about pre-tensioning precast concrete members now we're going to talk about pre-stressed post-tensioned concrete members that are cast in place of course we're going to start with the eye candy so here's some beautiful uh projects that are all of course done cast in place and post-tension. Anytime you have a high rise, it's most definitely done with post-tension concrete, whether it's the One World Trade Center, Burj Khalifa, or these pencil skyscrapers that have gone up around Central Park. They're known as Billionaire's Row because they're so extremely expensive. They're all concrete, they're all cast in place, and they're all post-tensioned. We'll see what the advantages of that are. Also, Ando's Museum in Fort Worth, the modern, is beautiful, cast-in-place, post-tensioned concrete. Okay. So, I know this slide, you're tired of seeing it. It's just so important that I cannot but repeat it every time. With pre-tensioned, pre-stressed concrete, we have simple supports. It's a roller or a pin, and each beam is its own person, and not very good in a seismic zone because it's heavy pieces, shaking, and without rigid connections. Versus post-tensioned concrete that is cast in place versus pre-cast. precast, okay, they're individual members and they're installed on the site versus cast in place, post-tensioned concrete is going to be monolithic. Okay, so let's look at the advantages of post-tensioned concrete. The concrete is going to be, the concrete member is going to be in more compression. When you pull those tendons, whether it's pre-tensioned or post-tensioned, the concrete is going into more compression. That's what it likes. And the tendons are in more tension. And therefore, on the member itself, there will be less moment because we increase the compression and reduce the tension on the concrete on the bottom of the beam. And therefore, it will result in less deflection and less cracking. It is cheaper than regular non-post-tension concrete because there's less rebar. You're replacing rebar with the tendons. And it's less concrete because post-tension members are shallower, therefore less concrete than non-post-tension members. So here's the real advantage. You can either have a shallower member or a greater span, or you can carry heavier loads. These are the options. So the advantages of post-tension concrete are many. And if you do have shallower members or longer spans, then you're going to have fewer columns and therefore a smaller foundation and therefore more sustainable. It's less taxing on the environment. So if you have a high rise and each floor plate is thinner than non-post tension, then you will end up with overall savings and a shallower overall height. Floor to floor is shallower. Everything is shallower. So it's lighter a building, therefore less foundation, therefore more sustainable. And money savings is great. Post-tension concrete is stronger than regular concrete members, and it's more durable. So disadvantages are, of course, it requires repetition. If you have repetition, high rise, then it's very cost effective. A disadvantage is that it is weather dependent. You can't cast when there's rain and whatnot, cold weather, etc. And, of course, you're introducing another trade post-tension crew on the site. There's two kinds of post-tension concrete. One of them is bonded. The other is unbonded. And bonded or unbonded refers to the concrete bonding with the casing of the tendons. The tendons are the wires inside the casing. So in the case of bonded concrete, it's more rigid. The tube or the duct, as it's called, is more rigid. It's still a little bit flexible to do those curvatures, but inside of it, there is tendons, and there is a gap between the duct and the tendons that will be filled with high-strength grout that protects the wires from corrosion. So, the other version is the unbonded post-tension concrete that we see a lot of in buildings. And this one is a sheath. This red stuff or the blue stuff is a sheath. And it has the post-tension tendons that are greased. So, there's grease between the tendon and the sleeve or the sheath. because it's going to be pulled after the concrete reaches a certain strength. Here we see this piece of wood with anchors coming down. Once the slab is poured, these anchors are for a CLP system of wood frame above a podium. So CLP is continuous load path. So we must tie the wood frame construction above the podium to the podium to secure it against uplift from wind loads. Anyway, here we can see that this is an unbonded concrete, post-tension concrete. We can tell because the hole is inward versus in the bonded variety, there's the duct. It sticks out of the concrete. And there is a gap, and that's where the grout goes, surrounds there. Sorry, bad graphics. Okay. Unbonded is much more popular in buildings than bonded. Bonded is more for civil works and bridges and larger construction. Okay, so this one is a duct versus a sheath. The sheath, of course, is much more flexible. This is an important slide. In cast-in-place concrete, everything is monolithic. Everything is moment-resisting. Unlike pre-stressed, pre-tensioned, rather, concrete members. So this is continuous. So we have tension on the top at the support and compression on the bottom. You can tell from the profile of this tendon. It's doing that. It's top is in tension. Then it does that. And now it's bottom is in tension. There is something called, where's the eraser? Here's the eraser. There's something called a point of inflection. That's a point where there is no moment. Before it, I had tension on the top, compression on the bottom. After the point of inflection, it reverses. Compression goes on the top. Tension goes on the bottom. When the tension is on the bottom, it's called a positive moment. When the tension is on the top, it's called a negative moment. At the point of inflection, it's not positive. It's not negative. The moment at the point of inflection is zero. So here it is because it's continuous. It's not just one bay. So we're going to see a point of inflection here, another one right after the support, right before the support. There is a point of inflection. At the support, the bending moment is negative. Why is it negative? Because that's a negative moment. That is a positive moment. So I'm seeing this guy doing that. The tension is on the top. Then I'm seeing it doing that. the tension is on the bottom. So in a continuous pour, whether it's a slab sitting on top of beams or a beam sitting on top of multiple columns, there is positive, negative, positive, negative, passing through zero. Very good. So with post-tensioning, once the wire is pulled, it's anchored on one end, so I can't go anywhere, but it's resisting the pull. Once the wire is pulled with a hydraulic jack, as we will see shortly, this flattens, it goes down. This goes up, this goes up, this goes down. So they're trying to achieve horizontal. They don't pull it to the extent of horizontal. But what's happening here is I'm reducing the tension. I'm making it that much. And I'm reducing the tension on the top by reducing it by pushing down on the concrete versus pushing up on the concrete. Very good. So the point is reduce the tension on the concrete and put it in more compression. That's what pre-stress is, whether post-tension or pre-tension. Very good. So here at the support, we can see that the post-tension cable is going from top and then dipping down. So tension on the top, tension on the bottom, and the post-tension cable is draped based on the moment diagram. So here, actually, the rebar is in compression in the red zones, and the rebar is in tension in the blue zones. And the post-tension cable is easy to manipulate to put it where the tension is and then pull it, reducing the tension on the concrete and increasing the compression. Very good. So this is more of the same, but looking at the unbonded sheath, it's got a cluster of tendons, one, two, three, four, five, six, seven, typically. And there's grease here surrounding so that it can be pulled easily. We saw this. This is the negative moment. You can see that red cable go down and then back up and then down and back up, as we will see in the next few slides. So with different height chairs, I can look at this diagram, and I can say there's tension here. There's tension on the bottom. Then it went back to the top. There must be tension on the top, tension on the bottom. So it's draped based on where the tension forces are. And pulling the wire reduces the tension and increases the compression. These are either tied to the formwork or tied to the rebar itself. They can anchor it in the rebar and pull from the other end. or to the formwork. Very good. So looking at this one and tracing it again, it's a roof cast. It's a formwork for a roof. I can tell because the rebars here of the column are bent, meaning there is not a floor to come. But looking at the post-tension cable, and unfortunately it is red in this project, sorry, it is black in this project, is going to the top of this beam. Then it goes down. Then it comes up again on top of the beam. Oops. Then it does that. And it follows the moment diagram. Wherever there's a support, tension is on the top. Between supports, the tension is on the bottom. And it keeps doing that because it's a is poor. So again, this is so important that I have dedicated multiple slides to it. Basically, it's hard to see, but the tendon is going down, then it comes up, then it goes down, then it comes up, then it goes down, comes up. It's one big poor. And there is a joist here, Therefore, the tendon is on the top. There is a joist. The tendon is on the top. Between them, it's spanning, etc. Okay. We saw anchorage, how it's anchored to the formwork or to the rebar. And then we can see here all the tendons sticking out. And then they will pull them to a certain degree. And that's good. So here we can see the duct. This is a duct. And there is grout in here to protect the tendons from corrosion with a bonded post-tension project. So in this example, it's tied to the formwork. They remove the formwork and they remove these plugs, these plugs. and they have wedges. It's a beautiful cylindrical piece, sorry, conical piece that is two halves that lock the tendon and prevent it from slipping. Then they will grout all the holes and then they will go on with their project. Here we are again with the PT cable anchored to the formwork and there is some paint on the formwork. There is paint here because it is extremely important to know where the post-tension cable is because any kind of boring through the slab or cut a hole or something like that, if that encounters any of these tendons, it is a disaster because it'll snip, it'll break, it'll kill people, It snaps out, it's under huge tension. So a lot of contractors do that where they will mark where the PT cable is. So don't mess with these areas, for example. That's what that paint on the formwork is doing. Again, this one shows me different height chairs can help me drape the post-tension tendon in a shallow thickness of a slab or a plate. It's 8, 10 inches, so I need different height chairs to position it in the top or the bottom of the plate. So they'll come back and they'll put these chalk marks. They pulled the sleeve off. They'll put chalk marks on the tendons. Then they come with a hydraulic jack and they anchor the tendon and they have the chalk marks and then they pull to a certain extent. And you can see that this cable used to, this PT tendon used to be here and then it was pulled. And there is a gauge that tells them how much to pull. Very good. So here we're looking at a two-way flat plate. Let's remember, two-way flat plate basically does not have beams underneath it, and it does not have a capital over the column. So it's just the plate itself, 10, 12 inches of plate. So we're worried about something called punching shear. With two-way systems, punching shear is the main concern. So if the slab is loaded too heavy, the column is supporting that much of the slab, the rest falls in shear, in two directions. That's why it's two-way shear versus one-way shear. So to counter that, in the same thickness, still we have 10 inches. We're not going to drop down with a beam or a capital over the column. They're going to use these stud rails, and they're going to lay them out in two perpendicular directions to prevent or to reduce the possibility of punching shear. So that's what we see here. In this image here, we can see the post-tension cable going over the support. There's a column here, and we can tell there's a column because right away there's extra reinforcement around the column, and that's what it looks like. These are sleeves for some kind of holes in the slab so that they can run plumbing or similar. Very good. So that's it for pulse tensioning.