Concrete: 7a Prestressed Pretensioned Precast Concrete

Okay, in this video and the next, I would like to discuss pre-stressed concrete, both pre-tensioned and post-tensioned. Both of those are considered pre-stressed. I have some beautiful examples here for you of pre-stressed, pre-cast, pre-tensioned. concrete examples the sydney opera house the pulse has architecture building at fiu florida international university in miami and the hemoroscopium house by ensemble studio in uh merida sorry in las rosas sorry about that merida is in mexico and that's another example Museo Internacional del Barroco in Puebla, Mexico. That's the one in 2016. And Little Island in New York City of 2021 by Heatherwick Studio. Okay. Don't get your hopes too high. Precast is mostly parking decks. Unless you have a lot of money, your client has a lot of money, and they're interested in good architecture. Otherwise, it's parking decks. Okay, so let's talk about pre-stressed concrete. The word pre-stress means before the loads are, the service loads, the final loads are deployed, the wire is stressed and it sends the concrete into more compression. By stressing the wire, either pre or post, you are sending the concrete into more compression and therefore less tension. That's the whole point of pre-stressing. So in pre-tensioned concrete, typically this is in pre-cast factories or pre-cast yards. You drape the wire or the tendon, as it's called. And it's just sitting there. It's bare. It's not in a plastic tube. It's exposed. And when you pour the concrete, it bonds with the concrete. After the concrete reaches a certain strength, they cut the wire and they take the member out of the form. Very good. So, precast, pretensioned concrete is always simply supported. We're going to put this on a column, and we're going to put the other one on a column. Then we might put the next beam on a column also. So, each beam is spanning between two columns. That's how precast works. And therefore, it'll require lateral stiffening with concrete. Probably a sheer wall is going to provide lateral support. Versus post-tensioned concrete. You can notice that there is a plastic sheath, as it's called, and the tendon is in the sheath so that it does not bond with the concrete when the concrete is poured. And this one, after the concrete reaches a certain strength, they pull the wire because that plastic sheath has grease in it in order to facilitate the tendon being pulled. So it is tensioned after the concrete reaches a certain strength. And the idea again is if you pull this wire from its ends, this wants to go up and it puts compression on the concrete. This part wants to go down. Once you pull the wire a little bit, it wants to go down here and therefore pushing again against the concrete, sending it to more compression. This area used to be in tension. This area used to be in tension. When you pull the wire, you're reducing the tension over the column. You're reducing the tension between the two supports and increasing the compression. And concrete loves compression. So it's very happy with pre-stressing. Excellent. Clearly, this one is done in a precast yard off-site. This one is cast in place on the site. So cast in place concrete, CIP, versus precast or precast. Those are the symbols for precast and cast in place. So cast in place concrete, whether it's post-tensioned or not, is always considered a rigid or a moment-resisting or a fixed connection between the beam and the column. It's continuous. You have one pour on top of 10 beams. That's one pour sitting on 10 beams. It's always considered cast in place concrete more often than not. it's considered continuous versus precast is always simply supported on a roller and a pin. Excellent. So being moment resisting, it does have quite a bit of lateral resistance. But if you have a tall building, maybe more than 20 floors, the moment frame is not, the cast in place moment frame is not enough lateral stability. So I might have to add shear walls and bracing. Okay, so let's go to the next slide, see what the next slide is. Advantages of precast, pretensioned concrete. This again is done in a factory, therefore it's weather independent and there is quality control. Unlike cast in place, cast in place, the inspector has to come and check on the reinforcing and everything else before the pour is executed. Then you get approval, then you can pour. So this one has quality control inside the factory, everything is taken care of. They're pouring flat on reusable forms, therefore it's a lot cheaper, As long as there's enough repetition. So the speed of casting is an important point here. They use high early strength cement that gives them high early strength concrete. So when I visited the precast factory, what happens is they pour the double tea. Then overnight, they take it out and they do formwork for the next one. They put the inserts, they put any embed plates and all the exceptions to the standard formwork. Then they pour the next day. So the turnaround time is one double T a day. So they have daytime crews and nighttime crews and they take shifts and they reinforce it. They do the formwork overnight and then the next day they cast. Very good. And then when it arrives on the site, it's very quick to install the members in place. So some of the disadvantages is it is difficult to customize sizes and shapes. And there's only a certain set of shapes that are produced in precast yards. so customizing for example a precast wall okay they can put some foam on the edges and make it different dimensions or put a window in it or put plates here or there but overall the dimensions are pretty standard so it could be maybe 10 foot tall 12 foot tall the maximum is 13 foot 4 and that's the width and it's based on the lane, the transportation restrictions. So this is a huge member that is very cumbersome to turn around, make a right turn, left turn, etc. Unless the factory is in a straight line to the site. This is a highway girder. That's another precast form. Then they're going to take it and install it. crane is waiting, et cetera. So transportation limits is a big deal. There is limits on weight. There's limits on width. The lane width is 12 feet, cannot exceed that. There is bridge clearances. There's all kinds of restrictions. And then the cost to have an escort, for example, if you have a double T that is longer than 60 feet and wider than 12 feet, then you need an escort and all of a sudden it's no longer affordable. So transportation limits is really the big thing. This of course doesn't exist in post-tensioning because it's done on the site. And then it's simply supported. That is important. Okay. Very good. So let's look at the profiles that are produced in precast, pretensioned, prestressed concrete. They make walls, they make columns, they make piles, they make highway girders, they make culverts. Each plant is specialized in a different product based on the formwork that they have. In all cases, when it is pre-tensioned, the wire is bare and it's anchored on one end and pulled from the other end. The same with post-tensioned, it's anchored on one end and then the live end is pulled. So let's take a look here at these walls being cast, they're cast flat. The formwork is just steel that is easy to clean between pores. And in this case, there is a window in this precast wall. Probably the wall ends somewhere here. That's another wall. Probably it ends here. That's another wall. And they're all standard height, that much, whatever that is. If the form is 12 feet or 13 foot four, then put some foam at the end so that you get a shorter wall. Please note that the pre-stress, pre-tensioned wire is running the full length of the plant because they're going to pour all of these walls at the same time, then they go back and they cut the wire and you have separate walls. Here we can see a wire going in the opening. They'll cut it and you have a clear opening. This is the only way to get insulation in a concrete wall is to have it precast because if it's on the site, you can't put the insulation and pour at the same time. The formwork is vertical. It becomes very cumbersome. So here they're putting the insulation. The way this works is, excuse me, For example, on a six-inch thick wall, they'll put a layer of concrete first, and then they'll put the insulation, and then they'll continue pouring. It's a quick process, and there is no cold joint between the two pours. So it is continuous, and it's one piece, and there it is stacked on the yard of the pre-casting plant. There's anchors that need to be embedded. That's what these yellow things are. I'll show them in another slide. So that the crane can lift the wall with these anchors. Very good. Quality control is there. It's easy to set where the rebar goes, where the tendon, the pre-stress tendon goes. And to put the inserts and to put any embed plates. And we can see here there is reinforcing between carbon fiber, reinforcing between two layers of foam insulation. Very good. So columns is another precast member. Typically, they will have a haunch to receive a beam, a precast beam. Like I said earlier, please, the big deal about precast is that it is simply supported. Two rollers, a roller and a pin, or two pins. But typically, it's a roller and a pin. The point I'm trying to make is this is totally unpopular in seismic zones. Very important to recognize that because it's a bunch of very, very, very heavy pieces that are attached barely, just sitting there. So in an earthquake, these very heavy pieces start shaking and they may cause collapse and fatalities. So precast concrete is not popular in seismic zones. They would much rather have cast in place, monolithic, so that the whole structure shakes as one piece. In this parking deck, the double Ts are supported. Here's one double T. And they're supporting the stems on the column. And the column is custom and it has two haunches. It looks like this. One haunch picks up one stem. The other haunch picks up the next stem. Versus the more standard column that has a haunch or a corbel that receives the beam. And the beam supports the double T or whatever member we have. Now, the way a precast column is attached to its foundation is interesting because there is always a notch here that allows the column to sit on the anchor bolts. And there is two nuts. One is below the plate. There is a steel plate embedded in a precast column. And there is a nut below the plate and a nut above the plate every time there is an anchor rod. And they're adjusted to make the column plumb in the north-south and in the east-west direction. It has to be plumb in two directions. Also, this must be an exposed column. That's why the corners are chamfered. So here we are again at the precast plant, and we can see that they can prepare the haunches ahead of time, put a metal plate in there, and have studs, steel studs sticking out to anchor into a negative plate that is cast in the column. Here's what it looks like after it's done, and the plate has to fit into, here's the formwork for that haunch, or else they can just cast them all at once. Here the haunches are being cast monolithic with the column. Here's a reinforcing and the end plate for the column. Here's the beginning of another column, and then they'll snip the wire in between. Of note that the formwork is steel and the whole bed vibrates at the same time. That's the vibration they have. It's very loud and it's very interesting because you don't need to come back with vibrators. The whole formwork is shaking. Another example of axially loaded members. I'm sorry I didn't say that. That was important. Axially loaded members, basically columns. and walls. Oh, I did say it. Okay. Sorry, I'm losing it. Another axially loaded member is precast concrete piles. Piles come in wood steel and precast concrete. So in this case, This is, I can't remember, 80, 85 feet, somewhere in that order. And it has the pre-stress wire in there. And it has a lift anchored so that they can lift it with the grain. They'll tip this thing vertical and then they'll start pounding it in the ground. This is when you have bad soil or weak soil or clay. In this project, this was in New Orleans, and I was watching them as the hydraulic jack was, the hammer was pounding the pile to go in the ground. They picked up the pile with the crane, and then they dropped it and went into the 30-foot marker. So it sank 30 feet just by its own weight. And then they started pounding it. We can see that this is 85 to 90 feet deep. So it's carrying a hefty load on basically a very weak soil. And so they are counting on skin friction along the length because there's not going to be any bearing ability. The soil is very weak. Unless they went 90 feet to reach bedrock, and then they'll get a little bit of end bearing, but it's mostly friction on the four sides of this pile that is keeping the load up. So in contrast to axial members, let's look at some spanning members. We have hollow core planks. Here we are in the factory. It's a linear process where this machine is dropping down some concrete already mixed, and you can see the hopper, it feeds the machine, and the machine travels on tracks. And this guy is following and screeding and making the surface nice. Then they'll go back and they'll rake finish it so that there has to be a topping that is cast on the site to tie all these individual precast members together. And then as the concrete drops, there's these cores that are rotating to maintain the hole. Where is it? Here. To maintain the holes in the hollow core member. And then there is a saw also on the track that comes in and cuts the hollow core and the pre-stress wire that's in there already. The pre-stress wire is typically on the bottom because it's spanning and the bottom is in tension. Typical dimensions are four foot and the length varies, but I saw eight inch hollow core planks, eight inch thick hollow core planks going 40 feet. So that's a pretty nice span range for relatively thin slabs. These are hollow, of course, and you can put some radiant flooring in them if need be. That's what it looks like, and we can see here, I need white, we can see the pre-stress wire, there it is. And there's going to be, here's the rake finish, there's going to be a cast in place topping that ties all of these guys together, and there's rebar sticking up so that it becomes monolithic with that CMU wall underneath it. Here is a steel frame, and instead of doing shoring and metal deck and formwork and then welded wire mesh between steel beams, they decided to put hollow core planks. It's just speed of erection, and considering weather, this will go a lot quicker. This was in Chicago. So you can do your metal stud framing on top of it and go on with your life. This is also a steel frame with hollow core planks sitting on steel beams. So other spanning members are beams. they will do a precast L beam and a precast inverted T beam so that it can receive double Ts on this ledge from this side and double Ts on this other ledge from the other side versus an L beam received from one end and not on the other. Here's some more precast L beams that are going to receive. I don't remember what they're going to receive, but again, here's an L beam. It's an end condition, or maybe it's a T. It's going to receive on both sides. I don't remember. I take my pictures usually. So here we have an inverted T that is receiving double Ts on one side and double T's on the other side. And we have the double T sitting on a column with two haunches, one for this beam and one for this beam. And the inverted T's are supporting the double T planks. Okay. Let me take a break here and talk about parking because double T's are typically for parking. So if we have a parking and another row of parking spaces, then my typical dimensions are 18 to 20 feet, 24 feet, and another 18, 36. So this is a 60 foot. Sometimes this is 20 and 20, and that takes it to 64 feet. So that is what the double T can do very comfortably and within transportation restrictions on the highway. So 60 feet is reasonable, more than 60 feet, 65 feet. You need an escort and it just becomes too expensive to afford. Another variation is a single T, and a single T is less popular than a double T. But if we look at this, it's very thin here and here, and it gets thicker over here. That's where the stem is. The stem is doing all of the work, and the flange is just there to stiffen. So here's one double T from here to here, and then another one is that much. That's a single T, and it is chamfered here so that it can come out of the formwork easily. So they chamfer it so that they can take it out of the form. Here's another single T in a parking deck, and you can see wherever the stem comes down, there is a column to support it. Okay. More popular, though, is the double T. So here's the double T stacked on the casting yard. And as we said earlier, the double T is going to sit on an L or a T inverted precast beam. And in this case, they've notched the stem so that it can rest on the inverted beam. And there is a bearing area. And then the span begins over here. Um, very good. So let's look at the formwork. Here's the formwork. It's a steel form, and it looks like this. It's infinitely long, 600 feet, something like that. It looks like this. It's chamfered here so that they can take it out of the form. Then it goes in the stem. Again, the stem is trapezoidal, so it can come out of the form. Then it's chamfered again, and then the second one, chamfer, and then the stem go up, chamfer go up. And there is the form of the double T. And this is really not the significant part. This is maybe two inches. The stem is where the pre-stress wire is. And of course, when you have depth is when you can span. So these things come in 16 inches, 18 inches, 20, 24, etc. in order to span more. And you will note that what's coming on top is basically minor reinforcing. It's welded wire mesh because the flange is not doing the work. This is the flange and it's the stem that is really doing the work. That's where the depth is. That's where the pre-stress wire is. So on one end, it is locked. On the other end, they pull it. They pull the pre-stress wire. Very good. So maybe I'll talk about that. Why don't I talk about that? Okay. Okay, so it is typical. First of all, this is a set formwork. They're not going to change it too much. They can customize a little bit. But if you want a different size double T, you have to go to a different plant that has formwork that does that size. So it's not easy to customize. But typically, in these double Ts, this is X, this is 2X, this is X. So some plants will have 1 foot, 2 foot, 1 foot, and a 4 foot double T. Some will have 1 1⁄2 foot, 3 foot, 1 1⁄2 foot, a 6 foot, or a 2 foot. And what is double of two is four. And a two foot and an eight foot. So they're standard dimensions. They're difficult to customize because that's the homework. Excellent. They can make precast stairs, precast, pretension. There's pre-stress wire in there. And it is bare. It doesn't have a sleeve in it the way post-tension does. So the big deal here is transportation and installation. And let's please remember that everything is either pin or roller, therefore very flimsy in an earthquake. So here it arrives on the site and I can see an escort truck. Therefore, this must have been a pretty long one. In fact, this was for a swimming pool at the YMCA. So it's probably longer than 60 feet. Pre-stress members will always have a camber. So once they snip the pre-stress wire, the concrete goes up a little bit. And then when the service loads are finally deployed, this comes back down to horizontal. So camber is like a positive deflection. Once loaded, it goes down to horizontal versus deflecting. It'll look funny. So precast members will have camber. So here's a wall arriving on the site with a big old opening in the middle. And it's tilted, must be wider than the flatbed. The flatbed is usually 8 feet. The lane is 12 feet. So this one is tilted. They lift it with a crane. Then they tip it up vertical. I can see some insert plates here. These are probably because there's going to be open web choice welded to them. Okay. So here it is on the lot. And again, I will repeat, this is important. The reinforcing or the pre-stress wire that bonds with the concrete is in the stem. And the flange itself has one function, which is essentially to be monolithic enough to tie the two stems together. And it has reinforcing, simple reinforcing against temperature cracking. So welded wire fabric or maybe number three bars in each direction making a mat. Okay. So here's the installation. This guy is putting a neoprene rubber pad, and then the double T is inserted, and it sits on this pad here. It's just rubber. It provides a lot of friction. So this thing is just sitting on the pad. There is no anchorage or anything sticking up into the precast member. So this is considered a roller, and a roller is basically vertical support. it can move this way. It's going to take a lot of force to move it this way because there's so much friction and the member is so heavy, but it is possible to move horizontally. So our roller support is unable to prevent horizontal movement, only vertical. And clearly when this member is loaded, that will rotate. The support will rotate one thousandth of a degree, nothing we see by the naked eye, but the fact is a roller support cannot prevent rotation, cannot prevent horizontal movement. Roller support. So when the double T comes in and sits on these two rubber pads, that's it. It's a roller support. So let's look at the sequence of insulation, and you will notice there's pockets in this wall, but in the opposite wall, the pockets are in the top of the wall. So the double T is lifted, and you will notice that it is chamfered. This part will go into the slot so that the span begins here, and the double T goes into the slot, and this guy is motioning it to come in. And once it reaches this stage, that's it. The crane will stop moving it in that direction. And this is done first. This wall, the double T is inserted in this wall first. And then they drop it through the top into the slots from the top of the wall. Again, there's rubber pads in each of these slots. and the double T sits on these rubber pads. Okay, so then they caulk it. They put a backer rod, and they caulk it all around. The top has a rake finish because there will be a three-inch, maybe, topping cast on top of these guys. There is a rod here where they weld the two pieces together. There is over here a plate inserted in that double T so that they can attach this wall by welding an angle. By welding an angle. That way it's attached to the wall and supported by the wall. There has to be lift points. The crane has to be able to lift the wall. So there's these lift points and there's these attachments that are cast into the wall. And once the concrete cures, they pop this cover off the yellow insert, and this is revealed. And then the crane can pick that up. We can see here that there is some kind of adjustment, vertical adjustment, because it's difficult with these huge members to get it precise. It's not like we draw it. They have to have some adjustment room. This other anchor is for when the panel is flat and the crane has to lift it out of its formwork. This anchor that is sitting in there and tied to the rebar, they'll pick up the panel easily with that. So precast concrete is always pin connected. So we can see here, the wall comes in, they erected it in place. The crane is still there holding the panel, and they put these neoprene pads to level the wall. And then there is a plate inserted into the precast member at the casting yard. And then they will bring this angle on the site and weld it. Sorry, I messed up. they drill a hole and they epoxy in there so that the anchor is in first, and then they weld it to the embed plate, and now we have a pin connection. So that's what this guy is doing. He's welding this plate to the existing plate after they drill a hole to tie it to its foundation. There is a wall foundation underneath this wall clearly. And then all the panels have a plate that is put in at the casting yard. And then another plate attaches them on the side. They weld these together. So it's stitched together. But these are huge, heavy panels. And this is a small piece of steel, maybe a quarter inch, maybe a half inch, still the mass of the concrete can very easily bend it if there's a lateral load. Okay, we'll look at post-tensioning next.