Concrete: 2. Cip Concrete Formwork

Concrete: 2. Cip Concrete Formwork

Concrete: 2. Cip Concrete Formwork - Full TranscriptionSo I'd like to begin the series on concrete with this video about formwork. There is so much to say about concrete. I love concrete, but there's too much to say, and I'm trying to limit it to 15 minutes each video. So bear with me. I hope you learn a lot. Okay, formwork. We're going to start with a comment here. Formwork is usually 50% of the total cost of the finished cubic yard of concrete. So the cost of the concrete, the cost of the rebar, the cost of placing the concrete, additives, admixtures, none of that matters. Not none of it matters. It of course matters. And the cost of rebar. But the cost of formwork is 50%. So the more standard the formwork, the cheaper it's going to be. Anything custom is going to cost an arm and a leg. That's typically the rule. So let's start with slabs. I'm talking about, first of all, a slab on grade. A slab on grade has no formwork. It's very cheap. You just back up a truck. First of all, they're going to put a gravel bed to protect the bottom of the slab from any moisture that may come up from the ground. A bed of gravel has a lot of surface area, so it'll take a lot of the moisture. And then they'll put the polyethylene barrier, six mil polyethylene barrier. It's just a sheet of plastic that is thick. It doesn't puncture very easily. And they're going to lift the welded wire mesh. Let's make sure we understand that. Which software am I in? Sorry. Okay, we can go smaller. Very good. So welded wire mesh or welded wire fabric, this is not real rebar. It's there just for temperature to keep the slab from cracking, expansion, contraction. That's what that reinforcing is for. The slab is not spanning. It's sitting on the ground. It doesn't need rebar. So they will either use a welded wire fabric, welded wire mesh, or a number three, number four bar, let's say at one foot on center in each direction. The thing is equal, grid, etc. Very good. What other pictures do I have here? There's chairs of different height to raise the rebar from the former to get cover. We're going to talk about cover soon. So they have the level, they have the bed of gravel, and then they put the plastic and then the welded wire mesh, and then they start pouring, then they level the concrete, they finish it, etc. But here's your formwork. It's nothing. It's a 2x6, something like that, 2x6, 2x8, depending on the thickness of the slab on grade. But the thing is, it's not spanning, so it's very cheap. There is no formwork here to speak of. Here's another example. Again, it's minor. No, nothing sophisticated or anything like that. The same may be said about tilt-up concrete walls. They're poured on top of a thick slab on grade because a crane has to walk on the slab on grade, pick these things, tip them up, and then put them in place. So this is another example where the formwork is very cheap. It's just a board, 2x6, 2x8, something like that. Then they put their inserts for vents or whatever it is through the wall. There's an opening here. And then they just pour. And there it is. They're finishing it. It's easy work. It's not on a vertical surface. It's on the ground. It's cheap. So this is your typical type 3 construction. where your perimeter is fireproof, and then your interiors are not so much. Type 3 and 3A. Okay, so that's it. Let's go to slabs above grade this time. If we look at the formwork for this one, it is, I don't see any beams. I don't see any drop panels at the columns. All I see is a platform of plywood, and I see some rebar cages for columns sticking up. If you don't see any beams or column capitals or anything like that, that's a flat plate. And it looks like that once it's cast. There's nothing around the column. There's no beams between the columns. So a flat plate is typically good for residential high-rise. By residential, I mean light live load. So your multi-story high-rise residential, more often than not, is a flat plate. This allows me a small floor-to-floor height. So I can add one floor every so often. So it's very economical in its height, and all your mechanical systems will go right underneath here. There's no beams to go around or anything like that. Versus this one, I see that there is a drop capital or something around the column, and then it's flat in the middle. But then again, around the column, there's going to be reinforcement. This one says, I'm a flat slab versus a flat plate. Plate means there's nothing dropping down. A flat slab looks like this. This is an ugly example, but it's still an example of a flat slab. It's got reinforcement around the column because the loads are heavy. Unlike the flat plate, the flat plate does light loads. The flat slab is there for heavier loads. I don't know. Storage, warehouse, something like that. industrial then you would have drop panels because the load is heavier so there's the formwork for it there's the drop panel and then between the drop panels there's nothing there's no beams it's just locally at the column and since the load is heavy we're worried about punching shear punching shear is where the slab is so heavy it'll just go down it'll leave that much of the column and it'll fall down and the column plus a little bit will stay up there. So dangerous for heavy loads. Even a flat plate will have a lot of reinforcing in the plate around the column. Again, punching shear. Two-way systems are very vulnerable to punching shear compared to one-way systems. One-way systems have beams. Okay, looking at this one, this one is a one-way slab, joist, beam, something like that. There's a lot of these guys, they look like they're joists, all of these, and then there's fewer of this one, and it's perpendicular. The joist is perpendicular to the beam or the girder. And maybe I won't use these terms. I will use the term secondary and primary. I mean, I could call the joist beam, and the big beam is the girder. There's some bolsters here. These guys, they're there to raise the rebar of the slab, which is the tertiary. So here's what that one looks like once the form is removed. That's not the same. Sorry, it's not the same picture. It's a different picture from a different site, but the logic is the same. There's beams. There's, okay, primary and secondary. The one that supports the other is called the primary. Very good. So what else can we say? Let's see. Form work, pans, slabs above grade, shoring, reshoring. Let's see what this is. So for this picture here, there's a platform, and on top of it are these pans that look like this, that overlap, that are trapezoidal, so they can come out of the form easy. And again, I see a lot of these must be the secondary, and this must be the primary. secondary. Sorry, that's primary and those are secondary. These are all secondary. They're sitting on a big one over here. Very good. So they lay these pans next to each other. They telescope them. There's a plug at the end and now you have reinforcing in here when they put the reinforcing and you have a beam or a joist. I don't care. The thing is I'm seeing a rectangle. I'm not seeing a square. So this is a one-way system. Had this been square pans, more like a waffle, it would have been a two-way system. Very good. So underneath this platform is all kinds of shoring, shoring up the platform. These are sometimes called jacks, a jack. These are jacks, in fact, individually, their jacks. But shoring is the system underneath the platform that supports it. And then they will remove the platform. They will remove the shores. They will remove the platform. And now we're seeing the pans on the underside. Again, they are trapezoidal so that they can come out very easily. And I'm seeing a beam here, another beam here, or a joist. And here's a beam, another beam between columns. So they'll pull these out, and they will have some repetitive joists in there. So here's the shoring underneath this platform. On top of it, there's pans making joists and beams, and then they'll put the rebar, then they'll pour. After they take the formwork out, they do some reshoring because they're in a hurry to get the formwork out to move to the next floor and start framing it, usually the code says you have to have three floors shored at the same time. So the fourth floor from the bottom, I'm taking it up four floors up to get started on the next one. Then they put some reshoring on critical members. This looks like it's a cantilever, so let's support it until later. So we might keep them in there for another two floors of pour. Here's some reshoring on the beams. They took out the formwork, everything, but then they went back with some jacks and they supported the beam until it cures enough. Then they will take them out totally. Very good. Sometimes the floor is very high. So you have a forest of shoring, such as these red shores. I need to switch apps here, so I need to go to this one. So here is a table form that is very interesting. What it does is it's the whole platform with the shoring underneath it all together as a package. It's picked up with the crane. We can see it over here. I can't write. Sorry. Anyway, on the left, you can see the images, the stills of this table. One component of it. It's a lot. It's time-saving, definitely. And the jacks or the verticals are adjustable. So you can adjust the right height to level it. You can come it. You can do everything you need to. Very good. So back here, this is in here. It's not related to formwork, but I thought I'd talk about it a little bit. When loaded, this slab wants to deflect. It wants to deflect it. You know, once there is a load, it might deflect, let's say. So there is this, there is a plate that is embedded in the concrete. They put it as they were pouring. then they came back with an angle and they welded it to that plate that is embedded in the concrete and this plate has slotted holes these holes so when the slab wants to go down when the slab wants to go down there is room for it to move and that's why this is an important detail this plate has that much ability to move. And the bolt, if you notice, is on the bottom of the hole to allow it to move down. Okay. Formwork for beams. Here we go. There is something called a form tie, which is this one. This one, when the concrete is wet and it's poured in here, it wants to push the formwork outward. It's called thrust. And so these form ties keep the form together. We have a chamfer strip, which is a strip of wood that is at 45, that is placed in the former to give you what concrete really looks like. It's chamfered. On exposed concrete, it's chamfered. So that corner doesn't break. It's like it's pre-broken. It's chamfered. Very good. Here's a little bit of nicer pictures versus the ones I'm showing you. I have to put something nice in there. 152 Elizabeth Street by Ando, 2016 residential apartment. Starts at who knows how many million. But you can see the form ties here. You can see the formwork. That's one piece of form. And then there's a heavier joint for the floor. You can tell where the floors are. and this is just the formwork. Very nice. Likewise, there's the Center for British Art at Yale by Lou Kahn. And again, we can see these dots, which are the form ties that are keeping the concrete together and not allowing it to explode. And here's a piece of formwork. Here's another piece. And then there's a heavier line here. So instead of a chamfer strip, probably it's a double chamfer. So it still comes out with the formwork. The formwork is here. And once the concrete goes in there, then you can pull this out because it's tapered when you pull the formwork. And you get a heavier line, a deeper line like this one. Very good. This concrete up here looks extra, extra, extra smooth. It looks like it's a steel form. The formwork looks like it's steel instead of wood with form ties and whatnot. So here's some beams, and they have stirrups, and they have rebars. And again, continuing with the idea of secondary and another secondary versus a primary. The primary is carrying the secondary. So, what they might do is they put this beam in first, this beam cage, reinforcing cage, in the pocket in the formwork for the beam. Then they probably tied this one individually. They put the rebars. This one was lifted by crane probably. It was tied outside of the hole. Sorry, outside of the pocket for the beam. And then this other beam is tied in place because I can't mesh them together if the other one's already in place. Okay. So looking a little closer, I see stirrups. These here are in charge of shear. Oops, not a good color. Shear. And the bottom rebars are in charge of tension. The top rebars are in charge of compression. together the compression and tension, make a couple. Here I see some, over here, I see some post-tension cable, the blue one, and it's anchored to the rebar. And then they pour the beam, then they come back later. This blue sheath, as it's called, has grease in it. They'll pull the tendons, and after the concrete cures, after it reaches a certain strength, not fully cured, They pull the tendons, and then that sends the concrete into more compression and less tension. That's the idea. So here, I'm looking at this beam, and I'm going, it's real weird. And the reason I'm saying that is, why are you not working? Oh, it's over there. Sorry. We need a little more pin weight here. How do I get that? Let me go here. Sorry. So I'm seeing this beam that does that. Beam usually wants depth, but it looks like this is like a drop panel. It's not a drop panel. It's called a flat beam. But basically, I've got it very wide because I don't have enough depth below the ceiling. So a flat beam versus a regular beam. These beams are very deep. They look like that, and they've got rebar on the top compression, rebar on the bottom tension, and a stirrup that ties it together and provides some shear resistance. Very good. So here we are with the beams, intersecting beams, and the column rebar is sticking out. So they could be tying the rebar cages for the beams on the ground. a crane picks them up, or else they could be tying them on the platform. But it's easier to pre-tie it than to tie it inside the pocket. Okay, good enough. What else do I have? Columns. Let's look at column formwork. So here's some column formwork, and you will notice that there is a chamfer strip in the corner, and this chamfer strip is going to give me what I need for the column not to have a 90 degree angle that'll break. So there's a chamfer strip. And this column formwork was just removed. It could have been a round form. It opens into two half cylinders. They put the rebar, they close it up, they pour from the top. Now, these rebars, let's see what color to use. These rebars are bent like that because there's another floor to come. And the next floor, what it will do is, let's make a little bit heavier line weight. They will pour up to here. And once that cures and they start forming the next floor, they will pick up another rebar gauge that they tie, and they splice to the red rebars. And that's why they're bent, so that the load is transferred directly onto the rebar, if that makes sense. Okay, good enough. So they put up the columns, and they need to make sure that each column is plumb. So they have these tiebacks, these two. They will adjust them so that this face is plumb. Then they will anchor them in the ground. Then they will go to this other one, other perpendicular face with two tiebacks. They will make sure that this one is plumb. Then they will tack them in the ground. And now this column formwork is ready to be pulled. They do that, of course, with a round column. I just need two directions to be plumb. Then the column itself is plumb. So they're going to put these 2x4s or whatever they are, wherever the column goes. And now they'll slip the column form inside of those pieces of wood. And okay. So, I have these column cages that are spliced. Are we going to see it here? It's too dark to see. I'm sorry. Okay. It's in another slide. We'll talk about it. But here, I see these guys that are bolsters. And what they're doing is they're keeping the rebar away from the formwork. That's what they're doing on all sides. because there has to be a minimum cover in concrete always. Whether it's a column, whether it's a beam, whether it's a slab, whether it's a foundation, there is ACI, American Concrete Institute, specifications for cover. Very good. So there's two kinds of columns. One is called a spaced tie. all of the ties are spaced evenly versus a spiral tie this one is going around it's usually used for round columns but this one is required in a seismic zone because right now I have a weakness here for the earthquake between stirrups but with a continuous spiral there is no such weaknesses very good. What is this picture about? Oh, they marked the slab to where they're going to put these pieces of wood. So they marked it on the ground on the slab and then they go in there. Oh, I think we can see it here. So these are the rebars. This one, this rebar came from the floor below. This rebar came from the floor below. This one came from the floor below, from the pore below rather. And then they brought the new cage and the new cage here is spliced, is tied to the ones that came from the lower pore to make it monolithic, continuous. So, there has to be an inspection. This column on the left is waiting for the inspector. Before any pour, there has to be an inspection. A code official comes out and says, yes, okay, you can pour. Then they close it up. And there's different proprietary systems, the DOCA is one of them, that they can easily tie the column in place with this formwork and just pour. So this is from a markup from one of my students. There has to be a cover, and that is maintained with those bolsters or chairs that we saw earlier to keep it away from the edge of the concrete. So typically in a column or a beam, it's one and a half to two inches, depending if it's interior or if it's exterior. Very good. What else? We talked about these for leveling in two perpendicular directions. We can see this column is going to get a splice later. And we see rebar sticking out from the wall to which the slab will probably be tight. So looking at cover requirements. Reinforcing bars that mesh to have concrete cover as follows. Footings, three inches. Formed concrete exposed to earth, two inches. Formed concrete exposed to earth for bars number five, smaller, one and a half inches, one inch, etc. So you don't need to memorize these. Just keep in mind if it's on the exterior, it needs a greater cover than if it were on the interior. Slabs and walls are not very thick, so it's a smaller cover. Very good. So here I have this image of this concrete column. The rebars are sticking up because there's another floor to come, but then some of the rebars are bent at this level so that when the slab is poured, it anchors rigidly to the column. Okay, we talked about this one. This is a pretty monumental column. It has rebars that are number 11, which is huge. Okay, here's a weirdo column. It's okay. It's nice. You can do that. I don't know how they did it. One of my former students sent it to me. But I imagine inside this form, there is another form. And the other form is probably doing the triangle. Why is this not working? There we go. So there's probably some kind of insert. And then we pour, and it doesn't go in the red areas, and it gives us this angled beam. Column, sorry, not beam. Walls, same thing. So here we go with trying to get the cover in place. So we have these bolsters that are keeping the formwork away from the rebar cage of the wall. there's again different proprietary systems of formwork but speed is important since time is money or whatever you got to do this and do it quick so standard formwork looks like that easy to assemble easy to disassemble and then there's the form tie it's coming it's going out on the other side and it's coming in on this side and it ties to the edge of the form, and then there's a rebar cage in there. Now, an embed plate like this with studded studs is going to be poured in this wall specifically because it looks like we need to attach some kind of steel to it on the other side. Once this form is removed, we have a steel plate that we can weld a joist or a beam or whatever it is to that piece. So here we are, the bolsters are in place, giving me enough cover. And I saw something, maybe it's not time to talk about it, but I'm going to anyway. There's thinner rebars in the front, if you see that. There's pretty hefty rebars behind in the second layer. That says that the yellow rebars are bigger and therefore there is tension on this face. These other rebars are smaller. It says that there is compression on this face. So this is a retaining wall basically and the dirt is going on this side and therefore it'll bend the stem and this will go into tension. This will go into compression. So we don't need as much rebar on the compression side. Okay. Here are the form ties being placed and you can see how that is done. There's one face of the form work. The space ties are in there. They'll put the rebar, then they'll get an inspection, close it up. And then that's what it looks like when it's done. This, of course, is the form we saw earlier, something like that. And here's the, we saw this one, but here's the form tie in there. And then they come in and they cut these guys off. Unless it's an exposed concrete wall, such as Ando or Khan or something like that, then they're going to spend a lot more time. But this one is just in the dirt, so it doesn't matter really what it looks like. It's going to be covered. Miscellaneous. Here's some formwork for a stair. They're working on it. And here's another one. Easy, cheap, dirty, egress stair versus a beautiful stair by Anto at Wrightwood 659 in Chicago. It's a beautiful building.