Concrete: 10 Types Of Joints

In this video I'd like to discuss different joints in building construction. Starting with control joints, I'll discuss both masonry and concrete control joints. Construction joints for concrete pores. Isolation joints, also in concrete. And expansion joints regardless masonry, concrete or steel. and also seismic joints. Okay, let's start with control joints. So control joints are two varieties, either tooled or sawed. And we see we're familiar with control joints. We've seen them all the time. The point is we're trying to control where the crack happens. So we put these grooves in the concrete soon enough so that a crack would happen in that location. It's done by making the slab a little bit thinner in a certain location, a little bit weaker rather, and that's where the crack would form. It's just very simply like perforating a piece of paper. It's easier to tear on the perforations because it's a little bit weaker. So we have a sawed variety and a tooled variety. So looking at the tooled variety here we see a sidewalk for example and they're tooling with these tools they're making the joint. Now with With control joints, it is typical to have a square bays or up to one, one and a quarter, one and a half maximum to place a control joint. And the depth of the groove needs to be at least one quarter the thickness. So that's the criteria for control joints. And in a long wall, they try to place them more frequently. And it's, of course, for temperature and shrinkage also. So that's what happens with control joints. And they're typically... Here's the sod variety. And of course it has to be done in a slab soon after it's poured and within 24 hours, not later than that. Otherwise cracks might form before then. Here we have control joints around this diamond blockout, which is in fact an isolation joint. So we're going to pour the slab on grade first. And it doesn't have to be first, but separately. And then the column is carrying roof load. The slab on grade is not. So the column has its own foundation because it's carrying a heavier load. So we don't want to pour them at the same time. Otherwise, the column would punch through the slab on grade. Excellent. Second one is the construction joints. And this one is to separate pores. Maybe one pore takes too much time or too much volume of concrete, so we break it into two. And there has to be rebar going from one pore to the next. And oftentimes, they will put a preformed shape in here that will make a keyway between the two pores. And that way it can move a little bit. And depending if the traffic is heavy or not on this slab on grate, maybe you don't even need this. If the traffic is not heavy, probably the aggregate between the two pores is enough to hold it together. But if there's heavier traffic, machinery or otherwise, then there needs to be rebar going from one into the other. And there needs to be some kind of keyway, like this one or this one. Here, they just put in a 2x in there, poured the foundation. This is what we see typically drawn like that. And then we see some kind of wall coming on top of that. So it's formed in the foundation. Isolation joints, we started to discuss a little bit, separating, for example, a column from a slab on grade or a wall from a slab on grade. And they're different loads. And so usually there is some kind of impregnated fiber board separating the two pores, rebar going from one end to the other. So with isolation joints, again, we're going to place these anchor rods in the concrete. and the one that is painted is usually the one that has the level and is measured, and then the other three, for example, follow that elevation. And this is poured separately, the diamond, as it's called, is poured separately, and there is control joints at each corner of the diamond, and an isolation joint, because these are two separate pores. Looking at expansion joints, that one is the most prevalent, especially for the ARE. We're going to see this in masonry and in concrete. So here we go, a detail in masonry, in CMU for example, there's this preformed rubber gasket that prevents water from propagating through the wall, and then they will put a mortar here, and it has to be continuous. all the way to the foundation. If they have reinforcing, they stop it on either side of this rubber gasket. And then we see something similar in brick, for example, in front of CMU. We see here the expansion joints, and then we see a control joint in the brick veneer. and the control joint is going to have some kind of filler. Typically, that's your backer rod and sealant, versus with the CMU, they're going to put mortar in there. Okay, so this is typically located near openings where a crack might be anticipated, and that's what happens in masonry, Versus in steel or in other forms of construction, for example, in this bridge, there's an expansion joint because steel moves a lot with temperature. It's very receptive to thermal differentials. And you notice, of course, that the rail is discontinued at that joint. Otherwise, it can't bridge that opening because it doesn't know which way to go. So, here's another example where you have one block of a building and the other block separated. They behave differently. This is an atrium and this is a two-story space. And so, it's good to separate them so they move differently. And we have the expansion joint. And here's a detail of what this might look like that I got from the web. and we can see clearly that this piece can move, and this piece can move, and then this is bridging the two, and then there's a cover plate. Here in this railing, we see that there's a little gap for expansion contraction. Looking at this bridge here, it's 120 feet, it's a pedestrian bridge, it's steel, and it wants to move here, expand and contract, And we can even see there's a little line there that is showing the different locations based on expansion contraction. This was shot in the winter. In the summer, it expands a little bit and maybe reaches this other line. So there's the detail of this one. There's a gap and there's a cover plate and it can move that much. I don't know. I don't know if I want to throw some math in here or not, but delta L equal alpha delta T times L0, where this is the original length, this is the change in temperature, and this is the coefficient of the material. And steel expands similar to concrete, but aluminum expands a lot more, and that is indicated with this one. So when we look at this formula, we see the original length is a big factor. So for a 120-foot bridge, the expansion contraction could be half an inch to an inch or something like that. And you've got to allow room for it. Otherwise, it's going to buckle. Okay. In concrete also, we have expansion joints. You can see there's a pair of columns in this parking deck. This is one structure. This is another structure. There's nothing that bridges the expansion joint. It's a gap that they fill with something soft that allows compression and expansion. If the load is heavy, then we need to do some alternative method to keep the two segments flush. How many times have you driven in a parking deck or something and you hear a bump there? Well, it doesn't matter as much compared to this airport, where the slab on grade is pretty thick because airplanes landing do impact load and whatnot. And we have these dowels sticking out from one pore to the next. And yes, this is a construction joint, but it's also an expansion joint because I'm seeing that these rebars are smooth. And there's these plastic sleeves that they'll put on top of this one and then just pour the next pour. And there is no bonding between the two pours. This stuff here, this plastic sleeve, looks like rebar so that it bonds with the concrete and it stays in the second pour. And the first pour has the rebar and it's green because it's coated anti-weather. so that when they throw the icing salts or something like that, this coated rebar is capable of resisting corrosion. If the load is heavy, they might use something called this diamond plate, where there's a plastic sleeve that is nailed to the formwork, and there it is inside this pore. And then there's a diamond, a steel diamond plate that goes in the second pour. And this allows expansion contraction, but it's also a construction joint as well. And this is to keep the two pours at the same level. This is, here's one pour, here's the other pour. If there's not something strong tying them, then that might happen, and we don't want that. So we have to link them in order to keep the level of the top of the slab intact. Okay, so diamond plates is another thing. We see here the control joint already has a crack. Finally, seismic joints. These are very complex, but they are pretty wide. They are very much like an expansion joint, typically wider, and it has some material that is compressible that can also expand because in an earthquake you're getting a seismic wave and therefore left, right, right, left, so it's got to be able to take both. So here are the irregularities as specified in the IBC code. we have horizontal irregularities or plan irregularities and vertical irregularities. And some of them are solved using a seismic joint. So I'd like to focus in on re-entrant corners. And I really like this table from the Structural Engineers Association of California. This is an old document from 1975. It was the precursor for the current code. And it's very graphically interesting to follow. So there's all these shapes that have re-entrant corners. This is a re-entrant corner. This is a re-entrant corner. This one. All of these are re-entrant corners. So you can put a seismic joint right here. And you have two blocks. And they're separated with something soft. So they can shake separately. Either here or here. Either here or here. You can put these seismic joints to separate into different blocks. So that's the re-entrant corner problem. Now in this image here, it would be nice to have a seismic joint here separating the structure into three separate blocks that shake separately. In an urban context, in a seismic zone, in San Francisco, Los Angeles, etc., If you're building with zero lot lines and you have to go against a neighbor, then there has to be a seismic joint there. And depending on the height of the building, that joint could be up to four feet. And, of course, it's less real estate to sell. So it ends up being pretty expensive in lost revenue, for example, in addition to cost of material and what have you. Here, with the split level, it would be nice to separate them. with a seismic joint. So looking in elevation at geometric irregularities, it would be nice to separate these into three blocks. So placement of a seismic joint is critical.