Infrastructure Projects - Construction Technology
A Practical Guide to Construction Techniques and Practice
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There are two kinds of hardware here. These are quick coupling device designed specifically for these panels. It joins the panels together into a gang. These rods are the tie rods that go through and through and carry the hydrostatic head of the concrete. Here's a typical arrangement on the right hand side. You have a completed set of forms. The tie rods have been protected here with these yellow plastic caps so that you don't have any kind of a sharp object protruding into the workspace. On the left hand side, you could just see the beginning of a set of formwork. In the middle there's an open space. The arrangement here is designed to pour alternate panels. This is a nice way to proceed, because it allows the panels to shrink somewhat. When you place concrete, there is an initial shrinkage, which is unavoidable. Then there is a long-term shrinkage. By pouring alternate panels that initial shrinkage takes place and when you pour the intermediate panel will minimize shrinkage. I've explained to you how these form panels are very popular, very efficient. It's the way the industry works, but this carpenter is actually working with loose sheets of plywood. He's building a form by hand. What is that all about? The ready-made panels are excellent for the back face and front face, but you need to close off the ends of the pours with what we call a bulkhead form. This is a very complicated form which has to be made by hand. The form has to allow all of the rebar to penetrate and pass through the form. The form has to be braced by hand. It carries the same hydrostatic head as the front face and the rear face. After the concrete is placed, the form panels are removed, but then somebody has to go into this space and remove the bulkhead form, which is very hard to remove, because a little concrete has seeped through all of the openings for the rebar. That form is pretty much cemented in place and you need wrecking bars. You need to literally tear it out. What we just saw is the typical two-face form, but you will run into situations where you use a single-face form. This is a project where the structural wall was poured up against a slurry wall. There is only access from one side. And the form in this case has to be extremely stiff and well braced, because there are no tie rods. |
The hydrostatic head of the wet concrete has to be carried by these inclined braces and each brace has to be anchored to the floor slab. Of course, you have all of the issues associated with the bulkhead form. There is a bulkhead form in place here and that too is being braced with these inclined braces.
In addition to forming walls, there are forms required for columns, forms required for the beams that span between the columns. This is an arrangement that's fairly typical. You can see there's a concrete column that's already been constructed. Now the form work for a concrete beam is being placed. The formwork here is being supported by pipe shores. The shores can be erected in any height, and with a screw adjustment on the top, they can be said exactly at the elevation that you need. This horizontal form can also be made strong enough so that it can span between two columns without the need for the pipe shoring. It's not clear whether the pipe shores here will be carrying the load of the wet concrete, or they may be used just temporarily to join together the segments of the formwork, which will then be freestanding. Either system is perfectly satisfactory. The forms for the beam is also attached to the previously poured column. You can get a better look at the column itself. An attempt has been made here to give the surface of the column some interest. There are horizontal and vertical ribs. There are rounded corners, and that's an excellent treatment. Without really adding significantly to the cost, you can get a much more pleasing look. Having said that, they have also attached this bracket, which will carry the form work and possibly even carry the weight of the concrete. You can see lot of the concrete has been chipped. I'm sure that the anchors attaching the bracket were drilled in later. In the course of drilling them in, they might hit some rebar, or just the action of drilling might cause the concrete to spall. No amount of patching is going to hide that defect. You're going to see that defect for the life of this structure. A better approach would be to cast inserts into the original column. This could easily have been done. In fact, you can see two holes here. I'm not sure what these holes are, but this is a good illustration of how this problem could have been solved. Inserts could have been installed, which were symmetrical with this groove pattern. They really would have been hidden in the finished product. In addition, when you cast the inserts into the concrete, they have much more capacity than a drilled-in insert. You would probably need far fewer inserts. The point here is that all of this needs to be planned in advance. That planning makes a world of difference in the finished product. You want that as cast concrete to look as good as you can possibly make it. That requires is a lot of attention to all of these details. |