Can you walk rebar? Of course I can, can you?
This was a question I was once asked on a job interview for an engineering firm.
It really is too bad I can’t do an audio for you. But if I had, you could hear my imitation Brooklyn accent. Can I walk Rebar? What did you say? I mean are you talking to me? I don’t see anybody else here so I’m under the assumption(and assumption is a big word in Brooklyn so you can tell I’m a little more than street wise….but probably not a wiseguy if you catch my drift) that your talkin’ to me. I mean I just finished workin’ on a sixty story tower. Of course I can walk rebar. Any other stupid questions? (And you have to make a face while you’re saying all of this. Like if another dumb question is asked your gonna get beat up.)
I didn’t answer my interviewer like that though. It was just an imitation. The dwarves, elves and yes even hobbits can walk rebar quite well. All are sure footed and have a good sense of balance. I have had my share of falls on rebar as well and it is dangerous if you’re not careful. That being said we tied our first rebar mat this weekend and got started on the footing and wall rebar grids.
For the uninitiated rebar serves many purposes in construction.(Just a word or two on that before we get started) Reinforced concrete is concrete that is reinforced with steel bars. The steel bars aka rebar. Just keeping it simple rebar ( and this is a general statement here) in footings helps keep the concrete together. In other words if you didn’t have rebar in the footings there is a chance that the footing might settle and crack. If that happens there is a chance for settlement issues to develop. The rebar holds the footing togther so it all acts as one unit. Any tension or pulling will be taken up by the rebar. Another good point just for educational purposes: Concrete is good in compression, steel is good in tension. Translation: Concrete can withstand tremendous downward pressure. The concrete that we will be using in the footings will be 3000 psi concrete. If you remember the last post, that is basically the weight of my Toyota Corolla on a single square inch of this footing. Rebar on the other hand is terrible in compression. Just imagine a single piece of spagetti being held vertically on your dinner table with your finger(A dry out of the box piece). If you apply pressure to the spaget it will bend and eventually snap if you apply too much pressure. Steel is terrific if it is in tension or being pulled. This is why suspension bridges are basically a bridge deck being hung from steel wire ropes or cables. Obviously not spagetti.
Reinforced concrete is the combination of the best parts of both materials. I hope I’m not boring too many of you out there but you can do some really cool stuff with reinforced concrete if you understand the way the materials work togther. So how is it all put togther?
First off I just want to show you the different bar sizes. I know we talked about this before but rebar (in the US) is called out by it’s bar diameter size in eigths of an inch. A 1/2 inch diameter bar is 4 eigths so it would be called a #4 bar, 1 inch diameter bars are 8 eigths and would be called a # 8 bar. Rebar is designed by its steel cross sectional area. So our design details call for a certain sized bar on certain centers. Like #4’s at 12 inches on center. But you could use # 5’s at maybe 16 inches on center as a bar substitution and still maintain the structural elements the engineeers want. The total area of steel remains the same.( We do this all the time in high rise) Here’s a picture of some cross sections.
So for the Hobbit house the rebar in the footings is really a big deal and is super important. It was one of the main concerns for the engineering firm who helped design this structure. So we have to follow the approved plans very carefully because this is going to be inspected for accuracy by the building department. Rebar cannot just be laid down on the insulation and tied, it has to be supported by chairs.(When you pour concrete on the steel the concrete is supposed to go all the way around the rebar. The chairs keep the rebar off the insulation the proper height) In this case I used epoxy coated rail chairs that are three inches high. A rebar mat or grid is laid on top of the chairs and tied with what we call tie wire. This way it doesn’t move as you pour the concrete. Here’s a picture.
So if you have everything you need the mat goes in pretty quick. The only problem is the rebar doesn’t come to the worksite cut for your job. Usually it comes in long lengths and you have to cut it. Here we cut the rebar using a demolition saw with a carborundom blade(not really sure how to spell that). It’s loud, hot, heavy work and you have to have ear and eye protection on while doing it. Sparks fly so you have to be careful where you’re cutting and what direction the sparks are going. I lit my pants on fire once cutting rebar. (Embarassing, but true.) Ethan was my cut man, here he is going at it.
Once I had the first mat in we had to secure the forms and install the shear key. Remember the banding wire we put in before the insulation? Well now that comes into play.First we nailed 2×4’s across the footings 4 foot on center and then wrapped the banding wire tensioned and clipped it. Jude took care of that.
The shear key is a structural element that helps prevent the poured concrete wall from moving when pressure is apllied laterally.(Like when we backfill the Hobbit house) It’s kind of hard to describe it but we used double 2x4s wrapped in plastic and this will make a depression in the top of the footing. When we pour the walls on top of the footing the shear key depression will help “lock in” the wall to the footing. We wrapped the 2x4s in plastic so they don’t get destroyed and it also is a bond breaker so we can get them out after we pour the concrete. I also tied the wall rebar to the shear key this helped speed up installation of the vertical rebars.
After the shear key it was just a matter of laying out and tying off the verticals as per the drawings.
Did you notice the banding wire? You can tell it’s in tension it can hold a lot of pressure. The 2×4’s look like they would hold the forms and in some instances they can but I have seen this situation go bad and when it does the screaming sesssion begins. When you have $4000 dollars worth of concrete showing up you only have a small window of time to mix and pour it. You better be ready.
Georgia and Terence were there finishing up a bunch of odds and ends.We got a lot done this weekend. Next weekend I should have no problem finishing up the rebar installation. After that we’ll get our inspection and then it will be pour day!!!!!!
Went to a graduation party at my brother’s house in Cold Spring Saturday afternoon. Which was good because it was really hot out Saturday-low 90’s. It actually cooled off nice by the time we got to the party. Just a quick shout out to my cousin’s husband Mike from Putnam Valley. We discussed it on the ride home Mike, and we all agree…Hobbits like pizza and a wood burning pizza oven might fit in quite well at Hobbit Hollow. You might get called in for your expertise on pizza ovens. Lunch is on me. Oh yeah I almost forgot….Mike…Netflix…The Fellowship of the Ring…The first ten minutes. That’s it…Then you’ll have a better idea of the back story here.