Greenbrae Chair Design

The chair mold
This entire mold is made out of melamine. I would get a little bit of bracing and reinforcing happening back here using 2x4s. This is just for the purpose of holding this wall, which is about 28 inches high, vertical and in position when we're placing the material. Aside from that, we don't have any other major reinforcing that goes into this mold. The D-FRC applies to the facing and hangs, basically, in position. This particular mold we're working on, you can see that it's kind of beat up and dirty. That's because this is about the tenth time we've actually used this mold. They’re easy to reuse because when we're demolding, these pieces come off and the chair just lifts right out. So, we've designed the mold to be able to cast multiple castings in the same mold.

Mixing the casting blend
I want to talk about the material that we're going to be mixing. This is the CHENG D-FRC. This is some of our activator, which we've already diluted and we've measured out. Here's a bag of the casting blend that we're going to be casting with. I've got a mixing bucket here, a bucket of fresh water, and my mixer, which is actually mounted on a stand here that takes a lot of the backbreaking work out of blending. Then over here, we're going to be using a spray gun to spray our first coat of the casting blend into the form. So these are all the tools we're going to be working with.

Now, let me get started. The first thing we've put into our mixing bucket is 1 gallon of what we call diluted activator. It's already been prediluted with water. For this project, we're going to be using the dark-base casting blend. We're just going to use this straight out of the bag without any pigmenting. If, let's say, I was going to add some color to this, at this point I would actually add the pigment to the activator and blend it in before I added my dry ingredients. This part of the process gets a little dusty, so I'm going to take a minute here and put on a particle mask and then I'll start filling this bucket. I've got my particle mask on, and we're going to start by adding half of what's in this bag to my bucket, and then we'll mix. That looks about right. Now I'm going to go ahead and add half of what's left in here, not all of it. This is my last addition of the D-FRC casting blend. I'm going to dump everything that's left in the bag, and I'm going to scrape the sides.

Now that the dusty part of that is over, I take my particle mask off. I want to talk to you about the consistency. Right now I've added my 50 pounds of the D-FRC casting blend and a gallon of the diluted activator. This is pretty stiff. I want to get it a little wetter before I put it into my sprayer. The reason is we need it to flow. I don't want it to be too runny, but I want it to have a good flowable consistency so that it will go through the gun. Each 50 pound bag of the D-FRC product allows me to add up to 16 ounces of additional water beyond that first 1 gallon of diluted activator. So for now I'm going to probably add about 8 ounces of this to my mix, reblend it, and just check my consistency and see if it will work.

OK, let's check again. See how it's a lot more flowable? Kind of falls right off of my margin trowel, leaving maybe about 1/8 inch of coating on the margin trowel. That's about the consistency that I want before I go ahead and actually put this in my spray gun. I'm going to do one last scrape of the sides and a final blend and then we'll go ahead and put this in the sprayer.

Getting ready to spray
OK, here we go. That's the casting blend mixed up to a consistency to place into my spray gun. I'm going to pull the sprayer over here. Now, a couple of notes at this point. We want to move pretty quickly once we've blended it, getting it into the sprayer and then actually start spraying. We don't want to lollygag around or take our time. We want to move pretty quickly or this material is going to start to set up making it harder to pass through the gun.

The instructional drawing for this Greenbrae chair can be found online (chengconcrete.com/dfrcplans).

Spraying the first coat
He's spraying this at about 1/8 inch thick. This first pass is the trickiest. If the material is too wet, it's going to flow down the wall and not stick. It looks like this is actually a pretty good consistency, to where he can get about 1/8 inch on the wall without it running. After we finish spraying the face and getting about 1/8 inch of the casting blend sprayed onto the face of our mold, we take a break. And we're actually going to use a chip brush here to brush out the surface. The purpose of this is basically to work out any small air bubbles that might have been trapped between the concrete material we were spraying and the face of our mold, so that we minimize the appearance of air bubbles on the surface of the concrete when we demold. You'll also notice that I'm going to spend a lot of time working the front edge of the brush into these corners, and that is to make sure that I have the wet D-FRC casting blend blended well with any sand that might have gotten trapped along that edge. So if I had a corner maybe where there was a lot of loose sand, I want to make sure that I get my wet material mixed with that so that when I demold it's all concrete and that there aren’t loose pits or pinholes or loose pieces of sand trapped in those corners. I'm going to go ahead, starting in the corners, just lightly dabbing the corners to make sure that the concrete material is worked into these tight corners. I'm using a pretty good amount of pressure. You can see how much pressure I'm using when I'm hitting those corners.

Brushing it into the form
The next step I'm going to do is where I'm actually brushing the surface out. Unlike the corners where I was jabbing pretty deep, pretty hard, to work that material, when I'm brushing the surface I'm actually using a pretty light pressure. It's not a lot of pressure, just enough to work the surface, level it out, and press the material to the face of the form. If I press too hard, I'm going to actually tear the material off of my form at this point. So I want to be gentle when I'm brushing the flat surfaces, a little firmer when I'm working it into the corners. This outside corner is actually a little tricky. I don't want to brush too hard here, or I'll pull material away from the corner, making my concrete thin there. This first coat I've sprayed in at 1/8 inch. I'm going to brush this out within the first 10 to 15 minutes of having sprayed it into the form.

OK. I've got this all brushed out. At this point, it's still a little on the soft side so I'm going to walk away from this for about 15 to 20 minutes and give this initial coat time to set up, to gel, because our next step in the process is to mix up the backing blend and apply it to this form to bring it up to the full 1-inch thickness that you see here on the edge. Before we can do that, though, I need this material, the casting blend that we sprayed in, to have a thick enough consistency, or to be set enough, to hold that backing blend in place without slumping down off the wall.

The instructional drawing for this Greenbrae chair can be found online (chengconcrete.com/dfrcplans).

Mixing the backing blend
I want to get started here, but before I do, just like with the casting blend, we're starting off with 1 gallon of the diluting activator. I also have a little additional water, up to 16 ounces, set aside over here. And the purpose of that is, if I end up needing to add a little more water to wet this out, I can add it, but I don't necessarily need to. So here, we've mixed it and I want to check the consistency. This looks pretty good. This is about the consistency you want. You can see how full of fiber it is. There are no clumps of fiber, it's all pretty well broken up. This is the consistency that we're going to use on our form, to actually build it up.

Applying the first layer
Now when I first start off, I want to start on one of these vertical sections, just taking a handful, kind of lightly pressing it into the form in a smearing action. I don't want to press too hard, or I'll press the back material through the face material. And I don't want it to be too thick, or I run the risk of it peeling this face material off the form. So I'm adding here maybe about a 1/8 to 3/16 inch layer. Remember, we're going to be building this up to 1 inch thick. But this first layer that I apply is going to be probably the thinnest one that I set.

Building up the layers
At this point we want to go ahead and actually finish adding our last layer to these vertical surfaces. Clay is working on this wall, and it's almost built up. I've got two layers of the D-FRC backing blend on this vertical wall, and I want to check and see how firm it was. I want this to be pretty firm so that I can rely on it to hold my next layer of D-FRC backing blend in place. We've got about another 3/8 inch that needs to be layered up here, and at this point, I can press into the surface. I press hard enough that I'll be able to leave a fingerprint in there, but generally speaking it's a pretty firm surface. Going to take some of my backing blend, and just like I did before, kind of work it into the surface, using firm pressure.

Now, when I get up high in the mold here, I want to be very, very careful not to shake or vibrate the mold to much because if I do this as this material gets built up, I've got a lot of weight that is resting on the material that I've placed down here. And if I go too high too fast, there'll be so much weight on here that it'll cause this to kind of slump down. So timing is of the essence here. You don't want to build it up too thick too quickly, so you kind of slow down at this point -- take our time and make sure that we're only applying as much material as what's below it can handle.

Before we wrap this up, the last thing that we do on this Greenbrae form when we're casting it is to clean this surface up, after I've built it to my 1-inch thickness. I want to come back around and actually clean this up so that it's a level, consistent finish. Just using a margin trowel, I'm going to cut the surface here. I'm going to get rid of any of my excess. We are going to finish applying the rest of our back coats here to fill this out on the two vertical surfaces, and then we're going to let this sit overnight before we go ahead and demold it.

The instructional drawing for this Greenbrae chair can be found online (chengconcrete.com/dfrcplans).

First off, you can see here along this front edge, I've actually ground down the backside of the concrete, to ensure that it's leveled with my side rails of the mold. This registry ensures that my edge profile from the side is going to appear a consistent 1 inch thick. If I hadn't done that -- there tends to be additional or extra material that's built up on the sides here -- it would make that edge profile look a little rough and varied.

At this point, I'm going to start unscrewing the mold, beginning with the edge form, and once I'm done removing the side forms, I'm then going to remove this entire back leg panel, and we should be able to lift the entire chair right out of the mold at this point.

Well, we've gotten the Greenbrae chair out of the mold. There are a few things that we can do before we're ready to seal it. I've got some rough edges in here and along the corners that we need to do a little bit of grinding or sanding on -- a few little air bubbles here and there that are going to need to be slurried. But generally speaking, the chair is done.

Just to recap, we've got 100 pounds of material in this chair -- 50 pounds of the casting blend and another 50 pounds of the backing blend. We're going to finish this up, get some sealer around this, and we'll be ready to install it onsite.

The instructional drawing for this Greenbrae chair can be found online (chengconcrete.com/dfrcplans).