So, I've been wanting to redo our countertops with polished concrete, but it's a significant job. I decided to first try my hand at making a concrete computer desk. My main struggle was that I don't want to invest in the polishing equipment for a one time project, so I set about trying to come up with a way to make a polished concrete look, without actually having to polish the surface. My first stop was instructables.com, where I found several references to Cheng Concrete, which led me to chengconcrete.com. These two resources helped me along my journey.
Here's what you'll need:
3 60 lb bags of 5000 psi concrete.
2 Boxes of Cheng Concrete Countertop Pro-Formula Mix.
1 3/4" sheet of plywood.
2 8' 2x2 or 2x4.
2 Pieces of scrap lumber to make molding radius.
1 8'x2-1/4" silicone concrete edge molding.
1 3'x4' dry-erase white board.
2 Sturdy sawhorses.
12 2" x 5/16" bolts.
4 1/2" x 27" hairpin legs.
1 2-1/4" piece of 1-1/2" pvc.
Step 1: Come Up With a Plan.
I struggled to find a way to develop a high-shine finish, without actually polishing the concrete. I wanted it to be as smooth as glass, which led me to my first idea, a pane of glass. However, after reading the information on chengconcrete.com and instructables.com, I realized that I was going to have to beat the heck out of the form in order to vibrate out all of the bubbles. And I know me. That would result in broken glass. My next thought was to use melamine and fiberglass the surface, but then of course, to get it perfectly smooth I would spend a ton of time sanding and polishing the fiberglass surface, which wasn't any easier than polishing the concrete surface, so that was out. Finally I settled on using a big white dry-erase board. Although they are available, I had a hard time sourcing a reasonably priced 4'x3' white board. If you do this, make sure that you see it in person (i.e. don't buy it on line) and make sure that the surface is as close to perfect as possible.
For the edge I decided to go for a split-face rock surface. I bought the silicone molding on eBay. I wanted the table to be about 28"x44", so an 8'x2-1/4" piece was perfect, since the back face of the desk would face the wall and needed to be flat. For that I used a 2-1/4" piece of the same dry-erase board cut to length.
Step 2: Create the Form.
It's important to make sure that the form will hold its shape, and you're going to need access to the bottom of the form to vibrate the concrete, so it needs to be strong and it needs to be elevated. I used some cheap plastic sawhorses to elevate it. Don't do that. Either build a table or use some good solid sawhorses. When this thing is done, it's flipping heavy. More on that later. The base of my form was a tripled up layer of 3/4 plywood. It likely didn't need to be that thick, but it's what I had. I cut a sheet in thirds, which gave me a surface of about 32"x48", plenty big for my desktop. The sides of the form were just 2x2 cut to length and screwed through the dry erase board into the plywood. The corners need to be radiused. I had some 2x6 lying around, so I decided to use that, making a 6" radius for the silicone molding, which was good enough, but next time I'd make it a little bigger, maybe a 10" radius.
I used a brad nailer to attach the molding to the form. It worked well, but there were a few brads that I had to drive deeper with a punch. Be safe using a brad nailer. I'd recommend gloves and obviously safety glasses are a must.
An option is to put a cord hole near the back. I don't have a picture of it but I took a piece of 1-1/2" pvc and pushed it in place once the concrete was placed. You can see the end result in the first picture in step 4 or the second picture in step 5.
Step 3: Place the Concrete.
Time to pour. I used Cheng Concrete Countertop Pro-Formula Mix, charcoal color. I had calculated out that my table should require about 1.3 cubic feet, so I mixed up 1.5 cubic feet (three 60 lb bags) of 5,000 psi concrete and used a box and a half of the Cheng mix. I also added a couple packets of admixture to retard the set of the concrete, hoping to ensure that it wouldn't go off before I finished working it. If you have a mixer, or can rent one, this may be a good place to spend a few extra bucks. I mixed it in a tub on the floor of my garage. This worked fine, but I made a real mess using a shovel to hog the mud up into the form. Also, if you are planning on making the desktop the way I did, without polishing it, you really need to make sure that you don't hit the form's bottom (desk's top) surface with the shovel or anything else for that matter. Anyway, just before placing the concrete, clean the dry-erase board one last time. I used windex and didn't find that it resulted in any flaws on the finished concrete surface.
Load the concrete in and start vibrating it to get the bubbles out. You could use a vibrating sander on the bottom of the form, but I didn't think it would be enough. I used a heavy deadblow hammer on the bottom of the form. It worked well. I wouldn't use a vibrator in the concrete, as you would likely scratch the form surface, which will translate directly to the finished surface. Once you are fairly confident that you've vibrated out the bubbles, place your reinforcement at the top of the concrete when you have the form almost all the way full. You don't want to put this in place before you vibrate the concrete, since it will naturally sink down and get very near the eventual top surface. That's not where you want your reinforcing tensile strength, of course. It needs to be near the bottom of the desktop. I'd shoot for about a half to 3/4" of cover.
As for reinforcement, I used 6x6 welded steel wire. Since the desktop is only 2-1/4" thick rebar wasn't a great option, though I'm sure you could do it. One last step is to set your bolts for the table legs. I decided to use a vintage style hairpin leg, so I made a wood form to set the bolts and sunk them once the concrete was all in place and the reinforcement was placed. After that, walk away. Stay away. I know concrete sets up quickly and you should be at 50% strength in 24 hours. I get that. Stay away. Let it strengthen in it's form for a few days or even a week. I promise you won't regret that. And the forms will come off no problem. Seriously, leave it alone.
Incidentally, I know that second picture isn't good. It's wet concrete, you can't expect much.
Step 4: Strip the Forms.
Make or break time. Two things to remember here:
1. It's heavy. Don't mess with it by yourself.
2. It's heavy. I'm not kidding.
I did something dumb and potentially unsafe. Focus not on what I did, but on what you should do. Have a buddy help you with this, or maybe two. The desktop is probably about 200 lbs, so two people can handle it, but it's nice to have someone else help you steady it as you flip it or move the sawhorses, etc.
Once you unscrew the 2x2 forms, you can pull the silicone molding off pretty easily. In fact, if you used the brad nailer like I did, the molding will come off with the 2x2. It's nice to see the edge, but the real money is in the surface. All this work is for naught if it's not pretty and smooth.
Not to worry, smooth as glass. Once you have stripped the form, securely store the slab in an upright position and use a water sprayer at least a few times a day to keep the surfaces wet. It's kinda cool to see the water bead up on the finished top surface, frankly.
Step 5: Install the Legs.
This step is pretty straightforward. Bolt on your legs. I used the 1/2" x 27" ones, mostly because they were on special, but they are the right length. Once they're tight, have a friend or two help you lift it into place. DO NOT TRY TO SLIDE IT. That won't work. Once it's in place, you should probably plan to leave it in place.
That's it, you're done. Congratulations. You've created a concrete computer desk with a high sheen, all without having to actually "polish" any concrete. Way to go. I'd love to have your comments, suggestions, or questions. Thanks for looking.