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Build a Concrete Computer Desk.

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Picture of Build a Concrete Computer Desk.
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.
Various tools.
 
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Step 1: Come up with a plan.

Picture of 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.  
David9711 months ago
For serious raging gamers!
Gilo11 months ago
Good work.
Where can I get the Cheng concrete mix and how much does it cost?

I believe you can use those 3/4" MDF boards w melamine surfaceand they are as smooth as formica sheets. You can wax and polish it to get it even smoother.

I plan to make a small laundry counter top w a utility sink in it.

Thanks for the instruction.
smassco (author)  Gilo11 months ago
Sorry, I forgot to answer your first question. I bought it from the Cheng Concrete website, concreteexchange.com. It was $22/box and I needed 2 boxes. Probably could have used one box for the 1.5 cubic feet of concrete, but I figured I'd stick with the recommended ratio.

http://store.concreteexchange.com/CHENG-Concrete-Online-Store_5/CHENG-Concrete-Countertop-Pro-Formula-Mix-and-Supplies_2/CHENG-Concrete-Countertop-Pro-Formula-Mix#largeview
smassco (author)  Gilo11 months ago
Yes, the melamine is available, but it isn't as smooth as glass (or a whiteboard). I like the laundry counter idea. Good luck. Post your project when you get a chance. I'd love to see it.
brucerray11 months ago
Also, I saw a TV program where they used a sheet of acetate to get the glassy surface. (I just happened to have a roll of that...so I see a shiny concrete outdoor bar top in my future!)
smassco (author)  brucerray11 months ago
I like the acetate idea. Just make sure whatever your substrate is, it's perfectly flat, since any imperfections (dimples or pimples) will translate directly to the finished surface.
brucerray11 months ago
Can you post a link (or links) to sources for your split-face rock edge molding?
I found some but they are REALLY proud of it! $$$
smassco (author)  brucerray11 months ago
Sorry, I don't have a link. It was on eBay and frankly, I felt like I overpaid, but the split face rock surface was cheaper than the other designs. I'll see if I can find the purchase and if the guy is still selling it, I'll add a link, but it will likely only be good while his current item is still up.
boxcarmj11 months ago
If you want to know what your project will weigh here is a formula you can use. Length times width times thickness divided by 1728 (cu foot in inches) times 140 (weight of a cu. foot of concrete). 28 x 44 x 2.25 = 2772 ÷ 1728 = 1.6 x 140 = 224 pounds.
Sorry for the errors.
smassco (author)  boxcarmj11 months ago
Yes, that's how I calculated the weight, but of course, not all concrete is exactly 140 pcf. It will get you close enough though, obviously. I've made some very lightweight concrete in my time. In college (building a concrete canoe) I made some that was right about 56 pounds/cubic foot, which is lighter than the density of water, so I was able to make a floating block of solid concrete. It wasn't strong at all, but it was still pretty cool. I used glass microspheres and perlite as "aggregate", an air entraining admixture, and a couple other tricks. It was very fun, but the compressive strength, even at 28 days, was terrible.
boxcarmj11 months ago
If you want to know what you your project will weigh here is a formula you can use. Length times width times thickness divided by 1728 (cu foot in inches) times 140 (weight of a cu. foot of concrete). 28 x 44 x2.25 = 2772 ÷ 1728 = 1.6 x 224 pounds.
woodNfish11 months ago
Very nice job. I especially like the mirror finish without grinding. Could you have made the top lighter by doing a double pour? 1. First pour about an inch deep, let harden, but still wet. block out the middle of the top with foam to create just an outside edge, then pour the edge. Would that work?
smassco (author)  woodNfish11 months ago
I think you could do something like that, but you wouldn't want the first pour to set up, or you'd have a cold joint. Also you'd want some reinforcement tying the two together. Like dowels in a concrete slab cold joint. If you used welded metal wire you could figure out a way to make it work. Not a lot of room to work in that one inch though. I think you could just do it without waiting for it to set up. You could probably save nearly 50 lbs by blocking out about 1"x18"x32".
travelfeet11 months ago
I'm in exactly the same place. I want to do counter tops but know I should do a practice run first.

Is the dry erase board usable for another form or it is shot after the first use?
smassco (author)  travelfeet11 months ago
Mine looked like it could have been reused. It would be worth a try on a smaller project. My fear would be that if it didn't release properly on the second project, you'd basically ruin your work. Of course, I suppose you could then just polish it like a regular polished concrete project.
jJohny11 months ago
That looks great. But heavy. Any numbers on final weight? Could I pour it's surface with this no-polish mix then pour the bulk of it's thickness with a low-density mix, or wouldn't they adhere well?

Maybe I've watched to much Modern Marvels but why not make a prestressed frame and set it within the pour and then use half the thickness or less?

Thanks and sorry for all the questions.
smassco (author)  jJohny11 months ago
I'd say the final weight including the legs is between 225 and 250 pounds. You could probably do that, but you don't want a horizontal cold joint. Also, the color would be tough to match with a different mix design, so that might show on the side. I suppose you could prestress the reinforcement, but I doubt it's worth it on a job this small. That's for some big stuff. You could certainly make it less thick, and that would save weight, I'm sure I could have gotten away with 1-1/2" or so, but it would still weigh over 150 lbs. I like what you're thinking though. A less dense mix would save weight to be sure, but my main goal was that shiny surface.
carp4612011 months ago
I saw a web page once where a guy used sheets of formica to achieve a smooth surface on fiberglass panels used in a boat hull. He built a flat table and bonded the formica onto it and vaccuum bagged the fiberglass onto the formica, which had been waxed beforehand. The panel looked like glass. This would probably work for concrete and the formica comes in 4x8 ft sheets. not sure about the cost. You might even bag it to help remove the bubbles.
travelfeet11 months ago
I'm in exactly the same place. I want to do counter tops but know I should do a practice run first.

Id the dry erase board usable for another form or it is short after the first use?
Buckshott0011 months ago
You might try NPN for no polish glass like surface. I hear it's good stuff if you use it right
http://concretesurfaces.com/gallery1.htm
CreativeGeek11 months ago
You could try setting the legs in the concrete while it's wet, actually, although if you're going to do that, some rebar may help.
smassco (author)  CreativeGeek11 months ago
Thanks. I had thought of that, but I wanted to be able to take them off when we move, so I just used embedded bolts. It would probably be a cleaner look if you wet set them though. I really liked the vintage look of hairpin legs, but of course you could use a different type. I love concrete and unfinished steel together, but I'd imagine that painted legs or wood legs would look cool too. Whatever you do, make sure it's strong. The desktop is pretty darn heavy.
No prob, and the little heart that was drawn in the wet concrete looked kinda cute in a way.
smassco (author)  CreativeGeek11 months ago
You noticed that, huh? I didn't think anyone would be able to see that. Maybe I hoped nobody would be able to.
X3 Still.
Gss201311 months ago
It loojs awesome
smassco (author)  Gss201311 months ago
Thank you!
beerhead50011 months ago
Looks amazing. I have been interested in trying something like this for the longest time. The site chengconcrete.com is a huge time suck because there is a ton of great work on there.
3 questions: Could you have used plexiglass and vegetable oil instead of expensive whiteboard? Also interested in the rock edging, anyway to do this with out the mold? I heard you could have purposely put "fault" lines in the concrete then chipped off with hammer. Finally how did you do the legs? Did you drill the holes after the fact? Great work and thanks for sharing.
smassco (author)  beerhead50011 months ago
Thanks. It was a fun project.
Probably plexiglass would have worked, but it might have been tougher to get that attached to the plywood backing. The whiteboard was easy. I think it cost $35 or so. Problem is that it's basically a one time use at that point. I'd imagine you could reuse the plexi.
I don't know how you would do it without the mold, but I suppose you could make a cold joing, or maybe even just cut a relief joint, then break it. However, I forgot to mention, I would actually not recommend using the split face rock edge. It's hell on your forearms. I'd look for something rounded, or maybe some dental molding. They have plenty of fancy molding for that.
As for the legs, they come with three holes pre-drilled in the bracket. I used those to drill through some 1/2 inch plywood, then used the plywood to set the bolts in the concrete.

I hope those answered what you were asking. If not, let me know.
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