This Instructable documents the construction of my new Modern desk with a polished concrete desktop!
Step 1: Plans!
Plan and layout the desk. Take measurements of the location you plan to place it. Nothing is worse than building something you wont be able to use. Make sure the concrete is broken up into manageable pieces both to prevent fractures, and to make sure that it is humanly possible to carry it to its final location! Also consider styling and other design elements. I decided to build a relatively minimal and modern corner desk.
Step 2: Buy Materials and Tools
about 30-35 board-feet of Lyptus wood,
a sheet of lyptus plywood,
some birch plywood,
2 sheets of 3/4" melamine,
2 94lb bags Portland cement,
6 50lb bags washed plaster sand,
acrylic concrete fortifier,
tube of silicone caulking,
Fiber optic cables,
other decorative elements.
air powered or waterproof angle grinder,
polishing pad set (from Ebay),
concrete working tools,
Step 3: Build the wood desk frame
I am not going to describe in depth this part, partly because I did not take enough pictures and partly because my dad mostly did this part. My desk is constructed of lyptus, which i found out is a genetically modified combination of Eucalyptus and Mahogany. It has two shelf/drawer cabinets ant the ends supporting a center frame with a pencil drawer. The main structural parts of the desk are double thicknesses of solid wood. The cabinets are made of panels holding a piece of plywood to cut down on the amount of solid wood necessary for this project.
Step 4: Construct Concrete Form
for the concrete portion of this project I followed doubleabattery's excellent concrete counter top Instructable here:http://www.instructables.com/id/Concrete-Countertops-for-the-Kitchen---Solid-Surfa/
The form was constructed out of 3/4" melamine sheet, with sides 2" high to ensure the concrete is strong enough. measure your desk frame and make the mold accordingly. make sure to test fit your desk before casting the concrete! It is easier to make changes now than later when it is literally set in stone. Use silicone in the corners to radius the edge of the concrete when it comes out of the mold.
Step 5: Add supports and embedded objects
Now is the time to add both the wire mesh support (I used K-lath) and any decorative crushed glass or embedded objects you wish to be exposed in the surface of the desk later! Also place any holes you want in the desktop later. wrap a piece of PVC pipe with some thin foam, then cover with packing tape to make a hole form. The pipe will tap free of the foam, then you can peel the foam/tape away. Use wire to suspend the mesh 1" above the surface of the form. this will place it exactly in the middle of the concrete. I wanted some crushed glass, so i bought some green, brown and clear glass, as well as uranium glass blobs. recycled bottles could be a good idea, any other rocks or other coins or other interesting objects can also work well! the possibilities are endless. I also bought some fiber optic cables to embed into the desk. I drilled some small holes through the melamine, inserted a fiber, then hot glued it on the other side. wire down the fibers and cables before pouring. bring the bundle of fibers together at the other end to go into a light box later. when unmolded and polished they will look like stars in the surface of the desk. Use spray adhesive to hold the glass in place in the mold so they dont get pushed around by the concrete.
Step 6: Pour Concrete!
Once everything is all prepared, mix up 1 part cement to 3 parts sand and mix well. Add water until the texture of thick oatmeal is achieved. Carefully plop into form. Add until about halfway full, and spread mix up sides. Vibrate VERY well to gt rid of air bubbles. now mix the same concrete mix ratio but add a good amount of glass fiber, and fill mold up the rest of the way. Screed over the top with a straight 2x4.
After a day of drying, cover with wet towels to slow the curing process and increase strength. It is safe to unmold after at least 2 days, preferably three. Unfortunately i didn't vibrate nearly well enough and got some major holes in mine, but all is not lost.
To unmold, i took off the sides, then use a shovel and some bricks as a lever to lift some sides. this caused it to peel off enough to get my fingers under the lip and peel it off the rest of the way.
Step 7: Grind and fill holes
Even if your slab has as many holes and voids as mine, the first thing you need to do is grind away the surface and expose the glass and embedded objects. The point of this is to get rid of the spry adhesive residue and stuff on the surface and to open up the tops of the holes to make it easier to fill them. Don't wear anything you don't want to get covered with concrete mud!
Next, let the slab dry and mix up some pure cement with some pure acrylic fortifier, until it is smooth and about the consistency of a shake. If it doesn't pour easily it is too thick. I found doing it in three stages worked best, as the stuff likes to shrink as it dries. spread one coat evenly over the surface, working it into the holes and voids. let it dry a bit until it has set up pretty well, then scrape all of the extra off of the surface. now repeat this process, filling in any holes you missed or holes that didnt fill all the way. Lastly repeat again, but do not scape off the extra. leave a layer of the mix on the surface in case it shrinks more than expected. let this cure overnight,then grind it off with the roughest polishing pad, probably 50 or 100 grit. inspect for any holes that didn't take the patch material well, and repeat the process if necessary. Surface should feel smooth, and the filled holes may be darker.
Step 8: Polish!
I don't have many pictures of this step since it is hard to hold the grinder, spray the hose, and take pictures simultaneously, but what you want to do is keep the polishing pads wet at all times, and slowly and evenly polish the whole surface with each pad until you reach 1500 grit. The polish pad set comes with a 3000 grit pad, but the sealer needs a small amount of roughness to adhere well. Be careful not to gouge the surface with the grinder or you will be able to tell in the shiny surface when you have finished. When it has been polished completely, apply sealer to the surface and let dry. A few coats may be desired. Buff the surface with paste wax and wipe off with a soft clean cloth or towel when dry.
Step 9: Assemble
Assemble the desk with bolts to hold the pieces together. set the concrete carefully on top, and add any drawer pulls, power strips, or other things to finish it off. Use zip ties to route wires.
Forthcoming Additions: diy light box build to light up fiber ends in different colors and patterns! also: pictures with less wire clutter visible.