Concrete #2

posted May 25, 2015, 2:24 PM by Paul Halliday   [ updated May 25, 2015, 4:11 PM ]
I hate dusting. More generally, I hate when something is difficult to clean or maintain. This actually bothers me so much that I have no problem throwing perfectly good things away if they meet this criteria.

This is what my office looked like a few weeks ago

What we see here are two Ikea desks that are confining, have very little usable desktop space, and are starting to show their age. Did I mention they are a pain to keep clean? (terrible finish for a desktop). Anyway, time for something new (don't worry, they were re-purposed).

My requirements for the replacement were pretty simple. I was looking for something that made the entire length of the room usable desk space and is durable and easy to keep clean. It just so happens that concrete is the perfect fit for this!

Building the form

After emptying the room I placed a 2x6 ledger board along the length of the wall. This is what the finished desktop will rest on so you want to make sure that you are using decent screws and hitting the studs as close to center as possible. After this was up I fastened a 2x4 the entire length, shooting long at the end so that I would have enough to complete the form. This is 3/4 inch down from the top of the ledger as this will support the plywood that will become the bottom of the form.

I ran a bead of silicon along the top of the 2x4 to keep the water in the form (and not make a huge mess). I used short screws sparingly to fasten the plywood because this is going to need to come apart easily when we are done. Notice that the top of the plywood is flush with the top of the ledger board.

The trough on the front is so that the finished product looks a little thicker than it really is. Regardless of whether you are going for this effect or not, you will still want to make it so that you can expose the face of the concrete early on so that you can clean it up before it sets (more on this later).

Once the form is done we need to add a grid of reinforcing bar (rebar). This is especially important for a piece as thin as this, it will crack and be significantly weaker without it. 

When you are laying the grid out you want to pay particular attention to the exposed edges, making sure that the bar is centered within these areas. You can use wire to suspend it within the form or just rest it on small spacers (I just used washers). The 2x6 legs are sacrificial. They are friction fit to support the the entire form while the concrete is poured and sets.

One last detail. This thing is going to be REAL heavy when it is done and the last thing we want is it coming away from the wall. To address this I drove lag bolts into the top of the ledger and into every stud along the length of the form. The small nails are there so that I can keep track of my depth as I do the initial screed.

Mixing and Pouring the Concrete

The total length for this project is a little under 13 feet in length with a depth of 30 inches. The finished surface will be 29 inches high and 1 5/8 inches thick. I am using 80 pound bags of high early cement which I added a charcoal dye to tone down the bright grey of typical bag mix. The finished product will weigh around 640 pounds.

NOTE: There is no turning back once you start mixing and pouring. You will be committed until you have screed, troweled, and edged the piece. Also, if you are working on anything bigger than a few feet you will want a few people with you; especially if you are mixing by hand. If you have even a hint of the need for the washroom, take care of that before you start.

Also, watch your water. If you are mixing it yourself follow the instructions. You are not making soup, you want stiff oatmeal.

Lastly, knockouts. If you have anything with wires there is a good chance you will want some of these. Unlike other materials though, they are pretty tricky to add after the fact so you need to build them into your form. I just used oak dowel here and if you have it, you can wrap these in something compressible (sill gasket is perfect) to make them easier to remove when everything sets.

Once the form was filled I wrapped the sides and bottom with a hammer for about 5 minutes. At this time I also used an edging tool to break the exposed edge.

That's it for now. This needs to rest for a while.

After about an hour I stripped the front of the form so that I could finish the edge. There is a good chance there will be some voids as can be seen below.

You can usually scrape enough slurry from the form piece that was removed to fill these. After filling these in I used the edging trowel to finish the face and round over the bottom edge. 

This was the first piece that I have done that was not going to be ground and polished with diamond pads. I was going for a very rustic look. 

Eliminating these steps doesn't necessarily get you off the labor hook though. I continued troweling the surface every few hours until the early hours of the next day. If I wasn't so exhausted I would have probably done a little more. I am happy with the results though.

Building the Legs

Nothing fancy here. Square tubing, a few small pieces of angle and jack post plates. They were 10 bux a piece but saved me a lot of time cutting and drilling.

The only problem with the plates is they were stamped and raised in the center. These mag angles helped me lay them out.

Ready for paint. The small angle is there so that I can fasten them to the floor as well. I used 1/2 inch drive pins to fasten these to the underside of the desk and the floor.

Finished (almost)

After the concrete rested for 7 days I stripped the form and cleaned up any sharp edges with a sanding block. I then applied 2 coats of a penetrating sealer. After this dried for about 2 hours I applied 6 coats of a water based satin finish (30 minutes apart).

I trimmed out the ledger with clear pine that I painted to match the legs,

The books are temporary and will be replaced by a 12" deep maple plank that will run the entire length of the desktop (waiting on my mill guy). There will also be maple plank shelving hovering just below the bottom of the desktop to support laptops, and yes there will be cable management too :)

 I will update this page once complete.