For our next larger project working our way towards doing the ceilings we built a desk in the library. The desk is a very shallow arch, a bridge spanning between the bedroom on the left and front door partitions along the east wall. This desk needs to be very strong as it will support a 500 lb fresh water aquarium so I did not do a plaster layer on the bottom like I had done with the bathroom lower shelf. The desk is 80″ long, 24″ wide, 7.25″ deep at the ends and 2.5″ thick in the middle of the arch. The cement is a high cement mix with added PVA fiber from Nycon and 661010 WWM reinforcing. Additionally there are rebar pins all around the ends joining the desk to the partition and side wall.
The support for the mold form is made of 2×4 joists with a 24″ long end plate. This sits on shims which sit on stacks of concrete blocks. The purpose of the shims is two fold: 1) to let me get the form support exactly level and 2) so that I can easily remove the shims when the concrete is hard such that the form support drops down out of the way and then I can peal the foam form mold off of the concrete.
The reason the middle joist is doubled is that neither of those pieces of wood were quite long enough. I’m loath to cut wood, or string, unnecessarily. It is a bit of a joke around here. “Just cut the string!” Holly will often say, even when we’re not talking about string, theoretical or otherwise. Well in this case I didn’t. Instead we put the two boards together so we didn’t have to cut another board shorter. It works beautifully and means one more board is saved for some future time when we’ll need the extra length. My mother says it’s a product of being a child of the Great Depression. I’m not. She is. I’m her child, the child of a child of the Depression. My father has another saying, “Buy it new, use it up, make it do or do without.” A modern similar mantra is “Reduce, Reuse, Recycle.” Of course, it could just be I’m a cheap penny pinching miser as someone once said.
The completed support frame for the foam mold with the adjustable cross support pieces that will be used to hold the shape of the curve. Only the middle upright 2×4 is toe-nailed into the joists to hold it in place. The other pieces are simply set on the joists so they can be slid to match the curved arch of the 1″ foam to come. The front 2×8 board was set in place for this picture but then I removed it so I could…
put the 1″ pink foam insulation sheet in place. I use pink foam because it is so slick the concrete does not stick well, especially if you demold it early. Spraying with a release oil makes it even smoother and easier to release but is not absolutely necessary as long as you don’t leave it in the mold too long, say a week. Generally I’ve demolded within a few days and the foam sheet just peels right off.
In this picture the cross supports have been positioned to support the foam so it won’t lose it’s shape as I add concrete. To further help I add the concrete a little at a time starting at the ends and working toward the middle.
A note on getting the right length on the piece of pink foam that makes the curve of the arch: You could calculate it mathematically. You could calculate it geometrically. You could use a tape measure of the actual curve and then transfer that measurement to a sheet for cutting. Or, as I did, you could bend the sheet of pink foam in place next to the wall and ding it with your finger nail in the spot to be cut. Simple and effective. I then used a right angle to cut it square and it fit perfectly.
Here is my son Will hammer drilling out the holes for the sill pins. I should have had him do this before I setup the forms but I forgot until later – mea culpa. In addition to three pins in the sill there were eight pins along the wall and the wall was roughed up a bit to make sure the desk would lock in. It will be heavy and be supporting a heavy aquarium so I don’t want any chance of it falling. The pin holes in the vertical walls are significantly larger than the rebar pieces so that the cement flows in around the pins and they are angled downward into the wall to lock tight.
After putting the front board on I used a bead of silicone rubber to seal the foam to the wood. This creates a little bit of a curve, after I ran my finger through it, and keeps concrete from dripping. I used a wood board with no foam on the front because we will peel the wood 2×8 off early to hand smooth and round the front edge of the desk after adding another complexity to the curve – details tomorrow.
The completed foam mold ready to accept concrete. It still needs pieces of 2″ PVC pipe in strategic places along the back so that electrical wires will be able to pass up along the wall. I put those in as we added the concrete.
The desk is curing eight buckets of concrete later, mixed by Ben and smoothed by Will and I. The white blobs in the back are the tops of the 2″ PVC pipe w
here wiring will pass through the desk. The setup took a lot of time but the actual pour went very quickly. Unfortunately it was snowing heavily outside, where we had setup the concrete mixer. When we had set it up things were nice but the weather changed quickly. For the next steps I’m going to move the cement mixer inside again – Ben will appreciate that.
The reason the top of the desk is rough is we’ll be adding another 1″ layer, with some fancy front curves and pigmenting it. But that is tomorrow’s project.
Note: I faked the gorgeous sunny views through the windows. While we were working on the library desk the weather was almost blizzard conditions outdoors and all you could see was white much of the time. So I Photoshopped in views from another post. Much more interesting and how I would have liked the weather to have been. Wouldn’t it be nice to be able to Photoshop the real world?
Outdoors: 37°F/18°F 8″ Snow, Sleet, Hail, Rain, Windy
Farm House: 72°F/53°F two logs
Tiny Cottage: 48°F/41°F door open too much and no sun