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Posted by aaron at 01:01AM, Wednesday, December 09th, 2009

My first attempt at casting concrete

I've wanted to play around with concrete for a while -- it seems pleasingly plastic, sturdy, and amenable to DIY. I finally found a good opportunity two weekends ago. Here's what I learned.

We've had this old, battered Saarinen Tulip chair for years. A couple months ago, the seat gave way -- the fiberglass just busted through. This struck me as practically impossible to repair, but I got to thinking that I had this nice aluminum Knoll base, which must be usable for something.

I decided to turn it into a DIY table. So I started researching concrete casting, and reading all about various recipes for different applications. I wanted to keep it simple to start, though, and just got a bag of Quikcreate premixed cement and aggregate.

I wanted a round or oval top, ideally with an angled edge, so I scoured the yard, basement, and a couple of hardware stores for suitable molds. I was tempted by an (enormous) kiddie pool on sale at the local Ace, and also by various garbage can lids, but ended up getting a cheap plastic flowerpot, with a top diameter of 24 inches -- about the minimum that would look suitable for this base. I sawed the top off about 4 inches down. Brian suggested I use a sheet of plywood from the garage as a base for the mold -- this is what would be the tabletop when the concrete was set.

I also wanted to affix the top to the base somehow -- it would be a heavy piece of concrete, and I didn't want it flopping off and breaking my foot when one of the cats ricocheted into it. The Tulip chairs, fortunately, have a hollow base, and the tops are generally attached by a bolt going through that. I figured the same thing would work here, so I got an eight-foot piece of threaded rod at the hardware store, hacksawed it to an appropriate length, and came up with a jig (i.e., a piece of scrap wood with a hole drilled through it) to hold the rod in place as the concrete set and cured.

So, on a Saturday afternoon, I mixed the concrete in a plastic trough, following the consistently vague directions I'd read online and on the back of the bag of mix: add an indeterminate amount of water, but don't let it get soupy. The slurry I came up with was on the gloopy, enormous-granularity-oatmeal side, rather than the soupy side of the scale. I tried to sort of shove it into the corners with my trowel to get (what would be) the top nice and smooth. Then I gently worked the threaded rod into my best guess at the middle of the top (Note to self: measure this with the rig next time!), trying to be careful not to go through all the way, but to leave 1/4" or so of concrete underneath the tip.

Here's what it looked like at this point:

My reading had indicated that I should probably try to vibrate some of the bubbles out. Suggested solutions for doing this ranged from using a purpose-made concrete vibrator (cue handy-lesbian jokes) to just pounding on the mold with a rubber mallet. I did a little of that, and stomped on the plywood base a lot, but I'd read about seeing bubbles float to the top, and I didn't see any of those. Maybe my mix was too dry.

Concrete likes nice hot weather to set and cure, the intertubes told me. Everything I read also concurred that high humidity was good for drawing out the curing process, which would make the concrete stronger. Fortunately, I started to do this in the middle of our week of torrential summer rain. I soaked a towel and hung it over the whole rig, and then put a garbage-bag tarp over that, to keep the mold from getting completely flooded.

Sunday morning I came out to check on it. When I peeled off the tarp and the towel, the concrete still looked shiny and wet. I poked it with my finger, and was surprised to find it was quite hard -- it felt like bona fide concrete!

So I left it for the next three days, as it continued to rain. Thursday it cleared up. When I checked on my baby, I noticed that the towel hadn't covered the top completely, and where there was a gap, the concrete had become much lighter. I re-soaked the towel and covered the whole thing carefully, and put it back under the tarp.

Saturday I decided it was time to break the concrete free of its mold. I nudged the top of the planter until the whole thing came free of the plywood, then carefully pulled it off. It was a lot heavier than I'd expected -- this thing was just less than 2 inches thick, but seemed to weigh a ton. Some bits around the edges crumbled -- concrete that had apparently oozed into the crevice where the rolled top of the pot met the plywood -- but the main shape seemed solid.

I was wondering whether the top would take the imprint of any of the lovely grain in the plywood. It did not -- clearly the mixture was too coarse. Also, while the top and edge surfaces were relatively smooth where they met the mold or the plywood (especially the sides -- that plastic made for a surprisingly smooth surface), there were lots of spots where the concrete *didn't* meet the mold: quarter-inch holes, and a few places where bigger bits of the aggregate came mostly, but not quite, to the surface, and seemed to have sucked the cement down a bit.

I'm not displeased with the result, which certainly looks like concrete. I think that if I wanted a more uniform, unpocked surface, I might start by slathering a very thin, slightly wetter layer, maybe with smaller aggregate, on the bottom of the mold, and then put the normal concrete mix over that. The underside of the table is very rough, which is fine, and uneven, which is a little surprising. Again, maybe this is a function of the relative dryness of my mix, but I had expected it to act more like a liquid, and sort of flow to a level. For a larger surface, or a more visible one, I'd be able to get in there with a trowel and smooth that out a little better.

Overall, I'm enthused. This seems like a versatile and pleasing material, and I love how simple it is to construct something so tough and, presumably, durable. I like the honesty of it, and the timelessness. I like the idea of being able to embed different aggregates -- glass, fiber optics, wood, or other materials which will change much faster than the concrete. I'm thinking about various materials I could co-opt into use as molds, and also thinking how much I'd love to have one of those cheap CNC lathes that are out now. The table is currently serving as a plant stand (after several unsuccessful attempts to lubricate the old, sheared-off bolt in the base in order to remove it, I ended up drilling it out; the bolt on the tabletop fits perfectly.)

Here are some links I found useful:

A detailed explanation of different aggregates, mixes, and reinforcements:

An article about making flowerpots at Make:

An article on making DIY concrete countertops for Ikea cabinets:
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