Yesterday I talked about the arch tests we’ve been doing. While all this arch testing was going on I made two large tiles which are to become the bases of the previously mentioned dog houses.
The tiles are also tests of pouring the ceilings for the front room, bathroom and bedroom. The basic idea is to pour the concrete onto a sheet of smooth plastic. This technique produces an almost glasslike finish. Masonite and various plastics were options but what I settled on was the 1/2″ plastic coated foam insulation. For one thing I already have it and work with it a lot so I’m familiar with it. The foam has a diamond pattern of threads in it that produces a very interesting pattern in the tile and thus the ceiling. Having a pattern like that helps to hide any flaws, bug splatter and the like.
We made the form by building a 1/2″ plywood frame on 2×4’s at 16″ centers. The 2×4’s sit on stones on the ground so the whole structure can be vibrated by the concrete vibrator to get the mix dense, eliminate bubbles and smooth it out. This worked very well. The few imperfections were due to experiments with aggregate and I know how to solve those. On top of the plywood we placed a sheet of the 1/2″ plastic clad foam and then silicone glued 2″ strips of foam around that to make two approximately 4’x4′ square molds for the test tiles.
What I didn’t do, and should have done, was run a bead of silicone around the joint inside the mold to seal it. I have done that on other projects but didn’t on this one so there was some concrete leakage in the mold joints. Not a big deal for this test but it would have been a bother for the real thing.
After the mold was ready I used tiny dabs of silicone to glue down some coins, stones and leaves. When we lift the dog house it will be interesting to see how those came out. Unfortunately I didn’t do this embedding technique on both of the tiles. Another little whoops.
We poured and floated both tiles in one morning. An easy six buckets of concrete. Ben, our mix master, liked that easy load as he felt he had gotten out of practice. We used a 0.65 gallons of water, 1.5 gallons of portland cement, 3 gallons of sand, small handful of PVA fiber, Aquron 300 mix for the base.
After getting the tiles well vibrated to remove air bubbles and densify the concrete we made wire cages of 661010 Welded Wire Mesh. We pushed these down into the concrete by about 1/4″ and then roughened up in each square to bind the wires. In addition to holding the wire this also gave better purchase for the second layer of the two part tile pour which came the next day after we put in the foam for the walls.
After that it was a matter of waiting and keeping the thin tiles moist. We covered them with light sheets and sprayed them regularly with a mist of water. The point of covering with the fabric is to keep the hot sun off the concrete so it won’t dry. Likewise the misting with water helps keep the concrete moist as it cures. Concrete cures, a chemical reaction, to reach full strength. Drying is not good or desirable.
That is one of the two tiles tipped up. I think it only weighs about 100 lbs as it is only about 1″ thick. After two weeks of curing with accelerant it is quite strong even though it is so thin. The surface came out wonderfully smooth just like I had anticipated. It’s always fun to have a test work out so well. The colors are experiments with using pigments. I was most interested in the black to simulate a marble look and that looks great. I had used about three tablespoons of black pigment folded into a five gallon bucket of already mixed white concrete. I then dumped it into the form and floated it to streak the pigment. The results look very much like the white and black streaked marble I have from the local quarry. Doing it again I think I would fold the mix a bit more.
The red pigmented areas are a bit too intense and I’m not fond of the color but it was interesting to see the results for the learning experience.
Another very interesting thing with this tile is that when I picked it up the colors looked great. Five minutes later there was a white powdery efflorescence over much of the surface. This is the first time we’ve seen efflorescence so it’s rather interesting. Thinking about the chemistry I concluded it was probably a result of the sudden exposure of the formed portion of the ceiling tile to CO2 in the atmosphere. The efflorescence appeared in both the grey and the white cement as well as the pigmented and non-pigmented areas. The effect may be slightly stronger in the white cement area. One interesting aspect is if you look closely there is a grid pattern. This grid exactly matches the 661010 WWM in the concrete tile. It appears that the WWM is dirty with some salt which has caused the efflorescence. The fact that the tile is so thin and the steel so close to the surface (0.75″) probably resulted in stronger efflorescence.
This gave us an opportunity to experiment with removing efflorescence. So far I have avoided using the recommended Muriatic Acid (a.k.a. HCL or Hydrochloric Acid). What we’re doing seems to be working and the HCL is a bit nasty. The photo above shows the tile after it has been washed several times. We are pressure washing the whole tile, then hand scrubbing the bottom left with a rag and plain water. The middle bottom half or so we are hand scrubbing with vinegar (Acetic Acid). After repeating this for four days the vinegar area looks the best but the efflorescence is greatly reduced from all the areas. Probably just washing it with clean water on a daily basis will allow the chemical reaction to use up the available free reagents and wash away the residue. I suspect plain water and a bit of time is the simplest, cheapest and safest solution.
Outdoors: 79°F/52°F Sunny
Tiny Cottage: 73°F/67°F