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Old 12-21-2019, 12:57 PM   #1 (permalink)
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Join Date: Jan 2008
Location: Placitas NM 20 miles north of Albuquerque
Posts: 1,439
Default Casting roof tile with ShapeCrete

My Alvarado Hotel used sheets of vacuum formed plastic tile for the roof.

The Alvarado as built

The roof did not stand up well in the hot sun of New Mexico so I wanted something more durable.
Roof damage

On MyLargeScale I had seen a mention of something called ShapeCrete which can be bought on line and delivered to Home Depot. It can be molded by hand like clay or cast in a form. It looked like something that I could use to make tiles for my buildings so I ordered some. There are several tutorials on line showing how to use it in addition to the instructions on the tub it comes in.

I decided to make a couple of test pieces to see how they worked and determine if I needed to make any changes in the process. A 3 X 8 inch form box was made of cardboard and tape.

A piece of the plastic tile was inserted upside down to form the contour of the casting. (The weights and c-clamp were to keep the box from bulging)

A backing was made of corrugated plastic and painted with concrete bonding agent. This was added to give the cast part more strength.

I filled the form with Quinoa to get an estimate of the amount of cement I would need.

To make the tiles the cement powder was mixed with water to a consistency of pancake batter so that it could be poured into the mold. I made a couple of test pours to see how they worked, the first with just the cement and the second with color added and a slightly thinner profile. While the cement was still wet I pressed the piece of coated corrugated plastic into the cement.

The first pour I did not use any mold release and it was hard to de-mold.

The second one I used some cooking spray on the mold and the casting came out very easily. The first casting is 3/8 inch thick from the top of the tile to the bottom of the concrete. The second one is 5/16 and with the plastic backing, both seem plenty strong.

In the future I will use a vibrator to remove the bubbles from the cement. Now I am ready to try casting roof sections for the Alvarado. The largest section will be about 11 X 15 inches and it will take 2 pieces to form the largest roof section. My original thought for the smaller parts was to cast the sections and then to bend them at the roof peak while the cement was still soft but that didn't work very well. I will post more details as I progress with this new roof.

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