Fire Table – Concrete Insert Part 2

Fire Table – Concrete Insert Part 2

If you’re keeping track around here, you know that the fire table I’m building requires a concrete insert. You’d also know that I’ve never worked with concrete before, so it shouldn’t be a big surprise that my first attempt failed. Time to regroup, take some lessons learned, and try again.

I think it is likely that the concrete expanded slightly as it dried, squeezing tightly into the first mold. The end result was a stuck concrete insert. The mold design relied on the concrete dropping out of the mold cleanly, which didn’t happen. I realized I needed to redesign the mold so that I could disassemble it after the concrete dried. It helped that the old mold was completely destroyed in the first attempt. 

With the goal of full disassembly after drying in mind, I picked up more melamine and started the new mold. The second mold design was basically the same as the first one, except I got rid of the large block in the middle for the insert hole. Instead, I replaced it with individual walls that I could pull off one at a time to release the mold. The outer walls were also made to be disassembled, so at the end there should be no issue pulling the concrete off the bottom board.

My original mold with the full, non-removable, block in the middle.
Mold number two, featuring individually removable walls.

The new mold should help take care of the major crack caused trying to get the mold out. I made one more adjustment to help prevent cracking. I extended the metal fencing I was using in the concrete to overlap in the insert corners. This should provide extra strength and support in the most vulnerable area.

Metal fencing to add strength, with fully overlapping corners.

Another issue was the number of small bubbles in the first insert top after pulling it out. If air gets trapped in the concrete while it dries, it causes little bubbles or air gaps. While pouring the first insert, I tried to tap on the sides of the mold with a hammer to pop all the bubbles and bring the trapped air to the surface, unsuccessfully apparently. This time, I decided to use all of the recommended water in the concrete mixture. This would make the concrete more liquidy and should make it easier to get the air bubbles out. It has the extra benefit of making sure it flows into every corner of the mold.

Second pour was much cleaner and easier to knock the bubbles out.

In the first attempt, I used Flow Control and Set Control in one bag of Rapid Set Cement All. In the second attempt, I got rid of the Set Control. I had plenty of time to pour the first batch without it setting, so it wasn’t necessary this time. With the more watery consistency of the concrete, I could see a lot more air bubbles coming to the surface as I tapped the sides with a hammer. It was a great sign that the next mixture was working much better. And this time, I could see steam coming off of the concrete as it cured. 

The chemical process during cure puts off a lot of heat. Note the steam in the sunlight.

I knew the concrete was done curing when it stopped putting off heat. I waited a bit longer this time before pulling it out of the mold, just to be sure. Drumroll please….

Really great results. Minor rough edges are no big deal for a concrete piece like this.

Perfect! The second attempt was a great success. I guess it is a good sign that I can still learn from my mistakes. The next post will be sealing the concrete, spoiler alert, I make a mistake there too! 

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