Fire Table – Concrete Insert

Fire Table – Concrete Insert

Almost every project I’ve completed in the past typically revolves around wood (with some exceptions). Wood is easily available, easy to cut, and easy to assemble. Unfortunately, it is also flammable. So when I made my fire table design, I added a concrete insert to act as a buffer between the flame and the wood. It is easy enough to included in a design, but now I had to try and build it. Welcome to the first episode of “Concrete Insert Building”!

A widely used method of making a shape out of concrete is to use a mold for the shape. This is exactly what I did as well. To start, I had to take the dimensions of my concrete insert.

SketchUp makes pulling dimensions easier.

Easy enough. Next up was to build a mold that would create that shape. For the mold, I used a melamine board. Melamine boards are strong, easily available, and smooth enough that the concrete shouldn’t stick to it. The initial mold design had a big rectangle in the middle of a board to create the empty space in the concrete insert. I’d say calling it an “initial” design was foreshadowing, but the main post picture already did that.

To match the 2×6 wood in the table, the insert had to be 1½” thick. Since the melamine is ¾” thick, stacking two pieces on top of each other was the perfect thickness. The outer edges of the mold were melamine strips ripped down to 1½” thick to match the inner mold piece. A few cuts on a miter saw and some rips on a table saw later, the mold was complete.

I pre-drilled everything to prevent cracking.

To finalize prep for pouring the concrete into the mold, I had to seal the joints of the mold. I did this using a silicone caulk. If I had to do this again, I would use a dark colored silicone instead of the clear that I used originally. This is just so that it would be easier to see and make sure the seams were clean and smooth. I also wrapped the inner blocks with packing tape to create a smooth surface instead of the rough particle board.

Seams caulked, inner block taped, ready for some concrete.

I used Rapid Set Cement All as the concrete. This “construction material” as they call it has very fine sand in it that means it will be very smooth when set. I added in a packet of Flow Control to help make the concrete more liquidy. This helps it to flow and settle everywhere within the mold. In addition, I added a packet of Set Control to give me a little more time to pour the concrete before it set. For this small of an insert, I only needed one 55-lb bag of concrete. 

I added slightly less than the maximum 5-quarts of water recommended for the mix. Then I added the concrete mix and the flow and set control packets, mixed it all up with a paddle mixer on my drill, and poured directly in the mold. I sprayed a light coating of WD-40 in the mold before pouring to help keep the concrete from getting stuck.

As the concrete went into the mold I tapped the sides of the mold with a hammer to agitate the mix and hopefully fill any air gaps and bubbles. For the top (the eventual bottom of the insert), I used a metal trowel to smooth out the concrete and remove any excess from my pour. 

I was a little worried about the cordless drill, but it had plenty of power for this.
Pouring the concrete. Note the metal fence piece I added after the pour for strength.
Troweling. Unfortunately, yes, my typical concentration face does include a wide open mouth.
I practiced my smooth troweling skills and was pretty pleased.

Time to let everything set. This concrete needs to be water cured. This means that as the concrete is curing right after pouring, it has to be kept wet. Which means that I hung out with my concrete insert for a couple of hours, constantly spraying it with a water bottle as it cured.

As the concrete dried, like the light gray parts above, I sprayed it down with water.

Time for the moment of truth! Once the concrete cured and stopped giving off heat, it was time to take the piece out of the mold. I flipped the mold over and the concrete slipped right out.

Few taps and it dropped right out…or not.

Just kidding, the concrete did not drop right out of the mold. So I took the sides off of the mold and then it came right out…

Surely this will work with the sides off…

Nope! What about after tapping it with a hammer and prying it out with pieces of wood?

I was still being fairly gentle at this point.

Also no. At about this point I was way too frustrated and stopped taking pictures. Full disclosure, I forgot the most important rule. Building things and learning skills is supposed to be fun. Definitely an area that I can continue to improve in. 

So the concrete didn’t come out of the mold cleanly. I did a lot of prying of the bottom of the mold, trying to get the concrete to drop out. Eventually, the mold broke, allowing me to pull everything off of the concrete in pieces. Great! However, the concrete cracked in a couple places during this process. There were also a ton of bubbles in what was supposed to be the top of the table. A pretty large fail. 

This was one of a couple cracks in this piece of concrete. So it became a testing piece for the fire pit.

Luckily, I regrouped and tried again, so the next post will detail what I learned and changed for the next attempt. Will it succeed? Tune in and find out next week on “Concrete Insert Building”.

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