This episode shows how I made a concrete fire pit out of Quikrete 5000; it takes about 20 hours and $120. I used my X-Carve CNC machine from Inventables to make reusable forms out of plywood. I created a modular design that includes embedded firebricks. I like this fire pit design because it is modular and can be relocated. This ability to move it in pieces makes it a great fire pit option for renters.

SUPPLIES:

¾” Melamine
Available at Home Depot
I used a scrap piece of ¾” melamine to use for the bottom of the molds.

¾” Plywood
Available at Home Depot
I made the forms out of ¾” furniture grade plywood.

Quikrete 5000
Available at Home Depot
I used Quikrete 5000 to cast this fire pit.

Silicone Caulk
Available at Home Depot
I used silicone caulk to seal the molds.

Fire Bricks
Available at Masonry Supply Store
I used fire bricks from a local masonry supply yard. They aren't the prettiest looking, but after a few fires they will turn black from the soot. Firebricks are heat-resistant and will protect the concrete from cracking.

Quikrete ¾" Gravel
Available at Home Depot
I used about 4 bags of Quikrete 3/4" gravel for the foundation layer.

Miniwax Polyurethane
Available at Home Depot
I sealed the forms with minwax polyurethane.

TOOLS:

Inventables X-Carve (Small)
Available at Inventables












RYOBI Orbital Sander
Available at Home Depot

INSTRUCTIONS:

STEP 1: Design
I used FormZ 3D modeling software to model the firebricks. I don’t recommend this software and am now switching to Fusion by Autodesk. I then arranged the firebricks into an octagon. I designed eight modular blocks to go around the firebricks. At first I designed the blocks with simple symmetrical points but then changed them to an asymmetrical design. I then designed the profile of a mold for the blocks and exported the outline of the pieces to easel software to get ready for CNCing. I made some of the pieces have tabs that would let me clamp the molds down to a base piece of melamine.













STEP 2: Cut the Pieces
I used my X-carve CNC machine by Inventables to cut out the pieces for the mold out of ¾” furniture grade plywood that I bought from Home Depot. The CNC machine leaves small tabs between the pieces so that the pieces don’t move when being cut. I used a handsaw to cut these small tabs and then used sand paper to smooth down the cut edges.







STEP 3: Glue the Pieces Together
I used wood glue to glue the pieces of plywood together and clamped them in place while the glue cured. I started with just a few pieces at a time and then glued those pieces together. This way it is easier to keep the pieces aligned.







STEP 4: Sand and Seal the Molds
Once the glue had cured I used my orbital sander to sand down the molds. I sealed the molds with 3 coats of minwax polyurethane. The parts of the mold that touch each other will be clamped together so I brushed on a coat of flex seal to give them a nice cushion that seals in compression.







STEP 5: Prep the Molds
I used a little bit of silicone to seal between the plywood molds and then clamped the down to apiece of melamine and to each other. I wrapped one of the firebricks to a piece of plywood with some wire. This way I can suspend the brick in the mold.









STEP 6: Mix and Pour the Concrete
I mixed some Quikrete 5000 in a bucket and then filled the molds halfway full. I then placed the brick and filled them the rest of the way. I used a stick to push the wet concrete into all the corners of the mold and I vibrated the full molds with my orbital sandpaper without a sanding pad on it. Vibrating the molds helps remove air bubbles in the concrete. I noticed that the brick was sucking moisture out of the wet concrete so for future blocks I soaked the bricks in water before putting them into wet concrete.









STEP 7: Remove the Mold and Trim the Wires
I let the concrete cure for 2 days before removing the molds and clipping the wires so that I could remove the plywood that held the brick. I then trimmed the wires.



STEP 8: Clean the Molds and Repeat
I used a putty knife to scrape the molds clean and then got them ready for making more blocks.



STEP 9: Install
I dug into the ground about 4 inches deep and then poured Quikrete gravel in the hole. I stamped the gravel down to make a nice firm base. I then placed the blocks one by one and added gravel as needed to make sure they were level.









I also made a bench to go with the fireplace using fast setting Quikrete concrete mix:



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