This modern concrete fire pit can be built over a single weekend and is a great centerpiece for outdoor entertaining. This isn't a difficult project, but it's time consuming and labor intensive. Concrete fireplaces and fire pits should be constructed carefully. When exposed directly to high amounts of heat, the moisture trapped inside concrete can expand and cause the concrete to crack and in extreme situations, explode. I lined the inside of the fire pit with fire brick and filled the bottom with lava rock to ensure that the majority of the heat didn't come in direct contact with the concrete. I built this fire pit in southern California, so I didn’t have to worry about putting in footings that go below the frost line. If you're building a fire pit in a cold climate, I recommend putting a sonotube footing under each of the four corners.
Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.
Step 1: Supplies + Tools
Quikrete 5000 (20-25 Bags)
QUIKRETE® 5000 Concrete Mix is a commercial-grade blend of stone or gravel, sand, and cement and is specially designed for higher early strength. This is the recommended concrete mix to use because of the high early strength and low price. It’s great for making tables, pavers, stools, and lamps and is typically found in 80-pound bags at Home Depot, but 60-pound bags are also available.
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. Fire bricks are heat-resistant and will protect the concrete from cracking.
I mixed my own mortar from a combination of Portland cement, mortar clay and sand. I followed the directions that came on the mortar clay bag.
I bought pre-cut rebar in 12" and 36" sections. I should've purchased 18" long sections for the verticals. I recommend using a combination of pre-cut 3/8" diameter rebar. Use 18" long pieces for the verticals and 36" long pieces for the horizontal pieces. Make sure you drive the vertical pieces through the gravel and into the ground so that the ends are at least 3.5" below the top of the wood forms. Wire the horizontal pieces to the vertical pieces so that the horizontal pieces will be embedded in each layer of concrete. This rebar frame will keep the different layers of concrete securely bonded to each other.
Quikrete 3/4" Gravel
I used about 7 bags of Quikrete 3/4" gravel for the foundation layer.
I used two 5 gallon buckets full of lava rock that I purchased from the masonry supply yard. The lava rock is heat-resistant and won't crack or shatter.
FOR THE FORMWORK:
2x4s + 2x6s
I used 2x4s and 2x6s held together with 2.5" deck screws to make the panels for the formwork.
I used L-brackets to join the panels at the corners. I recommend 6" L-brackets.
RYOBI 18 Volt Cordless Drill
RYOBI 18 Volt Circular Saw
RYOBI 10" Sliding Compound Miter Saw with Laser
Step 2: Download the Concrete Fire Pit Plan
Click here to download the concrete fire pit plan.
Step 3: Cut the Wood
The outer frame will be made of 4 panels that are 43.5" long. Each panel is made from 2 pieces of 2x4 and 2 pieces of 2x6. I used a circular saw to cut them, but a compound miter saw would make this task a little easier. If you do use a circular saw, I recommend using a speed square to make sure your cuts are nice and straight. I used 6" L-brackets to join the panels at the corners.
Step 4: Assemble the Panels
I used 2.5" deck screws and 15" long pieces of 2x4 to make the 4 large panels for the outer frame.
Step 5: Make the Inner Frame
The inner frame is made from 2 panels that are 23" long and 2 panels that are 20" long. Make sure to put the L-brackets on the inside corners of the frame since the outside ones will be buried in concrete.
Step 6: Place the Frame + Mark the Hole
Place the frame in the desired location and mark about 6" around the outside perimeter.
Step 7: Start Digging
Since I built the fire pit in southern California, I didn’t have to worry about putting in footing below the frost line. If you're building in cold climate, I recommend putting in footings that go beneath the frost line under each corner. I dug about 8" down and used a stamper to flatten and compact the soil at the bottom of the hole.
Step 8: Spread Some Gravel
I spread about 3.5" of Quikrete 3/4" gravel in the bottom of the hole and raked it as level as possible before stamping it down.
Step 9: Place the Formwork
Place the frames into position and measure the distance between the inner frame and outer frame to make sure the inner frame is properly centered.
Step 10: Secure + Level the Formwork
Once the frames are in the right position, use some scrap 2x3s and deck screws to lock them into place. Then use a 6 foot level to make sure that the formwork was level. I used a rubber mallet to knock down the high corners to make it more level.
Step 11: Place the Rebar
I drove the rebar through the gravel and into the ground. I should've used longer rebar instead of the short 12" long vertical pieces. I wired 3' long pieces horizontally about 2" from the bottom of the hole.
Step 12: Pour the First Layer of Concrete
I started with a 3.5" deep pour of concrete that would serve as the foundation of the fire pit. I mixed the Quikrete 5000 in a wheelbarrow and shoveled it into place. Renting a mixer would have made this a lot easier, but I enjoyed the exercise. Let the concrete cure at least 20 hours before laying the brick.
Step 13: Lay the Brick
Once the concrete has cured at least 20 hours, get ready to lay the fire brick. I mixed the mortar per the instructions on the bag of mortar clay. I started with a thick base of mortar about 1/2" around the inner perimeter of the formwork and then placed the bricks one at a time. I spread mortar on the side of the bricks before placing them so that they would stick together.
Step 14: Pour More Concrete
Once the mortar sets, mix and pour another 5 inches of concrete. I could've poured it all at once, but mixing concrete by hand is exhausting and doing it this way allows you to remove any extra braces that could be in the way of screeding before doing a final pour. I also added in additional rebar since my 12" bars where already covered. I wanted to make sure that the layers of concrete where bonded together.
Step 15: Pour the Final Layer
After letting the previous layer of concrete cure about 20 hours, I poured the final layer of concrete. I used a hoe to push the concrete down into all the corners and a wooden dowel to vibrate the concrete by hand.
Step 16: Screed the Top
I used a flat piece of wood to screed the top of the concrete. Work the screed back and forth to level the concrete.
Step 17: Use a Float
I let the concrete set about 30 minutes and then used a metal float to work the cream to the surface. I spent about 10-15 minutes working the surface.
Step 18: Steel Trowel
After using the float, I waited about 1 hour and then used a steel trowel to finish the concrete. I did my best to get the surface as smooth and flat as possible. It isn't perfect, but it looks great!
Step 19: Cover + Keep Moist
I covered the concrete with some boards and a sheet to make sure that nothing touched the wet concrete. I used a garden hose to keep the concrete moist over a 48 hour period.
Step 20: Remove the Formwork
Removing the outer frame was easy. The inner frame required some cuts with a circular saw. I should've made some diagonal cuts in the interior panels beforehand, but not a big deal either way. After about 20 minsutes with a pry bar, hammer and circular saw, I got the wood out. I then sprayed off the concrete with a hose to clean it.
Step 21: Pour in Some Lava Rock
I poured 2 buckets of lava rocks into the inside of the pit. This creates a nice, well-drained surface for starting fires and covers the 3.5" foundation layer of concrete.
Step 22: Before You Light a Fire
I recommend letting the concrete cure at least 30 days before lighting a fire. I also recommend not letting a fire burn for longer than 2 hours for the first 3 months. It takes a long time for concrete to fully cure, and you don’t want the moisture to expand inside and cause cracking. Be responsible and don’t make super large fires. The concrete is protected by the fire brick and lava rock, but if you make giant bonfires and let them burn for hours on end, the concrete could crack.
Good luck making your own concrete fire pit and please email or tweet photos to @benuyeda or firstname.lastname@example.org. For more DIY ideas and projects, visit us at HomeMade Modern.