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.

Supplies:

Masonry:

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.Fire Bricks – 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.

Mortar – 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.

Rebar – I bought pre-cut rebar in 12-inch and 36-inch sections. I should’ve purchased 18-inch long sections for the verticals. I recommend using a combination of pre-cut 3/8-inch diameter rebar. Use 18-inch 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-inch 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-Inch Gravel – I used about 7 bags of Quikrete 3/4-inch gravel for the foundation layer.

Lava Rock – 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.

Formwork:

2×4’s + 2×6’s – I used 2×4’s and 2×6’s held together with 2.5-inch deck screws to make the panels for the formwork.

L-Brackets – I used L-brackets to join the panels at the corners. I recommend 6-inch L-brackets.

Tools:

Cordless Drill

Circular Saw

Compound Miter Saw

Tamper

Metal Rake

Shovel

 

Level

Cement Float

Cement Trowel

 

Wheelbarrow

Hammer

Hoe

 

Instructions:

Click the image below to expand it:

STEP 1: Cut The Wood:

The outer frame will be made of (4) panels that are 43.5-inches long. Each panel is made from (2) pieces of 2×4 and (2) pieces of 2×6. 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-inch L-brackets to join the panels at the corners.

STEP 2: Assemble The Panels:

I used 2.5-inch deck screws and 15-inch long pieces of 2×4 to make the (4) large panels for the outer frame.

STEP 3: Make The Inner Frame:

The inner frame is made from (2) panels that are 23-inches long and (2) panels that are 20-inches 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 4: Place The Frame + Mark The Hole:

Place the frame in the desired location and mark about 6-inches around the outside perimeter.

STEP 5: 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-inches down and used a tamper to flatten and compact the soil at the bottom of the hole.

STEP 6: Spread Some Gravel:

I spread about 3.5-inches of Quikrete 3/4-inch gravel in the bottom of the hole and raked it as level as possible before tamping it down.

STEP 7: 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 8: Secure + Level The Formwork:

Once the frames are in the right position, use some scrap 2×3’s 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 9: 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-inch long vertical pieces. I wired 3-foot long pieces horizontally about 2-inches from the bottom of the hole.

STEP 10: Pour The First Layer of Concrete:

I started with a 3.5-inch 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 11: 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-inch 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 12: 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 leveling before doing a final pour. I also added in additional rebar since my 12-inch bars where already covered. I wanted to make sure that the layers of concrete where bonded together.

STEP 13: 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 14: Level The Top:

I used a flat piece of wood to level the top of the concrete. Work the wood back and forth to level the concrete.

STEP 15: 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 16: 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 17: 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 18: 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 minutes 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 19: 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 20: 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.

Resource:

Homemade Modern