Now that we know how to form and finish concrete we can bring together all the skills from this class and apply them to make a capstone project, a personal tabletop fireplace.
We'll test what we've learned and take concepts a bit further in this lesson, but after we'll have a beautifully finished concrete piece that's sure to be the centerpiece of the table at your next dinner party.
This capstone project will cover the concepts of making negative space in a mold, reinforcement, finishing the concrete surface, and applying concrete finishes. In addition, we'll explore how to embed into concrete by adding cast iron legs and feet to this fireplace.
Ready to cement your knowledge of concrete? Let's make!
As we covered in Lesson 4: Casting Molds With Voids, to make a concrete mold with a void we'll need a large container to create the mold with and a smaller container of some kind to displace concrete to make the void. For this tabletop fireplace I chose a large rectangular plastic storage container and found a smaller container that fit inside.
In the plumbing aisle of a hardware store there are all kinds of fittings in all different sizes. To go with the industrial look of concrete I chose ¾" cast iron fittings to make the legs and feet for this fireplace.
Each leg needs one mounting flange, one 45° elbow, one end cap, and two threaded nipples - all of the same size (I used ¾" fittings).
The cast iron flanges I chose were slightly too large for the bottom of the large plastic container I was using for the concrete mold, so there were modified slightly.
Using the angle grinder used to polish and grind the concrete Lesson 8: Concrete Finishing, we can trim the lip of the flange slightly to fit two flanges side by side in the bottom of the container used as the concrete mold.
The flange was marked and then securely clamped down to a sturdy work surface, then trimmed using the grinding disk on the angle grinder. Refer to Lesson 8 for how to change the wheel on the angle grinder.
Two of the flanges were trimmed to allow all 4 to fit snugly in the bottom of the large mold. Since I will be using the threaded portion of the flange it's important not to grind into threading and ruin the ability to connect the nipples later.
Check the flange sizing by placing them inside the mold to ensure a good fit.
Reinforcement might not be needed for this casting since it's small and will be quite thick, however adding the reinforcement guarantees they casting will stay strong no matter the load or span between the legs.
Any sturdy steel wire will work as reinforcement. I used animal wire fencing I got from my hardware store, which they sell by the foot. Using regular angle cutters I snipped off a shape roughly the size of the bottom of the casting mold, then refined the dimensions to fit exactly after.
As discussed in Lesson 2, steel reinforcement is a great compliment to the compression strength of concrete, but the steel needs to be completely entombed in concrete to prevent it from rusting and failing. It's important to make sure your reinforcement sits completely inside your mold with at least a ½" buffer around it for concrete.
Once the reinforcement is sized we can remove it from the mold and set it aside, we'll come back and insert it partway through our first pour.
Before the first pour the cast iron flanges need to be placed in the bottom of the mold. To ensure they stay still while pouring the concrete, and to prevent concrete from seeping under the flange and gumming up the threaded interior, a bead of hot glue was squirted on the top edge of threaded part of the flange, the flange was then placed glue side down into the mold.
Once all four flanges have been glued to the bottom of the mold the insides can be protected from concrete entering from the top. Paper wadding was used to fill the space inside the threaded interior, then the flange tops were sealed with heavy tape.
Repeat for all 4 flanges. The heavy tape will stop concrete from entering the threaded interior and the paper provides ballast so the tape doesn't cave in.
Once the flanges are glued and taped, sealing the threaded interior of the flange, we can start preparing the concrete mix for the first pour.
Mix a batch of concrete large enough to fill about half the mold. Carefully pour the concrete mix into the mold, ensuring to cover the flanges completely. Take small breaks to poke and tamp the concrete under and around the tight spaces at each flange.
Once all flanges are covered continue pouring until about ½" of concrete covers the top of each flange.
If you are using a clear container for your mold you can scoot one end of the container off the workbench to inspect the underside and ensure that the flanges haven't become unglued, and concrete is staying out of the threaded interior.
After the flanges are covered the reinforcement can be added. Carefully place the cut to fit reinforcement into the center of the mold on top of the poured concrete and gently press to seat it. Do not push the reinforcement down into the cement so it touches the flanges.
Next, carefully scoop more concrete on top of the reinforcement and completely cover with another ½" of cement.
At this point it's a good idea to gently vibrate your concrete to release any air bubbles that have been trapped.
Referring back to Lesson 2: mixing and Pouring to get pointers on the best ways to vibrate, gently tap your mold to degas.
Using the techniques from Lesson 4: Casting Molds With Voids, we'll use a second plastic container to make the void in this concrete casting that will hold the fire elements for this fireplace. I used a smooth sided plastic trough that was about 1" smaller on all sides than my large mold.
Release agent was sprayed on the outside of the smaller container and then the container was placed in the middle of the larger mold and centered.
Water was poured into the container to act as ballast and prevent the smaller mold from floating out of the concrete from being too buoyant. The smaller container was partway filled with water, then concrete was added around the container to the waterline, repeat alternating adding water and then concrete to the waterline until the concrete has filled the large container.
Vibrate the concrete again to degas, then cover with plastic wrap and leave to cure for 48 hours.
After curing the concrete cast can be removed. Depending on the shape of your mold, and if you used release agent, some molds are easier to remove than others.
The large mold was pliable enough to flex and remove the concrete casting from, however the smaller container used for negative space was very rigid and was impossible to remove despite the generous amount of release agent applied. Ultimately the inner container was destroyed in order to be removed.
As you can see in the top right of the picture above there was slight damage to the concrete casting from removing the negative space mold.
Flipping the casting upside down the flange threads can be examined.
Hot glue is easily removed from the rim of the flange threading, the paper removed, and any concrete flashing that managed to seep under the glue barrier can be cleaned off to expose the threads.
Once removed from the mold and the flange area has been cleaned up we can turn to finishing work on the concrete surface.
In Lesson 8: Concrete Finishing, we covered grinding and polishing using a variable speed inexpensive variable speed angle grinder and diamond polishing pads to expose the aggregate and create a smooth concrete surface, we'll use those skills now to bring depth to the otherwise dull casting.
As you can see from the arrows on the image above, the small section of polished concrete on the right looks much different than the original casting on the right.
Spend the time to carefully grind and then polish each face for the entire casting, the effort put in here makes the project really pop!
After exposing the aggregate and polishing the concrete to a smooth finish it's time to add some finish to seal the concrete and give it that wet look. Finishes for concrete were discussed in detail in Lesson 7: Concrete Finishes, and for this project I chose the "wet look" sealer, which will give the concrete a darker color and enhance the texture of the aggregate.
Wearing gloves, a liberal amount of sealer was poured onto a cloth and applied all over the casting. Excess was wiped off with a clean lint-free rag.
For each leg there are two threaded nipples, a 45° elbow, and an end cap - all fittings are ¾" cast iron pipe. The endcap and threaded nipple were tightly connected, then fit into the elbow as snug as you can by hand.
The second nipple was inserted loosely and then inserted into the flange on the underside of the fireplace, but not tightened.
Fit all legs into the flanges and then turn the fireplace right side up. Gently rotate each leg to tighten into the flange until it starts to tighten, then position the leg with the end cap facing outwards. Positioning the remaining legs the same way, making small rotations of each leg to keep the fireplace level.
The fuel for this tabletop fireplace are small 7-ounce Sterno cooking pods. These inexpensive pods give off a real flame, and are interchangeable when the fuel is depleted. These pods last 2 hours and are recyclable.
Place the pods inside the void of the fireplace and use some sifted aggregate saved from the drawer pulls made in the Lesson 3 project to fill in the space around each pod,
Make sure to leave the lids on the pods as you fill, otherwise you'll end up with rocks in the fuel.
All that's left is to wait until twilight and fire up your tabletop fireplace. Over the romantic glow of the fire you can wow your guests on your new knowledge all about concrete, and how this tabletop fireplace was made.
After going through this introductory concrete class there's lots more to explore with concrete. The projects and concepts described in this class are just the beginning. Concrete is a very versatile and creative medium to work with.
This capstone project encompasses all the skills we learned in this class. However, this is not the only project you can make with the skills learned, feel free to post any concrete project you've made below and share the results on your concrete creation!
Share a photo of your finished project with the class!
Nice work! You've completed the class project