Add GFRC to your furniture-making arsenal.
I have always loved aviation-inspired design. It’s something about everyday items inspired by planes that inspires me to wanderlust. I wanted furniture with that same aircraft vibe. This concrete and walnut wing table is the result.
Before I go further, let me answer the questions on everyones mind: Why concrete? Isnt this a woodworking magazine? If youre a woodworker, you already possess many of the skills and tools needed to work with concrete. Concrete will allow you to explore new design possibilities.
For this design, concrete was the perfect choice. Concrete is not any concrete. It’s white glass-fiber reinforced concrete. A standard concrete mix from a big-box store should be cast at least 1 / 5 cm thick. However, GFRC mixes can be cast at a thinner thickness. Glass fibers are embedded in the mix, and when cured, give it more strength than a typical aggregate-only concrete mix. GFRC enabled me to create the sleek, modern, and aircraft-inspired look I wanted by casting the waterfall tabletop in one 3 cm-thick section.
Building The Form
1. Cutting 30 support pieces for the waterfall base with a miter gauge on the table saw.
The first step in most precast concrete projects is building the form into which youll spray and/or pour the concrete. The form is basically an upside-down, reversed image of the piece you are making. Many materials can be used to make a concrete form. Concrete doesn’t stick to its smooth, waterproof surface and melamine is easy to shape using standard woodworking tools.
2. Hot glue is used to make the concrete form.
There are two approaches to building the melamine form. You can cut the base to match the shape of your final piece, then attach the sides to the base. You can also use a sheet of Melamine to form a base and glue the edges of your form to the base. The first method is great for creating rectangular slabs. However, I find the second approach easier for complex projects (such as this one). This allows you to draw the desired shape directly on the melamine base using a dry erase marker. Then, you can cut the side pieces to fit the sketch as you place them around it.
3. Attaching the sides of the form.
Two rectangular bases were cut to form my table. The larger of the two was slightly longer than the table’s length (including the waterfall angle), and the other being a little wider than the sides.
4. The concrete waterfall must be supported by the form.
The base of the waterfall must be strong enough to support concrete’s weight during pouring. I made triangular support pieces out of scrap melamine. The support pieces dont need to be pretty, so pocket holes work well to secure the support pieces to the base. However, melamine is slippery and the support pieces have to be aligned such that the two bases provide a perfect tabletop-to-leg transition. As such, I aligned the triangular supports using a straight edge and hot glued them to the base before I screwed them to the base. I could then clamp the waterfall base against the supports and secure it using hot glue and screws.
5. The sides of the waterfall form were shimmed in lieu of a complicated triple miter cut.
Pieces of the form that wont bear significant weight can often be joined using only hot glue on exterior seams, which greatly simplifies the process. Also, youll be much more efficient if you can let go of the woodworkers desire for perfect joints. Small gaps and misalignments at interior joints will be filled with silicone caulk when you seal the form, so they wont affect the final concrete product.
6. The outside of the form is not a problem as long as they are aligned with the corners.
Before cutting the sides for the form, I use a dry-erase marker to sketch the angular wing shape on the base of the form. I cut 3 cm pieces of melamine to size on my table. Then, I used my miter saw and trimmed the sides to fit the 3 cm strips. The side pieces were placed on the base of the form (using the sketch as a guide), and secured to the base using hot glue at exterior seams.
Compound Angles
7. For perfect caulk lines: a caulk gun, paste wax, 100% silicone caulk and a fondant ball tool.
The sides of the waterfall form were a bit tricky. Compound miter joints can make your head spin if you’re like me. You would need to make a triple compound joint to get the sides just right. This will allow you to take into account the inward angles of both the top and waterfall, as well the 15-degree outward slant. It doesn’t matter what the outside looks like, but it helps to let go of your woodworking instincts.
8. After applying paste wax to the form, a bead of caulk is applied.
To match the outward and inward angles of the waterfalls, I cut a double miter at 15 inward and 4.5 inward. To attach each side piece, I used one hand to hold the side so it tilted inward off the waterfall base, and the inside seams were flush with the tabletop side pieces, and used my other hand to shoot a few dabs of hot glue under the gap under the exterior edge of the side piece that results when the side is held in this position. To make sure it doesn’t move during pouring, I could use shims or more hot glue. The outside of the form isnt pretty, but it doesnt need to be!
9. Immediately after caulking, running a fondant ball tool over all the seams squeezes out excess caulk.
You will notice that the waterfall sides were a bit longer than I intended when attaching them. Once the waterfall sides had been attached, I was able to use the base of my form as a guide and return with a flush-cutsaw.
Sealing The Form
10. A caulk line that is almost perfect.
Once the form is completed, it’s time to seal it using 100% silicone caulk. Because the final concrete piece will show every detail of your form, I use black caulk. I apply a layer paste wax to the interior surfaces before caulking.
11-12. After the caulk has dried, you can remove the excess.
Then I apply a generous amount of caulk to all seams. Immediately after applying the caulk, I come back and run a metal ball tool over the caulk lines. This tool is often used to decorate cakes and cookies.
The metal ball pulls excess caulk from the seam leaving a perfect line at the center. The paste wax applied before caulking allows the squeeze out to be pealed away easily after the caulk cures, leaving perfect caulk lines.
Concrete Time
13. The first step in the concrete application process is spraying a coat of the concrete. Then, youll pack the form with the glass fiber and concrete mixture.
The concrete is the fun part. To get a smoother surface right out of the form, I used a two-part process where the form is first sprayed with a thin layer of GFRC mix without any glass fibers, before applying backer layers of GFRC mix with the glass fibers. The sprayed layer is also known as the beauty or face. Its applied using a drywall hopper gun powered by an air compressor. The GFRC mix contains #30-grit sand as the largest aggregate. This allows for a very smooth surface and minimal pinholes.
After spraying the face coat, I quickly (within seconds) use a chip brush to work the mix into the seams, and brush the entire surface lightly. The surface is buffed to remove any air bubbles trapped against it, reducing the possibility of pinholes. After the first coat had dried, I applied a second coat. However, this is an optional step.
14. Multiple steps are required to pack the form. Its not an exact sciencethe way the material behaves depends not only on the amount of water in the mix, but also the conditions in your shop.
The GFRC backer layer consists of the same GFRC mix as the beauty coat and includes alkali resistant (AR) glass fibers. (AR fibers shouldnt be confused with run-of-the-mill fiberglass, which would break down quickly due to the alkalinity of concrete.) The timing of the backer coat is important. You need to wait until the face coat is firm to the touch, but still wet, so the backer coat will stick to it.
Since this table is 3 cm thick, and my face coat was /, I measured out buckets of mix (by volume) to do two back layers of slightly less than / each. This form is very large and has a vertical surface (the leg). I used AR glass mesh to support it. The scrim will be placed between back-coat layers and provides extra strength. The scrim also helps reduce concrete slumping because the vertical parts are packed in. After I had cut the AR mesh to the shape of the form, I proceeded to mix the first coat.
Form
15. Getting the hang of packing the form happens pretty quickly. After the first layer is packed, you wait a bit for the concrete to set up so you can pack the second layer.
With the first backer coat mixed, its time to start packing the form. Concrete should be slightly wet like Play-Doh.
It must have enough mass to hold its form, but should be sticky enough that it adheres to previous coats. The directions on the bag of concrete mix give you a good starting point, but you may want to experiment a little if this is your first time using the material.
Packing the form is pretty straightforward. Pour a handful of the mix into the form, and use your hands to pack it in place. The first coat will be about a / thick. Keep that in mind while you work through the form. After you have put the first layer, wait for it to cure. This could take up to an hour depending on how the shop is set up. You don’t want to cure the entire thing. Just enough to allow the first layer of concrete to support the second layer while you pack the vertical leg surface. If its not cured enough, itll slump off the form, and its not fun (see sidebar on page 28).
16. You might be tempted to try and smooth out the concrete once the form is packed, but its much easier to grind this flat after its cured.
Make sure you place the AR mesh sheet in the form before packing the second layer. Continue packing the first layer as usual until you reach 3 cm in thickness. Although you may feel tempted to screed or smoothen the last layer once it is all packed, the concrete’s glass fibers make this difficult. The glass fibers tend to catch on the screeding board, which makes it difficult to achieve a level surface, and can sometimes pull the face coat away from the edges of the form. It is best to leave the form as it is, and make sure that it is smooth when it has cured.
Practice Patience
During my build, the back coat didnt go quite as planned. When hand-packing GFRC up a vertical surface, you want to pack in layers of / or less, allowing enough time for the previous backer layer to firm up before packing the next one. The concrete began to slump after I added the second layer because I was too busy with dinner plans. I was able to save the project, but Ill also be telling you what you should do to avoid some of the moments of panic I experienced.
I mixed the first coat of backer and spread it by hand over the vertical surface. It was working well. I then added the sheet of AR scrim Id cut earlier and massaged it into the back coat. The vertical was now packing well and not sagging. I still had 3 hours to go before dinner. So I waited and waited, until I finally realized that I had made a mistake. My shop was now 65 degrees outside. Concrete cure times are greatly affected by cold.
About an hour before dinner, the first backer layer was still not firm enough. I knew I had to choose between speeding up the project or causing trouble for my partner. This isn’t an option. So I went for it and added the second layer.
As soon as I started packing the second backer layer, I knew I was in trouble. If you pack your back layers correctly, GFRC can be hand packed vertically without a form. However, since the first layer hadnt firmed up enough, the weight of the additional layer caused the vertical layer to slump. I knew I needed to do something so I grabbed a piece of melamine scrap and clamped it against the concrete at the waterfall leg. There were a few small problems: glass fibers and tearaway in some places and extra grinding of the back of my waterfall leg after demolding. Fortunately, these issues were fixable with a bit of slurry later on. The moral of the story is to remember that concrete cures much slower in cold weather, and plan accordingly.
Demold, Sand And Seal
17. Dont remove the form immediately post-curing. I find it easier to do the rough grinding while the top is still supported by the form.
It’s like Christmas morning for the makers-waiting to see how your project will turn out can be frustrating! In most cases, GFRC can usually be removed from the form within 24 hours. However, I waited 48 hours because of the lower temperature in my shop.
18. If there are air pockets or gaps in the concrete, you can apply a slurry mixture of the concrete to fill things in.
Before demolding, I used the opportunity to grind the underside of my table while it was still in the form. The form’s sides can be used as a guide for grinding the underside to a uniform thickness.
19. The top is smoothed by wet sanding.
Theres really just one rule to demolding a concrete piece: dont touch the concrete with any tool. I typically use a cheap sacrificial chisel, a mallet and my hands, to pry the sides of the form away.
After demolting, I hand-sanded my tabletop with #400-grit paper. GFRC can be completed quickly and easily with high-grit, wet sand. Simply wet sand the area until the micro-texture is gone under your hands. This usually takes only a few seconds. After sanding, I apply a couple coats of concrete sealer with a clean microfiber cloth.
One note before we move on to the woodworking-depending on whether there are any flaws you want to fix, you may want to do a slurry coat (or two) to fill in voids and get a perfectly smooth surface. A slurry coat is just the same GFRC mix as used for the face coat, wiped in by hand to fill any voids or pinholes in the surface. In my case, I applied one slurry coat to patch up some places on the vertical surfaces where the slumping resulted in rough patches.
Concrete & Walnut Table Materials List
Wood:
4 bdft walnut
MOLD:
10 cmx8 sheet of melamine particle board
Sheet of AR glass mesh
1 / Tapcon concrete screws
Specialty Tool:
Buckets for mixing and measuring
Concrete paddle mixer
Exploded View
Making The Walnut Base
20-22. I cut the complimentary angles on the table saw. The leg is supported on a sled with my fence. Next, you will cut the complementary angle with the workpiece on top.
To support the other end of the table, I decided on a rhombus-shaped leg made from a single walnut board. I used a single board for a continuous grain pattern. (A rhombus is a parallelogram with opposite, equal acute angles, opposite, equal obtuse angles, and four equal sides. Yes, I had to look it up too.)
The rhombus slants at 30 in the opposite direction of the 15 leg, meaning the inner acute angles are 60, while the inner obtuse angles are 120.
From a design perspective, I chose a 30 angle because the sharper slant gives the table a more aggressive and dramatic look, while still being proportional to (double) the angle of the 15 waterfall leg.
23. To align the miters and strengthen the corners, I used domino joinery.
After milling the walnut lumber, I moved on to cutting the miters. Because of the rhombus shape, the four sides were cut exactly the same way. Each side has a 30 miter at one end and a 60 beier at the other. To make the 60 cuts, I used a vertical tenoning jig with the blade set to 30.
24. The dominos I had on hand were a little long, so I trimmed them with a hand saw.
I then left the blade in the exact same position, and used my miter gauge to make the 30 cuts. Pro Tip: When you are cutting quadrilaterals, first cut one side of each piece. Then, move the blade to the opposite side and make the next four cuts. This way, any error in the cut angle on one end will be offset by an equal and opposite error on the other end of each board, and youll get perfect miters off the blade.)
25. It can be a bit tricky to glue up and assemble the mitered base. A glue with extended open time helps.
Festools domino joinery was used for the joinery. The domino size was the most difficult part of this project. The only thing I had was 8mm x 40mm dominos. 40mm was too deep to make the sharp angles on the 60 miters. This was easily remedied by using a hand saw to cut the dominos down to 30mm and adjusting my plunge depth.
26. Keeping four mitered corners aligned and tight took some tricky clamping. Rehearse this step before theres glue on the joints.
27. To create a slight reveal at the junction of the top and floor, I beveled the edges.
Before applying finish, I gave the legs a 45 chamfer. I set up my router and routed the chamfer around the edges. The ground and table legs are separated by the chamfer. I then used a microfiber cloth to wipe on two coats of Maker Brand Simple Finish.
28. A simple oil and wax finish gives the walnut some depth.
I let the finish dry overnight, and then used construction adhesive and four 1 / Tapcon concrete screws to attach the wood base to the tabletop. With that, the table was done.
Michael Clifford is an American maker. You can see more of his work, including a video of building this table, at modustrialmaker.com.
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