So I had been asked about making concrete countertops and we decided to try a simple, outdoor table to go next to the grill as a practice run, since we had never made anything out of concrete other than setting fence posts and a shed floor. We also had a small undermount bar sink found at Habitat for $10 so we wanted to incorporate that into this table.
Step 1: Building the Form
First, we made a form out of a 4X8 sheet of 3/4" thick melamine. I angled the front corners because this would end up being head height for a child and, being concrete it is going to win a head-butting contest. My form was about 6' long X 24" deep by 2" thick
We also made a well through the form to hang a sink beneath. For that we used vinyl landscape edging (also a Habitat find). we cut the shape (a rectangle with rounded corners) for the sink well out of the melamine, and cut the landscape edging longwise so it would be the same height as the sidewalls of the form when done. This was to allow us to screed the concrete across the whole top of the form after pouring the concrete in. Hot glue was used to secure the the vinyl to the form with some scraps cut and placed inside to brace it on the ends and sides. Ultimately the edges were caulked with a thin bead of silicone and the well was filled with sand so it wouldn't compress when the concrete was added.
Step 2: Adding Lights
We found these Fiber-Optic lights on Amazon for $50. They have 150 Filaments.
Some wire concrete reinforcement was cut to fit within the mold, and we used a couple boards across the top of the form to suspend it so when concrete was added it wouldn't get pushed down against the bottom of the form, which will ultimately be the top surface of the table.
Then we arranged the light fibers into about 6 groups across the length of the table, with the trunk of all the fibers coming out near (too near in hindsight) the sink well. Next time the fiber trunk will come out near the middle, away from everything else including the future wooden legs which hold the table up. We used zip ties to secure the 6 bundles of fibers to the reinforcement wire so when the concrete was added there would be less stress on them. The fibers are pretty flexible, but if you bend them so much that they crimp over, most of the light will not get through that crimped area. In the end I also had 10-12 fibers coming out the bottom of the table with the rest of the trunk that got cut somehow. I'm not sure how they got cut, but I made the table thicker than I would if I did it again (see the section: Wish I didn't know now what I didn't know then) so there are 4 80# bags of concrete in my table plus the water (not sure if that adds to the final weight or not) so 320# at least in the top alone. I suspect they got crimped hard enough to break them in two while we were flipping the table or pouring the concrete.
Once the fibers were pretty evenly distributed across the length of the table, we drilled small holes in sort of a random 'starry sky' pattern in the bottom of the form, cut the fibers to length, dipped the ends in hot glue and glued them into the holes hoping when we added the concrete they would stay put. I think they all did or at least the vast majority did. The smaller fiber bundles and sometimes individual fibers were also zip tied to the reinforcement wire if it looked like it would help them stay put. We also put a few fibers the same way into the side of the form that would be the front of the table in the end, and through the vinyl wall of the sink well. Once all that was set, we were ready for the concrete.
Step 3: Adding the Concrete
So for this step, we were super-lucky to have a friend with a concrete mixer which he graciously brought over and helped us use. We mixed about 3-4 gallons of concrete at a time while we added it gently into the form. I used the Quickrete formulated specifically for countertops, which cost about twice as much as the normal. I've seen what look like good results online with the regular, I'm not sure I would spend the money for the countertop version again until I had tried the normal Quickrete.
Once the form was more or less full of concrete, we removed the boards suspending the reinforcement wire and used one of them to screed across the edges to level things out on what will eventually be the bottom. I have read since then that mixing the first few batches of concrete a little wetter than the rest (until you have covered the bottom of the form - remember this will be the top of the table) makes it smoother and I will try that when I do it again. The other thing I wish we would have done is rent a concrete vibrator. We tried the whole 'palm sander on the edge' thing but in the end had a lot of small pockmarks in my surface that I think were from not getting the concrete settled enough. We solved that with a slurry of concrete, but it was an extra few steps I would have been happy to avoid.
In the dark picture you can see the lights in what will be the front edge of this counter/bar top. The back wall is planned to go up against the house so we didn't bother putting any on that side.
Step 4: Curing the Tabletop
So concrete never stops curing. Tomorrow this table will be harder than it is today. That said, we still give it a head start at hardening before we started manipulating it. We covered in plastic and let it sit for a week or two. A week would have been enough, but life sometimes intrudes so it took a couple before we were ready to work on it again. We wet the concrete a couple/few times during the curing process and covered with plastic as shown or alternatively some folks recommend wet burlap sacks. You want it to cure (a chemical reaction) without drying out, especially on the surfaces where it might cause cracking if it dries too quickly.
Step 5: Un-forming
After curing the form was removed relatively easily. We had coated the form with a thin layer of commercial mold release to help in this, and it seemed to work pretty well. The literature also describes using vegetable oil wiped on with a paper towel. I would probably recommend one of those.
Because the fibers for the lights had been glued into the form boards, many of them extended up to 3/4 of an inch out from the surface (the thickness of the form). These were cut off at the surface with a sharp razor.
The un-forming revealed problem areas. Some areas of the surface and sides had issues where the concrete had not settled correctly. My takeaway was next time, rent a concrete vibrator. In the end these problems were solved by mixing a slurry of portland cement and rubbing into the surface by hand in 2 applications with some sanding in between.
The application of a slurry took to table finish in an unexpected direction. For reasons that aren't entirely clear to me now, I was a little worried that the slurry might obscure the lighted ends of the fibers. To counter this perceived threat, I mixed a small batch of epoxy and placed a drop on the end of each fiber. My thought was that these would sand off, but they turned out to be much tougher than I expected. The epoxy spread out more than I expected, and formed sort of a bubble lens over each light, which was kind of cool if unexpected. In the end, the (likely unnecessary) decision to epoxy the tips of the fibers, as well as remedying another, much more real problem area resulted in a decision to epoxy the entire surface of the table.
Step 6: Problem Area: the Drain Board
At almost the last minute, we found some pictures online of concrete counters with inset drain boards. We concocted a plan to add one to this project. We took a pack of wood shims, and ripped 3-4 of them longways on the table saw about 1/2" wide. We spaced them evenly with the thick ends butted up to the sink and glued them to the melamine form with spray adhesive (remember the bottom of the form is the surface of the table). The goal was a relatively flat countertop with an area of sloping grooves or channels down until they drain into the sink. In the first picture you can see the shims glued down to the melamine peeking between the two boards suspending the wire. The subsequent pictures show how it looked when we un-formed it.
This worked absolutely great as far as imparting the desired shape. The problem was we didn't seal the raw, dry wood shims and so the concrete in this one area ended up being more crumbly than the rest of the table. What we hoped would be crisp 90 degree angles ended up anything but. I think the moisture moved back and forth from the concrete to the wood and this was the result. I would definitely use this trick again, just seal the shims with several coats of polyurethane or epoxy first and I think it would work really well.
Step 7: Stain
Concrete stain comes in all sorts of colors. The gray concrete color is OK, but you could have a variety of other colors by using stain. We ended up choosing a gray just a bit darker than the natural concrete. It was called moonscape or something and since the goal with the lighting was 'random starfield' it sort of seemed pre-destined. I think the unevenness of the final color is characteristic of concrete stain. I didn't expect the brownish color, but was ok with the stain color overall. You can see as we neared completion how the epoxy drops on the light fibers look like little lenses and not having intended or being particularly happy with that look led to the decision to epoxy the whole table.
Step 8: Mounting the Sink
When pouring the concrete, we had set 4 threaded rods in the concrete at the corners of the sink, extending out several inches from the bottom to allow the attachment of a fabricated mount for the underhanging sink. I used some leftover oak flooring cut to the shape of the sink with holes for the threaded rods to come through and bolted the sink to the table. In the picture you can also see I am beginning to create the frame for the table legs and making sure everything fits.
Step 9: Epoxy Coat
In an effort to hide the epoxy 'lenses' over each light fiber, we hoped putting a layer of epoxy on the whole table would make them less obvious. It didn't really, probably because we had stained and otherwise impacted the appearance of the table in between. Additionally, we wanted to stabilize the drainboard and for this the epoxy functioned pretty well. The plastic-y appearance of the epoxy is not what we were aiming for in the beginning, but that's what we got in the end. To go with the 'random starfield' look of the lights and the 'moonscape' color, we sprinkled some mother-of-pearl flakes onto the wet epoxy. Mother-of-pearl sinks into epoxy, unlike glitter which floats on the surface so we got a little bit of sparkle but maintained the smooth surface.
Step 10: The Legs
We started the countertop project without a clear vision for what would hold it up. In the end there were 4 bags (80# each) of concrete used in the countertop so 320# at least. We threw together some legs out of treated lumber, including 6 4X4 legs to bear the weight. Planning this step ahead would have been much wiser, and I suggest if you try this you design the whole project ahead of time to help plan for clearances, overall appearance, functionality, storage, etc... By the time we built the legs I mostly just wanted my workbench back so the theme here is best described as, "quick and dirty". In the same spirit I epoxied a piece of scrap treated lumber to the bottom of the table to mount the light engine to the bottom of the table.
Step 11: I Wish I Didn't Know Now, What I Didn't Know Then....
Actually, learning is why we do these things, so here is a summary of the lessons that came at some cost in additional effort or sacrificed vision:
1: Rent a concrete vibrator. If it's only $20 or $30 rent a mixer too. The work saved with a mixer is less a factor than producing all the needed concrete quickly. The vibrator might have saved us several steps which took days (mostly drying/curing, only a few hours of working) to fix.
2. Bring the light trunk out of the bottom away from any other features or structures of the table. If I were doing it again, I would bring the trunk out more parallel to the bottom surface of the table rather than the virtual 90 degrees that this one was to soften the curve from the eventual intersection with the light engine which in our application was parallel to the bottom of the table. Plan your legs to ensure you have lots of clearance around the light trunk. I might even encase the trunk in PVC or something (at least tape it as a bundle) to help protect it. about 10-12 fibers got folded hard enough to crimp and break them at some point in the project. We had actually laid out the constellation Orion in 'stars' when we drilled and attached the lights to the mold. One of the light fibers that broke was the center star in Orion's belt. So the single potentially recognizable feature we tried to incorporate had one of its iconic points fail. You would never know if we hadn't told you, so it's just a lesson we wanted to share. In a truly random application, losing 10% of the lights wouldn't matter that much. If you're trying to shape the image, random failures can be a big deal.
3. All elements of the form need to be impermeable to concrete/moisture. Using bare wood was a rookie mistake. We hope you learn from ours instead of your own
4. We made our table 2" thick. Why? "It seemed like a good idea at the time." Aside from being very heavy, the planned faucet for the sink was not long enough where it passes through the surface to attach the nut that holds it in place (really, none of the commercially available faucets have throats long enough for a 2" thick mounting surface). We're working on a fix for this. Not planning the table legs ahead of time also resulted in a conflict that will need to be mitigated before a faucet can be attached. It seems like people are pretty consistently casting concrete counters 1" thick. Getting the reinforcement wire and optic fibers in place with a 1" space to work would be more difficult, but is probably worth it. The next one we try will be 1" thick.
5. Plan the base from the beginning. Setting some treated boards into the table bottom to aid in attaching the base would have been a good idea. Or some bolts to attach the base so it could be more secure. Not to mention making sure your sink, faucet, and light fiber trunk are out of the way of your structure.
Step 12: That's It!
We hope this helps anyone thinking about trying something like this. There is lots of inspiration online. Maybe too much as we explained. Would love to hear about your experiences if you take this on!