Intro: The Un-flippable Concrete Kids Table (Lessons in Form Making and Recovering From Failures)
A while back, we were given a small plastic table (the el cheapo variety), which my kids loved, but appeared to think they were in a western and would constantly flip the table over and hide behind it. This tends to be a problem when there are things on it, say food or paint... It was also too small to have four plates on at once, which tended to limit it's use. They also sought to constantly move the chairs to various rooms around the house. So I set out to build a new table that would resolve a few of the shortcomings of the original table.
1) Be unflippable with limited ability for the kids to rearrange the furniture
3) Able to have four plates and glasses
4) Work well for arts/crafts
The first two design considerations led me to select concrete as the material for the top. Additionally, the freedom of shape and form worked well for the arts and crafts drop in storage I added while pouring the top.
You'll see I made a few mods from the original model during construction as I considered a few extra features along the way.
Step 1: Make Your Concrete Forms
One of the most common materials in concrete form making (at least for smooth finished surfaces) is melamine plywood, it's pretty smooth (not perfect, but a good start) and relatively water impermeable on the melamine faces. You can pick up a sheet of 3/4" melamine from any big box home center.
I cut the tops for two benches and the table, with the knowledge that I would have to do two separate bench pours. The finished top is 36" x 36", so I added 3 inches to each side and made a 42" x 42" top. The benches finished top size was 24" x 12", so I made them 30" x 18". I then cut a whole bunch of stock for the sides at 2.5" wide. This dimension will be the thickness, so pick the size that you want taking into consideration the strength of the concrete you're going to pour, the total span, and of course how you want it to look. I wanted a beefy looking top, and was pouring relatively low strength concrete (4000 psi with minimal glass fiber for this project), so I went fairly thick at 2.5". I marked out where I wanted the edges of the finished pieces on the melamine as a guide for the edge bracing.
I made all the edges with the melamine and various plywood scraps I had laying around, forming a "L" brace, using pine pieces for screw and glue backing. Glue doesn't typically stick well the melamine, so I used epoxy (which still only sticks a little with some roughed up melamine). For extra holding power I used some trim nails to hold it in place while the epoxy set up. I set the blocks every so often, always with one on the start and end of the piece, so I could screw together the side supports. I also immediately filled the finish nail head holes with epoxy, then used a card scraper the next day to flush them up to the melamine. This both keeps the surface impermeable, as well as smooth.
Next drill and countersink holes on each block to hold the edge guides down, as well as to each other. Use the previously marked lines as guides while setting them. Find the right length screws, and screw it all together.
I also made a knockout in the center, which was just a 12" x 12" plywood piece, with a little frame screwed to it for a final thickness of 2.5" to be flush with the outside edges. It's quite helpful if they have the same height so you can screed off the wet concrete easily later. Then I used some air sealing tape (tyvek stuff and some stuff left over from underlayment) to seal exposed plywood edges. If unsealed, they wick up the moisture, swell, and either ruin your concrete, eliminate the ability to re-use the forms, or both.
I also knew I was going to use some angle aluminum to hold the drop in insert, so I cut some pieces and used silicon to hold it down.
Next I used black silicon to seal all the corners, which can be a bit of a mess, but is a quick and cheap way to seal and make a slight rounded over edge in the final piece. I've started using a better way now, but that'll be in a future instructable. Make sure there's no place for water to wick into the edges of your plywood pieces -- it'll wreak havoc in the future.
This would have been a great time to wax everything down as a mold release... but I didn't and you'll get to see my pain the next steps.
Step 2: Add Some Rebar
I bought a bunch of 3/8" rebar from my local home center to add some extra strength to the table top. I didn't really run any calcs on it to determine this, it just seemed like a good idea on the table top given the span (~30" unsupported in the center) with the knowledge that my kids would definitely be standing on this as a "stage" in the future. Once I got started, I figured I could throw it in the benches as well as insurance. It's obvious now that it was completely unnecessary in the benches.
I started off cutting it with a bolt cutter, but this was a bit too much work and the cut off pieces tended to shoot off at high velocity. I switched over to the angle grinder, which worked like a champ. Make sure your rebar doesn't get any closer than 1" from the edge, any closer and you risk having it settle and shift over during pouring and being exposed, or potential rust visibility in the future. Tie all the rebar together with some ties, making sure not to damage your melamine faces during dry fitting. I like to keep some xps foam insulation around for this.
Step 3: Pour the Concrete
Now comes the fun / high stress part. Concrete is cheap, but you just spent all this time prepping your forms so don't screw this up man. At least that's what I kept repeating in my head. I usually use the 5000 psi concrete, but I tried a different one this time... I'll be going back to quickcrete 5000 psi next time.
First, make sure your forms are good and clean of any debris, spray it out with some air if you have it.
Next, make sure to mix the concrete according to the directions, you definitely don't want it too wet or too dry. I like to add my integral color to the measured water and mix before pouring into the dry mix. Once you've added your water and color to the concrete, mix it thoroughly as any unmixed (and therefore uncolored) will haunt you as you scrape together the last part of your pour.
There's a lot of great resources online about how to do a good pour, youtube will be your friend. Unfortunately, since I generally work alone, it's not a great time to go all Ansel Adams and photo op this thing too hardcore.
Key's for me, mix well, pour in 1/2 the concrete, set the rebar in place, place the remainder of the concrete, use a concrete vibrator to get everything settled and remove air bubbles, screed, and then trowel the top surface (the bottom of the final piece since I'm casting everything upside down).
The red colorant made it look like a scene from Dexter, but the concrete had the desired color. Wash up good so you don't look so guilty covered in what closely resembles the color of blood. Or use a different color.
Step 4: Demold and Polish
Most of the demolding should go pretty well. I had no trouble removing the benches, even without adding any mold release. Just remove all the screws and gently separate the mold from the concrete using wooden shims or pulling.
The table top on the other hand posed some significant challenges. First off, turns out 3 bags of concrete in the table top weighs a lot (say 240 lbs... like it said on the bags...), one of the keys to a happy marriage is to limit the amount of heavy team lift items you try to rope your wife into lifting with you. So I spent a considerable amount of effort to demold it and flip it by myself. Then once I did, I realized my error in not using any mold release on the center section.
This is where you get to carefully but forcefully let it know who's boss. I at first tried to just pound it out. That was a bad idea and broke out a bunch of the concrete edges, so don't do that. Then I got out my drill and reciprocating saw and cut out the center, and made a series of edge cuts, stopping just short of the concrete, which doesn't saw well with a wood blade. That at least got the wood out. Then I struggled with the aluminum, since there was no mold release, it bonded quite well to the concrete. After trying various forms of hitting it with things, I switched to the old faithful blowtorch approach. Aluminum transfers heat well, and expands under heat, so apply a short burst of heat to the aluminum and it should pop free. It did break out some small concrete on the ends from the expansion, but it at least got it free of the mold.
Next, I brought it out into the yard and did my wet polishing. There's a lot of good resources here, and given the cold and complete mess with all the water, I didn't take any photos of this part. I bought my wet grinder on amazon for $225 - it's certainly not the best quality, I do get shocks from it, so I tend to wear a lot of rubber gloves while using it. The shocks aren't so bad, you get used to having an unintentionally strong grip on the tool. It supposedly has a GFCI on it, and I always plug it into a GFCI outlet, so the shocks must be pretty small since they never have tripped either GFCI. I took it all the way up to 3000 grit on the show sides.
Then while moving it back in, I dumped it off the wheelbarrow in the grass, and it was the season opener for deer hunting, so all my lifting friends were out hunting... So I had to break my marital harmony rule and ask my wife to help me get it back up safely.
Now you can seal and shine it all up. You need to wait a time period for the concrete to cure before sealing it, just read the label for your desired sealer to make sure you've waited long enough. I use a silicone based sealer from quickcrete, which works well. Flood all the surfaces several times until it stops taking in fluid, wipe it clean and then polish it. I have used automotive waxes with reasonably good results, Carnuba based waxes are the hardest, so use them if you can. A good helper (my 4 year old) can man the power buffer. I also melted some paraffin wax to fill some of the larger voids, I tried a skim coat, but those just never seem to adhere well, so I reverted to at least smoothing it out with wax.
Step 5: Make the Wood Bases
I used 2" x 4" and 4" x 4" Douglas Fir lumber for the bases, which you can pick up from most any home center. I typically stick to fine woods, but in going with the indoor/outdoor theme and the mass of the top, I wanted to keep the bases simple and sturdy. Be selective to try to pick out the best lumber with straight grain and few knots, it'll look better and be stronger, and since you only need 6 2x4's it shouldn't be too hard.
I cut out the legs on the benches on a 7 deg angle to give it some better front/back support and to give some better visual interest. I cut all the pieces to length, put them all on bench tops to see what legs went best together, then marked them accordingly so I could keep the inside faces straight and the matched sets together.
Next, I cut the joinery for all the pieces. I have the luxury of owning a Festool Domino Joiner XL, which works great for this sort of thing. You could also certainly use something easy like screws to join it all together. I really hate visible fasteners, and don't trust pocket screws much for structure - so the dominos worked great for me.
Before you glue it up, make sure to sand everything and round over all the corners, it only gets harder later.
If you end up with floating tenon joinery you can see I glued it, used a small brush to spread the glue, pound in the tenons (with an enthusiastic 4 year old's help and unfettered access to a mallet), and clamp it down. I did it in two stages to keep things reasonable.
Once everything was glued up I counter bored some holes for the tapcon fasteners with washers.
The same basic process was done for the table top, just without any angles, so nothing too exciting and I didn't remember to include any pictures.
Step 6: Seal and Finish the Wood
Now since I wanted to have this as an indoor/outdoor table, I wanted to make sure it would hold up to the elements. I looked into some clear penetrating epoxy sealer, which seems like a great product, but it wasn't readily available, so i ended up just using standard west systems epoxy, which I buy by the gallon for wood working anyway. Flood the end grain of both the top and bottom of all the exposed end grain. It draws it into the pours and permanently seals it. My hope is that it will prevent rot from starting by eliminating water wicking up the grain. It remains to be seen, but seems like a good thought.
Once the epoxy dried up, I sanded everything up well, and sprayed on some outdoor finish from General Finishes. I love General Finishes interior water based top coats, so I thought it would be good to try their outdoor. It's still too early to tell how it will do, but it applied easy with my HVLP sprayer.
Step 7: Make the Insert
I decided to build the insert out of 1/4" HDPE, which you can buy as sheet stock from Menards. It's impervious to water, and can take a beating without breaking. It's a pretty malleable plastic, so it worked well for this. It also has a surface texture that makes it easier to hide all the marks the kids will make on it in the future.
I built a box with two dividers, about 10" deep, I used pre-drilled the holes and counter sank them, then used #6 wood screws to join it together, which worked pretty well.
I also cut some new angle aluminum to size, drilled and tapped some holes it it to secure it to the box. I used #10-24 flat head screws to secure the box to the angle aluminum.
Step 8: Have Yourself a Good Cry, Then Figure Out How to Sorta Recover From Your Mistake
Now it's time to fill your life with regrets.
I borrowed a friends "bulldog" hammer drill, which turns out, has much more hammer than some of the previous multi-purpose hammer drills I've used before. I Clamped the bench to the concrete top, marked the depth with tape, and went about drilling the hole -- there were two problems.
#1 - The tape got concrete dust behind it and slowly moved upward, increasing my depth of drill without me noticing.
#2 - I intended to leave 1" of un-drilled concrete on the backside of the concrete, but even that was close.
So, I blew out 3 holes in one bench while learning my problems. There was much sadness throughout the land. Luckily I buy epoxy by the gallon, so I filled the holes with epoxy and made this an "interactive" bench where you can now see the rebar through the epoxy. Neat right? That's the story I tell people while trying to man up and not cry in front of people.
Once the epoxy dries, sand it up, polish again, and you're back in business.
Step 9: Final Assembly
Now this time around, pick slightly shorter fasteners, I ended up only going into the slab about 1", which doesn't give the strongest pullout strength, but I honestly only wanted it to keep the concrete from sliding off the wood parts. I also used a stop block to ensure you don't go too deep (the one in the picture was not my final one, but it give you an idea what I mean). Put in some tapcons and you're almost done!
I then added some felt sliders to the bottom so a mere mortal could move it around without a back injury. A good help is always good for this part too.
Step 10: Trick Your Neighbor Into Carrying It Inside With You, Then ENJOY!
Now this part is key, text your neighbor something ambiguous about needing some help "quick" and "shouldn't be too hard", once they're over there and committed to helping, then ease into the whole "made this out of concrete" part.
I'm not sure I'll be moving this around a lot, but the kids love it and it meets all of my design constraints. It's definitely a unique Christmas present for my kids.
I changed a few dimensions as I went, but I included my original SketchUp model for reference.
Thanks for reading and good luck with your own build!
Second Prize in the
Furniture Hacks Contest