I created this cantilevered coffee table from a live-edge spalted maple slab and a GFRC concrete mix made from scratch. The angled concrete post was formed by slicing two sections from a standard 12" concrete form tube with a 15 degree angle, attaching these sections to the live edge slab, and pouring the concrete through the slab to form a solid post base extending through the table top. To cut the 12” concrete forming tube at a 15 degree angle, I created a 12” miter box from scrap plywood.
As with my last project, I used a white glass-fiber reinforced concrete (GFRC) concrete mix, and this time, poured in the face coat for the table surface. For the backer mix, I used plasticizer to make a self-consolidating (SCC) type of GFRC.
Here is a link to my previous video with more details on GFRC and how to caulk concrete forms: https://youtu.be/sXyj0iHg7Lo
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Step 1: Materials
Figuring out everything you need for GFRC is probably the most daunting part for a DIYer, but in the end, is not too difficult. You can get the sand and cement locally from Home Depot, and place an order for everything else from an online supplier (I used Fishstone concrete, links in products below). You can also shortcut the process by buying GFRC pre-mixed bag, where you only add water and glass fibers.
Here is one good pre-measured mix for GFRC: http://www.concretecountertopsupply.com/Item/GFRC_...
As with past projects, I made my own mix for this project. I just followed the recipes available here and mixed my own GFRC face and backer from scratch. http://www.concretecountertopsupply.com/index.php?...
I personally think its rewarding to mix from scratch, and if you have any previous experience with concrete, I encourage you to go this route. Here are the ingredients you'll need.
Products for Concrete Mixes -- for this project you'll need 100 -120 lbs of mix, so calculate accordingly
1. ~30 mesh sand: ($5 for a 50 lb bag) I used Quikrete Commercial Grade Medium Sand from Home Depot.
2. Type I/II portland cement: ($10-20 for each 94 lb bag) I used white portland cement, which you'll need to source locally. If you have a White Cap / HD Supply near you, they carry it. Check with a masonry supply store if you don't have a white cap nearby. Or, you can just use grey portland cement available from Home Depot. The drawback to grey portland is you have less control over coloration. If your project needs more than 1 bag of portland and you use grey, you'll want to mix them together beforehand, since color can vary from bag to bag.
3. Acrylic Polymer: ($125 for 40 lb bucket) This is part of the magic of GFRC, which replaces water with acrylic polymer. This makes the concrete harder, less prone to shrinkage/cracking etc. I used a product called KongKrete, and highly recommend it.
4. Superplasticizer: ($42 for 1 gal) This makes the concrete mix more fluid, without requiring more water than useful. It is key in the GFRC face coat mix to popping a pinhole-free piece right out of the form. The amount you use will vary depending on your project. If you need a playdough like consistency for coating a vertical surface, you use very little. If you want a super fluid mix that fills a form and self levels, you use more. I used the Optimum superplasticizer from Fishstone.
5. Defoamer: ($42) This ingredient that prevents air bubbles from being trapped in the mix. Combined with the superplasticizer, it can result in pieces that pop right out of the form with few pinholes (and perhaps even perfect). I went with the C-64 defoamer from Fishstone. If you don't mind a few pinholes, or going back and doing a slurry coat later, you can probably get by without this.
6. Pozzolans: ($50 for 50 lb bag) These are not 100% required, but they were recommended to me, and I can tell you the mix I made with them was super easy to work with. I used a product called Alto-Pozz.
7. AR Glass Fibers (source online through Fishstone or Buddy Rhodes)
8. Water: you know where to get this.
See my previous GFRC instructables for more details and notes on ingredients and costs.
Other materials / tools:
12" concrete form tube: $10 (from big box store)
Saws: you can do this with only a circular saw, but a table saw and miter saw will make life easier. http://amzn.to/2wcDgnN
hot glue gun: assume you have one, $10 from amazon if not
black 100% silicon caulk - $6/tube - 1 tube should be enough
metal ball cake fondants - $7 - this tool allows for perfect edges in the form: http://amzn.to/2w0JCVx
paste finishing wax - $8, from Home Depot 1
lots of buckets. Get these from Home Depot. $2-3/each. I recommend getting some 5 gallon buckets, as well as 10 qt and 1 quart buckets
Abranet sanding pads, or wet/dry sanders 220 - 400 grit.
diamond polishing pads: http://amzn.to/2gmxUkt
Drill and Spiral or Helical Mixing Paddle. Since I have a few projects, I splurged for a mixer ($110 from Home Depot). You can also just use a heavy duty 1/2" corded drill if you have one, with a spiral mixing paddle ($15 from HD). Make sure you get a paddle where the bars are blade-like (like the one in the link), rather than tube-like, and are spiral or helical. The tube-like ones for paint won't move through the concrete as well. You can't make GFRC by hand. http://amzn.to/2wcC6bZ
Random Orbit Sander
Router Planing Jig (do a Google search to get instructions to make one, very easy to do)
Step 2: Flatten and Prepare the Live Edge Slab
I used a router planing jig to flatten both sides of the live edge slab. Just search Google or YouTube for "how to flatten a slab with a router" and you'll find plenty of instructions about how to make this simple jig and the process.
I then used a belt sander to remove lines left by the router planing process, and sanded again with an orbital sander up to 240 grit.
After flattening and sanding the slab, I cut a hole that I could pour concrete through to join the above-tabletop and below-tabletop portions of the concrete post. Make sure the hole is a decent size (e.g., 1.5-2" smaller than diameter of tube, so there is overhang to lock the slab in place between the upper and lower parts of the concrete post. At the same time, make sure it is big enough you can get your hand through it to spread the concrete in the top part of the form (which will be below the tabletop during casting, since it is being cast upside down).
IMPORTANT STEP: This isn't in the video, and should have been. Put a good thick layer of paste wax (or two) everywhere on the table, including the inner walls of your hole in the slab. This will prevent the wood from absorbing too much water from the concrete mix during the pour and curing process, and make it easier to remove the hot glue that you'll use to temporarily affix the concrete forming tube to the slab when you pour the post.
Step 3: Build the Form From a Concrete Tube and the Slab
The form is made from two sections of a concrete tube, which are sliced at a 15 degree angle. In order to make repeatable 15 degree cuts across the tube, and to cut the top and bottom of section of tube parallel to one another, I made a custom 12" miter box from scrap plywood. I'll try to add some pictures of the box, but for know, check out the video at the top of this instructable to see this process.
The top section of the form will actually be on the bottom during the pour, since we are casting upside down. I cut a piece of acrylic to form the bottom of the form (i.e., the top of the table), and attached the top of the form to it with silicon caulk on the inside, and hot glue outside. See my previous instructables for instructions on how to get the caulk lines perfect.
I think test fit everything on the form, and once I made sure everything was level, hot glued the thinner top section of the form (which is below the top during the pour) to the top of the slab.
IMPORTANT: Mark matching lines on the top part of the form (forming the bottom of the post) and the bottom part of the table where it lines up continuously with the other half of the form, but don't attach it to the table yet. You'll glue it to the table in the middle of the concrete pour, after you've poured the section to form the top of the concrete post.
Step 4: Mix & Pour the Concrete
Before you mix, cut some inserts out of pink 1.5" insulating foam (e.g., Foamular). You'll put these in the middle of the post to help reduce weight.
For GFRC mixing instructions, please check out my previous instructables -- there are detailed instructions there.
Here is the general step by step for mixing and filling the form.
1. mix a thin face coat (no glass fibers), and pour it through hole in wood. use your hand to reach through and work it into corners of the form and spread it around evenly.
2. let this face coat set up for 30 min
3. mix up your back coat of GFRC with AR glass fibers, and fill the rest of bottom part of the form (that forms top of the post) and the hole in your slab by hand. Make it an SCC mix with plasticizer (very flowable)
4. now you attach the top of the tube form (which makes the bottom of the post) using hot glue. remember to line the tube up with the markers on the bottom of the slab. This needs to be done pretty quickly.
5. fill up about 2" more of the tube, put in one of your foam inserts, fill around sides of foam insert, then repeat with additional foam inserts until about 3 or 4" from the top of the form (or bottom of the post).
6. now fill the rest of the tube with solid backer coat.
7. trowel the top (aka bottom of tube) to be as smooth as possible. It will be sitting on your floor and could scratch if you don't smooth it out.
Step 5: Demolding
After you let the mold sit for at least 24 hours (I'd recommend 48 in this case, since its thick), its time for the fun part, demolding. The demolding is pretty simple. Just tear the cardboard forming tube away and remove the hot glue and any excess silicon from the wood. You might have to use a razor to remove some of the hot glue, but be careful not to scratch the wood up too much. If you do, not a big deal, just sand the scratches down again before moving on.
Step 6: Finishing and Sealing the Concrete
I used diamond sanding pads to smooth out the edges of the concrete at the bottom of the post, but did no other type of surface work on the concrete. I then vacuumed the concrete and applied sealer..For this I used DuPont's high glass stone sealer. It is very economical if you have a lot of projects. I found a gallon of it for about $45 on amazon. A quart costs about $20. Cheng Concrete Countertop sealer is also very good, and I've used it, but it is a good deal more expensive. I only use it for projects that will see a lot of use (like countertops).
Step 7: Finish the Wood
I taped off the sealed concrete so as to avoid the wood sealer turning the concrete yellow. (This still happened to some extent, but it only dyed the surface sealer on the concrete instead of the concrete itself, so was easy to sand off later.) I then used four coats of Waterlox to seal the wood, sanding with 400 grit sandpaper between coats.