If you like this Instructable, I would greatly appreciate your vote in the Box contest. (I figure that the process to build the form for the concrete is essentially just building a box, so it seems appropriate for a box contest and thinking outside the box :) )
I made this table out of white glass-fiber reinforced concrete (GFRC), and a live-edge spalted maple slab. I got the wood from Ebay for about $10 -- it is thinner than a typical slab for a table top, but all that is necessary for an inlay in concrete (hint: search for taxidermy slabs and you'll find good deals for a project like this). The table top is 14"x14". A friend of mine made the steel legs, so this process is not documented in this instructable.
Step 1: Gather the Materials
This is probably the most daunting part of GFRC 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_...
However, I made my own mix for this project. I just followed the recipes available here: http://www.concretecountertopsupply.com/index.php... and mixed my own GFRC face and backer from scratch. 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
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.
8. Water: you know where to get this.
Note on Cost: You'll likely need to ship everything but the sand and cement. You can expect $60-100 in shipping costs. Since I have other projects, the mix cost for this small project was about $35. If you have other projects (and you likely will once you see how awesome GFRC is), then it is 100% worth it to invest in bulk materials. If you don't, use a GFRC pre-mix product instead.
Other materials / tools
1. Hopper gun for face coat: $100 (optional depending on shape of form)
2. Air compressor: any pancake compressor you have will do, assume most people have one
3. 4x8 melamine board: $28/each (plenty leftover for other projects, or you can buy 15" wide melamine shelves for closets)
4. Saws: you can do this with only a circular saw, but a table saw and miter saw will make life easier.
5. hot glue gun: assume you have one, $10 from amazon if not
6. black 100% silicon caulk - $6/tube - 1 tube should be enough
7. metal ball cake fondants - $7 - this tool allows for perfect edges in the form: http://amzn.to/2w0JCVx
8. outdoor double sided carpet tape - $5-8 - available at Home Depot
9. paste finishing wax - $8, from Home Depot
10. 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
11. Syringes for measuring out defoamer and plasticizer. $3
12. Abranet sanding pads, or wet/dry sanders 220 - 400 grit.
13. 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.
14. Random Orbit Sander
Step 2: Prepare the Wood Slab
I kept this simple. I purchased a slab that was planed already, so flat enough to use.
I used the base of the form to mark where to cut the ends of the slab (so it extends the length of the table and is flush against both ends), then cut it on a miter saw. (I did steps 2 and 3 concurrently -- see step 3 for form instructions.) . A circular saw would also work.
Then I sanded it up to 240 grit, and put a few layers of polycrylic on it. I also sanded the sides and used a chisel to remove any parts of the sides that might catch and prevent removal of the slab from the concrete, after the concrete cured. (generally, you want your sides sloping inward, from top to bottom of slab, so removal from the concrete is simple.)
It is important to finish the slab first, before pouring, so the slab doesn't absorb water from the concrete when the mix is poured over it.
Step 3: Build the Form
This is a pre-cast design, so the table is being cast upside down.
To make the base form you'll need to do the following:
(a) Cut melamine strips for the sides of your form to the height of your table + 3/4". In my case, the table was 1.75" thick so the sides were cut at 2.5" It is easiest to cut these strips on a table saw, but it can be done with a circular saw if you are very careful to make cuts consistently. Cut all the strips at once, so the height is identical. Then use your miter saw to cut the strips to the lengths you need. Cut each side a few inches longer than the base (e.g., 17" or so for a 14" side of table top). The extra overhang gives you leverage to pull the sides away from the concrete when you demold.
(b) Cut a base piece of melamine to the size of your table, in my case 14" x 14". Note that I did this first, and then used the base to mark where to cut my wood slab, before attaching the sides.
-BEFORE PROCEEDING TO (c), DO STEP 2
(c) pre-drill holes, and then screw your sides to your base. I used 1.25" drywall screws for this.
(d) place your wood slab in the form upside down (with top of slab against bottom of form).
(d) Apply paste wax to the inside of your form and bottom/sides of slab.
(e) caulk the inside edges using black silicone caulk and the cake fondants. This is a very easy way to get perfect silicon caulk lines. TAKE YOUR TIME AND DO THIS RIGHT - WATCH THIS VIDEO:
The video explains it better than I can in words here. After the caulk cures and you pull the excess to leave clean caulk lines, your form is ready.
Make sure to caulk between edges of wood slab and form too. You don't want any place for mix to flow between a visible surface of the slab and the form.
Step 4: Mix & Pour the Concrete
Preparing for the Mix
I used one 10 lb batch of face coat (actually was too much), and a 30 lb batch of backer coat, but it was way more than needed, so I also used the leftover to make this cool bowl. :)
This step involves first figuring out how much mix you'll need to make (i.e., the overall weight), and then pre-weighing and measuring out all the ingredients into buckets.
(a) Calculate the total weight of the face coat mix and back coat mixes you need for your project. To figure the total weight of face coat mix you'll need, calculate the square footage of your project, including top and sides, and figure about .25" thick, at most for the calculation. There are plenty of concrete weight calculators online to help you do this. Then measure out your materials proportionally to the recipes here: http://www.concretecountertopsupply.com/index.php?...
You can also just use the pre-mixed GFRC bag mix I linked to above to simplify things (in which case, just follow those instructions, and skip everything below).
Mixing & Spraying
Once you have all your materials prepared in buckets, it is go time. I have tried mixing a couple ways, and order doesn't seem to matter all that much, so long as you add dry material to water in multiple phases (i.e., do NOT add water to your dry material, or mix everything together all at once). You also need to allow 5 minutes after mixing your face coat for it to set up. Here is what I found easiest for the face coat mix:
1. Mix together your dry ingredients (sand, portland cement, alto-pozz, pigment (if using powdered pigment)). Use your mixer to make sure the dry ingredients are blended well. Wear a dusk mask, you don't want to breath in portland. 2. mix your wet ingredients in a separate big bucket (polymer, water, defoamer, plasticizer). Err on the low side with water, you can always add a bit more if needed. 3. add 50% of your dry ingredients to the bucket with the wet ingredients, then mix thoroughly 4. add another 25% of your dry ingredients, mix thoroughly again. 5. scrape the sides of the mix bucket with a trowel, mix thoroughly again. 6. add remainder of dry ingredients, mix thoroughly, scrape sides again, mix thoroughly again. 7. wait 5 minutes. This is important since the sand takes a bit to absorb all water it can. As sand absorbs water, the mix gets thicker. Thus, if you don't wait, you run the risk of your face mix thickening mid-spray, and clogging your hopper gun. 8. after 5 minutes, test the consistency of your face mix with a trowel (see videos below). Add a touch of water if needed, and mix again. 9. load 5-10 lbs of face mix into your hopper gun and spray the form. As soon as you spray, use a chip brush to remove any air bubbles from seams and face. 11. Wait for the face coat to firm up before adding backer coat.
Pouring the Back Coat
The timing of your backer coat is important, so make sure your face coat has firmed up enough (but not completely dried out), before you start your backer coat. Usually this only takes 30-60 minutes,
You can wait until the face coat is ready to begin mixing your back coat, so long as you have everything measured out in buckets (so mixing only takes a few minutes). The process is similar to mixing the face coat, except you add glass fiber at the end.
1. Mix together your dry ingredients (sand, portland cement, alto-pozz, pigment (if using powdered pigment)). Use your mixer to make sure the dry ingredients are blended well. Remember to wear your dust mask. 2. Mix your wet ingredients in a separate big bucket (polymer, water, defoamer, plasticizer). Err on the low side with water, you can always add a bit more if needed. 3. add 50% of your dry ingredients to the bucket with the mixed wet ingredients, then mix thoroughly 4. add another 25% of your dry ingredients, mix thoroughly again. 5. scrape the sides of the mix bucket with a trowel, mix thoroughly again. 6. add remainder of dry ingredients, mix thoroughly, scrape sides again, mix thoroughly again. 7. Add 25% of your glass fiber, mix thoroughly, and repeat until all your glass fibers are mixed in. 8. (Dryer Back Coat) If you made a dryer backer mix (with less superplasticizer), hand pack your backer mix in, gently pushing it into the face coat. If you have vertical surfaces (I had 3.5" vertical faces), pack the seams at the bottom first, then work your way up the vertical packing. You want to pack the backer up and over the edges at the top of the sides of your form to make sure you don't have cracks or even bottom corners . (This allows you to come back with a grinder and grind the bottom perfectly level using the sides of your form as a guide.) . Also, don't pack too thick at once. I like to pack 1/4" - 3/8" thick layers. If you are making multiple backer batches (e.g., 2-3 60 lb mixes), then try to cover all your surfaces with each batch (applying a thinner layer over everything first helps with this). Make sure to carefully but firmly work the backer mix into the corners as you pack. OR (Flowable or Self-Consolidating (SCC) backer mix) If you make your mix more flowable (with more superplasticizer), then you can pour your mix in, work it into the corners and seams and let it cure. With a flowable self-leveling mix, you have to make sure your form is perfectly level. If it isn't, the bottom of your end product won't be level either. Have some shims nearby in case you notice it flowing to one side after you pour.
Step 5: Demolding
After you let the mold sit for at least 24 hours, its time for the fun part, demolding. The demolding is pretty simple. Unscrew the screws from your form, then use your hands to pull the side walls away. If the side walls don't come off easily, use a rubber mallet to gently tap the sides of the form away from the concrete. If you flipped the piece over before demolding, it should be fairly easy to remove the bottom of the form (which is now on top). If it doesn't come off, then use a plastic putty spackle (not metal or anything sharp that could scratch the concrete!) to gently pry the bottom off. If it is really tough, then there is likely a vaccum between the form and concrete. In this case, you can use an air gun attached to your air compressor to shoot air between the form and concrete. This will break the seal created by the vacuum, so you can remove the bottom of the form.
Step 6: Finishing and Sealing the Concrete
The table came out of the form with very few pinholes, so I decided not to use a slurry coat and to simply sand and seal it. I did a quick wet sand with 400 grit abranet pads by hand. Sand until you feel the creme layer coming off and you can see a bit of the sand coming through. It shouldn't take long, a spent about a minute sanding the whole piece by hand. I wanted a bit more shine, so I then sanded with a 600 grit pad. I then rinsed it off (use a squeegee if you have one to clean the surface), vacuumed it, 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).
Third Prize in the
Box Contest 2017