GFRC Floating Concrete Hearth for Concrete & Wood Fireplace

This instructable is going to focus on the construction of the concrete (GFRC) hearth. I haven't seen many instructables related to glass fiber reinforced concrete (aka GFRC), so I thought this would be useful. GFRC takes a bit more planning to obtain materials and make your mix, but makes other parts of the process much easier for a DIYer. The weight reduction by 50% or more makes it easier to move precast pieces around. It pops out of the form basically pinhole free, so can be polished and finished without messy grinding (this is a HUGE reduction in labor and hassle involved). And its way stronger, so no rebar or other reinforcement is necessary. Lastly, while I used a semi-overmold to integrate the plywood support structure and create a floating effect, you don't need one to do vertical surfaces, so the form can be greatly simplified. (If I were to do this again I would not have integrated the plywood support.)

The other pieces of the fireplace makeover were done using techniques that are well documented elsewhere, but I'll provide a quick overview here, before getting into details of casting the GFRC hearth.

(1) The durock/drywall work to create the nooks was done by a contractor. (There are some things, like drywall work, that I don't like to do, and are fairly cheap, so I hire those out.)

(2) I used the Henry Feather Finish (available at Home Depot), aka Ardex Feather Finish, to put 3 coats of concrete microtopping on the walls of the fireplace. Do a search for Ardex concrete fireplace and you'll find a good tutorial. Only thing I did differently is apply it directly over existing drywall, which worked great. After 3 coats, I finished it with Dupont Stonetech high gloss finishing sealer. I just used a microfiber rag to apply the sealer. I like this finish because it gives a shiny, hard, stone-like appearance to the concrete wall. If I were to do this again, I might use Ardex SD-M (special order product), which can accept pigment. This way I could color match the concrete hearth to the concrete wall.

(3) I used Schluter 3/16" black metal Jolly trim and Stikwood Vandyke peel n' stick wood in the nooks. I attached the trim with 2-sided carpet tape (works fine because Stikwood is laid over part of it and holds it in place), then applied Stikwood per instructions on their website. I'll do a separate instructable on this later, since I haven't seen anyone else post one with the trim added to a wood wall (which gives it a super-clean modern finish).

Now on to the GFRC hearth....

This 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 face coat mix, GFRC backer mix, along with liquid polymer. This simplifies the mixing process, since you don't have to weigh out ingredients. You just mix one bag of mix, one gallon of polymer, and water. However, this premix can be very expensive, every 60 lbs of mix will be about $65 + shipping. Here is a source if you want to go the pre-mixed GFRC route: http://store.concreteexchange.com/Concrete-Counter...

POST-PROJECT UPDATE: Fishstone just emailed me about a new pre-mixed "just add water" GFRC product, that I wish had been available. This will GREATLY simplify things for the DIYer, and is much less expensive than any other pre-mixed GFRC product I've seen. It has powdered acrylic polymer already in the mix, so you don't have to pay the lofty shipping charges to ship 5 gal buckets of liquid polymer to add to your mix. Here is a link: http://www.concretecountertopsupply.com/index.php?...

Returning to what I actually did for this project, I 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 is a key 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.

6. Pozzolans: ($50 for 50 lb bag) It is my understanding 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. All told, I spent about $650 on mix materials. However, this is enough to make about 1000 lbs of mix. I only used 180 lbs for the hearth, and am going to use the rest for other projects. Since I have other projects, the mix cost for this project was about $130. 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 this amount of materials. If you don't, you could use a GFRC pre-mix products. If you only plan on one project, it would be more economical to get pre-mixed products, with the exact amount you need. For this project, you could get 3 bags of Fishtone's GFRC mix, 5 lbs of glass fiber, and it will be somewhere around $200-250 including shipping.

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(s): $28/each - I only need one for the hearth which was a trapezoid with 6 ft back and 4 ft front.

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 silicon caulk - $6/tube - 1 tube should be enough

7. metal ball cake fondants - $7 - this is my secret tool, and allows for perfect edges in the form

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 6x 5 gallon buckets, 6x 10-quart paint bucket (with measuring lines), and 6x 5-quart paint buckets (with measuring lines).

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 on this 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. Angle grinder

15. Random Orbit Sander

This is a pre-cast design, so the hearth is being cast upside down. The form had two parts: (a) the melamine base form and (b) the plywood and foam insert. The plywood and foam insert was designed to be left in the concrete after it cured, to form a base for the hearth to rest. The plywood insert was made to be 2.25" taller than the height of the concrete (which is dictated by the height of the melamine sidewalls in the base form). In short, I do not recommend doing it this way. I would suggest making the base form as I did and that is it. Making the plywood insert and pouring concrete around it, turned out to be a huge hassle. As explained later, you can still create a hollow hearth by packing the backer coat up the sides like play-doh. Then, build your base however you like to raise the hearth up to your fireplace (you can use plywood or 2x4s painted black for the base).

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 hearth. In my case, the hearth needed to have a height of 3.75" (with 2.25" of space between the bottom of the concrete and the floor so that my hearth would be even with the bottom of my fireplace, which is 6" from the floor). 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.

(b) Cut a base piece of melamine to the size you need for your hearth. The base of the form should be 5.5" wider and 5.5" longer than the longest/widest dimensions of your hearth. For example, my hearth was 72" long and 15" wide, so my base needed to be at least 77.5" x 20.5". The extra room is so you have room 3/4" melamine sides and 2x3 or 2x4 supports (which are 1 5/8" thick) on either side of the form.

(c) If you have any non-90 degree angles on your sides, miter the edges of your side strips to the exact angle. It is important that the inside of the form is formed entirely by the white melamine (no endgrain). In my case, I had a 6' back and 4' front on my hearth, with 15" depth. I cut the 6' and 4' melamine sides to the exact length needed, then laid out longer pieces for the sides. I made sure the 6' and 4' sides were centered and 15" apart, then laid out longer strips next to them to figure out the angle with my angle finder. For these dimensions, the angle turned out to be 53". I then used my miter saw to cut all the sides of the form to create the 53 degree angle.

(d) Double check that your melamine side strips are perfectly flat (re-cut if not).

(e) Before permanently attaching the side strips, lay out the sides on the base exactly as you want them. If you have right angles, use a square to make sure they are all perfect. Check any other measurements you want to make sure are accurate. In my case, I measured to make sure the centers of the 4' front and 6' back strips were perfectly aligned, and that the angle on both corners was 53 degrees. Now, use a pencil and mark the outside edge of the strips on the form.

(f) Remove the side strips and attach short 2x2, 2x3, or 2x4 pieces to the outside of the side strips. To attach them, I used 2" construction screws. You can use whatever you have handy, just make sure to pre-drill your holes so you don't split the support or the melamine sides. One or two screws for each support is enough. Your support pieces should be shorter than the height of your side strips. I had 3.75" H side strips, so 2x3s and 2x4s worked. The bottom of the 2x2/3/4 supports should be as close to flush to the bottom of the melamine strips as possible, BUT, whatever you do, make sure the bottom of the support doesn't extend past the bottom of the melamine strip. This will cause your strip to angle inward, and result in an edge of your hearth not being square. (NOTE: if your height is 1.5" or less, you can skip this step, because the carpet tape, silicon, and hot glue will be enough to hold your side forms in place)

(g) Apply the double sided carpet tape to the underside of the side strips. If the tape is wider than the melamine (mine was 2"), use a razor to cut the tape down so it is flush with the melamine. Then stick your side forms to the base form using the lines you drew in step (e) as guides.

(f) With your hot glue gun, go around the outside edges (NOT the inside) and glue your side strips to the base form. Since you are gluing your 2x3 or 2x4 supports as well, you don't need to screw them to the base form.

(g) Apply paste wax to the inside of your form, then caulk the inside corners using black silicone caulk and the cake fondets. 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.

As mentioned above, I don't recommend building the plywood insert into the mold like I did, so I am not going to go into details of this here. If you are a glutton for punishment and want to use this technique to "float" the hearth, leave a message for me in the comments.

I used one 30 lb batch of face coat (actually was too much), and two 60 lb batches (120 lbs total) of backer for the hearth. 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 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. There are plenty of calculators online to help you do this. Mix 2 lbs of face coat for every square foot of surface you need to cover.

Since my hearth was about 12 sq ft., I needed 24 lbs. I actually mixed 30 lbs of face coat, but this turned out to be way more than I needed. I sprayed my face coat pretty thick (about 1/4"). You can go thinner, and use less face coat, but for your first project erring on the side of too much face mix and a thicker face coat is better.

(b) Calculate the total weight of the backer mix you need for your project.

To figure out how much backer mix you'll need, use the square footage of your project you calculated in (a). Then figure out how thick you want your hearth to be. For most GFRC projects 3/4" concrete is plenty strong. If you like to (WAY) overbuild, then you can bump it up to 1", but honestly this isn't really necessary unless you have large cantilevers or overhangs. If you are doing 3/4", then take your total square footage and multiply by 8 lbs / sq ft. (which is the approximate weight of 1 sq ft of GFRC at 3/4" thick) to figure out the total weight of your mix. If you are doing 1" thick, then do the same calculation, but you 10.67 lbs / sq ft instead of 8 lbs /sq ft. (Note: This will actually build in a cushion, since your backer is actually 1/8" - 1/4" less thick, depending on the thickness of your face coat. For sake of simplicity and building in a cushion with my calculations, I'm pretending the backer is 3/4", when in reality, it will be less since the total thickness of my face coat plus backer will be 3/4".)

My hearth was about 12 sq ft., and I planned to have 3/4" concrete, so I calculated 12 sq ft * 8 lbs/sqft to be 96 sq ft. Multiply by 1.1 and I needed ~106 lbs. I rounded up to 120 lbs, but it turned out not to be necessary as I had excess mix. (You'll notice a huge savings in material and weight, which makes GFRC less expensive than it might seem initially. To do this with Quikrete you would need to go 1.5" - 2" thick, and since GFRC weighs less per sq ft than mix with gravel aggregate, you more than cut your weight in half.)

(c) Measure out ingredients in buckets.

The key to making the process of spraying the face coat and pouring/packing the back coat go smoothly, is measuring out your mix ingredients ahead of time in buckets. You'll want to measure out multiple batches, so you'll have a row of buckets measured out for each batch. For the 30 lbs of face coat I needed just 1 batch, since this easily fits in 5 gallon bucket. For backer mix, if you are using 5 gallon buckets to mix, I don't recommend going higher than 60 lbs per batch. (I tried measuring ingredients out for an 80 lb patch, and it would have made the 5 gal bucket so full that mixing the ingredients would have been difficult, so I re-measured into 60 lb batches). For my project, I measured out 3 rows of buckets for one 30 lb face coat batch, and two 60 lb backer coat batches.

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. There is a technique to this, so watch the videos I pasted below. The explain better than I can in words.

10. As soon as you spray, use a chip brush to remove any air bubbles from seams and face. Again, watch the videos below, as they explain it well.

11. Wait for the face coat to firm up before adding backer 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, but it was really cold in our basement (probably 50 degrees F) and it took forever. After an hour and a half I got impatient and added backer while the face coat was still malleable. The result was a few visible spots in the final product where the backer came through. Watch this video to get a better idea when your face coat is ready:

You can wait until its ready to mix, 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. Make sure to pour all the way up to the edges (and perhaps slightly over the edges, to make sure you have nice bottom edges). This is much easier if you don't have vertical surfaces. However, an SCC won't work if you have vertical surfaces you need the backer to cling to. (I used an SCC backer coat on the 1.75" thick bartop I made to match the hearth. Pics of this bartop, its form, etc. are mixed in throughout this project, and I'll try to make a separate simple countertop instructable for it at some point.) Also, 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.

OPTIONAL: You can also use a GFRC standard roller to work in the backer back and align the fibers. While it isn't necessary using a roller to align fibers this will make your piece stronger, and ensures a good bond between the face coat and backer. The idea is that have 2 or more layers of backer coat, where you roll in different directions in each layer, so fiber layers are aligned perpendicular to one another. This crosshatch pattern of fibers adds significant flexural strength. When I realized that I really liked Fishstone's mix products (over other products I've tried, such as Surecrete and various home depot mixes) and was going to be recommending them, I asked if they might send me some rollers to try out so I could add them to this instructable. (While they did not charge me for the rollers, I paid for everything else, and was planning to recommend their products regardless of whether they sent me the GFRC rollers.) They were kind enough to send me a variety, including the corner roller, 1x3 roller, and 2x6 roller. For the average DIYer, I would recommend getting the 2x6 roller, but would not recommend the 1x3 roller. The design of the 1x3 roller was different from the 2x6 roller. The 1x3 roller has continuous ridges extending around its entire circumference, whereas the 2x6 roller has individual nodules all over it, which I found did a much better job. The corner roller was nice, but I did not think it was necessary. In short, go for the 2x6 roller if you are an average DIYer, you really don't need much less. Here is a link to GFRC rollers if you'd like to buy one: http://www.concretecountertopsupply.com/Category/...

After you let the mold sit for at least 24 hours, its time for the fun part, demolding. It is like Christmas morning when you were a kid (at least for me). I demolded upside down, but it probably would have been better to flip the piece over onto a foam board and demold right side up. The demolding is usually 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.

The hearth 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. Then rinse it off (use a squeegee if you have one to clean the surface). Inspect, and if you like how it looks, go ahead and apply a sealer. I have used a couple different sealers with success. 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).