This will also be a reference as to problems that could arise, mistakes that could happen (read, mistakes we made), and fixes/work-arounds in case they happen to you. You don't want to spend the time building something like this only to have it end up as a small patio in your backyard instead of the countertop you wanted. Believe it or not, the concrete is much more forgiving than you'd think.
On to the good stuff...
Step 1: Gather Your Materials, Have a Plan
- Additional melamine boards for the sides of the form
- Sturdy and LEVEL sawhorses to build on (our finished top weighed around 400 lbs)
- 3/8 inch rebar for inner support
- Remesh for more inner support
- Wire for attaching the rebar and remesh to the form
- Screws for building the form (we used 3 inch and 1 5/8 inch drywall screws)
- Drill (you MUST pre-drill the particle board to avoid splitting)
- Saw(s), circular hand saw and/or table saw to cut the form sides. We also used a chop saw.
- Silicone caulk in a color easily seen on your Melamine (we used black)
- Concrete tools consisting of float(s) and trowel(s)
- Long screed board
- Hacksaw/bolt cutter/wire cutters for cutting your rebar, remesh, and hanging wire
- Rubbing alcohol
- Concrete (of course). We used Quikrete 5000 without additives like fibers or water reducer.
- Pigment of choice. We used a little black so the natural color would just darken a bit.
- Concrete mixer. Ours was rented and we mixed and filled everything in 4 hours.
Before you build your countertop, you should definitely build a test form. We built two 1 x 2 foot forms with rebar and remesh to test pouring consistency, color, technique etc. This also allows you to test finishing techniques on something other than the real countertop. Please don't skip this step.
Plan the size and shape of your top and mark it out on the Melamine sheet. We didn't need to make any templates since ours was a simple rectangle and was going on an island and not against a wall. Be sure to take into account the thickness of the boards being used for the walls when drawing the guidelines.
*While our sawhorses were both sturdy and level, they were only on the ends of the mold base. After curing, we noticed a very slight bow in the countertop. In retrospect, we should have had two additional supports across the length. *
Step 2: Build the Form
Also, be sure to add the additional length for abutment. Since we used 3/4 inch melamine we added 1.5 inches to the end boards. You could, of course, just let those boards run as long as you'd like. An additional couple of inches of overrun would add support for corner brace boards.
Since the length of our sides was 74 inches, there was a tendency for them to want to bow. That's why it's important to have a guideline and clamp the board in a couple of places to assure it remains straight.
Step 3: Clean and Seal the Form
Tape off the insides of the walls and base using blue painter's tape. Leave about 1/8 inch on either side of the seams. Apply the caulk and run your finger down the line to press it in and smooth it out. (What...you thought you were going to do a project like this and not get dirty?)
Remove the tape as soon as all the caulking is applied. You don't want it to dry on the tape or you risk it tearing as the tape is removed. Let the caulk cure for a day then clean up the inside of the form with rubbing alcohol.
Step 4: Prepare the Inner Reinforcement
We set the rebar on styrofoam blocks about an inch thick to keep them in place as they were tied off. We used 1 5/8 inch drywall screws in the base, spaced every 16 inches or so. Use any type of sturdy wire to tie onto the rebar then hook onto the screws. The wire should be twisted around the rebar several times then one end clipped off. The other end of the wire hooks to the support screw and will be cut after the the concrete is poured and the end is just pushed down into the wet concrete.
In order to add additional structural support and to hold the rebar in place after the styrofoam is removed, remesh is added. This is tied off with wire to the rebar at several points. If the remesh is cut to where there isn't a solid wire that rests against the rebar, tie it off at the first one available. (See pictures 2 and 4)
Once the grid is fully secured, you can remove the styrofoam supports and the entire grid should hang in place nicely. If it sags at any spot, you probably just need more ties between the rebar and remesh.
When everything is secured and ready, do another thorough cleaning with alcohol. It's a little tougher with the rebar in place, but it's very important since any specks of dirt or debris will put a blemish in the finished product.
Step 5: Pour the Concrete!
You definitely want at least two people when pouring. One works with the mixer and shovels the concrete into the form while the other(s) use their hands to push it into place. Make sure you don't push it too hard over the support grid. You actually want to kind of scoop it under to make sure the remesh is supported.
*One thing we learned is that it's not a bad idea to vibrate the air bubbles out at a couple of different points before all of the concrete is added. You obviously don't want to spend a lot of time because you don't want the concrete to start setting up between pours, but even one time in the middle will help. If you have more people available at this step, one or two can be tapping the form while others are working the concrete.*
Fill the form until it looks like there's a little too much. You will be screeding it off next and don't want low spots.
Step 6: Screed and Float the Concrete
Now is when you'll want to cut your support wires. Don't worry about messing up the concrete, just follow the wires as deep into the concrete as you can (an inch should be fine) and clip them off. Then use your fingers to make sure the remaining wire isn't going to show or protrude. Once they're all clipped, you can go back and fill the spaces and re-screed.
Now you'll use a concrete float to begin finishing. The float is drawn across the surface with the leading edge raised just a bit so as not to cut into the concrete. You'll probably notice water coming to the surface which is fine. The concrete shouldn't be so dry that the float tears it up rather than smooth it out. Float it several times if necessary to get a rather smooth surface.
*Don't be afraid to pull concrete out and re-mix if it's too dry. Our first test form was too dry and ended up with large voids from air pockets that couldn't be vibrated out. If you notice the concrete is too dry when pushing it into the form, pull it back out and put it back into the mixer with a little more water.*
Step 7: Vibrate the Form to Remove Air Pockets
*When using this method, be sure you don't pound too hard. We had three star pattern cracks in the finished top due to the melamine cracking while being hammered.*
You can also use a palm sander without sand paper to vibrate the form, or even a large vibration machine hooked to the form. The longer and more evenly the form is vibrated, the better it will turn out. Our top was pretty well vibrated (apart from the 3 cracks), but the sides could have been done a little more. Just don't underestimate the importance of this step. It's probably the most important when it comes to obtaining a smooth finished surface without voids. Also, fewer voids means less potential for cracking.
We will probably use a professional vibrator if we build another one. This can be attached to the form at the start and run even while the concrete is being poured to keep everything vibrating the whole time.
Step 8: Trowel the Concrete to Smooth It Out
After several hours, you can also remove the sides of the form to trowel the sides if you'd like. For a rounded edge you can use a rounded corner trowel on the top and/or bottom. We just left the corners sharp (relatively speaking) and left the form sides in place for a couple of days. We figured the surface of the sides wouldn't get any smoother with us messing around with them. You'll want to remove the sides of the forms after a couple of days of curing anyway to allow the sides to dry more fully.
When removing the sides of the form, be sure you've taken out all the screws. The boards should begin to pull away with gentle pressure at a corner. Do NOT put any type of prying tool on the concrete or you can mar the surface. You can pry against the adjacent corner board which should give you enough room to grab the board and gently pull it back. If a board really seems to be stuck, double check that there's not still a screw hiding under a spot of concrete on the form board.
Step 9: Flip the Countertop Out of the Form
Step 10: Examine Your Work...Take a Deep Breath, It Will Turn Out OK.
We had also decided that by this step it would only cost us about $150 to build an entirely new countertop so, worst case, we could always build another later if this one didn't end up to our liking.
We began by going over the entire surface with a wire brush. This opened up any small air pockets that were hiding beneath a very thin layer of concrete. This may seem like going backwards, but it enabled the small pinholes to be filled in the next step.
Step 11: Slurry Fill Any Holes or Cracks
Step 12: Seal and Install
To install the countertop, we attached a sheet of 3/4 inch plywood to the top of the cabinets by screwing it into the corner braces from inside. This allows for easy removal of the top later if we decide to replace it. We applied a liberal amount of construction adhesive to the plywood and set the countertop onto it. Again...not a step you want to do without several people assisting. After the adhesive has dried, you'll want to wax the top for the final finish.