Intro: Basic Concrete Countertop
This instructable shows how to build a simple concrete countertop. There are several sites on the net with good instruction and lots of details, including one here on Instructables. This project is much more basic for those who may be interested in trying to build one but want to start with something simple (relatively speaking). The countertop was for my friend's new kitchen island and was built by just the two of us with no prior experience. There are no bells and whistles, just a plain rectangular top 74 inches x 37 inches, 2 inches thick.
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
The basics for making a concrete countertop are simply building your mold, pouring and finishing the concrete. There are several acceptable materials for building the form, but we chose cheap and easily obtainable...melamine coated particle board. You'll need a piece larger than your desired finished dimensions, we chose a 4 foot x 8 foot piece 3/4 inch thick. Other items needed are:
- 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
Once you've measured and marked out your countertop on the melamine, it's time to set the sides. Building a simple rectangle made this pretty easy. Naturally, you'll want the sides to be as tall as your finished thickness, in this case 2 inches. The sides were cut to length, clamped in place, and screwed down to the base using 3 inch drywall screws. Pre-drill the walls or they WILL split. Be sure the heads of the screws are sunk low enough that your screed board won't catch on them. The particle board was soft enough that we didn't bother using a countersink bit to drill the holes, but if you have one, go ahead and use it.
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
Now that the sides are ready, you need to seal all of the inside seams with silicone caulk. Pick a color that stands out against the melamine.
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
Next you're going to set your reinforcement grid. This is one of the longest and most important steps. We were planning for a 10 inch overhang from the cabinet base on three sides so we added 3/8 inch rebar around the edges. You'll probably want to do this regardless, especially with something this large. We didn't bother with trying to bend the rebar around the corners, we just cut each piece about 4 inches shorter than the side. We wanted to maintain a 2 inch setback so we didn't get ghosting when the concrete cured.
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!
Now it's time for the concrete. While small projects can be hand mixed in a 5 gallon bucket (as we did with our test forms), you'll want a concrete mixer for this size. We rented ours from Home Depot and were able to get it back within their 4 hour time frame. The concrete should be mixed to the "consistency of oatmeal." But...how thick do you eat your oatmeal? Basically, you want the concrete to be dry enough to minimize potential cracking during curing, but thin enough to work with. We found that it was better to err on the side of being a little wetter than too dry. (And I said a little wetter). There are different ways of coloring the concrete, we added pigment during the mixing.
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
Using a long, straight board, you now screed across the concrete with a sawing motion. This pushes the concrete into the form more and levels everything out. There will be spots where the screed board will come up to the wire ties, at this point you can just lift over them and continue screeding. You'll be knocking off a lot of extra concrete and probably finding some low spots or holes. Use the overflow to fill in those spots and continue screeding until it all looks uniform.
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
Now it's time to make sure your finished countertop looks nice. While the concrete is still wet, you need to get the air pockets out. This can be accomplished in different ways, the most low tech being a rubber mallet. Simply go around the entire form and gently tap on the sides and bottom to bring the bubbles to the surface. You'll have to spend a bit of time in every area to assure that they come up. As they do, you'll see a bubble(s) grow until they pop. These can then be re-floated to fill and smooth them.
*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 the concrete has begun to set up, it's time to trowel it. We used an aluminum rectangular trowel with gentle pressure to remove any lines left from floating. This will end up being the bottom of the countertop, but you still want it as smooth as possible so it will sit level on your cabinet base.
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
After 7-10 days of curing, you need to get the countertop face up so you can finish it (and in our case, do any patch-up work necessary). Again, our finished top weighed about 400 pounds, so we needed four guys to do this. We placed some cardboard against one side and tipped the countertop up onto its side on the cardboard. This allowed us to slide it to the other side and then gently lower it back down, now resting top up.
Step 10: Examine Your Work...Take a Deep Breath, It Will Turn Out OK.
Hopefully you've learned from our mistakes and your countertop looks perfect at this point. If you decided to build yours exactly like ours(mistakes and all), you'll need to patch up any cracks. These three cracked areas resulted from the melamine being tapped too hard during the air bubble removal. The surface looked great otherwise. It dried with an almost granite look to it from the black tint we added during mixing. (The large lighter spots in the pictures are from the flash). While these cracks were concerning at first, we decided it would be relatively easy to attempt to repair them, as well as being a good learning tool for others who may experience something similar.
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
It's important to know your color to concrete ratio if you're using tint in the mix so you can mix a slurry with the same ratio. We did two slurry passes: the first made with the same Quikrete 5000, the second with Fu-Tung Cheng's slurry mix. This is applied with a trowel or putty knife, allowed to dry, then sanded down to smooth it. The holes and cracks filled nicely, and while the top was nice and smooth when we finished, the thing we noticed was a lack of the same "granite" look to the finish in the 3 large cracked areas.
Step 12: Seal and Install
The final step is applying the finish of your choice. We used a food safe polyurethane.
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.
Step 13: Finished!
Hopefully this has encouraged you to go ahead and try to build your own concrete countertop. We learned a lot through the process and are considering possibly doing another one in 6 months or so. While this project does take a fair amount of time as far as drying and so forth, overall, we didn't have a lot of man hours in the actual construction. Have fun, get creative, and read about additional techniques for personal touches you can add to your top. No matter how it turns out, there's a lot of satisfaction in knowing you built it yourself.