The trickiest part of making concrete counter tops is building the mold. After that all that is required is manual labor and patient finishing.
Concrete kitchen countertops can be made either in place on top of the existing cabinetry or can be poured into a mold and finished off-site before installing on top of the cabinets. This article will describe the basic process for making concrete countertops in molds.
A. Building the Concrete Counter Top Mold
Making a concrete counter top mold means building a mold that is upside down and backwards from what the finished countertop will look like. This is because the concrete will be poured into the mold and then left to sit for a couple of days before being turned over and out of the mold. The concrete resting on the bottom of the mold will become the finished surface of the installed countertop.
Make a pattern for the concrete mold by laying down strips of cardboard or thin strips of wood onto the perimeter of existing cabinetry. Allow for an overhang over the front of the cabinets. Use a hot glue gun to attach the strips together.
Allow 1/8” gap between the mold pattern and any walls to allow a bit of wiggle room when installing. Also lay down strips to mark where any openings for sinks will be. Carefully label pattern, indicating top, left, right, front and back of countertop. Indicate location of any faucets or soap dispensers as well as the centerlines of any sink knockouts.
Each length of counter will require its own mold. Ideally, no molds should be longer than ten feet long. Even at this length a few people will be required to flip the concrete-filled mold.
The easiest material to use for mold making is white 3/4” melamine-covered particle board. The final counter top surface will be very smooth without any troweling required. Flip the pattern upside down onto an appropriate size piece of melamine and trace the outline. Saw along the outline with a saw blade appropriate for melamine.
Use only eraseable pencil to write on the melamine. Some felt pen markings will transfer to the finished counter. On the edges of the mold, transfer the pattern’s markings of left, right, front and back, and label inside with an arrow. Be careful not to scratch or nick the melamine since all imperfections will transfer to the countertop surface.
Every five feet or so there should be a break in the concrete. Pieces longer than this will be more difficult to install and will be prone to stress cracks. To create a break in the concrete, vertically insert a piece of painted aluminum flashing into the mold. To do this, route out a channel into which the flashing can be inserted. Cut the flashing so that its top is flush with the mold’s side walls. As long as the flashing is painted, the concrete will not stick to it.
1. Creating Concrete Countertop Knockouts and Openings
For sink openings, trace the pattern piece that came with the sink onto a piece of dense styrofoam such as that used in insulating walls. The styrofoam will need to be the exact thickness of the finished countertop. With a band saw, carefully saw along the line drawn on the styrofoam.
If an undermount sink will be installed, try to keep the sawn edge as smooth as possible. Wrap the sawn edge of the styrofoam with packing tape, keeping the tape completely smooth.
Any wrinkles in the tape will create small dented lines in the finished exposed sink edge. Have the tape start and finish on the edge of the sink knockout that will be closest to the front of the counter. Imperfections in the final counter will rarely be seen here.
Make all faucet soap dispenser mold knockouts out of PVC pipe or wood of the diameter required for the fixtures. Keep in mind that the faucet knockouts will need to be big enough on the underside of the counter to accommodate wrenches for installing sink fixtures.
Oftentimes, it will be necessary to use two different diameter pipes stacked one on the other – the first, sitting on the bottom of the mold, is a narrower size to fit the actual faucet hardware and the latter sitting on top of the narrow one, is the widest to accommodate installation tools.
2. Attaching the Concrete Counter Top Mold Sides and Sealing
Once all voids have been created and attached to the mold surface, it is time to attach the side walls. Cut melamine sidewall pieces to fit around the perimeter of the counter mold. The sidewall height should be such that the inside mold height is the same as the desired counter top thickness. Pre-drill holes and turn in drywall screws every 6” along the outer edge of the mold and along the corner butt joints.
Now the mold must be silicone sealed so that water does not leak out the edges of the mold after the concrete is poured. Using painter’s masking tape, mask all around the inside edges of both the mold surface, the sidewalls and any dividing channels. Leave about 1/16” gap between the tape and the edges all the way around.
Using a caulk gun, carefully squeeze silicone into the inside edges of the mold. Use just as much as is needed to fill the space between the two adjacent pieces of tape. Use a moistened finger to smooth the silicone to a gentle convex shape. Any bumpy spots in the silicone will result in a bumpy edged counter. Work quickly since once the silicone starts to skin it can not be worked any longer. Remove tape once the silicone has been smoothed.
3. Pouring the Concrete Kitchen Countertop
The time spent making a mold accurate and well sealed will reflect directly on the quality of the finished concrete counters. Once the mold is complete, it is time to move on to pouring the concrete counter tops.
B. 15+ Concrete Kitchen Countertops
#1. Mid-sized trendy limestone floor and gray floor open concept kitchen design
#2. Farmhouse kitchen
#3. Contemporary kitchen
#4. Rustic kitchen idea with light wood cabinets and concrete countertops
#5. Contemporary kitchen idea with concrete countertops and stainless steel appliances
#6. Contemporary dark wood floor eat-in kitchen remodel with flat-panel cabinets and white cabinets
#7. Urban l-shaped dark wood floor kitchen with concrete countertops, stainless steel appliances and an island
#8. Outdoor Light Brown Concrete Kitchen Countertops
#9. Rustic kitchen with distressed cabinets, stainless steel appliances, an integrated sink and shaker cabinets
#10. Mountain style
#11. Inspiration for a timeless kitchen remodel with concrete countertops
#12. Mid-sized industrial single-wall concrete floor and gray floor kitchen idea
#13. Elegant kitchen with concrete countertops
#14. Mid-sized tropical backyard concrete patio kitchen
#15. Industrial kitchen
C. How to Cure, Grind, and Polish a Concrete Counter Top
The care taken in finishing concrete counters can result in a beautiful, highly polished show piece for the home.
1. Curing Concrete Counters
Once the concrete has been placed in its mold and all air bubbles have been released, it is time to let the concrete cure for a couple of days.
If the environment is particularly dry, tent the mold over with some plastic sheeting to keep the concrete from curing too quickly. Rig something up so that plastic hovers over the mold anywhere from a few inches to a few feet depending on just how dry and hot the air is. Avoid direct sun on the curing concrete.
After two days, place two pieces of wood on a flat surface and carefully turn the mold over upside down onto the wood.
Unscrew the mold and pry off the side pieces. Do not pry against the concrete, it is still too soft at this point.
Lightly mist the concrete then let sit for another day. The concrete will continue lightening in color over the next several days but once the concrete is sealed is will take on a darker color – close to what the cured concrete looks like around day three (this is a rough approximation and varies widely depending on the pigments and concrete mix used).
After the concrete has been out of the mold for two full days, grinding and slurrying in any voids can commence.
2. Grinding and Polishing the Concrete Counter Top
There are many possibilities for finishing the counter top. The simplest is to do no grinding at all but to simply slurry in any pinholes or voids that there may be on the top surface of the counter top and then to polish the concrete starting with a 200 grit diamond polishing pad.
The other option is to grind the surface to expose the aggregates (rocks and sands) within the mix. By grinding with a 50 grit diamond polishing pad the larger aggregates can be exposed. By starting with 100 grit, the sands can be exposed. It is a matter of taste as to what looks best. With a practice piece it is possible to try different styles in different parts of the same concrete slab.
If the decision is to expose the aggregate, start the wet grinding two days after the slab is flipped out of its mold. If the piece will only be polished wait for day five.
All grinding is done with water streaming onto the counter top. There are a few ways to accomplish this. If a water-fed grinder is not being used one method for keeping the surface water fed is to poke holes in the bottom of a large plastic container (a large vinegar container works well), place it on the counter top and feed a hose into the opening with the water trickling in. A constant stream of water will wash out of the bottom holes onto the counter top.
For exposing the rocks, work slowly with a 50 grit circular diamond pad attached to the grinder across all areas of the surface until the desired aggregate exposure is obtained. Switch to a 100 grit pad and repeat the process until the desired finish is achieved. Hose off the surface well. Slurrying can be done the next day.
2. Slurrying Concrete Countertop Pinholes and Voids
Usually there will be some pinholes and occasionally a small divot from an imperfection due to a nick on the mold. These will need to be filled with a slurry made from Portland cement, pigment and a bonding agent.
The pigment can be dosed in the same ratio as was used with the concrete mix or a lesser or greater concentration of pigment can be used to create a contrasting color. Use one part bonding agent to 10 parts cement. (For details on the bonding agent, see the countertop supplies article listed towards the bottom of the page.)
Mix the cement powder, bonding agent, the pigment and just enough water to get a consistency a bit thicker than sour cream. Work the slurry into the holes using gloved hands and a putty knife. Some people find simply rubbing the slurry in with hands works best then further working it in with the putty knife. Work quickly as the bonding agent will make the slurry set up fairly quickly. Use the putty knife to scrape all excess slurry off the surface, otherwise it will need to be ground off.
Wait one day, then polish with the next grit of polishing pads, most likely 200 grit. Hose the surface off well after polishing. It may be necessary to do another round of slurrying the next day. One day after the last slurrying, wet grind with the 400 grit and 800 grit polishing pads. Then let the piece sit until it has been out of the mold for a total of 10 days before polishing with the higher grit pads.
3. Final Polishing of Cement Counter Tops
Slowly work through all the grits of diamond polishing pads all the way up to 1500 grit, making sure that all parts of the slab get the same attention. Use the hand pads wherever the electric grinder can’t reach.
The slab should now be incredibly smooth and ready ready for sealing. Once it is sealed the concrete will take on a coloration half way between its dry unsealed state and what it looked like when wet polishing.