The trickiest part of making concrete counter tops is building the mold.
After that all that is required is manual labor and patient finishing.
This article will describe the basic process for making concretecountertopsin molds.
In this article
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The concrete resting on the bottom of the mold will become the finished surface of the installed countertop.
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.
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 molds side walls.
As long as the flashing is painted, the concrete will not stick to it.
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.
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.
Imperfections in the final counter will rarely be seen here.
Cut melamine sidewall pieces to fit around the perimeter of the counter mold.
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.
Once the mold is complete, it is time to move on to pouring the concrete counter tops.
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Avoid direct sun on the curing concrete.
Do not pry against the concrete, it is still too soft at this point.
Lightly mist the concrete then let sit for another day.
Grinding and Polishing the Concrete Counter Top
There are many possibilities for finishing the counter top.
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 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.
A constant stream of water will wash out of the bottom holes onto the counter top.
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.
These will need to be filled with a slurry made from Portland cement, pigment and a bonding agent.
Use one part bonding agent to 10 parts cement.
Work the slurry into the holes using gloved hands and a putty knife.
Work quickly as the bonding agent will make the slurry set up fairly quickly.
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.
Use the hand pads wherever the electric grinder cant reach.
The slab should now be incredibly smooth and ready ready for sealing.
