Owner Building a Strawbale House
Polished Concrete Benchtops
Well, you get to the kitchen and think "granite would be nice, or perhaps stainless steel". Then you get these options priced, at $3,000 for granite or $4,000 for stainless steel. Next, solid wood would be great, but also expensive. Roll in the option of concrete.
Concrete benchtops can be cast (molded) to any shape you desire, polished to a mirror finish, you can leave them raw, or treat them with natural sealants (waxes and oils), or do the full 2 part epoxy finish. We prefer to reduce the number of nasty chemicals in the house, so no epoxy for us, but we will use a combination wax and oil finish. The benchtop can include a splashback, drainage board, anti-spill lip, and any fancy thing you want. You should note that a concrete benchtop has to be thick - about 60mm.
We bought a book "Concrete Countertops" by Fu-Tung Cheng (check his website) and used it as the reference for our kitchen benchtops.
Our kitchen cupboards were built with the knowledge that a heavy benchtop was going ontop. This information need to be passed on to your kitchen builder, as the flimsiest kitchen cupboards will not support a concrete benchtop. Our biggest benchtop will weigh about 340Kg.
The mold is built from 19mm melamine sheet. This provides a surface that the concrete will not stick to in a cheap (disposable) format. It is easy to work with, and readily available. Internal corners are filled with a small fillet of silicon, shaped into a curve. Exposed chipboard edges are covered with plastic packaging tape. The mold is made to disassemble, to release the finished concrete benchtop.
Reinforcing is installed using deformed bar and some metal mesh, tied together with thin wire (standard reinforced concrete methods). The reo is suspended in the mold with scraps of wire, which are later cut off below the surface of the concrete (while wet, but after vibrating and most working). Like all concrete, the reo needs to be about 2 finger widths (25mm) from the edge of the concrete in all directions. Note that if you have a flick-mixer tap and a sink they need to be close together, and the concrete needs to be reinforced, in which case you ignore the minimum distance ruling and get some reo in there. We have two round pillars coming up from our benchtop (service ducts to the roof) so we kept some offcuts of the pipe and used them in our mold, padded a few extra millimetres with closed cell foam.
You need to set the mold up on a flat, level surface - use a spirit level, winding sticks, and tripple check everything. Also remember how much weight is involved.
Add Interest to the Benchtop
You can imbed objects into the concrete (such as fossils) to be exposed when you polish the surface. This can add a personal touch to your benchtop. You can also use selected aggregate (coloured, opal chips, etc.) to add interest. Coloured concrete is an option too, and con be enhanced by using either standard (grey) cement or more expensive off-white cement. We wanted to insert some brass inlay, specially shaped, but ran out of time in our first benchtop.
In our first benchtop we used white aggregate, so we put just a few black pebbles into the bottom of the mold, which is the top of the bench, to be exposed when we grind and polish the bench. The cement we used was off-white, so the final piece should turn out quite a light white colour.
We also added crushed glass into the concrete mixer (nothing special, smash a few wine bottles into small pieces), and placed a few selected bits (the wine bottle necks) into the mold to create "rings" in the top surface of the bench.
The Concrete Mix and Pour
The benchtop must be crack free, so you want a very stiff (not too much water) concrete mix, and some extra reinforcing helps. To get a stiff mix we used a "water reducer", something that acts as a plasticiser, and an early high strengthening agent and reduces the volume of water you need to add to get the mix to flow correctly in the mold. We used Reobuild 1000. For extra reinforcing we used poly fibres, which are small plastic fibre mesh offcuts designed to be added to the mixer and spread through the wet concrete to reduce and control micro-cracks.
You put the concrete into the mold, spread it around and settle it in (and remove air bubbles) using a vibrator - you could hit the mold with a hammer or hire or buy a real concrete vibrator, but you can use an orbital sander. We wrapped some plastic around the sanding pad and used it directly on the concrete as well as on the mold. After you have removed air bubbles and roughly filled the mold you screed it just like any other concrete and smooth off the surface (which will become the bottom of the bench). Pay attention to the edges as most of these will be partly visible in the finished bench.
Polishing and finishing off
You grind and polish the concrete with a wet grinder. This is usually electric with short circuit protection (earth leakage trip) built in, and tripple earthed. The grinder has a rigid backed disk with velcro hooks on it. The grinding disks are a plastic/rubber compound with diamonds embeded in them, and a velcro cloth backing. You run water through the centre of the pad, too much water and the whole pad "floats" on water (useless), too little and the plastic/rubber wears out really fast.
The process for polishing is to wait about three days after pouring, extract the benchtop from the mold, grind it (course pads 50 to 400 grit), burn off any fibres sticking up (propane torch or similar), work some cement slurry into the voids, wait about 6 days, polish it (finer pads 400 up to 3000 grit).
After about three days of curing, grind the test piece with 50 grit to see if it is hard enough. If it is too soft the cement and sand will wear away faster than the aggregate. If you leave the concrete too long it will get quite hard and will wear out your pads faster. Test grind each day until the sand/cement is going to grind flush with the aggregate, then attack your benchtop.
The benchtop needs to be removed from th mold and turned over, so you are working on the finished top surface (which has been next to the melamine, and comes out of the mold mimmicking the melamine finish). The raw concrete benchtop looks OK, but magic appears with a bit of effort. Work the grinder side to side and then up and down over a smallish area, then move on to the next bit. Work through 50 grit to expose aggregate and get the bench close to the finished look that you want, The next 100, 200, 400 grit passes wil remove the scratches from the previous grinding, but should not have to remove significant concrete. Look at the bench with a layer of water on it to get an idea of the finished product (remember it will get lighter as the concrete dries out over the next month).
They are heavy, so be prepared! Our lightest bench was 105Kg, the next 135Kg, and the heaviest was 185Kg. The 105Kg was ok to move by myself (one end at a time) - but really you want more people helping and probably a lifting device. We used my trusty old engine lifter (see the series of photos). Getting them in place is only part of the battle.
Once put in place (for a trial fit) you need to secure them. For our cooktop bench we used silicon around the edges (where it butted up against other cupboards) for waterproofing and adhesion, plus the cooktop also clamped down, squeezing the benchtop between it and the melamine layer underneath. For the island bench we used "liquid nails" to glue the cement bench doen to the melamine layer below, plus silicon at some edges and silicon to join the two halves together. It also turned out that the island benchtops needed some packers (I used strips of stainless steel and aluminium that I had lying about) to level up the adjoining edges and to ensure good support on the small overhang we have at one end (see the overhaning end). The undermounted sink was installed into a rebate in the melamineunderlayer, and siliconed up to the concrete benchtop.
We chose to seal our concrete (to resist stains) with tung oil and a final polish of wax (just as you would for a wooden benchtop) - although you can choose to use and concrete sealer you want (topical - forming a layer above - or penetrating - soaking into the cement). Lots of options, look around for advise you trust, and weight up hygiene and maintenance and environmentally friendly products.
After that the tap and cooktop was installed, and the benchs were finished.
We were quoted around $1000 to make our benchtops in laminated particle board, about $3000 to make them out of synthetic granite, and $4000 to make them out of stainless steel. So how did we go with our benchtop construction costs ?
That is over $1000 to set ourselves up (ready to make lots of benchtops).
And that is less than $600 for the materials consumed to make one cooktop benchtop (1550*750mm) and a two-piece island benchtop (4000*700mm).
We are ready (and plan) to make more benchtops (bathroom, laundry, patio), for which the material cost will rise marginally (as we run out of old reo), but there is a lot of life left in our diamond abrasive pads, so we will make large savings by not having to buy many (if any) of them. None of this includes any costs attributed to our labour, and it does take some time to do (although most of the elapsed time is spent waiting for the concrete to cure).