|C O L I N L E W I S B o n s a i A
W O R K S H O P P R O J E C T S
|C O N S T R U C T I N G C O N C R E T E S L A B S|
Why a cement container?
As western creativity and tastes play an increasing role in bonsai design, it is inevitable that we should look for alternative, one-off, container designs. Very few have the facilities to fire ceramics, and fewer have kilns large enough to take even medium-sized bonsai pots.
Additionally, large group or forest plantings, as well as some single-tree designs, require a slab rather than a pot. Good slabs of the right thickness and shape are very hard to find.
Making them with cement couldn't be easier!
|What kind of cement?
Ordinary Portland cement is perfectly strong enough for this work if properly reinforced . However, it takes days to become dry enough to handle without crumbling, during which time any slight movement will weaken the inherently thin structure.
Far better to use a "quick-drying" cement. This is what builders and construction types call high aluminum cement (or high alumina cement, depends where you live). This is much easier to obtain. Any builders merchant will be able either to sell you some or point you in the right direction. It hardens sufficiently for the piece to be handled very quickly, anything from twenty minutes to an hour.
QUIKRETEŽ Quick-Setting Cement (No. 1240) dries in 10 - 15 minutes, which may be a little too fast for our purposes here. If you can't find it conveniently, or if you prefer something that dries a little slower,, here's a trick I learned: Get a bag of QUIKRETEŽ Fast-Setting Concrete Mix (No. 1004) - the stuff in the red bag that's used for anchoring fencing posts, available in all hardware stores. Sift out about a few cups full of the fines and mix this with half a bucket of your regular DRY Portland cement. This will be enough to cause the regular cement to harden in about an hour.
What else will you need?
Reinforced concrete derives its strength from the combined high compression strength of the concrete and the high tensile strength of the steel reinforcement. Imagine a concrete slab with steel rods running through it, just below center (diagram A). If a force applied from above is to break the slab, the steel rod must stretch and the concrete must compress. Since neither will oblige, the slab will not break.
If the steel rods are too close to the surface, the slab breaks easily because its strength now relies on the weakest properties of both materials. The steel rods will bend and the concrete will fracture (diagram B).
kind or reinforcement?
Furthermore, fixing relatively dense fabrics such as fiberglass over the wire to support the cement will in effect create two separate layers, which will have no strength at all.
Ideally you need a galvanized steel mesh, or 'hardware cloth' with, say, a half by one-inch dimension. Then go find yourself some tulle. Some what? Some tulle - it's a very fine fiber, usually synthetic these days, woven to an extremely open, delicate gauze. It's the stuff that ballerinas' frilly things are made of. You can use any very fine netting, but the type used for grannie's curtains is too closely woven and will weaken the piece. A 2mm mesh is ideal.
Making the reinforced framework
Constructing the mesh frame and fixing the netting will take several hours at the first attempt, depending on the complexity of shape. Don't rush this stage - make sure the shape's right before "casting it in stone"....
of all you will need to cut the mesh to size and mold it
to the desired shape. It is a good idea to get your
experimenting done beforehand by cutting paper or card patterns. Use
your imagination by all means, but try to keep the shape
It may be necessary to join two or more pieces of mesh together. This is perfectly alright and won't weaken the job, provided that you remove all excess wire and loose ends. Join the pieces with very fine wire, such as single strands of telephone cable. Too many wires bunched together will create a weak point.
Any free ends around the rim should be bent to form a more or less continual line, allowing for about 9mm of cement to be applied before the final dimension is reached.
Coat the wire with quick-drying adhesive. Any kind will do so long as it will stick to the wire. Smooth the netting over the mesh, ensuring it sticks reasonably uniformly, especially around the edges. It doesn't need form a really firm bonding, just enough to stay put for the time being.
Some people stitch the fabric to the mesh. This is entirely unnecessary and mind numbingly tedious.
Trim the edges flush with the mesh and wait till dry.
Before you proceed
Apply the first layer ...
cement is applied, it adds weight to the structure and
will soon begin to bend the wire mesh out of shape. To
minimize this problem, work
around the edges first and stop as soon as the mesh
begins to distort. Wait until the first section has
become firm (but not dry to the touch) and then fill in
the remaining areas, bit by bit if necessary. Using
blocks or wedges to support the center of the framework may help. When almost dry, lightly
abrade the surface with an old brush
to provide a key for the next layer.
... And the second ...
As you work
around the edges, begin to extend the rim beyond the
mesh framework. When the slab is finished, the cement
should extend beyond the wire by 6 - 9mm. Too much and
it may break off easily. Too little and the
temperature-induced expansion and contraction of the
wire may eventually fracture the cement. (see
...And the third ... and the fourth ...
part was easy - the next bit's trickier!
Whatever you decide, don't over-do it. The idea is to make the slab look natural. Too much texture or too strong a color will will make that already difficult task almost impossible. Resist the temptation to try to make your slab look like a mountainside on your first attempt. It's very difficult and the result often ends up as plain old kitsch. Try to make your artificial slab look like a real one. Once you have gained experience with finishes and textures you can become more creative.
final textured layer has become almost dry (lighter in
color but not as light as the trial piece you did last
week) you can color the surface if you wish. Use very
dilute spirit-based paints or wood stains, applied in
several thin coats. This will soak into the
surface of the uncured cement and will be more or less
permanent. Try a flat layer of one color, followed by
patchy layers of other colors - browns, greys, muted
greens, even a splash of purple here and there. But
always VERY dilute, you want to color the
cement, not coat it.
Is that it?
Not quite. There's one more thing you have to do ... Wait!
As I explained earlier, cement takes several weeks to cure (in fact, in theory, it never completely 'dries'. It continues to harden slowly throughout its lifetime). You must wait at least a month - the longer the better - before planting on your slab. The larger the slab, the longer you should wait. Leave the slab outdoors so the rain will wash the surface clean of colorant residues. Some recommend sealing the cement to stop lime from entering the soil. These days there is very little significant free lime in cured concrete. Anyhow, if you make your slab in the autumn and leave it out in the weather all winter, it will certainly be ready for planting come spring.
Oh, by the way, this may sound daft, but keep the slab frost-free for a month or so. If the tiny amount of moisture in the concrete freezes before it's cured, hairline fractures may occur. These won't be a problem at first, but in years to come they will inevitably begin to deteriorate.