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How to make dome-shaped SanPlats
SanPlats can be made in different shapes and
sizes. The size will depend on the moulds used. The method for making them is, however,
the same.
Left--Circular SanPlat 1.5m
diameter; dome-shaped SanPlats are used where wood is in short supply
Right--Circular
SanPlat 1.2m diameter
Moulds
The following moulds are required; the number depends on the daily production:
- Girdle moulds (one per slab and day)
- Drop-hole moulds (one per slab and day)
- Arch moulds (one or two should be enough)
- Foot-rest moulds (one or two should be enough)
Tools
As for the ordinary SanPlat, a set of normal mason's tools is a minimum requirement. The
number of tools needed depends on the daily production.
Ordinary mason's tools are used for making SanPlats
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- One or two shovels or a hoe for mixing the concrete
- Mason's trowels, one large and one small
- A steel-float (floor trowel) for finishing off the surface
- A wheelbarrow for transport of material
- One or two buckets for measuring the material
- A hammer for various purposes
- A hacksaw or a chisel for cutting the reinforcement bars (they
can also be cut with a hammer against the edge of a pick-axe or any other sharp edge)
- A piece of a water pipe is useful when bending rein-forcement
bars for the handles.
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Material
The SanPlat slab is made of unreinforced concrete. To avoid unnecessary breakages the
material should be clean and of good quality.
The quantities will depend on the dimensions. You will need:
- Normal cement (standard Portland)
- River sand
- Gravel (12mm or similar)
- Plaster sand (if cheaper than river sand) can be used for the
moulding. Plaster sand can also be used to 'modify' the river sand if it is too coarse
- A roll of sisal cord or any other string for tying the ends of
the girdle mould together unless the gridle clip is used. (Iron wire is not recommended as
it will damage the girdle mould when it is pulled with the pliers.)
- 6mm mild steel reinforcement (for the handle of the lid).
The casting yard
Before you start, make an assessment of the area and the use of the space in your casting
yard. You will need a flat, hard and smooth surface for mixing the concrete and later you
will need space for spreading out the slabs for making foot-rests and lids. Finally you
may need some space for storing finished slabs.
The example illustrated is designed for a daily production of five to
ten SanPlats of 1.5m diameter. In selecting a site you should try to find a place which
people normally pass by. Seeing the slabs being cast makes people curious and is very
effective publicity. In planning the site, you must consider carefully where and how to
off-load the site vehicles, as heaps of sand are difficult to move. Pay attention to the
vehicle turning radius and to the strength of the road as manoeuvering large heavy
vehicles may cause damage. The mixing area should be horizontal and fiat. To make cleaning
easy, it should be very smooth arid have no elevated edges.
Note:
Remember that testing is required for all domeshaped sanplats.
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Making the slab
Good workmanship is always appreciated. It is also a quality which is important for
hygiene and cleaning, as people prefer to care for a SanPlat which is smooth and well
shaped. No part of any surface must be rougher than the surface of a well-stretched-out
palm of a hand and the shape should always be the same as that of the moulds.
Keep the moulds clean. Remove concrete leftovers before they get too hard.
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1. Start by
placing the peripheral girdle mould in a circle on the ground and tie the ends together
with a piece of string and fill with sand. |
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2. Place the
drop-hole mould in the middle on a heap of damp sand and check its position with the arch
mould. |
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3. Well-positioned,
you should be able to turn the arch mould around within the girdle mould. |
| 4. The tips of the
arch should move just inside the edge of the girdle mould. Compact the sand and add or remove sand as necessary until
you have just the right shape. Be careful that the two wings of the arch mould are resting
on sand all the time. |
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5. Now
take the drop-hole mould out of the sand... |
| 6. .... And cover
the hole gently with sand without disturbing the shape of the mounded sand. |
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7. Finally
check the height of edge (the thickness of the SanPlat) and remove excess sand as
required. The thickness at the edge should be 4cm (1.5in) which can be best checked with a
piece of wood of the same thick-ness. |
| 8. Cover the sand
with paper from cement bags or with newspaper starting from the sides and finishing at the
top. The papers should cover the sand as tiles cover a roof to prevent concrete later
flowing between the papers. Fix the papers temporarily with stone or cement lumps so that
they do not blow away. |
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9. Now
place the drop-hole mould in the centre on top of the paper and check the position with
the arch mould. If another slab should be cast on top it is important that the mould is
placed exactly in the middle. If not you will experience problems when rotating the arch
mould. |
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10. You are now
ready to cast the slab with concrete in the proportions 1+2+2 of cement, sand, and
half-inch (12rnm) clean gravel (or finer). Start by filling very care-fully around the
drop-hole and pound gently with the edge of the trowel to make sure that the concrete
fills well and you get a smooth edge around the hole. |
| 11. Spread out the
concrete with the help of a straight batten to ensure correct thickness of the concrete. |
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12. The
inclination inwards around the drop-hole is made by hand 'digging' with a mason's trowel
down to the threshold of the drop-hole mould and possibly 1 or 2mm below that level when
you finish the surface, to make sure that the edge does not break when you later remove
the mould. Pay special attention to the inclination in front of and behind the mould where
some extra concrete may be needed. |
| 13. Now the
surface of the slab can be worked out to final finish, preferably using a steel-float
(floor trowel). You may need to wait for some of the water to be soaked up by the
underlying paper and the sand in order to get a good surface. Some cement powder sprinkled
on the surface may also be helpful. |
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14. After
an hour or two, when the concrete is stiff, the drop-hole mould can be removed. This is
done more easily if you hit the two ends of the mould gently with a light hammer. |
| 15. This
is the right time to make the inscriptions in the slab. Each slab should have the concrete
mix, the casting date, the number of the slab and the initials of the slab maker written
on the concrete as indicated in the illustration on page 00. (The numbers of the slabs
should be consecutive numbers starting from 1, independent of the size and shape of the
SanPlat.) |
| 16. Before
the concrete becomes too hard you should scratch the surfaces of the foot-rests to get a
rough surface for the foot-rests themselves to stick better. Use the foot-rest mould as a
template. |
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Multiple slab making
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A number of slabs can be cast on
top of each other. This saves space in the casting yard and facilitates curing as only the
top slab will be exposed to the sun, and the moisture from the fresh concrete will prevent
the under-lying slabs from drying during the curing time. |
| 17. To make another slab
the girdle mould can be pulled up with a pair of pliers. Before you start to put on more
sand as a base for the next slab, place the drop-hole mould in the centre, in the hole of
the underlying slab. |
| 18. Now you can continue to
make one or more slabs on top of the first one, following the same principles as for the
first. |
Curing
The curing is best done while the slabs are still stacked.
The recommended curing time is one week.
- Make a hole in the paper through the drop-holes and add water.
The sand will soak up the water until the whole stack is well soaked.
- Even if the top slab will receive some water from beneath, the
surface must be protected from drying out. Pull up the girdle mould a bit and cover with
more sand and water. Keep the stack wet for one week by watering at least once a day.
- After one week the slabs can be removed from the stack and put
flat on the ground for completion with foot-rests and lids for testing.
Making the lid
The lids and the foot-rests are made in the same way as for the small SanPlats. (See
beginning of this chapter for further details.)
Test for safety
| When at least one week
old, the SanPlats should be test-loaded with the weight of six people, when supported by
four diagonally placed wooden wedges as in the diagram (click on the top photo for an
enlarged view of this. All slabs should be test-loaded. When the slab has cured for at
least seven days (check with date inscrip-tion) it can be test-loaded with six people on
top of it, standing in a row along the length of the slab. Start by checking the date
inscription. With a piece of chalk or charcoal, mark out places for a set of wedges to be
inserted, first along the length axis and then at right angles. Insert the wedges,
checking that all four wedges provide good support for the slab. Ask people to stand in a
row on the slab. If the slab does not break, it has passed the test and should be marked
with a small "s" (for security tested). |
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Congratulations on finishing your Domeshaped SanPlat.
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