Making Domeshaped SanPlats
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.
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.
- 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.
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.
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
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.
Place the drop-hole mould in the middle on a heap of damp sand and check its position with the arch mould.- Well-positioned, you should be able to turn the arch mould around within the girdle mould.
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.- Now take the drop-hole mould out of the sand...
- ... And cover the hole gently with sand without disturbing the shape of the mounded sand.
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.
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.
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.
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.- Spread out the concrete with the help of a straight batten to ensure correct thickness of the concrete.
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.- 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.
- 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.
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.)- 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.
- 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.
- Now you can continue to make one or more slabs on top of the first one, following the same principles as for the first.
Multiple slab making
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.
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).