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The context for the construction of the SUDU – according to scientific director of the EiABC, Dirk Hebel – is an “international research laboratory” for the development of this low-cost, sustainable housing prototype. But what constitutes the aims and the challenges of an International Research Laboratory? The outcomes are as much embedded with concerns of the technical as they are with the cultural.
One may recognize this phenomenon in the simple terms of critical materials acquisition: The thin-tile or timbrel vault construction technique relies on the use of a rapid-setting plaster-of-paris (or gypsum) mortar, which allows the first course of masonry to be set in cantilever from the masonry surface without the support of formwork from below.
Photo by C.Lippuner
If a cement mortar, for instance, were used for such purposes, the mason would have to stand in one place on the scaffolding for several days to support the brick while the mortar dried. This is perhaps an absurd picture, which nevertheless conveys the importance of plaster mortar in this technique: because the mortar cures so fast, no formwork is required from below. Nevertheless, if a team of masons could only mix the mortar for one brick at a time, it would take months to built a vaulted structure. The efficiency of the method lies in its rapid-setting properties, which may be strung in small batches to maximize efficiency in the work of the masons. Thus, the properties most ideal for the raw plaster material is that it sets rapidly, while remaining workable as long as possible to set many bricks in one batch.
In most places of the world, plaster is very a common construction material – indeed, plaster is also produced in Ethiopia. The properties of acquired materials, however – very much like cement – are simply not reliable without engaging the levels of both technical and cultural investigation. With all industries, in the West as in the developing world context, there are manufacturers with reasonably trustworthy products and manufacturers of the lower order where one cannot rely on material properties. The best respected of plaster manufactures in Addis Ababa is a company called “Sede”. However, the constraints of production, and the competition in the market significantly complicate the acquisition of this material. If one goes to Merkato (the largest market in Africa, based in Addis Ababa), and one purchases plaster in packages labelled “SEDE”, one is sure to have acquired a knock-off using the name of Sede to sell its product. This is common knowledge to the local who works in construction. Sede, though producing the best plaster, does not produce paper bags for its packages. It may use plain brown paper packages, but it often – as in our case – uses the packages of the plaster manufacturer “Mughal”. In this respect, it is quite impossible to find the material one is actually specifying in Addis Ababa, unless one is to investigate more fully.
Plaster is most produced in Ethiopia for wall plastering, the dominant use in many other regions of the world. Yet the working qualities for wall plastering is indeed quite different from the qualities required for timbrel vaulting. The former, for instance, is often combined with fine sand and is optimized by both manufacturing and working technique to dry very slowly. Since the properties of this material are so critical for our purposes – and since the first tests failed to adhere even one brick with such low-order, sand-doctored plaster as acquired in the markets – we invited a representative of Sede to the construction, so that he could observe our method of working and thus more fully understand our required working properties. With tall graduated glass cylinders from the laboratory, the plaster was mixed to demonstrate the poor results. I then mixed the plaster brought from Sede, certainly a pure plaster, and the results were better, though not entirely uplifting. It would take a long time to train inexperienced masons which such a finicky material. The Sede employee then demonstrated the techniques recommended for their typical wall application, which seems to have the exact opposite effect as desired, setting was very slow but drying was very fast, meaning that the plaster rapidly lost its plasticity, while still taking a very long time to fully set. With these experiments in mind, and with a better understanding of his part on how we needed the material to perform, we discussed possible manufacturing techniques which would allow the material to set more rapidly and remain plastic. I suggested that the material be burned longer, or alternately at a higher temperature. He concurred that the company’s techniques to produce the best slow-setting plaster could be reversed to produce more rapid-setting qualities. It should be dually noted here, that the Sede representative spoke no English, and I spoke no Amharic, though we fortunately had some reliable translation.
And, with the days counting down in our limited period for construction – it was a tremendous relief that this little experiment worked. One material sourcing crisis was averted, but the implications of future construction is clearly enough demonstrated. The research that must be done to implement this technique at a larger scale – well, it must be pursued also across cultural lines.
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