Building upwards – concrete ceiling slabs

Kurt Rhyner   

It is a well-known fact that the large cities in most developing countries are spreading at a dangerous rate. Formerly rural areas are being developed or simply invaded, and the urban sprawl limits the possibility of its inhabitants for natural recreation activities. It also creates incredible costs for the urban society. The services that eventually have to be provided (roads, water, sewage, electricity, security) are much more costly in a horizontal growth pattern than in a vertical one. Over the centuries most towns have grown vertically as much as horizontally, and it is only in recent decades that many societies have adopted the “North American Style” of ever growing suburbia and of relinquishing the inner cities to business and low level / low cost apartments.

However, for many people in cities, as well as small towns, it is a fact that they have to expand their living space vertically. When they plan to build a second or third level on their house, one of the most difficult problems is to find a ceiling that is affordable and easy to build. Timber ceilings have become expensive in most places and they do transmit a lot of sounds unless they are very well done. Concrete slabs are complicated, slow and expensive. Existing systems of “beams and filling elements” tend to be in the hands of big industry and in most cases are not easily available.

Thus, several members of EcoSouth have put their minds toward creating an easier and less expensive system. We would like to present to our public some of the solutions developed and put in practice.

Jorge Acevedo at CECAT in Cuba developed a modular system that provides a concrete slab, based on beams and vaults between them. As the beams are poured on site, together with the concrete that forms the vaults, the result is a slab that acts monolithically, which is an important safety factor in earthquakes.

This system actually saves on steel and on concrete in relation to all other systems that we know. As the total height of the slab is high (typically 18 to 20cm), a relatively small amount of steel is needed, but at the same time the vaults save on concrete. The final appearance of the ceiling from underneath is very good, and in first commercial applications in Nicaragua and the Dominican Republic simple painting rendered an attractive look.

The forms for the vaults are of microconcrete produced on a “tevi” vibrating table. They should be about 12 to 14mm thick and are placed on a mould that gives them the ideal curvature. Several experiments were made and it was found advisable to use those microconcrete forms as lost casing and not demould them from the slab, as they guarantee a smooth ceiling.

The builders found it relatively cumbersome to place formworks for the beams and then the vault forms on top, but after some initial training they mastered the technology. However, Martin Melendez of Sofonias Nicaragua introduced the partial prefabrication of the beams in a second building. The beams are prepared on the construction site and then placed on the walls. Of course they need to be supported until the concrete has hardened. Now the installation becomes much more efficient and easier.







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Consultants

Pedro Seijo
Cuban
Professor of Civil Engineering
Spanish, English
Teaching, ecomaterials workshop management, research (municipal waste)

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