Prefabrication experiments - 63 - Tunnel forms for poured concrete

Achieving visual quality, structural efficiency and long term durability in Concrete construction relies on three interrelated factors: the mixture and ingredients’ appropriate proportions, quality formwork and a monitored hydration process. An imperfection in one of these elements impairs the finished product, which is only revealed after it is complete. This makes quality control in concrete important but somewhat of a hit-or-miss. Conceivably the most demanding of the three constituents is quality formwork, as it is dependent not only on precise, robust and informed shapes and materials but on the workmanship of carpenters and steelworkers.

Developments in formwork, from vertical slip forms, to collapsible forms to tunnel forms and reusable forms, industrialized the production of concrete structures and made on-site construction more efficient. Although industrialization frequently connotes building in a factory, in the case of concrete construction, the construction site often becomes the factory. Particularly in slab and panel structures the site’s flexibility eliminates obstacles such as transportation and plant dimensions. Furthermore, hydration and curing time requires large spaces and produces a logistical issue in the factory or on the factory’s site.

Tunnel forming is a particularly efficient process as walls and floors are poured in one single step. This type of formwork came into productive use during the second half of the twentieth century, notably in relation to France’s concrete building culture and its important social housing projects. Tunnel forming uses an inverted u-shaped rectangular prism. Juxtaposed form-units generate inhabitable shelf-like spaces. The U-shape allows for vertical and horizontal planes to be poured at once. Rectangular Spans vary from 5-6 m in width to 10-12 m in depth with an approximate thickness of 200mm. Once preliminary hydration is completed and the concrete reaches necessary strength, the tunnels could be moved, raised or glided into their new position.

Patented in 1974 by the Societe Outinord-St-Armand (US 3979919 A), their system for a mobile a prismatic form combined the advantages of slip forms with tunnel forming to produce repetitive units transferring the advantages of mass production to the construction site. Although this repetitive building was not what modernity sought out for it to become, an answer to society’s problems, it did however influence an on-site factory model for concrete construction.

Patent drawing from patent US 3979919 A