Auxetic Steel Grids as Flexible Formwork for Concrete Shells

Abstract

The present climate emergency demands a reduction in carbon emissions, with the expanding construction industry contributing a substantial portion of global emissions. Floor systems with concrete shell designs can reduce replacing the conventional flexural load transfer with efficient membrane action, but their curved geometries are difficult and time-consuming to construct with conventional formwork methods. Characterised by a negative Poisson’s ratio, auxetic grid geometries exhibit synclastic behaviour where curvatures in perpendicular directions are in the same direction. Concrete shells can be optimally designed through form-finding to select the shape where the entire structure is purely working in compression under the applied load. Since there exists a range of synclastic geometries which satisfy form-finding algorithms for shells suitable for floor systems, auxetic grids can be a promising alternative for simple flexible formwork for concrete shells. This study numerically and experimentally explores the possibility of using auxetic steel grids as a flexible formwork for shape-optimised concrete shells. An auxetic grid manufactured from a flat plate was used as a flexible formwork for a concrete shell, allowing the grid to deform under the self-weight of concrete. The experimental geometry was compared against form-found best-fit compression-only shell geometries to assess the suitability of the proposed novel formwork system for shape-optimised concrete shells. A flat auxetic steel grid restrained at four corners transforms into a compression-only shell geometry under the self-weight of fresh concrete due to the synclastic behaviour of auxetic grids, providing a promising simplified method for the construction of compression-only concrete shells.