Design and construction of concrete shells using semi-flexible auxetic grids as formwork

Concrete shells can be efficiently designed through form-finding techniques to create shapes where the structure functions predominantly in compression under the applied load. While such concrete shell floor systems can reduce material consumption by replacing traditional flexural load transfer with efficient membrane action, their construction is challenging with conventional formwork methods. This study conceptualises, develops, and experiments the use of flat auxetic grids as formwork for casting compressive-dominant concrete shells. Due to their negative Poisson's ratio, auxetic meshes exhibit dome-like synclastic behaviour, curving towards the same side in all directions when subjected to out-of-plane deformations. Therefore, the feasibility of transforming a flat auxetic grid into the most efficient shapes for concrete shells solely under the self-weight of concrete or with additional constraints is explored in this study. A parametric nonlinear finite element model linked to an evolutionary optimisation algorithm is developed to design a flat auxetic grid that deforms to approximate a form-found compression-dominant shell geometry. The construction of concrete shells using the designed auxetic geometry is experimentally demonstrated. A form-finding algorithm coupled with an evolutionary algorithm is also used to verify that the experimental geometries and the deformed finite element models of different auxetic formwork designs approach compression-dominant shell geometries. Results show that semi-flexible flat auxetic grids can be feasible, versatile, reusable, and less bulky as a formwork system for casting concrete into cast compression-dominant shell geometries.