This research presents an alternative approach to constructing compression-only structures by introducing an innovative in-situ construction approach. This approach involves assembling lightweight 3D-printed plastic formworks into a compression-only configuration for the casting of concrete. To overcome the issue of hydrostatic pressure, instead of casting the entire structure at once, this research adopts sequentially casting concrete. This is achieved through developing: 1) a new construction method based on the assembly of lightweight 3D printed plastic formwork in compression-only form, which is then sequentially cast with concrete in place; 2) an integrative formwork system informed by structural and casting logics (i.e., flowability, the height of the casting point in relation to hydrostatic pressure, placement of formwork interface in alignment to internal forces in the structure;  3) a parametric model for translating force-form diagram to integrative formwork. To verify this research method, Cavity Shell, a 1:1 scale compression-only table leg structure measuring 1.4 meters in radius and 0.8 meters in height, was designed and built. This ultra-lightweight formwork took less than 48 hours to assemble and sequentially cast by two people. This research demonstrates the potential to rethink the construction of compression-only structures by minimizing the material used and improving economic and environmental efficiency in their construction life cycle.

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