3D printing of cementitious materials has emerged as a promising technology for transforming the construction industry by reducing material waste, minimizing manual labour, and eliminating the need for conventional formwork through robotic fabrication. From a design perspective, it enables unprecedented geometric freedom, allowing the development of structurally optimized components with significant material savings compared to traditional construction methods. Nevertheless, practical limitations related to early-age material behavior and the geometry of the toolpath itself hinder the direct realization of optimized designs, and addressing these challenges is a central focus of our research. Building on our recent work on optimizing three-dimensional prestressed concrete beams and slabs [1], we develop a shape-optimization--based computational framework to realize these designs by partitioning structures into 3D-printed and cast components, with the printed parts serving as formwork. The approach includes segmentation into printable units and the generation of buildable toolpaths that respect prestressing cable geometry. Buildability is ensured by incorporating material mix properties and robotic fabrication parameters, resulting in an integrated framework that links structural form, material behavior, and robotic fabrication. To further extend the framework and enable topological optimization of the 3D-printed concrete layers, we develop a hybrid formulation that combines a continuous density field with a skeletal representation to optimize printing toolpath topology without relying on casting as a filling material. The method addresses key geometric challenges to ensure numerically robust, fully printable, continuous, and non-overlapping toolpaths, while accounting for prestressing integration and incorporating buildability constraints related to early-age concrete behavior. The resulting optimized designs constitute an additional step toward bridging optimization procedures with the practical fabrication of 3D-printed concrete structures.