Concrete remains one of the most environmentally impactful materials in global construction; however, emerging strategies can mitigate its environmental footprint and even enable the creation of carbon-sequestering concrete elements. Soft Shells is a design research project that explores the integration of digitally controlled fabric formwork and CO₂-sequestering concrete mixes to develop low-carbon, high-performance architectural elements. As an initial exploration into sustainable alternatives, this work proposes a locally implementable and globally relevant approach for reducing embodied carbon while expanding the expressive potential of concrete architecture. The formwork system uses stretchable textiles whose knitting patterns are designed computationally to deform predictably under the gravitational hydrostatic pressure of wet concrete. By adjusting local material properties via CNC knitting or incorporating localized reinforcement, we can guide how the fabric stretches, sags, and reacts under load. This digitally informed form-finding process generates complex, doubly curved shell geometries that are structurally efficient and formally intriguing. Crucially, these geometries increase the exposed surface area, significantly enhancing the rate of CO₂ absorption during and after curing. Fabric formwork improves fabrication efficiency and sustainability because it is lightweight, reusable, and uses minimal material, reducing both waste and casting costs. Its flexibility and compatibility with digital workflows make it well-suited for resource-limited construction settings, as well as customizable or modular systems such as panels, vaults, and compression shells. To evaluate environmental performance, we tested a series of carbon-absorbing concrete mixes, each employing a distinct sequestration additive. Each additive represents a unique approach to CO₂ uptake. Multiple concrete shell specimens were fabricated using these additives, along with a set of control specimens using traditional mixes without additives for comparative evaluation. We conducted CO₂ absorption tests using calibrated CO₂ meters. The results confirm the hypothesis that surface-area amplification through fabric-formed geometry, in combination with targeted additive use, measurably improves post-cure carbon sequestration.

Thin concrete shells are efficient with their capabilities to carry loads through geometry rather than material mass. However, they are rarely built today due to the costly, wasteful formwork. This paper tests a simple, CAD/CAM-friendly alternative that programs a standard woven textile. The main goal is to cast shells that become thick where demand is high and thin elsewhere. We start from a 2D structural field (topology regions) indicating where material should accumulate. Those regions are converted to stitch paths and stitch densities. Then they’re sent to a CNC lock-stitch machine. The stitched textile is tensioned on a bending-active plywood frame and used as the casting surface. Stitches locally stiffen the fabric and steer wet concrete during the pour. Areas with few or no stitches sag and thicken, while dense stitched areas stay thin. We fabricated one unstitched control and three stitched one-way shell strips at different stitch densities (1:20 scale; expansion cement (Rockite) mix held constant). After curing, each piece was captured with a mobile, image-based 3D scan. Five transverse sections and one longitudinal section per prototype were captured to check whether thickening occurred under the intended stitch regions. Results show that stitching can guide section changes. The medium-density specimen produced a clear mid-span rib that matches the analysis band and remains continuous along the span. It supports pocket-controlled pooling without blocking flow. The low-density specimen showed weak ribs (stiffness too low), while the high-density specimen sometimes bridged over stiff bands (stiffness too high). Overall, the method provides results for repeatable, analysis-aligned thick-thin sections using inexpensive equipment and standard fabric. Limitations include qualitative thickness reads and the need to tune base-fabric elasticity versus stitch density. Future work will add quantification, refine stitch spacing to avoid bridging, and couple section control with load testing and vaulted-slab components.

Concrete is the most widely used construction material globally. Despite its versatility, it is typically poured into stiff, rectilinear formwork that restricts formal exploration and leads to considerable material waste and higher carbon output. Fabric formwork offers an alternative in which flexible textiles shape fresh concrete into structurally efficient geometries such as thin shells and catenary arches. However, a persistent challenge remains that forms optimized in tension under gravity often crack when rotated into their final compression orientation. Previous research has focused on form-finding and fabrication workflows, with little attention to damage-free reorientation. This paper addresses this gap through two contributions: a CNC-milled repositionable frame with soft-to-rigid connection details enabling controlled tilt-up reorientation without damage, and a scalar reframing that embeds small repeating catenary units within larger building components such as walls and slabs. The research pursues three objectives: (1) to design and refine compatible textile–concrete combinations, with particular focus on non-woven geotextiles; (2) to develop a CNC-cut, repositionable frame system that redistributes stresses during reorientation; and (3) to devise robust soft-to-rigid connection details that permit safe demolding and handling. Through material testing and iterative prototyping, the study identifies concrete paste–geotextile pairings that produce high-quality surface finishes. A tilt-up method was developed where the frame rotates with the arch, minimizing tensile stress. Results demonstrate that catenary arches can be cast, released, and reoriented without cracking or damage. These findings advance fabric-formed concrete toward low-tech, materially efficient structures with reduced environmental impact.