Rheology and application of buoyant foam concrete for digital fabrication

Abstract

In the fresh state, conventional lightweight foam concrete (LWFC) has low yield stress which challenges shape retention and buildability in digital construction. Several literatures attempt to address the rheological performance of 3D printable lightweight foam concrete (3DP-LWFC) in essence. This research presents a comprehensive rheological characterisation with controlled shear rate tests and flow curve tests over different foam volume fractions and densities of 700, 1000 and 1400 kg/m3. LWFC is appropriately adapted for extrusion-based 3D printing in the experimental program by incorporating a small amount of nanopowder (replacing 2% of cement mass) for increased yield shear stress, and calcium sulfoaluminate cement replacing 10% of cement mass for improved thixotropy in the fresh state. Accordingly, this raises the yield stress to 347–812 Pa for 700–1400 kg/m3 LWFC compared to static yield stress below 100 Pa of conventional LWFC, improves thixotropic performance in terms of the rate of reflocculation (Rthix 0.21–3.15 Pa/s) and rate of structuration (Athix 0.06–1.02 Pa/s), viscosity (2.5–3.4 Pa⋅s), and elastic shear modulus evolution. Foam volume is found to significantly influence the rheological properties. To analyse the constructability, shape retention and buildability are investigated, resulting in up to 15 deposited filament layers to be reached in a buildability test. Lastly, a practical example is presented whereby a façade element is printed with 3DP-LWFC at a wet density less than 1000 kg/m3, yielding a lightweight element with buoyant characteristics that further expands the application potential of 3D concrete printing.