Forming Heterogeneities: Fabricating Graded Low-Carbon CharCrete Building Elements

As the construction industry seeks sustainable alternatives, functionally graded concrete (FGC) offers a promising approach to optimising structural performance alongside environmental performance. This paper investigates multi-material fabrication strategies for building elements with a low carbon footprint made of CharCrete, a cementitious composite material leveraging the carbon sequestration capability of biochar. The design and fabrication method “Grading Carbon” is part of a larger study that aims to balance structural integrity with embodied carbon reduction. The paper focuses on two fabrication strategies: “print and cast,” using a cementitious 3D-printed formwork, and “partition and cast,” where physical partitions facilitate simultaneous multi-grade casting. Experimental results highlight the critical role of bonding between grades, with sequencing and deposition timing proving essential for preventing delamination. Computational design tools and Finite Element Analysis (FEA) are used to simulate the performance of building elements with a heterogeneous materiality. Combining BCM_B10 (high-performance printable) and BCM_B30 (carbon-sequestering) grades can achieve a balance of mechanical strength and environmental impact, resulting in elements with a net neutral carbon footprint. CharCrete 3D printing (CC3DP) has been developed and evaluated. The assessment indicates that lower biochar grades (BCM_B0–B20) are suitable for printing, while higher biochar grades necessitate casting due to increased water content and longer setting times. Mechanical testing confirms that the “print and cast” method is more efficient compared to the “partition and cast” one as it offers lower fabrication costs, greater customisation, and reduced environmental impact. Computational design workflows integrate embodied carbon considerations into the design-to-manufacturing process, providing a scalable approach for sustainable construction. The findings contribute to the advancement of graded material systems, demonstrating the viability of biochar-enhanced FGC for adaptive, high-performance building applications.