Despite the increasing interest in 3D Concrete Printing (3DCP) in construction, limited research has quantitatively investigated the environmental impact that 3DCP brings. The existing Life Cycle Assessment (LCA) studies in the field can be criticized for a lack of clearly defined functional units of comparison, especially considering load-bearing structures. This paper investigates the potential environmental benefits of 3DCP over conventional concrete construction in a case study of structural beams based on a cradle-to-grave comparative LCA. Unlike in the existing LCA studies on 3DCP, this paper employs a carbonation model to account for the potential carbon offsetting from the use-stage of 3DP concrete, which showed to be significant for the results. The assessment includes three-beam designs, each analyzed for both prefabrication and on-site construction scenarios. While 3DCP generally has a higher environmental impact due to the larger quantity of cement employed in the process, the reduction of materials through shape and infill optimization results as valid design principles to reduce emissions. Results show that, while standard designed 3DCP beams perform worse than their equivalent cast beams, lightweight designs for printed beams are promising in terms of reducing the environmental impacts from construction. The paper draws recommendations for future research on material development, e.g. integration of larger aggregates and low-clinker cement, and carbonation-efficient 3DCP design of load-bearing structures, e.g. considering the environmental benefits of carbonation in the design optimization process.