Designing 3D printable concrete by integrating the influence of aggregate characteristics

This study systematically investigates the effects of aggregate type, aggregate content, and paste rheology on the printability of concrete mixtures. The intricate relationship between printability, paste rheology, and excess paste content in concrete mixtures is investigated by incorporating three different types of aggregate. Concrete mixtures with higher yield stress paste demand greater amounts of excess paste to maintain the fluidity required for printability, thereby limiting the aggregate content. The binder content required to form the paste for filling voids between aggregate particles and coating their surface is determined by the packing density and the specific surface area of the aggregate. Interestingly, the study reveals that despite having higher packing density, if the aggregate type is characterized by high fine content and low sphericity, the mortar mixtures are printable only at very low aggregate content and require higher excess paste content. Aggregates with irregular shapes and large surface areas demand more paste to ensure flowability, overcome friction, and provide adequate coating, thereby affecting the overall printability of the mixture. Although a linear relationship between paste yield stress and excess paste content is observed in printable concrete mixtures, the relationship is found to vary with the aggregate type. A unique linear relationship is found between the yield stress of the paste and the ratio of paste volume fraction to the surface area of the aggregate normalized by the square of sphericity. This relationship is systematically validated across mixtures with multiple aggregate types, resulting in guidelines for proportioning the printable concrete mixtures. The paste requirement in printable concrete mixtures is determined by the yield stress of the paste and aggregate characteristics, including surface area and shape.