Developing Extrudable Cementitious Binders
Additive Manufacturing (AM), or 3D Printing
Additive manufacturing (AM) or 3D printing is an emerging technology for building 3D structures from a digital model. 3D concrete printing has gained significant attention in the recent past, primarily because of its ability to eliminate formwork for concrete construction, thereby enabling the construction of complex and sophisticated, structurallly- and functionally-efficient geometries. Our work evaluates the rheological characteristics of 3D printable concretes, in an effort to develop better, more efficient 3D printed binders, and to forward sophisticated test methods and models that can capture the effect of time dependence and property development at early ages. Our work also elucidates the influence of printing parameters on mechanical and durability properties of 3D printed binders.
Pre-print and Post-print Properties of 3-D Printable Binders
The study is broadly classified into two sections: pre-print and post-print properties. The fresh properties include rheological (yield stress, adhesion) and extrusion characteristics (steady state extrusion pressure) of the printable binder. The post-print studies include the rheology of the extruded material, stability and mechanical properties of the printed shape, and comparing the actual CAD model to the printed output. Hence, a performance based mix design can be proposed to consider these parameters in deciding an appropriate mix for 3D printing.
Modelling the Extrusion Process Using DEM and Analytical Models
The preliminary study formulated cementitious pastes amenable to extrusion by using a combination of ordinary portland cement and supplementary cementitious materials. The microstructural packing of these mixes (with and without superplasticizers) are defined based on a simulated fresh state microstructure. Ram extrusion experiments are used in conjunction with phenomenological and analytical models to predict the rheological properties under pressure for the cementitious materials. Discrete element modelling is used to simulate the ram extrusion process using numerical simulations. These methods are correlated and established as means of predicting the extrusion response and material properties.
Properites of 3D Printed Concrete
3D printed concrete exhibits significantly different fresh and hardened properties as compared to conventionally cast concretes. Layer deformation in the fresh states cause unexpected and uncommon failure modes of these systems, and influences the buildability of 3D printed concrete. Techniques to determine these effects in real time are necessary. Once hardened, the layer interfaces influence the properties in a significant manner. Both the number of layers and the orientation of the inter-filament and inter-layer interfaces influence the properties.