Date of Award
Doctor of Philosophy (PhD)
Dr. Prasad Rangaraju
Dr. Bradley Putman
Dr. Amir Poursaee
Dr. Ian Walker
In comparison with other industries that have incorporated larger automation and digitalization towards their manufacturing process, the construction industry is significantly behind the curve even though it accounts for 13% of the industrial expenditure worldwide. Also, the construction industry significantly lags behind other industries in terms of annual growth in productivity at only 1%. The introduction of digital technologies like 3D printing of concrete into the construction sector can improve product performance and logistical issues in construction. 3D printing or additive manufacturing is a technique, wherein materials are assembled layer-by-layer to form a 3D object or element, without the need for any formwork. The main challenges faced by this technology are developing a well-characterized and tunable rheology of materials used, integrating reinforcement into the construction, developing appropriate standards for design and printing of concrete structures that are well-calibrated and accepted by the construction industry, etc. There is a growing interest in investigating additive manufacturing processes using cement-based materials in the construction industry.
Of fundamental interest in designing cementitious mixtures that are suitable for 3D printing is the ability to control not only the rheology of the mix but also other printability characteristics such as buildability, open time, shape stability and bond between the layers. These characteristics can be affected by using a combination of mineral and/or chemical admixtures.
The focus of the present study was to examine the influence of binary and ternary mixtures of cementitious materials containing silica fume (SF), meta-kaolin (MK) and ground granulated blast furnace slag (slag) with portland cement on rheological and 3D printability characteristics. These materials were used in combination with specific chemical admixtures such as superplasticizer (SP), viscosity modifying agent (VMA), set retarder and additive, polypropylene fibers. In this study, the replacement levels of SCMs such as SF, slag and their combinations have been studied for various properties like flow, open time, setting time, rheology and mechanical properties. Findings from this study indicate that achieving a flow between 126% and 133% based on standard flow test (ASTM C1437) was essential in obtaining a mixture that is extrudable. The time-gap effect, i.e. the gap in time between the casting of two adjacent layers, on the properties of the printed layers was studied by determining the flow behavior, rheological and hardened properties of the material which were measured at different time gaps of 0min, 5mins, 10mins, and 20mins. The effect of aggregate shape on the rheological and mechanical properties of these 3D printed assemblages was studied by using various replacement levels of natural sand (more rounded shape aggregate particles) with manufactured sand (more angular shaped particles) in the cementitious mixture. The bond between the layers of the cementitious materials, i.e. interlayer shear strength in the fresh state, has been evaluated for all binary and ternary blends containing Portland cement, MK and slag using a shear test developed in house, known as J-shear test.
The findings from this study indicate that achieving a finite flow value in the fresh state, along with adequate rheological properties (yield stress and plastic viscosity) are essential in developing a feasible 3D printable cementitious mixture. In general, as long as the bond between the layers is adequate, the mechanical properties of the 3D printed assemblages are not significantly different from the monolithic casting of the mixtures. For a given set of cementitious material combinations, the shape of aggregate particles has a definite influence on the rheological characteristics of the mix which can impact the 3D printability of the mixtures. It can be concluded that the presence of higher percentages of angular particles in the mix will lower the flow of the mix and increase the yield stress and plastic viscosity of the mix.
The impact of vibration on the flow behavior of all the mixtures was investigated in this study. It can be concluded that inducing vibration in fresh mixtures significantly enhanced the flow of the mixtures and improved the rheological properties of all mixtures, to allow for 3D printing. However, the duration of vibration needed to achieve adequate flow depended on individual mixtures. Mixtures with higher contents of fine SCMs such as MK and SF, and mixtures with more angular aggregate particles needed longer duration of vibration to achieve adequate flow values needed for suitable 3D printing.
In conclusion, the principal findings from this research study indicate that various parameters such as mixture proportions, types of SCMs and chemical admixtures, aggregate characteristics and physical manipulation factors such as induced vibration can impact the 3D printability of a cementitious mixture. By suitably tuning of all of the elements, a 3D printable cementitious mixture can be produced that has adequate printability, extrudability, buildability and shape retention. Additional research is needed to investigate and calibrate the findings from this study to validate the 3D printability of larger elements and full-scale structures.
Nekkanti, Haripriya, "Influence of Selected Supplementary Cementitious Materials on Properties of 3D Printable Cementitious Mixture for Application in Additive Manufacturing" (2020). All Dissertations. 2629.