Date of Award
Doctor of Philosophy (PhD)
Technological advancements and automation in the last two decades have made additive manufacturing of cementitious systems a reality. Among all the additive manufacturing techniques used for cementitious materials, layer-by-layer extrusion-based printing is the most used technique. For a layer-by-layer printing process, the requirements of cementitious materials vary from those of a conventional construction process. Typically, the cementitious materials are pumped to a print head using a positive-displacement pump. The pumpability of the cementitious material for a specific pumping system depends on its rheological properties. For effective pumping, the yield stress and plastic viscosity of a cementitious system should be low. On the other hand, for building ability (i.e., buildability), the cementitious system should have enough yield stress and plastic viscosity to resist deformation under self-weight and be able to support the weight of subsequent printed layers. The conflicting rheological requirements for pumpability and printability make the palette of mix (material constituents and proportioning) available for additive manufacturing limited and the tolerance to variation in the mix is very small. One of the methods to deal with the opposite needs of pumpability and buildability of a 3D printed material is to develop an active control system. The active control for an additive manufacturing process of cementitious materials can be through use external stimuli to control in-line rheological properties (active-rheological control) or through use of a chemical intervention at the nozzle head to change the hydration kinetics and in turn the gain in yield stress and plastic viscosity of the cementitious materials.
In this study, both active rheological control (ARC) and active stiffening control (ASC) systems have been developed and studied. In ASC system, a dynamic inline mixer capable of mixing chemical additives inline at the nozzle head was developed. A parametric study was carried out on the five accelerators which included one chloride-based, two non-chlorides based, two physical accelerators and a shotcrete accelerator. In the parametric study, various conditions viz. water-to-cement (w/c) ratio, sand-to-cement (S/C) ratio, cement type, temperature, and admixture type and dosage were varied. The results indicated that the behavior of all accelerators varies significantly as compared to each other as a function of above-mentioned variables. For ASC, the aluminum sulfate-based accelerators (shotcrete accelerators) were best suited to change the onset and rate of solidification at the nozzle head. Further study on the interaction of aluminum-sulfate-based accelerator with viscosity modifying agents (VMAs) and high-range water-reducing (HRWR) admixtures was carried out. The results indicate that the addition of the aluminum sulfate-based accelerators significantly increases the demand for the high-range water reducers, however, the onset of solidification for aluminum sulfate-based mixes is a function of the dosage of HRWR and VMA.
The active rheological control was achieved by using controlled vibration as external stimuli. In this study, two active rheological approaches were developed and used. In the first approach, an EN445 flow cone was modified by mounting a frequency-controlled vibration setup and in this configuration, the flow under gravitation forces was studied under non-shear conditions. The effect of vibration on the flow initiation, flow topology and flow rate of cementitious systems with respect to water-to-cement ratio, two mineral admixtures (silica fume and metakaolin), a high range water reducer (HRWR) and a viscosity modifying agents (VMA) was studied. The results from this study suggest a strong correlation between yield stress, and the frequency and amplitude of vibration. However, to understand the fundamental rheological properties under vibration and implications on the pumpability and buildability, a rheometer test setup was designed and developed. The rheometer setup was designed to simulate the controlled external (on-pipe) vibration. The influence of vibration on the yield stress and plastic viscosity of the cementitious materials were investigated for cementitious mortars with water-to-cement ratio, sand-to-cement content, vibration frequency and amplitude, and admixture type and dosages as variables. The results from the study suggest that the rheological behavior of the cementitious system changes between Bingham Plastic, non-linear Newtonian fluid, and near Newtonian fluid based on the paste content, fluidity of the mix, and the characteristics of vibration.
Peerzada, Abdul Basit, "Active Rheological and Stiffening Control of Cementitious Systems for Additive Manufacturing" (2022). All Dissertations. 3185.
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