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MULTISCALE THERMAL AND MECHANICAL ANALYSIS OF DAMAGE DEVELOPMENT IN CEMENTITIOUS COMPOSITES
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The exceptional long-term performance of concrete is a primary reason that this material represents a significant portion of the construction industry. However, a portion of this construction material is prone to premature deterioration for multi-physical durability issues such as internal frost damage, restrained shrinkage damage, and aggregate susceptibility to fracture. Since each durability issue is associated with a unique damage mechanism, this study aims at investigating the underlying physical mechanisms individually by characterizing the mechanical and thermal properties development and indicating how each unique damage mechanism may compromise the properties development over the design life of the material.
The first contribution of this work is on the characterization of thermal behavior of porous media (e.g., cement-based material) with a complex solid-fluid coupling subject to thermal cycling. By combining Young-Kelvin-Laplace equation with a computational heat transfer approach, we can calculate the contributions of (i) pore pressure development associated with solidification and melting of pore fluid, (ii) pore size distribution, and (iii) equilibrium phase diagram of multiple phase change materials, to the thermal response of porous mortar and concrete during freezing/thawing cycles. Our first finding indicates that the impact of pore size (and curvature) on freezing is relatively insignificant, while the effect of pore size is much more significant during melting. The fluid inside pores smaller than 5 nm (i.e., gel pores) has a relatively small contribution in the macroscopic freeze-thaw behavior of mortar specimens within the temperature range used in this study (i.e., +24 °C to -35 °C). Our second finding shows that porous cementitious composites containing lightweight aggregates (LWAs) impregnated with an organic phase change material (PCM) as thermal energy storage (TES) agents have the significant capability of improving the freeze-thaw performance. We also find that the phase transitions associated with the freezing/melting of PCM occur gradually over a narrow temperature range (rather than an instantaneous event). The pore size effect of LWA on freezing and melting behavior of PCM is found to be relatively small. Through validation of simulation results with lab-scale experimental data, we then employ the model to investigate the effectiveness of PCMs with various transition temperatures on reducing the impact of freeze-thaw cycling within concrete pavements located in different regions of United States.
The second contribution of this work is on quantification of mechanical properties development of cementitious composites across multiple length scales, and two damage mechanisms associated with aggregate fracture and restrained shrinkage cracking that lead to compromising the long-term durability of the material. The former issue is addressed by combining finite element method-based numerical tools, computational homogenization techniques, and analytical methods, where we observe a competing fracture mechanism for early- age cracking at two length scales of mortar (meso-level) and concrete (macro-level). When the tensile strength of the cement paste is lower than the tensile strength of the aggregate phase, the crack propagates across the paste. When the tensile strength of the cement paste exceeds that of the aggregate, the cracks begin to deflect and propagate through the aggregates. As such, a critical degree of hydration (associated with a particular time) exists below which the cement paste phase is weaker than the aggregate phase at the onset of hydration. This has implications on the inference of kinetic based parameters from mechanical testing (e.g., activation energy). Next, we focus on digital fabrication of a cement paste structure with controlled architecture to allow for mitigating the intrinsic damage induced by inherent shrinkage behavior followed by extrinsic damage exerted by external loading. Our findings show that the interfaces between the printed filaments tend to behave as the first layer of protection by enabling the structure to accommodate the damage by deflecting the microcrack propagation into the stable configuration of interfaces fabricated between the filaments of first and second layers. This fracture behavior promotes the damage localization within the first layer (i.e., sacrificial layer), without sacrificing the overall strength of specimen by inhibiting the microcrack advancement into the neighboring layers, promoting a novel damage localization mechanism. This study is undertaken to characterize the shrinkage-induced internal damage in 7-day 3D-printed and cast specimens qualitatively using X-ray microtomography (μCT) technique in conjunction with multiple mechanical testing, and finite element numerical modeling. As the final step, the second layer of protection is introduced by offering an enhanced damage resistance property through employing bioinspired Bouligand architectures, promoting a damage delocalization mechanism throughout the specimen. This novel integration of damage localization-delocalization mechanisms allows the material to enhance its flaw tolerant properties and long-term durability characteristics, where the reduction in the modulus of rupture (MOR) of hardened cement paste (hcp) elements with restrained shrinkage racking has been significantly improved by ~ 25% when compared to their conventionally cast hcp counterparts.