Multi-Scale High-Resolution X-ray Synchrotron Characterization of Air Voids in Concrete

The crystallization of ice in porous materials can lead to a significant decrease in durability. This is particularly important for reinforced concrete structures where ice formation can compromise the civil infrastructure. Much progress has been made in how to reduce the damage, but to develop new improvements, it is necessary to characterize the multi-scale dimensional structure of air-entrained concrete exposed to low temperatures. Synchrotron radiation laboratories supply their equipment with high flux high brightness light, offering phenomenal and varied exploration for fine-tuned and precise characterization techniques that are unafforded, in comparison, by standard stand-alone laboratory equipment with significantly limited source illumination. The use of synchrotron radiation has opened exciting new lines of research, and this dissertation uses cutting-edge techniques to image, for the first time, the in-situ ice formation inside a hydrated air-entrained cement paste using hard x-ray synchrotron microtomography (µCT). The results provide clear evidence of the importance of air-entrainment to mitigate freezing damage, and the ability to observe undisrupted internal paste ice formation opens the possibility to non-destructively observe and evaluate air entrainment variables in action. This novel technique opens new lines of research on the in-situ quantification of ice crystallization, a critical information for the micromechanical modeling, including cryo-suction of porous materials exposed to freezing conditions. Using a custom-made loading device mounted on the loading stage of the µCT, it was possible to record, for the first time, the in-situ formation and propagation of microcracks in entrained air void mortar pastes subjected to compression and tension. It is well-known that the presence of distributed small air voids in paste protects the matrix against ice expansion, but the enhanced durability comes at a cost of reduced mechanical strength. However, the mechanism of the reduction of strength has not been studied. The in-situ mechanical tests clearly show how the cracks form near or at the air void and develop a complex cracking pattern connecting the voids. The studies provide physical evidence for improvement of topology of the air void system in the cementitious matrix. Even though the use of surfactants has been used to successfully entrain air in concrete, the understanding of their mechanism is far from complete. This lack of understanding prevents the development of optimized surfactants. For this reason, this dissertation presents a sub-micron scale chemical speciation of the shell structure formed around entrained air voids in hardened entrained cement paste using scanning transmission x-ray microscope (STXM). Some detailed location specific information is revealed in this study, such as the state of carbonation, the identification and composition of the hydration products, and the level of hydration at and near the air void shell. It is found that the air void shell is made mostly of calcium silicate hydrate (C-S-H) with higher Ca/Si ratio in composition than the C-S-H that exists in the matrix, which contradicts previous studies performed using a scanning electron microscope energy dispersive spectroscopy (SEM-EDX). The study of ancient Roman concrete has been receiving attention because of its long durability. One of the open questions is how the Roman concrete survived frost action. For this reason, ancient concrete samples collected both from marine and land structures were collected and subjected up to 20 cycles of freeze-thaw. Their varied resistance to freeze damage was documented using µCT and while some patterns were identified, it was not possible to establish a universal law to predict the frost resistance of Roman concrete.

Advisor: Monteiro, Paulo JM