Thesis (M. S.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Jacobs, Laurence; Committee Co-Chair: Kurtis, Kimberly; Committee Member: DesRoches, Reginald.

The objective of this research is the pursuit of a better characterization method for the air voids - particularly air-entrained voids - in fresh cement-based materials through the use of ultrasonics. The use of air-entraining agents (AEA's) to incorporate a stable network of air voids into fresh cement paste is common practice in the concrete industry. These particular air voids significantly improve durability of hardened cement paste through mitigation of stresses associated with freezing and thawing cycles. It is understood that the performance of entrained air voids in cement paste is dependent on their size and distribution, or spacing factor. However, conventional methods for qualifying air content, such as the Pressure, Volume, and Gravimetric Methods, only measure total air volume and cannot assess size or spacing. In this investigation, using matched pairs of transducers, ultrasonic pulses were transmitted through fresh cement paste specimens (0.0\% up to 0.6\% AEA by weight of cement). The received signals were recorded every five minutes during the first six hours and then every fifteen minutes thereafter. Analysis shows strong distinctions between specimens with and those without the AEA. Further research is needed into the distinctions among specimens with the AEA. However, the data suggest correlations between Vicat setting times, heat of hydration, and autogenous strain and ultrasonic metrics such as pulse velocity and peak frequency of the signal. The findings of this research should be most appropriate as a foundation for an inversion process and improved air-entrainment detection methods.

Ordinary Portland cement (OPC) is the main constituent of concrete works as a principal binder for aggregates and intrinsically transmits the brittleness into concrete through the formation of hydration crystals in the cement microstructure. A number of nano cementitious composites were developed in recent years to offset the brittleness with newly discovered nanomaterials and the most prevalent among those is the graphene oxide (GO). The main objective of this PhD research work is to develop nano graphene cementitious composites (NGCC) using low cost, two dimensional (2D) graphene nanoplatelets (GNPs) and one dimensional (1D) graphited carbon nanofibres (GCNFs) with unique conical surface morphology. The GNPs were sourced synthesised in an environmental friendly way via plasma exfoliation whereas, GCNFs were manufactured through catalytic vapour grown method. The project further investigated the effect of these nanomaterials in regulating the distinctive microstructure of cement matrix leading to enhance its mechanical properties. Three different types of high-performance NGCC namely NGCC-Dot, NGCC-Fnt and NGCC-CNF, are developed by activating pristine GNPs (G-Dot), functionalised GNPs (G-Fnt) and graphited nanofibers (G-CNFs) into the cement matrix respectively. It is found through various characterization and experimental techniques that both GNPs and GCNFs regulated the cement microstructure and influenced the mechanical properties of NGCC uniquely. A remarkable increase in the flexural and the tensile strength of newly developed NGCC has been achieved and that could be attributed to the formation of distinctive microstructure regulated by catalytic activation of these nanomaterials. The shape (1D, 2D) and unique morphology of these nanomaterials played a vital role in the mechanism of crystal formation to regulate the cement microstructure. Based on the observations of test results and comprehensive characterization, the possible mechanisms of crystal formation and development of distinctive microstructure of NGCC has been established which has then proceeded to the development of a physical model for NGCC development.