This career award evaluated the performance and durability of randomly oriented nano/microfiber reinforced cement-based materials and investigated the potential for new materials being developed by the nanoscience community to contribute to stronger and more resilient concrete structures well into the future. The research focused on the degradation mechanisms from the nanometer to the engineering scale as a function of various weathering forces and mechanical stresses. Understanding how to enhance and predict the performance of cement-based materials can enable a transformation from traditional materials towards multi-functional materials tailored for specific applications.

Motivation.
Concrete structure degradation, one of the most significant problems faced by
civil infrastructure, is a large burden on the US economy. Composite cement-based materials provide an excellent alternative for amelioration, protection, and deterioration prevention of civil infrastructure based on a demonstrated resistance to corrosion, control of crack propagation, and exceptional strength to weight ratios. These materials have been shown to improve a variety of mechanical properties and have the potential to lower the total life cycle cost for civil engineering applications. Nano and microfibers can impart additional, unique properties, including low electrical resistivity, electromagnetic field shielding, self-sensing capabilities, high ductility and self-control of cracks. Understanding the long-term performance and durability of these materials in response to inevitable, complex, time-dependent and multi-scale, environmental, weathering forces (e.g., moisture, temperature, acid rain, and chemical attack) is important because of the critical needs that they can potentially fulfill to improve infrastructural health and function.

Intellectual Merit. An integrated experimental and computational approach
at multiple size scales, nano to micro to macro, was used with an emphasis on
the chemical mechanisms and failure modes at the nano/micro-reinforcement-cement interface. As part of the research, the use of vapor grown, carbon nanofibers (CNFs) as a nano-reinforcement, or nano-rebar, to replace steel rebar, a main cause of concrete degradation was evaluated. CNFs are multi-wall, highly graphitic structures with diameters ranging from 70 to 200 nm and lengths up to a few hundred microns that present a very large, useful surface area.

The work showed the challenges in obtaining disaggregation and uniform dispersion of CNFs at the individual fiber level in the cement paste and clearly showed that the dispersion state of the CNFs in solution was not indicative of the final dispersion state in the hydrated cement paste. Microscale CNF agglomerates were seen in the cement matrix regardless of the initial degree of CNF dispersion in solution and became more prominent with increasing CNF loading. The distribution of the individual CNFs was not uniform within the composites with the presence of CNF rich and CNF poor regions. The best CNF dispersion was, nevertheless, found when polycarboxylate-based high range water reducer was used with increases in flexural strength up to 65% seen with the addition of 1% CNFs per weight of cement even with the presence of CNF agglomerates. The research furthermore revealed that the addition of CNFs to the cement matrix increased the percentage of high-stiffness calcium silicate hydrates (C-S-H), the main glue of cement-based materials, at the expense of low-stiffness C-S-H and that CNF agglomerates acted as flaws within the cement matrix with no reinforcing effects.

A strong correlation was found between the dispersion state of CNFs, the microstructural evolution of the composites during exposure to weathering forces, and the composite mechanical properties. For e...

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