Anagi M. Balachandra

Enhancement of Ultrahigh Performance Concrete Material Properties with Carbon Nanofiber

Ultrahigh performance concrete (UHPC) realized distinctly high mechanical, impermeability, and durability characteristics by reducing the size and content of capillary pore, refining the microstructure of cement hydrates, and effectively using fiber reinforcement. The dense and fine microstructure of UHPC favor its potential to effectively disperse and interact with nanomaterials, which could complement the reinforcing action of fibers in UHPC. An optimization experimental program was implemented in order to identify the optimum combination of steel fiber and relatively low-cost carbon nanofiber in UHPC. The optimum volume fractions of steel fiber and carbon nanofiber identified for balanced improvement of flexural strength, ductility, energy sorption capacity, impact, and abrasion resistance of UHPC were 1.1% and 0.04%, respectively. Desired complementary/synergistic actions of nanofibers and steel fibers in UHPC were detected, which were attributed to their reinforcing effects at different scales, and the potential benefits of nanofibers to interfacial bonding and pull-out behavior of fibers in UHPC. Modification techniques which enhanced the hydrophilicity and bonding potential of nanofibers to cement hydrates benefited their reinforcement efficiency in UHPC.

Optimization of ultra-high-performance concrete with nano- and micro-scale reinforcement

Abstract Ultra-high-performance concrete (UHPC) incorporates a relatively large volume fraction of very dense cementitious binder with microscale fibers. The dense binder in UHPC can effectively interact with nano- and microscale reinforcement, which offers the promise to overcome the brittleness of UHPC. Nanoscale reinforcement can act synergistically with microscale fibers by providing reinforcing action of a finer scale, and also by improving the bond and pullout behavior of microscale fibers. Carbon nanofiber (CNF) and polyvinyl alcohol (PVA) fiber were used as nano- and microscale reinforcement, respectively, in UHPC. An optimization experimental program was conducted in order to identify the optimum dosages of CNF and PVA fiber for realizing balanced gains in flexural strength, energy absorption capacity, ductility, impact resistance, abrasion resistance, and compressive strength of UHPC without compromising the fresh mix workability. Experimental results indicated that significant and balanced gains in the UHPC performance characteristics could be realized when a relatively low volume fraction of CNF (0.047 vol.% of concrete) is used in combination with a moderate volume fraction of PVA fibers (0.37 vol.% of concrete).

Monitoring of Sulfate Attack in Concrete by ²⁷Al and ²⁹Si MAS NMR Spectroscopy

AbstractSi29 and Al27 magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy techniques were employed towards evaluation of hydrated cement paste and concrete subjected to accelerated sulfate attack. Quantitative analyses of the Si29 NMR spectra of specimens subjected to different periods of sulfate attack indicated that the chain length CL of SiO4/AlO4 tetrahedra was lowered by sulfate attack. Al27 NMR spectra indicated that sulfate attack produced a significant rise in the intensity of ettringite and a sharp drop in the concentration of monosulfate hydrate, which are some primary indications of sulfate attack on concrete. Fourier-transform infrared (FTIR) spectroscopy method was employed in order to verify the result of NMR spectroscopy.

Reinforcement Efficiency of Modified Carbon Nanofiber in High-Performance Concrete Nanocomposite

Graphite nanomaterials offer distinct features for effective reinforcement of cementitious matrices in the pre-crack and post-crack ranges of behavior. In the work reported herein, carbon nanofiber was chosen for use in high-performance concrete. Synergistic actions of carbon nanofibers and polyvinyl alcohol (PVA) fibers in high-performance concrete were also investigated. Carbon nanofiber surfaces were modified by introduction of hydrophilic groups in order to improve their dispersion and interfacial interactions in cementitious matrices. An experimental program was conducted in order to assess the contributions of modified carbon nanofiber to diverse engineering properties of high-performance concrete. A statistical optimization program was implemented in order to identify optimum dosage of nano- and micro-scale reinforcement systems in high-performance concrete. The experimental results verified that optimum reinforcement systems comprised both carbon nanofiber and (micro-scale) PVA fiber. The gains in concrete engineering properties realized with optimum (nano- and micro-scale) reinforcement could not be matched by those provided by nano- or micro-scale reinforcement used alone. This finding supports the hypothesis that nano- and micro-scale reinforcement play complementary/synergistic roles in concrete by providing reinforcing effects at different scales and are also due to the benefits rendered by nanomaterials towards interfacial stress transfer and pullout behavior of fibers.

Piezo-driven self-healing by electrochemical phenomena:

Self-healing structures mimic the ability of biological structures (e.g. bone) to redistribute their structural mass in response to dynamic service loads and damaging effects. The self-healing features yield enhanced levels of structural efficiency and safety in dynamic service environments. In this study, the piezoelectric effect was used to convert the dynamic mechanical energy applied to the structure into electrical energy that, in turn, was used to drive electrochemical self-healing phenomena within a solid electrolyte. A theoretical framework was developed for self-healing materials, and experiments were conducted to verify the fundamental principles of the approach. The theoretical models confirmed that: (1) the piezoelectric effect can, within the geometric and mechanical constraints of actual structural systems, generate sufficient electric potential and charge (through harvesting the available mechanical energy) to enable electrochemical mass transport within a solid electrolyte; and (2) the redistribution of structural mass in dynamic service environments can occur within viable time frames. The fundamental principles of the new self-healing materials were validated through the demonstration of piezo-induced electrolytic phenomena in solid electrolytes and by verifying the gains in mechanical performance associated with such phenomena.

Polymer nanocomposites processed via self-assembly through introduction of nanoparticles, nanosheets, and nanofibers

Nanocomposites offer the theoretical potential to achieve mechanical properties surpassing those of conventional (micro-scale) composites. The underlying reasons for the high potential of nanocomposites include the uniquely high mechanical attributes of nano-scale reinforcement, effective control of defect size and growth by nano-spaced interfaces, and interactions between the polymer matrix and the large surface areas of nanomaterials. Attempts to produce nanocomposites via conventional processing techniques have encountered challenges associated with thorough dispersion and effective interfacial interactions of nano-scale reinforcement with the polymer matrix. In order to address these challenges, materials were processed into polymer nanocomposites via electrostatically driven layer-by-layer self-assembly. Electrostatically dispersed nanomaterials and oppositely charged polyelectrolytes were sequentially built upon a substrate (cellular scaffold). The self-assembled nanocomposites, after complementary cross-linking, provided a unique balance of strength and ductility, which surpassed those of conventional (micro-scale) composites. Self-assembly was found to be an effective approach to producing nanocomposites embodying uniformly dispersed nanomaterials with controlled interfacial interactions. This approach is highly versatile and enables introduction of diverse nanomaterials into polymer nanocomposites. The work reported herein evaluated introduction of diverse categories of nanomaterials incorporating nanoparticles, nanosheets, nanotubes, and nanofibers. This investigation also evaluated the potential for a biomimetic approach to processing of light-weight structural systems by self-assembly of polymer nanocomposites onto cellular scaffolds.

Characterization of ASR in Concrete by ²⁹Si MAS NMR Spectroscopy

AbstractSi29 MAS NMR spectroscopy was employed for the evaluation of the alkali-silica reaction (ASR) in laboratory and field concrete specimens. A series of NMR data was collected on the individual constituents of cement as well as on samples subjected to accelerated aging in a carefully controlled laboratory setting. Peaks associated with ASR were assigned and quantified. In spite of the spectral complexity due to the diverse constituents and the heterogeneous nature of concrete, changes due to ASR, including increased polymerization of C-S-H and A-S-H formation, could be identified and quantified. The trends established through Si29 NMR spectroscopy of the laboratory specimens were used to identify the presence and extent of the alkali-silica reaction in samples from operational bridges. Both bridge samples exhibited spectral evidence of ASR. Fourier transform infrared spectroscopy was used to verify the Si29 NMR spectroscopy observations.

High-recycled-content hydraulic cements of alternative chemistry for concrete production

ABSTRACT Sustainable hydraulic cements based on alkali aluminosilicate chemistry were developed with market-limited wastes acting as the primary aluminosilicate precursors. Two sets of formulations were developed with waste brick or impounded coal ash used at relatively large volume as aluminosilicate precursors. Supplementary materials were used to produce raw materials of viable chemistry. Blends of raw materials were transformed into hydraulic cements by input of mechanical energy without resorting to elevated temperatures. The resultant hydraulic cements exhibited viable strength development characteristics. FTIR, SEM, EDS and NMR analyses were conducted in order to gain insight into the structure of the hydraulic cements and their hydration products. The results pointed at the prevalence of calcium aluminosilicate hydrates among the products of hydration which thoroughly bound the non-hydrated cores of cement particles.

Effects of the Duration of Landfill Disposal on the Physicochemical, Mineralogical and Toxicity Characteristics of Coal Ash

ABSTRACT The quantities of coal ash disposed of in landfills over the past decades add up to billions of tons. Weathering in landfill would impact the mineralogical, chemical, physical and toxicity characteristics of coal ash. In order to investigate these effects, ashes were obtained from different depths of landfills representing different durations of disposal. These ashes were subjected to a range of physical, chemical, and toxicity tests. Aging in landfill was noted to produce significant changes in various ash properties. The trends in these changes with increasing disposal duration were identified. These trends were explained based on the weathering and leaching effects experienced by the coal ash in landfills. In addition, hydraulic activity and reactivity of the landfill coal ashes were determined to assess their potential use in alternate cement production.

Surface grown copper nanowires for improved cooling efficiency

Abstract The interactions between heat sink surfaces and coolant play important roles in cooling methods. This study relies upon controlled nanostructuring of heat sink surfaces that produces orders of magnitude increases in surface area, excites local vortexes and improves the phase change mechanisms to enhance cooling efficiency. A scalable, economical and environmentally benign technique to grow copper nanowires with a strong/conductive base-anchorage on the surface of copper and related materials is described. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to monitor the reduction and morphology of the nanowires. Transmission electron microscopy (TEM), electron diffraction (ED) and X-ray diffraction (XRD) were employed to understand the structure of the as-grown copper hydroxide nanowires and reduced copper nanowires. The convective heat transfer of nanostructured surfaces was measured in the laboratory and compared to a theoretical treatment of the nanowire array effects on convective heat transfer. The various surface treatments tested showed heat transfer increases of up to 93% in good agreement with a theoretical analysis.