Amirpasha Peyvandi

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.

A New Self-Loading Locomotion Mechanism for Wall Climbing Robots Employing Biomimetic Adhesives

A versatile locomotion mechanism is introduced and experimentally verified. This mechanism comprises four rectangular wheels (legs) with rotational phase difference which enables the application of pressure to each contacting surface for securing it to the surface using bio-inspired or pressure-sensitive adhesives. In this mechanism, the adhesives are applied to two rigid plates attached to each wheel via hinges incorporating torsional springs. The springs force the plates back to their original position after the contact with the surface is lost in the course of locomotion. The wheels are made of low-modulus elastomers, and the pressure applied during contact is controlled by the elastic modulus, geometry and phase difference of wheels. This reliable adhesion system does not rely upon gravity for adhering to surfaces, and provides the locomotion mechanism with the ability to climb walls and transition from horizontal to vertical surfaces.

Structural Design Methodologies for Concrete Pipes with Steel and Synthetic Fiber Reinforcement

Improved structural designs were developed and experimentally verified for concrete pipes. These designs make effective use of discrete synthetic fiber reinforcement to lower the reinforcing steel ratio, and thus allow for increasing the protective cover of concrete on steel for improved durability. The new concrete pipe design also enhances the toughness and damage resistance of pipes. The work reported herein covers theoretical modeling, design, and experimental verification of concrete pipes with synthetic fiber and conventional steel reinforcement. The focus of the theoretical models was on the flexural strength and load-carrying capacity of concrete pipes. These models account for the contributions of fibers to the tensile behavior of concrete via fiber pullout or rupture. These models were used to develop new concrete pipe designs, which made complementary use of synthetic fiber (polyvinyl alcohol [PVA]) and conventional steel reinforcement. Full-scale pipes embodying the new design were fabricated and experimentally evaluated. The experimental results were used to refine the theoretical models and design procedures. Test results confirmed that the number of steel reinforcement layers in concrete pipes can be reduced with the use of synthetic fibers. This allows for increasing the protective cover of concrete on steel, which is a major advantage towards increasing the service life of concrete in aggressive environments, including sanitary sewers, where microbial- induced corrosion is a major concern.

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.

Characterization of alkali-activated nonwood biomass ash-based geopolymer concrete

AbstractThe combustion ash of a common nonwood biomass (wheat straw) was evaluated for value-added use in production of geopolymer concrete where alkali aluminosilicate hydrates are the primary bin...

A Fundamental Assessment of Graphite Nanoplatelet Effects on Progress of Alkali-Silica Reactions

Graphite nanoplatelets were used to control alkali-silica reactions in concrete by enhancing the barrier qualities of cementitious binders and providing local reinforcing effects against the damaging expansive phenomena. Nuclear magnetic resonance (NMR) spectroscopy was used to monitor the changes in chemical environment that reflect upon alkali-silica reactivity. Laboratory experiments were conducted to evaluate the effects of graphite nanoplatelets on alkali-silica reactions (ASRs). Reactive flint aggregates were used to induce ASR. The ²⁹Si MAS NMR spectroscopy technique was employed for evaluating the changes in chemical environment by monitoring different silicate tethrahedra (Qⁿ) as a basis to quantify the progress of ASR in concrete materials prepared with and without graphite nanoplatelets. Quantitative evaluations of different Qⁿ species present in anhydrous cement, calcium silicate hydrate (C-S-H), and ASR products indicated that introduction of graphite nanoplatelets lowered the degree of polymerization of silicate tetrahedral in C-S-H. The reduction in the breakdown of the networked structure of silicate tetrahedra under accelerated ASR indicates that graphite nanoplatelets reduce the extent of ASRs.

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.