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2. Collisional-radiative modeling of shock-heated nitrogen mixtures.

Author

Aiken, Timothy T. and Boyd, Iain D.

Subjects
*EXCITED states, *ELECTRONIC excitation, *NON-equilibrium reactions, *SHOCK waves, *NITROGEN, *MIXTURES
Abstract

A three-temperature collisional-radiative model for shock-heated nitrogen–argon mixtures is developed to facilitate the study of nonequilibrium electronic excitation and ionization behind strong shock waves. Model predictions accurately reproduce measurements of N 2 dissociation for mixtures of 2%–10% N 2 in argon, with some discrepancies observed for 20% N 2 mixtures. Potential causes of the discrepancies are discussed. Net dissociation in mixtures containing 20% N 2 is significantly impacted by the dissociation of N 2 (A), the first excited electronic state of N 2 , indicating that molecular electronic excitation can affect net dissociation in shock-heated nitrogen flows. The collisional-radiative model successfully predicts the three-stage behavior and induction time observed in concentration measurements of atomic nitrogen in its fourth excited state, the 3 s 4 P level, behind reflected shocks. Mechanisms for the observed behavior are discussed, which deviate from those inferred using a simpler kinetic model. Excited state number density predictions are strongly influenced by the modeling of radiation self-absorption and the inclusion of the measured non-ideal pressure rise. At higher N 2 concentrations, the measured data indicate increased efficiency of atomic nitrogen electronic excitation in collisions with N as compared to collisions with N 2 and Ar. A global sensitivity analysis of the excited state predictions is then performed, identifying the processes in the kinetic model that most sensitively influence the predicted excited state time history and further clarifying the dominant mechanisms affecting the experimental observables. [ABSTRACT FROM AUTHOR]

Published
2024
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6. Durability of geopolymer materials in aggressive aqueous environments : acid attack, steel reinforcement corrosion and freeze-thaw attack

Author

Aiken, Timothy Adam

Abstract

Geopolymer or alkali-activated concrete is a novel alternative to conventional Portland cement (PC) concrete. This thesis aims to advance the knowledge available by investigating the resistance of geopolymers against acid attack, steel reinforcement corrosion and freeze-thaw attack. The resistance of geopolymers to acid attack was investigated by exposing samples to mineral and organic acids as well as silage effluent. Mortar samples were used to assess the effect of acid exposure on physical properties and equivalent paste samples were used to assess the effect of acid exposure on leaching behaviour and microstructural properties. The effect of slag content and alkaline activator dosage on the acid resistance of fly ash geopolymers was also studied. It was discovered that neat fly ash geopolymers have significantly greater resistance to acidic media than blends of fly ash and slag. Furthermore, all geopolymer mixes investigated displayed superior performance when compared with PC. This was due to higher calcium content and several phases present in hydrated PC which are vulnerable to acid attack. The mechanisms of acid attack were also considered. The steel reinforcement corrosion was assessed by obtaining the chloride migration coefficient and carrying out a ponding test on concrete slabs which had embedded steel reinforcement bars. It was discovered that increasing the slag content and alkaline activ?tor dosage significantly improves the steel reinforcement corrosion resistance of fly ash geopolymers. The PC concrete performed much better than mixes with high fly ash content but mixes with high slag content provided comparable performance with PC concrete. Freeze-thaw resistance was investigated by performing a scaling test in the presence of both water and NaCl solutions. This revealed poor resistance of fly ash geopolymer concrete which improved as the slag content increased to be almost similar to that of PC concrete.

Published
2018

22. Comparison of lithomarge and cement-based mortars performance in aggressive aqueous environments

Author

Kwasny, Jacek, Aiken, Timothy A, Soutsos, Marios N, McIntosh, Andrew J, and Cleland, David J

Subjects
Acid attack, Civil Engineering, Lithomarge, Geopolymer mortars, Portland cement mortars, Sulfate attack
Abstract

The resistance of room temperature cured geopolymer mortar (GPM) against chemical attacks, i.e. sodium and magnesium sulfate solutions, and sulfuric and hydrochloric acid solutions, was evaluated. GPM was formulated using a lithomarge precursor (low-purity kaolin) to achieve 28-day characteristic compressive strength of 60 MPa. Its performance was compared with an equivalent Portland cement mortar (PCM) having the same paste volume and strength grade. 28-day old bar samples were stored in 0.352 mol/L sulfate solutions for 52 weeks whereas 28-day old cube samples were exposed for 8 weeks to acid solutions with concentration of 0.52 mol/L. GPM showed superior performance against sulfate attack when compared to PCM. No visual deterioration was observed in GPM, the length changes were relatively small, and no changes to the microstructure were detected – in contrast to severely deteriorated PCM. As confirmed by visual observations and lower mass loss, GPM showed better resistance to attack by both acids than PCM. GPM provided a better quality (lower permeability) of an acid-degraded layer, lowering the degree of further deterioration. The main mechanisms of the matrix deterioration of GPM in both acids was dealumination of the hardened binder, with a higher degree of changes detected for sulfuric acid.

Published
2019

23. Comparison of lithomarge and cement-based mortars performance in aggressive aqueous environments

Author

Kwasny, Jacek, Aiken, Timothy, Soutsos, Marios, Cleland, David, and McIntosh, Andrew

Subjects
Portland cement mortars, Lithomarge, Geopolymer mortars, Acid attack, Sulfate attack
Abstract

The resistance of room temperature cured geopolymer mortar (GPM) against chemical attacks, i.e. sodium and magnesium sulfate solutions, and sulfuric and hydrochloric acid solutions, was evaluated. GPM was formulated using a lithomarge precursor (low-purity kaolin) to achieve 28-day characteristic compressive strength of 60 MPa. Its performance was compared with an equivalent Portland cement mortar (PCM) having the same paste volume and strength grade. 28-day old bar samples were stored in 0.352 mol/L sulfate solutions for 52 weeks whereas 28-day old cube samples were exposed for 8 weeks to acid solutions with concentration of 0.52 mol/L. GPM showed superior performance against sulfate attack when compared to PCM. No visual deterioration was observed in GPM, the length changes were relatively small, and no changes to the microstructure were detected – in contrast to severely deteriorated PCM. As confirmed by visual observations and lower mass loss, GPM showed better resistance to attack by both acids than PCM. GPM provided a better quality (lower permeability) of an acid-degraded layer, lowering the degree of further deterioration. The main mechanisms of the matrix deterioration of GPM in both acids was dealumination of the hardened binder, with a higher degree of changes detected for sulfuric acid.

Published
2018

24. Difference Tests for Distributions of Ratings.

Author

Aiken, Lewis R. and Aiken, Timothy A.

Abstract

Three exact probability tests, counterparts of t tests for single sample, independent samples, and dependent samples, are described for data obtained from ratings on "m" scales by a single rater or on a single scale by "n" raters. (LMO)

Published
1986

29. Medical malpractice: eliminating the myths.

Author

Gesler, Alan E., Aiken, Timothy J., Gleisner, William C., Domnitz, Merrick R., and Antoine, Virginia M.

Subjects
Medical malpractice insurance -- Economic aspects, Physicians -- Malpractice
Published
1985

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