Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

The hydration of cement and water is an exothermic reaction capable of generating significant amounts of heat. Unaccounted for, the heat generated can alter the chemical reaction of the cement, producing massive cracks in the hardened concrete that sacrifice the integrity of the structural element. Alternatively, the heat produced can create thermal gradients capable of cracking the concrete and exposing the reinforcing steel to chlorides. To prevent either of these events from occurring, a software program known as ConcreteWorks was created as part of a previous research project funded by the Texas Department of Transportation. ConcreteWorks gives TxDOT engineers, contractors, and inspectors the ability to manage the structural design, mix proportions, and construction processes in order to minimize maximum concrete temperatures as well as temperature gradients. The free program has seen successful on several non-TxDOT projects, however, it has failed to become incorporated into standard TxDOT practices and specifications. The goal of this research, funded by TxDOT, was to promote widespread use and acceptance of the program within the DOT. In pursuit of this goal, a four-hour hands-on training class was developed and taught throughout the state of Texas, construction projects were selected for the use and validation of the software program, and a few modifications were suggested to make the program more helpful and easy to use. This thesis primarily focuses on the results of the validation of ConcreteWorks on mass concrete and precast applications. In total, four precast beams and two columns were instrumented. With regards to existing methods of predicting temperatures, the program was fairly accurate for mass concrete applications. The program was also very useful for precast elements; however, the lack of variables to match the model to the actual structure likely limits the software program from producing a more accurate prediction. / text

The enormous biodiversity of microalgae as well as their high photosynthetic rates can be exploited for a wide variety of applications including the production of high value chemicals, nutraceuticals, aquaculture feed, and most recently, biofuels. Regardless of the application, the productivity of the microalgae culture must be optimized in order to make the systems economically feasible. One environmental factor that greatly affects the productivity of mass cultivation systems is temperature since it can be prohibitively expensive to control in outdoor systems. Temperature affects microalgae growth rates both directly by its effect on metabolic rates, and indirectly, by changing the bioavailability of the inorganic carbon present in solution. In the first part of this research, the effects of dissolved inorganic carbon (DIC) concentration (varied by sparging CO₂-enriched air) and temperature on the growth of a model microalga species (Nannochloris sp., UTEX LB1999) were investigated in a turbidostat bioreactor. The results indicate that increasing DIC concentration yields higher microalgae growth rates up to an optimum value (around 3 mM for Nannochloris sp.) but higher concentrations actually inhibited growth. Since increasing the temperature decreases the DIC concentration for a given gas pCO₂, it is necessary to adjust the pCO₂ to maintain the target DIC concentration in the optimal range for growth. In the next phase of the research, the effect of average light intensity (Gav) and temperature on the growth rate of two microalgae species (Nannochloris sp., UTEX1999. and Phaeodactylum tricornutum, UTEX646) was investigated. Growth rates were measured over a range of average light intensities and temperatures using a turbidostat bioreactor. A multiplicative model was developed to describe growth as a function of both average light intensity and temperature. In the third phase of this research, both microalgae species were grown together to explore the effects of temperature fluctuations on the population dynamics of the co-culture. It was observed that Nannochloris was inhibited by the presence of P. tricornutum in the medium, probably due to the excretion of secondary metabolites into the medium that affected Nannochloris growth (allelopathic effects). The temperature and average light intensity model developed under monoculture conditions was modified to incorporate the allelopathic effects observed. The resulting model provided a reasonable fit to the dynamic behavior of a Nannochloris/P. tricornutum co-culture subjected to temperature variations in chemostat experiments. / text

Firefighters routinely have to go through buildings with reduced visibility due to smoke. Moving through even the simplest apartment building can become a perilous task when you remove visual sensing, and introduce fires, toxic gasses and extremely high temperatures. While a number of tools, both low and high tech, exist to aid firefighters, none of them are perfectly able to solve this problem. This thesis proposes using sonar to supplement those tools, and documents the development of a flame-penetrating sonar. To the authors knowledge, no previous effort has been made to develop sonar for firefighting applications. Traditional ultrasonic range finders were found unable to penetrate flames, and thus a pulse-compression based sonar is presented here. A prototype sonar is developed to allow experimental testing of this technique. A number of experiments were conducted to understand the limitations of this device. This sonar was able to overcome the scattering of the flame, and even detect the extents of the flame. A number of applications of this technology can be imagined other than assisting firefighters. Military personnel, or anyone else needing to navigate obscured environments could use this technology. Other applications could be two-dimensional and three-dimensional temperature field reconstruction for industrial applications. The author believes combination sensors using sonar, thermal-imaging, global positioning system, dead reckoning, etc, are the only way to solve the problem of firefighter navigation. / text

Controlling the placement and hydration temperature of concrete is important to concrete durability. Thermal gradients and delayed ettringite formation (DEF) result in cracking when concrete in the plastic state becomes too hot. Cooler placement temperatures slow hydration reaction, increase working time, reduce the maximum temperature in the concrete member, and reduce thermal gradients. Furthermore, cooler concrete achieves better long-term strength and microstructural development. Concrete producers have been using multiple methods of reducing the placement temperature of concrete, such as cooling mixtures with ice or chilled water, shading aggregate piles, placing concrete at night, and using evaporative cooling of aggregate piles. More recently, concrete producers have turned to liquid nitrogen for cooling fresh concrete. The objective of this research was to determine the effects of liquid nitrogen on concrete hydration, microstructural development, and performance. The following concrete mixture properties and methods were investigated: cement type, the effects of selected supplementary cementing materials and chemical admixtures, placement temperature, and the time at which liquid nitrogen dosing occurs (delayed dosing). Concrete performance was tested in terms of slump, setting time, yield, compressive and splitting tensile strength, elastic modulus, rapid chloride permeability, and hardened and fresh air void analysis. Hydration and microstructural development were monitored by isothermal calorimetry, semi-adiabatic calorimetry, x-ray diffractometry, inductively coupled plasma, and environmental scanning electron microscopy. Additional testing was performed on concrete mixing drums to determine the effects of liquid nitrogen on the durability of steel mixing drums. The results indicate that performance, hydration, and microstructural development of fresh concrete are relatively unaffected when cooled with liquid nitrogen to room temperatures. Significant findings show that the slump of liquid nitrogen cooled concrete is similar to hot concrete mixtures and not room temperature mixtures. Additionally, setting time results show that liquid nitrogen dosing of hot concrete can be delayed for up to 1 hour and setting times will still be similar to room temperature mixtures. Based on findings from this research study, liquid nitrogen is recommended as a primary cooling option to reduce the placement temperature of fresh concrete.

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published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Civil Engineering / Master / Master of Philosophy