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600451-00053-1 Dissertation Abstract

Modeling Deformation of Freezing Concrete: Towards the Identification of D-cracking Susceptible Aggregates and Construction of all Concrete LNG Tanks

Syeda Farhana Rahman, December, 2016

For many decades, degradation of concrete by freezing actions has been a primary interest of research for civil engineers. Past studies mostly relied on expensive and time-consuming experimental or semi-empirical investigations to identify the source of damage that is attributable to substandard aggregates, inadequate entrained air content, highly porous mortar or cement matrix, and use of deicing salts. Theoretical works developed in recent years do not incorporate all these factors in one single model. Very recently, concrete has gained widespread popularity as a cheap alternative to traditional material utilized for containing liquefied natural gas (LNG). Most studies documenting concrete behavior at cryogenic temperatures are obscure. Therefore, poroelastic theory, capable of incorporating aggregate and mortar properties, pore solution characteristics, air void spacing, and environmental exposure has been utilized to model damage triggering stresses and strain in concrete used for two purposes: 1) concrete pavement exposed to freezing and thawing cycles, and 2) concrete walled tanks containing LNG. The solid-liquid phase transformation equilibrium has been redeveloped to demonstrate the effect of pore solution speciation and disjoining pressure on the deformation of freezing concrete.

The modeled trends are in good agreement with experimental results obtained from literature. It has been found that the damage initiating tensile stresses, exhibited at the aggregate-matrix boundary for both the air-entrained and non-air-entrained concrete, can be exacerbated by the Mandel-Cryer effect induced by the delayed relaxation of the pore pressure from the aggregate center. The model suggests that high-porosity, low-permeability aggregates are the most vulnerable to D-cracking. Concrete with low-porosity, low-permeability mortar matrix, typical of mortar containing supplementary cementitious materials and/or low water to cement ratio, can withstand freezing deformation even with a spacing factor larger than the recommended value. In addition, thermodynamic analysis shows that the disjoining force favors crystal growth, while the dissolved ions suppress the freezing point but are still capable of building high hydraulic pressure in the pore network. We believe that implementation of these models will help practitioners select appropriate combinations and proportions of concrete mixture constituents to build safe, economic, and durable concrete structures.

Keywords: Concrete, Deformation, LNG Tanks, Freeze Thaw Cycle

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