This research presents the results on an experimental investigation to identify the significant factors influencing horizontal cracking in continuously reinforced concrete pavements (CRCP). An in-depth analysis of the microstructure, morphological characteristics of the interfacial transition zone (ITZ) and the observation of cracking using the environmental scanning electron microscope (ESEM) was done. Characterization of oxides using Fourier transform infrared spectroscopy (FTIR) and electron dispersive x-ray spectroscopy (EDS) was also performed. Water to cement ratio (w/c) and rebar temperature had a significant influence on the rebar-concrete bond strength. The 28-day shear strength measurements showed an increase in rebar-concrete bond strength as the water to cement ratio (w/c) was reduced from 0.50 to 0.40. There was a reduction in the peak pullout load as the temperature increased from 14oF to 252oF for the corroded and non-corroded rebar experiments. The corroded rebar pullout test results showed a 20-50 % reduction in bond strength compared to the non-corroded rebars. FTIR measurements indicated a presence of lepidocrocrite (γ -FeOOH) and maghemite (γ -Fe2O3) on the ITZ. ESEM images showed the existence of microcracks as early as three days after casting with the bridging of these cracks between coarse aggregate locations in the interfacial zone propagating through the mortar.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第17914号 / エネ博第286号 / 新制||エネ||59(附属図書館) / 30734 / 京都大学大学院エネルギー科学研究科エネルギー変換科学専攻 / (主査)教授 木村 晃彦, 教授 星出 敏彦, 教授 小西 哲之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

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Chloride-induced stress corrosion cracking (CISCC) has been identified as the main degradation mechanism for spent nuclear fuel dry storage canisters. This type of induced cracking is complex and depends on several factors, such as material composition, exposure temperature, relative humidity, applied tensile stress, and atmospheric salt concentration. An accelerated experiment was designed to simulate marine environments in a controlled fogging chamber to examine 304 and 304L stainless steel U-bend and welded U-bend samples. The samples were exposed to chloride rich and humid fogging in a corrosion chamber at 35℃ continuously for 4 weeks, 8 weeks, and 12 weeks. The same experiment was repeated at 50℃ for 4 weeks, 8 weeks, and 14 weeks to study the sensitivity of CISCC to temperature changes. A qualitative evaluation of optical micrographs from a 3D Surface Profiler was performed for 16 corroded samples and compared with 2 reference samples. Cracking was observed on 12 out of 16 samples exposed to 35℃ and 50℃ for durations ranging from 8 to 14 weeks. Likely cracking observations were noted on 4 out of 16 samples. A quantitative statistical analysis was also performed using surface profile depth (valley) data from corroded and reference samples. The quantitative analysis examined the effect of temperature, welding, exposure duration, and material composition. The quantitative results were compared with the qualitative results and literature published in CISCC. / Master of Science / Most nuclear power plants are currently using dry storage canisters (DSCs) which are made of a concrete vault and a stainless steel canister that houses the spent nuclear fuel (SNF) assemblies. Multiple conditions must be present simultaneously for chloride-induced stress corrosion cracking (CISCC) to develop, such as the presence of a susceptible alloy, high relative humidity, high temperature, high atmospheric salt concentrations, and applied tensile stresses. DSCs are typically made from 300-series austenitic stainless steels which are susceptible to this type of corrosion during long-term storage near marine environments. Therefore, understanding of the factors leading to CISCC is critically important for proper management and mitigation and to estimate the service life of DSCs for the safe long-term storage of SNF. An accelerated experiment was designed to examine the effects of temperature, exposure duration, and welding on pitting and cracking for 304 and 304L U-bend samples. The experimental results concluded that stainless-steel grades 304 and 304L are susceptible to CISCC when exposed for 8 weeks or longer to fogging at temperatures between 35℃ and 50℃, 95% relative humidity, and 5% salt concentration. This study also concluded that increasing exposure duration from 8 to 12 weeks or the temperature from 35℃ to 50℃ had no significant effect on the acceleration of CISCC. Also, unwelded samples were deemed more susceptible to CISCC than welded samples and the susceptibility of 304 and 304L grades were relatively similar.

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In addressing the need for a site specific corrosion model for concrete structures, Bazant’s time to first cracking model was used as the basis for an experimental research study. The theoretical model, which expresses the time to first cracking as a function of the corrosion rate and physical properties of the reinforced concrete, was never experimentally validated. This research study focuses on examining Bazant’s model and its validity using commercially available corrosion rate measuring devices. With validation of the model, the present condition of the structure could be assessed, damage rate predicted, and repair or rehabilitation methods selected. The experimental program examined 18 test blocks and 52 test slabs constructed to simulate sections of a reinforced concrete bridge deck. The test blocks and slabs contained a single layer of electronically isolated steel reinforcement embedded below the surface of the specimens. The design variables included cover depth, bar spacing, bar diameter, and corrosion rate. Six chloride concentrations 0.0 to 9.6 lb/yd³ (0.0 to 5.7 kg/m³) added directly to the fresh concrete were used to vary the corrosion rate. Two environmental conditions were also considered: outdoors in Blacksburg, Virginia and indoors with near constant moisture and temperature. The slabs were designed with two reinforcing mats. The lower mat of fiberglass reinforcement was placed in both directions and the upper mat was one layer of fiberglass and one layer of steel. Thus, the corrosion rate a single layer of electrically isolated steel bars was monitored. The corrosion rates were monitored from the date of casting for approximately one year using the 3LP and Gecor corrosion rate devices. The amount of metal loss of the bars was determined both destructively and non-destructively to allow a comparison between the two. The metal losses were compared to the predictions made according to Bazant’s equations as well as to other existing time to first cracking criteria to determine their validity. Tests were performed to verify the chloride contents of the slabs. The effect of temperature on the corrosion rate was also investigated. Preliminary results indicate that temperature strongly influences the corrosion rate. To date no cracks have formed in the surface of the test specimens. The corrosion rate of the slabs has been progressing at a maximum rate of 1.24 mils/yr (31.5 microns/yr) for over one year. The test blocks and slabs are to be monitored until cracking occurs at which time Bazant’s model will be either validated or adjusted to reflect the predictions derived from the commercially available corrosion rate devices used in this experimental program. Additional tests must also be performed to further quantify the effects of temperature and other variables. / Master of Science

Las condiciones ambientales de trabajo de los aceros estructurales y de conducciones en instalaciones energéticas y petrolíferas han puesto en evidencia la necesidad de encontrar una metodología de caracterización de su resistencia a la fisuración, tanto en régimen de dominio elástico como elastoplástico. Este trabajo recoge una metodología experimental y analítica adecuada para ser aplicada a los procesos de fisuración estudiados sobre probetas y estructuras, en particular a aquéllos asociados a la presencia de ambientes agresivos, como corrosión bajo tensión o fisuración inducida por hidrógeno de aceros microaleados. Una vez aplicada y validada, la metodología ofrece resultados de gran interés en la caracterización cuantitativa del comportamiento de fisuración y su correlación con los micromecanismos de rotura presentes. Esta tesis se ha mostrado eficaz en la caracterizacióny establecimiento de un modelo de comportamiento de aceros microaleados utilizados en condiciones de fisuración inducida por hidrógeno, como es el caso de las plataformas petrolíferas en alta mar (estructuras off-shore). / The environmental conditions of employment of structural steel and energy pipelines and oil facilities have highlighted the need to find a methodology for characterizing their resistance to cracking, both in the elastic and the elastoplastic regime. This work includes experimental and analytical methodologies that are appropriate to be applied to cracking processes studied on specimens and structures, in particular to those associated with the presence of aggressive environments, such as stress corrosion cracking or hydrogen induced cracking of microalloyed steels. Once applied and validated, the methodology provides results of great interest in the quantitative characterization of the cracking behavior and its correlation with the fracture micromechanisms taking place. This thesis has been shown to be effective in the characterization and establishment of a behavioral model of microalloyed steels used in conditions of hydrogen-induced cracking, such as oil platforms at sea (off-shore structures).

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