Seung Yup Jang

Development of high-performance concrete having high resistance to chloride penetration

The resistance to chloride penetration is one of the simplest measures to determine the durability of concrete, e.g. resistance to freezing and thawing, corrosion of steel in concrete and other chemical attacks. Thus, high-performance concrete may be defined as the concrete having high resistance to chloride penetration as well as high strength. The purpose of this paper is to investigate the resistance to chloride penetration of different types of concrete and to develop high-performance concrete that has very high resistance to chloride penetration, and thus, can guarantee high durability. A large number of concrete specimens have been tested by the rapid chloride permeability test method as designated in AASHTO T 277 and ASTM C 1202. The major test variables include water-to-binder ratios, type of cement, type and amount of mineral admixtures (silica fume, fly ash and blast-furnace slag), maximum size of aggregates and air-entrainment. Test results show that concrete containing optimal amount of silica fume shows very high resistance to chloride penetration, and high-performance concrete developed in this study can be efficiently employed to enhance the durability of concrete structures in severe environments such as nuclear power plants, water-retaining structures and other offshore structures.

Charge states and magnetic ordering in LaMnO3/SrTiO3 superlattices

We investigated the magnetic and optical properties of [(LaMnO{sub 3}){sub n}/(SrTiO{sub 3}){sub 8}]{sub 20} (n = 1, 2, and 8) superlattices grown by pulsed-laser deposition. We found that a weak ferromagnetic and semiconducting state developed in all superlattices. An analysis of the optical conductivity showed that the LaMnO{sub 3} layers in the superlattices were slightly doped. The amount of doping was almost identical regardless of the LaMnO{sub 3} layer thickness up to eight unit cells, suggesting that the effect is not limited to the interface. On the other hand, the magnetic ordering became less stable as the LaMnO{sub 3} layer thickness decreased, probably due to a dimensional effect.

Modeling Mechanical Behavior of Reinforced Concrete due to Corrosion of Steel Bar

Expansion of a steel bar due to corrosion causes tensile stresses in concrete around the reinforcing bar and may induce cracking through the concrete cover. The purpose of the present study is to explore realistic mechanical properties of the corrosion layer, including the pressure-free corrosion strain and the stiffness of the corrosion layer. The corrosion products that penetrate into the pores and cracks around a steel bar have been considered in the calculation of expansive pressure due to steel corrosion. A realistic relationship between the expansive pressure and the average strain of the corrosion product layer has been derived and the representative stiffness of the corrosion layer was also determined. A concept of free-expansion (pressure-free) strain of the corrosion product layer was newly introduced to describe the relationship between the expansive pressure and the corrosion layer strain. The comparisons were made between the theory and test data on the surface strains of the concrete cover according to the corrosion amount for various concrete strengths and cover thicknesses. The proposed theory reasonably agrees with experimental data and may be a good base for the realistic durability analysis/design of concrete structures.

Effects of oxygen-reducing atmosphere annealing on LaMnO3 epitaxial thin films

We investigated the effects of annealing on LaMnO3 epitaxial thin films grown by pulsed laser deposition (PLD) and propose an efficient method of characterizing their stoichiometry. Structural, magnetic and optical properties coherently indicate non-stoichiometric ferromagnetic and semiconducting phases for as-grown LaMnO3 films. By annealing in an oxygen-reducing atmosphere, we recovered the antiferromagnetic and insulating phases of bulk-like stoichiometric LaMnO3. We show that non-destructive optical spectroscopy at room temperature is one of the most convenient tools for identifying the phases of LaMnO3 films. Our results serve as a prerequisite for studying LaMnO3 based heterostructures grown by PLD.

Double polarization hysteresis loop induced by the domain pinning by defect dipoles in HoMnO3 epitaxial thin films

We report on antiferroelectriclike double polarization hysteresis loops in multiferroic HoMnO3 thin films below the ferroelectric Curie temperature. This intriguing phenomenon is attributed to the domain pinning by defect dipoles which were introduced unintentionally during film growth process. Electron paramagnetic resonance suggests the existence of Fe1+ defects in thin films and first principles calculations reveal that the defect dipoles would be composed of oxygen vacancy and Fe1+ defect. We discuss migration of charged point defects during film growth process and formation of defect dipoles along ferroelectric polarization direction, based on the site preference of point defects. Due to a high-temperature low-symmetry structure of HoMnO3, aging is not required to form the defect dipoles in contrast to other ferroelectrics (e.g., BaTiO3).