Kyung Joon Shin

Chloride Permeability of Damaged High-Performance Fiber-Reinforced Cement Composite by Repeated Compressive Loads

The development of cracking in concrete structures leads to significant permeability and to durability problems as a result. Approaches to controlling crack development and crack width in concrete structures have been widely debated. Recently, it was recognized that a high-performance fiber-reinforced cement composite (HPFRCC) provides a possible solution to this inherent problem of cracking by smearing one or several dominant cracks into many distributed microcracks under tensile loading conditions. However, the chloride permeability of HPFRCC under compressive loading conditions is not yet fully understood. Therefore, the goal of the present study is to explore the chloride diffusion characteristics of HPFRCC damaged by compressive loads. The chloride diffusivity of HPFRCC is measured after being subjected to various repeated loads. The results show that the residual axial strain, lateral strain and specific crack area of HPFRCC specimens increase with an increase in the damage induced by repeated loads. However, the chloride diffusion coefficient increases only up to 1.5-times, whereas the specific crack area increases up to 3-times with an increase in damage. Although HPFRCC shows smeared distributed cracks in tensile loads, a significant reduction in the diffusion coefficient of HPFRCC is not obtained compared to plain concrete when the cyclic compressive load is applied below 85% of the strength.

Chloride Resistance of Concrete with Marine Blended Cement Using Corrosion Resistant Mineral Admixture

In this study, the chloride resistant properties of concrete with MBC (marine blended cement) were evaluated by an accelerated corrosion test of reinforced concrete. The half-cell potential measurements indicated that the chloride resistance of MBC concrete is approximately 32% larger than that of TBC (ternary blended cement) concrete. As for the reinforcement corrosion area, while the TBC specimens exhibited 5.32-6.11% corrosion at the final 40th cycle, the MBC specimens did not show any sign of steel corrosion. The chloride penetration depth of the MBC specimens was approximately 41-79% smaller than that of the MBC specimens.

Damage Detection with FBG Sensors for Pre-Stress Concrete Girders

In this paper, possibility to detect damage on post-tensioned concrete girders was investigated through an experimental program with 6 m long specimens containing smart tendons where FBG (Fiber Bragg Grating) sensors were embedded. Total six specimens were fabricated and tested, and test variables were prestressing tendon’s profile and web thickness. All the specimens were subjected to 3-points loading, and they exhibited shear failure. Through the test, it was observed that tendon strains were successfully measured through FBG sensors regardless of the test variables. It was also observed that tendon strains within the pure span significantly increased while ones nearby the anchors beyond the pure span were constant. When the specimen was cracked, FBG sensors nearby cracks showed relatively drastic increase on tendon strain. Since strain variation along tendons cannot be detected by conventional equipment such as a load-cell, the test results indicated that actual tendon strains can be easily measured with FBG sensors. These results showed that FBG sensors can be useful to check whether prestressed concrete members were significantly damaged. In addition, it is expected that FBG sensors can be helpful on more reasonable maintenance of PSC girders.

The Effect of Specimen Size on the Results of Concrete Adiabatic Temperature Rise Test with Commercially Available Equipment

In this study, adiabatic temperature rise tests depending on binder type and adiabatic specimen volume were performed, and the maximum adiabatic temperature rises and the reaction factors for each mix proportion were analyzed and suggested. The results indicated that the early strength low heat blended cement mixture had the lowest maximum adiabatic temperature rise (Q∞) and the ternary blended cement mixture had the lowest reaction factor (r). Also, Q and r varied depending on the adiabatic specimen volume even when the tests were conducted with a calorimeter, which satisfies the recommendations for adiabatic conditions. Test results show a correlation: the measurements from the 50 L specimens were consistently higher than those from the 6 L specimens. However, the Q∞ and r values of the 30 L specimen were similar to those of the 50 L specimen. Based on the above correlation, the adiabatic temperature rise of the 50 L specimen could be predicted using the results of the 6 L and 30 L specimens. Therefore, it is thought that this correlation can be used for on-site concrete quality control and basic research.

Frictional Loss of Prestress Caused by Deflected Tendon

Bending of a prestressing tendon by construction error or the radius of curvature at the continuous joint of PSC girders cannot be avoided. However, this kind of prestress loss is not considered in design and construction processes. This study proves that prestress loss occurs at the continuous joint due to local bending of the tendon which is induced by construction error or the radius of curvature. The result shows that a maximum 3 % of prestress loss occurs at the continuous joint for a single tendon.

The Effect of Concrete Deformation on Displacement of an Axially Loaded Drilled Shaft

This article presents the settlement of drilled shafts resulting from their structural deformations. Although drilled shafts are widely used as foundations for settlement-sensitive structures such as bridges and high-rise buildings, the structural deformations of drilled shafts are not typically taken into account in the design process. However, if unexpected structural deformations of drilled shafts cause additional settlement to the foundation, the serviceability of the superstructure can be jeopardized. Unfortunately, very few research efforts have been made to quantify the structural deformation of drilled shafts; this needs to be addressed to accurately predict the settlement of drilled shafts. In this study, we investigate the effect of structural deformation on displacement of axially loaded drilled shafts. Finite element analyses were performed to quantify the structural deformation of drilled shafts. The analysis results indicated that the structural deformation of drilled shafts could be quite significant for long drilled shafts. The main factors that affected the structural deformation of drilled shafts were found to be pile length, the material properties of drilled shafts, and the relative humidity of surrounding soil. An approximate equation is proposed to estimate the long-term deformation of drilled shafts.

Chloride Ion Penetrability of HPFRCC after Loading

Cracking is one of the most important factors in the serviceability as well as durability performance of concrete structures. Recently, it was recognized that a high performance fiber-reinforced cementitious composite (HPFRCC) provides a possible solution to this inherent problem of cracking by smearing one or several dominant cracks into many distributed microcracks. The purpose of the present study is to explore the ductility characteristics of HPFRCC. The permeability of HPFRCC after subjected to different load levels were measured to identify the effect of reduced cracking among the mixtures. It was confined that the permeability of proposed mixtures was lower than that without microfibers. This means that the proposed materials can reduce the crack width greatly at the same applied loads

Chloride Ion Diffusion Coefficient of HPFRCC after Loading

Various methods have been used to reinforce cementitious material such as mortar and concrete that have weak tensile strength. Fiber Reinforced Concrete is one of the reinforcing methods that mixes a matrix with fibers that have strong tensile strength. Recently, High Performance Fiber Reinforced Cementitious Composites (HPFRCC) have been developed. HPFRCC provides a possible solution to this inherent problem of cracking by smearing one or several dominant cracks into many distributed microcracks. The present study explores the ductility characteristics of HPFRCC by measuring chloride diffusion coefficients after load is applied.

On the Hypothesis of Effective Stress in Consolidation and Strength for Unsaturated Soils

The hypothesis on effective stress of unsaturated soils is validated by consolidation strength results of triaxial tests for the compacted residual soil. The effective stress can describe the unsaturated soil behavior, which was defined from shear strength or from soil water characteristic curves. Since the effective stress from consolidation agrees with that from the shear strength, the effective stress from soil water retention curve could describe the unsaturated behavior consistently on both consolidation path and stress at failure. The effective stress can describe the entire unsaturated behavior from consolidation to failure.

Flexural Toughness of Micro-Fiber Reinforced Mortar

Various methods have been used to reinforce cementitious materials which have a weak tensile strength. A major reinforcing method is to mix the matrix with fibers which have a strong tensile strength. Recently, micro-fiber reinforced mortar has been studied. This type of mortar has the coarse aggregates removed and uses micro-fibers in order to homogenize the matrix properties and maximize the performance of the fibers. Micro-fiber reinforced mortar can exhibit different cracking characteristics, showing many distributed cracks. Hence, the flexural performance is different from that of conventional fiber reinforced mortar. This paper investigates the flexural behaviors of micro-fiber reinforced mortars which have different fiber types and mixture proportions. The flexural behavior, flexural toughness, and number of cracks are explored with respect to the fiber types and contents.