Young-Chul Song

Enhanced durability performance of fly ash concrete for concrete-faced rockfill dam application

The main purpose of this research was to enhance the durability in both the design and construction of dams. Especially, in case of rockfill dams, the durability of face slab concrete in a concrete-faced rockfill dam (CFRD) is achieved by optimizing the fly ash replacement for cement. The effect on durability corresponding to the increasing replacement of fly ash was evaluated, and the optimum value of fly ash replacement was recommended. The results show that 15% of fly ash replacement was found to be an optimum level and demonstrated excellent performance in durability.

Creep prediction of concrete for reactor containment structures

Since the biggest time-dependent prestress loss of a prestressed concrete nuclear reactor containment structure is due to the creep of concrete, creep is one of the most important structural factors to be considered for the safety of a reactor containment structure during design, construction and maintenance. Creep in concrete has also recently been considered in evaluation of the crack resistance of concrete at an early-age in the durability examination of massive concrete structures like reactor containment structures. Existing empirical formulas on creep prediction show errors in their predictions due to simplified consideration of mixture proportions, and they also show large discrepancy among their predictions. In addition, they do not consider early-age behaviors of concrete and thus are mainly for the prediction of long-term creep at hardened concrete. In this paper, the creep characteristics of the reactors both early-age and hardened reactor concrete made of type V cement are examined by carrying out both early-age and long-term creep tests. Then, the creep of the reactor concrete is predicted by using major creep-prediction equations of the AASHTO LRFD design specification, the Japanese standard specification for concrete structure, the ACI Committee 209 and the CEB/FIP model code and the Bazant and Panulas model, and the predicted results are compared with the test results. From the comparison, the applicability of the creep-prediction equations for the concrete of a reactor containment structure at both early-age and hardened stages is discussed.

Study on the application of fiber Bragg grating sensors for the containment structures of nuclear power plants

In this paper, a Fiber Bragg Grating (FBG) sensor system for smart structures is described. FBGs are well-suited for long term and extremely severe experiments, where traditional strain gauges fail. In the system, a reflect wave-length measurement method which employs a tunable light source to find out the center wave-length of FBG sensor is used. The real field test was performed to verify the behaviors of fiber Bragg grating (FBG) sensors attached to the containment structure in Uljin nuclear power plant as a part of structural integrity test which demonstrates that the structural response of the non-prototype primary containment structures within predicted limits plus tolerances when pressurized to 115% of containment design pressure, and that the containment does not sustain any structural damage. The system works very well and it is expected that it can be used for a real-time strain, temperature and vibration detector of smart structure.

An Analytical Method for the Evaluation of Micro-cracking in Concrete Shrinkage Induced

Korea Institute of Construction Materials, Construction Material Research Center, Seoul 137-707, KoreaABSTRACT The majority of research that has been performed on cracking potential of concrete by shrinkage has assumed thatconcrete acts as a homogeneous material. However, with this approach, it is not able to evaluate the micro-cracking behavior inconcrete due to autogenous shrinkage under unrestrained boundary condition (free boundary condition) nor to understand the crack-ing behavior properly because of the heterogeneous nature of concrete. To better understand the micro-cracking behavior of con-crete induced by autogenous shrinkage, series of experiments were performed measuring the length change and acoustic emissionenergy. As an analytical approach, this research uses an object oriented finite element analysis code (OOF code) to simulate thebehavior of the concrete on a meso-scale. The concrete images used in the simulations were directly obtained from mortar samples.From the experiments and simulation results, it was able to better understand the micro-cracking behaviour of concrete due toshrinking of paste phase and internal restraint by aggregates.Keywords : unrestrained condition, micro-cracking, acoustic emission, finite element analysis, internal restraint

MEASUREMENT OF CRACK DEPTH LOCATED UNDER REINFORCEMENT IN REINFORCED CONCRETE SPECIMENS USING ULTRASONIC METHOD

The objective of this study is to determine crack depth located under reinforcement in concrete specimens using ultrasonic method. Experimental studies were performed on concrete specimens containing vertical and inclined surface-opening cracks with known depths. Experimental results have shown that the crack depth can be effectively measured when the distance between the probes is less than the crack depth. The effect of reinforcement on crack depth estimation is studied through a model by considering P-wave diffraction at the tip of crack and reinforcement. In addition, experimental results show that the ultrasonic method is one of useful methods to evaluate the crack depth in reinforced concrete.

Experiments on the Detection of Delamination in FRP Reinforced Concrete

With a growing concern about the state of infrastructure worldwide, the demand for the development of reliable nondestructive testing techniques (NDT) is ever increasing. Among possible NDT techniques, microwave method is proven to be effective in fast and non-contact inspection of concrete structures and inclusions inside concrete. It is also found that the microwave method has a potential in detecting the delamination between fiber reinforced polymers (FRP) plate and concrete. On the other hand, ultrasonic method can be another way to find the delamination. In this paper, the research work needed for the development of a reliable microwave method and ultrasonic method is studied in the measurements of concrete specimens reinforced with FRP. Concrete specimens are made with FRP and artificial delamination inside. A microwave measurement system with hom antennas with high center frequency and broad frequency bandwidth are used to image inside concrete specimens for the detection of debonding between concrete and FRP. Also, ultrasonic method is used for the same condition. Both results are compared with each other.