Papers by Author: Seung Hun Kim

Abstract: This study conducted a comparative evaluation of bond behavior according to the steel reinforcing type and substitution rate of recycled coarse aggregate. Mixed as high-strength concrete of design strength 50MPa in order to verify the applicability of recycled coarse aggregate concrete as a structural member. Total of 24 specimens were manufactured with variables of recycled aggregate substitution rate (0%, 30%, 50%, 100%), steel reinforcing arrangement direction (vertical, horizontal top·bottom) and steel reinforcing types (SD400 D16, D19). The specimen was produced according to CSA S802-02 and ASTM 234. specimens using D16 reinforcing, the vertical reinforcing showed similar bond stress regardless of the recycled aggregate substitution rate and slip amount, and for the horizontal reinforcing, the bottom specimen showed similar bond stress as the vertical reinforcing and slip amount. The top specimen showed slightly less bonds tress than vertical and horizontal bottom specimens. This is thought to have been caused by the subsidence of the aggregate. The specimen using D19 reinforcing showed similar bond stress in vertical · horizontal top · bottom, horizontal top reinforcing maximum bond stress, horizontal bottom reinforcing maximum bond stress and showed no difference due to aggregate subsidence like in D16. As the result, Bond stress in this study is larger than 2.0fck, the value proposed by CEB-FIP code. The specimen using D16 reinforcing bar showed 4.5fck and the specimen using D19 reinforcing bar was near 3.9fck. Therefore, since the standard value is satisfied regardless of replacement rate of recycled aggregate, natural aggregate in high strength concrete can be substituted by recycled aggregate.

Abstract: In order to maintain the existing concrete structures in a safe and usable state, an overall maintenance management is necessary regarding structure aging from quality management of new construction. Thus, non-destructive testing is needed to estimate the structure damage, defect, or proper construction without damaging the structure. In U.S., there is a standard for non-destructive test (ACI 228.2R-98), and also in Japan, the non-destructive test method and compressive strength estimation manual was prepared by the Architectural Institute of Japan in 1983, and there are active researches in the ground field, but it lacks verification in architecture field. Thus, in this study, a technique that can estimate the depth of concrete column member using the Impact Echo method which is one of the non-destructive test methods shall be reviewed and evaluated for applicability to the architecture field. The specimen was mixed with design strength of 30MPa. The equipment used in testing is Freedom Date Pc Platform Win.TFS 2.5.2 by company Olson of U.S., and the experiment involved leveling the top surface of the concrete member, installing the equipment and applying impact 9 times, and taking the average of the reverberation values obtained. The estimated average depth of concrete column member using Impact Echo method was 304mm for IEC-300, 398mm for IEC-400, and 484mm for IEC-500, and the relative error rate compared to the actual size was 1%~3%. Through this study, the applicability of estimation of depth in concrete column members using impact echo method could be confirmed.

Abstract: This study used the ultrasonic pulse velocity method, one of the non-destructive test methods that does not damage the building for maintenance of to-be-constructed concrete structures using recycled aggregates in order to estimate the compressive strength of high strength concrete structure using recycled coarse aggregate and provide elementary resources for technological establishment of ultrasonic pulse velocity method. 200 test pieces of high strength concrete 40, 50MPa using recycled coarse aggregate were manufactured by replacement rates (0, 30, 50, 100%) and age (1, 7, 28, 180days), and air curing was executed to measure compressive strength and wave velocity. As the result of compressive strength measurement, the one with age of 180day and design strength of 40MPa was 43.69MPa, recycled coarse aggregate replacement rate of 30% 50% 100% were 42.82, 41.22, 37.35MPa, and 50MPa was 52.50MPa, recycled coarse aggregate replacement rate of 30% 50% 100% were 49.02, 46.66, 45.30MPa, and while it could be seen that the test piece substituted with recycled aggregate was found to have lower strength than the test piece with natural aggregate only, but it still reached the design strength to a degree. The correlation of compressive strength and ultrasonic pulse velocity was found and regression analysis was conducted. The estimation formula for compressive strength of high strength concrete using recycled coarse aggregate was found to be F_c=0.069V_p^4.05, R²=0.66

Abstract: This study uses the ultrasonic pulse velocity method, one of the non-destructive testing methods that do not damage the structure, to analyze the correlation of wave velocity and compressive strength of normal strength and high strength concrete to propose an estimation formula. And a concrete member is produced to estimate the compressive strength, and provide elementary data for establishing a site-applicable standard for strength estimation formula. In order to analyze the correlation of concrete compressive strength and wave velocity, total of 144 specimens were produced per age, and six concrete members were produced for wave velocity measurement. In this study, the Ultrasonic Pulse Velocity method was used to propose a compressive strength estimation formula of normal strength and high strength concrete, which was applied to concrete members for verification, and the following conclusion was obtained. The correlation between wave velocity and compressive strength was found to be F_c=0.0952V_P^3.5, R²=0.82 for normal strength concrete, and F_c=0.0028V_P^6.1, R²=0.83 for high strength concrete. Result of comparing the estimated strength of the concrete member of the relative error rate was standard strength 18.7% on average, the high-strength 8.7%

Abstract: Recently, the interest in maintenance and repair of existing concrete structures have increased, and it is typical to use non-destructive testing methods such as rebound hardness test or ultrasonic pulse velocity method to execute maintenance and repair of structures efficiently. Many non-destructive testing methods are being used in practice such as at construction sites, but verification for site applications are quite inadequate. Thus, this study intends to evaluate the applicability of Impact Echo Method which is one of the non-destructive testing methods using stress wave. Total of four specimens were planned and produced. The thickness of concrete slab members was estimated using I.E(OLSENs Freedom Data PC with Win.TFS Software Version 2.5.2). The estimated materials of concrete members by IE was found to be IE-1 specimen 178mm, IE-2 specimen 197mm, IE-3 specimen 191mm, and IE-4 specimen 263mm, and the error rate was found to be 4.22%~18.67% (average 9.6%), showing that they are relatively well in agreement. In this study, the experiments were executed with the objective of estimating the thickness of concrete slab members using Impact Echo Method. Through this study, the applicability of thickness estimation in concrete slab members using impact echo method could be confirmed.

Abstract: Non-destructive testing methods, unlike typical destructive testing methods that deconstruct or cut the building in case of issues such as pores, heterogeneous material, cracks or any such equivalent issues inside/outside the building. And refer to the testing methods for pores, heterogeneous material, or defectiveness occurring in the specimen without changes or destruction of internal structure using ultrasound, radiation, electromagnetism, fluid, heat, or light. In this study, among such non-destructive testing methods, the impact echo method was used for an experiment to estimate the steel rebar location and thickness in the concrete mock member. The mix was made with design standard strength of 30MPa, and for the steel rebar, diameter 22mm was used on the specimen of 300×370×200 to install spacer on the ground surface, and after separating by 40mm, it was arranged with 130mm and 150mm from the top of the specimen to the top of the rebar in 1 column and 3 rows. The specimen for thickness estimation was manufactured with total length of 1800×300 and 6 varying thicknesses of 150mm, 180mm, 210mm, 240mm, 270mm, and 300mm. As the result of rebar location estimation, the maximum resonant frequency was found to be 11269Hz, 9453Hz,and the rebar location estimates were 127.8mm and 151.8mm, which was relatively accurate with error rate of 1.72% and 1.19% from the actual value. In case of thickness estimation specimen, the error rates comparing actually measured thickness and the average value were 2.2%, 2.2%, 4.6%, 0.9%, 3.8%, and 4.7%, which were relatively accurate with average of 3.1%. Through this study, the applicability of steel rebar location and thickness estimation in concrete members using impact echo method could be confirmed.

Abstract: This study evaluates the flexural performance of reinforced concrete beams with GFRP(Glass Fiber Reinforced Polymer) bars and RCA(Recycled Coarse Aggregates). A total of four specimens with various replacement ratios of RCA (0%, 30%, 50%, and 100%) were tested. An investigation was performed on the influence of RCA with various replacement ratios on load-carrying capacity, post cracking stiffness, cracking pattern, and ductility. The test results showed that replacement ratios of RCA had not a bad effect on concrete compressive strength or flexural strength of beams. They were compared with the design flexural strength and the nominal moment predictions of ACI Code.

Abstract: Stainless steel has been utilized on structural members of building due to significant characteristics in its superior corrosion resistance, durability, aesthetic appeal etc. Recently, structural behaviors and curling effect in single shear bolted connection fabricated with thin-walled plane plates have been studied by T.S. Kim. In this paper, finite element analysis (FEA) has been conducted based on the existing test result of channel bolted connections in austenitic stainless steel. The validation of numerical approach was verified to predict the structural behaviors such as fracture mode, ultimate strength and curling occurrence of bolted connections. Curling (out-of- plane deformation) also occurred in the connections with a long end distance like bolted connections assembled with plane plates. The curling caused the ultimate strength reduction and ultimate strength reduction caused by curling has been estimated quantitatively through the comparison of FEA results of FE models with free edge and restrained curling. Additional parametric analysis for FE models with extended variables has been performed. Therefore, the ultimate strengths were compared with current design strengths and reasonable strength formulae for connections with no curling effect were proposed.

Abstract: Recently, many structures which were built about 30 years ago are watched by reconstruction. Demolished concrete is occurred in the process and these quantity increase about 10% more than the preceding year. Fortunately, recycled aggregates are produced from demolished concrete, whereas the recycled aggregates are not used often because there are not many researches which have been verified by experts or researchers about strength when reinforced concrete is made with recycled aggregates. In this paper, high strength reinforced concrete is valued with potential applications and check change of strength when it made by recycled aggregates. For this, flexural tests of 4 high strength reinforced concrete beams with recycled aggregates were performed, and the high strength reinforced concrete beams were tested within the limits such as compressive strength, flexural strength, ductility, strain, and curvature. The current test data were examined in terms of flexural strength, along with the data from previously tested reinforced concrete beams with recycled aggregates.

Abstract: As the industrial development brings the structures growth, the consumption of the natural aggregate is increasing annually. On the other hand, the amount of the potential aggregate is decreasing with the huge amount of construction waste from the concrete demolition. The annual total amount of natural aggregate consumption estimated 370million tons. And the experts are concerned about the depletion of aggregate which is presumed to be exhausted in 20 years, considering the annual consumption, which is approximately 250million(m³) in construction industry. Along with this issues, the government promotes to use recycled coarse aggregate from the construction waste in consideration of social, economic and environmental aspects. In conclusion, the purpose of this study is to promote to use of recycled coarse aggregate as a future substitute resource and to increase awareness of it as well. This study conducts the experiments by comparing high-strength concrete for the substitution of chosen two companies.