Papers by Author: Sung Wook Kim

Abstract: This study investigates the effects of the compressive strength and area ratio of coarse aggregate of concrete for offshore construction on the erosion depth. To evaluate the erosion resistance of concrete, test was performed according to ASTM C 1138 using a plain mix with design strength of 30 MPa using only Portland cement and five other mixes including different quantities of fly ash and blast furnace slag. The test results showed that the erosion depth is influenced not only by the compressive strength but also by the area ratio of coarse aggregate.

Abstract: The placing of anti-washout underwater concrete for the increasing construction of offshore foundations of long-span bridges at relatively deep sea is accompanied with significant loss of the quality of concrete as well as of the durability and reliability of the structure due to limitations brought by the workability of the high-pressure pump and the performance of the anti-washout agent. Therefore, the improvement of the performance of the underwater anti-washout agent and the development of high fluidity anti-washout underwater concrete exhibiting improved packing ability and fluidity should be implemented. Accordingly, this study focuses on the concrete for foundations applied in the RCD (Reverse Circulation Drill) method. The adequate mix proportions of a high fluidity anti-washout underwater concrete satisfying a design compressive strength of 35 MPa, slump flow larger than 600 mm and with less than 50 mg/ℓ of suspended solids is derived and the corresponding characteristics are evaluated.

Abstract: Ultra high performance concrete (UHPC), characterized by a high strength and high ductility, is also subjected to large shrinkage due to its low water-to-binder ratio and its large content in high fineness materials. The large amount of autogenous shrinkage of UHPC can induce crack on structural member when it was restrained with reinforcement and form. However, shrinkage of UHPC in plastic state is not generating confining stress, which is the main cause of initial crack. Normally, the setting time in concrete is an index to distinguish shrinkage which occur confining stress or not. An estimation of setting time is conducted in compliance with ASTM C 403 till now however, that test standard reveals error of results due to discordance of test condition as following with concrete type. This study therefore evaluated setting time of UHPC through the modified test method which was proposed by KICT. Test results and analyses proved a discrepancy of setting time between ASTM and proposed method. The proposed method put faith in evaluation of setting time in accordance with UHPC.

Abstract: Since ultra-high performance concrete (UHPC) is subject to large occurrence of shrinkage at early age due to its low water-to-cement ratio, the mixing of large quantities of powdered admixtures and the absence of coarse aggregates, UHPC presents large risks of shrinkage cracking caused by the restraints provided by the form and reinforcing bars. Accordingly, this study intends to evaluate the shrinkage behavior of UHPC under restrained state by performing restrained shrinkage test using ring-test. The test results reveal that increasing thickness of the inner ring increases the tensile creep at early age leading to the reduction of the average strain and residual stress of the inner ring.

Abstract: This study, in order to identify the effects that the mixed-use of long and short fiber has on the mechanical characteristics of UHPC, we used a steel fiber of 65, 82 and 98 in a shape ratio (l/d) alternating from the length of 13, 16,3 and 19.5 mm respectively in the short fiber with a diameter of 0.2 mm and used the hooked-type high-elasticity steel fiber with a diameter of 0.375 and the length of 30 mm of the long fiber. As a result, it was found that, in constructability and compressive behavior in the same fiber mixing ratio, the more the mixed amount of short fiber, the higher the compressive strength, but it was analyzed that the increase of width was very narrow. According to the results from the review on the flexural tensile behavior of UHPC to make a hybrid from the straight-short and hooked-long fiber, it was analyzed that the use of hybrid fiber does not significantly contribute to the enhancement of the flexural strength of UHPC, however, it greatly contributes to the enhancement of the flexural toughness. In the case of using the fiber in this method, it was analyzed that it would be possible to manufacture UHPC which is superior in workability and mechanical characteristics and economically feasible.

Abstract: Since ultra-high performance cementitious composite (UHPCC) exhibits remarkable compressive strength and ductility, its application for structural member provides significant reduction of the section. The use of low water-to-binder ratio and admixtures with high fineness induce large shrinkage strain, leading to the possibility of large occurrence of shrinkage cracks due to the restraints of the form and reinforcing bars. Therefore, this study admixes a combination of expansive and shrinkage reducing agents as a solution to reduce the shrinkage of UHPCC. The eventual appropriateness is computed to evaluate the free shrinkage and restrained shrinkage behaviors. From the test results, the admixing of a combination of expansive and shrinkage reducing agents is seen to achieve a reduction of the free shrinkage by 30% to 50% at 28 days and a reduction of the strain of the external steel by about 19%.

Abstract: This intends to examine the flexural behavioral characteristics of hybrid UHPC using a mix of steel fibers with different lengths. Three types of fibers are adopted with fixed diameter of 0.2 mm and lengths of 13, 16.3 and 19.5 mm (aspect ratio of 65, 82 and 98, respectively). Comparative analysis of the flexural strength, load bearing capacity, deflection and toughness is performed adopting a mix use of these 3 types of steel fibers with ratio of 2% and 1.5%. The results show that the hybrid use of steel fibers improves significantly the flexural strength and flexural toughness compared to the use of a single type of fiber. When steel fibers with lengths of 16.3 mm and 19.5mm are admixed at a rate of 1% each, UHPC develops a flexural strength larger by 27% (maximum 50%) than conventional UHPC admixed with 2% of steel fiber with length of 13 mm. Moreover, flexural strength similar to that of conventional UHPC is secured when steel fibers with lengths of 16.3 mm and 19.5mm are admixed at respective rates of 0.5% and 1% (total rate of 1.5%).

Abstract: Ultra High Strength Steel-Fiber Reinforced Concrete (UHS-SFRC) is characterized by very high compressive and tensile strength that is about 8 times of ordinary concrete, and high ductility owing to the addition of steel fibers. This paper investigates the relationship existing among the direct tensile strength, flexural tensile strength and splitting tensile strength of UHS-SFRC. Differently from ordinary concrete, it is found that the first cracking strengths in UHS-SFRC obtained through direct tensile test and splitting tensile test are similar, while the strength obtained from flexural tensile test is significantly larger than those from other tests. Based on the experimental results, relationships between the direct tensile strength and flexural tensile strength, between the first cracking strengths in direct tensile test and in flexural tensile test, and between the first cracking strength in direct tensile test and the flexural tensile strength are proposed.

Abstract: The fibers alignment in steel fiber reinforced high strength concrete (SFR-HSC) has naturally significant influence on the mechanical properties of concrete. Fiber-reinforced concrete being manufactured by means various kinds of specimen shape and diversified filling methods and directions, these variables are likely to produce effect on the fibers alignment leading to large differences in its mechanical properties. This study intended to evaluate the effect of placing and flow direction not only on the fibers alignment but also on the tensile behavior of SFR-HSC. Section analysis using photographic shooting was adopted to investigate the fiber alignment and revealed considerable difference in the fiber alignment according to the placing and flow direction. The best alignment appears to be achieved when placing is done in the direction of the flexural tensile stress and the alignment was change with the flow distance although the same flow direction. Such placing and flow direction produce little difference in the first cracking strength but significant discrepancy up to 50% in the ultimate tensile strength.

Abstract: This paper estimated the influence of volume of steel fiber on the tensile softening behavior in Ultra High Performance Concrete. Tensile softening curves were obtained from Three-Point Bending Test(TPBT) with notched beam. Inverse analysis method by Uchida et al. was introduced to obtain the tensile softening behaviors from the results of TPBT. We could find out that the increase of volume fraction of steel fiber makes the tensile strength higher but all of the curves converged on an asymptote with crack width. We proposed the equation of softening curve expressed by combination of plastic area and exponential descending area considering the volume fraction of steel fiber and ω0, which is corresponding to the maximum crack width of plastic area. We also carried out the crack propagation analysis using finite element method with smeared crack model and confirmed that the proposed equation had a good agreement with the experimental results.