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CONTENTS
Volume 65, Number 2, January25 2018
 

Abstract
Post-tensioning (PT) tendons are commonly used for the assembly of modularized concrete members, and tension is applied to the tendons during construction to facilitate the integrated behavior of the members. However, the tension in a PT tendon decreases over time due to steel corrosion and concrete creep, and consequently, the stress on the anchor head that secures the PT tendon also diminishes. This study proposes an automatic detection system to identify tension reduction in a PT tendon using pulsed-eddy-current (PEC) measurement. An eddy-current sensor is installed on the surface of the steel anchor head. The sensor creates a pulsed excitation to the driving coil and measures the resulting PEC response using the pick-up coil. The basic premise is that the tension reduction of a PT tendon results in stress reduction on the anchor head surface and a change in the PEC intensity measured by the pick-up coil. Thus, PEC measurement is used to detect the reduction of the anchor head stress and consequently the reduction of the PT tendon force below a certain threshold value. The advantages of the proposed PEC-based tension-reduction-detection (PTRD) system are (1) a low-cost (< $ 30), low-power (< 2 Watts) sensor, (2) a short inspection time (< 10 seconds), (3) high reliability and (4) the potential for embedded sensing. A 3.3 m long full-scale monostrand PT tendon was used to evaluate the performance of the proposed PTRD system. The PT tendon was tensioned to 180 kN using a custom universal tensile machine, and the tension was decreased to 0 kN at 20 kN intervals. At each tension, the PEC responses were measured, and tension reduction was successfully detected.

Key Words
post-tensioning tendon; tension reduction detection; pulsed-eddy-current measurement; post-tensioned concrete; nondestructive testing and evaluation

Address
Ji-Min Kim, Jun Lee and Hoon Sohn: Department of Civil Engineering, Korean Advanced Institute for Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea


Abstract
In this study, the bend-buckling strength of the web in longitudinally stiffened plate girder was numerically investigated. The buckling strength of the reinforced web was evaluated through an eigenvalue analysis of the hypothetical model, in which the top and bottom junctions of the web to the flanges were assumed as simple support conditions. Major parameters in the analysis include asymmetrical cross-sectional property, aspect ratio of the web, stiffener locations, and bending rigidity of the stiffeners. The numerical results showed that current AASHTO LRFD specifications (2014) provides the buckling strength from considerably safe side to slightly unsafe side depending on the location of the stiffeners. A modified equation for buckling coefficients was proposed to solve the shortcomings. The bending rigidity requirements of longitudinal stiffeners stipulated in AASHTO were also investigated. It is desirable to increase the rigidity of the stiffeners when the aspect ratio is less than 1.0.

Key Words
plate girder; longitudinal stiffener; in-plane bending; web bend-buckling strength; rigidity of stiffener

Address
Hee Soon Kim, Yong Myung Park and Byung Jun Kim: Department of Civil Engineering, Pusan National University, Busan 46241, Republic of Korea
Kyungsik Kim: Department of Civil Engineering, Cheongju University, Cheongju 28503, Republic of Korea

Abstract
A smart glass fiber reinforced polymer (SMFRP) reinforcing bar with a fiber Bragg grating (FBG) sensor was fabricated using a pultrusion technique, while ribs were formed to improve bonding between concrete and SMFRP. Then, strain of SMFRP bars were measured for a uniaxial tension test of an SMFRP bar, and a four-point bending test of concrete beams reinforced with SMFRP bars. The results of a uniaxial tension test illustrate that the strain obtained from an FBG sensor agrees well with that obtained from electrical resistance strain gauge (ERSG). Additionally, concrete beams reinforced with SMFRP bars were fabricated, and actual flexural test were performed while the strain of with an FBG sensor was compared with that of ERSG. The experimental results demonstrate that SMFRP bars can be used as reinforcement of concrete member while providing deformation information. Furthermore, SMFRP bars may provide stronger durability and smart monitoring to reinforced concrete members under corrosive environments during a service life.

Key Words
SMFRP bar; fiber Bragg grating (FBG) sensor; strain, reinforced concrete; four point bending test

Address
Minkwan Ju and Kyoungsoo Park: Department of Civil and Environmental Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea
Doyoung Moon: Department of Civil Engineering, Kyungsung University, 309 Sooyoung-ro, Nam-gu, Busan, 48434, Republic of Korea
Cheolwoo Park: Department of Civil Engineering, Kangwon National University, 1 Joongang-ro, Samcheok-si, Kangwon, 25913, Republic of Korea
Jongsung Sim:Department of Civil and Environmental Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do,15588, Republic of Korea

Abstract
Plain concrete is a brittle material with a very low tensile strength compared to compressive strength and critical tensile strain. This study analyzed the dynamic characteristics of high-performance fiber-reinforced cementitious composites based on slurry-infiltrated fiber concrete (SIFCON-based HPFRCC), which maximizes the steel–fiber volume fraction and uses high-strength mortar to increase resistance to loads, such as explosion and impact, with a very short acting time. For major experimental variables, three levels of fiber aspect ratio and five levels of fiber volume fraction between 6.0% and 8.0% were considered, and the flexural strength and toughness characteristics were analyzed according to these variables. Furthermore, three levels of the aspect ratio of used steel fibers were considered. The highest flexural strength of 65.0 MPa was shown at the fiber aspect ratio of 80 and the fiber volume fraction of 7.0%, and the flexural strength and toughness increased proportionally to the fiber volume fraction. The test results according to fiber aspect ratio and fiber volume fraction revealed that after the initial crack, the load of the SIFCON-based HPFRCC continuously increased because of the high fiber volume fraction. In addition, sufficient residual strength was achieved after the maximum strength; this achievement will bring about positive effects on the brittle fracture of structures when an unexpected load, such as explosion or impact, is applied.

Key Words
SIFCON-based HPFRCC; flexural strength; flexural toughness; aspect ratio; steel-fiber volume fraction

Address
Seugnwon Kim, Haekook Jung, Yongjae Kim and Cheolwoo Park: Department of Civil Engineering, Kangwon National University, 346 Choongang-ro, Samcheok 25913, Republic of Korea

Abstract
In this paper, finite element (FE) model updating based on multi-objective optimization with the surrogate model for a steel plate girder bridge is investigated. Conventionally, FE model updating for bridge structures uses single-objective optimization with finite element analysis (FEA). In the case of the conventional method, computational burden occurs considerably because a lot of iteration are performed during the updating process. This issue can be addressed by replacing FEA with the surrogate model. The other problem is that the updating result from single-objective optimization depends on the condition of the weighting factors. Previous studies have used the trial-and-error strategy, genetic algorithm, or user\'s preference to obtain the most preferred model; but it needs considerable computation cost. In this study, the FE model updating method consisting of the surrogate model and multi-objective optimization, which can construct the Pareto-optimal front through a single run without considering the weighting factors, is proposed to overcome the limitations of the single-objective optimization. To verify the proposed method, the results of the proposed method are compared with those of the single-objective optimization. The comparison shows that the updated model from the multi-objective optimization is superior to the result of single-objective optimization in calculation time as well as the relative errors between the updated model and measurement.

Key Words
FE model updating; surrogate model; multi-objective optimization; pareto-optimal front

Address
Yongmoon Hwang, Seung-seop Jin, Ho-Yeon Jung and Hyung-Jo Jung: Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology,
291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
Sehoon Kim and Jong-Jae Lee: Department of Civil and Environmental Engineering, Sejong University,
209 Neungdong-ro, Gwangjin-gu, Seoul 05006, Republic of Korea

Abstract
The prestressed concrete (PSC) technology that was first developed by Freyssinet has significantly improved over the past century in terms of materials and structural design in order to build longer, slender, and more economic structures. The application of prestressing method in structures, which is determined by the pre-tension or post-tension processes, is also affected by the surrounding conditions such as the construction site, workforce skills, and local transportation regulations. This study proposes a prestressed concrete girder design based on a hybrid segment concept. The adopted approach combines both pre-tension and post-tension methods along a simple span bridge girder. The girder was designed using newly developed 2400 MPa PS strands and 60 MPa high-strength concrete. The new concept and high strength materials allowed longer span, lower girder depth, less materials, and slender design without affecting the lateral stability of the girder. In order to validate the applicability of the proposed hybrid prestressed segments girder, a full-scale 35 m girder was fabricated, and experimental tests were performed under various fatigue and static loading conditions. The experimental results confirmed the feasibility of the proposed long-span girder as its performance meets the railway girder standards. In addition, the comparison between the measured load-displacement curve and the simulation results indicate that simulation analysis can predict the behavior of hybrid segments girders.

Key Words
hybrid segment girder, pretension, post-tension, fatigue and static loading test

Address
Hong Jae Yim: Department of Construction and Disaster Prevention Engineering, Kyungpook National University, 2559 Gyeongsang-daero, Sangju, Gyeongsangbuk-do, 742-711, Republic of Korea
Jun Mo Yang and Jin Kook Kim: Product Application Center, POSCO, 100, Songdowahak-ro, Yeonsu-gu, Incheon, 406-840, Republic of Korea

Abstract
The corrosion environments in a steel structure are significantly different depending on the individual parts of the members. To ensure the safety of weathering steel structures, it is important to evaluate the time-dependent corrosion behavior. Thus, the progress and effect of corrosion damage on weathering steel members should be evaluated; however, the predicted corrosion depth, which is affected by the corrosion environment, has not been sufficiently considered until now. In this study, the time-dependent thicknesses of the corrosion product layer were examined to quantifiably investigate and determine the corrosion depth of the corroded surface according to the exposure periods and corrosion environments. Thus, their atmospheric exposure tests were carried out for 4 years under different corrosion environments. The relationship between the thickness of the corrosion product layers and mean corrosion depth was examined based on the corrosion environment. Thus, the micro corrosion environments on the skyward and groundward surfaces of the specimens were monitored using atmospheric corrosion monitor sensors. In addition, the evaluated mean corrosion depth was calculated based on the thickness of the corrosion product layer in an atmospheric corrosion environment, and was verified through a comparison with the measured mean corrosion depth.

Key Words
corrosion depth; corrosion product layer; weathering steel; atmospheric exposure test; electro-magnetic coating thickness tester

Address
Shigenobu Kainuma: Department of Civil Engineering, Kyushu University, 744, Motooka, Fukuoka 8190395, Japan
Yuya Yamamoto: Planning Department, Japan Expressway Holding and Debt Repayment Agency, 1-1-2, Takashima, Yokohama 2200011, Japan
Jin-Hee Ahn: Department of Civil Engineering, Gyeongnam National University of Science and Technology,
33 Dongjin-ro, Jinju, Gyeongnam 52725, Republic of Korea
Young-Soo Jeong: Seismic Simulation Test Center, Pusan National University, 49 Busandaehak-ro, Yansan, Gyeongnam 50612, Republic of Korea

Abstract
This paper introduces the development process of NCC(New Concrete Cutting) system and analyzes first verification test. Based on the first verification test results, some problems of NCC system have been newly modified. We carry out the second verification test. We tried to verify cutting performance and dust control efficiency of NCC system through the cutting test of concrete bridge piers. In particular, this verification test strives to solve the problem of concrete dust, which is the biggest problem of dry cutting method. The remaining dust problems in cutting section tried to solve through this verification test. This verification test of the NCC system shows that the dust problem of dry cutting method is closely controlled and solved. In conclusion, the proposed NCC method is superior to the dry cutting method in all aspects, including cutting performance, dust vacuum efficiency and cooling effect. The proposed NCC system is believed to be able to provide eco-friendly cutting technology to various industries, such as the removal of the SOC structures and the dismantling of nuclear plants, which have recently become a hot issue in the field of concrete cutting.

Key Words
concrete cutting; NCC method; water-cooled cutting; dry cutting; dust vacuum efficiency; temperature

Address
Park Jong-Hyup: Technical Research institute, Ok Dang Industry Co., Ltd, Ace Hightech II, 25, Seonyu-ro, 13-gil, Yeongdeungpo-gu, Seoul, Republic of Korea
Han Jong-Wook: Hybrid Structural Test Center, Myong-Ji University, Young-In City, Republic of Korea


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