JOURNAL OF ADVANCED CONCRETE TECHNOLOGY (Japan Concrete Institute)
Effects of Severe Accident Conditions on Integrity of RPV Pedestal of Fukushima Daiichi Nuclear Power Plant
Osamu Kontani, Takashi Okayasu, Keishi Kawasumi, Shunsuke Ishikawa, Hiroshi Masaki, Norihiko Tanaka, Yasuyuki Goto, Shinichi Ishioka
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.20, 2022.
(https://doi.org/10.3151/jact.20.444)
Abstract
The Fukushima Daiichi Nuclear Power Plant lost its core cooling function due to the massive tsunami generated by the 2011 off the Pacific coast of Tohoku Earthquake, which caused core meltdown, resulting in high temperature inside the containment vessel and exposing the RPV pedestal, a reinforced concrete structure, to an unusually high temperature environment. After the earthquake, water was poured into the containment vessel to cool the molten core, and the con-crete structure was gradually cooled in the process. Since it will take at least 40 years to remove the fuel from the core, the integrity of the RPV pedestal is a major concern for the decommissioning of the Fukushima Daiichi Nuclear Power Plant. In order to assess the long-term integrity of the RPV pedestal, a horizontal loading test was conducted using a 1/6 scaled model of the RPV pedestal of Unit 1 considering the effect of the high temperature heating and subsequent wet conditions. And then, the static stress analysis of the RPV pedestal was performed considering the degradation phenomena revealed by the experiments. As a result, it was confirmed that the RPV pedestal of Unit 1 would be structurally sound for 40 years against the current design basis earthquake even if the material degradation due to severe accident and aging was considered.
Multiscale Numerical Simulations of Static Shear and Long-Term Behavior of RC Members Considering Corrosion of Reinforcing Bars
Sun-Jin Han, Tetsuya Ishida, Satoshi Tsuchiya
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.20, 2022.
(https://doi.org/10.3151/jact.20.492)
Abstract
In this study, a finite element (FE) analysis is conducted to investigate the effects of corrosion in tensile reinforcement on the shear performance of reinforced concrete (RC) members. In this regard, a multiscale chemo-hygral computational system is adopted, and its rationality is verified by comparing the FE analysis results with the shear test results of corroded RC beams. Based on the verified FE model, a parametric analysis is performed to examine the static shear and long-term behaviors of RC members according to the corrosion damage. The analysis results show that when the tensile reinforcement is simply straight anchored in the member, the ratio of reduction in shear strength due to corrosion decreases with the shear span-to-depth ratio. Meanwhile, when the tensile reinforcement is fully anchored, the shear strength of the corroded member increases owing to the formation of arch action despite the occurrence of splitting cracks caused by corrosion, and this tendency is more prominent as the shear span-to-depth ratio decreases. In terms of the long-term behavior of corroded RC members, it is shown that as corrosion progresses gradually over time, failure occurs with a rapid increase in deflection, including at low sustained load levels.
Extended Multi-directional Crack Model Applied to the RC Precast Joint Interface with Shear Dowel
Edom Zewdie, Masoud Soltani, Koichi Maekawa
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.20, 2022.
(https://doi.org/10.3151/jact.20.535)
Abstract
The extended multi-directional non-orthogonal crack model was applied to the joint interface area between RC members connected by shear dowel and the experimental verification was conducted in use of the low cycle fatigue experiments. The splitting tension field induced by the dowel action around the reinforcing bar is successfully reproduced with the coupled joint crack model and the beam finite elements, and the splitting tension cracking under cyclic fatigue loading and the joint degradation were fairly captured computationally. The effect of spiral steel confinement on the shear dowel performance can be quantitatively evaluated by the generic full 3D nonlinear analysis.
Experimental Study on Prestressing Force of Corroded Prestressed Concrete Steel Strands
Jingyuan Li, Tomohiro Miki, Qiuning Yang, Mingjie Mao
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.20, 2022.
(https://doi.org/10.3151/jact.20.550)
Abstract
This study presents the effect of corrosion of prestressed concrete (PC) strands on the prestressing force and the reduction mechanism of the uniaxial force. A total of 14 strands was tested by combining a rigid frame testing machine with an electrified accelerated corrosion device. The specimens were divided into two groups having prestressing levels of 70% and 50% of tensile strength of PC strands. The expected degree of corrosion of the specimens, which was defined by the mass loss, was calculated from controlled electric current and time. A rigid frame testing machine was used to sustain the tensile force of the PC strands, and the prestressing force and deformation of the strand were continuously measured during the corrosion test. The test results indicate that the prestressing force decreases with the increase of corrosion. After the corrosion tests, tensile loading tests were carried out on the specimens that did not rupture during the corrosion test. It was found that corrosion led to the deterioration of the tensile properties of the PC strands, and the ultimate tensile capacity of the corroded PC strand was related to the fracture condition.
Biomineralization Analysis and Hydration Acceleration Effect in Self-healing Concrete using Bacillus subtilis natto
Nguyen Ngoc Tri Huynh, Kei-ichi Imamoto, Chizuru Kiyohara
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.20, 2022.
(https://doi.org/10.3151/jact.20.609)
Abstract
Since concrete with bacteria incorporated in the matrix shows promising results in initial studies, more research has focused on using bacteria for the strength and durability enhancement of concrete. Bacterial concentrations varying from different positions on the crack surface were recorded and evaluated in conjunction with the amount of self-healing product formed. The change in bacteria concentration with the survival time in concrete was investigated and lasted for two years. The degree of mineralization of calcium carbonate from the microbial activity is also closely related to the level of survival and reduction of bacterial concentrations in concrete compared to the initial amount. These processes were tracked and analyzed through phase composition analysis and microstructure analysis. The role of nucleation sites of bacteria for accelerating mineral deposition was also investigated. The change in the content of hydrated cementitious minerals can be seen in groups of samples with different bacteria regarding cracking age (7-90 days). The increase in C-S-H content in the bacterial samples at early cracking age was significant compared with the control group. The effect on healed crack parameters through microscopic observation contributed to supporting and demonstrating the hypothesis of the combination of the formation of the calcium carbonate crystals around the bacterial cell as crystallization nuclei and the promotion of hydration for C-S-H formation.
Performance of Superplasticizers in Alkaline Environment of Self Compacting Geopolymer Mortar
Muhammad Talha Ghafoor, Chikako Fujiyama
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.20, 2022.
(https://doi.org/10.3151/jact.20.676)
Abstract
Self-compacting geopolymer concrete (SCGC) is a revolutionary concept in the construction industry. This study fills the research gap by determining the impact of six different types of commercially available superplasticizers on the flow properties, dry density and compressive strength of SCGM. It was investigated that the relationship between rela-tive flow area (Gm) and relative flow speed (Rm) of SCGM depends on the type of superplasticizer. The test results ex-hibited that a polyaryl ether-based superplasticizer performed better in increasing the flow deformability and flow speed of SCGM in comparison to melamine-based and naphthalene-based superplasticizers. The excessive increase in air content of SCGM paste with the addition of some superplasticizers had a negative impact on the flow properties. The increase in sodium oxide to silica molar ratio (Na2O/SiO2) and sodium oxide to water molar ratio (Na2O/H2O) both had a negative impact on the flow properties of SCGM. It was determined that beyond a trigger value of Na2O/SiO2 of 0.171, the impact of the superplasticizer in separating the flowing particles of SCGM became insignificant, resulting in the flash setting. Moreover, the addition of each type of superplasticizer had a negative impact on the dry density and compressive strength of SCGM.
Effects of Particle Size Distribution on the Performance of Calcium Carbonate Concrete
Ngoc Kien Bui, Ryo Kurihara, Wataru Kotaka, Hikotsugu Hyodo, Miku Takano, Manabu Kanematsu, Takafumi Noguchi, Ippei Maruyama
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.20, 2022.
(https://doi.org/10.3151/jact.20.691)
Abstract
The study investigates the effects of the particle size distribution (PSD) of aggregates on the properties of calcium carbonate concrete (CCC). Fifteen different types of aggregate PSDs were designed to select the appropriate PSD for CCC, based on the experimental data. Notably, the compressive strength of the CCC depends on the aggregate PSD, fineness modulus of the aggregates, packing ratio, and the amount of solution moving throughout the specimen. After one-day CCC processing, its compressive strength reached 7.2 MPa with an appropriate PSD. In addition, the X-ray diffraction and scanning electron microscopy analysis revealed that aragonite was the dominant contributor to the development of CCC strength. The amount of aragonite was influenced by the amount of solution passing through the specimen and the PSD of the aggregate blend.
Influence of Transverse Sectional Pre-crack on Shear Failure Behavior of RC Slender Beams Based on Experimental Loading Test
Li Fu, Lijia Zhang, Qiangsheng Yin, Hikaru Nakamura, Dengfeng Wang
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.20, 2022.
(https://doi.org/10.3151/jact.20.732)
Abstract
For the RC members under cyclic loading, flexural cracks commonly propagate in two opposite transverse directions at the zone under high bending moment and possibly form the cracks penetrating through the entire cross sections (pre-crack), which is considered as a crucial factor for flexure-shear failure of member. This paper clarified the influence of imitation pre-crack on the shear failure behavior of RC slender beams by three point bending test. Shear span depth ratio (a/d=3.14, 4.69) and location and thickness of pre-crack were set as main variables. As the important findings, it was revealed that the influence of pre-crack on the shear strength of RC slender beams is relatively small because it does not affect the formation of the critical diagonal crack and the mode of diagonal tension failure. It was also noted that the pre-crack at 2d (the section of pre-crack is 512 mm, which is twice of the effective depth d, far away from the loading plate center) leads to a reduction of shear strength with increasing pre-crack width, and the maximum reduction is 21.4%. Based on the detailed analysis of crack propagation, it was clarified that pre-crack may result in two patterns of critical diagonal crack, and in the condition that the pre-crack plane vertically intersects with the lateral splitting part of diagonal crack, the lateral splitting part becomes more severe and thereby reduces the shear strength and deformation ability.