Researcher Database

Kohei NAGAI
Faculty of Engineering Civil Engineering Infrastructure and Management
Professor

Researcher Profile and Settings

Affiliation

  • Faculty of Engineering Civil Engineering Infrastructure and Management

Job Title

  • Professor

Degree

  • Doctor of Philosophy(2005/03 Hokkaido University)

Research funding number

  • 00451790

J-Global ID

Research Interests

  • Structural Engineering   Concrete Engineering   Mesoscale Simulation   Maintenance of Infrastructure   International Cooperation   

Academic & Professional Experience

  • 2023/10 - Today Hokkaido University Faculty of Engineering Professor
  • 2011/07 - 2023/09 The University of Tokyo Institute of Industrial Science Associate Professor
  • 2019/10 - 2021/03 University of California, Davis Visiting Researcher
  • 2008/11 - 2011/07 The University of Tokyo The Graduate School of Engineering Project Lecturer
  • 2007/04 - 2008/11 The University of Tokyo The Graduate School of Engineering Assistant Professor
  • 2005/04 - 2007/03 Swiss Federal Institute of Technology in Zurich (ETH) JSPS Post Doctoral Research Fellow

Education

  • 2002/04 - 2005/03  Hokkaido University  Graduate School of Engineering  Department of Civil Engineering
  • 2000/04 - 2002/03  Hokkaido University  Graduate School of Engineering  Department of Civil Engineering
  • 1996/04 - 2000/03  Hokkaido University  School of Engineering  Department of Civil Engineering

Research Activities

Published Papers

  • Naoki Suzuki, Kohei Nagai
    Soils and Foundations 64 (1) 0038-0806 2024/02 
    The pile bearing capacity varies even within the same site. This study aims to gain a better understanding of the effect of pile penetration techniques on the variability of bearing capacity. The study uses data from 83 jack-in test piles to explore the variations in penetration resistance at the pile head, base, and shaft. The semivariance increases with an increase of the horizontal distance, so we focus on the semivariance between 5 m range, whose error was defined as the error due to piling workmanship. The analysis shows that the errors follow a lognormal probability distribution, and the coefficient of variation (COV) in penetration resistance at the heads and bases is about 10%, while the COV in extracted resistance varies widely, with a range of 5–25%. Little difference in the variations due to piling workmanship was observed among different penetration motions, speeds, and soil types.
  • Yi Wang, Ridho Surahman, Kohei Nagai
    Engineering Fracture Mechanics 295 0013-7944 2024/01/23 
    Normal polyvinyl alcohol-engineered cementitious composites (PVA-ECC) exhibit high mechanical performance but are susceptible to shear cracking. It has been recently found that the shear resistance can be largely improved by including coarse aggregates. However, under complex stress conditions, the mechanism of shear movement and the contribution from coarse aggregate remain unclear. This paper presents an experimental investigation of the shear cracking behavior in pre-damaged PVA-ECC beams with a mixture that includes coarse aggregate. Symmetric and antisymmetric loading tests were carried out to examine the effects of coarse aggregate under different stress rotation fields. To clarify the shear cracking mechanism, digital image correlation (DIC) was applied to observe crack development and strain localization in the shear span of the PVA-ECC beams. The results obtained from DIC and conventional measurements showed good agreement, confirming the reliability of monitoring complex fracture processes using the DIC technique. The shear cracking mechanism of the PVA-ECC beam under complex stress conditions was analyzed. The collaboration of coarse aggregate was observed from the orientation of secondary cracks, which showed less dependence on the presence of precracks. Crack opening and sliding were significantly limited due to the reduced anisotropic effect of the predamage. Therefore, using the DIC technique, the positive effect of coarse aggregates was verified, and the shear cracking mechanism of pre-damaged PVA-ECC beams was revealed through direct observations.
  • Takeru Kanazawa, Kohei Nagai, Koji Matsumoto
    Structures 59 2024/01 
    For ascertaining the structural longevity of commonly used half-joint girder-to-girder connections in ageing concrete bridges, accurate strength assessments are crucially important. Existing strut-and-tie models (STM) as well as code provisions underestimate resistance to diagonal tension failure from a re-entrant corner crack, perhaps leading to unnecessary maintenance expense. We develop a more accurate equilibrium-based strength analysis of diagonal tension failure based on a faithful representation of the failure surface. We formulate equilibrium equations to represent the internal force system; strength derivation is then optimised for these equations and yield criteria. The accuracy of the new analytical method was validated through comparison with experimentally obtained strengths from 30 specimens. The analytical predictions show good agreement with the test results, with an average error of less than 2%, which is superior to STM-based prediction. Not only strength capacities but also internal force-resisting components are quantified using this analytical method. Results indicate a dependency among all internal force-resisting components and a non-dimensional parameter of a1/hd.
  • Jie Luo, Kohei Nagai
    Construction and Building Materials 408 0950-0618 2023/12/08 
    The alkali-silica reaction (ASR) is one of the most serious durability issues affecting the safety of concrete structures. Numerical simulation is useful for monitoring the internal condition of an RC structure with ASR damage and predicting its long-term behavior. A mesoscale three-dimensional rigid body spring model (3D RBSM), a type of discrete analysis model, has previously been developed to simulate ASR expansion by modeling aggregate and mortar separately. The element size in simulations using this model (known as the aggregate model) is 2–3 mm, so the number of elements is huge and computation time correspondingly long. An equivalent model with lower number of elements is required, so in this study a concrete model based on RBSM is proposed. The aggregate is not separately modeled and expansive elements are included to simulate ASR expansion, allowing a larger element size (1–2 cm). The number of elements and computation time are less than 1 % compared with the aggregate model. This proposed model is used to simulate free ASR expansion, ASR expansion under confinement, and the mechanical properties of ASR-damaged concrete, as well as pullout behavior after ASR expansion. The simulation results are compared against previously reported results obtained using the aggregate model, as well as against experimental results. This comparison confirms equivalency between the concrete model and the aggregate model, opening the possibility of simulating ASR damage in RC structures in the future.
  • Dheeraj Waghmare, Dawei Ren, Punyawut Jiradilok, Kohei Nagai
    Case Studies in Construction Materials 19 2214-5095 2023/12 
    The precise prediction of moisture variations within concrete is of great significance because various deterioration phenomena spanning corrosion to carbonation are induced by mass transfer facilitated by the presence of moisture. Under actual environmental conditions, concrete is exposed to numerous wetting and drying cycles. Thus, to accurately predict moisture variations it is essential to consider the effect of these conditions. This study investigates the effect of alternate wetting and drying cycles on moisture penetration through concrete at the mesoscale using a 3D discrete network model based on a Rigid Body Spring Model (RBSM). For modeling moisture transport during wetting, the non-linear diffusion equation is used and the effects of capillary suction in moisture transport are also considered. On the other hand, moisture transport during drying is modeled as an evaporation-diffusion process. The simulation results obtained using this discrete network model are validated against the available experimental outcomes. During the wetting cycle, the moisture content in regions close to the exposed surface rapidly increases and approaches the maximum saturation. The effect of subsequent drying is seen only until a certain depth below the exposed surface, known as the influential depth. It is also observed that moisture continues to penetrate deeper into a specimen even during a drying cycle.
  • Suhas S. Joshi, Vikas Singh Kuntal, John E. Bolander, Kohei Nagai
    Engineering Fracture Mechanics 292 0013-7944 2023/11/15 
    Rebar corrosion estimation is essential for maintenance of reinforced concrete structures against deterioration. Currently available non-destructive methods of evaluating corrosion do not clearly indicate the internal corrosion condition along the rebar and its effect within the concrete. In prior research, an inverse analysis scheme has been proposed in which the model predictive control (MPC) optimization method is utilized within a rigid body spring model (RBSM) to estimate the corrosion-induced expansion distribution along the rebar from corresponding surface crack width data. In this study, an innovative approach is introduced to verify the precision of MPC-RBSM outcomes and systematically examine the major contributing factors. To facilitate this verification, target cracking is generated by precisely controlling the corrosion expansion within the RBSM framework. In total, three sets of simulations are considered: two cases with varied expansive strain distribution profile along a straight rebar, and third in which a bent rebar arrangement is subject to corrosion. The simulated expansion distribution is compared against the original expansion used to generate the crack width data to validate the reproducibility of the MPC-RBSM results. The inverse analysis results show that the automated expansion scheme in MPC-RBSM successfully reproduces the target surface cracking. In the inverse analysis process, the necessity to account for corrosion distribution is identified. When appropriate corrosion distribution criterion is used to estimate corrosion-induced expansive strain levels along the reinforcement, the results are in good agreement with the assumed corrosion-induced expansive strain profiles (including both uniform and non-uniform profiles). Additionally, simulations of bent rebar corrosion are performed. The results indicate that surface cracking local to the bent region is mainly influenced by corrosion-induced stresses along the straight lengths of the rebar, which complicates the corrosion analysis of bent rebars via this form of inverse analysis.
  • Yi Wang, Sha Xie, Zhao Wang, Xiaoda Li, Fuyuan Gong, Kohei Nagai, Jun Deng, Tamon Ueda, Wenguang Hu
    Cement and Concrete Composites 143 0958-9465 2023/10 
    In cold regions, concrete structures suffer from low temperatures, where damage will initiate due to the expansion of ice formation and accumulate with water supply to keep penetrating into the pores and cracks. Meanwhile, it is found that the mechanical properties of cementitious materials can be enhanced owing to the pores filled by ice. Therefore, it is important to evaluate the freezing process, in which the freezing action can have both strengthening and damaging effects on the mechanical properties of concrete material, depending on several environmental and material factors. Besides, if temperature remains negative, the capacity of cementitious materials in service with ice presented inside pores and cracks are also important. However, there is still lack of effective evaluation methods for the mechanical properties of frozen concrete. In this study, cementitious materials with/without air entraining agent (AEA) of different water-to-cement ratios (W/C) are prepared and tested at low temperatures (−10, −20, −30 and −40 °C). The results show that the mechanical properties of the materials (compressive strength, splitting tensile strength, dynamic elastic modulus) are closely related to electrical resistivity, which is further explained by the impact of air voids and pore structures from a multiscale scheme. It proves that the change of resistivity could be adopted as the index to evaluate the performance of frozen concrete, and an empirical model is derived based on the experiment results.
  • Naoki Suzuki, Kohei Nagai
    Applied Sciences (Switzerland) 13 (18) 2023/09 
    The use of data in the construction industry is growing rapidly. However, projects that do not have multiple stages, such as pile foundation and cantilever wall construction, are difficult to reinforce based on the data of observation. It cannot be said that the design–build construction process is optimized by piling data and active learning. In this paper, a new data-driven framework is proposed so that it can be used even for construction under single-stage conditions. The proposed method adopts a lower safety factor (SF) in the preliminary design than that in the conventional methods, and checks the performance after the building using piling data. Countermeasures are conducted to satisfy the target reliability, if necessary. Focusing on the expected total cost, the parametric studies reveal that the proposed method can reduce the expected total cost under specific conditions, such as lower countermeasure cost, higher failure cost, and higher relative costs of safety measures. Furthermore, our method exhibits robustness, as even with low initial safety factors, the expected total cost does not become excessively larger compared to the conventional methods. The findings highlight the potential benefits of piling data for optimizing construction projects under single-stage conditions.
  • Bennie Hamunzala, Koji Matsumoto, Kohei Nagai
    Remote Sensing 15 (14) 2023/07 
    The construction year of road bridges plays an important role in bridge management systems. Based on the age of road bridges and other factors, deterministic and probabilistic deterioration models can be used to calculate deterioration rates and predict the future physical condition of road bridges. Two new techniques are proposed in this manuscript for estimating the construction year of road bridges by analyzing the normalized difference water index 2 (NDWI_2). Technique 1 uses both the target bridge point (TBP) and a selected optimal reference control point, while Technique 2 uses only the TBP. Landsat 5 Thematic Mapper NDWI_2 data were analyzed at all 44 road bridges in Nago City, Japan of the bridges’ overall length ≤ 100 m and construction year between 1990 and 2006. The sequential t-test analysis of the regime shift method, at a significance level (Formula presented.) = 0.05 and cutoff length l = 2 to l = 27, was used to interpret the estimated construction year from the NDWI_2 for both techniques. Both techniques successfully determined the estimated construction year, which was statistically significant with p-values < 0.05, except for seven road bridges in Technique 1 and one road bridge in Technique 2. The correlation and comparative analysis of the actual and estimated construction years yielded (Formula presented.) = 0.24 and (Formula presented.) = 0.33, as well as an average deviation of S = 5.81 years and S = 4.08 years for Technique 1 and Technique 2, respectively. The findings suggest that Technique 2 is more accurate and provides a better estimate than Technique 1. It was observed that, as the cutoff length l increased, the absolute error between the actual and estimated construction year increased. Therefore, as a measure of accuracy, the upper limit of cutoff length l was set to (Formula presented.) 12. It was also observed that the increase in the bridge’s overall length and forested area contributed to the accuracy of the results. By using the construction year as one of the inputs into bridge management systems, bridge managers can make more informed decisions about how best to maintain and improve road bridges to ensure user safety and road bridge preservation for the future.
  • Eam Sovisoth, Vikas Singh Kuntal, Prakhar Misra, Wataru Takeuchi, Kohei Nagai
    Infrastructures 8 (4) 2023/04 
    Inspection data can be used to comprehend and plan effective maintenance of bridges. In particular, the year of initial construction is one of the most important criteria for formulating maintenance plans, making budget allocations, and estimating soundness. In an initial survey of bridges in Cambodia, it was concluded that the year of construction of only 54% of 2439 bridges surveyed is known, with the remaining 46% remaining unknown. In this research, Landsat satellite data is used to estimate the year of construction of these bridges. Landsat provides spatial spectral reflectance information covering more than 30 years, and for longer bridges this can be used to estimate the year of construction by visual judgement. However, limited image resolution means this is not possible for shorter bridges. Instead, a method using the Landsat Normalized Difference Water Index (NDWI) is used to estimate the year of construction. Three pixels are selected from Landsat image data in such a way that one lies on the current location of a bridge and two other reference pixels are placed on similar terrain at a certain distance perpendicular to the bridge axis. NDWI values are plotted over time for the three pixels and the difference in value between the bridge pixel and the two reference pixels is then compared. Before the bridge is constructed, all three pixels should have similar NDWI values, but after construction the value of the target bridge pixel should differ from the other two because the NDWI value of a bridge surface is different from that of the surrounding vegetation. By looking for this change, the year of construction of a bridge can be estimated. All the bridges in the Cambodian database are classified into three categories based on length (which affects their visibility in Landsat images) and year of construction is estimated. The results show that estimated year of construction has the same accuracy in all three categories.
  • Patrick Nicholas Hadinata, Djoni Simanta, Liyanto Eddy, Kohei Nagai
    Applied Sciences (Switzerland) 13 (4) 2023/02 
    Monitoring damage in concrete structures is crucial for maintaining the health of structural systems. The implementation of computer vision has been the key for providing accurate and quantitative monitoring. Recent development uses the robustness of deep-learning-aided computer vision, especially the convolutional neural network model. The convolutional neural network is not only accurate but also flexible in various scenarios. The convolutional neural network has been constructed to classify image in terms of individual pixel, namely pixel-level detection, which is especially useful in detecting and classifying damage in fine-grained detail. Moreover, in the real-world scenario, the scenes are mostly very complex with varying foreign objects other than concrete. Therefore, this study will focus on implementing a pixel-level convolutional neural network for concrete surface damage detection with complicated surrounding image settings. Since there are multiple types of damage on concrete surfaces, the convolutional neural network model will be trained to detect three types of damages, namely cracks, spallings, and voids. The training architecture will adopt U-Net and DeepLabV3+. Both models are compared using the evaluation metrics and the predicted results. The dataset used for the neural network training is self-built and contains multiple concrete damages and complex foregrounds on every image. To deal with overfitting, the dataset is augmented, and the models are regularized using L1 and Spatial dropout. U-Net slightly outperforms DeepLabV3+ with U-Net scores 0.7199 and 0.5993 on F1 and mIoU, respectively, while DeepLabV3+ scores 0.6478 and 0.5174 on F1 and mIoU, respectively. Given the complexity of the dataset and extensive image labeling, the neural network models achieved satisfactory results.
  • Cheng Jiang, Kumar Avadh, Kohei Nagai
    Composite Structures 304 0263-8223 2023/01/15 
    Although there are extensive investigations on fiber reinforced polymer (FRP)-strengthened concrete structures, reliable numerical simulation is still one of the major challenges in the analysis of such structures. This paper has successfully developed a rational FRP-to-concrete model based on the 3D rigid body spring model (RBSM). This is the first work to study the bond problem by involving FRP in the 3D RBSM. All the parameters in the developed model are proposed with clear and precise physical meanings and straightforward calculation methods. The simulation results are in good agreement with both the results of tested specimens and analytical solutions. Based on the proposed model, parametric studies are conducted to study the effects of the adhesive-concrete interfacial strength and width factor. From the simulation results, the relationship between the adhesive-concrete interfacial strength and failure mode is revealed. It is also found from the simulation results that the width factor has an upper limit, which is ignored by all the existing models. When the concrete width (bc) is greater than a certain value (e.g., FRP width (bF) + 30 mm in this work), the bond increase due to the width factor is insignificant.
  • Jie Luo, Shingo Asamoto, Kohei Nagai
    Engineering Fracture Mechanics 277 0013-7944 2023/01 
    The alkali-silica reaction (ASR) is a durability concern that can lead to expansion and cracking of concrete structures. ASR damage affects the physical characteristics of the concrete itself, although the load-carrying capacity of an RC structure may be maintained because the reinforcement continues to confine the core concrete. However, if the combined effect of expansion damage and external loading were to result in reinforcement rupture, this would prejudice the safety of a structure. To study this phenomenon, the discrete numerical simulation method 3D RBSM is used to carry out a parametric study of the compressive failure of concrete with three ASR damage levels under confinement. Stirrups with normal yield strength and with elastic behavior are investigated. The ASR expansion and cracking behavior for concrete with different confinement scenarios are presented and discussed. Following that, the compressive failure of concrete with ASR damage and the effect of stirrup confinement are studied. The simulated stress–strain relationships for cases without ASR damage are well fitted by a theoretical prediction model. As for elastic modulus and compressive strength, it tends to decrease with increasing ASR expansion whatever the confinement conditions. For models confined by stirrups, the degradation of compressive strength is delayed. Visualizations of internal stress, concrete crack propagation and stirrup strain development are obtained from the simulations and these are discussed. It is found that the confinement effect of elastic stirrups is greater than that of stirrups with normal yield strength. Further, the strain distribution on inner and outer surfaces of stirrups which is hard to be monitored in experiments is extracted. Stress concentrations arise at the inner radius of the stirrup bends, which is consistent with observations of real structures and could lead to rupture at these locations.
  • Haoyu Peng, Kumar Avadh, Kohei Nagai
    Journal of Advanced Concrete Technology 21 (1) 42 - 57 1346-8014 2023/01 
    Change in the mechanism of post corrosion bond behavior was investigated in a previous experimental study by analyzing the deformations in concrete using digital image correlation (DIC). Using their DIC data, this study focuses on the local strain non-uniformity in reinforcing bar during the uniaxial tensile test due to the effect of rib height reduction and corrosion layer. The results show that, the local strain below the rib tip is higher than that below the flat part between ribs, due to compressive stress resulting from mechanical interaction at the rib tip. Furthermore, the non-uniformity in local strain is less pronounced in highly corroded reinforcing bar because less compressive stress is transferred due to gentler rib slope. In highly corroded reinforcing bar, the appearance of non-uniformity in local strain is delayed since the accumulated corrosion product on the reinforcing bar surface delays the mechanical interaction and stress transfer between the concrete and the reinforcing bar.
  • Seyed Yaser Mousavi Siamakani, Kohei Nagai, Punyawut Jiradilok, Raktipong Sahamitmongkol
    Case Studies in Construction Materials 17 2214-5095 2022/12 
    Concrete breakout failures with acutely damaged concrete are one of the main problems caused by expansion anchors under tensile loading. In this study, the prevention of concrete breakout is investigated by post-installed reinforcement (PRs). Experimental and numerical studies were conducted to look into the effect of the PRs on the expansion anchors subjected to tensile loads. The 3D Rigid Body Spring Model (RBSM), which is based on discrete analysis at the mesoscale, was used as supporting numerical research. On pullout capacity, displacement, failure mechanism, and concrete breakout geometry, both experimental and numerical analyses were undertaken. The numerical analysis also looked at internal stress, strain, and concrete cracking. In general, the anchors with the PRs had steady displacement (ductile) and less brittle failure compared to the anchors without the PRs. The significant finding was in the failure mode, where severe concrete damage and concrete breakout failures were prevented by the PRs. The internal tensile stress of concrete was concentrated around the wedges of the anchors (interlocked area). The contribution of the PRs was shown by high strain values at around their mid-length. Additionally, due to the PRs, concrete cracking was less, mostly generated in the region between an anchor and its PRs. The utilization of PRs could be a method to protect concrete from breakout failure.
  • Jie Luo, Shingo Asamoto, Kohei Nagai
    Construction and Building Materials 351 0950-0618 2022/10/10 
    The bond performance between rebar and concrete is essential for the safety of RC structures. There is a need for research on bond deterioration due to the alkali-silica reaction (ASR) and delayed ettringite formation (DEF) damage which affects real structures. In this paper, a parametric study is conducted to quantitatively study the effect of slight-to-severe ASR/DEF damage in the presence of stirrup confinement on the pullout behavior between concrete and reinforcement using three-dimensional rigid body spring model (3D RBSM) simulation. It is found through the simulation that the bond stress in ASR damaged cases increases when ASR expansion before pullout is small and then decreases as the damage level rises, while the bond stress in DEF damaged cases keeps decreasing from the beginning. Besides, higher stirrup confinement effectively mitigates bond deterioration in both ASR and DEF damaged cases when damage is serious. More importantly, stress development and crack propagation during the expansion stage and the pullout stage are visualized. Interface cracking condition varies according to the damage type (ASR or DEF) and the level of stirrup confinement due to different cracking mechanisms. However, the tendency for the number of cracks in the concrete to increase, which reflects the overall damage level in ASR and DEF damaged cases, is similar, resulting in a similar trend for bond stress to decrease.
  • Kumar Avadh, Kohei Nagai
    Materials 15 (18) 2022/09 
    The buildup of corrosion products over a reinforcing bar and associated reduction in rib height lead to degradation of the bond between reinforcement and concrete. The authors have previously used digital image correlation (DIC) to visualize and quantify load-induced cracking at the interface in specimens with varying degrees of corrosion. The results obtained in that study are used here to simulate the post-corrosion local bond behavior. A bond degradation model is incorporated into the discrete analysis tool, 3D Rigid Body Spring Model (RBSM) for the simulation. This analysis method allows the shape of the reinforcing bar to be directly modeled, and concrete cracking behavior is simulated by using a randomly shaped mesh. The magnitude of opening and sliding over the tips of ribs in the simulation, in which the reduction in rib height could not be modeled, is significantly lower than observed in the experiment. The results demonstrate that reduction in rib height is an important factor in post-corrosion behavior, and needs to be included in simulation models. It is also understood that in order to gain a better understanding of local post-corrosion bond behavior, de-bonding between reinforcement and concrete needs to be modeled in a discrete analysis framework.
  • Kumar Avadh, Punyawut Jiradilok, John E. Bolander, Kohei Nagai
    Construction and Building Materials 339 0950-0618 2022/07/11 
    Corrosion-induced bond degradation leads to changes in deformation characteristics, cracking patterns, and loss in tension stiffening in structural members. Since the induced damage is dependent upon multiple inter-related parameters, prediction of post-corrosion deformation behavior requires sophisticated numerical simulations. This study integrates corrosion expansion and bond degradation models into a discrete analysis framework, 3D RBSM (Rigid Body Spring Model), to simulate post-corrosion loss of tension stiffening. Uniaxial tensile loading is applied to reinforced concrete models with different degrees of corrosion to obtain plots of load versus average strain and surface cracking patterns. Simulated surface cracking patterns due to corrosion and uniaxial loading in uncorroded and corroded models are similar to experimental results. As the degree of corrosion increases, the number of transverse cracks on the concrete surface decrease and the load at first cracking also decreases. Further, internal stress and bond stress investigation directly illustrate the decrease in stress transfer from reinforcing bar to concrete due to corrosion.
  • Mohamed Adel, Koji Matsumoto, Tamon Ueda, Kohei Nagai
    Construction and Building Materials 339 0950-0618 2022/07/11 
    This study experimentally investigates the influence of several material parameters on steel fiber reinforced concrete (SFRC) structural beams and their crack-bridging degradation over constant and variable amplitudes of flexural cyclic loading. Crack-bridging stress is evaluated using inverse analysis and its degradation against maximum rebars strain is derived. Four series of SRFC beams are prepared and characterized with different concrete compressive strengths, hooked-end steel fiber shapes in two different volume fractions, and two longitudinal reinforcement ratios. The results reveal lower crack-bridging degradation rates, and associated longer fatigue life and higher residual capacity, for beams with higher concrete strength. Further, SFRC fracture energy is shown to have a significant effect on fatigue response, with higher fracture energy associated with lower rates of crack-bridging degradation. However, steel fibers with double hooks are shown to have a relatively insignificant effect on controlling crack-bridging degradation rates and the flexural cyclic responses of SFRC beams as compared to single hook fibers. Also demonstrated is that a lower longitudinal reinforcement ratio leads to higher rebar strain rates with no influence on crack-bridging stress. Crack-bridging degradation diagrams are proposed for SFRC beams that quantitatively capture several material parameters effects based on their mechanical properties over the fatigue life.
  • Jie Luo, Yi Wang, Shingo Asamoto, Kohei Nagai
    Cement and Concrete Composites 129 0958-9465 2022/05 
    One of the most serious serviceability concerns for reinforced concrete (RC) structures is expansion and cracking resulting from the alkali-silica reaction (ASR), which has a negative effect on material properties as well as the bond between reinforcement and surrounding concrete. In a typical RC member, ASR induced concrete expansion and cracking are restrained by internal reinforcement as well as the boundary conditions. The mechanism is complex and difficult to understand through experimental study, so predicting the residual capacity of a damaged RC structure is not easy. In this study, the authors use a 3D Rigid Body Spring Model (RBSM) comprising mortar, aggregate and steel elements and which is able to simulate ASR expansion in reinforced concrete. To study the complex interactions among multiple parameters and quantify the effect of ASR damage on structural behavior, previously reported experiments on ASR induced expansion under internal and external restraint are simulated, along with experiments on pullout behavior after ASR expansion. The effect of restraints on macroscopic ASR expansion is well modeled in each case, and how the development of internal stresses and concrete cracking influenced by the restraint can be explained from the simulations. The simulations enable discussion of the number of ASR-induced cracks and the internal stress condition in various cases. The peak bond strength of concrete with different ASR damage levels, as reported in pullout experiments, is predicted accurately and the load-displacement curves of ASR damaged concrete are discussed.
  • Suhas S. Joshi, Kumar Avadh, Vikas Singh Kuntal, Punyawut Jiradilok, Kohei Nagai
    Construction and Building Materials 325 0950-0618 2022/03/28 
    In real structures, corrosion cracking patterns tend to be complex as a result of congested rebar arrangements and non-uniform corrosion distribution. It is challenging to understand the internal corrosion cracking behaviour based only on the limited amount of visible surface cracks. The present study attempts to use a meso-scale discrete element method, the three-dimensional rigid body spring model (3D-RBSM), to simulate factors that affect corrosion cracking. Factors considered in reinforced concrete panels with multiple corroded rebar includes, corroded rebar location, corrosion order among rebars and rebar arrangement. Three different cases are simulated and the results are compared with previously published experimental results. In one set of simulations, the non-uniform rebar corrosion profiles obtained in the experiments are utilized, with the results showing similar cracking patterns to the experimental ones. In a second set of simulations, uniform corrosion is assumed so as to conduct a parametric study of the same specimens for an accurate understanding of each influencing factor. It is observed that when all rebars corrode simultaneously, the initial cracks propagate along the outer rebars because this is where the confinement effect is minimum. Cracking along inner rebars is delayed due to strong compression zone resulting from mechanical interaction among corroded rebar stresses. However, this compression zone is reduced when the rebars corrode in two stages and cracking occurs at all the rebar locations. Cracks that initially form along the inner rebars can close up when corrosion of the outer rebars occurs later.
  • Hiroshi Yokota, Kohei Nagai, Koji Sakai
    Acta Polytechnica CTU Proceedings 33 644 - 650 2022/03/03 
    A concrete structure should be sufficiently planned, designed, executed and maintained to ensure its requirements during the life cycle. However, structures suffering from serious deterioration in structural members and sometimes subsequent loss in sustainability have been often found due to various reasons. One of the reasons is lack of total management for the structure. To meet these facts, it is extremely important to pursue coordination of engineering work in the design, execution and maintenance stages. The life cycle management is an organized system to support engineers decision to realize sufficient sustainability of the structure in the design, execution, maintenance, and all related work during its life cycle. The life cycle management is implemented based according to the life cycle management scenario in which balance of several sustainability indicators should be considered with ensuring overall sustainability. The sustainability indicators will be determined from the social, environmental and economic points of view. The scenario should be regularly reviewed based on the PDCA cycle and be updated if necessary. This paper deals with the concept and framework of the life cycle management of concrete structures to ensure sustainability during the structural life.
  • Mohamed Adel, Hiroshi Yokoyama, Hitoshi Tatsuta, Takanori Nomura, Yuki Ando, Takuro Nakamura, Hiroshi Masuya, Kohei Nagai
    Engineering Structures 246 0141-0296 2021/11/01 
    Reinforced concrete (RC) bridge decks suffer from cracking damages caused by traffic loading and environmental-related defects, such as the alkali-silica reaction (ASR). These require inspections involving measuring crack width and density followed by essential maintenance and repair works, however, there are no signs for fatigue failure. In this study, the out-of-plane shear deformations which cause small delaminations (pits) along surface cracks are proposed as an early indicator for fatigue failure. Thus, un-damaged and ASR-damaged RC deck slabs are tested under moving wheel loading and, using image-recognition for surface cracks detection, the pits along surface cracks are captured using an artificial intelligence (AI) model. The results show that, while both crack and pit density increase over the fatigue life of un-damaged slabs, there is an earlier sudden increase in pit density. In the case of the ASR-damaged slab, surface cracking is almost constant over the fatigue life until a sudden increase just prior to failure. Pit density, however, increases over the fatigue life with an earlier rapid increase before failure. The density of pits along cracks would be, therefore, a significantly earlier indicator of fatigue failure than crack density, offering the potential for more efficient and automatable bridge inspections.
  • Vikas Singh Kuntal, Punyawut Jiradilok, John E. Bolander, Kohei Nagai
    Cement and Concrete Composites 124 0958-9465 2021/11 
    In corroded reinforced concrete (RC) structures, there is a need to understand the degree of internal corrosion of the reinforcing bars in order to evaluate residual performance and ensure a rational maintenance regime. The authors have developed a computational approach for estimating the distribution of corrosion along the length of a reinforcing bar from the observed surface crack widths. The approach is based on mesoscale simulations of reinforced concrete, using rigid-body-spring models (RBSM), guided by Model Predictive Control (MPC). The current study extends that research to account for differing (smaller) crack openings, multiple instances of localized corrosion, and added confinement due to the presence of stirrups. Accuracy of the proposed approach is demonstrated through comparisons with laboratory tests of steel corrosion within concrete. The crack distribution observed on the concrete surface is automatically reproduced by optimized application of internal expansion within the mesoscale model, from which the corrosion distribution is estimated. The estimated corrosion distribution is accurate when compared to the measured corrosion of bars removed from the test specimens. In the case of specimens with stirrups, crack distributions and openings are accurately simulated by the MPC-RBSM system. Estimated corrosion levels are close to the measured values with some variations. The main reasons for these variations are the greater crack widths and higher corrosion levels in the stirrup-confined specimens. The simulated crack patterns are similar to those observed on the experimental specimens, indicating that specimen damage is simulated well. The internal stress and cracking distributions simulated by this method of meso-scale discrete analysis provides useful information for evaluating damage level that cannot be observed at the surface.
  • John E. Bolander, Jan Eliáš, Gianluca Cusatis, Kohei Nagai
    Engineering Fracture Mechanics 257 0013-7944 2021/11 
    Discrete models of solids have been motivated, in large part, by the discontinuous and heterogeneous nature of material structure and its breakdown under loading. The capabilities of discrete models have evolved over the past several decades, offering novel means for investigating material structure–property relationships. However, lack of understanding of both the utilities and disadvantages of discrete models limits their further development and applications. This paper reviews relevant features of discrete approaches applied to modeling the mechanical behavior of geomaterials, concrete materials in particular. The discrete models are classified according to their form and abilities to represent elastic and fracture behaviors in the presence of large-scale material heterogeneity. Discretization of the material domain plays a large role in this respect. Emphasis is placed on particle-based lattice models. The relative merits of various strategies for introducing reinforcing components, which are essential for many applications, are outlined. Recent advances are highlighted, including the use of discrete models for coupled, multi-field analysis. The merits of discrete approaches are summarized in the conclusions.
  • Kumar Avadh, Punyawut Jiradilok, John E. Bolander, Kohei Nagai
    Cement and Concrete Composites 123 0958-9465 2021/10 
    Corrosion induced rebar deterioration and concrete cracking alter the local bond interactions in reinforced concrete. An experimental methodology for investigating changes in the local deformation at the interface, caused by rib height reduction and corrosion layer is proposed. The effect of concrete cracking is eliminated by re-casting corroded rebars into new specimens. Local bond interaction is examined using uniaxial tensile tests and recorded through an observation window. The recorded interaction was analysed using Digital Image Correlation (DIC). Prior to visible crack formation, strain distribution maps revealed diagonal cracks indicating that the bond failure is associated with mechanical anchorage until a degree of corrosion of around 12%. Measurements of local opening and sliding behavior at the interface confirm that the slipping interactions become dominant with increase in corrosion. Differences in the shape of individual rib tips and flat lengths in specimens with higher corrosion cause significant variation in local bond interaction.
  • Mohamed Adel, Koji Matsumoto, Kohei Nagai
    Composite Structures 272 0263-8223 2021/09/15 
    A few studies have demonstrated a degradation in the performance of the steel-fibers in bridging cracks over the lifetime of steel-fiber reinforced concrete (SFRC) structures but only considered for constant amplitude cyclic loading. However, SFRC structures in practice are subjected to variable amplitude loading, thus their crack-bridging would be influenced by the preceding loading history. This concern has so far remained unexplored. This study presents experimental and analytical flexural cyclic responses of SFRC structural beams under constant and variable amplitudes. The crack-bridging stresses are evaluated using a proposed inverse method based on sectional analysis calculations. Besides, the maximum rebar strain level is correlated with the crack-bridging degradation law, aiming at reflecting the beam's macroscopic response. The results show that crack-bridging stress increases if the fatigue load level increases, indicating that new fibers play a role in bridging cracks along the crack length. Further, decreasing the fatigue load level leads to an instant decrease in crack-bridging, which is followed by a stabilization as the pullout stress applied to the bridging fibers is reduced. Finally, a diagram of crack-bridging degradation and evolution is proposed regarding the maximum rebar strain, which is supposed to be a valuable tool in the SFRC structural design.
  • Hafiza Fatima Zahid, Punyawut Jiradilok, Vikas Singh Kuntal, Kohei Nagai
    Construction and Building Materials 283 0950-0618 2021/05/10 
    Reinforced concrete (RC) structures are widely used, yet prone to damage induced by reinforcement corrosion which causes susceptibility of concrete spalling and eventually a reduced serviceable life. The visual inspection has been performed usually for structural assessment, but this does not account for the local interactions between rebars. Consequently, detailed scrutiny is required to consider the effects of various parameters on the initiation and propagation of corrosion-induced cracking. This experimental study considers the effect of multiple and multidirectional rebars on the development of corrosion cracking. Additionally, the effect of the location of corrosion and the change in order of corrosion occurrence is investigated in this research. For this purpose, reinforced concrete panels with unidirectional and multidirectional rebar arrangements and varying cover depth are tested using an accelerated corrosion technique. The results show a significant role of confinement in controlling the widths of cracks provided by both reinforcement arrangements. Moreover, the effect of boundary conditions along with increased confining pressure alters the generation of cracks in a unidirectional reinforcement arrangement. In the absence of enough surrounding concrete, cracks along non-corroding rebars can also form because of the expansion of adjacent corroding rebars. The inclusion of multidirectional reinforcement alters the crack direction and causes cracking along non-corroding rebars. Further, previously generated cracks may close considerably by the pressure from adjacent rebar corrosion. This study also facilitates the comprehension of the impact of influencing factors affecting corrosion cracking in real structures.
  • Vikas Singh Kuntal, Punyawut Jiradilok, John E. Bolander, Kohei Nagai
    Computer-Aided Civil and Infrastructure Engineering 36 (5) 544 - 559 1093-9687 2021/05 
    Corrosion of the steel reinforcing bars in concrete structures is one of the major maintenance problems. Corrosion results in expansive pressure on the surrounding concrete, which causes internal damage that may become visible as surface cracking. Such damage may degrade structural safety and serviceability. Effective maintenance requires the evaluation of residual performance based on estimates of spatially nonuniform levels of corrosion, which are typically obtained through surface measurements only. In this study, the authors have developed a simulation system for estimating the levels of internal corrosion along the reinforcing bar length from surface crack information. This innovative system is produced by integrating the technique of Model Predictive Control (MPC) with Rigid-Body-Spring Models (RBSM) of corrosion-induced cracking at the concrete mesoscale. In this study, MPC controls the simulated surface cracks such that they match the observed cracks by optimizing the internal expansions of springs representing the steel-concrete interface within the RBSM. The applicability of the system is verified using both synthetic crack width data and crack data collected from in-house laboratory testing. In the laboratory testing, corrosion levels were quantified by 3D scanning of the extracted reinforcing bars. The simulation results agree with the corrosion measurements, demonstrating the potential of the MPC-RBSM system for predicting the corrosion distribution along reinforcing bars using surface crack data.
  • Kumar Avadh, Punyawut Jiradilok, John E. Bolander, Kohei Nagai
    Cement and Concrete Composites 116 0958-9465 2021/02 
    The confining effect of concrete cover and stirrups reduces the rate of bond deterioration due to corrosion. However, the large dispersion in recorded experimental data makes it difficult to clearly separate the influence of cover depth and stirrup confinement on bond degradation. This study utilises the discrete 3D Rigid Body Spring Model (RBSM) to conduct a meso-scale investigation regarding the effect of cover thickness and stirrup confinement on internal crack evolution and pull-out behaviour in corroded reinforced concrete models. The simulation scheme is divided into two stages. In stage 1, different degrees of corrosion are introduced, producing cracking in the cover concrete; in stage 2, the corroded main reinforcement is pulled out from the damaged concrete. 3D RBSM is advantageous because the concrete is randomly meshed to reduce mesh bias on crack propagation and the actual geometry of the deformed bars is modelled. The simulation results clarify that the presence of thicker cover delays crack initiation but increases the rate of crack opening. Stirrups do not have any significant effect on crack initiation but effectively restrict crack growth. An investigation of the internal stress in the simulation models shows that tensile stresses generated in stirrups during corrosion are responsible for reactionary confining pressure that restricts crack propagation. Load-displacement curves show reductions in pull-out capacity, stiffness and ductility with increasing corrosion damage. The relative influence of crack opening and stirrup volume on the rate of bond degradation with respect to average surface crack width are discussed and compared with published experimental results and an empirical equation.
  • Ahmad Aki Muhaimin, Mohamed Adel, Kohei Nagai
    Materials 14 (3) 1 - 20 2021/02/01 
    The application of reinforced concrete for permanent and temporary deep ocean structures has recently become more prevalent; however, the static and dynamic effects of high water pressure on concrete remain unexplored. This paper investigates the influence of high water pressure (60 MPa) on four series of concrete cylinders with and without an embedded steel bar under sustained and cyclic loading conditions. The residual compressive strength, bond strength, and associated evolution of surface and internal damage are evaluated after exposing concrete cylinders to a water pressure of 60 MPa. The first series is exposed to sustained water pressure for 7 and 60 days, while the other series is tested under repeated water pressure for 10, 20, 30, 60, and 150 cycles. The results reveal that residual compressive strength falls immediately by 16% within 7 days of sustained high water pressure, but the strength then remains stable up to 60 days. Under repeated high water pressure, residual compressive strength gradually reduces by up to 40% until 60 cycles, after which it remains reasonably stable until 150 cycles as crack propagation is arrested at a certain depth within the concrete cylinders. The bond strength between the steel bar and matrix is observed to decrease considerably under repeated cycles of 60 MPa water pressure up to 26%. The damage gradually propagates at the matrix/steel bar interface under the repeated water pressure, resulting in a reduction in residual pullout capacity.
  • Liyanto Eddy, Kohei Nagai, Punyawut Jiradilok
    Lecture Notes in Civil Engineering 132 555 - 563 2366-2557 2021 
    External steel plates anchorage is one way to reduce the reinforcement congestion inside the beam-column joint because the reinforcing bars of the beam and column are anchored using steel plates located outside the joint. In this study, the failure behavior of the joint is investigated through the study of internal stresses using a discrete analysis method called 3D Rigid Body Spring Model. Simulation results are compared with experimental results. The simulation results show that in BCJ-Hook (conventional hook bar anchorage), the capacity of the beam-column joint in open case is significantly lower than that in close case. Meanwhile, in BCJ-Plates (external steel plates anchorage), the maximum load in open case is roughly the same as that in close case with only 6–14% reduction in maximum load. Higher capacity in BCJ-Plates is caused by the confinement from the steel plates.
  • Y. Wang, P. Jiradilok, K. Nagai
    Lecture Notes in Civil Engineering 101 1903 - 1913 2366-2557 2021 
    In this study, a 3 dimensional rigid body spring model (RBSM) was used to simulate the combined effects of alkali-silica reaction (ASR) and delayed ettringite formation (DEF) induced concrete damage. To model the mechanical reduction of concrete after deterioration, expansion was considered as the index and the initial strain was introduced as the damage history. Firstly, for each damage type, the simulated cracking patterns and mechanical reduction of concrete were compared with the existing experimental findings, good agreement was reached which verified the reliability of the proposed simulation method. Then the para- metric study was conducted to understand the combined effects of ASR and DEF since they may coexist in site. Surface cracking and internal crack numbers were analyzed to correlate the mechanical reduction. More importantly, the quantitative interaction of the two type damages was further discussed to clarify the mechanical properties loss of concrete under the combined effects, which is hard to achieve from experimental studies. The proposed method is efficient in simulating of durability problems of concrete and the simulated results are significant to the concrete structure design and durability evaluation under harsh environmental exposure.
  • M. Henry, K. Matsumoto, H. Yokota, K. Nagai
    Bridge Maintenance, Safety, Management, Life-Cycle Sustainability and Innovations - Proceedings of the 10th International Conference on Bridge Maintenance, Safety and Management, IABMAS 2020 4084 - 4088 2021 
    In response to the aging and deterioration of its infrastructure systems, Japan has invested in new technological and institutional systems for comprehensive infrastructure asset management. These approaches range from the construction of highly durable structures to the strategic maintenance of existing structures. Human resource development plays a critical role in tackling emerging challenges acrosss all stage of the life cycle, and Japan has been carrying out domestic training programs and education to increase the capability of infrastructure managers and engineers. However, there are limitations to the number of people who can participate in these training programs, so the transfer of acquired knowledge from the participants to their coworkers and organizations becomes important for improving capability on the whole. In this paper, the means by which participants in infrastructure maintenance training programs shared acquired knowledge will be examined through the lens of knowledge management, with a study on the transfer of knowledge by engineers in regional Japanese training programs introduced as an exploratory example.
  • Kumar Avadh, Kohei Nagai
    fib Symposium 41 - 44 2617-4820 2021 
    The degradation of bond due to reinforcement corrosion degrades the tension stiffening capacity of structural members. This study utilises 3D RBSM (Rigid Body Spring Model) discrete analysis framework to investigate the effect of degree of corrosion on tension stiffening, change in internal stress conditions and crack distribution over the surface of corroded reinforced concrete specimen on uniaxial loading. Behavior of corroded reinforced concrete is directly simulated by implementing corrosion expansion model along with bond degradation model. The only parameter varied in this study is the degree of corrosion. It was observed that as the degree of corrosion increases, the number of transverse cracks on the surface due to uniaxial load decreases. The load-displacement curves also show decrease in capacity and reduction in ductility with increase in corrosion.
  • Seyed Yaser Mousavi Siamakani, Punyawut Jiradilok, Kohei Nagai, Raktipong Sahamitmongkol
    Construction and Building Materials 262 0950-0618 2020/11/30 
    The utilization of post-installed anchors is growing fast to connect both structural and non-structural members. Adhesive anchors are one type of post-installed anchors which are formed as threaded bars. They can be post-placed into concrete with the help of epoxy adhesive. On the one hand, improvement of the performance of post-installed anchors, unlike cast-in-place anchors, has not drawn attention much. On the other hand, due to the complexity of their performance, a numerical study can be a supportive method for the experimental study. This paper addresses the effect of post-installed reinforcement (PR) on the adhesive anchors under tensile loading in both experimental and numerical studies. Reinforcing steel bars as post-installed reinforcement (fully embedded in concrete) were used to ameliorate the performance of the adhesive anchors under tensile force. Rigid Body Spring Model (RBSM) was developed as a simulation that is based on discrete analysis at mesoscale. Pull-out capacity, displacement, failure mode, and concrete cone geometry were measured by the experiments and the RBSM simulation. In addition, internal stress, strain, cracking patterns were investigated by the RBSM simulation. Moreover, some comparisons were conducted through the existing standard. In general, PR assist the anchors to have higher pull-out capacities, lower displacement as well as leading smaller size of concrete cone failure. Due to PR, the internal tensile stress of concrete changed to compressive stress resisting greater pull-out force. The higher strain is obtained at near the top of the anchors and at the mid-span of PR. Furthermore, cracking patterns in concrete were mostly within the area between the anchor and PR. Post-installed reinforcement can be considered as a method to improve the performance of adhesive anchors under tensile loading. This is validated by the RBSM simulation, which agrees with the experimental results.
  • Mohamed Adel, Punyawut Jiradilok, Koji Matsumoto, Kohei Nagai
    Composite Structures 244 0263-8223 2020/07/15 
    Steel fiber reinforced concrete (SFRC) at the structural scale exhibits an enhanced fatigue performance compared to conventional reinforced concrete (RC) with lower stress levels and longer fatigue life. The steel fibers contribute to the crack-bridging strength of the concrete, but how this degrades in the tensile stress zone of an SFRC structural beam during flexural cyclic loading remains unknown. This makes the fatigue design and safety verification of SFRC beams an unexplored area. In this work, the degradation in crack-bridging strength of SFRC structural beams with 1.5% by volume of hooked-end steel fibers under different flexural fatigue stress levels is evaluated over the fatigue life using an inverse analysis method. The experimental flexural response is monitored during static and fatigue tests, and compared with the calculated one from the section analysis calculations through the execution of the inverse analysis method. Based on the results, the crack-bridging strength is shown to degrade gradually at different flexural fatigue stress levels over the fatigue life. Further, the residual flexural capacity at the end of fatigue life is shown to be little different from the original capacity obtained in static loading when the flexural fatigue stress level is low.
  • Hosein Naderpour, Masoomeh Mirrashid, Kohei Nagai
    Engineering with Computers 36 (3) 1083 - 1100 0177-0667 2020/07/01 
    In this study, a new computational approach for determination of the bond strength of fibre reinforced polymer (FRP) strip-to-concrete joints is presented based on adaptive neuro–fuzzy inference system (ANFIS). For this purpose, 150 experimental data were gathered from the literature. The number of 120 data was used to train the system, and the other 30 were applied to the test. Six parameters including the compressive strength of the concrete, width of the concrete prism, FRP thickness, FRP modulus of elasticity, FRP bond length and FRP width were utilized to determine the bond strength. The results of the proposed ANFIS show high accuracy in the model. A comparison study with other published equations was also done, and it was concluded that ANFIS had less error and also had better results in comparison with other existing methods. Finally, a sensitivity analysis was done to investigate the relative importance of each input parameter on the target.
  • Yi Wang, Zong Woo Geem, Kohei Nagai
    Applied Sciences (Switzerland) 10 (14) 2020/07 
    Bond strength assessment is important for reinforced concrete structures with rebar corrosion since the bond degradation can threaten the structural safety. In this study, to assess the bond strength in concrete-corroded rebar interface, one of the machine learning techniques, artificial neutral network (ANN), was utilized for the application. From existing literature, data related to the bond strength of concrete and corroded rebar were collected. The ANN model was applied to understand the factors on bond property degradation. For the input in the ANN model, the following factors were considered the relative bond strength: (1) corrosion level; (2) crack width; (3) cover-to-diameter ratio; and (4) concrete strength. For the cases with confinement (stirrups), (5) the diameter/stirrups spacing ratio was also considered. The assessment was conducted from input with single parameter to multiple parameters. The scaled feed-forward multi-layer perception ANN model with the error back-propagation algorithm of gradient descent and momentum was found to match the experimental and computed results. The correlation of each parameter to the bond strength degradation was clarified. In cases without confinement, the relative importance was (1) > (2) > (4) > (3), while it was (2) > (1) > (3) > (5) > (4) for the cases with confinement.
  • Hosein Naderpour, Kohei Nagai
    Structural Design of Tall and Special Buildings 29 (9) 1541-7794 2020/06/25 
    The aim of the present study is to propose innovative predictive models for shear capacity of reinforced concrete (RC) exterior joints in terms of multiple soft computing techniques. Existing models were evaluated and by a preliminary sensitivity analysis, seven parameters including compressive strength of concrete, product of the yield stress and the reinforcement ratio of the joint stirrups, the effective width of the joint panel, cross-sectional column width, beam tensile longitudinal reinforcement ratio, beam compressive longitudinal reinforcement ratio, and column longitudinal reinforcement ratio were considered. Then, a large data set having the details of experimental programs on shear capacity of exterior RC beam–column joints was provided. The experimental data were utilized in developing the proposed models. After verification of the new models against available database, their efficiency compared with existing models was confirmed. Finally, a sensitivity analysis was performed in order to find the relative importance of each input parameter on the shear strength of RC joints. The results indicated that the beam reinforcement is the most important factor in shear capacity estimation of exterior RC beam–column connections.
  • Yi Wang, Punyawut Jiradilok, Kohei Nagai, Shingo Asamoto
    Engineering Fracture Mechanics 232 0013-7944 2020/06/01 
    Real concrete structures can suffer simultaneously from the alkali-silica reaction (ASR) and delayed ettringite formation (DEF), resulting in expansion of the concrete and surface map cracking. This damage can lead to serious durability problems. The respective effects of ASR and DEF have been studied, but the cracking behavior of concrete when they act together remains unclear. In this study, a three-dimensional rigid body spring model (3D RBSM) is used to simulate the cracking and mechanical property degradation of concrete under coupled ASR and DEF. A parametric study is conducted to understand various combinations of ASR and DEF. In line with previous studies by the authors on independent ASR/DEF damage, expansions caused by ASR and DEF are, respectively, introduced as strains at the mortar-aggregate interface and in mortar elements. Due to a confinement effect, specimens subjected to coupled expansion may suffer less expansion or cracking than when only one of the degradations is acting. The model clearly visualizes the cracking process and stress development as the degree of coupled expansion increases. More importantly, based on the simulated results, the interactions of the two types of expansion are revealed by correlating the degradation in mechanical properties with internal cracking behavior. Further, the confinement effect arising on application of the second type of expansion is quantitatively examined. While there is no significant change in stiffness, compressive strength in some cases increases with expansion as larger cracks close up under the confinement effect. This kind of investigation of coupled effects is difficult to quantitatively analyze through experimentation, but this work demonstrates that it is possible using RBSM simulations.
  • Ahmed Okeil, Koji Matsumoto, Kohei Nagai
    Cement and Concrete Composites 109 0958-9465 2020/05 
    This study is an effort to gain a better understanding of local bond behavior between a reinforcing bar and concrete or cement paste. Axial tension tests were applied to deformed steel bars that had been embedded in specimens of concrete and cement paste. The specimens were formed with openings to allow for full and continuous observation. The results of digital image correlation (DIC) analysis unveiled differences in the phenomena of crack propagation from the surface of the steel rebar inside concrete and cement paste. In concrete, large deformations tend to concentrate around the interface between mortar and aggregate particles, especially big particles. The results also show that a similar interface zone of almost 1–2 mm around the deformed bar is present in both concrete and cement paste. Finally, it is shown that there are deformations normal to the surface of the steel bar in concrete and cement paste which result in opening, not only sliding, at the concrete-steel interface.
  • Liyanto Eddy, Punyawut Jiradilok, Koji Matsumoto, Kohei Nagai
    Engineering Structures 210 0141-0296 2020/05/01 
    This paper presents an analytical investigation of the role of the reinforcing bars in the perpendicular beams of beam-column knee joints with mechanical anchorages. A computer simulation using 3D Rigid Body Spring Model (3D RBSM) is used for the analysis of RC beam-column joints in consideration of the effect of multidirectional arrangement of reinforcing steels. Simulation results provide beneficial information for understanding the macroscopic behavior because the internal stresses and crack conditions can be investigated. In this study, the beam-column joint simulation models are varied in the number of reinforcements to study the role of the reinforcing bar in perpendicular beam. Simulation results show that the capacity can be increased when reinforcing bars are added in the perpendicular beams. Through the study of the internal stresses and cracks using numerical simulation, the function of the longitudinal reinforcement in the perpendicular beams is to transfer the stresses from the joint to the perpendicular beams by means of the dowel action which is obtained by utilizing the multiple layers meshing of reinforcement and the local strain distribution. Ultimately, this research also shows that 3D RBSM can be a useful tool for the analysis of the RC beam-column joints with multidirectional arrangement of reinforcing steels based on a case-by case basis that cannot be addressed by the design code and conventional numerical methods.
  • Punyawut Jiradilok, Yi Wang, Kohei Nagai, Koji Matsumoto
    Construction and Building Materials 236 0950-0618 2020/03/10 
    Rebar corrosion can lead to significant deterioration in bond strength between steel and concrete. However, since confinement is lost as the concrete cracks, the changes in frictional bonding and mechanical bonding resulting from corrosion remain unclear and there is a need for an efficient tool that can evaluate the bond performance of corroded concrete structures. In this study, an innovative pull-out test is used to study changes in bonding in reinforced concrete after rebar corrosion, then a discrete meso-scale model based on the Rigid Body Spring Model (RBSM) is developed to simulate the observed bond behavior. To isolate the effects of concrete cracking, after an initial pull-out test, the corroded rebars are obtained and used as the reinforcement in newly cast sound specimens that are subject to a further pull-out test. A corrosion acceleration method is used to obtain corroded rebars. Different types of reinforcement (round rebars and deformed rebars) are studied. The results are used to study changes in frictional bonding and mechanical bonding at different degrees of corrosion. It is found that, without concrete cracking, the frictional bond in specimens with both round and deformed rebars significantly increases with the amount of corrosion, while total bond strength of specimens with deformed bar is almost unchanged because mechanical bonding is weakened. In the presence of concrete cracking, for specimens reinforced with a corroded round rebar, bond capacity increases initially (up to 3% corrosion) and then falls as corrosion increases, while bonding in those with a corroded deformed rebar shows a consistent decrease. In the RBSM-based simulations, the effect of corrosion is modeled by adjusting springs at the corroded interface. Constitutive models of shear and normal springs are developed to simulate corrosion and reflect the changes on frictional and mechanical bonding. Simulated results for load-displacement relationship, pull-out capacity and crack pattern are in good agreement with the experimental results. The numerical simulation is used to visualize internal stress development and strain distribution along the rebar as the degree of corrosion increases, a result that cannot be obtained by experimental methods. The results demonstrate that RBSM is a useful tool for evaluating the mechanical performance of corroded reinforced concrete structures.
  • Wenlong Deng, Yongli Mou, Takahiro Kashiwa, Sergio Escalera, Kohei Nagai, Kotaro Nakayama, Yutaka Matsuo, Helmut Prendinger
    Automation in Construction 110 0926-5805 2020/02 
    Structural Health Monitoring (SHM) has greatly benefited from computer vision. Recently, deep learning approaches are widely used to accurately estimate the state of deterioration of infrastructure. In this work, we focus on the problem of bridge surface structural damage detection, such as delamination and rebar exposure. It is well known that the quality of a deep learning model is highly dependent on the quality of the training dataset. Bridge damage detection, our application domain, has the following main challenges: (i) labeling the damages requires knowledgeable civil engineering professionals, which makes it difficult to collect a large annotated dataset; (ii) the damage area could be very small, whereas the background area is large, which creates an unbalanced training environment; (iii) due to the difficulty to exactly determine the extension of the damage, there is often a variation among different labelers who perform pixel-wise labeling. In this paper, we propose a novel model for bridge structural damage detection to address the first two challenges. This paper follows the idea of an atrous spatial pyramid pooling (ASPP) module that is designed as a novel network for bridge damage detection. Further, we introduce the weight balanced Intersection over Union (IoU) loss function to achieve accurate segmentation on a highly unbalanced small dataset. The experimental results show that (i) the IoU loss function improves the overall performance of damage detection, as compared to cross entropy loss or focal loss, and (ii) the proposed model has a better ability to detect a minority class than other light segmentation networks.
  • Michael Henry, Kohei Nagai, Koji Matsumoto, Hiroshi Yokota
    Journal of Disaster Research 15 (3) 368 - 376 1881-2473 2020 
    To ensure the long-term performance of bridges in Myanmar, the Japan International Cooperation Agency (JICA) initiated a capacity development project to enhance the quality control capabilities of engineers in the Myanmar government. Such expertise will be transferred to a group of “core trainers,” who will be responsible for sharing their acquired skills and knowledge with other engineers in Myan-mar. The effective transfer of Japanese expertise is thus crucial for realizing the project’s goal of improv-ing the quality of bridges and structures managed by the government of Myanmar. This research aims to explore the perspectives and expectations of the core trainers’ for training transfer after the project using the results of a survey questionnaire and to evaluate the effect of various factors on their expectations for transfer using partial least squares path modeling. It was found that the core trainers had positive expectations regarding the benefits of training transfer as well as generally positive perspectives on the factors affecting such transfer. The results of statistical mod-eling, however, failed to reveal any significant relation-ships between the modeled factors and the expectations for training transfer. This may be attributable to the sample size, which is limited by the scope of the capacity development project; a mixed method approach is therefore proposed as a more appropriate method in this context. Nonetheless, the results generally suggest that the work environment is fundamental in facilitating effective training transfer, and further research is necessary.
  • Punyawut Jiradilok, Kohei Nagai, Koji Matsumoto, Takeshi Yoshida, Tetsuro Goda, Eiji Iwasaki
    Journal of Disaster Research 15 (3) 360 - 367 1881-2473 2020 
    After the collapse of Myaungmya Bridge in April 2018, the safety of infrastructure became an urgent task and attracted increased public interest in Republic of the Union of Myanmar. After the incident, monitoring systems were installed in several suspension bridges, of the same structure type as Myaungmya Bridge, recording unwanted bridge movements. The observations indicate that a large deformation oc-curred, rendering the current bridge condition different from its original design, though the safety of the current condition is unclear. Therefore, in this study, numerical models are simulated utilizing the data from detailed observations made by the monitoring system, to confirm the safety of the bridge. Twantay Bridge was chosen as the bridge analysis tar-get. Linear time history analyses were performed on Twantay Bridge, in which the seismic performance was evaluated by comparing the stresses generated in the main members with the allowable stresses. More-over, by comparing the analysis results obtained from two models – a design drawing model of the Twantay Bridge, and a model that reproduced the present condition of the bridge – the current bridge performance was confirmed. The information obtained from this analysis is useful in the field of maintenance and will allow Myanmar officials to effectively plan and take corrective action for the requisite maintenance of the damaged bridge.
  • Ahmed Okeil, Koji Matsumoto, Kohei Nagai
    Proceedings of the International Conference on Structural Dynamic , EURODYN 2 3343 - 3354 2311-9020 2020 
    A part of reinforced concrete structures are always subjected to dynamic repeated loading and simultaneously subjected to liquid water from an external source such as rains. These structures, especially the old ones, have numerous surface cracks that allow water to penetrate concrete and reach steel-concrete interface and subsequently affect the bond between concrete and steel bars. Against this issue, a fatigue pull-out test was carried out with supplying of water to investigate experimentally the bond behavior between the concrete and a deformed bar by using digital image correlation technique (DIC). A fatigue pull-out test was applied to a deformed steel bar which was embedded in a concrete specimen. The specimen had a trapezoidal "window, " which allows full continues observation and measurements of concrete or cement paste displacements and subsequently strains by recording high-resolution digital video which was later analyzed by DIC software. The experiment focuses on the failure mechanism of the deformed bar from the concrete and the mechanism of the sludge 's formation in the vicinity of the deformed bar. The results show that the actual deformations of concrete in case of fatigue test were captured well in both positive and negative direction of loading by using the DIC technique and they matched the past results. Also, the experimental results show that the pull-out failure of the deformed bar occurred due to the failure inside the concrete, not through the interface between the concrete and the deformed bar.
  • Juan Jose Rubio, Takahiro Kashiwa, T. Laiteerapong, Wenlong Deng, K. Nagai, Sergio Escalera, Kotaro Nakayama, Yutaka Matsuo, Helmut Prendinger
    Computers in Industry 112 0166-3615 2019/11 
    Structural Health Monitoring (SHM) has benefited from computer vision and more recently, Deep Learning approaches, to accurately estimate the state of deterioration of infrastructure. In our work, we test Fully Convolutional Networks (FCNs) with a dataset of deck areas of bridges for damage segmentation. We create a dataset for delamination and rebar exposure that has been collected from inspection records of bridges in Niigata Prefecture, Japan. The dataset consists of 734 images with three labels per image, which makes it the largest dataset of images of bridge deck damage. This data allows us to estimate the performance of our method based on regions of agreement, which emulates the uncertainty of in-field inspections. We demonstrate the practicality of FCNs to perform automated semantic segmentation of surface damages. Our model achieves a mean accuracy of 89.7% for delamination and 78.4% for rebar exposure, and a weighted F1 score of 81.9%.
  • Yi Wang, Yushi Meng, Punyawut Jiradilok, Koji Matsumoto, Kohei Nagai, Shingo Asamoto
    Cement and Concrete Composites 104 0958-9465 2019/11 
    Internal stresses induced by the alkali silica reaction (ASR) and delayed ettringite formation (DEF) can cause cracking and degrade the mechanical properties of concrete, but the relationship between these phenomena remains unclear. In this study, crack patterns and the loss of compressive strength and elastic modulus of concrete after ASR and DEF expansion are simulated at the mesoscale using a three-dimensional Rigid Body Spring Model (3D RBSM). Concrete expansions induced by ASR and DEF are introduced in the model by applying initial strain on the springs of interfacial transition zone (ITZ) and mortar elements as damage history respectively. After verifying the model with single aggregate, further analysis of differences in concrete cracking due to ASR and DEF damage is carried out. The effect of the percentage of reactive aggregate is studied in the cases of ASR damage, while different intensified expansion areas are considered in the case of DEF. The simulated losses of mechanical properties of the concrete are compared with the experimental results and good agreement is obtained. It is found that the loss of mechanical properties is dominated by internal cracks rather than the surface crack pattern. Compressive strength is highly dependent on the development of large cracks while elastic modulus is closely related to the total number of cracked faces. More importantly, regardless of the intensified expansion area, the mechanical properties are consistent with expansive cracking damage.
  • P. Jiradilok, K. Nagai, K. Matsumoto
    Engineering Structures 197 0141-0296 2019/10/15
  • H. Naderpour, K. Nagai, P. Fakharian, M. Haji
    Composite Structures 215 69 - 84 0263-8223 2019/05/01 
    There are several methods for predicting experimental results such as empirical methods, elasticity and plasticity theory. Among these methods, the use of soft computing has been expanded due to good capabilities and high accuracy in predicting the target. Soft computing contains computational techniques and algorithms to provide useful solutions to deal with complex computational problems. In this study, three methods including Artificial neural networks, Group method of data handling and Gene expression programming are utilized to predict the compressive strength of columns confined with FRP. Total of 95 experimental data were selected to form the model. The height of the column, the compressive strength of unconfined concrete, the elastic modulus of FRP, the area of longitudinal steel, the yield strength of longitudinal steel and confinement pressure provided by FRP and transverse steel were considered as input parameters, while the compressive strength of FRP-confined columns was considered as the target. The proposed methods are compared with the existing models and provide great accuracy in predicting the results. Among the utilized methods, the ANN model showed the highest accuracy.
  • Hosein Naderpour, Kohei Nagai, Mohammad Haji, Masoomeh Mirrashid
    Expert Systems 36 (4) 0266-4720 2019 
    In this study, a fuzzy logic-based model for predicting the ultimate strength of FRP-confined circular reinforced concrete (RC) columns is presented. The adaptive neuro-fuzzy inference system (ANFIS) model was generated using valid experimental data with seven input variables. Input parameters were considered in such a way that all the parameters affecting the compressive strength of the column were simultaneously involved. Different models for compressive strength of fiber reinforced polymer (FRP)-confined concrete including the model in American Concrete Institute (ACI), to calculate the maximum stress endured by the column under axial load, were presented and compared with the results of the ANFIS model. Also, for similarity to other models, the ACI equation for calculating the maximum compressive strength tolerated by a column was considered without reducing coefficients as ACI-N and was compared with other models. The results obtained from the ANFIS model were compared with results from other models. ANFIS model showed the highest accuracy among all models in predicting the experimental results.
  • Koji Matsumoto, Carlos Arturo Linan Panting, Nuntikorn Kitratporn, Wataru Takeuchi, Kohei Nagai, Eiji Iwasaki
    Journal of Bridge Engineering 23 (10) 1084-0702 2018/10/01 
    A performance assessment of the Twantay Bridge, Myanmar, which is a suspension bridge that has been damaged by the movement of anchorages due to the soft soil conditions, was conducted based on simple monitoring, spatial measurements by three-dimensional (3D) terrestrial laser scan, and structural analysis with the finite-element method (FEM). Simple monitoring can capture the present progress of main-tower inclinations. This study proposes a method for capturing the overall deformation of bridges based on spatial measurement by 3D terrestrial laser scan. The FEM analysis was able to appropriately reproduce the deformation history of the suspension bridge after the construction, which was caused by movement of the anchorage blocks. Thus, the deformation of the main towers obtained with FEM was found to almost agree with that measured by the 3D terrestrial laser scan. In addition, by giving external loads, ultimate capacities were evaluated. This study clarified that a combination of simple monitoring, spatial measurements by 3D terrestrial laser scan, and structural analyses by FEM is useful for the safety assessment of and maintenance strategy for damaged suspension bridges.
  • Koji Matsumoto, Hirofumi Yamaguchi, Kohei Nagai
    Cement and Concrete Composites 91 198 - 208 0958-9465 2018/08 
    To investigate the pull-out behavior of deformed bars subjected fatigue loading in the simultaneous presence of liquid water, fatigue pull-out tests were carried out. Experimental parameters were the presence/absence of liquid water, matrix type, loading pattern, and the presence of a mechanical anchorage. It was found that the matrix in the vicinity of the deformed bar was converted into sludge by the complex interaction of liquid water and fatigue, and pull-out failure of the deformed bar occurred without accompanying macro cracks, resulting in significantly reduced fatigue life. Pull-out failure associated with the formation of sludge occurred at a comparatively low load level, and tended to occur when the water-cement ratio was high and under reversal loading. A further finding is that mechanical anchorages have the effect of reducing the conversion to sludge and have the beneficial effect of preventing pull-out failure.
  • Chika Yamasaki, Michael Henry, Kohei Nagai, Koji Matsumoto, Hiroshi Yokota
    Engineering Journal 22 (3) 243 - 255 0125-8281 2018/06/28 
    It is necessary to increase the capability of engineers to carry out maintenance management for keeping infrastructure safe and serviceable. Many regional training programs have been established to develop engineers into “maintenance experts.” While small and medium size construction companies and engineering consultants play an active role in infrastructure maintenance, they have limited human and economic resources for joining the training programs, so the sharing of knowledge acquired from the programs becomes important for improving the capability of the company as a whole. The objective of this research is to investigate how engineers who participated in regional training programs shared their knowledge inside their organization using an online questionnaire survey. It was found that leadership is an important factor for driving knowledge sharing activities, particularly for adapting the knowledge acquired from the training program to the company’s environment. However, the lack of experience serves as a barrier to the deeper transfer of knowledge, such as on-the-job training or seminars, as these methods of knowledge sharing require expert knowledge. For organizations to improve their knowledge sharing practices, it is necessary to consider the type of knowledge to be shared and the appropriate method for sharing, along with the improvement of the leadership for knowledge sharing in the organization.
  • Liyanto Eddy, Takeshi Miyashita, Koji Matsumoto, Kohei Nagai, Win Bo
    Journal of Disaster Research 13 (1) 80 - 87 1881-2473 2018/02 
    Myanmar officials have observed damage and unwanted movement of some bridge structures situated within the Yangon, Myanmar region. Low bearing capacity soils (termed “soft soil” herein) may be responsible for the observed damage and unwanted movement. The observations may indicate that the current bridges have not been adequately designed and constructed to fully accommodate the soft soils. Several damaged bridges were repaired; however, it was not clear if these repairs halted the unwanted movement of the structures. We conducted comprehensive observations of the existing conditions to fully understand the contributing factors of the damage and unwanted movements. We utilized our detailed observations to develop a simple and straightforward independent monitoring system to assess whether the bridge repairs were successful in arresting further unwanted movement. The Thakhut Bridge and Twantay Bridge were selected as the subjects of our study. A monitoring system consisting of two displacement transducers and a portable data logger were installed on the Thakhut Bridge to monitor movement of the central pier. The monitoring results indicated that the central pier of the Thakhut Bridge moved 3.5 mm over an eight month period along the direction aligned with the bridge axis. Because the movement of the central pier in Thakhut Bridge behaves linearly with time, the future relative displacement can be predicted to be on the order of 5–6 mm per year. A monitoring system consisting of an inclinometer and a portable data logger were installed on the Twantay Bridge to monitor the inclination of the south main tower. Our observations indicated that the inclination of the south main tower of the Twantay Bridge ranged from 0.00◦ to 0.04◦ (in the direction of the bridge axis). The very small angles of inclination that were observed led us to conclude that the south main tower was in a stable condition. The implementation of a simple and straightforward independent monitoring system will permit Myanmar officials to effectively plan and implement the requisite maintenance and corrective measures for damaged bridges.
  • Nuntikorn Kitratporn, Wataru Takeuchi, Koji Matsumoto, Kohei Nagai
    Journal of Disaster Research 13 (1) 40 - 49 1881-2473 2018/02 
    In Myanmar, defects and possible deformation were reported in many long-span suspension bridges. The current state of bridge infrastructure must be inspected, so that deterioration can be stalled and failure can be prevented. A 3D laser scanner, specifically the terrestrial laser scanner (TLS), has demonstrated the ability to capture surface geometry with millimeter accuracy. Consequently, TLS technology has received significant interest in various applications including in the field of structural survey. However, research on its application in large bridge structure remains limited. This study examines the use of TLS point cloud for the measurement of three deformation behaviors at the Pathein Suspension Bridge in Myanmar. These behaviors include tower inclination, hanger inclination, and deflection of bridge truss. The measurement results clearly captured the deformation state of the bridge. A comparison of the measurement results with available conventional measurements yielded overall agreement. However, errors were observed in some areas, which could be due to noise and occlusion in the point cloud model. In this study, the advantages of TLS in providing non-discrete data, direct measurement in meaningful unit, and access to difficult-to-access sections, such as top of towers or main cables, were demonstrated. The limitations of TLS as observed in this study were mainly influenced by external factors during field survey. Hence, it was suggested that further study on appropriate TLS surveying practice for large bridge structure should be conducted.
  • Michael Henry, Chika Yamasaki, Kohei Nagai, Koji Matsumoto, Hiroshi Yokota
    Journal of Disaster Research 13 (1) 88 - 98 1881-2473 2018/02 
    While the creation of new infrastructure has played an important role in Myanmar’s rapid economic growth, maintenance of the existing infrastructure is also necessary to ensure its continued safe development. The objective of this paper is to clarify the current practices and technologies for the maintenance and life cycle management of road bridges in Myanmar by reporting and analyzing the results of a series of surveys carried out with Myanmar’s Ministry of Construction. It was found that the maintenance cycle in Myanmar is not fully functional, as there are no standardized methods for inspection and repair, and the results of the inspections are not recorded in a database. However, efforts are under way to prepare a comprehensive manual for road bridges, and a database is being developed as part of a Japanese capacity development project. Analysis of technology transfer activities for maintenance found that the mismatch in conditions between Japan and Myanmar made it difficult to adapt the knowledge acquired in training programs to the local conditions. Furthermore, low awareness of maintenance and an inadequate maintenance budget also hinder opportunities to practice maintenance activities. To achieve sustainable transfer of maintenance technology, future activities should consider financial support for opportunities to acquire practical on-site maintenance experience in Myanmar.
  • L. Eddy, K. Matsumoto, K. Nagai, Z. Wang
    Computational Modelling of Concrete Structures - Proceedings of the conference on Computational Modelling of Concrete&amp;amp;amp;amp;amp;amp;amp;amp;amp;nbsp;and Concrete Structures, EURO-C 2018 755 - 766 2018 
    In this study, local bond model using 3D RBSM is proposed in which the reinforcing bar with its ribs in 3D shape is modeled accurately. In 3D RBSM, the local failure including cracks is predicted by checking for the discontinuous deformation of concrete and interaction between concrete and the reinforcing bar at the meso-scale. To verify the availability of the proposed approach, the simulations of a massive concrete specimen with a single reinforcing bar and under tension loading are conducted. The simulation results are compared with the experimental results which show a good agreement in terms of the strain distribution of the reinforcement. Other simulations of the tension stiffening behavior in a RC element are also conducted. The results of these simulations are also discussed through the comparison with the experimental results with good agreement in terms of macroscopic responses and the strain distributions of the reinforcing bar. The effect of porous concrete adjacent to the reinforcing bar is taken into the consideration. The simulation results indicate that the macroscopic responses are not affected by the bond model, but the internal stresses and cracks are different due to different bond models. The proposed approach is a suitable method when the effect of the local bar ribs-concrete interaction needs to be considered, while the conventional bond stress-slip relationship is applicable for the conventional structural analysis when the element size is over 10 cm.
  • Punyawut Jiradilok, Kohei Nagai, Koji Matsumoto
    Proceedings of the 12th fib International PhD Symposium in Civil Engineering 467 - 475 2617-4820 2018 
    In reinforced concrete structures, the residual capacity of corroded structure mostly depends on the severity of local corrosion damage. Even the same steel weight loss ratio, different corrosion pattern can result different local behavior and macroscopic behavior. In this study, a numerical model for simulating the spatial damage of reinforced concretes are developed based on Rigid Body Spring Model (RBSM). The previous experimental steel weight loss data is utilized for modelling the reinforcement steel properties along the bar length. The reduction in steel bar effective cross-section area, deteriorated bond capacity and expansion of corrosion product are quantitatively modelled based on the experimental data at every 5 mm. Beams with different corrosion pattern are simulated and analyzed. The simulation results show a good agreement with the previous experimental results.
  • Daisuke Hayashi, Kohei Nagai, Liyanto Eddy
    Journal of Structural Engineering (United States) 143 (7) 0733-9445 2017/07/01 
    To investigate anchorage performance in a multidirectional arrangement of reinforcement bars, simulations are carried out using a three-dimensional discrete model, specifically the three-dimensional rigid-body-spring model (3D RBSM). In 3D RBSM, a material is partitioned into an assemblage of rigid bodies interconnected along their contact boundaries through discrete springs. In this study, for example, RC is meshed into rigid bodies with a size of 1-2 cm, where the opening, closing, and sliding of cracks can be modeled by the deformation or transmission of internal forces between two rigid bodies through the use of nonlinear springs along their contact boundaries. Simulations are conducted, one with the number of transverse reinforcement bars as a parameter and another varying the clear space between column reinforcement and embedded reinforcement in a congested joint. The results of these simulations are evaluated through comparison with experimental data; good agreement is observed with respect to anchorage capacity, crack pattern, and failure mode. The simulation results indicate the concrete strength is reduced if the reinforcement spacing between the column reinforcement and the embedded reinforcement is very close because of a nonhomogeneous behavior of concrete, where in the previous experiment large voids around reinforcing bars were observed. Simulations also suggest that the propagation of local cracks depends on the arrangement of reinforcing bars. This research shows how mesoscale analysis using the 3D RBSM can be a useful technique to consider the effect of a multidirectional arrangement of reinforcing steel that results in a complex stress and strain state due to the 3D bond transfer mechanism. It can be used for the design of a reinforcement arrangement in congested areas that is not addressed by the design code. As a result, a rational reinforcement arrangement can be designed on a case-by-case basis.
  • Carlos Arturo Liñan Panting, Kohei Nagai, Eiji Iwasaki, Thein Nu
    Journal of Disaster Research 12 (3) 406 - 414 1881-2473 2017/06 
    This study conducted a simple monitoring for a deformed suspension bridge in Myanmar to confirm advances in deformation and verified the structural performance by FEM. The bridge is Twantay Bridge those main towers were inclined due to the movement of anchor blocks, that resulted in lowering of the road surface level. Therefore, inclinometers were set for simple monitoring to confirm advances in the deformation. The result of measurements performed for 50 days did not indicate any significant advances in inclination with respect to the main towers. The FE analysis to evaluate the structural performance estimated the current condition of the bridge by reproducing the construction process of the bridge and simulating the setting and removal of concrete blocks used as bridge railings as well as the sliding of anchor blocks. At each step of the analysis, the results were compared with the measured data to verify the appropriateness of the analysis. The findings confirmed that the current tensile force of the cable did not result in hazardous conditions that led to the rupture of the cable.
  • Kazuo Kyuma, Yozo Fujino, Kohei Nagai
    Journal of Disaster Research 12 (3) 394 - 395 1881-2473 2017/06
  • Ram Chandra Neupane, Liyanto Eddy, Kohei Nagai
    Fibers 5 (2) 2017/06/01 
    Poor detailing of the position of bearing pad over reinforced concrete (RC) corbel may lead to premature failure, which is undesired and structurally vulnerable. An appropriate retrofitting solution is necessary to ensure the functionality of such RC corbels. Considering the growing popularity of external carbon fiber-reinforced polymer (CFRP) in retrofitting, this research examines the effectiveness of an externally wrapped unidirectional CFRP sheet and compares its performance against traditional retrofitting methods. Moreover, it is intended to fulfill the lack of extensive research on external CFRP application for corbel strengthening. A total of eight medium-scale corbel specimens were tested on vertical load. Observed premature failure due to placing the bearing pad near the edge of corbel was verified and the effectiveness of the proposed structural strengthening solutions was studied. Experimental results show that although the loading capacity of the damaged corbel due to the poor detailing of bearing pad position could not be fully recovered, the external CFRP wrapping method demonstrated superior performance over RC jacketing and was able to prevent localized failure. Further study based on non-linear 3D finite element analysis (FEA) was carried out to identify the governing parameters of each retrofitting solution. Numerical studies suggested important parameters of various retrofitting alternatives for higher capacity assurance.
  • Liyanto Eddy, Kohei Nagai
    Engineering Structures 126 547 - 558 0141-0296 2016/11/01 
    There is as yet no clear design method for a rational reinforcement arrangement in a beam-column knee joint with mechanical anchorages because internal stresses, internal cracks, and the failure process have not been well understood. Previous experiments have shown that local reinforcement along the anchorages or the placing of an additional concrete block on the top surface of the joint can prevent anchorage failure due to local stresses from the anchorage plates. In this study, a meso-scale discrete analysis using 3D rigid body spring model (RBSM) is conducted to investigate the failure process and to investigate the effect of these measures. By studying the internal stresses and cracks in a beam-column joint with mechanical anchorages, the failure process is revealed: with bonding along the development length of the anchorages, diagonal cracks begin to occur and these cracks propagate to the top surface of the beam-column joint where they open at the surface, and finally, the opening of diagonal cracks takes place. Local reinforcement placed along the anchorages is shown to have two effects: to increase bond performance along the development length of the anchorages and to restrict the opening of the diagonal cracks. An additional concrete block placed on the top surface of the beam-column joint also has two effects: to increase bond performance along the development length of the anchorages and to restrict crack penetration to the top surface of the joint.
  • Koji Matsuinoto, Tao Wang, Daisuke Hayashi, Kohei Nagai
    Journal of Advanced Concrete Technology 14 (9) 573 - 589 1346-8014 2016/09 
    Pull-out tests and numerical analyses for deformed bar were performed on test specimens having cracks in concrete along the axial direction of the bar. The parameters for the experiment and analysis were: The maximum width of the initial crack, the presence or absence of a mechanical anchorage, the presence or absence of bond on either side of the deformed bar. and the presence or absence of a transverse bar. In both the experiments and the analyses, the pull-out load was applied at the end of the deformed bar after introducing the initial cracking by means of preloading. The results showed that the pull-out performance was greatly reduced by cracking along the axial direction of the bar Also, it was seen that cracking along the deformed bar tended to occur only on one side of the bar. In the particular case of mechanical anchorage it was found that bending stresses occurred near the anchorage due to eccentric loading. The transverse bar increases the residual crack width when the same initial crack width was generated before unloading and also inducing spalling of the concrete cover due to crackmg of the bond, resulting the reduction in pull-out strength.
  • Tao Wang, Koji Matsumoto, Kohei Nagai
    Proceedings of the 11th fib International PhD Symposium in Civil Engineering, FIB 2016 485 - 492 2617-4820 2016 
    In this paper, anchorage performances with T-headed bar in the damaged situation are investigated by experiment and analysis, affected by the transverse bars. In the experiment, there are two cases: with placing the transverse bar and without it near the T-head. Bending cracks are induced to represent the damages. The direction of cracking is perpendicular to transverse bar along the T-headed bar. The width of residual crack is more serious in case of using transverse bar than without it. In the damaged conditions, pull-out tests are conducted. As a result, the capacity of pull-out load is reduced 22% by using the transverse bar than without it. Internal stress condition and failure pattern are investigated by the discrete analysis. Yielding of transverse bar near the T-head makes the crack more difficult to close than without it. In the pull-out stage, the case without transverse bar can transfer the stress better from T-headed bar to concrete, which improved the pull-out capacity.
  • K. Nagai, D. Hayashi, L. Eddy
    Advances in Structural Engineering 17 (6) 861 - 869 1369-4332 2014/06/01 
    Nowadays, seismic design code in Japan is becoming more stringent. To satisfy the strict requirement, larger numbers of reinforcements must be placed, resulting in increased reinforcement congestion. To reduce the reinforcement congestion, mechanical anchorage is becoming popular in use instead of conventional hook rebar. However the behavior of mechanical anchorage placed in thin cover zone is not well understood, and the use of this is still limited. In this study, the discrete element method was used to investigate the performance of mechanical anchorage placed in thin cover depth zone, and the pull-out test in previous research was simulated. To assume that anchor plates welded to bar end are difficult to place side by side due to closely-spaced reinforcements, the position of anchor plate was taken as main parameter. The result shows that the anchorage performance changed according to the transverse bars effect and the shift of the mechanical anchorage. The simulations show good agreement with experiment data in terms of the anchorage capacity, crack pattern and failure mode.
  • Benny Suryanto, Koichi Maekawa, Kohei Nagai
    Journal of Advanced Concrete Technology 11 (2) 35 - 48 1346-8014 2013/02 
    This paper describes a method for estimating creep deformations of PVA-ECC under high stress levels from short-term tests. To obtain necessary data, a series of accelerated bending and compression creep tests under a sequence of increasing loads were carried out. Of particular interest was to study the evolution of plasticity and damage under varying load levels, and thus allow the rate of plasticity and fracturing as functions of evolving strain and fracture to be determined. Based on these behavioral aspects, predictions of creep of ECC at high stress levels were made. It is found that creep rupture in flexure and compression occurs at nearly the same order of lifetime in a logarithm scale, being the rupture at 90% of flexural strength about one order longer than that under compression. The tensile and compressive strains at rupture, when the load level is decreased from 90% to 75% of the short-term strength, are 1.1-1.4 and 1.6-3.5 times the short-term tensile and compression strain capacities, respectively. Copyright © 2013 Japan Concrete Institute.
  • Daisuke Hayashi, Kohei Nagai
    Engineering Computations (Swansea, Wales) 30 (6) 815 - 824 0264-4401 2013 
    Purpose - To solve the reinforcement congestion, mechanical anchorage is increasingly popular in use instead of conventional hook rebar. However, the bond performance between the rebar and concrete and the range of stress transfer between the two are still not well understood. The purpose of this study is to study the bond performance and failure mechanisms between reinforcement and concrete around an anchorage zone in a structural element. Design/methodology/ approach - In this study, simulations were carried out by 3D RBSM (Rigid Body Spring Model). This approach divided a problem of interest into elements, namely concrete and steel elements. And to simulate the failure of anchorage of RC, the steel element size is set according to the geometry complexity of the reinforcing bar. By using this method, two simulation cases of anchorage failure were carried out. Findings - This paper shows that simulations demonstrated good agreement with experimental data in terms of anchorage capacity, crack pattern, and failure mode. This indicates that RBSM analysis can simulate the failure behavior governed by complex cracks. Originality/value - This paper indicates the analytical approach to investigate the anchorage performance of RC. © Emerald Group Publishing Limited.
  • Jian Guo Dai, Tamon Ueda, Yasuhiko Sato, Kohei Nagai
    Computer-Aided Civil and Infrastructure Engineering 27 (6) 406 - 418 1093-9687 2012/07 
    Allowing for the tension stiffening effects resulting from the bond between steel reinforcement and surrounding concrete leads to effective deformation analysis of reinforced concrete (RC) members when using a nonlinear finite element analysis modeled on the smeared crack concept. Nowadays, externally bonded fiber reinforced polymer (FRP) composites are widely used for strengthening existing RC structures. However, it remains unclear to what extent the tension stiffening of postcracking concrete is quantitatively influenced by the addition of FRP composites, as a result of the bond between the FRP and the concrete substrate. This article presents a discrete model, which is based on rigid body spring networks (RBSN), for investigating the tension stiffening behavior of concrete in FRP-strengthened RC tensile members. A two-parameter fracture energy-based model was deployed to represent the bond-slip behavior of the FRP-to-concrete interface. The reliability of the RBSN model was verified through comparisons with previous test results. Further parametric analysis indicates that the tension stiffening of concrete is hardly influenced by the addition of FRP composites before the yield of steel reinforcement has occurred although concrete crack patterns and crack widths may be influenced by the bond-slip behavior of the FRP-to-concrete interface. © 2011 Computer-Aided Civil and Infrastructure Engineering.
  • B. Suryanto, K. Nagai, K. Maekawa
    RILEM Bookseries 2 239 - 246 2211-0844 2012 
    This paper investigates the post-cracking behavior of steel fiber rein- forced concrete (SFRC) panels from an analysis perspective based on a smeared, fixed crack approach. The analysis results show that the addition of hooked steel fibers improves the average tensile stress of the concrete and, when added beyond 1% by volume, limits the amount of crack-shear slip in the concrete effectively. The analysis reveals that experimentally observations of smaller angles of inclination of concrete principal strain than those of concrete principal stress at intermediate load levels is due to this limited crack slip. Finally, the analysis identifies that hooked steel fibers tends to be less effective than transverse reinforcement in confining shear cracks, thereby resulting in a lower shear transfer capacity. © RILEM 2012.
  • Kohei Nagai, Benny Suryanto, Koichi Maekawa
    ACI Materials Journal 108 (2) 139 - 149 0889-325X 2011/03 
    This paper focuses on the numerical modeling of high-performance fiber-reinforced cementitious composites (HPFRCCs), specifically polyvinyl alcohol engineered cement composites (PVA-ECCs) in the context of a space-averaged, fixed-crack approach. Compression, tension, and shear models are proposed. The compression and tension models include internal unloading and reloading paths. The shear model considers the shear stress transfer contributed by surface friction and fiber bridging in a phenomenological manner. The applicability of the models is verified against recent experiments on precracked PVA-ECC plates subjected to principal stress rotation, demonstrating that the proposed models replicate various responses of the plates. The degradation of initial stiffness and the overall strength of plates with precracks at different angles is represented well. Finally, this paper demonstrates the ability of the models to replicate the average strains spanning bidirectional multiple cracks occurring at the bottom surface of the precracked plates. Copyright © 2011, American Concrete Institute. All rights reserved.
  • Yu Inoue, Kohei Nagai
    Procedia Engineering 14 1165 - 1173 1877-7058 2011 
    This study investigates a mechanical behavior of T-headed bar for anchorage when it is applied to thin cover depth place. Firstly, an experimental program has been carried out. Five cylinderical specimens were subjected to uniaxial tensile loading. Main parameters include cover depth (30 mm and 60 mm), anchorage-end type (with and without T-headed end), and bond. The results show that fracture pattern is different by cover depth and anchorage-end type. Secondly, simulation of the specimen response was performed using the two and three-dimensional numerical analysis, Rigid Body Spring Model (RBSM). The analysis results simulate the crack development of the specimens during loading. Three dimensional analysis can reproduce the longitudinal cracking pattern at failure though it requires a calculation time. Furthermore, it is useful to evaluate the internal stress carrying mechanism nearby the anchorage end.
  • Benny Suryanto, Kohei Nagai, Koichi Maekawa
    ACI Materials Journal 107 (5) 450 - 460 0889-325X 2010/09 
    An experimental program was conducted to investigate the effects of simultaneous opening-sliding of multiple cracks on the behavior of high-performance fiber-reinforced cementitious composites (HPFRCCs). For this purpose, 12 HPFRCC plates were tested in bending and under two constitutive principal stress directions. To facilitate reorientation of the stress fields, the plates were precracked and then sawn with certain orientations. Finally, the plates were retested in bending to failure. The results showed that the change in principal stress direction had a substantial effect on macroscopic plate behavior, as marked by reductions in strength and initial stiffness. The effect of stress field reorientation on the cracking pattern was, however, minimal. Regardless of the orientation of the new principal stress direction to that of precracking, a somewhat orthogonal crack pattern always appeared. To characterize the mechanisms involved, the stressstrain relationship within the constant moment span of each plate is presented and discussed. Copyright © 2010, American Concrete Institute. All rights reserved.
  • Benny Suryanto, Kohei Nagai, Koichi Maekawa
    Journal of Advanced Concrete Technology 8 (2) 239 - 258 1346-8014 2010/06 
    This paper describes an attempt to predict the response of shear-critical ECC members that exhibit strong anisotropic stress and strain fields. The ECC members investigated include pre-cracked ECC plates under stress field rotation, orthogonally-reinforced ECC (R/ECC) panel under pure shear, and shear-critical R/ECC beams under reversed cyclic loading. To achieve a simple yet accurate prediction, the mechanics of the ECC are represented by smeared models using a fixed crack approach. The applicability of these models is demonstrated through a simulation of ECC plates and R/ECC panel responses. This demonstrates the importance of an appropriate shear transfer model in representing essential behaviors of ECC in an anisotropic field. Predictions of these models were then compared against experimental results of shear-critical R/ECC beams with a M/Vd ratio of 1.0 and 0.5. For beams with a M/Vd ratio of 1.0, a good agreement is observed in terms of hysteretic response, crack pattern, and failure mechanisms. For beams with a 0.5 M/Vd ratio, the analysis somewhat underestimates the beam capacity, although it does predict a correct failure mechanism. Overall, this paper demonstrates that practical application of nonlinear finite-element analysis to ECC structural members is possible. Copyright © 2010 Japan Concrete Institute.
  • Kohci Nagai, Benny Suryanto, Koichi Maekawa
    Advances in Concrete Structural Durability - Proceedings of the 2nd International Conference on Durability of Concrete Structures, ICDCS 2010 653 - 662 2010 
    Space-averaged models are presented henceforth the behaviour of cracked High- performance Fibcr-rcinforced Cementitious Composites (HPFRCCs), specifically Polyvinyl Alcohol Engineered Ccmcnt Composites (PVA-ECCs), may be investigated. The models include the compression, tension, and shear transfer models and are suitable in use in a fixed crack scheme. The applicability of the models is verified against recent experiments on pre- crackcd PVA-ECC plates subjected to principal stress rotation. The verification shows that the proposed models replicate various responses of the plates reasonably. The degradation of initial stiffness and strength of the plates is simulated well. The ability of the models proposed to replicate the in-plane strains across multi-directional cracking at the bottom of the plates is demonstrated. ©2010 by Hokkaido University Press All rights reserved.
  • Benny Suryanto, Kohei Nagai, Koichi Maekawa
    Journal of Advanced Concrete Technology 8 (3) 315 - 326 1346-8014 2010 
    This paper presents the verification of analytical modeling for reinforced Engineered Cementitious Composite (R/ECC) in the context of a smeared, fixed crack approach. Verification is provided through the analysis of six R/ECC panels subjected to pure shear. The results demonstrate that the proposed models are capable of replicating various responses of the panels well, provided that tensile property of the ECC is calibrated against those obtained from the panel tests. These responses include load-deformation responses, the magnitudes and directions of principal stress and principal strain, and failure modes. The results also demonstrate the possibility of representing the average crack-shear transfer in the ECC with an explicit smeared model. Finally, this paper includes predictions of the shear capacity of R/ECC panels with a wide range of reinforcement ratios and concludes with discussions regarding factors influencing shear strength. Copyright © 2010 Japan Concrete Institute.
  • Tamon Ueda, Muttaqin Hasan, Kohei Nagai, Yasuhiko Sato, Licheng Wang
    Journal of Materials in Civil Engineering 21 (6) 244 - 252 0899-1561 2009 
    Mesoscale constitutive models of frost-damaged concrete are developed in this study through numerical simulation using a two-dimensional rigid body spring model (RBSM). The aim of the simulation is to predict the macrobehavior of frost-damaged concrete subjected to mechanical loading. The models also clarify the difference in failure behavior of concrete with and without frost damage. Zero strength elements and the concept of mesoscale plastic tensile strain are introduced into the normal RBSM springs to consider the experimentally observed cracking and plastic deformation caused by frost damage. The difference in the effect of frost damage on compression and tension behavior as found in the experiments is clearly predicted. Finally, analysis of a notched beam subjected to bending after different degrees of frost damage is carried out. The resulting load-deflection curves agree well with those obtained in the experiments. These good correlations confirm the applicability of the mesoscale model for predicting the macrobehavior of frost-damaged concrete. © ASCE 2009.
  • K. Nagai, B. Suryanto, K. Maekawa
    Creep, Shrinkage and Durability Mechanics of Concrete and Concrete Structures - Proceedings of the 8th Int. Conference on Creep, Shrinkage and Durability Mechanics of Concrete and Concrete Structures 1 557 - 563 2009 
    To develop a space averaged constitutive model under multi-directional cracking condition of High Performance Fiber Reinforced Cementitious Composite (HPFRCC), bending loading test of HPFRCC plate with initial crack is carried out. The experimental result shows different behavior in bending depends on an angle of the initial crack direction where the degradation in strength and initial stiffness are observed in certain cases. Based on the experimental result and the previous researches, the constitutive model is developed and verified by FEM with Mindlin plate element. Through a discussion of result of analysis, it is clarified that some part of the developed model are necessary to be modified for the accurate prediction of HPFRCC behavior. © 2009 Taylor & Francis Group.
  • B. Suryanto, K. Nagai, K. Maekawa
    Creep, Shrinkage and Durability Mechanics of Concrete and Concrete Structures - Proceedings of the 8th Int. Conference on Creep, Shrinkage and Durability Mechanics of Concrete and Concrete Structures 1 495 - 500 2009 
    An experimental program is undertaken to investigate the influence of damage on cracking behavior of Ductile Fiber Reinforced Cementitious Composite (DFRCC) plates under four-point bending test. The DFRCCs tested herein were the Engineered Cementitious Composite (ECC) and Ductal. The damage was of flexural cracking with different levels and orientations. After pre-damaged and further loaded, a bidirectional crack pattern was always observed; initial cracks and then the secondary ones. The initial cracks were uniform and continuous, whereas the secondary cracks were discontinuous. The effect of initial-crack orientation on the orientation of secondary-crack in the former material was minimal as indicated by a relatively orthogonal crack pattern, whereas its influence on the later material was more substantial as evidenced by larger deviation of secondary-crack from orthogonality. Moreover, the initial damage was also found to influence the plate behavior in which the degradation of initial stiffness and strength were evident. © 2009 Taylor & Francis Group.
  • K. Nagai, B. Suryanto, K. Maekawa
    EASEC-11 - Eleventh East Asia-Pacific Conference on Structural Engineering and Construction 2008 
    The recent attempt to predict the behavior of pre-cracked HPFRCC plates is reported. A brief overview of the experiment is firstly described. In the experiment, a significant influence of pre-cracks was observed that was mainly caused by the orientation of the pre-cracks. In certain experimental cases, the degradation of strength and initial stiffness were evident. To simulate the behavior of pre-cracked HPFRCC plates, three spatially-averaged constitutive models (compression, tension, and shear models) of the COM3, a finite element analysis for reinforced concrete structure, were modified. Several analyses were then carried out to verify the accuracy of the incorporated models. Through the discussion of the analysis results, it is found that the incorporated models still need to be further upgraded to improve the accuracy of the simulation.
  • Kohei Nagai, Yasuhiko Sato, Tamon Ueda
    Journal of Advanced Concrete Technology 3 (3) 385 - 402 1346-8014 2005 
    Concrete is a heterogeneous material consisting of mortar and aggregate at the meso scale. Evaluation of the fracture process at this scale is useful to clarify the material characteristic of concrete. The authors have conducted meso scale analysis of concrete over a past few years by Rigid Body Spring Model (RBSM). In this study, three-dimensional analyses of mortar and concrete are carried out, which is necessary for the quantitative evaluation of concrete behavior especially in compression. Constitutive models at the meso scale are developed for the 3D RBSM analysis. Failure behaviors and strengths in compression and tension of mortar and concrete are predicted well by the analysis. In biaxial compression test of concrete, crack in normal direction to plane of specimen is simulated that cannot be presented by two-dimensional analysis. Copyright © 2005 Japan Concrete Institute.
  • Kohei Nagai, Yasuhiko Sato, Tamon Ueda
    Journal of Advanced Concrete Technology 2 (3) 359 - 374 1346-8014 2004/10 
    Concrete is a heterogeneous material consisting of mortar and aggregate at the meso level. Evaluation of the fracture process at this level is useful to clarify the material characteristic of concrete. However, the analytical approach at this level has not yet been sufficiently investigated. In this study, two-dimensional analyses of mortar and concrete are carried out using the Rigid Body Spring Model (RBSM). For the simulation of concrete, constitutive model at the meso scale are developed. Analysis simulates well the failure behavior and the compressive and tensile strength relationship of mortar and concrete under uniaxial and biaxial stress conditions. Localized compressive failure of concrete is also simulated qualitatively.

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Research Grants & Projects

  • 日本学術振興会:科学研究費助成事業
    Date (from‐to) : 2023/04 -2026/03 
    Author : 浅本 晋吾, 長井 宏平, 高橋 恵輔, 宮本 慎太郎
  • 日本学術振興会:科学研究費助成事業
    Date (from‐to) : 2022/04 -2025/03 
    Author : 長井 宏平, 松本 浩嗣, 酒井 雄也, 浅本 晋吾
  • 日本学術振興会:科学研究費助成事業
    Date (from‐to) : 2021/10 -2024/03 
    Author : 長井 宏平, 松本 浩嗣, 鎌田 知久, 金澤 健
     
    海外共同研究者となる深セン大学の上田教授,カリフォルニア大学デイビス校のBolander教授と研究の進め方について確認した。 研究のコアとなる微細構造解析プログラムに連続繊維シートのモデルを組み込み,引張試験による付着挙動の解析を行った。微細スケールにおいてはコンクリート側にもひび割れが進展し損傷が蓄積されることでマクロな付着挙動にも影響を与える。また,コンクリートには繊維軸方向から横方向にも応力が広がるが,これを3次元解析により再現可能である。解析により,局所の損傷と全体の応答を再現し,解析の妥当性の検討を続けている。 水分移動のモデル化についても,既往の水分移動,物質移動モデルから,本解析システムに組み込むモデルの高度化を進めている。特に,水分供給初期にコンクリートの乾燥度が高い場合と湿潤状態では,条件により水分の浸透速度が異なるので,適切なモデル化が必要である。 鉄筋腐食については,腐食後の付着挙動をより正確に再現するために,研究代表者が過去に行った試験の観察動画から画像相関法を用いて界面の挙動をより詳細に分析し,鉄筋からコンクリートへの応力伝達の変化と鉄筋表面の腐食生成物の影響について調査をしている。 凍結融解作用については,水分の供給と温度の影響のモデル化の方法と,劣化後に材料力学特性がどのように低下していくかについて,既往の文献を参考に,モデル化の方法を検討している。特に温度履歴のモデルへの反映を,解析可能なものとして行うことが重要となる。
  • 日本学術振興会:科学研究費助成事業
    Date (from‐to) : 2021/04 -2023/03 
    Author : 長井 宏平, JIANG CHENG
     
    受入れ研究者が開発している鉄筋コンクリート構造解析のための三次元微細構造解析システムを発展させ,新たに連続繊維補強コンクリートにより補強した際の構造性能を評価できるシステムとすることに取り組んだ。具体的には,解析システムにおいてコンクリート表面に連続繊維補強材(FRP)をモデル化し,FRP要素に繊維の材料特性を,FRPとコンクリート要素の界面には付着特性を組み込むことで構造性能を表現することを可能にした。解析の検証として,既往の実験を解析対象にFRPの引張破壊の再現を行った。微細構造解析では要素サイズが5mm程度と小さく,ローカルな付着と滑りの積分として全体の引抜挙動が表される。FRPの応力状態,コンクリートとの界面の付着,さらにコンクリートにひび割れが発生し進展していく複雑な破壊プロセスを実験と同様に適切に再現することができ,妥当性が確認された。また,三次元解析であるので,FRPの外側に広がるコンクリートの応力やひび割れも解析ができ,汎用性の高いシステムとなった。通常の実験では,FRPの塗布の状況などにより,観察される結果にばらつきも大きくなるが,数値解析により各因子の全体挙動への影響度もパラメトリック解析から推定できるようになった。 Jiang氏が,研究実施期間中にオーストリアの大学に職を得たために,研究を終了することとなったが,上述のように解析システムにFRP要素を新たに組み込み,実験との比較やパラメトリック解析により,解析の妥当性を確認したうえで新たな知見を得られるシステムへと拡張がされた。拡張された解析システムを用いて,研究室の学生が研究を継続しておりJiang氏との連携を継続する予定である。
  • 日本学術振興会:科学研究費助成事業
    Date (from‐to) : 2020/11 -2023/03 
    Author : 長井 宏平, JIANG CHENG
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2020/07 -2022/03 
    Author : Nagai Kohei
     
    As a method to estimate the construction year of bridges, we proposed to estimate the bridge construction year from the past satellite images by tracing back the satellite data records. It was possible to confirm the construction of long bridges by visual images from past satellite data. Since it is difficult to confirm this for short bridges, we proposed a method to estimate the construction year using the Normalized Difference Water Index (NDWI) obtained from satellite wavelength data and confirmed the accuracy of this method.
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2019/04 -2022/03 
    Author : Nagai Kohei
     
    We have developed a computational approach for estimating the distribution of corrosion along the length of a reinforcing bar from the observed surface crack widths. The approach is based on mesoscale simulations of reinforced concrete, using rigid-body-spring models (RBSM), guided by Model Predictive Control (MPC). It is extended to account for differing (smaller) crack openings, multiple instances of localized corrosion, and added confinement due to the presence of stirrups. Accuracy of the proposed approach is demonstrated through comparisons with laboratory tests of steel corrosion within concrete. The crack distribution observed on the concrete surface is automatically reproduced by optimized application of internal expansion within the mesoscale model, from which the corrosion distribution is estimated. The estimated corrosion distribution is accurate when compared to the measured corrosion of bars removed from the test specimens.
  • 日本学術振興会:科学研究費助成事業
    Date (from‐to) : 2018/11 -2021/03 
    Author : 長井 宏平, WANG YI
     
    6か月の研究期間で、前年度に投稿した論文の査読対応を行い、公表された。ASRとDEFという異なるコンクリートの膨張劣化現象に対してそれぞれのモデル化を行い,強度と剛性の低下を適切に表現できる解析システムを構築することができた。また、遅れていた補修材のモデル化を実施し、簡易な解析ができるようになったが、損傷後に補強を行い、その性能評価を実施するには至らなかった。 2年間の研究機関全体を纏め、開発した解析プログラムを整理して、研究室として今後も継続して解析が行えるような体制を整えた。
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2017/04 -2020/03 
    Author : Asamoto Shingo
     
    This study investigated the damage around constitutive materials of concrete and the subsequent change of material properties when the high temperature over 70 C is exposed to concrete at early ages. It is suggested that the high temperature can cause the micro damages at the interfacial zones depending on the materials and the resistivity of concrete to mass transfer of harmful materials may be decreased due to the damage. It is also found that the volumetric change and deformation characteristics can increase according to the materials and initial temperature history. The experimental and numerical analysis indicated that the swelling arising from the high temperature at early ages can be accelerated by carbonate ion. The long-term inhibition of the swelling by the coal ash was also experimentally confirmed.
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2018 -2020 
    Author : NAGAI Kohei
     
    We have developed a simulation system for estimating the levels of internal corrosion along the reinforcing bar length from surface crack information. This innovative system is produced by integrating the technique of Model Predictive Control (MPC) with Rigid-Body-Spring Models (RBSM) of corrosion-induced cracking at the concrete meso-scale. MPC controls the simulated surface cracks such that they match the observed cracks by optimizing the internal expansions of springs representing the steel-concrete interface within the RBSM. The applicability of the system is verified using both synthetic crack width data and crack data collected from in-house laboratory testing. In the laboratory testing, corrosion levels were quantified by 3D scanning of the extracted reinforcing bars. The simulation results agree with the corrosion measurements, demonstrating the potential of the MPC-RBSM system for predicting the corrosion distribution along reinforcing bars using surface crack data.
  • 日本学術振興会:科学研究費助成事業
    Date (from‐to) : 2016/10 -2019/03 
    Author : 長井 宏平, EDDY LIYANTO
     
    最終年度の半年間で,数値解析については,鉄筋コンクリートの付着のモデルの適用性の検討,膨張モデルの向上のためのパラメトリック解析を行うとともに,解析システムのインターフェイスを改善し,より解析モデル作成や解析結果の表示が円滑に行えるようにした。 また,アジア域の環境や,建設時に鉄筋かぶりが不足した場合のコンクリート表層品質の変化を計測する実験を継続し,論文として纏めた。同様の実験をタイとミャンマーで計画し,タイでの計測が開始された。
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2016/04 -2019/03 
    Author : NAGAI Kohei
     
    Three dimensional meso scale analysis system was expanded by introducing the local bond model for corrosion damage where the constitutive model was developed by the experiments. The local corrosion can be directly considered as the input of simulation that enable to evaluate the residual structural performance of damaged reinforced concrete. This system was applied to the case of corrosion damage at the anchorage part. The influence of anchorage damage on the overall structural performance was examined. For the expansion problems such as ASR or DEF, the expansion model was introduced to the simulation system and the residual strength reduction can be simulated.
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2013/04 -2015/03 
    Author : NAGAI Kohei
     
    A meso-scale analysis of reinforced concrete members by a 3D discrete element analysis, called RBSM, was conducted in this study especially focusing on the beam-column joint part where the reinforcement bar arrangement is very complex. The study by a 3D meso-scale discrete analysis is useful since the reinforcement arrangement can be modeled in an accurate manner, local failure can be predicted precisely. In this study, analysis was conducted in order to investigate the effect of the stirrups arrangement on the failure of beam column joints with mechanical anchorages through the comparison with the experimental results. The simulation results were in good agreement with the experimental results. Based on the simulation results, the causes of major cracks were described. Eventually, the failure process of beam column joints with mechanical anchorages has been revealed through the study of the internal stress and the cracking pattern of RBSM.
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2012/04 -2015/03 
    Author : NAGAI Kohei, FUJIYAMA Chikako
     
    Experiments of fiber reinforced cementitious composite were conducted at both material and structural scale for a development of robust material for structural material. To improve the shear performance, aggregate and/or steel fiber are added to a conventional PVA/ECC. From the experimental results, it was observed that reduction of the capacity was controlled by adding these materials. The effect was confirmed by the image data analysis and calculation of shear strain obtained from the deformation locally measured. The shear sliding was restricted by the inclusion of aggregate and steel fibers.
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2011 -2012 
    Author : NAGAI Kouhei
     
    Three-dimensional mesoscopic analyses were conducted to evaluate the anchorage performance of reinforced concrete under multi-directional reinforcement arrangement.Also, particle model was adopted to simulate the flow of fresh concrete into the congested region where the concrete was modeled by two-phase model, mortar and aggregate.
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2011 -2012 
    Author : SUZUKI Naofumi, MATOUS Petr, NAGAI Kohei
     
    This study attempted to appraise the process that community-based organizations aimed towards regeneration of a depopulating rural community has grown and increased their importance within the community, using a social network approach. It employed an approach that combined the established methods of Name Generator and Resource Generator to measure social capital, and estimated the importance of the organizations in terms of the extent to which their members provided resources to the community. The result showed that the members of the organizations were significantly more relied upon as resource providers compared to nonmembers, and the number of years they lived in the community or the fact that they were born somewhere else did not affect their ability to provide resources for others. This might suggest that those organizations could be useful vehicles for those who moved in from other areas to acquired trust and respect within the community and become integrated.
  • 日本学術振興会:科学研究費助成事業
    Date (from‐to) : 2011 -2011 
    Author : 前川 宏一, 内村 太郎, 長井 宏平, 長山 智則, 千々和 伸浩
     
    水が浸透したコンクリート系橋梁床板の早期疲労劣化問題を解明するため、コンクリート構造物のひび割れや接合界面などに介在する凝結水の動態と、コンクリート構造の変形応答・破壊進展に現れる速度依存性に着目し、地盤分野で多くの成果を挙げているBiotの固液二相理論を既往のコンクリートの非線形有限要素解析プログラムに組み込んだプロトタイプモデルを作成した。これにより、多方向分散ひび割れ構成則と対数時間積分法に基づく全履歴追跡型の疲労解析法と水の動態を、ひとつの数値計算システムで解くことが可能となった。作成した数値解析モデルの感度解析を実施するとともに、検証実験の詳細設計を行った。意図的にひび割れを生じさせた鉄筋コンクリート試験体で、載荷速度をパラメータとし、ひび割れ内部の水圧変動、試験体の変形(変位)、鉄筋のひずみおよび荷重の動的測定を行い、凝縮水の圧力変動と構造物の応答変化に関するデータを得ようとするものである。また、ひび割れを有するコンクリートとひび割れ内に貯留される凝縮水に着目し、既往の研究(ひび割れを有するコンクリートの透水係数同定実験)のデータに基づき、本研究で提案する数値モデルによる異方性透水係数の検証方法を策定した。 本研究は3年間の研究期間を想定していたが、期間途中で申請者の基盤研究Sが採用されたため、重複制限により研究廃止措置となった。よって、平成23年5月末日までの2カ月に行った研究について報告するものである。
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2010 -2011 
    Author : NAGAI Kouhei
     
    This research was conducted to develop a new robust fiber reinforced cementitious material under the multi directional stress field based on HPFRCC that shows high ductility in tension with multiple cracking. Coarse aggregate and steel fiber were put in the HPFRCC to improve the shear performance after the cracking. Through the experimental investigations at the material scale, appropriate ratio of the inclusions was proposed. To apply this material to structure, static and fatigue tests of beam were carried out to verify the applicability.
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2008 -2010 
    Author : MAEKAWA Koichi, ISHIDA Tetsuya, NAKARAI Kenichiro, NAGAI Kohei
     
    The multi-scale simulation model to quantify the micro damage of cracked/uncracked cementitious composites as well as structural performance made by massive concrete was developed. The model is capable of successfully simulating a long-term behaviors of concrete structure subjected to repeated thousand~million cycles of loads in its life. Furthermore, ion resolution from concrete, such as calcium, boron etc. and re-absorption model for underground concrete structure was provided. With coupling the macro and micro models, this study enables to predict long-term mechanical and thermo-dynamic performances of structural concrete and its environmental impact.
  • 日本学術振興会:科学研究費助成事業
    Date (from‐to) : 2008 -2009 
    Author : 石田 哲也, 前川 宏一, 長井 宏平
     
    (1)重力式堆肥かくはん・切返し装置BioMY-BOXの効率実証試験 堆肥のかくはん・切返しに供するBioMY-BOXの効率を実証するため,堆肥材料を模擬した人工材料を用意し,BioMY-BOXに投入する複数種の混練状況を,切り替えしパターンを変えながら画像解析により分析した.その結果,BioMY-BOX単体を用いるのみでも複数種材料を一体的に混練することが可能であるが,発酵槽からBioMY-BOXへの投入方法によっても,天地返しなどのバルク移動が生じることが明らかとなった.構成要素の組み合わせを工夫することで,堆肥化プラントの高効率化が可能であることを示すと同時に,施設設計に向けたノウハウを得ることができた. (2)堆肥混合物内部の温度多点計測と堆肥化過程シミュレーション技術BioDuCOM改良 堆肥化プロセスを数値解析で高精度に追跡・評価するために,堆肥混合物内部の温度について多点測定を行った.その結果,表面近傍では酸素供給が十分にあるため発酵温度が初期から速やかに上昇すること,また内部では嫌気環境となるため温度上昇が鈍化することが明らかとなった.季節,外気温,発酵材料の種類によって,発酵速度と酸素消費,また水分逸散条件が変わるため,堆肥混合物全体の発酵を促進させるための条件が変わることが明らかとなった。同時に,開発を進めているシミュレーション手法BioDUCOMにおいても,同様の傾向を解析しうることが分かった.
  • Japan Society for the Promotion of Science:Grants-in-Aid for Scientific Research
    Date (from‐to) : 2008 -2009 
    Author : NAGAI Kohei
     
    This research focuses on the mechanics of fiber reinforced cementitiou composites especially under multi-directional cracking condition. To clarify the performance, experimental investigation was firstly conducted and the degradation of strength and stiffness were confirmed that were depending on the initial cracking angle and damage level. Based on the experimental result and the previous researches, the constitutive model was developed and verified by FEM. Furthermore, analysis was expanded to structural level especially in shear critical member. Through the results, applicability of the developed model were verified.
  • 日本学術振興会:科学研究費助成事業
    Date (from‐to) : 2007 -2007 
    Author : 長井 宏平
     
    損傷を受けたRC構造物の性能を照査するため,補修補強材料として実績があり,近年は新規構造物への適用も行われている繊維補強コンクリートに着目し,損傷を受け多方向ひび割れが発生した際の挙動を実験的に明らかにし,空間平均構成モデルの構築を試みた. 繊維補強コンクリートは高い一軸引張性能を有するものの,補修補強材料や構造部材として用いられた場合に複雑な応力下にさらされ多方向ひび割れが発生する可能性があり,その際の挙動は明らかにされていない.本研究では繊維補強コンクリートとして高じん性繊維補強セメント複合材料(DFRCC)を用い,損傷を受け多方向ひび割れが発生した際の挙動を把握するための実験方法を新たに考案した.薄板DFRCCの曲げ載荷により第一ひび割れを導入し,除荷後に薄板を回転させ主応力方向を変化させ曲げ載荷することで第二ひび割れを発生させる.これにより第一・第二ひび割れの開口とずれが混合した複雑な挙動を実現することに成功した.市販のDFRCCであるECCとDuctalを対象とし,第一ひび割れの損傷程度と第二ひび割れとの角度の全体挙動への影響を調べた.実験結果より,第一ひび割れ角度が小さいほど初期剛性が低下し,逆に角度が大きいほど強度が低下することを確認した.また,ひび割れ性状として,第二ひび割れは主応力方向に垂直には発生せず,第一ひび割れを介しながら第一ひび割れに垂直に近い角度で発生し,終局時の局所化したひび割れは第一及び第二ひび割れが混合し全体として主応力垂直方向に近い角度を伴うことを明らかにした.また実験結果を基に多方向ひび割れ発生時の空間平均化モデルを構築し,有限要素法プログラムに導入し,実験結果の再現を試みた.これらの実験および解析結果を2編の論文に纏め公表するに至った.


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