The mechanical nonlinearity of a masonry structure depends on debonding and slip of masonry joints and the fracture of blocks like bricks and concrete. The present study proposes a behavioral simulation to idealize masonry joints by allocating three orthogonal planes, which are governed by the existing multi-directional crack model, and to represent the damage of blocks by using another three crack planes. The shear stiffness of the joint changes due to the disintegration of the infilling mortar caused by the shear slip of the joint, as well as the confining pressure dependence of the shear strength. Shear response analysis of a masonry structure considering this disintegration was carried out, and the applicability of the analysis model was validated. The following three types of structures were selected for validation: 1) masonry structures in which both joints and masonry blocks are damaged, 2) structures in which masonry blocks are primarily damaged but deformation is not concentrated in the joints, and 3) structures in which mortar joints are exclusively damaged but the masonry blocks are exempt of damage. It was confirmed that the proposed analysis model can analyze the damage mode of masonry structures. It evaluates yield strength as well and deformability is estimated on the safe side of an engineering viewpoint.
Electrochemical realkalization has been applied to many concrete structures deteriorated by carbonation of concrete. As the repair effect of this method, it has been reported that the pH value of the carbonated concrete recovered by the electrolysis reaction at the steel in concrete and the electro-osmosis of the electrolyte solution in the anode system set on the concrete surface. However, the protection effect of the realkalization against the steel corrosion in concrete has not been clarified enough. Therefore, this study investigated the steel corrosion behavior in concrete after the application of realkalization in the cases of different degree of deterioration and different kind of electrolyte solution by measuring content profiles of several sort of ions and electrochemical indicators for evaluating steel corrosion. As a result, the electrochemical removal of Cl- ions from carbonated concrete and the electrochemical penetration of K+ ions from the electrolyte solution into carbonated concrete were both promoted compared with the case of non-carbonated specimen. Moreover, the protection ratio calculated by the corroded area ratio of steel bars in the electrochemically treated specimens and non-treated specimens subjected to the cyclic drying and wetting storage for 400 days after the period of realkalization achieved around 80% regardless of the difference of the deterioration condition before applying realkalization. This paper is an extended and enhanced version of an earlier work under different title [Takahashi, H., Ueda, T., Nanasawa, A., Nakayama, K. and Tsukagoshi, M., (2020). “Repair effect of realkalization for reinforced concrete with different degree of deterioration.” In: Proceedings of the 6th International Conference on Concrete Materials - Performance, Innovations, and Structural Implications (ConMat’20), Fukuoka, Japan 27-29 August 2020. Tokyo: Japan Concrete Institute, 1065-1075].
The purpose of this study was to provide insights into deeper understanding of the combustion performance of cement-bonded particleboards. Cement-bonded particleboard produced from Chinese fir processing residues is fabricated and its combustion performance is further evaluated by cone calorimeter analysis. Obtained results revealed that combustion performance of cement-bonded particleboard positively depends on the density of which, and higher density imparts corresponding particleboards reduction in heat release rate, total heat release, and mass loss rate, along with elevated time to ignition. With the density of particleboard increasing from 0.7 to 1.0 g/cm3, peak and average heat release rate, total heat release and average mass loss rate of specimens were decreased by 55.9, 73.4, 77.8 and 60.3%, respectively. Furthermore, time to ignition of particleboard increased by more than 20 times as the density increasing. Residual mass of specimens was also remarkably elevated along with higher yields of CO and CO2 as the density of particleboard increasing. We believe that proposed investigation is beneficial for the value-added utilization of wood processing residues and further developing products fabricated by wood processing residues.
This study proposes a theoretical evaluation equation for the capacities of beam action in reinforced concrete (RC) beams. The equation reflects the dowel action of the main reinforcements for the development of rational evaluation methods for the shear capacity. To verify its accuracy, a static loading experiment and finite element (FE) analyses were conducted for the beams with 2.1 to 9.4% of the main reinforcement ratio. The fundamental equation that reflects the dowel action of the main reinforcement was developed using the equilibrium condition of the free body in beam action. The decrease in crack width by the main reinforcement, which was required to calculate dowel force, was derived by the assumption of perfect bonding between concrete and rebars. The governing equations were formulated to derive the experimental values of the contributing forces of arch action and beam action in RC beams with cross sections of multilayered rebars. The calculated values indicated that changes in load-carrying mechanisms could be explained from the equation. The accuracies of the proposed equation and evaluation equations of shear strengths based on design codes were compared against the capacities of beam action based on the analytical results. As a result, for the values based on the proposed equation, the coefficient of variation was 7.7% and the highest accuracy was observed in all the cases examined. It was confirmed that the proposed equation accurately reflected the dowel action of the main reinforcements and guaranteed rationality owing to its theoretical background.
The true strain-rate effect for roller compacted concrete (RCC) removing the structural effect is a primary concern to achieve a reliable response prediction of RCC structures. In this study, a series of laboratory tests were carried out to study the dynamic compressive strength of RCC specimens with three different sizes subjected to various high-rate loadings. Based on the traditional decoupling method, the strength enhancements due to the inertial and end friction confinements were numerically evaluated and removed from the experimental DIF data. However, the mean values of material properties are generally employed in numerical simulation, which cannot reflect the high variability due to fast-successive constriction and mix proportion. This brings subjectivity and may result in incorrect evaluation of the structural effect and high variability in corrected DIF data. To solve this problem, the fractal dimension was introduced to quantify the fracture degree of impact-induced concrete fragments and describe the strain rate effect without significant variability. At last, a fragmentation-based approach was proposed to derive the true strain-rate effect of RCC and verified by comparison with the numerical and semi-theoretical methods.
Decommissioning of the Fukushima Daiichi Nuclear Power Station (F1NPS) in a proper manner requires assessment of the contamination levels and mechanisms for contamination in the concrete structures. Between January 2018 and March 2020, Japan’s Ministry of Education Ministry of Education, Culture, Sports, Science and Technology (MEXT) conducted a project called “The Analysis of Radionuclide Contamination Mechanisms of Concrete and the Estimation of Con-tamination Distribution at the Fukushima Daiichi Nuclear Power Station”. In this review, we outline the results of this study. The experimental results from the first project indicate that concrete carbonation, Ca leaching, and drying condi-tions affected the adsorption behaviors of Cs and Sr and therefore, their penetration depths. Additionally, the studies showed that α-nuclides precipitated on the surface of the samples because concrete causes a high pH. A reaction transport model was developed to assess further the adsorption characteristics of Cs and Sr in carbonated cement paste and on concrete aggregates. The model used real concrete characteristics from the materials used at F1NPS and historical boundary conditions at the site, including radionuclide concentrations and penetration profiles within the turbine pit wall. Capillary water suction resulting from dried concrete was evaluated by considering structural changes in cement hydrates using X-ray CR and 1H-NMR relaxometry.
Thermal cracking is a severe problem faced with massive concrete structures especially in early ages. In order to control the temperature distribution and lower cracking risk of a large-dimension reinforced concrete wall during construction, air pipe cooling was applied to experimental concrete walls with embedded corrugated pipes. Three levels of inlet airflow velocity (about 4, 8, and 12 m/s) were studied and compared with the wall without pipe cooling. Temperature distribution and strain development were obtained with embedded sensors. Besides, surface crack depth development with age was monitored with the ultrasonic testing analyzer. Moreover, stress distributions were derived through thermo-mechanical analysis to illustrate the strain and cracking trend of concrete walls under the specific cooling scenarios as in the experiment. According to the results, cooling efficiency was closely related to pipe intervals. In the pipeintensive region, air pipe cooling effectively reduced the peak temperature and concrete volumetric deformation. However, higher air velocity induced more considerable tensile stress both surrounding the pipe and on the outer surface in the region of larger pipe intervals. It indicates that the proper airflow velocity should be carefully investigated for the purpose of cracking control under a given pipe arrangement.