Regarding the spatial distribution of the corrosion process it is important to distinguish between macrocells and microcells, and to measure them quantitatively when experimentally clarifying the mechanism of corrosion of steel reinforcement embedded in concrete to decide an appropriate countermeasures. Therefore, this study is aimed to develop a method for measuring macrocell and microcell currents while dividing the rebar embedded in a concrete specimen into segments. The macrocell current is defined as the current flowing between rebar segments, whereas the microcell current flows within a single segment. First, it can be confirmed that the segment length does not influence the magnitude of the total corrosion current, which is the total of the macrocell anodic current and the microcell current. In addition, it can be confirmed that the magnitude of the macrocell current increases as the segment length decreases. From the experimental results obtained, the magnitude of the macrocell anodic current or the microcell current became constant when the length of individual segments was 15 mm or less. Also, the measured value of the macrocell current flowing between the neighboring segments could evaluate the macrocell current flowing between all elements. Additionally, the amount of steel loss derived from the corrosion current using electrical measurement methods became equal to the total corrosion weight loss.
The pore structure of concrete is highly complex and random. Fractal theory is used to describe the characteristics of its pore structure. Ds (surface fractal dimension) is commonly used to characterize the surface roughness of the pores. However, the existing Ds model is limited to two dimensions and there are unreasonable assumptions about the shape of the pores and the method of measuring pores is not applicable. Therefore, this paper proposes a three-dimensional (3-D) model for calculating the Ds. The new parameter Dss represents the chaotic degree of element morphology in the 3-D pore system. When Dss=0, the pore morphology tends to be the same. The larger Dss value is, the more divergent and diversified pore morphology is. The specimens in two states of standard curing and high temperature drying were prepared and the pore structure parameters of the specimens were calculated by combining CT scanning, Deep Learning and 3-D reconstruction. By comparing Dss and Dv (volume fractal dimension) and other parameters, it is found the 3-D pore system of concrete is more chaotic and becomes more complex under the effect of high temperature drying. It also confirms the reliability of the 3-D fractal dimension model proposed in this paper.
Compared with conventional two-part alkali-activated materials (AAMs), one-part AAMs have several advantages, especially the simple production using only water similar to ordinary Portland cement. However, one-part AAMs are much less researched, especially in terms of rheology. Understanding the rheological behavior of such binders is a crucial step for their production, placement, and application in the construction industry. This paper studies the rheological properties of one-part AAMs through evaluating the dynamic yield stress, static yield stress, and thixotropic index. The effects of solid activator content, besides the effect of two types of fly ash (containing different amounts of carbonaceous impurities) with varying contents, are explored. The possible mechanisms are also discussed.
The Fukushima Daiichi Nuclear Power Plant lost its core cooling function due to the massive tsunami generated by the 2011 off the Pacific coast of Tohoku Earthquake, which caused core meltdown, resulting in high temperature inside the containment vessel and exposing the RPV pedestal, a reinforced concrete structure, to an abnormally high temperature environment. In order to cool the molten core, water was poured into the containment vessel, and the concrete structure was gradually cooled in the process. Since it will take at least 40 years from the earthquake to remove the fuel from the core, the long-term integrity of the RPV pedestal is a major concern for the decommissioning of Fukushima Daiichi. In this study, the effects of high temperature exposure and subsequent wetting conditions on concrete properties were experimentally investigated. As a result, it was confirmed that the strength of concrete decreased by heating at high temperature, but recovered under subsequent wetting conditions.
Though the thermosetting FRP (FRTS) is commonly used, its cost is very high and has not been widely used in actual structures. On the other hand, thermoplastic FRP (FRTP) can be mass-produced and the cost can be reduced. However few studies have focused on the use of composites with concrete. This study clarifies the applicability of FRTP made of carbon fiber or glass fiber to reinforcing bars in concrete. In the case of the FRTP rod alone, the tensile strength and elastic modulus before and after exposure to water at room temperature and before and after immersion in a highly alkaline aqueous solution were evaluated. In the case of FRTP rods and concrete complexes, the variations in the pull-out test were evaluated. Based on these results, we summarize the applicability of FRTP rods to concrete reinforcements. Finally, the bending strength of concrete beams with embedded FRTP rods was evaluated experimentally and theoretically.
There has been a recent trend in Japan toward using precast segments to reduce the burden on construction sites and improve productivity. In large-scale precast segment structures such as bridges, the deformation of precast segments must be controlled during yard storage. This paper uses the multi-scale integrated chemo-hygric computational system (DuCOM-COM3) developed and extended by our group to numerically evaluate dimensional stability during yard storage of precast concrete with fly ash for a large-scale precast segmental bridge. To demonstrate the sensitivity to deformation behavior, simulations are conducted under three hypothetical cases of outdoor air temperature, outdoor air humidity, and temperature at the top surface of the segment, including the harsh temperature conditions expected in the Shikoku region. In the sensitivity analysis using the climatic conditions obtained from the field monitoring data as input, the reproducibility of the surface temperature and shrinkage of the concrete segment is improved by introducing the absorption of shortwave radiation by solar radiation on the segment top surface as an input parameter in the simulation.
Basic magnesium sulfate cement (BMSC) is a new type of gas-hardening magnesium cementitious material. In this study, basic magnesium carbonate was used as the raw material to obtain different active magnesium oxides by calcination at different temperatures to prepare BMSC with MgO:MgSO4:H2O = 5:1:20. The effect of different active magnesium oxides and different mineral admixtures and their content on the compressive strength of BMSC before and after soaking was analyzed, and the influence of MgO activity on the water resistance of BMSC was explored. The experimental results show that the activity of MgO has a significant impact on the water resistance of BMSC, and the water resistance of BMSC prepared from low-activity MgO is better, mainly because the hydration products of BMSC prepared by low-activity MgO are 5•1•7 phases. The MgSO4 in the 5•1•7 phase is insoluble in water; consequently, the free MgSO4 in BMSC is scarce; thus, the phase remains unchanged. After selecting low-activity MgO (T1000) and adding mineral admixtures to study its water resistance, it was found that the BMSC softening coefficient increases with an increase in the mineral admixture content. Both fly ash and silica fume were found to improve the water resistance of BMSC and increase its water resistance.
This paper is a state-of-the-art report on the performance assessment of cementitious and related materials as components of engineered barrier systems for radioactive waste management. In this paper, (1) the concept of safety functions is reviewed as the engineering background of discussion, (2) an overview of the postclosure performance assessment for Belgian low- and intermediate-level short-lived radioactive waste disposal is provided, and (3) a modeling methodology for engineered barrier systems is analyzed using the concept of “mandala for durability mechanics”. According to these works, authors present technical suggestions for technical stakeholders of Japanese low-level radioactive waste disposal.
The results of the first-ever in-situ monitoring of a large mortar specimen at a depth of 3515 m in the Nankai Trough are presented in this study targeted at creating a technology platform for in-situ monitoring and evaluation of cement-based materials at the seabed to realize deep-sea infrastructures. We successfully monitored in situ the development of strain and hydraulic pressure in the specimen. In addition, the short-term behavior of the specimen can be explained by hydraulic confinement and stress relaxation due to water infiltration. Some contraction strain remained in the specimen even after approximately an exposure to the deep sea condition for one year, causing microstructural damage. The pore entry volume was enhanced toward the center of the specimen, and a decrease in compressive strength and Young’s modulus were observed in the specimen after exposure due to the microstructural damage. Further improvement of the in-situ measurements is required to ensure the waterproofing and pressure resistance of the strain and pressure gauges.