Journal of Advanced Concrete Technology最新文献

The crucial part of nuclear waste storage is the construction of sealing structures made of reliable, safe and well–understood materials. We present an extended analysis of long-term multi–sensory monitoring and non–destructive testing (NDT) inspection of two laboratory specimens aiming at potential materials for sealing structures for nuclear waste repositories. Specimens with a volume of 340 litres made from newly developed alkali–activated materials (AAM) and established salt concrete (SC) were analysed using embedded acoustic emission and wireless radio-frequency identification (RFID) sensors, ultrasonic echo imaging, active thermography, and X–ray computed tomography. The monitoring analysis showed lower heat of reaction and 50% less acoustic emission events in AAM compared to SC. However, due to the surface effects of the AAM material, the number of acoustic emission events increased significantly after approximately two months of monitoring. Subsequently performed NDT inspections reliably located embedded sensors and confirmed the absence of major cracks or impurities. The presented laboratory results show the feasibility and potential of comprehensive NDT monitoring and inspection to characterise cementitious and alternative materials as well as the need for multi–parameter long–term monitoring. Thus, our study demonstrates that tailored NDT investigations will help to develop safe sealing structures for nuclear waste repositories.

The passivation of steel bars in concrete is determined by the characteristics of the concrete pore solution. However, ion evolution in the pore solution of alkali-activated slag/limestone powder (AAS/L) systems is not yet well known. Therefore, in this study, ion evolution in real and simulated pore solutions of alkali-activated slag/limestone powder mortars is investigated using high-pressure and solid-liquid extraction methods. The study focuses on investigating the passivation behaviour of steel bars when exposed to the simulated pore solution. The results demonstrate that the contents of Ca, Na, Al, Si, and S₂O₃^2- first increase and then decrease with time, whereas the contents of K and SO₄^2- consistently increase. Among the simulated pore solutions, the one with a solid-liquid ratio of 1 : 1 and an age of 14 days closely resembles the actual pore solution. In addition, the presence of a high S content resulting from the dissolution of S in the slag inhibits the passivation process of the steel bars in the simulated pore solution of AAS. However, the passivation film of the steel bars formed in the simulated AASL pore solution is strengthened by the addition of the limestone powder.

Concrete slurry waste is generated at concrete plants and generally re-used in new batches of concretes. Due to the presence of hydrated cement paste it has the potential to be carbonated prior to re-use in order not only to store CO₂, but also to enhance its reactivity in blends with cement. In this study, a concrete slurry waste obtained at a ready-mix concrete plant was investigated. For accelerated carbonation, a wet process was used at laboratory scale. The carbonated product was dried afterwards, characterized and used as supplementary cementitious material. When carbonated, the hydrate phases of the concrete slurry waste decomposed to calcite, gypsum and a silica-alumina gel. When blended with Portland cement (30% replacement by mass) early hydration kinetics was accelerated by the carbonated concrete slurry waste. The pozzolanic reaction of the silica-alumina gel consumed a significant part of the portlandite and showed a slightly positive contribution to compressive strength compared to inert quartz powder and to the uncarbonated concrete slurry. This offers the use of carbonated concrete slurry waste to store CO₂ in concrete with the additional benefit of reducing the cement content.

The focus on sustainable construction materials has prompted research into alternatives to reduce the carbon dioxide emissions associated with the production of Portland cement. Several studies have attempted to minimise the use of Portland cement in structural concrete mixtures. However, traditional formulations and prediction models are not applicable to low-carbon concrete. Therefore, this study aimed to evaluate low carbon concrete mortar matrices, focusing on cement replacement by supplementary cementitious materials (SCMs), such as fly ash, natural pozzolans and electric arc furnace slag, increasing the compactness of the mixtures, and limiting the water-binder ratio. Different binder powder contents were studied (350 and 400 kg/m³) in two types of concrete for prefabrication: dry consistency and plastic consistency. The results showed that dry consistency mixtures with a higher fly ash content allow higher compactness. Higher compactness promotes an overall increase in Young's modulus, up to 16%, for all eco-mixtures. The analysis enabled the estimation of the Feret coefficients for each combination of cement with SCMs and their correlation with a power function, dependent on the water-cement ratio. This allows the future estimation of the strength for binder pastes of mixtures incorporating different percentages of these additions.

The structural performance of reinforced concrete (RC) members affected by alkali–silica reaction (ASR) is difficult to predict because of the multi-scale phenomena. Recent structural tests reveal that the performance of RC members also depends on ASR-induced crack patterns, including localized cracks and dispersed microcracks. Additionally, microscopic factors, such as crack-filling by gel and presence of microcracks, are relevant. To explore this in detail, a computational system for finite element analysis of ASR-damaged RC members was developed. This study numerically investigated the structural behavior of ASR-affected RC members based on localized/dispersed crack patterns and microscopic factors. The applicability of the developed computational system was verified by comparing the analysis results with experimental data. The analysis results showed that ASR-damaged RC members with dispersed microcracks exhibited highly ductile behavior, while those with localized cracks failed in shear. This is because the dispersed crack pattern prevents the shear crack propagation and enhances the mechanical contribution of gel filling cracks, while the localized ASR cracks facilitate critical crack propagation, leading to failure, and minimize the gel-filling effect. Through the analytical investigations, it was found that the localized ASR cracks can result in significant loss of structural performance; thus, this study recommends the assessment of structural capacity of RC members in the case where the localized cracks were observed.

This study aims to discuss the effects of calcium-magnesia compound expansive agent on the mechanical strength, shrinkage, and creep of concrete. On this basis, the expansive mechanism is investigated by using XRD, TG measurement, and MIP with percolation and backbone fractal dimension analysis. It is shown that replacing fly ash with calcium-magnesia compound expansive agent has little effect on the axial compression strength, but significantly reduces the elastic modulus. Moreover, increasing the amount of expansive agent from 0% to 12% leads to a reduction in autogenous shrinkage values by 9.1% to 14.8% at 180 days while causing an increase in creep by 24.5% to 37.8%. Concurrently, the porosity of the concrete increases with the addition of the calcium-magnesia compound expansive agent, and the proportion of gel and medium capillary pores in the total porosity also increases. However, fractal dimension analysis shows that it also decreases the complexity of overall connectivity of interconnected pores. Furthermore, the results obtained from TG also indicate that the calcium-magnesia compound expansive agent is beneficial for the formation of gel hydration products.

Cementitious materials generally have large carbon footprints because of the high CO₂ emitted during Portland cement production. This is because limestone is used as an essential CaO resource, and its decomposition by calcination emits CO₂. From this perspective, the concrete in urban buildings can be considered an urban mine of CaO resources. In this study, we propose obtaining a solidified product by crushing all the waste concrete, carbonating it, pressurizing it with a calcium bicarbonate solution, and drying it. The experimental results show that the bicarbonate solution, high-temperature conditions, and extended loading period produce a higher strength. In addition, neck growth at the contact surfaces of the carbonated concrete fines was confirmed using scanning electron microscopy. Consequently, the proposed method indicates that the hardening mechanism is the cold sintering of calcium carbonate on the surface of fine-carbonated concrete particles. This method allows the developed blocks to be used semi-permanently with relatively low energy consumption through repeated crushing and re-pressurization.

It is effective for the maintenance of existing structures to understand the current status, such as the residual stress of the materials used, and a portable X-ray diffractometer is used as a technique for measuring steel stress. Although measurement methods and pre-treatment for steel bars with low strength have been revealed, the influence of the strength on the X-ray stress measurement and the appropriate measurement method has not been clarified. In this study, residual stress measurements using the X-ray diffraction are performed on steel bars, the differences in the results for various steels with different strengths are discussed, and measurement conditions for accurate measurement are reported. In addition, the cause of the tendency of the X-ray stress measurement to be smaller than the actual increment value under high stress is investigated. The experimental results show that the appropriate measurement conditions vary depending on the rebar strength, and that the X-ray stress measurement under high stress also varies. The metallurgical structure of the steel was shown to be related to the cause of this phenomenon through microstructural observation and analysis of the deformation behavior.

One carbon neutralization measure applied in the concrete sector is the use of artificial carbonate in concrete for immobilization. This CO₂ reduction technology corresponds to the CO₂ emitted during concrete production. When considering the marketability of these technologies, especially for newly developed products in the carbon market, it is essential to quantify the amount of CO₂ fixed as inorganic carbonate. Additionally, as a representative test specimen for concrete containing aggregate, a φ100 × 200 mm cylinder specimen is conventionally used for physical property evaluation. To evaluate the amount of CO₂ fixed in one batch of concrete, a mass far from that of the conventional chemical analysis sample may need to be analyzed. Therefore, in this study, we investigated a pulverization process for concrete analytical materials. We also propose a new analytical apparatus that can be used to measure large cylinder specimens. Experimental results showed that the newly developed analyzer, equipped with a mass balance and CO₂ and H₂O gas analyzer for large cylinders, exhibited excellent analytical variability and measurement speed performance. It was also inferred that the homogenization process is necessary to grind the entire cylindrical concrete specimen into a fine powder and homogenize it to improve the representativeness of the concrete.

Moisture transport is the key phenomenon indicating the deterioration of the durability and structural performance of concrete structures. Although various studies have attempted to evaluate moisture transport in concrete, an anomalous behavior, which does not follow the root-t law compared to other porous material, was not explicitly taken into account. To quantitatively evaluate anomalous moisture transport, this study developed a couple of numerical methods between the truss-network model (TNM) and the rigid-body-spring model (RBSM) for this purpose. The colloidal behavior of calcium-silicate-hydrate (C-S-H), which is the major phase of cement-based material, was introduced to consider the anomalous behavior and mechanical response regarding the microstructural change of cement paste as well as cracks that significantly accelerate the moisture transport in concrete. The numerical results indicated that both microstructural change of cement paste and rapid absorption through cracks cause anomalous behavior. In addition, the numerical results suggest that volumetric change of cement paste should rely on water content related to the colloidal behavior of C-S-H in order to reproduce the realistic expansion and the closure of cracks during a rewetting process that affects structural performance and durability of concrete.