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Summary: Two-dimensional (2D) layered materials and their heterostructures have recently been recognized as promising building blocks for futuristic brain-like neuromorphic computing devices. They exhibit unique properties such as near-atomic thickness, dangling-bond-free surfaces, high mechanical robustness, and electrical/optical tunability. Such attributes unattainable with traditional electronic materials are particularly promising for high-performance artificial neurons and synapses, enabling energy-efficient operation, high integration density, and excellent scalability. In this review, diverse 2D materials explored for neuromorphic applications, including graphene, transition metal dichalcogenides, hexagonal boron nitride, and black phosphorous, are comprehensively overviewed. Their promise for neuromorphic applications are fully discussed in terms of material property suitability and device operation principles. Furthermore, up-to-date demonstrations of neuromorphic devices based on 2D materials or their heterostructures are presented. Lastly, the challenges associated with the successful implementation of 2D materials into large-scale devices and their material quality control will be outlined along with the future prospect of these emergent materials.

Understanding the wear behavior of Cu-based brake pads, which are used in high-speed railway trains and aircraft, is essential for improving their design and safety. Therefore, the wear mechanism of these pads has been studied extensively. However, most studies have focused on the changes in their composition and not the effects of their manufacturing conditions. In this study, we fabricated commercial Cu-based brake pads containing Fe, graphite, Al2O3, and SiO2 using spark plasma sintering under different conditions. The microstructures and mechanical properties of the pads were investigated. The pads were tribo-evaluated using the ball-on-disc test under various load conditions. Their worn surfaces were analyzed using X-ray diffraction analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and confocal microscopy in order to elucidate their wear mechanism. In addition, the dynamometer test was performed to confirm whether their wear behavior would be similar under actual conditions. Finally, a comparative analysis was performed using the ball-on-disc test. The results indicated that the brake pads with the same composition but fabricated under different sintering conditions exhibited different wear characteristics. We believe that this research is of great significance for understanding the wear mechanism of Cu-based brake pads and improving their design and hence their performance.

As opposed to asphalt emulsion waterproofing membrane, Synthetic Rubber Polymer Gel (SPRG) waterproofing materials are not heated prior to installation in concrete structures. SPRG materials are typically required to undergo a screw-mixing process to temporarily reduce the high viscosity and facilitate membrane installation on a concrete surface. However, there is no standard regulation on the duration of screw-mixing time during SPRG construction. Reported construction cases indicate that SPRG are left under constant screw mixing and are reused after hours or days of rest without being replaced with fresh products. When installed in this condition, SPRGs are subject to waterproofing performance degradation. In this study, SPRG viscosity properties are measured after five different screw-mixing procedures (no screw mixing, 10, 20, 30 and 60 min) and are set to rest in storage (2 h, 1, 2, 3, and 7 days). Specimens prepared under the respective screw mixing and storage times are evaluated for their changes in waterproofing properties through a series of ISO TS 16774 standard evaluation methods. A correlative comparison of the property evaluation results is presented to provide the changes to SPRG property and waterproofing performance. These results are then used to propose a general guideline for selecting optimal screw-mixing time with respect to maintaining adequate waterproofing performance and the viscosity recovery property of SPRG.

In the field of waterproofing concrete structures, the use of self-adhesive waterproofing sheets has become a popular technique for ensuring long-term waterproofing performance. One important characteristic of such sheet materials is maintaining their stable adhesion strength over an extended period. Adhesion testing serves as a crucial method for assessing the long-term adhesion performance. However, a standardized test method, including equipment and criteria, for directly measuring the adhesion strength of waterproofing sheets applied to onsite concrete walls has not yet been established. Therefore, reliance on laboratory evaluation results has been the only option thus far. In this study, a field-applicable adhesion measurement device was developed in an effort to provide a quality control method for self-adhesive waterproofing sheets and to demonstrate the validity of a standardized evaluation method utilizing this device. The developed adhesion measurement device was designed based on the principle of the peel-out test in compliance with the requirements outlined KS F 4934, where the test specimen backing plate can slide into a 1:1 ratio at the same distance as the rise of the tension jig when the tension jig is raised at a 45° angle. By utilizing this device, the adhesion strength values of self-adhesive waterproofing materials applied in the field were compared with those obtained using a laboratory universal testing machine (Salem, MA, USA) (UTM) on the test specimens. Comparative analysis yielded that the standard deviation of the results varied for the tested waterproofing materials, and the overall standard deviation for the UTM measurements was 0.17, while it was 0.18 for the portable field measurement equipment. The results of the comparison indicated that when limited to the scope of the KS measurement method specifications, the possibility for a wider scope of usage can be made possible with more data and studies.

In this study, the relationship between viscosity of synthetic polymer rubberized gel (SPRG) used for waterproofing and adhesive strength on varying degrees of wetted surfaces is examined. First, SPRG specimens of different ratios of viscosity modifiers were prepared (3% to 10%, interval of 1% per specimen type). These specimens were installed onto substrate surfaces of varying degrees of humidity ratio (0%, 10%, 20%, 30%, 50%, 70% and 100%). Upon following the standard installation procedure of the SPRG specimens, standard pull-off tests were conducted. Results of the test showed that a consistent relationship exists between the viscosity of synthetic polymer rubberized gel (SPRG) and its adhesive strength on different degrees of wetted surfaces. Linear regression analysis and subsequent calculation of the respective coefficient of determination leads to the result that viscosity is a relevant factor for achieving stable adhesion when it comes to the usage of SPRG waterproofing. The findings of this study could have significant implications for the development and application of adhesive gel waterproofing materials in various industries.

Facility walls with high relative humidity, such as bathrooms or kitchens installed with tiles by spot-bonding methods, become far more prone to defect or adhesion failure when using large or heavy tiles and insufficient application area of adhesive but is still continued to be practiced due to their low costs in the material. Most importantly, if this practice is to be continued, the changes in adhesion strength of the tiles based on different adhesive application areas of adhesives must be clarified such that the very least secure application can be achieved even by using spot-bonding methods. In this regard, an experiment was conducted in this study where tile-adhered specimens with different adhesive-applied area ratios (AR) of 60 ± 2%, 80 ± 2%, and 100% were prepared. Tile adhesion strength was subsequently measured, after sectioning the entire surface of the tile into 40 pieces. Experimental results showed that the adhesion strength above the standard criteria could be achieved for about 75% of the entire tile with AR 100% conditioning, followed by 30% of the entire tile with AR 80 ± 2% conditioning, and 20% of the entire tile for AR 60 ± 2% conditioning. Further analysis showed that with AR 80 ± 2% and AR 60 ± 2% conditions, the overall adhesion strength decreased by the range of about 59–67% compared to the AR 100% application conditions. The results of the study intended to provide an analytical basis of guidelines and risks with the potential usage of spot-bonding and should only be used if AR 100% application is planned.

Track impact factor (TIF) is a coefficient for estimating the increase of train dynamic load and is a major factor for the evaluation of dynamic stability of railway tracks. Many kinds of TIF in ballasted and ballast-less slab tracks consider only speed as a variable in different regions of the world. Because of this lack of clarification, nations such as Korea and those in the Middle East, and South East Asia are implementing overly simplified calculation methods for TIF, resulting in inconsistent maintenance of different railway track types. A comparative analysis of theoretically track support stiffness (TSS) and trackside measurement calculated TSS at wheel-rail contact point shows different values (depending on speed) between ballasted and ballast-less slab tracks. Based on this finding, this paper investigates a TIF formulation method that considers both dynamic wheel load and speed as variables, and a separate application of TIF formulation depending on track types is proposed. A clarified distinction of how these parameters (TSS and TIF) interact differently between ballasted and ballast-less slab track structures can be an important factor in case of track designing and maintenance aspects.

Leakage in below-grade concrete structures are repaired using various types of grout-injection materials, but the selection of optimal material types with a consideration of the environmental degradation factors are not conducted. Different degradation factors can act on the waterproofing membranes or grout-injection materials simultaneously. Especially in the early stages of installation, the injected grout materials in the cracks for leakage repair or for reforming damaged waterproofing layers are subject to complex forms of degradation factors. In such cases, physical property changes to the materials can reduce the waterproofing performance of the grout-injection materials. In this study, a technical evaluation regime is proposed for selecting the optimal repair material to be used in underground concrete structure leakage cracks. In this study, six environmental degradation factors (thermal stress, chemical corrosion, erosion due to ground water flow, hydrostatic pressure, substrate movement, and humidity on concrete surface) are identified. Corresponding evaluation methods based on the ISO TS 16774 test method series were used for each factor to assess the performance evaluation of four different types of grout-injection materials (acrylic resin, epoxy resin, polyurethane foam, and synthetic polymerized rubber gel). Based on the test results, a new comprehensive evaluation regime is presented that allows a quantitative performance comparison between each type of grout-injection material.

Synthetic Polymer Rubber Gels (SPRG) are used in Composite Modified Bituminous Waterproofing (CMBW) systems for waterproofing in concrete structures. A problem with the current design of SPRG is that when the emulsion breaks, the segregated oil layer can percolate through concrete cracks and cause oil leakage. This study proposes a new evaluation method which assesses the practical performance of SPRG products by testing their susceptibility to filler content settlement and oil leakage. Eight SPRG products used in Korea were used to demonstrate this evaluation method. Each product was installed in a concrete substrate specimen with an artificial crack made in the specimen’s bottom substrate. The specimens were observed for 28 days, and mass of leaked oil, total duration of leakage, average rate of sedimentation, and rheological threshold of initial leakage point were examined as part of the evaluation criteria.