In this study, a modular FRP DSCT(Modular Fiber-Reinforced Polymer Double-Skinned Composite Tubular) column was suggested, and bending performance was experimentally evaluated. Two specimens were designed, a monotype and a modular type, and quasi-static tests were conducted to investigate the bending strength of the modular type in order to compare its strength with the monotype. To calculate the number of bolts needed and the bolt diameter for the modular type, the maximum stress result acting on the FRP tube from nonlinear analysis was applied. The test was carried out by increasing the drift ratio, and as a result of the test, the moment, lateral load, displacement, and dissipation energy were calculated. The test results showed that there was no significant difference in maximum displacement between the monotype and the modular type, but the modular type showed a strength approximately 30% lower than that of the monotype. For the displacement ductility and the energy ductility, it resulted in values 1.4% and 3.5% higher than those of the monotype, respectively. Therefore, it is necessary to develop an enhanced connector or to consider a reduction factor in designing a modular DSCT column.

Lightweight-foamed soils are promising new geomaterials that can be used when the lateral earth pressure and settlement need to be reduced. Lightweight-foamed soil typically consists of dredged soils, cement, and air foam. Extra additives could be used to enhance the performance of lightweight-foamed soils, and waste fishing nets are materials that can be used for tensile reinforcement of geomaterials. In this study, a waste fishing net was used in lightweight-foamed soil, and several laboratory tests were conducted to optimize the composition of lightweight-foamed soils in terms of water content, cement ratio, air foam ratio, and net type. A dredged soil in-situ was sampled to use and a waste fishing net was retreated for laboratory experiments. The waste fishing net cut into pieces and some were unthreaded. A form generator was used to produce air foam. The comprehensive evaluation of lightweight-foamed soil reinforced with waste fishing net was performed in terms of the sample preparation time, rheology, unit weight, and strength associated with lightweight-foamed soils. The results showed that lightweight-foamed soils reinforced with waste fishing net provided a peak strength similar with unreinforced one, but having higher critical strength. Finally, the normalized factor (NF) was used to develop a design guideline for lightweight-foamed soils.

Lightweight-foamed soils are mixed soils with foam and cement to enhance the solidity and lightness of soils. Marine wastes, especially waste fishing nets, can be additives to reinforce the engineering properties of lightweight-foamed soils. In this paper, lightweight-foamed soils reinforced with waste fishing nets were investigated. Dredged soil and waste fishing nets were collected and pre-processed for testing. For optimization, the water content, foam ratio, cement ratio, net ratio, net conditions, and curing days were evaluated with respect to workability, unit weight, and strength. The variables were narrowed down based on the performance criteria. The results found that a water content of around 100%, cement ratio of 20%, foam ratio of 5%, and net ratio of 4% with shredded nets provide the best engineering performance of lightweight-foamed soils. The use of nets presented a superior increase in critical strength rather than an obvious increase in peak strength. A normalized factor was used to predict the required strength of lightweight-foamed soils. Finally, this study proposes field implementation methods in terms of the initial conditions of soils and optimal conditions of soils, resulting in the depletion of waste fishing nets.

A large-scale ultrasonic washing instrument was developed, and its performance was evaluated using an ultrasonic instrument and mechanical washing for desalination in sea sand. Preliminary experiments with a portable ultrasonic device confirmed that increasing the ultrasonic power intensity and application time increased effectiveness of salt washing in sea sand. On the basis of these findings, a large-scale washing instrument with a maximum output of 1500 W, an ultrasonic frequency of 28 kHz, and a 25 L capacity washing tank was developed. Parametric experiments via the developed system were performed to investigate the effects of the sea-sand-to-washing water ratio, ultrasonic intensity, and washing time on washing effectiveness. The greatest reduction in salinity was observed when both ultrasonic treatment and mixing of sea sand with water were applied. The optimal washing conditions were an ultrasonic intensity of 1200 W, a washing water-to-sea sand ratio of 2:1, and a minimum washing time of 3 min. The results confirm that the developed equipment is effective for removing salt from sea sand and that further optimization of the ultrasonic intensity and mixing techniques can increase the washing efficiency.

Ultrasonic desalination has emerged as a promising physical technique for chloride removal from sea sand; however, the influence of water temperature on its performance remains insufficiently elucidated under practical operating conditions, as the water temperature during the process varies seasonally and is often not actively controlled. In this study, we examined the effects of water temperature on ultrasonic desalination performance using a laboratory-scale system under low (10–15 °C), ambient (~ 25 °C), and elevated (30–35 °C) temperatures. The residual estimated chloride content was evaluated against the (≤ 0.04%). The results indicated that under the tested laboratory conditions, ultrasonic power was the primary factor influencing estimated chloride reduction, while water temperature modulated the mass-transfer stability. The residual estimated chloride content generally decreased with increasing ultrasonic power across the tested temperatures. Ambient temperature exhibited the most stable and monotonic reduction, achieving residual estimated chloride levels as low as 0.014–0.017%. In contrast, low-temperature conditions showed non-monotonic behavior and greater variability, requiring extended treatment time to satisfy the same criterion. Elevated-temperature conditions achieved desalination performance comparable to that of ambient conditions only at the highest power level tested, indicating limited benefit from additional heating. These preliminary findings suggest that ultrasonic desalination can effectively lower estimated chloride concentrations under representative seasonal temperatures without active temperature regulation, provided operating times are suitably adjusted.