The aging of concrete structures is a threat to public safety; therefore, maintenance and repair of these structures have been highly emphasized. However, regular inspections to detect concrete cracks that rely on operators lack objectivity and consume a lot of time. To overcome this limitation, high-resolution image processing and deep learning have been adopted. Nevertheless, cracks on structure surfaces are still challenging to detect owing to the variety of shapes of cracks and the dependence of recognition performance on image conditions. Herein, we propose a new concrete crack detection method that applies the semantic segmentation technique using 1196 concrete crack images and labeled images produced in this study. A new segmentation algorithm is developed using a hierarchical convolutional neural network to improve speed, and a multi-loss update method is proposed to improve accuracy. The performance of the proposed network is evaluated in terms of accuracy and speed. The results show that the proposed network produces a 2.165% increase in the intersection over union of crack, 65.90% decrease in the average inference time, and 99.90% decrease in the number of parameters compared with the best accuracy results using existing segmentation networks. It is expected that the application of this improved crack detection method will result in faster and more accurate crack detection and, consequently, improved safety, thereby making it suitable for application in structure safety inspections.

This study investigates the geomechanical, hydraulic and thermal characteristics of natural sandy sediments collected during the Ulleung Basin gas hydrate expedition 2, East Sea, offshore Korea. The studied sediment formation is considered as a potential target reservoir for natural gas production. The sediments contained silt, clay and sand fractions of 21%, 1.3% and 77.7%, respectively, as well as diatomaceous minerals with internal pores. The peak friction angle and critical state (or residual state) friction angle under drained conditions were ~26° and ~22°, respectively. There was minimal or no apparent cohesion intercept. Stress- and strain-dependent elastic moduli, such as tangential modulus and secant modulus, were identified. The sediment stiffness increased with increasing confining stress, but degraded with increasing strain regime. Variations in water permeability with water saturation were obtained by fitting experimental matric suction-water saturation data to the Maulem-van Genuchen model. A significant reduction in thermal conductivity (from ~1.4–1.6 to ~0.5–0.7 W·m⁻¹·K⁻¹) was observed when water saturation decreased from 100% to ~10%–20%. In addition, the electrical resistance increased quasi-linearly with decreasing water saturation. The geomechanical, hydraulic and thermal properties of the hydrate-free sediments reported herein can be used as the baseline when predicting properties and behavior of the sediments containing hydrates, and when the hydrates dissociate during gas production. The variations in thermal and hydraulic properties with changing water and gas saturation can be used to assess gas production rates from hydrate-bearing deposits. In addition, while depressurization of hydrate-bearing sediments inevitably causes deformation of sediments under drained conditions, the obtained strength and stiffness properties and stress-strain responses of the sedimentary formation under drained loading conditions can be effectively used to assess sediment responses to depressurization to ensure safe gas production operations in this potential target reservoir.

Abrasive waterjets are being increasingly used in civil engineering for rock and concrete cutting, particularly for the demolition or repair of old structures. The energy of an abrasive waterjet is primarily provided by the accelerated abrasive. The momentum transfer during mixing and acceleration determines the abrasive velocity, which affects the cutting performance. Meanwhile, the geometry of the focus at which mixing occurs influences the momentum transfer efficiency. In this study, the effects of the focus geometry on the optimum abrasive flow rate (AFR) and momentum transfer characteristics in hard rock cutting were investigated. Experiments were conducted using granite specimens to test the AFR under different focus geometry conditions such as diameter and length. The results show that the focus geometry significantly affects the maximum cutting depth and optimum AFR. The maximum cutting energy was analyzed based on the cutting efficiency of a single abrasive particle. In addition, the momentum transfer parameter (MTP) was evaluated from the empirical relationship between the maximum energy and the cutting depth for granitic rocks. Accordingly, a model for estimating the MTP based on the AFR was developed. It is expected that the results of this study can be employed for the optimization of waterjet rock cutting.

In this study, we investigated the characteristics of abrasive erosion considering the material properties of abrasives and targets. An abrasive particle erosion model considering energy transfer due to hardness differences was developed based on energy conservation using the correlation between volume removal and effective kinetic energy. To obtain the effective erosion kinetic energy of an abrasive, an acceleration model was derived for the abrasive particles, including terms describing the properties of the abrasive and fluid. The applicability of the suggested model was verified by comparing the brittle erosion results obtained using a previous theoretical approach to those of the present numerical analysis. The results obtained using the developed model exhibited good qualitative agreement with the brittle material erosion results. By evaluating acceleration and the erosion characteristics of an abrasive, the erosion performance could be predicted and optimized.