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In recent years, overlapping hoop, consisting of a U-shaped stirrup and a crosstie, has gained popularity because it could enhance construction efficiency of concrete composite beams. The U-shaped stirrups (U-shaped stirrup with 135°/180° hooks, namely A and S) and crossties (crosstie with 135° (90°)/90° hooks, namely J and C) are typical in the application. This study focused on four corresponding combinations of overlapping hoops, i.e., A+J, S+C, A+C and S+J. Ten full-scale concrete composite/monolithic beams were tested under cyclic loading, with parameters of the above four overlapping hoops and traditional enclosed hoop. The results revealed that all overlapping hoops had no fracture or early detachment prior to the peak load. The hysteresis curves of the four overlapping hoop composite beams were similar in full shape, demonstrating a good energy dissipation capacity. The four overlapping hoop composite beams had differences in flexural strength within 8% and 1% with the enclosed hoop composite beam and monolithic beam, respectively. The displacement ductility of the four overlapping hoop composite beams ranged from 6.1 to 6.8, while that of the enclosed hoop composite beam and monolithic beam was 7.3 and 6.3, respectively. Ten specimens exhibited similar stiffness degradation laws, and their three-linear restoring force models were proposed. Overall, the concrete composite beams reinforced with the four different overlapping hoops demonstrated similar seismic performance to that of the two enclosed hoop concrete beams. Considering the integrity and workability, the overlapping hoop A+C, consisting of a U-shaped stirrup (two 135° hooks) and a crosstie (two 90° hooks), would be the preferred combination. • Cycle test results on ten full-scale concrete composite/monolithic beams reinforced with overlapping hoops are presented. • The property of four types of overlapping hoops is studied and the preferred combination of overlapping hoops is proposed. • The effect of hooks on the integrity property of hoops is discussed. • The difference in seismic performance between concrete composite/monolithic beams with overlapping hoops is investigated. [ABSTRACT FROM AUTHOR]

Natural gas hydrates are crystalline solids found in sediments beneath the seabed and permafrost, which significantly affect the mechanical behavior of sediments. The effect of hydrates often varies for different sediments, which has been captured in some laboratory studies. But the particle-scale information involved is obscure and the underlying mechanisms often rely on assumptions. In this study, discrete element method (DEM) was used to improve understanding of the hydrate effect on the mechanical behavior of different sands. Triaxial compression tests were carried out using DEM on the digital hydrate-bearing sands (HBS) with different grain sizes. Simulation results shows that the large-grain samples often yield earlier and tend to strain-hardening behavior, while hydrate occurrence induces strain-softening behavior. In terms of stress-strain response and porosity changes, the hydrate strengthening on the small-grain samples is much greater than the large-grain samples. Particle-scale observations indicate that particle rotation usually occurs in regions with small particle displacement and is more intense on shear slip surfaces. Hydrate bonds are mainly subjected to tensile failure, and large grain size may hinder bond shear failure. Hydrates restrict the development of radial fabrics and enhance the stress-induced anisotropy, rendering a more stable framework for sediments. • Large-grain sediments often yield earlier and tend to strain-hardening behavior. • Hydrate strengthens small-grain sediments much greater than large-grain sediments. • Significant particle rotation occurs on the shear slip surface. • Hydrate enhances the stress-induced anisotropy in sediments. [ABSTRACT FROM AUTHOR]

Natural gas hydrates are commonly found in onshore permafrost and seabed regions. Hydrates often exhibit different pore habits in different regions, which may affect the mechanical behavior of hydrate-bearing sands (HBSs). In this study, pore-filling, load-bearing and cementing HBSs were prepared using discrete element method (DEM). The numerical biaxial compression tests show that the digital cementing HBS specimen well capture the stress-strain response of the gas-saturated HBS specimen in experiments under σ c = 3 MPa, which justify the DEM model in this study. The pore-filling hydrates contribute little to the stress-strain response of sediments, while the load-bearing and cementing hydrates significantly enhance the failure strength and stiffness, and also induce strain-softening behavior and higher volume dilation. Hydrate cementation hinders the lateral fabric development in HBS, and renders orderly particle displacement and abundant strong force chains. The yielding of HBS is dominated by the buckling of strong force chains. Particle sliding, dislocation and rotation are responsible for the formation and development of shear bands, which governs the volume dilation and non-uniform deformation of HBS. Comprehending the geomechanical behavior of HBS under different hydrate habits facilitates the optimization of engineering solutions for hydrate exploitation. • Pore-filling, load-bearing and cementing hydrate-bearing sands (HBS) were prepared using DEM. • Digital cementing HBS specimen well captured the stress-strain response of gas-saturated HBS. • Pore-filling hydrate contributes little to the stress-strain response of sediments. • Load-bearing and cementing hydrates induce strain-softening behavior and volume dilation. • The buckling of strong force chains dominates the yielding of HBS. [ABSTRACT FROM AUTHOR]

• Sediment skeleton and gas permeability during hydrate decomposition were monitored. • The number and size of pores dominate the permeability of the sediment. • Hydrate morphology from load-bearing to cementing has more influence on permeability. • A relative permeability model related to hydrate saturation is proposed. Research data and visual evidence to understand the interaction between hydrate dissociation and micro-seepage in sediment are currently lacking. In this study, X-ray computed tomography (CT) is used to reveal the dynamic interaction process between sediment pore-scale structural changes and permeability during hydrate dissociation. After Xe hydrate formation in spherical quartz and then decomposed the hydrate by depressurization to obtain the real-time permeability and 3D structure of hydrate-bearing sediments (HBSs) with CT scans. The results show that the heterogeneity of hydrate spatial distribution does not change the power function relationship between permeability and hydrate saturation. The ball-and-stick model shows that the number and size of flow channels dominate the flow resistance of the fluid at high hydrate saturation, while the size of flow channels dominates it at low hydrate saturation. The saturation cut-off point is between 16.35% and 25.37%. The CT scan images show that the hydrate morphology changes from patchy to load-bearing, and then to cementing. The transition process from load-bearing to cementing has the greatest impact on sediment permeability. Further PNM simulations show that the size and number of flow channels and capillary forces are the main factors affecting the ability of gas–water two-phase flow in the sediment. Most importantly, a relative permeability model related to hydrate saturation parameters is proposed based on image processing technology. The good agreement between the model and available laboratory/field measurements proves its practicability. The method of proposing this model provides a new idea to establish new relative permeability models. [ABSTRACT FROM AUTHOR]