Showing papers on "Slab published in 2021"

Comparison of Fly Ash, Pva Fiber, Mgo and Shrinkage-Reducing Admixture on the Frost Resistance of Face Slab Concrete via Pore Structural and Fractal Analysis

Abstract: Face slab concrete suffers from serious frost damage in the cold regions in China. How to improve the frost resistance of face slab concrete in cold regions is one of the important issues in concre...

An analytical method with numerical results to be used in the design of optical slab waveguides for optical communication system applications

Abstract: This study develops an analytical method with numerical results for the design of optical slab waveguides for optical communication system applications. An optical slab waveguide structure made of silicon on silicon dioxide material is designed and analyzed. The effective index of the mode is studied against variations in the waveguide dimensions. Transmission and reflection coefficients are studied and compared to the wavelength and dimensions of the waveguide. Variations are sketched with the x-axis, in addition to the electric field intensity distribution and effective refractive index. Waveguide bending loss is also studied with waveguide thickness and length variations within three waveguide transmission windows of 850 nm, 1300 nm, and 1550 nm.

Influence of reactivity and dosage of MgO expansive agent on shrinkage and crack resistance of face slab concrete

Abstract: Enhancing cracking resistance of face slab concrete is essential for the structural integrity and normal operation of concrete-faced rockfill dams (CFRDs). In this study, the effects of MgO with three reactivities and three dosages on the shrinkage and crack resistance of face slab concrete were systematically investigated by slab test, restrained drying shrinkage test and temperature stress test machine (TSTM). The results indicate that: (1) 7 %–25% of the total drying shrinkage and all (or most of) the autogenous shrinkage can be compensated by adding 5 %–10% MgO. The reactive MgO (M60) starts to compensate the drying and autogenous shrinkage at 1 day and compensates more shrinkage at early age than the moderate reactive MgO (M150) and the weak reactive MgO (M300), while M300 begins to compensate the shrinkage at about 50 days and produces larger compensation than M60 and M150 at late age. (2) M60 performs better than M150 in improving the crack resistance of face slab concrete to constraint and thermal stress. The increase in dosage of M60 and M150 from 0 to 10% prolongs the initial cracking time of face slab concrete by 10.0–27.5 h, increases the cracking strain by 9.2 %–25.7%, enhances the cracking tensile stress (σ) by 4.7 %–18.8% and lowers the cracking temperature (Tc) by 2.6–7.4 °C. Conversely, M300 weakens the cracking resistance. (3) The reactive MgO with relatively high dosage is suggested to eliminate the early shrinkage and to improve the cracking resistance of face slab concrete.

Hydrodynamic effects of the flow on the slab of the stand in the presence of cavitation

Abstract: This (article) work is devoted to one, relatively unbalanced issue of the dynamics of hydraulic structures - the determination of hydrodynamic loads on the slabs of a high-pressure spillway in a cavitating flow in the presence of erosion-free energy absorbers. Working on the implementation of these studies, the authors simultaneously studied in laboratory pulsation loads on a real structure - elements of the downstream spillway devices, below – the Kafirnigan hydroelectric complex.

Slab temperature evolution over the lifetime of a subduction zone

Abstract: The thermal evolution of subducting slabs controls a range of subduction processes, yet we lack a robust understanding of how thermal structure develops over a subduction zone’s lifetime. We investigate the time-dependence of slab thermal structure using dynamically consistent, time evolving models. Pressure-temperature (P-T) conditions along the slab Moho and slab top exhibit substantial variability throughout the various phases of subduction: initiation, free sinking, mature subduction. This variability occurs in response to time-dependent subduction properties (e.g., fast vs. slow convergence) and thermal structure inherited from previous phases (e.g., due to upper plate aging). At a given depth, the slab cools rapidly during initiation, after which slower cooling occurs. In the case of the Moho, additional cooling occurs during the free sinking phase. We explore the implications of time-dependent thermal structure on exhumed rocks and slab dehydration. Modeled slab top P-T paths span much of the P-T space associated with exhumed rocks, suggesting a significant component of recorded variability may have dynamic origins. Coupling our P-T profiles with thermodynamic models of oceanic lithosphere, we show that dehydrating ultramafic rocks at the slab Moho provide the bulk of hydrous fluid at subarc depths during the earliest phases. Over subsequent phases, these rocks carry fluids into the deeper mantle, and it is mafic crust along the slab top that releases water at subarc depths. We conclude that varying subduction conditions, and non-steady-state thermal structure, challenge the utility of kinematically-driven models with constant subduction parameters, particularly for investigating thermal structure in the geological past.

The big mantle wedge and decratonic gold deposits

Abstract: The Circum-Pacific subduction zone is a famous gold metallogenic domain in the world, with two important gold metallogenic provinces, the North China Craton and Nevada, which are related to the destruction of the North China Craton and the Wyoming Craton, respectively Their ore-forming fluids were possibly derived from the stagnant slab in the mantle transition zone The oceanic lithospheric mantle usually contains serpentine layers up to thousands of meters thick During plate subduction, serpentine is dehydrated at depths of 300 km The overlying big mantle wedge is hydrated during the breakdown of these hydrous facies in the mantle transition zone The dehydration of the subducted slab in the big mantle wedge releases sulfur-rich fluid, which extracts gold and other chalcophile elements in the surrounding rocks, forming gold-rich fluid Because the cratonic geotherm is lower than the water-saturated solidus line of lherzolite, the fluid cannot trigger partial melting Instead, it induces metasomatism and forms pargasite and other water-bearing minerals when it migrates upward to depths of less than 100 km in the cratonic lithospheric mantle, resulting in a water- and gold-rich weak layer During the destruction of craton, the weak layer is destabilized, releasing gold-bearing fluids that accelerate the destruction The ore-forming fluids migrate along the shallow weak zone and are accumulated at shallow depths, and subsequently escape along deep faults during major tectonic events, leading to explosive gold mineralization The ore-forming fluids are rich in ferrous iron, which releases hydrogen at low pressure through iron hydrolysis Therefore, decratonic gold deposits are often reduced deposits

Dynamic slab segmentation due to brittle-ductile damage in the outer rise.

Abstract: Subduction is the major plate driving force, and the strength of the subducting plate controls many aspects of the thermochemical evolution of Earth. Each subducting plate experiences intense normal faulting1–9 during bending that accommodates the transition from horizontal to downwards motion at the outer rise at trenches. Here we investigate the consequences of this bending-induced plate damage using numerical subduction models in which both brittle and ductile deformation, including grain damage, are tracked and coupled self-consistently. Pervasive slab weakening and pronounced segmentation can occur at the outer-rise region owing to the strong feedback between brittle and ductile damage localization. This slab-damage phenomenon explains the subduction dichotomy of strong plates and weak slabs10, the development of large-offset normal faults6,7 near trenches, the occurrence of segmented seismic velocity anomalies11 and distinct interfaces imaged within subducted slabs12,13, and the appearance of deep, localized intraplate areas of reduced effective viscosity14 observed at trenches. Furthermore, brittle–viscously damaged slabs show a tendency for detachment at elevated mantle temperatures. Given Earth’s planetary cooling history15, this implies that intermittent subduction with frequent slab break-off episodes16 may have been characteristic for Earth until more recent times than previously suggested17. Numerical subduction models used to determine the consequences of bending-induced plate damage show that slab weakening and segmentation can occur at the outer-rise region of the subducting plate.

The Hindu Kush slab break-off as revealed by deep structure and crustal deformation.

Abstract: Break-off of part of the down-going plate during continental collision occurs due to tensile stresses built-up between the deep and shallow slab, for which buoyancy is increased because of continental-crust subduction. Break-off governs the subsequent orogenic evolution but real-time observations are rare as it happens over geologically short times. Here we present a finite-frequency tomography, based on jointly inverted local and remote earthquakes, for the Hindu Kush in Afghanistan, where slab break-off is ongoing. We interpret our results as crustal subduction on top of a northwards-subducting Indian lithospheric slab, whose penetration depth increases along-strike while thinning and steepening. This implies that break-off is propagating laterally and that the highest lithospheric stretching rates occur during the final pinching-off. In the Hindu Kush crust, earthquakes and geodetic data show a transition from focused to distributed deformation, which we relate to a variable degree of crust-mantle coupling presumably associated with break-off at depth. Here, the authors document active slab break-off and the crustal response during continental collision under the Hindu Kush, a rarely observed process since it happens over geologically short time spans.

Stagnant slab front within the mantle transition zone controls the formation of Cenozoic intracontinental high-Mg andesites in northeast Asia

Abstract: The geochemistry of Cenozoic intracontinental high-Mg andesites (HMAs) in northeast Asia, together with regional geophysical data, offers an opportunity to explore the genetic relationship between the formation of intracontinental HMAs and subduction of the Pacific plate. Compared with primary HMAs in arcs, Cenozoic intracontinental HMAs in northeast Asia have lower Mg# [100 × Mg/(Mg + Fe2+)] values (53–56) and CaO contents (5.8–6.6 wt%), higher alkali (Na2O + K2O) contents (5.15–6.45 wt%), and enriched Sr-Nd-Hf isotopic compositions (87Sr/86Sr = 0.7056–0.7059; εNd = −4.9 to −3.4; εHf = −4.7 to −2.6) as well as lower Pb isotope ratios (206Pb/204Pb = 16.76–19.19; 207Pb/204Pb = 15.42–15.45; 208Pb/204Pb = 36.71–37.11). These Cenozoic intracontinental HMAs are similar to Cenozoic potassic basalts in northeast China with respect to their Sr-Nd-Pb-Hf isotopic compositions but have higher SiO2 and Al2O3 contents and lower K2O, MgO, and light rare earth element contents. These features indicate that these Cenozoic intracontinental HMAs originated from the mantle, where recycled ancient sediments and water contributed to partial melting of peridotite. Combined with the presence of a large low-resistivity anomaly derived from the mantle transition zone (MTZ) near these intracontinental HMAs, and their occurrence above the stagnant slab front within the MTZ (at 600 km depth) in northeast Asia, we conclude that the stagnant slab front, with high contents of recycled ancient sediments and water, has controlled the formation of Cenozoic intracontinental HMAs in northeast Asia.

Lattice Discrete Particle Modeling of Size Effect in Slab Scratch Tests

Abstract: For a long time, geomechanicians have used scratch tests to characterize the compressive behavior and hardness of rocks. In recent years, this test has regained popularity in the field of mechanics, especially after a series of publications that highlighted the potential capability of the scratch test to determine the fracture properties of quasi-brittle materials. However, the complex failure mechanisms observed experimentally in scratch tests led to scientific debates and, in particular, raised the question of the size effect. This article intends to provide a better understanding of the problem by using numerical tools and fracture mechanics considerations. To narrow the investigation area, this study focuses on slab scratch tests of quasi-brittle materials and adopts two different numerical methods: (i) the lattice discrete particle model (LDPM) that includes constitutive laws for cohesive fracturing, frictional shearing, and nonlinear compressive behavior, and (ii) the meshless method based on Shepard function and partition of unity (MSPU) implementing linear elastic fracture mechanics (LEFM). The numerical results are further analyzed through Bažant’s size effect law (SEL) with an appropriate mixed-mode fracture criterion. Fracture properties are then calculated and compared to the results of typical notched three-point bending tests. The results show that mixed-mode fracture considerations are of paramount importance in analyzing the fracture process and size effect of scratch tests.