Numerical study of the biaxial compressive strength of high strength concrete based on a yield design micromechanical approach     

Ahmed Naija  Ghazi Hassen  Oualid Limam  Karim Miled 《国际地质力学数值与分析法杂志》2020,44(6):772-781

This paper presents a numerical study of high strength concrete microstructure effects on its uniaxial and biaxial compressive strengths. Concrete is first represented as a set of angular aggregates interacting within a cement paste matrix. Then, a yield design kinematic approach is conducted at the mesoscopic scale in order to determine the concrete compressive strength for a given loading path. The proposed model, having a low computational cost, is able to capture the main microstructure effects already observed in literature on concrete uniaxial compressive strength, in particular, the aggregates volume fraction and maximal size effects. Finally, the proposed model also predicts the biaxial failure envelope of high strength concrete and confirms some experimental trends observed in literature.  相似文献   

Impact of Saharan dust on PM10 concentration in the Mediterranean Tunisian coasts     

Karim Bouchlaghem  Blaise Nsom  Noureddine Latrache  Houda Haj Kacem 《Atmospheric Research》2009,92(4):531-539

This paper presents samples of Saharan dust outbreak affecting the Mediterranean Tunisian coasts and its impact on PM10 (Particles with an aerodynamic diameter below 10 µm) surface concentrations measured at seven monitoring stations during summer 2006. During the events, the daily PM10 levels at all stations exceeded EU and Tunisian air quality standard limits which are equal to 50 µg/m³. The maximum values ranged from 200 µg/m³ to 300 µg/m³ depending on the monitoring station. The impact is even more dramatic on PM10 hourly concentrations leading to maximum hourly peaks ranging from 400 µg/m³ to 850 µg/m³ again depending on the monitoring station (industrial or residential, traffic and commercial). Comparison between backward air masses trajectories reaching Tunisian coasts and satellite imageries vis-à-vis the PM10 hourly concentrations measured at the monitoring stations during 2006 evidenced the influence of the Saharan dust outbreaks on surface concentrations. The origin of the air masses is found to be from South-West direction under the influence of air masses from the Algerian Saharan desert.  相似文献   

Organic matter heterogeneities in 2.72 Ga stromatolites: Alteration versus preservation by sulfur incorporation     

Kevin Lepot  Karim Benzerara  Marine Cotte  Gordon E. Brown Jr.  Pascal Philippot 《Geochimica et cosmochimica acta》2009,73(21):6579-47

The stromatolites of the weakly metamorphosed 2.72 Ga Tumbiana Formation present abundant organic globules that resemble in size, shape and distribution the microorganisms observed in modern stromatolites. In order to evaluate the significance of these cell-like organic materials, we characterized organic matter in-situ down to the nanoscale using a combination of Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), Raman microspectroscopy, scanning transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM).These analyses revealed the occurrence of two distinct types of organic matter forming μm-scale textural and chemical heterogeneities distributed in distinct mineralogical laminae of the stromatolites. Type A organic matter, which is by far the most abundant, consists of sulfur-poor organic matter that is located in mud-type laminae at grain boundaries, mostly in association with silicate minerals. In contrast, Type B organic matter is rare and preserved as inclusions in the core of calcite grains forming laminates. It occurs as micrometer-sized cell-like globules containing variable amounts of organic sulfur likely in the form of thiophenes.Different scenarios may account for these compositional heterogeneities in the kerogen. Based on textural and compositional analogies with modern stromatolites, it is argued that Type B sulfur-rich globules may represent microbial cells protected by mineral encapsulation and selectively preserved through polymerization by early diagenetic sulfurization. In modern sediments, this reaction is fuelled by bacterial sulfate reduction (BSR). This metabolism has been widely considered as a major driver in modern stromatolites calcification and could thus have played an important role in the formation of the Tumbiana Formation stromatolites. In contrast, Type A sulfur-poor organic matter corresponds to either fossil extracellular polymer substances (EPS) or recondensed kerogen. This pool was likely not sulfurized due to either local and/or timely variations in the concentrations of H₂S or adverse pyritization driven by the availability of iron. Our observations thus show the need to use spatially-resolved techniques to complement organic geochemistry analyses and provide a detailed analysis of the organic carbon pools composing Archean stromatolites.  相似文献   

Multiscale characterization of pyritized plant tissues in blueschist facies metamorphic rocks   总被引：2，自引：0，他引：2  

Sylvain Bernard  Karim Benzerara  Olivier Beyssac 《Geochimica et cosmochimica acta》2010,74(17):5054-5068

Pyritized plant tissues with well-preserved morphology were studied in rocks from Vanoise (western Alps, France) that experienced high-pressure, low-temperature metamorphic conditions in the blueschist facies during the Alpine orogeny. Organic and inorganic phases composing these fossils were characterized down to the nanometer scale by Raman microspectroscopy, scanning transmission X-ray microscopy and transmission electron microscopy. The graphitic but disordered organic matter composing these fossils is chemically and structurally homogeneous and mostly contains aromatic functional groups. Its original chemistry remains undefined likely because it was significantly transformed by diagenetic processes and/or thermal degradation during metamorphism. Various mineral phases are closely associated with this organic matter, including sulphides such as pyrite and pyrrhotite, carbonates such as ankerite and calcite, and iron oxides. A tentative time sequence of formation of these diverse mineral phases relative to organic matter decay is proposed. The absence of traces of organic matter sulphurization, the pervasive pyritization of the vascular tissues and the presence of ankerite suggest that the depositional/diagenetic environment of these metasediments was likely rich in reactive iron. Fe-sulphides and ankerite likely precipitated early and might have promoted the preservation of the fossilized biological soft tissues by providing mechanical resistance to compaction during diagenesis and subsequent metamorphism. In contrast, iron oxides which form rims of 100-nm in thickness at the interface between organic matter and Fe-sulphides may result from metamorphic processes. This study illustrates that it may be possible in some instances to deconvolve metamorphic from diagenetic imprints and opens new avenues to better constrain processes that may allow the preservation of organic fossils during diagenesis and metamorphism.  相似文献   

Modelling hydrological connectivity of tropical floodplain wetlands via a combined natural and artificial stream network          下载免费PDF全文

Fazlul Karim  Anne Kinsey‐Henderson  Jim Wallace  Paul Godfrey  Angela H. Arthington  Richard G. Pearson 《水文研究》2014,28(23):5696-5710

The ecological condition and biodiversity values of floodplain wetlands are highly dependent on the hydrological connectivity of wetlands to adjacent rivers. This paper describes a method for quantifying connectivity between floodplain wetlands and the main rivers in a wet tropical catchment of northern Australia. We used a one‐dimensional hydrodynamic model to simulate time‐varying water depths across the stream network (i.e. rivers, streams and man‐made drains). The timing and duration of connectivity of seven wetlands (four natural and three artificial) with the two main rivers in the catchment were then calculated for different hydrological conditions. Location and areal extent of the wetlands and the stream network were identified using high‐resolution laser altimetry, and these data formed key inputs to the hydrodynamic model. The model was calibrated using measured water depths and discharges across the floodplain. An algorithm was developed to identify contiguous water bodies at daily time steps, and this gave the temporal history of connection and disconnection between wetlands and the rivers. Simulation results show that connectivity of individual wetlands to both rivers varies from 26 to 365 days during an average hydrological condition. Location, especially proximity to a main river, and wetland type (natural stream or artificial drain) were identified as key factors influencing these levels of connectivity. Some natural wetlands maintain connection with the river for most or all of the year, whereas the connectivity of some artificial wetlands varies from 26 to 36 days according to their patterns of network connection to adjacent rivers – a result that has important implications for the accessibility of these types of wetland to aquatic biota. Using readily available river gauge data, we also show how connectivity modelling can be used to identify periods when connectivity has fallen below critical thresholds for fish movement. These connectivity patterns within the floodplain network are central to the setting of river flows that will meet environmental requirements for biota that use floodplain wetlands during their life history. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

A new pan-sharpening method using multiobjective particle swarm optimization and the shiftable contourlet transform     

Jamal Saeedi  Karim Faez   《ISPRS Journal of Photogrammetry and Remote Sensing》2011,66(3):365-381

In this paper, a novel approach based on multiobjective particle swarm optimization (MOPSO) is presented for panchromatic (Pan) sharpening of a multispectral (MS) image. This new method could transfer spatial details of the pan image into a high-resolution version of the MS image, while color information from the low-resolution MS image is well preserved. The pan and MS images are locally different because of different resolutions, and therefore we cannot directly combine them in the spatial domain. For this reason, we generate two initial results, which are more appropriate for a weighted combination. First, the pan and the MS images are histogram matched. Then we use the shiftable contourlet transform (SCT) to decompose the histogram-matched pan and MS images. The SCT is a new shiftable and modified version of the contourlet transform. In this step, an algorithm based on the SCT is used to generate two initial results of the high-resolution MS images. Our objective is to produce two modified high-resolution MS images, in which one has high spatial similarity to the pan image and the other one has high radiometric quality in each band. Therefore, we have used two different fusion rules to integrate the high-frequency contourlet coefficients of the pan and MS images to generate two initial results of high-resolution MS image or the pan-sharpened (PS) image. Finally, we can find the optimal PS image by applying the MOPSO algorithm and using the two initial PS results. Specifically, the PS image is obtained via a weighted combination of the two initial results, in which the weights are locally estimated via a multiobjective particle swarm optimization algorithm to generate a PS image with high spatial and radiometric qualities. Based on experimental results obtained, the produced pan-sharpened image also has good spectral quality. The efficiency of the proposed method is tested by performing pan-sharpening of high-resolution (Quickbird and Wordview2) and medium-resolution (Landsat-7 ETM +) datasets. Extensive comparisons with the state-of-the-art pan-sharpening algorithms indicate that our new method provides improved subjective and objective results.  相似文献   

Incorporating climate change into water resources planning for the town of Polokwane,South Africa   总被引：2，自引：0，他引：2  

James Cullis  Ken Strzepek  Mark Tadross  Karim Sami  Beyers Havenga  Burgert Gildenhuys  Joel Smith 《Climatic change》2011,108(3):437-456

Incorporating climate change into water resource planning is crucial to ensuring sustainable growth and development. The aim of this study was to investigate how the proposed steps to incorporating climate change into project planning developed by USAID could be applied in practice using the town of Polokwane in northeastern South Africa as a case study. Two Regional Circulation Models (RCMs) and statistical downscaling from a range of General Circulation Models (GCMs) were used to produce a set of monthly climate scenarios for 2025 and 2050. These scenarios were used to estimate the impact on surface water runoff in the Olifants River catchment and the Letaba River catchment, and groundwater recharge in the Sand River Aquifer. The impact on the potential yields from the existing Flag Bashielo Dam and other proposed dams in the Olifants River catchment were determined using the Water Resources Yield Model (WRYM). The results of the analysis were discussed at a regional stakeholders workshop. The workshop participants concluded that the higher priority for now should be on water conservation and demand management (WCDM) and to consider a phased approach to developing additional storage capacity. The results of this study show that the six-stage process for incorporating climate change into resource planning can be applied in practice and that the potential threat of climate change can give emphasis to existing win–win approaches such as WCDM and phased implementation that should form part of any sustainable water resource planning approach.  相似文献   

Ocean acoustic noise and passive coherent array processing     

Philippe Roux  W.A. Kuperman  Karim G. Sabra 《Comptes Rendus Geoscience》2011,343(8-9):533-547

Broadband correlation processing for extracting time-domain Green's functions and coherent wavefronts from random ocean noise has been demonstrated recently using experiments and numerical simulations that are consistent with theoretical predictions. Ocean acoustic noise processing presents additional challenges over its seismological counterpart. Mainly, the ocean environment is temporally non-stationary and it is spatially heterogeneous. Further, in the lower underwater acoustic frequency regime of about 20 to 500 Hz, space-time episodic shipping is the dominant noise source. The data from different publications and research groups are gathered here, with the goal being to review recent underwater acoustic research that demonstrates the viability and potential applications of passive coherent array processing in the ocean.  相似文献   

Review of constitutive models for describing the time dependent behaviour of soft clays     

Md. Rajibul Karim  C. T. Gnanendran 《Geomechanics and Geoengineering》2014,9(1):36-51

Different classes of constitutive models have been proposed to capture the time-dependent behaviour of soft soil (creep, stress relaxation, rate dependency). This paper critically reviews many of the models developed based on understanding of the time dependent stress-strain-stress rate-strain rate behaviour of soils and viscoplasticity in terms of their strengths and weaknesses. Some discussion is also made on the numerical implementation aspects of these models. Typical findings from numerical analyses of geotechnical structures constructed on soft soils are also discussed. The general elastic viscoplastic (EVP) models can roughly be divided into two categories: models based on the concept of overstress and models based on non-stationary flow surface theory. Although general in structure, both categories have their own strengths and shortcomings. This review indicates that EVP analysis is yet to be vastly used by the geotechnical engineers, apparently due to the mathematical complication involved in the formulation of the constitutive models, unconvincing benefit in terms of the accuracy of performance prediction, requirement of additional soil parameter(s), difficulties in determining them, and the necessity of excessive computing resources and time.  相似文献   

Development of a three-dimensional basin model to evaluate the site effects in the tectonically active near-fault region of Gölyaka basin,Düzce,Turkey     

Yousefi-Bavil  Karim  Koçkar  Mustafa Kerem  Akgün  Haluk 《Natural Hazards》2022,114(1):941-969

Natural Hazards - The high seismicity and tectonic activity of the study area located in a near-fault region in Gölyaka, Düzce, results in a bedrock geometry highly complex in the sense...  相似文献