混凝土冻融劣化后动态单轴抗压特性试验研究

为研究混凝土冻融循环后的动态力学性能,利用10 MN动静三轴仪对不同冻融劣化程度的混凝土在不同加载速率下进行单轴压缩试验,同时采集声发射数据分析试验过程中能量释放及损伤演化规律。最后,选用Weibull-Lognormal损伤本构模型,对试验数据进行拟合分析,并分析不同工况混凝土的损伤随应变的发展规律。研究结果表明:①相同冻融劣化程度时,随应变速率增加,峰值应力增大,峰值应变减小;冻融劣化程度会影响峰值应力的率敏感性;②在应变速率相同时,随冻融循环次数增加,峰值应力随之减小,峰值应变随之增大。③混凝土冻融劣化后应力应变关系峰前服从Weibull统计分布,峰后服从Lognormal统计分布。④在相同应变速率下,随着冻融循环次数的增加,混凝土的累计塑性应变整体呈增大趋势。     

冻融循环下考虑塑性损伤演化的饱和岩石本构模型

针对冻融循环下饱和岩石,提出冻融损伤与岩石基质受载塑性损伤耦合的损伤变量计算方法。之后基于修正Drucker-Prager帽盖屈服准则,推导出考虑岩石基质塑性损伤演化的本构模型,而后考虑冻融对岩石力学性能的劣化作用,建立了冻融循环下考虑塑性损伤演化的饱和岩石本构模型。最后采用冻融循环下花岗岩单轴压缩实验结果对所提模型进行了验证。结果表明：(1)冻融循环会造成岩石的初始损伤,且初始损伤随冻融循环次数的增大而增大;(2)岩石基质受载损伤与冻融损伤两者耦合,会增大试件的损伤程度,进而加速岩石力学参数的劣化;(3)模型预测岩石单轴抗压强度随冻融循环次数的增大而降低,理论值与实验值吻合良好,证明了该模型的有效性。     

持载与湿热环境下ＣＦＲＰ加固高强混凝土梁结构性能研究

夏季湿热的大气环境对我国大部分地区的ＣＦＲＰ（碳纤维增强聚合物）加固钢筋混凝土结构性能 有不利影响。为探讨湿热环境与荷载耦合作用对ＣＦＲＰ加固高强钢筋混凝土梁结构性能的影响,共制作 16根ＣＦＲＰ加固钢筋混凝土梁试件,在不同温湿度环境下,对荷载与湿热环境耦合作用下ＣＦＲＰ加固钢 筋混凝土梁进行试验研究与理论分析。试验结果表明:随湿热环境作用时间增加,ＣＦＲＰ加固混凝土梁 剥离破坏界面由混凝土层逐渐转移到树脂胶层;荷载与湿热环境的耦合作用对ＣＦＲＰ加固钢筋混凝土梁 极限承载力影响较明显;湿热环境对加固钢筋混凝土梁的结构性能有不利影响。     

盐浸-干湿-冻融耦合作用下混凝土坝强震开裂机理

为探究混凝土坝在盐浸侵蚀、干湿循环、冻融循环因素影响下的强震破坏规律,通过材料劣化试验和数值模拟方法开展大坝强震开裂机理研究。针对坝体不同位置开展盐溶液侵蚀、干湿循环以及冻融循环劣化试验,基于材料试验结果,构建混凝土重力坝扩展有限元模型,模拟混凝土材料劣化前及劣化后混凝土坝强震开裂破坏情况。结果表明：多因素耦合作用导致混凝土材料劣化,强震作用下会降低大坝承载能力,增加大坝倒塌破坏风险。考虑混凝土多种劣化因素影响,所得结论可用于高寒地区混凝土坝的抗震设计。     

粉煤灰掺量对混凝土抗冻性能影响的试验研究

文中采用快速冻融循环试验方法,探讨了不同混凝土强度等级、相等水胶比情况下粉煤灰掺量对混凝土抗冻耐久性的影响规律。试验结果表明:随着冻融循环次数的增加,相对动弹模量逐渐降低,混凝土的质量损失率与冻融损伤累积度逐渐增大。结合损伤力学和数值分析的方法,基于相对动弹模和质量损失率劣化规律,建立混凝土冻融损伤模型和剩余使用寿命预测模型。     

冻融劣化混凝土压剪作用下力学特性及破坏准则

为研究冻融劣化混凝土动静态抗剪性能,进行了不同冻融循环次数(0,10,25,35和50次)后混凝土在不同法向应力(0,3,6,9和12 MPa)下的压剪强度试验,混凝土强度等级为C30。研究分析了冻融循环次数和法向应力对混凝土剪切强度、峰值应变和黏聚力与摩擦系数的影响。分析结果表明:①随着冻融循环次数的增加,混凝土在相同法向应力状态下的剪切强度均逐渐降低,而且法向应力越大,剪切强度随冻融循环次数的增加而降低的程度越小;当冻融循环次数相同且法向应力不大于单轴抗压强度50%时,剪切强度随法向应力的增大而增大,且冻融劣化程度会影响该增幅效果;②在法向应力相同时,剪切峰值变形随冻融循环次数的增加呈线性增长趋势,对某一冻融循环次数,法向应力的存在增大了混凝土的剪切峰值变形;③摩擦系数和黏聚力都随冻融劣化程度的加深而降低,黏聚力大幅度降低是由于冻融劣化作用起主导作用所致。基于上述试验分析和八面体应力空间二次抛物线形式的压剪破坏准则,构建了平面应力状态下考虑冻融循环次数的混凝土压剪破坏准则。     

冻融循环与疲劳荷载作用下混凝土损伤研究

为研究冻融循环与轴向疲劳荷载作用下混凝土力学性能损伤演化规律,对混凝土试件分别进行冻融循环、疲劳加载、先冻融循环后疲劳加载和先疲劳加载后冻融循环4种损伤试验,以试件经历损伤后的抗压强度劣化作为损伤评价指标研究混凝土的损伤特性和机理,同时研究相对动弹性模量和质量的劣化规律。研究结果表明:不同联合作用下,混凝土的相对动弹性模量均呈现降低趋势。冻融循环单一因素作用下,混凝土强度随冻融次数的增加逐渐降低;疲劳荷载单一因素作用下,混凝土强度随疲劳次数增加呈先升后降的趋势,疲劳4万次时,混凝土的损伤度为1.8%;先冻融循环后疲劳加载作用下,即混凝土先受冻融循环作用,再受1万次应力水平(0.1 f_c~0.5 f_c)的疲劳荷载作用时,随疲劳次数的增加,试件的强度均呈现升高趋势;先疲劳加载后冻融循环作用下,随冻融次数的增加,混凝土的力学性能损伤显著,历史疲劳次数为0.5万次和1万次,再经历75次冻融循环作用时,其损伤度分别为19%和24.2%。研究成果可为建立符合实际工程的混凝土结构耐久性设计理论提供较可靠的基础依据。     

冻融循环对钢纤维混凝土动力性能的影响研究

通过对冻融循环劣化后的钢纤维混凝土试件进行动态三轴压缩试验,分析了冻融循环及钢纤维掺量对混凝土轴向极限抗压强度、轴向峰值应变和应力-应变曲线的影响。结合扫描电子显微镜(SEM)分析冻融前后钢纤维混凝土的微观结构。目的在于探明冻融循环对钢纤维混凝土动态力学性能的影响规律,为寒冷地区钢纤维混凝土在实际工程中的应用提供理论参考。结果表明:增加冻融循环次数导致轴向极限抗压强度下降且100次冻融循环后下降速度明显增大,而轴向峰值应变基本呈线性增大。应变速率的增加导致轴向极限抗压强度增大且轴向峰值应变逐渐减小。冻融循环破坏了钢纤维混凝土内部结构,导致应力-应变曲线包围面积减小,钢纤维混凝土吸收能量的能力降低。钢纤维掺量对冻融劣化后混凝土动力性能影响较大,本试验中1%的钢纤维掺量下冻融劣化后混凝土最优。SEM微观结构揭示了钢纤维增强混凝土抗冻性的强化机理,以及过量掺入钢纤维对抗冻性的弱化机理,与宏观试验结果一致。     

不同冻融循环次数混凝土单轴压缩试验

为研究冻融循环作用对混凝土力学性能的不利影响,分别对混凝土进行0,10,25,35和50次快速冻融循环,并利用10 MN大型多功能动静力三轴仪对混凝土历经40%f_c的荷载历史作用后(f_c=40 MPa为普通混凝土单轴抗压强度),以10-4/s的应变速率进行单轴压缩试验,得到冻融循环后混凝土的单轴抗压强度,并分析其损伤演化规律与破坏机理。结果表明:随着冻融循环次数的增加,历经相同加载历史作用后的混凝土的单轴抗压强度逐渐降低,且峰值应力随冻融循环次数的变化呈二次曲线关系;选用修正后的Weibull-Lognormal分段式损伤本构模型,经验证能够较好拟合冻融劣化混凝土历经荷载历史作用后单轴应力应变曲线;此外,冻融循环次数越多,对混凝土造成的损伤程度越大,在损伤发展的后期阶段,冻融程度较大的混凝土损伤路径大幅度延长且趋于扁平化,直至进入破坏阶段。     

冻融环境下不同预制裂缝混凝土断裂性能研究

对不同相对切口深度的预裂缝混凝土试件进行冻融循环试验,循环次数分别为0、25、50、75、100次。对冻融循环后的试件进行三点弯曲试验,观察试件的表观变化,并利用扫描电子显微镜和立体显微镜观察试件裂缝的情况。分别从宏观力学性能、损伤劣化规律、细微观结构等方面对冻融环境下混凝土断裂力学性能进行研究,并根据混凝土的损伤理论,定义了混凝土基于微缺陷及动弹性模量的损伤变量。结果表明:混凝土冻融循环损伤是初始缺陷发展、劣化并累积的过程;混凝土断裂韧度与初始裂缝相对深度无关,与抗弯强度具有良好的线性相关性。     

Call for Papers

正As the eldest and most distributed professional journal of water industry,Water Wastewater Engineering(W WE)gained very wide acceptance from the colleagues working in this field in this country since start publication in 1964.Now it is administrated by the Ministry of Construction of the People’s Republic of China and sponsored by the China Civil Engineering Society etc.     

地下厂房施工过程的三维非线性数值模拟分析

根据冗各电站地下厂房开挖支护的施工顺序,以隐式杆单元模拟锚杆,以壳单元模拟混凝土喷层,采用三维非线性有限元技术,对洞室群的开挖支护动态过程进行了模拟计算。通过三维非线性有限元计算分析得到:洞室群动态开挖过程中围岩应力变形的分布规律,围岩塑性区的发展规律,锚杆受力变化情况。计算结果表明:整个开挖过程中,洞周围岩位移变化规律正常,量值较小,塑性区主要分布在洞室周边,围岩稳定性较好;但在洞室交叉口处围岩的变形较大,出现较大范围的塑性区,在此基础上提出了支护措施优化方案,并对优化方案的效果进行了评价。     

Evaluation of freshwater mussel relocation as a conservation and management strategy

The relocation of unionacean mussels is commonly used as a conservation and management tool in large rivers and streams. Relocation has been used to recolonize areas where mussel populations have been eliminated by prior pollution events, to remove mussels from construction zones and to re-establish populations of endangered species. More recently, relocation has been used to protect native freshwater mussels from colonization by the exotic zebra mussel Dreissena polymorpha. We conducted a literature review of mussel relocations and evaluated their relative success as a conservation and management strategy. We found that 43% of all relocations were conducted because of construction projects that were forced to comply with the Endangered Species Act 1973 and that only 16% were monitored for five or more consecutive years. Most (43%) relocation projects were conducted from July to September, presumably a period when reproductive stress is relatively low for most species and the metabolic rate is sufficient for reburrowing in the substrate. The mortality of relocated mussels was unreported in 27% of projects; reported mortality varied widely among projects and species and was difficult to assess. The mean mortality of relocated mussels was 49% based on an average recovery rate of 43%. There is little guidance on the methods for relocation or for monitoring the subsequent long-term status of relocated mussels. Based on this evaluation, research is needed to develop criteria for selecting a suitable relocation site and to establish appropriate methods and guidelines for conducting relocation projects.     

基于模型试验的河床砂砾石层基本特性研究

为了经济可靠地确定深厚覆盖层的基本特性参数,以大渡河双江口水电站坝基河床砂砾石层基本特性研究为工程背景,进行大尺寸模型试验,研究河床砂砾石层的旁压模量和动探击数随密度、级配以及上覆压力的变化规律。根据室内模型试验结果,结合现场旁压试验及钻孔试验成果,推断出坝址区河床砂砾石相关层位的密度,并依此进行砂砾石室内力学试验,获得设计所需力学参数。研究成果可为深厚覆盖层基本特性测试提供新的方法。     

SPATIAL AND TEMPORAL WATER QUALITY VARIABILITY IN AQUATIC HABITATS OF A CULTIVATED FLOODPLAIN

The floodplains of lowland rivers contain diverse aquatic habitats that provide valuable ecosystem services but are perturbed when intensively cultivated. Hydrologic, water chemistry and biological (fish) conditions in five aquatic habitats along the Coldwater River, Mississippi, were measured for more than 4 years: the river, two severed meanders that functioned as backwaters, a managed wetland and an ephemeral channel draining cultivated fields. Off‐channel habitats were connected to downstream regions 0.10% to 32% of the dry season and 24% to 67% of the wet season. The median temperatures for the five monitored sites ranged from 18°C to 23°C, the median total solids concentration for all sites was 135 mg L^?1, the median total phosphorus was 0.29 mg L^?1 and the median total nitrogen was 1.56 mg L^?1. Chemical and physical water quality displayed strong seasonal differences between the wet winter/spring and the dry summer/fall periods so that temporal variation consisted of gradual seasonal trends superimposed on strong diurnal variations. All off‐channel habitats exhibited periods of hypoxia and temperatures >30°C during the dry season. Between‐site gradients of water and habitat quality were strongly coupled to water depth and runoff loading. The rehabilitation of one backwater by increasing water depth and diverting agricultural runoff was associated with improved water quality and fish species richness relative to an adjacent untreated backwater. The diversion of polluted runoff and the use of water control structures to maintain greater water depth were observed to be effective management tools, but the former reduces the water supply to habitats that tend to dry up and the latter reduces connectivity. Published in 2011 by John Wiley & Sons, Ltd.     

Automated system for removal and pneumatic transport of fly ash from electric precipitator hoppers

A system for removal and pneumatic transport of fly ash is examined, in which air pulses act on batches (pistons) of ash formed in a duct. Studies are made of the effect of several physical parameters on the force required to displace a piston of ash and these serve as a basis for choosing a system for removal and pneumatic transport of ash simultaneously from several hoppers of an electric precipitator. This makes it possible to separate the ash particles according to size without introducing additional components. Formulas are given for calculating the structural and dynamic parameters of this system and measurements of indirect dynamic parameters are used to calculate the input-output characteristics of the system. In order to optimize the system, configurations for summing several ducts into a single transport duct for pneumatic ash transport are proposed. Some variants of dry ash utilization and the advantages of producing of size-separated particles are considered. __________ Translated from élektricheskie Stantsii, No. 2, pp. 26–30 (2008).     

The flood pulse advantage and the restoration of river-floodplain systems

The ‘flood pulse advantage’ is the amount by which fish yield per unit mean water area is increased by a natural, predictable flood pulse. Evidence for this increase is presented from tropical and temperate fisheries. It is argued that increasing multispecies fish yield by restoring the natural hydrological regime is consistent with increasing production of other trophic levels and with restoration from ecological and aesthetic viewpoints. When applied to a river-floodplain system, this restoration would provide a large, self-sustaining potential for recreation, commercial exploitation, and flood control. An interim ‘natural flood pulse’ restoration approach is proposed for systems modified for navigation. This approach approximates the natural hydrological regime in a river reach and is intended as a first step in the long process of restoring the watershed.     

Spatio‐temporal variation of gross CO2 and CH4 diffusive emissions from Australian reservoirs and natural aquatic ecosystems,and estimation of net reservoir emissions

Carbon dioxide (CO₂) and methane (CH₄) diffusive emissions were measured during two field surveys in Queensland and Tasmania, Australia, using the floating chamber method. Bubbling and degassing emissions in 2010 were estimated in Koombooloomba Dam reservoir using only inverted funnels and gas concentrations, respectively. A total of 14 reservoirs and 16 rivers and lakes were sampled from 2006 to 2010. Spatial variation was substantial within each water body, as well as between them. The main drivers of diffusive emission variation were physiographic region and climate, with a clear demarcation being observed between diffusive emissions from tropical Queensland and temperate Tasmania, and between the humid West Coast Range (Tasmania) and dry Central Plateau (Tasmania). Higher CO₂ and CH₄ diffusive emissions were observed during the dry season, when long water residence times would promote organic matter degradation. Estimated total gross emissions, including diffusive, bubbling and degassing emissions, for Koombooloomba Dam reservoir were about 1.5 × 10⁶ t CO₂eq km² per year, or 24 × 10⁶ t CO₂eq per year. This corresponds to a plant emission factor of 3.18 kg CO₂eq MWh^?1. Using an estimate of terrestrial emissions derived from literature data for the Tully River catchment area, rough estimated net emissions from the catchment area are about 44 kt CO₂eq per year, or 5.83 kg CO₂eq MWh^?1, which is in the lower range of the studied reservoirs.     

基于BP神经网络-马尔科夫链模型的隧道围岩位移预测

在隧道工程施工中,围岩位移预测起着很重要的作用。将BP神经网络-马尔科夫链模型引入到隧道围岩位移预测中来,通过对训练样本的学习,利用BP神经网络实现了对位移时间序列的滚动预测,同时得到了实测值与预测值的相对误差;在此基础上利用马尔科夫链对相对误差进行修正,有效地提高了预测结果的精度。并将该模型应用于某公路隧道拱顶下沉位移时序预测中,结果表明该模型具有精度高、科学可靠的特点,为隧道围岩变形的预测提供了新的途径。     

Short communication. Pipelines as a refuge for freshwater snails

In the lower River Murray, Australia, populations of gastropods have declined markedly over the past 40 years. Of the native fauna of about 18 species, only the ancylid Ferrissia petterdi remains common (but in patches), and another five species feature in sporadic local resurgences. Until recently the viviparid Notopala was thought to be extinct. The introduced physid Physa acuta is widespread, but in low numbers. The decline probably is associated with flow regulation, high turbidity and the introduction of common carp. Snail populations have persisted, however, in irrigation pipelines fed from the river, where they periodically cause blockages. This paper reports the discovery of the only known populaion of Notopala hanleyi in a pipeline where the inner wall accumulates microbial and other organic matter, providing high quality food for snails. In the Murray before regulation most of the periphytic biomass was probably microbial, as fluctuating water levels would have maintained littoral communities in a highly productive, early state of succession his postulated that, by stabilizing seasonal water levels, regulation has promoted the production of filamentous green algae, an inferior food source, and so displaced snail populations from the open aquatic environment.