基于核磁共振成像的混凝土冻融损伤特征

为研究多次冻融循环过程中材料内部孔隙分布的变化特点,采用低强度磁场核磁共振成像扫描仪对经过0,50,100和200次冻融循环的混凝土试件进行扫描,获得了弛豫时间T₂分布谱和扫描图像。研究结果表明:随着冻融循环次数的增加,表征材料孔隙率大小的T₂谱面积呈现初期增速较快,后期增速减小的规律;小孔隙含量随循环次数增加而减小,大孔隙含量随循环次数增加而上升;不同损伤程度的混凝土试件的核磁共振(NMR)扫描图像与T₂分布谱的变化特点具有较好的同步性,体现了扫描结果的准确性。研究成果有助于加深对混凝土在冻融循环过程中结构损伤演化规律的认识。     

引江济淮试验工程软岩崩解特性及其影响因素试验研究

以引江济淮试验工程为依托,开展砂岩与泥岩的崩解特性试验研究。通过静态烘干-浸水崩解试验,研究不同风化程度的软岩在干湿循环作用下的崩解性状,同时进行耐崩解试验以及点荷载强度试验,揭示崩解性与强度、失水率的相关变化规律。试验表明:干湿循环作用和风化作用均会减弱岩石耐崩解能力;风化岩石的耐崩解指数与干燥岩石点荷载强度成正相关,与岩石的失水率成线性负相关;岩石的耐崩解性与岩石结构类型关系密切。     

冻融循环作用下砂岩力学特性与电镜扫描试验研究

通过对取自新疆阿勒泰某隧道的砂岩开展冻融循环试验,在-25℃下冻结9 h,(20±2)℃下融解5 h,为一个冻融循环过程,对岩样分别进行0、5、10、20、30、40、50、60、80次冻融循环。然后,进行单轴压缩试验和电镜扫描试验,得到经历不同次数冻融循环后的饱和砂岩破坏特征、应力-应变曲线、弹性模量、泊松比,并分析该砂岩物理力学特性和破坏断口微裂纹演化与冻融条循环次数之间的关系。结果表明:随着循环次数的增加,该砂岩岩样弹性模量呈不同程度降低趋势,峰值应变呈逐渐增大趋势,而泊松比呈先增加后减小趋势,微裂纹长度、宽度及面积不同程度呈增大趋势,使砂岩损伤劣化不断加剧。     

改良黄土作为高速铁路路基填料的试验研究

为保证西北地区修建的宝兰客运专线铁路黄土路基的强度,必须对黄土填料进行改良.在黄土中掺入不同配比的矿渣脱硫石膏生石灰,养护28 d进行冻融循环试验和浸水试验后测试其无侧限抗压强度,结果表明改良后的黄土强度能够满足铁路路基的强度要求,最后提出作为路基填料的改良黄土的最优配比.     

活性粉末混凝土冻融循环前后NaCl溶液的吸入特性

为更好地理解混凝土盐冻破坏机理,研究了NaCl溶液中NaCl质量分数对活性粉末混凝土溶液吸入量的影响规律,并比较了试件表面及芯部吸入面溶液吸入特性的差异;分析了冻融循环1 000次对溶液吸入特性的影响,给出了不同吸入面的毛细吸液系数.研究表明:活性粉末混凝土毛细吸液量随着NaCl质量分数的增大而减小;15~180 min和180~540 min两阶段溶液吸入量均与t1/2呈线性关系;试件表面毛细吸液系数远小于芯部;冻融循环1 000次后溶液吸入量显著降低,仅为冻融循环前的0.33倍;冻融循环1 000次后距离试件表面越远的吸入面毛细吸液系数越大.     

冻融损伤混凝土重复受压本构关系

为研究冻融损伤对混凝土重复受压力学性能的影响,完成了两种强度等级混凝土试块的快速冻融及重复受压试验.结果表明,冻融损伤混凝土重复受压力学性能显著降低,动力本构特征曲线也发生改变,包络线及再加载曲线在峰值应力之前因存在明显压实效应而表现出下凹形态,卸载曲线应变滞后现象严重且曲线逐渐趋于重合.在试验研究基础上,探讨了冻融循环次数、混凝土强度等级与冻融后混凝土力学性能参数及本构模型参数之间的关系,建立了冻融损伤混凝土动力本构模型.     

Ultrasonic testing and microscopic analysis on concrete under sulfate attack and cyclic environment

The damage process of concrete exposed to sodium sulfate attack and drying-wetting cycles was investigated. The water to binder（W/B） ratio and the concentration of sulfate solution were taken as variable parameters. Through the experiment, visual change, relative dynamic modulus of elasticity（RDME） and the surface damage layer thickness of concrete were measured.Furthermore, SEM and thermal analysis were used to investigate the changing of microstructure and corrosion products of concrete.The test results show that the ultrasonic velocity is related to the damage layer of concrete. It approves that an increase in damage layer thickness reduces the compactness and the ultrasonic velocity. The deterioration degree of concrete could be estimated effectively by measuring the surface damage layer and the RDME of concrete. It is also found that the content of gypsum in concrete is less than that of ettringite in test, and some gypsum is checked only after a certain corrosion extent. When the concrete is with high W/B ratio or exposed to high concentration of sulfate solution, the content of ettringite first increases and then decreases with corrosion time. However, the content of gypsum increases at a steady rate. The content of corrosion products does not correspond well with the observations of RDME change, and extensive amount of corrosion products can be formed before obvious damage occurs.     

A new view of anti‐windup design for uncertain linear systems in the frequency domain

This paper presents a new perspective on the stability problem for uncertain LTI feedback systems with actuator input amplitude saturation. The solution is obtained using the quantitative feedback theory and a 3 DoF non‐interfering control structure. Describing function (DF) analysis is used as a criterion for closed loop stability and limit cycle avoidance, but the circle or Popov criteria could also be employed. The novelty is the combination of a controller parameterization from the literature and describing function‐based limit cycle avoidance with margins for uncertain plants. Two examples are given. The first is a benchmark problem and a comparison is made with other proposed solutions. The second is an example that was implemented and tested on an X‐Y linear stage used for nano‐positioning applications. Design and implementation considerations are given. An example is given on how the method can be extended to amplitude and rate saturation with the help of the generalized describing function, and a novel anti‐windup compensation structure inspired by previous contributions. Copyright © 2015 John Wiley & Sons, Ltd.     

新型水工混凝土表面防护涂层抗冻融性能研究

西部高寒地区地理位置独特,水文地质气候条件复杂,自然环境恶劣,造成该区域内水利工程混凝土材料易发生冻融劣化破坏。对此,选取课题组已研发的3种新型水工混凝土表面防护涂层材料(耐候型环氧涂层、不饱和聚酯树脂和聚脲涂层体系),进行室内加速冻融试验。试验结果表明,以环氧胶泥为底漆,聚脲材料为面漆的水工混凝土表面防护涂层的抗冻性能最优。随后在西藏地区水利水电工程进行了现场生产性试验,通过观测发现,新型聚脲材料起到了很好的抗冻防护效果。该材料有望应用于西部高寒地区水工建筑物的表面防护,提高水利水电工程建筑物的耐久性。     

硫酸（亚硫酸）盐渍土有荷载多次冻融盐冻胀发育规律试验研究

通过对天山北麓水磨河流域细土平原区硫酸（亚硫酸）盐渍土有荷载多次冻融盐-冻胀试验研究,发现不同硫酸钠含量、不同压力土体多次冻融盐-冻胀变形随冻融次数的变化规律是土体膨胀力、土体抵抗力、外压力综合作用的结果。不同压力、不同硫酸钠含量土体冻融循环次数与土体盐-冻胀干密度和土体盐-冻胀率均符合二次抛物线规律。