承高水压山岭隧道混凝土衬砌渗透损伤机理与模型研究

山岭隧道一般都处于高水压环境中,衬砌损伤特性需要考虑水压渗透作用。采用Weibull统计分布函数,建立混凝土整体初始渗透系数及损伤场变量计算公式;求解外水压力下圆形衬砌内渗透孔隙水压力;在此基础上,建立混凝土圆形衬砌渗流-应力-损伤耦合模型,并通过ABAQUS有限元分析软件进行算例分析,说明承水压山岭圆形混凝土衬砌损伤与渗流及应力之间关系密切。     

高性能水泥基纳米胶凝材料渗透性能及孔径分布试验研究

为解决混凝土脆性开裂引发的结构渗漏问题,该文提出一种新型高性能纳米改性水泥基复合材料(简称HPNCC)。借助砂浆抗渗仪和压汞仪,着重研究HPNCC材料在恒定水压作用下的渗透性能和微观孔隙分布。试验结果表明,HPNCC材料的相对渗透系数仅为3.64×10-9cm/h,其抗渗性能是传统纤维增强水泥基复合材料(UHTCC)及普通混凝土的1000倍以上,微观孔隙尺寸更小,结构更为致密,具有优异的抗渗性能。     

承水压混凝土结构损伤机理与渗透特性研究综述

研究承水压混凝土结构的损伤与渗透问题有利于地下工程结构的安全运营,譬如山岭隧道、城市地下结构等。从构成材料损伤理论框架的三个主要方面(场变量的定义、损伤演化方程、本构行为关系)系统地阐述混凝土的损伤机理,并宏细观地分析了混凝土损伤的基本特性。分析了水压下的混凝土渗透行为特性,并针对渗透对混凝土结构的破坏机理进行阐述。剖析混凝土渗流与混凝土损伤之间的耦合机理,建立了渗流-损伤耦合模型。     

渗透作用下开裂混凝土的材料损伤模型

混凝土的渗透性在控制混凝土的质量及混凝土结构的行为上起着关键性的作用,而开裂混凝土在渗透作用下的损伤则直接控制了结构的长期工作性能.从开裂混凝土的渗透机理、开裂水压务件、开裂应力出发,求出开裂混凝土中裂缝开度与水压、拉伸及压剪作用下裂缝开度与应力的关系并给出计算公式;然后定义了混凝土开裂损伤问题,通过有效承载面积损失来表达开裂混凝土的损伤;最后通过上述方式将混凝土开裂渗透作用与混凝土的损伤结合起来,求得渗透作用下开裂混凝土的损伤表达.     

不同加载速率下饱和混凝土的劈拉试验研究及强度变化机理

利用大型液压伺服试验机研究了不同加载速率下饱和混凝土与干燥混凝土的劈拉强度变化情况,试验研究表明:在准静态加载时,与干燥的混凝土相比饱和混凝土的劈拉强度有所降低,随着加载速率的升高,饱和混凝土的的动态劈拉强度有较大的提高。通过对干、湿混凝土强度变化的比较表明:孔隙及裂缝中的自由水对混凝土强度有所影响。根据细观层次上、不同加载速率下裂纹中孔隙水压力的作用形式(即低加载率时孔隙水的“楔入”作用和高加载率时的“Stefan效应”),利用断裂力学来探讨不同加载速率下饱和混凝土劈拉强度的变化机理。结果表明:提出的机理可以较好地解释饱和混凝土强度的变化情况。     

基于孔结构分析的多尺度聚丙烯纤维混凝土耐久性

为研究不同尺度纤维对混凝土耐久性的影响,借助压汞法技术测定多尺度聚丙烯纤维混凝土内部孔隙的孔径大小及分布情况,通过孔径分布、最可几孔径、平均孔径等孔隙特征参数的变化情况,探究多尺度聚丙烯纤维在抗渗试验、抗硫酸盐侵蚀循环试验、抗冻融试验中对混凝土耐久性的影响.研究结果表明:不同尺度聚丙烯纤维的掺入,均使得混凝土内部多害孔...     

多壁碳纳米管增强混凝土的抗氯离子渗透性能

采用快速氯离子迁移系数(RCM)法和自然浸泡法对多壁碳纳米管(MWCNTs)增强混凝土的抗氯离子渗透性能进行研究,测量混凝土试件纵向断面上的氯离子扩散深度,据此计算氯离子扩散系数。试验结果表明：当MWCNTs掺量为0.15wt%时,混凝土28天的氯离子扩散深度、氯离子扩散系数分别降低了25.7%、19.1%;在4种不同侵蚀龄期的自然浸泡下,掺入MWCNTs的混凝土,内部氯离子浓度始终低于对照组。结合两种方法分析得出：混凝土内部各深度的自由氯离子浓度随着MWCNTs掺量的增加而降低,致使氯离子扩散系数随着MWCNTs掺量的增加而变小,MWCNTs的掺入提高了混凝土的抗氯离子渗透性能。此外,通过SEM和压汞(MIP)测试进一步探究MWCNTs对混凝土抗氯离子渗透性能的微观增强机制,分析结果表明,MWCNTs具有一定的桥接和填充效应,这可能使混凝土裂缝扩展受到抑制、孔隙更加细化,从而改善混凝土的微观结构,提高混凝土的抗氯离子渗透性能。     

基于统计损伤理论的饱和细粒砂岩本构模型研究

为了更好地揭示饱和细粒砂岩在不同孔隙水压力作用下的损伤本构关系,基于损伤力学/概率统计方法和有效应力原理,采用MC强度准则假定微元强度服从对数正态分布,建立了基于统计损伤理论的饱和细粒砂岩本构模型,修正了统计损伤模型参数F_0、S_0与孔隙水压力之间的关系表达式,建立了能够更加客观地反映饱和细粒砂岩损伤状态的本构模型。理论与试验对比研究表明所建立的模型具有一定的可靠性,在此基础上,研究了统计损伤模型参数F_0、S_0对饱和细粒砂岩物理特征的影响,探讨了饱和细粒砂岩在不同围压、不同孔隙水压力下的损伤演化规律,为实际工程案例中饱和细粒砂岩的损伤变化提供了参考。     

再生混凝土抗冻性能试验研究及孔隙分布变化分析

再生混凝土抗冻耐久性与混凝土结构内部孔隙分布变化密切相关.为研究再生混凝土结构内部孔隙分布与抗冻耐久性的定量关系,选取再生粗骨料取代率为0% 、25% 、50% 、75% 、100%的普通再生混凝土和引气再生混凝土作为研究对象,进行冻融循环试验和核磁共振试验,测试混凝土试件质量、动弹性模量、抗折强度以及结构内部孔隙分布情况.结果表明:在冻融循环试验中,加入引气剂可有效改善试件内部的中孔(0. 01～0. 05 μm)和大孔(0. 05～1 μm)的占比,从而提高其抗冻性能;在10种不同配比中,引气天然骨料混凝土的抗冻性能最佳,其次是再生粗骨料替代率为50%的引气再生混凝土,其内部孔隙结构相比再生粗骨料替代率为25% 、75%和100%的引气再生混凝土更加稳定;再生混凝土冻融循环后的抗折强度变化与结构内部孔隙的分布和占比密切相关.     

硅灰对再生混凝土界面过渡区的影响

为改善再生混凝土的力学和耐久性能,以硅灰为增强材料对再生混凝土进行改良。研究了硅灰对再生混凝土3 d、28 d、90 d抗压强度和28 d、90 d抗氯离子渗透性能的影响。结合扫描电镜、显微硬度等微观观测手段,分析了28 d再生混凝土试样微观结构和性能变化。采用压汞法测试了再生混凝土的孔结构参数,探究硅灰对再生混凝土孔隙性能的影响。结果表明：硅灰可以提升再生混凝土的抗氯离子渗透性能,随掺量的增加提升效果先增后减;掺入硅灰可以改善再生混凝土多重界面过渡区结构,增加界面过渡区(ITZ)显微硬度,降低孔隙率。再生混凝土内部存在较多有害孔隙,硅灰可以细化孔隙结构,降低孔隙率,掺量为6%时效果最佳。     

孔隙水对湿态混凝土抗压强度的影响

湿态混凝土受压时会产生孔隙水压力,孔隙水压力的大小与混凝土的变形和裂纹的扩展速度相关。孔隙水压力的产生给混凝土的力学性能带来一些影响,但是目前这方面的理论研究甚微。尝试利用断裂力学的方法,来探讨湿态混凝土在承受单轴压缩荷载时,孔隙水压力对混凝土开裂、扩展和抗压强度的影响。研究结果表明:混凝土中的孔隙水压力减小了阻碍混凝土开裂的摩阻力,相当于楔体的“楔入”作用,加速了混凝土的损伤和微裂纹的扩展,与干燥态的混凝土相比,湿态混凝土的开裂应力和抗压强度都有所降低。     

Axisymmetric thermomechanical constitutive and damage modeling for airfield concrete pavement under transient high temperature

An axisymmetric thermomechanical damage model is proposed for airfield concrete pavement under very rapid heating and cooling processes due to high-temperature exhaust gas from vectored thrust engines. This is typical of advanced aircraft during their short vertical take-off and landing routines. The temperature and pore pressure distributions are investigated inside the airfield concrete pavement along the radial and vertical directions. In addition, we derive the three-dimensional thermoelastic stress-strain laws accounting for spherical void effects. Since the temperature range in this study is very large, thermal properties of concrete pavement are treated as functions of temperature. The spatial-temporal temperature field of the airfield concrete pavement is calculated numerically by the explicit finite difference method. Subsequently, the pore pressure distribution is predicted based on the ASME Steam Tables and the foregoing temperature distribution. Within the framework of linear thermoelasticity, the stress distributions are computed as functions of locations and time by the finite element method. Further, Newman's crack growth model is applied to estimate the delamination (thermal spalling) time of the airfield concrete pavement at various locations due to the internal pore pressure.     

About drying effects and poro-mechanical behaviour of mortars

The drying of mortar and concrete generally affects their elastic properties and strength. Such property variations are of interest, since they affect the durability of structures designed with these materials. Previous studies, conducted on two mortars with different W/C ratio, showed that drying such material led to decreases in their elastic modulus and their Poisson ratio. Although generally attributed to micro-cracking, these decreases are nevertheless consistent with simple poro-mechanical effects i.e. transition from undrained behaviour toward drained behaviour. The main objective of this experimental study was therefore to state whether or not pore fluid pressure played a direct role in the variation of elastic properties measured on saturated or dried material. Poro-mechanical measurements of drained and undrained bulk moduli, Biot and Skempton coefficients were carried out. They actually revealed that poro-mechanical coupling effects could explain a part of the differences observed between elastic properties. Ethanol was chosen as the pore fluid to avoid possible unexpected chemical effects on the materials tested (often observed with water). To conclude about the saturation effects, a complementary experimental procedure was conducted in two steps: compression tests were performed on previously dried samples, which were then re-saturated with ethanol before being mechanically tested again. The results obtained are unambiguous and showed that poro-mechanical effects are not involved in the change in elastic properties. On an other hand, the major roles played by microcracks induced by drying and by capillary suction are highlighted: (a) The decrease in elastic properties, observed as drying occurs, is shown to be due to induced micro-cracking; (b) the increase in capillary pressure, brought about by drying, leads to an increase in the mortar failure strength.     

Study on mechanism of strength distribution development in vacuum-dewatered concrete based on the consolidation theory

The strength and hardness of a concrete slab surface are considered to be significantly affected by concrete bleeding. Vacuum dewatering is reported to be quite effective in imparting high density and strength. However, in Japan, in contrast with concrete work in civil engineering applications, concrete work in the field of building construction has not been successfully treated by this method. In an earlier report, the authors pointed out the strong relationship between strength distribution and density distribution in vacuum-dewatered concrete, both of which gradually decrease from the top surface to a depth of about 15 cm. The main purpose of the present study is to discuss the mechanism of the occurrence of such distribution of strength and density, based on consolidation theory. In an experiment, pore water pressure distribution in concrete is measured by means of an original measuring system. The results of the experiment confirm that the consolidation theory is quite effective in explaining the internal properties of vacuum-dewatered concrete as well as those of press-dewatered concrete. A prediction method for the strength improvement of concrete by vacuum dewatering is also discussed. It was considered likely that pore water pressure distribution generated by vacuum dewatering could be attributable to the influences of capillary tension and viscous resistance. This mechanism was verified by model experiment.     

Comparative study of different test methods for reinforced concrete durability assessment in marine environment

There are many different test methods to assess reinforced concrete durability. As in marine environment reinforcement corrosion due to chloride attack is the most important degradation process, chloride penetration rate has been compared with different durability tests results (concrete strength, porosity, water absorption, water penetration depth under pressure, capillarity, water and oxygen permeability) carried out on concrete cores obtained from the caissons of seven Spanish wharves. Data have been studied separately, depending on concrete location (chloride penetration rate is faster in submerged concretes than in tidal zone concretes) and cement type (mineral admixtures reduce permeation rate due to pore size refinement). Results show that it is advisable to control concrete water tightness through water penetration under pressure test; additionally, in order to make sure a slow corrosion rate, it should be advisable to control oxygen permeability in tidal zone concretes.     

蒸压加气混凝土的孔结构及表征方法研究进展

蒸压加气混凝土因其质轻、保温性好、环保等优点而受到人们的重视。作为一种典型的宏观多孔建筑材料,蒸压加气混凝土的孔结构特征与其微观结构和性能有密切的关系。钙硅比、水料比、铝粉及工艺参数是影响蒸压加气混凝土孔结构的重要因素,孔隙率、孔径分布等孔结构特征与蒸压加气混凝土的强度、吸水性、干燥收缩、导热性能及耐久性等性能有紧密的联系。本文重点阐述了蒸压加气混凝土的孔结构特征及影响孔结构的主要因素,孔结构对蒸压加气混凝土性能的影响,并简述了蒸压加气混凝土孔结构现阶段主要采用的表征方法。     

内水压作用下粘弹性饱和土-隧洞衬砌相互作用

混凝土衬砌既有粘弹性性质,又有渗透性。实际工程中内水压力值由衬砌和孔隙水共同承担,该文通过引入与孔隙流体体积分数有关的应力系数合理地分配了衬砌和孔隙水分别承担的内水压力值。根据衬砌和土体界面处衬砌中流体速度和土体中流体速度相等以及应力和位移连续性条件建立了部分透水边界条件。将衬砌和土体分别视为多孔粘弹性材料和液固两相介质,采用饱和多孔介质理论和粘弹性理论,在频率域内给出了内水压力作用下粘弹性饱和土-衬砌相互作用时饱和粘弹性土位移、应力和孔压和衬砌的位移和应力解析表达式。进行了参数研究,表明:应力系数以及衬砌和土体相对渗透系数对系统动力响应影响很大。另外,应力系数合理地确定了边界衬砌和孔隙水分别承担的内水压力值。     

Influence of limestone powder used as filler in SCC on hydration and microstructure of cement pastes

In recent years, self-compacting concrete (SCC) has gained wide application in the construction industry. As for high performance concrete (HPC) and traditional concrete (TC), the microstructural properties of SCC are the main factors, which determine the material properties, i.e. the mechanical properties, transport properties and the durability behaviour.In order to investigate the development of the microstructure of SCC, the microstructural parameters of the paste including porosity, pore size distribution and phase distribution are determined by means of mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The thermogravimetric analysis (TGA) and the derivative thermogravimetric analysis (DTG) are used to identify the phase constituents. These parameters as studied for self-compacting concrete are compared with high performance concrete and traditional concrete. The specimens of self-compacting cement paste (SCCP) are made with water/binder ratio 0.41 and 0.48, the high performance cement paste (HPCP) with w/c 0.33 and traditional cement paste (TCP) with w/c 0.48. The measurements are performed at different hydration stages, i.e. at 1, 3, 7, 14, 28 and 56 days.The result of this research shows that the pore structure, including the total pore volume, pore size distribution and critical pore diameter, in the SCCP is very similar to that of HPCP. The fact that limestone powder does not participate in the chemical reaction was confirmed both from thermal analysis and BSE image analysis.     

Thermo-mechanical properties of Penly reactor envelope at temperatures up to 200°C

This paper reports on an experimental investigation conducted to study the effects of high temperatures of up to 200°C on the phase composition, pore structure development and physico-mechanical properties of concrete at the PENLY nuclear power plant (France). The concrete specimens were manufactured under laboratory conditions from identical materials used at the construction site in PENLY, and then stored at 20°C/100% R.H. for 28 days and exposed to temperatures of 40°C, 60°C, 100°C, 200°C, and 20°C/60%R.H., respectively. Test results revealed that an intense structural integrity degradation of PENLY concrete occurs between 100°C and 200°C due both to a loss of water bound in hydrated cement minerals and to subsequent air void formation. This phenomenon is related to an increase in the median pore radius and total porosity values, as well as to a decrease in the dynamic and static moduli of PENLY concrete. The reduction in volume of the hydrate phase is believed to be the reason behind the rapid expansion, over a short time interval, due to a time-limited moistening of the specimen by released water with a sudden rise in temperature, followed by a stabilised period of shrinkage and creep.