荷载作用下饱和水泥浆体中氯离子扩散性能研究

混凝土类水泥浆复合材料中各种尺度的孔隙,如凝胶孔、毛细孔、掺入的气体气泡以及微裂纹等影响着氯离子的扩散性能。孔隙结构参数(如孔隙率)在外荷载作用下会产生变化,进而影响了水泥浆体中氯离子扩散性能。外荷载作用对氯离子扩散行为的影响,可以等效为外荷载所引起的孔隙率的改变对氯离子扩散性能的影响。从微观角度出发,将饱和水泥浆体看作由水泥浆体基质(其孔隙率为零)和孔隙水夹杂相所组成的两相复合材料介质。基于弹性力学理论推导并获得了饱和水泥浆体达到其强度前(即未产生新裂纹前)当前孔隙率与材料初始孔隙率及体应变之间的定量关系,得到了水泥浆体中氯离子扩散系数与这些参数的定量关系。基于Fick第二定律分析了外荷载(体应变)和孔隙率变化对氯离子扩散性能的影响。研究表明:氯离子在饱和砂浆中的扩散系数随孔隙率增大而显著增大;氯离子在砂浆中的扩散系数随压缩体应变的增大而减小,随拉应变增大而增大。     

骨料级配对二维模型混凝土单轴抗拉强度影响的理论研究

混凝土尺寸效应及其宏观力学非线性根源于其材料细观组成的非均质性。结合混凝土细观结构形式,将混凝土看作由骨料颗粒、砂浆基质及界面过渡区组成的复合材料。采用双线性弹性损伤模型来描述砂浆基质及界面过渡区的力学行为,假定骨料颗粒为弹性体而不发生破坏,进而推导并获得了单轴拉伸条件下不同骨料颗粒级配混凝土断裂裂缝扩展路径长度及其抗拉强度的理论解。最后,对比了建立的理论公式结果与细观尺度数值模拟结果,验证了构建的关于裂缝长度及抗拉强度理论解的准确性和合理性。     

考虑过渡区界面影响的混凝土宏观力学性质研究

混凝土材料的宏观力学特性及破坏机理由其细观组分来决定,界面过渡区是影响混凝土断裂破坏路径及宏观力学特性的重要因素。认为界面过渡区是区别于远处砂浆基质的一层含较高孔隙率的近场砂浆材料,采用“两步等效法”得到了混凝土细观单元的等效本构关系模型。最后基于细观单元等效化方法分析了在单轴拉伸、单轴压缩及弯拉载荷条件下混凝土试件的破坏过程及宏观力学性质,探讨了界面过渡区对混凝土力学特性的影响,并与随机骨料模型分析结果进行了对比。结果表明:界面相的存在对混凝土的弹性模量、强度及残余强度等力学性质有很大影响,在对混凝土宏观力学特性及细观断裂破坏过程进行研究时不可忽略其影响。     

循环荷载下混凝土徐变特性的宏细观分析

为研究循环荷载加大混凝土徐变的内因,以循环荷载和恒荷载下混凝土徐变试验结果为基础,基于多孔介质理论的混凝土徐变模型,编制了徐变模拟分析程序,在细观尺度下模拟了受循环荷载作用的混凝土内部骨料、砂浆与水的耦合过程,得到了宏观尺度下模型内部节点位移过程图、细观尺度下水分流速分布图、孔隙压力消散过程图、等效应力变化图。研究结果表明:(1)数值模拟方法可以描述细观尺度下混凝土内部骨料、砂浆、水分的运动规律;(2)与恒定荷载相比,循环荷载通过影响水分的运动,加快了骨料与砂浆的内力重分布速度,推进了混凝土固相的受力均匀化的过程,从而加大混凝土徐变。     

考虑初始缺陷影响的混凝土梁动态弯拉破坏模式分析

考虑到混凝土细观非均质性的影响,从细观角度出发,认为混凝土是由骨料、界面过渡区、砂浆基质及初始缺陷组成的四相复合材料,建立了混凝土简支梁的二维随机骨料模型。采用耦合材料应变率效应的塑性损伤本构模型来描述砂浆基质及界面的力学性能;假定骨料不产生损伤破坏,设定为弹性。对无缺陷、2%和5%孔隙率的混凝土梁进行弯拉破坏数值研究,探讨初始缺陷及加载速率对混凝土梁弯拉破坏模式、弯拉强度及宏观应 力-应变关系的影响。数值结果表明:混凝土弯拉破坏模式及宏观力学性能具有明显的加载速率相关性;初始缺陷的存在对混凝土破坏模式及宏观力学性能具有很大的影响。     

基于细观尺度的再生混凝土多相导热系数理论模型

为分析再生混凝土导热系数变化机理,进行了再生混凝土导热系数理论模型研究。在细观尺度上,将再生混凝土看作由新硬化砂浆、天然骨料、界面过渡区、孔隙相组成的四相复合材料。采用等效化方法,建立了界面过渡区导热系数模型并进行了验证,同时定义了等效界面过渡区(NITZ)影响系数η,η与再生混凝土导热系数有显著的线性关系。基于复合材料导热系数计算模型,利用η建立再生混凝土导热系数理论模型,计算结果与测试结果吻合较好,误差小于6%。在此基础上,根据该模型分析了各相组分导热系数、水灰比、骨料取代率、孔隙饱和度等因素对再生混凝土导热系数的影响,揭示了再生混凝土与普通混凝土传热差异,为再生混凝土传热机理的进一步研究提供了理论基础。     

核电厂预应力混凝土安全壳中氯离子扩散系数的预测方法EI北大核心CSCD

氯离子扩散系数是评价沿海环境中核电厂预应力混凝土安全壳耐久性的重要参数。该文基于两尺度方法,通过分析双轴等压下水泥浆基体、界面和混凝土细观结构,建立了水泥浆基体和界面毛细孔隙率与预应力之间的定量关系。为了量化预应力对微裂纹闭合、产生和扩展的影响,提出了临界毛细孔隙率与预应力之间的经验公式。将混凝土模拟成由骨料、界面和水泥浆基体组成的三相复合材料,获得预应力混凝土氯离子扩散系数比。通过与试验结果比较,校正了经验公式中的两个参数。用三组试验数据初步验证了该预测方法的有效性。     

基于多尺度分析的再生混凝土有效氯离子扩散系数预测

将再生混凝土视为由砂浆相、再生骨料相和二者砂浆之间的界面过渡区相(ITZ-2)组成的非均质复合材料，其中砂浆相由细骨料、细骨料-新砂浆界面过渡区(ITZ-1)、硬化水泥浆体三相组成，而再生骨料相由旧骨料、附着砂浆及旧骨料与旧砂浆的界面(ITZ-3)三相组成。基于N层球夹杂理论，考虑微观相的影响，建立了再生混凝土有效氯离子扩散系数预测的五相多尺度模型，通过硬化水泥浆体、砂浆和再生混凝土的稳态扩散系数实测值与模型预测值对比分析，验证其模型的准确性和有效性；最后，进一步讨论了氯离子侵蚀时间、再生骨料体积分数和附着砂浆含量等关键参数对其有效扩散系数的影响。结果表明：有效扩散系数预测值与试验值吻合较好，说明模型对预测再生混凝土的有效氯离子扩散系数具有普适性，为氯盐环境下再生混凝土耐久性评估与寿命预测提供理论依据。     

钢筋混凝土柱偏心受压力学性能的细观数值研究

钢筋混凝土构件的宏观力学性能由其组分-钢筋和混凝土两部分的力学性能决定。结合混凝土细观结构形式,认为混凝土是由骨料颗粒、砂浆基质及界面过渡区组成的复合材料,假定钢筋与混凝土之间完好粘结,基于钢筋混凝土柱偏心受压试验,建立了钢筋混凝土柱偏心受压加载下力学特性及破坏行为研究的细观尺度力学分析模型。通过对混凝土方形和矩形试件进行受压力学特性模拟,采用反演法确定了界面的力学参数,进而模拟了钢筋混凝土柱偏心受压加载下的宏观力学性能。结果表明,相比于宏观尺度模型,细观数值分析模型能够充分体现材料的非均质性,能够较好的模拟试件的宏观力学性能,并且能够细致的描述裂缝发展及试件破坏过程,与试验结果吻合良好。该文建立的细观尺度分析模型与方法,为钢筋混凝土构件层次宏观力学非线性及其尺寸效应研究提供了理论支持。     

纤维增强混凝土氯离子扩散系数的多尺度预测模型

纤维增强混凝土材料属于多相非均质复合材料,其宏观耐久性能由微观和细观的组分占比和夹杂关系共同决定。为考虑纤维增强混凝土材料不同尺度下的非均质性对整体氯离子扩散系数的影响,本文基于从微观到宏观的多尺度方法选取了不同层级代表单元,建立了纤维增强混凝土氯离子扩散系数多尺度预测模型。模型在充分考虑微观水泥水化过程和阈值效应的基础上,分析了细观尺度下纤维、骨料及其与水泥浆体的结合界面对混凝土宏观扩散性能的共同影响,并探究了纤维尺寸、纤维-浆体界面过渡区厚度等因素与扩散系数之间的影响关系,且通过第三方试验对模型的可靠性进行了验证。参数化分析的结果表明,当水灰比大于一定限值(约为0.45),水泥浆体的氯离子扩散系数与水灰比呈指数增长,而在细观层级上,纤维-浆体界面过渡区是影响混凝土整体扩散性能的主要因素：纤维掺量的增加和纤维直径的减小都会增大界面过渡区的体积,而较高的纤维过渡区体积占比和纤维界面扩散系数都会增大纤维增强混凝土的宏观氯离子扩散系数。结果还表明混凝土扩散系数与纤维掺量之间并无直接关系,而需要综合考虑纤维直径、界面过渡区厚度等各种因素的影响。本文所提模型能够有效预测纤维增强混凝土的扩散系数...     

Influence of internal curing using lightweight aggregates on interfacial transition zone percolation and chloride ingress in mortars

The microstructure of the interfacial transition zone (ITZ) between cement paste and aggregate depends strongly on the nature of the aggregate, specifically its porosity and water absorption. Lightweight aggregates (LWA) with a porous surface layer have been noted to produce a dense ITZ microstructure that is equivalent to that of the bulk cement paste, as opposed to the more porous ITZ regions that typically surround normal weight aggregates. This ITZ microstructure can have a large influence on diffusive transport into a concrete, especially if the individual ITZ regions are percolated (connected) across the three-dimensional microstructure. In this paper, the substitution of LWA sand for a portion of the normal weight sand to provide internal curing (IC) for a mortar is examined with respect to its influence on ITZ percolation and chloride ingress. Experimental measurements of chloride ion penetration depths are combined with computer modeling of the ITZ percolation and random walk diffusion simulations to determine the magnitude of the reduced diffusivity provided in a mortar with IC vs. one with only normal weight sand. In this study, for a mixture of sands that is 31% LWA and 69% normal weight sand by volume, the chloride ion diffusivity is estimated to be reduced by 25% or more, based on the measured penetration depths.     

Modeling the elastic properties of concrete composites: Experiment,differential effective medium theory,and numerical simulation

Concrete is a mixture of cement, water and aggregates. In terms of microstructure, besides the cement paste matrix and aggregate inclusions, there is a third phase, which is called the interfacial transition zone (ITZ), which forms due to the wall effect and can be thought of as a thin shell that randomly forms around each aggregate. Thus, concrete can be viewed as a bulk paste matrix containing composite inclusions. To compute the elastic properties of a concrete composite, a differential effective medium theory (D-EMT) is used in this study by assigning elastic moduli to corresponding bulk paste matrix, ITZ and aggregate. In this special D-EMT, each aggregate particle, surrounded by a shell of ITZ of uniform thickness and properties, is mapped onto an effective particle with uniform elastic moduli. The resulting simpler composite, with a bulk paste matrix, is then treated by the usual D-EMT. This study shows that to assure the accuracy of the D-EMT calculation, it is important to consider the increase in the water:cement mass ratio (w/c) of the ITZ and the corresponding decrease in w/c ratio of the bulk matrix. Because of this difference in w/c ratio, the contrast of elastic moduli between the ITZ and the bulk paste matrix needs to be considered as a function of hydration age. The Virtual Cement and Concrete Testing Laboratory (VCCTL) cement hydration module is used to simulate the microstructure of cement paste both inside and outside the ITZ. The redistribution of calcium hydroxide between ITZ and bulk paste regions can further affect the elastic contrast between ITZ and bulk paste. The elastic properties of these two regions are computed with a finite element technique and used as input into the D-EMT calculation. The D-EMT predictions of the elastic properties of concrete composites are compared with the results measured directly with a resonant frequency method on corresponding composites. This comparison shows that the D-EMT predictions agree well with experimental measurements of the elastic properties of a variety of concrete mixtures.     

Prediction of the chloride diffusion coefficient of concrete

It has been experimentally verified that the structure of the interfacial transition zone (ITZ) in concrete differs from that of bulk cement paste. As such, concrete should be modeled as a three-phase material at a mesoscopic level. This paper presents a three-phase composite model for predicting the chloride diffusion coefficient of concrete. Taking the inclusion as aggregate and the matrix as cement paste, the composite circle model is established by adding an ITZ layer in between the inclusion and the matrix. Solving the asymmetrical problem analytically, a closed-form solution for the chloride diffusion coefficient of concrete is derived. After verifying this model with experimental results, the effects of the aggregate area fraction, the chloride diffusion coefficient of ITZ, the ITZ thickness, the maximum aggregate diameter and the aggregate gradation on the chloride diffusion coefficient of concrete are evaluated in a quantitative manner. It is found that the chloride diffusion coefficient of concrete decreases with the increase of the aggregate area fraction and the maximum aggregate diameter, but increases with the increase of the chloride diffusion coefficient and thickness of ITZ. It is also found that the aggregate gradation has a significant influence on the chloride diffusion coefficient of concrete.     

混凝土杨氏模量预测的三相复合球模型

提出了混凝土杨氏模量预测的三相复合球模型。在细观水平上将混凝土看成是一种由分散相骨料、中间相界面和连续相水泥浆所组成的三相复合材料。应用混凝土细观结构模拟技术, 获得了任一级配骨料的界面体积分数。模拟结果表明, 李氏近似法高估了界面体积分数, 界面体积分数在很大程度上取决于界面厚度、最大骨料直径和骨料级配。引入三相复合球模型, 给出了混凝土杨氏模量的解析解。将解析解与实验结果进行比较, 证实了本文模型的有效性。数值结果也表明, 对于给定的骨料体积分数, 混凝土杨氏模量随着最大骨料直径和界面杨氏模量的增大而增大, 但随着界面厚度的增大而减小, 骨料级配对混凝土杨氏模量也有较大影响。     

Computational homogenization of diffusion in three-phase mesoscale concrete

A three dimensional (3D) mesoscale model of concrete is presented and employed for computational homogenization in the context of mass diffusion. The mesoscale constituents of cement paste, aggregates and interfacial transition zone (ITZ) are contained within a statistical volume element (SVE) on which homogenization is carried out. The model implementation accounts for ITZ anisotropy thereby the diffusivity tensor depends on the normal of the aggregate surface. The homogenized response is compared between 3D and 2D SVEs to study the influence of the third spatial dimension, and for varying mesoscale compositions to study the influence of aggregate content on concrete diffusivity. The computational results show that the effective diffusivity of 3D SVEs is about 40 % greater than 2D SVEs when ITZ is excluded for the SVE, and 17 % when ITZ is included. The results are in agreement with the upper Hashin–Shtrikman bound when ITZ is excluded, and close to the Taylor assumption when ITZ is included.     

The ITZ in concrete – a different view based on image analysis and SEM observations

The traditional picture of the ITZ in concrete involves an approximately 30 μm zone around each aggregate, within which the porosity increases as the aggregate interface is approached. The results of the writers' extensive image analysis investigations, and examinations of SEM specimens from various concretes provide a very different picture. While the “wall effect” excluded much of the ground cement from the vicinity of the aggregates, the great increases in pore content within a few μm of the aggregate, up to approximately 30% porosity, that have been reported by others, are not found. On average, only modestly higher porosities are observed within the ITZ than in the bulk paste. This is true even in the innermost areas immediately adjacent to the aggregates. In part, the extra space produced by the wall effect is filled in by CH deposits, many of which are anchored directly on the surfaces of aggregates and are essentially non-porous. Strong indications exist that the ITZ contains as high a proportion of C–S–H per unit volume as the bulk paste; thus some of the extra space that was created by the wall effect is filled in by through-solution deposits of C–S–H derived from elsewhere in the system. It is considered that the structure of the ITZ in ordinary concretes is not different enough from that of the bulk cement paste to provide any basis for significant effects on permeance or mechanical properties.     

An n-layered spherical inclusion model for predicting the elastic moduli of concrete with inhomogeneous ITZ

An n-layered spherical inclusion model is presented in this paper for predicting the elastic moduli of concrete with inhomogeneous interfacial transition zone (ITZ). In this model, concrete is represented as a three-phase composite material, composed of the aggregate, bulk paste, and an inhomogeneous ITZ. An analytical solution for the ITZ volume fraction is derived for the general aggregate gradation. By constituting a semi-empirical initial cement gradient model, the local water/cement ratio, degree of hydration, and porosity at the ITZ are estimated. The inhomogeneous ITZ is then divided into a series of homogenous concentric shell elements of equal thickness. The elastic moduli of concrete are determined by solving the n-layered spherical inclusion problem. Finally, the validity of the model is verified with three independent sets of experimental data and the effects of the maximum aggregate diameter, aggregate gradation, and ITZ thickness on the Young’s modulus of concrete are evaluated in a quantitative manner. The paper concludes that the proposed n-layered spherical inclusion model can be used to predict the elastic moduli of concrete.     

考虑不均匀界面时混凝土弹性模量预测

提出了考虑不均匀界面时混凝土弹性模量预测的解析法。根据界面层上水泥颗粒的分布特性, 给出了界面层上任一点处的局部水灰比和孔隙率。将不均匀界面层划分成一系列同心球壳单元, 通过反演方法确定了每个球壳单元和水泥石基体的弹性模量。将三相混凝土分解成一系列两相复合子结构, 应用两相复合球模型的正确解导出混凝土弹性模量。通过与文献中的两组实验结果比较验证了本文方法的有效性。数值结果表明, 对于给定的骨料体积分数, 混凝土弹性模量随着最大水泥颗粒直径和水灰比的增大而减小, 但随着最大骨料直径的增大而增大, 骨料级配对混凝土弹性模量也有一定的影响。      

Crumb rubber aggregate coatings/pre-treatments and their effects on interfacial bonding, air entrapment and fracture toughness in self-compacting rubberised concrete (SCRC)

The interfacial-bonding, interfacial transition zone (ITZ), and porosity are regarded as the key factors affecting hardened concrete properties. The aim of this study was to experimentally improve the bonding between the rubber aggregate and cement paste by different methodologies including water washing, Na(OH) pre-treatment, and both cement paste and mortar pre-coating. All methods were assessed by determining mechanical and dynamic properties, then correlating this with ITZ porosity and interfacial gap void geometry, along with quantification of the fracture energy during micro crack propagation using fractal analysis. The results indicated that pre-coating the rubber by mortar gave the best results in terms of fracture toughness and energy absorption showing good agreement between observations made at both micro and macro scales.