Investigation of the Interfacial Transition Zone between Aggregate-Cement Paste by AC Impedance Spectroscopy

Three different types and sizes of coarse aggregate were chosen,and the alternating current(AC) impedance of cement paste samples with and without aggregate was measured at different curing ages.Based on Song's equivalent circuit model,the electrical properties from the AC impedance results were obtained,and the resistance of connected pores RCCP was used to characterize the microstructure of the interfacial transition zone(ITZ).The results show that the RCCP of concrete sample with aggregate is lower than that of cement paste sample,which indicates that the introduction of aggregate in cement paste makes the ITZ porous.Furthermore,for the same type of aggregate,an increase in particle size leads to a more porous ITZ,which accounts for the "water effect" and a larger aggregate would accumulate a thicker water film around it.In addition,for the same size of aggregate,the physical interaction between aggregate and cement paste is dominant in early age,and the microstructure of the ITZ around limestone aggregate is denser,which mainly depends on its rough surface and high water absorption.However,the microstructures of the ITZ around granite and basalt aggregates are denser in later age,which may be due to their higher chemical activity,and the chemical interaction between them and cement paste resulting in the generation of more hydrates.AC impedance spectroscopy thus proves to be powerful for evaluation of the microstructure of the ITZ.     

Quantitative Analysis and Affecting Factors of the Overlapping Degree of Interfacial Transition Zone between Neighboring Aggregates in Concrete

Based on the nearest surface function formula, a quantitative formula to measure the overlapping degree between the interfacial transition zone (ITZ) of neighboring aggregate particles was put forward. The formula was further deduced to quantitatively analyze the influence of the volume fraction of aggregate, ITZ thickness and the maximum aggregate diameter on the overlapping degree between neighboring ITZ. The volume of ITZ was quantitatively calculated in actual concrete by comparing the nearest surface f...     

Nanoindentation Characteristics of Cement Paste and Interfacial Transition Zone in Mortar with Rice Husk Ash

The nanostructure of cementitious materials has important effects on concrete properties. The effects of rice husk ash(RHA) on cement hydration product phases and interfacial transition zone(ITZ) in mortar were investigated from the nano-scale structure perspective. The experimental results indicate that, with the increase of RHA dosages of samples, the volume fraction of high-density calcium-silicate-hydrate(HD C-S-H) in porosity and hydration product phases increases. The volume fractions of HD C-S-H in C-S-H of samples show an increasing trend with the increase of RHA dosages. RHA decreases the thickness of ITZ and increases the matrix elastic moduli of samples, however, the RHA dosoges hardly affect the thickness and elastic moduli.     

Improvement of the FRP sheet bonded to concrete substrate by silane coupling agent

We experimentally studied the fine lightweight aggregate with the particle size range of 3.15-4.75 mm used as functional bridge between FRP sheet and concrete substrate. However, problems would appear and how to deal with the interfacial transition zone (ITZ) and make it stronger is the key point for this concept. Considering that silane coupling agent (SCA) can provide a better bond on a silicon-containing material surface, it was introduced as a modifying material to further improve the bond quality of the ITZ between lightweight aggregate and cement paste. Results indicated that the water absorptivity of lightweight aggregate can be controlled by SCA solutions, and the pull-off bond strength, mechanical strength, and microhardness were increased, which was attributed to the optimized microstructure under the condition of an appropriate concentration of SCA.     

老混凝土中骨料-水泥界面过渡区(ITZ)(Ⅱ)——元素在界面区的分布特征

利用电子探针（EPXM）对骨科颗粒和水泥浆体界面过渡区（ITZ）进行了研究，研究的重点是化学元素在ITZ附近的分布情况，试验结果表明：Ca、K和Fe有不同程度地集中于界面过渡区的趋势，而Si的浓度在界面处相对比较低，但随着远离界面其浓度逐渐升高。Mg、Al等元素在ITZ的局部出现，且有不连续分布的特点。通过对老混凝土中ITZ元素分布的分析，可以部分地了解混凝土内部所发生的离子和物质迁移，骨科和水泥浆体之间发生的化学反应，从而为评估混凝土的化学稳定性乃至其耐久性提供依据。     

Mechanical Properties and ITZ Microstructure of Recycled Aggregate Concrete Using Carbonated Recycled Coarse Aggregate

The effect of carbonation treatment and mixing method on the mechanical properties and interfacial transition zone (ITZ) properties of recycled aggregate concrete (RAC) was investigated. Properties of recycled concrete aggregate (RCA) were tested firstly. Then, five types of concretes were made and slump of fresh concrete was measured immediately after mixing. Compressive strength and splitting tensile strength of hardened concrete were measured at 28 d. Meanwhile, the microstructure of RAC was analyzed by backscattered electron (BSE) image. It was found that the water absorption ratio of carbonated recycled concrete aggregate (CRCA) was much lower when compared to the untreated RCA. Comparatively, the apparent density of CRCA was not significantly modified. The concrete strength results indicate that the mix CRAC-2 prepared with CRCA by adopting two-stage mixing approach shows the highest compressive strength value compared to the other mixes. The microstructural analysis demonstrate that the mix CRAC-2 has a much denser old ITZ than the untreated RAC because of the chemical reaction between CO₂ and the hydration products of RCA. This study confirms that the ITZ microstructure of RAC can be efficiently modified by carbonation treatment of RCA and encourages broadening the application of construction and demolition wastes.     

超细矿粉对混凝土界面区硫酸盐腐蚀的影响

为了研究骨料与水泥石界面区(ITZ)的组成及结构对混凝土抗硫酸盐腐蚀性能的影响,利用紫外分光光度计绘制了硫酸钠溶液浸泡混凝土试样的SO2-4浸入浓度/深度曲线,并辅以扫描电镜(SEM)和电子探针能谱(EDS)等对ITZ进行观测;同时利用XRD定量分析研究了混凝土本体相和界面相的水化产物富集的差异.结果表明:Ca(OH)2、钙矾石(AFt)等晶体富集于硫酸钠溶液浸泡后试样的ITZ,引起AFt二次结晶型的硫酸盐腐蚀破坏.在适量石膏的激发下,掺入超细矿粉(GGBS)可阻止AFt转变成单硫型水化硫铝酸钙(AFm),减少AFt二次结晶的生成,明显提高抗硫酸盐侵蚀能力,试样的同深度硫酸根浓度最低,抗硫酸盐侵蚀性能最好.     

再生细骨料及其混凝土的微观结构特征

采用扫描电镜、能谱分析、X射线衍射、显微硬度和氮吸附等微观测试方法研究了再生细骨料及其混凝土的微观结构特征.研究结果表明:再生细骨料是一种组成复杂的、具有一定水化活性的和高渗透性的人造骨料,其主要矿物相为SiO2、CaCO3以及少量的C2S.再生细骨料混凝土内部水泥石孔隙较多,结构密实性较差,同时其与再生细骨料间存在较为明显的界面过渡区,该界面过渡区宽度较大,且界面过渡区两侧的骨料和水泥石的显微硬度均较低.再生细骨料的多孔结构,以及再生细骨料混凝土内部水泥石和界面过渡区微观结构缺陷是导致其大孔增多的主要原因,大孔的增多会对混凝土抗渗性产生不利影响.     

混凝土材料的细观结构数值模拟与性能分析

在细观上将混凝土看作是由粗骨料、水泥砂浆和界面三相介质组成的复合材料,根据体视学原理并应用蒙特卡洛方法模拟出混凝土的二维圆形骨料分布;应用理论公式计算出界面厚度,并将界面看作是包围在骨料外面的一层圆环,在细观上模拟生成混凝土的三相组成;基于骨料随机模型,应用有限单元法提出了混凝土稳态氯离子扩散系数的计算方法并与试验结果进行了对比。结果表明,该模型用于计算混凝土稳定氯离子扩散系数是有效的。     

钢筋非均匀锈蚀引发的混凝土保护层开裂细观数值研究

混凝土保护层锈裂严重影响钢筋混凝土结构的耐久性。为了研究混凝土保护层的锈裂行为,考虑到混凝土细观结构的非均质性以及钢筋锈蚀的非均匀性,将完好混凝土视为由骨料、砂浆和界面过渡区组成的三相复合材料,建立了细观随机骨料模型。在模型中,钢筋的非均匀锈蚀行为以施加非均匀径向位移的方式模拟,骨料的力学行为假定为弹性,砂浆和界面过渡区的力学特性采用塑性损伤模型来描述。在此基础上进行了中部钢筋非均匀锈蚀引发的混凝土保护层开裂行为的细观数值模拟;分析结果与已有文献中的试验结果吻合良好。另外,对比了均质模型和非均质模型中钢筋均匀锈蚀和非均匀锈蚀导致的保护层开裂模式;并探讨分析了保护层厚度和钢筋直径对保护层开裂模式、钢筋锈胀压力及保护层开裂时钢筋锈蚀率的影响。     

再生集料混凝土的微观结构特征

研究了两种不同类型的再生混凝土集料(RC集料)和一种花岗岩集料配制的混凝土的微观结构特征，尤其是集料和水泥浆体之间的界面过渡区的某些结构细节。结果表明，界面过渡区的微观结构特征与集料的密实程度有关，过于密实(天然集料)和过于疏松(低标号混凝土)的集料可引起界面区的多孔性或聚集粗大颗粒的水化产物，而适中密实程度的集料可形成较为密实的界面区。     

水泥基复合材料界面过渡区体积分数的定量计算

为了计算水泥基复合材料的界面过渡区体积分数,采用最邻近表面分布函数和骨料比表面积2种方法,结合实际骨料的筛分曲线,给出了界面过渡区体积分数定量关系式．结果表明：单位体积水泥基复合材料中,当骨料的体积分数〈0．5,界面厚度〈50μm时,2种方法计算的误差约为10％;界面区厚度越小,2种方法计算的误差也越小．     

Modificatin of ITZ Structure and Properties of Regenerated Concrete

1Introduction Increasingattentionispaidtotheinvestigationand applicationofregeneratedconcrete(RC)forthesustain abledevelopmentofsociety[13].TheRCispreparedwith demolitionwasteconcreteoroldconcreteastherecycled aggregate(RA).RAcontainsacertainamountofhard enedcementmortar,someofwhichhaveaformofinde pendentlumps,andothersadhereontothesurfaceofnat uralaggregate.SinceRAhasagreaterporosityanda higherwaterabsorptioncapacitythannaturalaggregate,whichhasagreaterinfluenceontheperformanceofRC;conse…     

开裂混凝土中氯离子扩散行为的细观数值模拟

为了研究氯离子在开裂混凝土中的扩散性能,考虑混凝土细观结构的非均质性,将含有裂纹的混凝土试件视为由砂浆基质、骨料、界面过渡区以及裂纹组成的四相复合材料,建立了带有单条裂纹的二维细观有限元模型.数值模型中,骨料不具有渗透能力,砂浆基质和界面中的氯离子扩散性能由水灰比确定,裂纹相中氯离子扩散系数与裂纹宽度的定量关系由文献中已有试验数据的拟合获得.研究了骨料分布形式对氯离子扩散行为的影响,分析并探讨了不同裂纹宽度下混凝土中的氯离子扩散行为.数值结果与试验数据的良好吻合说明了该方法的准确性.研究结果表明:骨料的随机分布形式基本不影响混凝土中氯离子的扩散行为;裂纹宽度小于50μm时,混凝土中氯离子的扩散行为基本不受裂纹的影响;裂纹宽度介于50~170μm时,氯离子的扩散随裂纹宽度增大而明显加剧;裂纹宽度大于170μm时,氯离子在裂纹附近的扩散趋于二维扩散形态.     

Test method to simulate the influence of the interface on the concrete carbonation process

A trial test method is introduced to form and magnify regular interface. Through researching on the carbonation of the magnifying interfacial transition zone (ITZ), the practical carbonation of the concrete can be simulated. Because the diffusion rate of CO₂ in the ITZ is several times greater than that in the bulk paste, the diffusion rate and direction of CO₂ will change and form a new carbonation front line. An interfacial effect zone caused by the ITZ will change the distribution of the complete carbonation zone and the partial carbonation zone. One of the important reasons for the formation of the partial carbonation zone was the existence of the interfacial effect zone. Consequently, the method mentioned in this paper provides a new way for researching on the microstructure of the cement based materials during the carbonation process.     

混凝土界面体积百分比的计算方法

讨论了混凝土界面体积百分比的计算.根据混凝土细观结构模拟技术,提出了混凝面体积百分比的计算方法.计算结果表明,对于给定的骨料体积百分比,界面体积百分比着骨料最小直径的减小而增大;对于给定的骨料最小直径,界面体积百分比随着骨料体积百分比的增大而增大.基于这些模拟结果,给出了界面体积百分比的计算公式.     

基于XFEM的骨料分布对混凝土有效模量的影响

混凝土的弹性模量是结构设计中最基本的参数,在计算方法上需要保证足够的准确性。基于现有的3层球壳解析模型,将骨料、界面过渡区和薄层砂浆组成的整体先行处理为等效夹杂体,并根据解析模型获得相应的弹性模量,随后将混凝土视为由此等效而来的夹杂体与基体砂浆组成的二相复合材料。在固定背景网格体系中,采用无附加自由度的扩展有限元方法计算不同级配和骨料含量的混凝土等效弹性模量。当混凝土的骨料分布发生变化时,无附加自由度的扩展有限元方法能够在程序设计上避免整体刚度矩阵的维度变化问题。此外,为了能在获得不同的骨料分布的同时获得较高的骨料含量,混凝土的级配采用固定的粒径分布形式。结果表明：骨料含量同为47.12%的3种级配的混凝土弹性模量基本相同,说明级配对混凝土弹性模量的影响可以忽略;解析解可较准确地预测稀疏骨料分布情况下的混凝土等效弹性模量,但骨料增加后其预测值出现较大偏差;混凝土弹性模量随硬骨料含量的增加线性增大,随软骨料含量的增加线性减小。     

Multi-scale modeling of the effective chloride ion diffusion coefficient in cement-based composite materials

N-layered spherical inclusions model was used to calculate the effective diffusion coefficient of chloride ion in cement-based materials by using multi-scale method and then to investigate the relationship between the diffusivity and the microstructure of cement-basted materials where the microstructure included the interfacial transition zone（ITZ）between the aggregates and the bulk cement pastes as well as the microstructure of the bulk cement paste itself.For the convenience of applications,the mortar and concrete were considered as a four-phase spherical model,consisting of cement continuous phase,dispersed aggregates phase,interface transition zone and their homogenized effective medium phase.A general effective medium equation was established to calculate the diffusion coefficient of the hardened cement paste by considering the microstructure.During calculation,the tortuosity（n）and constrictivity factors（D_s/D₀）of pore in the hardened pastes are n≈3.2,D_s/D₀=1.0×10^-4respectively from the test data.The calculated results using the n-layered spherical inclusions model are in good agreement with the experimental results;The effective diffusion coefficient of ITZ is 12 times that of the bulk cement for mortar and 17 times for concrete due to the difference between particle size distribution and the volume fraction of aggregates in mortar and concrete.     

混凝土界面过渡区不均匀特性研究

在新拌混凝土制备过程中,因混凝土内部的微泌水效应和宏观泌水作用会对处于不同位置的骨料周围区域内产生水分的不均匀分布,进而影响界面过渡区的均匀性。利用显微硬度测试技术研究了混凝土界面过渡区的不均匀特性。结果表明,单骨料上、下以及侧面等不同界面处显微硬度值存在较大差别,其中上界面显微硬度值最大。表层骨料周围界面过渡区宽度略大于内部骨料界面过渡区;硅灰的掺加能明显改善混凝土界面过渡区的不均匀性。     

再生混凝土ITZ结构与性质的研究

采取降低水灰比 ,掺入适量粉煤灰、高效减水剂和对再生集料表面进行处理等措施 ,通过 SEM研究了再生混凝土界面过渡区 (ITZ)的结构与性质。试验结果显示 :降低水胶比 ,掺入 2 0 %的粉煤灰和 2 .5 %高效减水剂 (对水泥 ) ,可使再生混凝土 ITZ结合更加紧密 ,抗压强度得以提高。在同样条件下 ,采用 1% PVA溶液处理过的再生集料 ,有利于提高新拌再生混凝土的流动性 ,同时 ,硬化混凝土的强度也有所提高