混凝土中邻近集料表面最近间距分布的计算机模拟

混凝土中邻近集料问界面过渡区的相互影响程度、混凝土中原生裂纹的尺度范围以及中心质假说中集料效应圈的范围等都涉及到混凝土中最邻近集料表面间距分布的问题。由于常规的实验方法无法给出混凝土中集料空间分布的信息,同时以往的计算机模拟方法由于采用随机分布方式分布粒子,导致无法得到较高集料体积分数的模型混凝土结构,而采用具有粒子动态混合密实功能的SPACE系统,模拟了高集料体积分数混凝土的结构。在假定模型混凝土中集料的最小粒径为1mm的前提下,以符合Fuller分布为例,研究了模型混凝土中集料粒径分布和集料体积分数对邻近集料表面最近间距分布的影响。     

计算混凝土中平均最邻近集料表面间距的正十二面体模型

带状模型是以解析解的方式研究混凝土中集料尺寸分布和集料体积分数对平均最邻近集料表面间距影响的模型。但是,由带状模型的计算公式计算浆体的体积无法得到正确的结果。鉴于上述缺点,提出采用单一粒径球形粒子体系的正十二面体模型。以符合Fuller分布集料为例,结合数量基平均粒径方法,采用正十二面体模型研究了集料尺寸分布和集料体积分数对平均最邻近集料表面间距的影响。研究结果表明:随着集料体积分数的增加和集料细度的提高,平均最邻近集料表面间距减小;对所研究的集料体积分数(60％～75％)和集料尺度分布(Fuller分布)而言,平均最邻近集料表面间距值在90～232μm之间变化;在给定的集料分布下,集料体积分数在55％～75％范围内,平均邻近集料表面间距与集料体积分数之间近似呈线性关系。     

集料形状和尺寸对混凝土边界效应的影响

为了探讨集料对混凝土边界效应的影响机理,利用计算机模拟技术,分析不同形状和尺寸的单尺度椭圆形集料随机堆积对混凝土边界效应的影响。结果显示：椭圆形集料的形状和尺寸的变化不影响集料固相体积分数曲线的形状,均可以分为先上升、后下降、再上升、最后趋于稳定的曲线,趋于稳定时的集料固相体积分数约等于集料的面积分数。通过统计和分析得...     

微乳液法制备无定形纳米二氧化硅

采用Triton X-100／正己醇／环己烷／氨水体系配制反相微乳液，在碱性条件下正硅酸乙酯在反相微乳液中发生受控水解，合成了具有无定形结构的球形二氧化硅纳米粒子。通过红外光谱（FT-IR）、X衍射（XRD）、透射电镜（TEM）分别对样品的结构及形貌尺寸进行了表征和分析。结果表明：改变表面活性剂加入量可以得到不同粒径（50～110nm）、不同粒度分布及不同分散程度的球形二氧化硅纳米粒子。随着表面活性剂在微乳液中体积分数的增大，二氧化硅纳米粒子的粒径先减小后增大，团聚程度也呈现先减小后增大的趋势。当表面活性剂在微乳液体系中的体积分数为20％时，所合成的二氧化硅纳米粒子粒径最小（50nm），粒度均匀且呈现出良好的分散性。     

聚合物改性水泥砂浆界面过渡区的交流阻抗谱研究

水泥砂浆可视为由水泥浆体、细集料和水泥浆体－－休料界面过渡区（interfacial transition zone，简称ITZ）所组成，聚合物改性水泥砂浆中，较小的聚合物颗粒有利于休料表面的紧密堆积，用交流阻抗方法研究了不同砂子粒径和不同砂子体积分数的聚合物改性砂浆在不同龄期的表现，结果表明：交流阻抗方法是研究聚合物改性砂浆界面过渡区发展变化的一种新手段，聚合物改性砂浆的阻抗谱随龄期延长出现二次明显变化，这种特征分别与聚合物在砂浆界面过渡区的聚集、凝结、形成紧密堆积的颗粒结构以及聚合物颗粒相互扩散渗透，形成具有一定力学强度的过程有关。同时，聚合物在界面过渡区的成膜情况与砂子粒径及其体积分数有关，从而在交流抗谱上可以观察到相应的响应。     

水泥基复合材料浆体厚度分布的定量表达

依据统计物理学领域中的弦长密度函数,提出了水泥基复合材料邻近集料间浆体厚度分布的定量估计方法.借助计算机模拟方法生成的模型结构验证该理论方法的可靠性.通过浆体厚度的数量基概率密度曲线和累计概率曲线的比较发现,理论解和模拟结果之间能较好吻合.在常用集料体积分数下,用该解析解预测了集料符合Fuller分布的砂浆和混凝土中浆体厚度分布及其平均值随集料体积分数和集料细度的变化关系.结果发现:邻近集料间浆体厚度平均值的变化范围为190～860 μm.     

高岭土悬浮液的流变性质

<正>1引言高岭土在水中分散形成絮凝悬浮液,此体系中固相的体积分率、粒子形状和粒径分布、表面电荷和流动类型等,都在不同程度上影响其流变特性。有关此问题的研究工作尚不多。文献中已有的悬浮液流变模型大都是针对刚性球形粒子形成的悬浮液,粒于之间无相互作用,此时影响悬浮液表观粘度的主要因素是固相体积分率。     

骨料对氯离子在水泥基复合材料中扩散系数的影响

为了确定骨料对氯离子在水泥基复合材料中扩散系数的影响,利用压汞技术和稳态电迁移法分别对含不同类型、不同粒径分布、不同体积分数骨料的砂浆和混凝土试样,进行了孔结构和氯离子扩散系数的测试,并根据试样配合比、骨料的粒径分布以及界面过渡区(简称界面区)厚度进行了界面区体积分数计算,最后提出了界面区有效扩散系数的预测模型。结果表明:氯离子在水泥基复合材料中的扩散系数由基体扩散系数、界面区扩散系数、骨料以及界面区的体积决定,而界面区的体积分数主要取决于骨料的粒径分布、骨料体积和界面区厚度;骨料改变了水泥基复合材料中浆体的孔结构,其稀释效应和曲折效应降低了氯离子的传输性能;界面区特殊的微观结构增加了传输性能,其中,界面区效应要大于其曲折和稀释效应。根据试验结果采用回归分析法,得到氯离子在砂浆和混凝土界面区的扩散系数分别是基体的13.26倍和18.45倍。     

聚丙烯纤维增强水泥基复合材料界面过渡区的纳米力学性能

研究了不同水灰比条件下聚丙烯纤维增强水泥基复合材料界面过渡区(ITZ)的纳米力学性能。通过纳米压痕试验测试界面过渡区及其附近区域的载荷–深度(P–h)曲线,运用Oliver-Pharr方法得到各测点弹性模量和压痕硬度,采用高倍显微镜成像与扫描电子显微镜技术绘制了界面过渡区微结构表征云图。利用反褶积法对水泥基体及界面过渡区的弹性模量及压痕硬度进行多峰拟合,得到了各相的频率分布。结果表明:不同水灰比条件下界面过渡区的厚度均处于10~20μm之间,且其弹性模量和压痕硬度均低于水泥基体的;随着水灰比的变化,界面过渡区水化产物的成分及体积分数也产生相应改变。     

再生集料及再生混凝土界面过渡区研究进展

再生集料及界面过渡区的性质对再生混凝土性能有重要影响.基于国内外再生混凝土的研究现状,分析了再生集料性质与界面过渡区的微观结构对再生混凝土物理力学性能的影响,总结了再生混凝土性能的改善措施.根据再生混凝土现阶段存在问题,提出今后应注重再生集料加工工艺优化、再生集料分级评估、界面过渡区微观结构及再生混凝土耐久性等方面的研究.     

Influence of aggregate size, water cement ratio and age on the microstructure of the interfacial transition zone

This paper presents the results of an investigation on the effect of water-cement ratio (w/c), aggregate size, and age on the microstructure of the interfacial transition zone (ITZ) between normal weight aggregate and the bulk cement paste. Backscattered electron images (BSE) obtained by scanning electron microscope were used to characterize the ITZ microstructure. The results suggest that the w/c plays an important role in controlling the microstructure of the ITZ and its thickness. Reducing w/c from 0.55 to 0.40 resulted in an ITZ with characteristics that are not distinguishable from those of the bulk paste as demonstrated by BSE images. Aggregate size appears to have an important influence on the ITZ characteristics. Reducing the aggregate size tends to reduce the ITZ porosity. The evolution of the ITZ microstructure relative to that of the bulk paste appears to depend on the initial content of the unhydrated cement grains (UH). The results suggest that the presence of a relatively low amount of UH in the ITZ at early age may cause the porosity of the ITZ, relative to that of the bulk paste, to increase with time. The presence of relatively large amount of UH in the ITZ at early ages may cause its porosity, relative to that of the bulk paste, to decrease with time.     

Prediction of diffusivity of concrete based on simple analytic equations

Proposed is a simple analytic model that can predict realistically the diffusivities of concrete and mortar. The basic concept of the model comes from the relation between the diffusivities and the microstructure of concrete. The microstructure that affects the diffusivity includes the interfacial transition zone (ITZ) between aggregates and cement pastes as well as the microstructure of cement paste itself. The general effective media (GEM) equation was introduced to derive the diffusivity of cement paste. The effective diffusivity of concrete is derived on the basis of the composite sphere assemblage model, which considers the diffusivities of both ITZ and cement paste. The main parameters in the proposed model are the microstructural properties of cement paste such as capillary porosity and pore structure parameter, solid phase diffusivity, aggregate volume fraction, and interfacial zone properties. To validate the proposed model, many series of concrete and mortar specimens have been tested to measure the diffusivities. The major test variables include the water-to-binder ratios, the types and amount of mineral admixtures on the diffusivities. The effects of compressive strength, water-to-binder ratio, and mineral admixtures have been investigated comprehensively. The comparison of the proposed theory with the test data exhibits reasonably good correlation. The proposed model allows more accurate prediction of diffusion process and, thus, more realistic durability design of concrete structures.     

An inverse method to determine the elastic properties of the interphase between the aggregate and the cement paste

Concrete is a three-phase material consisting of cement paste matrix, discrete inclusions of rock (aggregate), and an interfacial transition zone (ITZ) between the matrix and the inclusions. We model the material as a composite formed by a matrix with embedded spherical particles; each surrounded by a concentric spherical shell. Effective elastic moduli of this composite are evaluated on the basis of the generalized self-consistent scheme (GSCS). This formulation is used to solve the inverse problem of determining the elastic moduli of the ITZ from experimentally known elastic properties of the composite.     

Influence of lightweight aggregate on the microstructure and durability of mortar

The results of an investigation on the effect of dry and prewetted lightweight aggregates on the microstructure and durability of mortar are presented in this paper. The results are compared with those obtained for normal aggregate mortar. There appears to be only a small difference in the microstructure of the interfacial transition zone (ITZ) between dry and prewetted lightweight aggregate mortars. The porous ITZ surrounding lightweight aggregate appears to extend for about 10 and 15 μm from the aggregate surface for dry and prewetted lightweight aggregates, respectively. The ITZ for dry and prewetted lightweight aggregates seems to be surrounded by dense paste that extends from 10 to about 50 μm from the aggregate surface. This dense paste has lower porosity than that observed in the bulk paste located 50 μm and farther from aggregate surface. The normal aggregate mortar prepared with the same water/cement ratio appears to have porous ITZ that extends beyond 35 μm from the aggregate surface. The dry and prewetted lightweight aggregate mortars seem to have a lower sorptivity and electrical conductivity than does the normal aggregate mortar. Lightweight aggregate mortars also appear to have excellent resistance to sulfate attack as compared with normal aggregate mortar.     

Assessing the influence of ITZ on the steady-state chloride diffusivity of concrete using a numerical model

In this study, the influence of the aggregate-cement paste interfacial transition zone (ITZ) on the steady-state chloride diffusivity of mortars and concretes was examined using a semi-empirical, three-phase composite sphere model. Mortars and concretes were modelled as three-phase composites consisting of the aggregate, bulk cement paste and an inhomogeneous ITZ. The latter was divided into a series of homogenous concentric shell elements of equal thickness. The initial porosity and cement gradients at the ITZ were first estimated from the overall water/cement ratio (w₀/c). The evolution of the porosity, solid hydration products and remnants of unreacted cement were then calculated from the hydration degree and local water/cement ratio (w/c) using Powers' empirical model. Based on the Laplacian equation, an element transfer matrix was derived analytically to predict the steady-state chloride diffusivity. The model was calibrated using the available experimental data and then applied to perform a sensitivity analysis to evaluate the effects of aggregate content, water/cement ratio, curing period, ITZ width, maximum aggregate size and aggregate gradation on diffusivity. Some of these variables are impractical to quantify by laboratory experimentation. Implications of the findings with regard to the role of ITZ on mass transport properties are discussed.     

Water-cement ratio gradients in mortars and corresponding effective elastic properties

Water-cement ratio gradients are modeled through the interfacial transition zone (ITZ) of a mortar with spherical inclusions. The model is a function of the over-all water-cement ratio, volume fraction and radius of sand, specific gravity of cement and thickness of ITZ. Based on experimental data from the literature, the dependence of saturated, homogeneous cement paste is modeled as a function of water-cement ratio. Subsequently, the effective bulk and shear moduli for mortars are determined using a generalized self-consistent method. Finally, application of the model to data in the literature pertaining to elastic wave speeds in saturated mortars composed of 20-30 screened sand with an overall water-cement ratio of 0.3 yielded a mean ITZ thickness of 48.3 μm.     

A multiscale model for effective moduli of concrete incorporating ITZ water-cement ratio gradients, aggregate size distributions, and entrapped voids

This paper develops a model for the effective elastic properties of concrete, which is a function of the volume fractions, size distributions, and elastic properties of fine aggregate (FA) and coarse aggregate (FA) and entrapped voids. Furthermore, the model is a function of the overall water-cement ratio and specific gravity of cement. Explicitly modeled are the water-cement ratio gradients through the interfacial transition zone (ITZ), which, in turn, affect the variation of the cement paste elastic properties through the ITZ, while maintaining the total fractions of cement and water consistent with the overall water-cement ratio. The ITZ volume is also conserved.     

自养护路面混凝土抗盐冻性能及疲劳特性

为有效增强路面混凝土耐久性能,基于盐冻试验、盐冻前后的断裂性能试验及弯拉荷载疲劳试验,探索了高吸水性聚合物(SAP)自养护路面混凝土抗盐冻性能及疲劳特性随SAP掺量、粒径的变化规律,并结合自养护水泥浆体孔隙参数、微观形貌及骨料-水泥石界面过渡区(ITZ)特征,揭示了性能影响机理。结果表明:小粒径SAP形成的残留孔洞能有效释放拉应力,降低结冰点,细化孔结构,从而增强路面混凝土抗盐冻性能;当SAP粒径为100目(150 μm),掺量为0.145%(质量分数)时,路面混凝土在冻融30次时的断裂韧度损失率、断裂能损失率分别比基准组降低了25.25%、10.51%;小粒径SAP对疲劳寿命的提升程度随应力水平的提高而增大,当应力水平为0.80时,自养护组的疲劳寿命相比基准组提升了2.65倍;SAP能够有效提升水泥混凝土结构内部密实度,吸持ITZ区域部分水分,增强水泥石和骨料之间的粘结性,从而改善混凝土抗盐冻性能和疲劳特性。     

Microstructural characterization of ITZ in blended cement concretes and its relation to transport properties

The improvements in the overall performances of concrete with blended materials were often ascribed to the modification of its hardened paste in general. In this paper, the effects of limestone filler (LF) and slag (GGBS) on chloride migration and water absorption of concretes with systematically varied aggregate properties were evaluated from the view point of ITZ by using BSE image, EDS, and MIP analysis. It was found that the incorporation of moderate amount of LF and GGBS would compact the microstructure of both ITZ and bulk cement matrix. The reduction in the pore volume (> 100 nm) contributes to the largest decrease in total porosity. Additionally, incorporating GGBS avoids the build-up of Ca(OH)₂ within ITZ and provides a more uniform microstructure. The mechanism for the improvement in limiting water and ions penetration was found to be mainly related to the densification of bulk cement matrix rather than the modification of ITZ.     

Aggregate-cement paste transition zone properties affecting the salt-frost damage of high-performance concretes

The influence of the cement paste-aggregate interfacial transition zone (ITZ) on the frost durability of high-performance silica fume concrete (HPSFC) has been studied. Investigation was carried out on eight non-air-entrained concretes having water-to-binder (W/B) ratios of 0.3, 0.35 and 0.42 and different additions of condensed silica fume. Studies on the microstructure and composition of the cement paste have been made by means of environmental scanning electron microscope (ESEM)-BSE, ESEM-EDX and mercury intrusion porosimetry (MIP) analysis. The results showed that the transition zone initiates and accelerates damaging mechanisms by enhancing movement of the pore solution within the concrete during freezing and thawing cycles. Cracks filled with ettringite were primarily formed in the ITZ. The test concretes having good frost-deicing salt durability featured a narrow transition zone and a decreased Ca/Si atomic ratio in the transition zone compared to the bulk cement paste. Moderate additions of silica fume seemed to densify the microstructure of the ITZ.