砂浆中邻近集料表面最近间距分布的数值模拟

采用具有粒子动态混合密实功能的SPACE 系统, 实现了高集料体积分数模型砂浆结构的生成。从而, 以3 种细集料粒径分布的模型砂浆(其中一种的粒径范围为0.125～1.34 mm , 另外两种的粒径范围为0.25～5.00 mm) 结构为例, 研究了集料细度和集料体积分数(φ= 40 %～70 %) 对邻近集料表面最近间距分布的影响。结果表明, 集料细度增加和集料体积分数增大都会使大尺度邻近集料表面最近间距出现的概率减小、小尺度邻近集料表面最近间距出现的概率增加, 且峰值概率的位置向小间距方向偏移。另外, 邻近集料表面最近间距的分区段累计概率结果分析表明, 3 种模型砂浆结构的邻近集料表面最近间距分布的57 %以上小于10μm。最后, 邻近集料表面最近间距的平均值的分析结果显示, 当集料体积分数在40 %～70 %之间变化时, 邻近集料表面最近间距的平均值在54.6～1.1μm 之间变化; 当砂浆中集料的体积分数在50 %～70 %之间变化时, 邻近集料表面最近间距的平均值与集料的体积分数基本上呈线性关系; 单位砂浆体积下集料的表面积的变化对邻近集料表面最近间距的平均值有影响, 但二者之间的比值并非常数。     

新拌水泥浆体中邻近胶凝材料粒子表面最近间距分布的解析解

依据Torquato的最邻近表面函数公式, 推导了新拌水泥浆体邻近水泥粒子表面最近间距分布的解析解, 并采用计算机模拟生成了1个水泥浆体结构模型, 选取邻近粒子表面最近间距概率密度分布曲线、 区间概率曲线和累计概率分布曲线这3个函数检验模拟结果与理论结果之间的差别。结果表明, 该解析解与计算机模拟实验结果吻合良好。利用该解析解, 以Rosin-Rammler分布为例, 研究了胶凝材料细度以及水胶比对邻近水泥粒子表面最近间距分布及其平均值的影响。结果显示, 水泥细度对邻近粒子表面最近间距平均值的影响高于水胶比的影响。邻近粒子表面最近间距解析解的获得, 不但为基于不同粒径分布以及尺度范围粉体之间搭配的高粉体初始堆积密度的实现提供了依据, 而且对多尺度模拟方法中选取各级模型的尺度、 量化粉煤灰浆体中粉煤灰颗粒的微集料效应、 了解邻近集料间的交互作用程度以及界面重叠程度等问题具有参考价值。     

截面分析法对任意凸形粒子周围界面过渡区厚度过高估计的解析解

借助于几何概率中的一些相关理论, 从理论分析的角度, 对表面为凸形曲线或曲面的二维和三维任意形状粒子建立了表观界面过渡区厚度统计平均值和实际界面过渡区厚度之间的定量关系。结果表明, 对于二维凸形粒子, 截面分析法所得的表观界面过渡区厚度的统计平均值与实际界面过渡区厚度之间的比值小于π/ 2 。精确比值与粒子的形状有关。如果实际界面过渡区厚度与粒子的尺寸相比可以忽略的话, 截面分析法所得的表观界面过渡区厚度的统计平均值与实际界面过渡区厚度之间的比值约等于π/ 2 。对于三维凸形粒子, 截面分析法所得的表观界面过渡区厚度的统计平均值与实际界面过渡区厚度之间的比值小于2 , 比值的精确解与粒子的形状有关。如果实际界面过渡区厚度与粒子的尺寸相比可以忽略的话, 截面分析法所得的表观界面过渡区厚度的统计平均值与实际界面过渡区厚度之间的比值约等于2 。      

超早强聚羧酸对低坍落度混凝土流变与气泡结构经时变化的影响

分别采用两种超长侧链聚羧酸减水剂与一种常规聚羧酸减水剂制备低坍落度混凝土,通过测试分析湿筛砂浆流动度、流变性、气泡结构参数以及三维重构气泡形态,对比研究了超长侧链聚羧酸减水剂对混凝土工作性能与气泡特征经时变化的影响.结果表明:低坍落度混凝土湿筛砂浆符合宾汉姆流体特征,流动度与屈服应力具有强线性相关性,减水剂种类对其影响较小,但超长侧链聚羧酸对砂浆流动度的经时保持作用明显弱于普通聚羧酸.超长侧链聚羧酸在砂浆中的引气数量和小孔径气泡占比均低于普通聚羧酸,并且超长侧链聚羧酸加速了砂浆中小孔径气泡向大孔径气泡的转变.CT三维重构试验直观地证实了砂浆中相邻小气泡聚并成大气泡现象的存在,超长侧链聚羧酸使得砂浆中气泡聚并发生的时间比普通聚羧酸更早.     

一种大型锻件外形尺寸在线测量新方法

提出了一种大型锻件外形尺寸的在线测量新方法.首先通过激光扫描仪扫描锻件表面的信息,将获得的扫描仪到锻件表面的距离信息转化为坐标信息,然后进行点云拼接、数据精简和曲面重构处理,最后导入SolidWorks软件中构建锻件的三维模型,从中提取截面,给出锻件的尺寸信息,实现大型锻件外形尺寸的在线测量.     

基于计算机模拟的正常晶粒长大的三维特征和二维截面表征

用适当的三维ＭｏｎｔｅＣａｒｌｏ技术模拟了较完整的单相材料正常晶粒长大过程，获得了晶粒长大动力学和拓扑学的全面信息，对正常晶粒长大的三维特征和相应的二维截面有征进行对应分析表明：二维截面上测定的时间指数，具有三维系统一致随时间向稳态晶粒长的大理论值（ｎ＝０．５）趋近的特征，由二维截面单位面积上的晶粒人与时间的定量关系可以推断三维系统晶粒长大是否达到准确稳态，二维截面上的晶粒尺寸分布不能代表真实的三     

异形热态锻件几何尺寸测量方法的研究

提出一种异形热态锻件几何尺寸测量方法。基于线激光器、CCD和伺服系统构建线激光扫描测量系统,通过提取图像上被锻件表面轮廓调制的激光条纹二维信息,经过坐标变换及点云三维重构得到锻件三维尺寸信息。针对被测异形锻件形貌特征,建立基于摄像机运动的扫描测量模型,并提出一种基于棋盘参考平面的摄像机光心轨迹求解方法。根据平面上特征点在CCD图像不同帧中的坐标拟合摄像机的运动轨迹,结合扫描测量模型实现对异形锻件完整轮廓尺寸的测量。通过实验对封头进行扫描测量,得到球冠部分截面测量直径误差小于4 mm,满足热态锻件测量要求,验证了该方法的可行性。     

基于Pro／E的人体特征尺寸提取

根据三维服装CAD系统对三维数字化人体尺寸特征信息的需要,提出了一种提取人体特征尺寸的新方法.该方法可根据人体扫描数据重构人体曲面模型,而后将模型导入Pro/E中,通过创建水平基准面与人体表面求交,找出人体主要特征尺寸如颈围、胸围、腰围、臀围所在的截面轮廓曲线特点,以此设定搜索这些特征截面轮廓曲线需满足的条件,利用Pro/E的测量分析和优化特征分析功能进行求解,从而获得满足设定条件的人体截面轮廓曲线所在的位置,最终计算截面轮廓曲线的长度以得到人体特征尺寸.该方法具有操作简便,易于掌握的特点.     

基于二维朗奇光栅的表面形貌测量

本文提出一种采用二维朗奇光栅测量物体表面三维形貌的方法及装置,用一块二维朗奇光栅可同时获取被测表面在两正交方向上的梯度信息,从而仅需一张朗奇光强图便可重构出被测表面的形貌.文章介绍了方法的原理及二维朗奇光栅的结构设计,并用本方法研制的仪器进行了静止的和缓变的表面形貌的测量,取得了良好的测量结果.     

基于CT图像重构模型的MEMS结构尺寸检测方法

微机电系统(MEMS)结构尺寸检测技术已成为其设计和加工生产的重要保证手段,但现有技术都是对MEMS表面结构进行二维测量,不能准确反应其三维形貌.对此,提出了一种基于CT图像进行模型重构的MEMS结构尺寸检测方法.该方法通过CT扫描的断层图像建立MEMS结构的三维模型,然后通过特征点提取、边缘检测和特征区域划分识别出结构的几何特征区域,然后用最小二乘法对提取的特征数据进行拟合,提取其基本参数信息,最后计算MEMS结构关键几何参数,并提出了一种微结构表面质量参数的计算方法.该方法摆脱了传统方法对三维结构的检测限制,可实现MEMS结构的三维检测.     

Air void system in concrete containing circulating fluidized bed combustion fly ash

The increased use of advanced coal-burning technologies for power generation, such as circulating fluidized bed combustion (CFBC), results in new waste products. The potential for using CFBC fly ash in air-entrained concrete was investigated in order to assess the influence of CFBC fly ash on the microstructure of air voids in hardened concrete. A special specimen surface preparation technique for contrasting the image and enabling measurements of air voids size and distribution using an automated image analysis procedure was used. The microstructure of air voids was evaluated on the basis of the total air content, the spacing factor, and the specific surface of air voids. It was found that a satisfactory air void system in concrete could be produced when using CFBC fly ash for partial replacement of cement. The air-void system was characterized by a decreased specific surface of voids and an increased spacing factor.     

Air void structure and frost resistance: a challenge to Powers’ spacing factor

This article compiles results from 4 independent laboratory studies. In each study, the same type of concrete is tested at least 10 times, the air void structure being the only variable. For each concrete mix both air void analysis of the hardened concrete and a salt frost scaling test are conducted. Results were not originally presented in a way, which made comparison possible. Here the amount of scaled material is depicted as function of air voids parameters: total air content, specific surface, spacing factor, and total surface area of air voids. The total surface area of air voids is proportional to the product of total air content and specific surface. In all 4 cases, the conclusion is concurrent that the parameter of total surface area of air voids performs equally well or better than the spacing factor when linking air void characteristics to frost resistance (salt frost scaling). This observation is interesting as the parameter of total surface area of air voids normally is not included in air void analysis. The following reason for the finding is suggested: In the air voids conditions are favourable for ice nucleation. When a capillary pore is connected to an air void, ice formation will take place in the air void, being feed from the capillary, but without pressure build-up in the capillary. If the capillary is not connected to an air void, ice formation will take place in the capillary pore, where it can generate substantial pressure. Like this, frost resistance depends on that capillary pores are connected to air voids. The chance that a capillary pore is connected to an air void depends on the total surface area of air voids in the system, not the spacing factor.     

Fracture mechanics of air-entrained concrete subjected to compression

Air voids are entrained in concrete for protection of constructed elements, especially highway pavements, against freeze-thaw damage. Entrained air void systems inadvertently reduce the compressive strength of the concrete. The present study describes development of an analytical model for evaluation of the effects of entrained air void system on the compressive strength of concrete. The model developed here will assist in predicting the compressive strength of concrete for specified mix designs. The constitutive relationships for air-entrained concrete were established by considering a micro cracked porous material with randomly distributed circular air voids and uniformly oriented cracks from the air voids. Linear elastic fracture mechanics was employed to explain the evolution of damage due to the individual voids and cracks that emanate from such voids. The damage model considers the interactions among the voids and cracks during various stages of loading. The analytical results from this study were evaluated through an experimental program for comparison of the computed and measured compressive strengths. A wide range of samples were examined that included concretes with air contents ranging from 2% to 13% air by volume of concrete. The experiments involved microscopic determination of air content and spacing factors as well as compressive strength tests for all the concrete samples.     

Discrete modelling of ductile crack growth by void growth to coalescence

Ductile crack growth is analyzed by discrete representation of the voids growing near a blunting crack-tip. Coalescence of the nearest void with the crack-tip is modeled, followed by the subsequent coalescence of other discretely represented voids with the newly formed crack-tip. Necking of the ligaments between the crack-tip and a void or between voids involves the development of very large strains, which are included in the model by using remeshing at several stages of the plastic deformation. The material is here described by standard isotropic hardening Mises theory. For a very small void volume fraction the crack-tip tends to interact with one void at a time, while larger void volume fractions lead to simultaneous interaction of multiple voids on the plane ahead of the crack-tip. In some cases a change from one of these mechanisms to the other is seen during growth through the many voids represented here. First uniformly spaced voids of equal size are considered, but also a few computations for a random distribution of the void spacings or of the void sizes are carried out.     

Measurement of entrained air-void parameters in Portland cement concrete using micro X-ray computed tomography

The entrained air-void system in concrete is closely related to freeze-thaw durability in concrete pavements or other structures. For either research or forensic purposes, reliable and economical methods for the quantification of entrained air are desirable. This study explores the potential of using micro X-ray computed tomography (μCT) to measure entrained air-void parameters in concrete. A series of small cores (6 mm dia.) were retrieved from larger (100-mm-dia.) cores from two different concrete pavements, representing both adequate and marginal air contents, and scanned at a resolution of 7.5 μm/pixel. A systematic procedure based on image processing is proposed to address practical difficulties such as void/solid thresholding, air-type discernment (entrained air-voids vs. voids in aggregate) and the separation of bubbles within close proximity to each other (e.g. clustered air-voids). Air content and specific surface were measured directly from the three-dimensional (3D) reconstructed X-ray images, while values for paste content were derived from manual point counts performed on two-dimensional (2D) slices obtained from the 3D images. The derived values for air content, specific surface and paste content were used to calculate Powers’ spacing factor. To assess the issue of local fluctuations of material constituents and the limited dimensions of the small cores, uncertainty associated with the sample volume of concrete under measurement was also estimated. Based on the results in this study with regard to the work involved in sample preparation, data analysis and uncertainty bounds, μCT has been found to be a viable option for measurement of spacing factor and specific surface, but due to limitations imposed by the dimensions of the sample size (6-mm-dia. cores), the method is not appropriate for bulk air content determination.     

Computations of permeability tensor coefficients and anisotropy of asphalt concrete based on microstructure simulation of fluid flow

Asphalt concrete is the most widely used material for building the surface layer of pavements. It is a porous material that consists of a non-uniform arrangement of asphalt binder, aggregate particles and air voids. One of the primary factors controlling pavement performance is the fluid flow characteristics within the surface asphalt concrete layer.

This paper focuses on the numerical simulation of fluid flow in the three-dimensional (3-D) microstructure of asphalt concrete, and the calculation of permeability from the flow field. The asphalt concrete microstructure was captured using the non-destructive X-ray computed tomography (CT) technique. X-ray CT images were processed in order to identify and retain interconnected air voids and eliminate isolated voids. This image processing enhanced the efficiency of the model as it does not have to solve for flow in isolated voids that do not contribute to fluid flow. The X-ray CT images were analyzed and the results were used to determine the relationship between air void distribution and permeability directional distribution or anisotropy.

The computed permeability values were found to have good correlation with the experimental measurements. The major and minor principal directions of the permeability tensor were found to correspond to the horizontal and vertical directions, respectively. The results indicated that the non-uniform spatial distribution of air voids created more open flow paths in the horizontal directional than the vertical direction, and hence was the much higher permeability in the horizontal directions.     

An algorithm for virtual fabrication of air voids in asphalt concrete

Motivated by the virtual testing of asphalt concrete, the North Carolina State University research team has developed an algorithm to computationally generate air voids. After examining the X-ray tomographic images of real asphalt concrete microstructure, we concluded that the air void's shape and size are affected primarily by the surrounding local aggregate structure. Building on this observation, we developed an algorithm to generate random but representative air void configurations inside a given microstructure. By applying the algorithm to scanned aggregate structures, we show that the generated air voids not only look visually similar to actual air voids, but also are effective in capturing modulus reduction. The algorithm is included in a virtual aggregate structure generation framework, resulting in a streamlined virtual fabrication procedure for asphalt concrete that can qualitatively capture the effects of accelerated degradation due to the presence of air voids.     

Fracture toughness prediction using Weibull statistical method for asphalt mixtures containing different air void contents

Brittle fracture of asphalt mixtures at low temperatures is one of the main deterioration modes of pavements. Hence as an important design parameter, it is required that a reliable value for fracture toughness of asphalt mixtures is known. However, because of natural inhomogeneity of asphalt mixtures and inherent sources of scatters such as random distribution of ingredients and preparation process, the use of statistical analyses might provide better estimations for the crack growth resistance of asphalt mixtures. In this paper by conducting several low temperature fracture toughness experiments on three types of asphalt mixtures with different air void contents, the effects of air void percentage on mode I fracture toughness are studied statistically. Fifty six edge cracked semi-circular bend specimens containing 4, 5 and 7% air voids were tested, and the corresponding two and three-Weibull distribution parameters were determined for each set of data. It was shown that the Weibull model can be used successfully for predicting the statistical nature of tensile cracking phenomenon in asphalt mixtures. The mean fracture toughness values and the Weibull parameters were reduced by increasing the air void content. Furthermore, the distribution parameters obtained experimentally for the mixtures containing 4% and 5% voids were also predicted quite well in terms of the Weibull parameters of a reference mixture containing 7% air void.