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,strength, and permeability of wet-mix shotcrete before and after shotcreting operation: The influences of silica fume and air-entraining agent

It is well known that the air-void structure of hardened concrete has substantial effects on the mechanical properties and durability of concrete. In this study, laboratory evaluations were conducted to quantify the effects of air-entraining agent (AEA) and silica fume on the air-void characteristics of wet-mix shotcrete (WMS) before and after shotcreting process. For this purpose, a high-resolution image analyzer capturing elaborate graphical layouts of air-void structure using the linear transverse method was employed. Also, this study examined the effects of air-void characteristics, such as air content and spacing factor, on the strength and permeability of WMS. Based on the findings of this study, it can be concluded that: (1) shotcreting process considerably reduces overall air contents in WMS; (2) incorporating AEA with a 4.5% silica fume replacement ensures both satisfactory spacing factor and good retention of small entrained air bubbles even after shotcreting, which may improve the freeze-thaw and scaling resistance; (3) the compressive and flexural strengths of WMS were reduced as the air content increased and average spacing factor decreased; and (4) the air content affected the permeability of WMS, but no consistent correlation was found between spacing factor and permeability.     

Laboratory investigation of rheological properties and scaling resistance of air entrained self-consolidating concrete

An experimental investigation was undertaken to analyze the influence of various admixtures on the rheological properties and scaling resistance of self-consolidating concrete. Such concrete is intended for use as a repair material for filling highly restricted areas, such as forms with closely spaced reinforcing steel bars. Several self-consolidating concrete mixtures having slump flow of 550+50 mm were prepared with water-to-cement ratios varying between 0.35 and 0.41. The mixtures were cast with 0 and 3 percent silica fume, with and without air-entraining admixture. All concretes incorporated superplasticizer and viscosity-modifying admixture to enhance deformability and stability. Rheological parameters (yield value and plastic viscosity) were measured using a concrete viscometer. The air content, unit weight, and consistency were also determined. The consistency was assessed using the slump flow and L-Flow methods. Tests performed on hardened concrete included compressive strength at 28 days (ASTM C 39), scaling resistance (ASTM C 672), durability to freezing and thawing (ASTM C 666) and measurement of the air-void parameters (ASTM C 457). Relationship between the simple slump flow and yield value and plastic viscosity measurements determined using a concrete viscometer are also discussed. In general, the laboratory test results indicate that it is possible to produce a frost durable, self-consolidating concrete with low yield value and high plastic viscosity (for such fluid concrete) which can be use as a repair material to fill highly restricted areas.     

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.     

冻融循环作用下玄武岩纤维-矿渣粉-粉煤灰混凝土压拉强度试验与细观结构

对玄武岩纤维-矿渣粉-粉煤灰混凝土(BF-SP-FAC)进行了单轴抗压试验、劈裂抗拉试验、冻融循环试验、气孔结构测试试验和SEM分析.研究了不同冻融次数下BF-SP-FAC冻融损伤量、抗压强度、抗拉强度的变化,分析了气孔结构参数(含气量、气孔比表面积、气泡间距系数和气泡平均弦长)与BF-SP-FAC抗压强度、抗拉强度、...     

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.     

Les effets du séchage sur la résistance à l'écaillage du béton avec et sans fumée de silice

Twelve mixtures were prepared to investigate the influence of drying on the scaling resistance of concrete to freezing and thawing cycles in the presence of deicer salts. Six mixtures were made with an ordinary Portland cement (Canadian type 10), and six with a silica fume blended cement. The other parameters of this series of tests were the water/binder ratio (0.25, 0.35, and 0.45), the characteristics of the air void system (an air-entrained and a non-air-entrained and a non-air-entrained mixture were made with each combination of type of cement and water/binder ratio), and the drying temperature before the salt scaling tests (20°C, 40°C, and 105°C). The deicer salt scaling resistance was determined using the ASTM C 672 standard test. Test results indicate that drying at 40°C and 105°C can considerably reduce the deicer salt scaling resistance of concrete. Test data also show that the use of silica fume and the reduction of the water/binder ratio both significantly reduce the negative influence of drying. In addition, test results indicate that air entrainment is no longer required to obtain a satisfactory scaling resistance when the water/binder ratio is equal to 0.25, irrespective of the type of binder or the drying temperature.     

Distances between air-voids in concrete by automatic methods

In order to access to the air-void distances in concrete by automatic image analysis, three different methods were proposed, used and discussed: half-distances between two voids nearest neighbours, called count-dilation; half-distances between all neighbours, called SKIZ; and distance function. They were utilized in a euclidean and geodesic way. Some of them can replace favourably the manual methods proposed in the different standards.     

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.     

Frost resistance and microstructure of Engineered Cementitious Composites: Influence of fly ash and micro poly-vinyl-alcohol fiber

One of the most damaging environmental conditions to concrete structure is cyclic freezing and thawing. This paper discusses the influence of the high volumes of fly ash (FA) and micro poly-vinyl-alcohol (PVA) fibers on the cyclic freeze-thaw resistance and microstructure of the Engineered Cementitious Composites (ECC). ECC mixtures with two different FA-cement (FA/C) ratios (1.2 and 2.2 by weight), and at constant water-cementitious materials (fly ash and cement) ratio of 0.27 are prepared. To compare the behavior of ECC with ECC matrix, all of the preceding properties are also investigated for ECC matrix mixtures of same composition without PVA fiber. For frost resistance, mixtures are exposed to the freeze and thaw cycles up to 300 cycles in accordance with ASTM C666, Procedure A. Experimental tests consist of measuring the residual mechanical properties (flexural strength, mid-span beam deflection and flexural stress - deflection curve), ultrasonic pulse velocity and mass loss. The roles of PVA fibers and FA are discussed through the analysis of microstructure and fiber-matrix interactions as function of frost exposure. The microstructural characterization by measuring pore size distributions is examined before and after exposure to frost deterioration by using mercury intrusion porosimetry (MIP). The air-void characteristics of mixtures are also studied using linear transverse method. Test results confirm that both ECC mixtures with high volumes of FA remain durable, and show a tensile strain capacity of more than 2% even after 300 freezing and thawing cycles. On the other hand, before completing 300 freezing and thawing cycles, matrix (ECC without fiber) specimens have severely deteriorated, requiring removal from the freeze-thaw machine. Therefore, results indicate that the addition of micro PVA fiber to the ECC matrix substantially improved the frost resistance. The results of freeze-thaw tests also indicated that the reduction of residual physical and mechanical properties with increasing number of freeze-thaw cycles is relatively more for ECC mixture with FA/C ratio of 2.2 than for ECC mixture with FA/C ratio of 1.2.     

Accelerated tests for assessing the potential exhibited by concrete aggregates for alkali-aggregate reaction

A number of tests are available for assessing the potential of aggregates for alkali-aggregate reaction. These are listed, for example, in ASTM C33-93. The majority of these test procedures have been widely used in researching aggregates, but there are serious reservations concerning the application of some of these tests to the selection of materials for structures. In recent years, many papers have been published detailing investigations of the tests. An examination of the literature shows that most of the tests are of little value for making engineering decisions. This paper reviews the literature relating to three commonly used tests, the ASTM C-289 Rapid Chemical Test, the ASTM C-227 Mortar Bar Test and what has become ASTM C-1260-94 Accelerated Mortar Bar Expansion Test. Attention has been given to a detailed practical study of the accelerated mortar bar tests since these are becoming very widely used. The experimental work, in essence, represents a determination of the errors to be expected from application of the test to a single aggregate and the factors that influence these errors. The repeatability of the tests suggests an upper bound standard deviation of 0.0365% expansion. This figure is used to establish discriminant criteria for the recognition of potentially damaging materials. It is evident that diagnosis of potentially highly reactive materials can be made with confidence, but that a large number of tests are required to assess marginal types. Petrographic evaluation of the mortar following the test is essential, particularly where the results are borderline, though the presence of traces of gel has to be interpreted with caution. Much of the uncertainty in the test results derives from inherent aggregate variability. A test schedule can be designed that has small contributions to the error from the experimental steps, but the error due to aggregate variability remains. This can lead to the requirement for the manufacture of multiple batches of mortar for establishing acceptance criteria for even a single aggregate batch, and further consideration is needed as to the procedures necessary for the assessment of aggregate sources.     

基于图像处理的碳纤维增强树脂基复合材料固化压力-缺陷-力学性能建模与评估

利用热压罐成型工艺制备了不同固化压力条件下的碳纤维增强树脂基复合材料层合板,分析了超声相控阵C扫描图像与微观缺陷的对应关系,研究了固化压力、孔隙缺陷及力学性能之间的关联规律。结果表明：利用超声C扫描图像差异能够表征孔隙等缺陷含量,在本实验条件下,固化压力由0 MPa提高到0.6 MPa,复合材料孔隙率降低96.7%,拉伸强度（TS）和层间剪切强度（ILSS）分别提高56.1%和68.8%。在此基础上,对不同固化压力条件下制备的复合材料层合板的超声相控阵C扫描图像进行图像处理并定义成型质量指数,实现了基于C扫描图像对孔隙缺陷的定量表征。最后,通过对孔隙缺陷检测、力学性能测试及图像定量化评价结果进行数学拟合,建立了基于图像处理的固化压力-缺陷-力学性能之间的数学关联模型（CPDMP模型）,并给出了成型质量指数阈值为81%,及可接受的孔隙率应不高于1.1%,相应的固化压力应不低于0.35 MPa。     

Investigations on the influence of fly ash on the formation and stability of artificially entrained air voids in concrete

In concrete, fly ash is applied to a task-oriented improvement of different properties. Besides the advantages, e.g. the improvement of the rheology of the fresh concrete or the density of the hardened concrete, some investigations and the experience from practice indicate that some fly ashes probably influence the formation and stability of artificially entrained air voids. The reason lies presumably in the fraction of unburned carbon, a minor component of the fly ash. To identify the causes, seven fly ashes from European power plants were investigated. The fly ashes were characterized and mortar and concrete tests were conducted to identify specific fly ash parameters which might be responsible for the impaired formation and stability of the air voids. Furthermore, it was examined whether the foam index test is applicable for the assessment of the air entraining agent demand and whether an adequate accuracy of the results is given. On the basis of the results it was also examined whether the mortar tests or a fly ash specific parameter can be applied as an alternative prediction tool to assess the air entraining agent demand for an air entrained concrete.     

Bead fusion in polystyrene foams

A quantitative technique has been developed to measure the extent of fusion between expanded beads in molded polystyrene foams. Experiments were conducted with ASTM D638 tensile test specimens that were molded under conditions to produce various levels of bead fusion in the foam. The tensile properties of the foam for various levels of bead fusion were measured according to ASTM D638 standards. The fracture surfaces of the tensile specimens were analysed by quantitative image analysis techniques to measure the degree of bead fusion in the sample. This technique was then used on a commercial pattern to map the variations in bead fusion at numerous locations in the molded part. The results indicate that there is a good correlation between the tensile properties and the measured bead fusion. Significant variations in bead fusion may be present in a single molded part.     

Precision of method for determining resistance of bituminous mixtures to de-icing fluids

This paper presents the results of a Round-Robin test to estimate the precision of European method EN-12697-41 “Test methods for hot mix asphalt-Resistance to de-icing fluids”. The purpose of the project was to determine precision data according to ISO 5725, ASTM E691 and ASTM C802. The examined test method is intended for use in requirements specifications for airfield de-icing chemicals and/or as a tool for development of such products. Precision statistics, repeatability and reproducibility standard deviations, are based on observed values from six laboratories and six levels, each level comprising four samples. From a general statistical analysis, which was conducted in addition to precision determination, it could be concluded that the most damaging de-icing agents (treatments) were identified by all participating laboratories both in terms of absolute values and by ranks.     

Methods for threshold optimization for images collected from contrast enhanced concrete surfaces for air-void system characterization

Several automated procedures for the characterization of the air-void system of hardened concrete rely on a contrast enhancement step to make air-voids appear white and aggregates and paste appear black. Pixels in the digital image darker than a selected threshold level are classified as non-air, pixels brighter are classified as air. Laboratories that perform air-void testing typically have a large number of samples with corresponding results from manual operators. Proponents of automated methods often take advantage of this fact by analyzing the same samples and then comparing results. A similar iterative approach is described here where scanned images collected from a significant number of samples are analyzed and the threshold optimized to best approximate the results of the manual operator. The results of this calibration procedure are compared to an alternative approach based on more rigorous digital image accuracy assessment methods employed by the remote sensing/satellite imaging community.     

引气混凝土气泡尺寸分布的三维重构

基于体视学和几何概率理论给出了引气混凝土三维气泡尺寸重构方法,由二维平面上气泡截面圆的直径分布计算气孔的实际尺寸分布,并生成了一个多尺度分布的立方体模型结构验证了该三维重构方法的合理性.然后,使用邻近粒子表面最近间距的解析解研究了气泡细度和混凝土含气量对邻近气泡表面最近间距平均值的影响,并与用传统方法得到的气泡间距因子进行了比较.结果表明,在含气量相同的条件下,用传统方法得到的气泡间距因子是邻近气泡表面最近间距平均值的3-4倍.该方法的给出,为从二维截面上获得的引气混凝土中的气泡截面圆信息获取实际气泡在三维空间中的气泡间距信息提供了依据.     

Mode-I fracture toughness testing of thick section FRP composites using the ESE(T) specimen

Experimental and numerical analyses are performed to determine the translayer mode-I fracture toughness of a thick-section fiber reinforced polymeric composite using the eccentrically loaded, single-edge-notch tension, ESE(T) specimen. Finite element analyses using the virtual crack closure technique were performed to assess the effect of material orthotropy on the mode-I stress intensity factors in the ESE(T) specimen. The stress intensity factors for the proposed ESE(T) geometry, are calculated as a function of the material orthotropic parameters. The formula is validated for a class of thick composite materials. The thick composite tested in this study is a pultruded composite material that consists of roving and continuous filament mat layers with E-glass fiber and polyester matrix materials. Data reduction from the fracture tests was performed using two methods based on existing metallic and composite ASTM [ASTM E 1922, Standard Test Method for Translaminar Fracture Toughness of Laminated Polymer Matrix Composites, Annual Book of ASTM Standards, 1997; ASTM E 399, Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials, Annual Book of ASTM Standards, 1997] fracture testing standards. Criteria for assessing test validity and for determining the critical load used in calculating the fracture toughness were examined. Crack growth measurements were performed to determine the amount of stable crack growth before reaching critical load. The load versus notch mouth opening displacement, for different crack length to width ratios is affected by material orthotropy, nonlinearity, and stable crack propagation. The mode-I translayer fracture toughness and response during crack growth is reported for ESE(T) specimen with roving layers oriented both, transverse and parallel to the loading direction.     

泡沫沥青冷再生混合料成型方法

采用振动成型设备以压实度为指标确定了泡沫沥青冷再生混合料振动成型的振动时间。振动成型、静压成型、马歇尔成型及旋转压实成型试件在总空隙率相同的条件下,对比了不同成型方法混合料的力学强度,分析了强度差异产生的原因。基于CT检测手段,分析了不同成型方法混合料内部空隙分布状态、空隙分形维数、最可几孔径及平均孔径。结果表明:旋转压实的"揉搓作用"显著增大了冷再生混合料的间接拉伸强度,振动成型次之,静压成型与马歇尔成型混合料间接拉伸强度(ITS)较接近,而四种成型方法混合料的无侧限抗压强度差别不大;旋转压实及振动成型会影响泡沫沥青冷再生混合料内部细颗粒的分布,进而影响混合料内部空隙的分布状态,这两种成型方式混合料内部空隙数目多且等效半径小。不同压实方法的压实机理的差异导致混合料内部空隙平均孔径、空隙维数及最可几孔径存在差异。空隙特征的不同是引起冷再生混合料力学强度差异的本质原因。     

微机电硅膨胀阀在变频空调器中的应用研究

为提高变频空调器能效及安全稳定性,就硅膨胀阀在变频空调器中的应用展开了研究。对硅膨胀阀流量特性的实验表明在最大流量的20％～70％范围内,其流量与占空比之间线性特性较好,其正向流动和反向流动的流量特性重合度高,响应特性线性好。同时对其在KFR-25GW/Bp变频空调器中进行性能实验,实验表明相对电子膨胀阀而言,其变频空调器的制冷量、制热量以及相应的能效比更高。因而,硅膨胀阀可以作为变频空调器节流元件。