Durability characteristics of self-consolidating concrete designated for repair applications

This paper presents test results carried out to study the influence of key mixture parameters on frost durability, scaling resistance, and transport properties of self-consolidating concrete (SCC) that can be used in structural repair. Regardless of the w/cm, binder type, or admixture combination, properly designed SCC can develop high resistance to freezing and thawing with frost durability factor greater than 80%. The optimized mixtures had 56-day rapid chloride-ion permeability (RCP) values of 200–900 Coulomb. On the average, SCC made with 0.42 w/cm developed 20% higher capillary porosity, 20% lower compressive strength, and 30% greater RCP value compared to similar SCC prepared with 0.35 w/cm. The type of blended cement in use had considerable influence on transport properties. For a given mix design, concrete with 180 mm slump consistency exhibited similar RCP value of 505 Coulomb compared to 630 Coulomb for the same concrete with greater dosage of HRWRA to secure a slump flow consistency of 670 mm.     

钻床动力头轴承室的刷镀修复工艺

简单介绍电刷镀技术及其特点和在修复零件方面的应用。结合刷镀技术修复钻床动力头轴承室的典型实例,重点介绍刷镀工艺的原理、方法和效果。提出了发展电刷镀技术修复机械零件的可行性及其深远意义。     

Frost salt scaling resistance of concrete containing CFBC fly ash

The possibility for using coal combustion by-products in concrete exposed to frost-salt aggression was investigated. The research was aimed to assess an influence addition of circulating fluidized bed combustion (CFBC) fly ash on frost-salt scaling of air-entrained concrete. For evaluation of the resistance of concrete to frost salt scaling the test called “depth sensing indentation” (DSI) was applied. The DSI test method was implemented on a universal testing frame using a standard Vickers indenter. Experimental tests were performed on cement paste specimens and concrete specimens designed with partial replacement of cement with coal combustion by-products. The mass of scaled material in standard frost salt scaling resistance tests on concrete was inversely proportional to the Vickers hardness of the paste containing CFBC fly ash; the best-fit arithmetic relationship is provided.     

非均质混凝土材料破坏的三维细观数值模拟

在随机骨料模型的基础上,采用特征单元尺度对网格进行剖分,建立了非均质混凝土材料损伤破坏及宏观力学特性研究的三维细观单元等效化模型。对单轴拉伸、单轴压缩条件下湿筛混凝土试件的破坏过程及宏观力学性能进行了模拟分析;研究了混凝土梁的三分点弯拉力学特性,并与平面模型结果作了对比。研究表明：1) 与平面计算模型相比,三维模型更真实地模拟混凝土材料在外荷载作用下的损伤破坏过程,更准确的描述非均质混凝土材料的宏观力学性能,且与实验结果吻合;2) 骨料空间分布形式基本不影响混凝土材料的宏观弹性模量及强度,但影响其破坏过程和破损路径;3) 与随机骨料模型等细观力学方法相比,该方法具有高效性。     

Repair and strengthening of reinforced concrete square columns using ferrocement jackets

A series of 10 one-third scale square reinforced concrete column specimens were cast; preloaded under axial compression up to various fractions (0%, 60%, 80%, and 100%) of its ultimate load; repaired using ferrocement jackets containing two layers of Welded Wire Mesh (WWM) encapsulated in high strength mortar; and then retested to failure. The overall response of the specimens was investigated in terms of load carrying capacity, axial displacement, axial stress and strain, lateral displacement, and ductility. The test results indicated that jacketing reinforced concrete square columns with this form of ferrocement provided about 33% and 26% increases in axial load capacity and axial stiffness, respectively, compared to the control columns. The test results also indicated that repairing similar reinforced concrete columns (after preloading them to failure) with the same ferrocement jacket almost restored their original load capacity and stiffness. Furthermore, the repaired columns failed in a ductile manner compared to the brittle failure exhibited by the control columns.     

Parameters affecting the debonding risk of bonded overlays used on reinforced concrete slab subjected to flexural loading

This research aims at better understanding the mechanisms involved in the cracking behaviour of bonded overlays used on reinforced concrete beams and slabs. The project involves the testing of reinforced concrete beams (1.8 m × 0.2 m × 0.2 m) repaired with concrete overlays using four different types of surface preparation and subjected to cyclic flexural loading. The concrete beam specimens were simply supported with a point load at midspan. Structural capacity (evolution of the apparent rigidity, maximum deflection, failure mode) and cracking behaviour (flexural cracking and interface debonding) were monitored. Technical data indicate that good adhesion alone is insufficient to guarantee bond durability. A total of 20 beam interfaces were prepared by scarification, sandblasting, chipping with a light jackhammer, and water jetting. A number of cores were tested to evaluate interface strength by direct shear and direct tension. Roughness was characterized quantitatively using a newly developed optometric method. The results indicate that cracking behaviour depends on the bond strength and on the surface roughness produced by a specific surface treatment. The relationships between adhesion, structural behaviour, and roughness were evaluated, and an updated debonding mechanism is proposed to take into account the influence of roughness. To achieve a monolithic behaviour, the surface treatment must generate critical adhesion and roughness levels. In addition to this roughness, the debonding risk decreases rapidly and monolithic behaviour is reached.     

Efficiency of RC square columns repaired with polymer-modified cementitious mortars

This paper regards the axial behavior of reinforced concrete columns repaired by polymer-modified cementitious mortars. Tests were performed on eight columns with square cross-section: six were repaired with three types of polymer-modified cementitious mortars on all faces, two were in non-damaged and non-repaired condition (control elements). Tests were repeated varying mechanical properties (elastic modulus and compressive strength) of repair materials, maintaining the same repair thickness, including the reinforcement bars. Comparisons between repaired and control elements showed that polymer-modified cementitious mortars cannot restore the original load-bearing capacity of columns. In spite of this, selection of mortar mechanical properties plays a significant role. Among the three types of repair mortar tested in this experimental study, using the material with the most similar elastic modulus and higher compressive strength than that of the concrete substrate is recommended.     

表面技术在模具中的应用

简要述概述了表面处理技术在模具发展中的重要作用。并介绍了模具中应用成熟的表面技术。     

Physical property control for innovative powder processes – Rheological control of concentrated slurry and wet granules

The manufacture of materials using slurry-based processes, such as the electrodes of Lithium ion secondary batteries and fuel cells, can be made more productive by increasing the concentration of the slurry or adopting wet granules with extremely high concentrations.Shear thickening, which is a particular issue for such highly concentrated slurries, is derived from collisions between particles, it is important to control the size, size distribution, and concentration of particles, as well as the solvent viscosity. Furthermore, the controlling of electrostatic interaction between particles and mixing of different size particles have direct impacts on shear thickening, the concentration of salt in the slurry and size and mixing ratio of particles are also important characteristics. It may be possible to control rheological behavior using these characteristics.The amount of solvent is reduced beyond the level in a concentrated slurry, the slurry changes from a so-called liquid state to a solid state of wet granules. Fully utilizing the properties of these wet granules would maximize various advantages in the production process. Flowability is the most important characteristic for use of highly concentrated wet granules. This research found that flowability is extremely low when the strain is low, but that higher wet granule concentrations flow more easily under high strain.Our latest research about rheological behavior of concentrated slurry and wet granules open up to the possibilities for ultimate form of the electrode manufacturing process might involve film-creation or forming using particles without any liquid content at all. Resolving issues such as the uniformity of the blended powder and flowability control may enable conventional slurry processes to be replaced by powder processes.     

Performance evaluation of structural concrete using controlled quality coarse and fine recycled concrete aggregate

This paper presents the fresh, mechanical, and durability performance, of a structural concrete mix classified as C-1, by the Canadian Standards Association (CSA) made with controlled quality Recycled Concrete Aggregate (RCA). Five mixes with water-to-cementing material (w/cm) ratio of 0.40 were produced with various RCA contents and tested against two 0% RCA control mixes made with General Use (GU) cement, and General Use Limestone cement (GUL). The RCA contents in the mixes were 10%, 20%, and 30% by coarse aggregate volume replacement, as well as 10% and 20% fine and coarse (granular) aggregate volume replacement. All evaluated mixes met the specifications from the CSA for fresh, mechanical, and durability properties. The coarse RCA mixes performed better than the granular RCA mixes in terms of flexural and splitting tensile strengths, linear drying shrinkage, water sorptivity, and rapid chloride-ion permeability, where the test results were significantly affected by the ultra fines present in the granular RCA.     

An optimal geometric process model for a cold standby repairable system

In this paper a cold standby repairable system consisting of two identical components and one repairman is studied. Assume that each component after repair is not ‘as good as new'. Under this assumption, by using a geometric process, we consider a replacement policy N based on the number of repairs of component 1. Our problem is to determine an optimal replacement policy N* such that the long-run expected reward per unit time is maximized. The explicit expression of the long-run expected reward per unit time is derived and the corresponding optimal repair replacement policy can be determined analytically or numerically. Finally, a numerical example is given.     

45钢磨削硬化的试验研究

在平面磨床上,采用干磨削方式对45钢进行了磨削硬化处理,研究了磨削硬化层的组织与性能.结果表明,在磨削温度场和应变场的综合作用下,完全硬化区马氏体相呈现"细-粗-细"的变化规律,且相对粗大马氏体相出现在次表层;而过渡区组织的形成主要在于磨削热循环温度场的作用.磨削深度或原始组织对显微硬度无显著影响,硬度值均在700～750HV之间;但磨削硬化层深度随着磨削深度的增加或原始组织均匀性的提高,从0.9mm增大到1.4mm.     

混凝土结构用CFRP筋的感知性能试验研究

对混凝土结构用CFRP筋及其加筋混凝土梁的力学性能和感知性能进行试验研究。自行研制开发了不同直径和表面处理形式的CFRP筋。在此基础上,制作了三组各6个标准受拉试件,分别测试了CFRP筋的弹性模量、极限抗拉强度和电阻变化率,得到了相应的变化曲线;其次制作了两组各3根CFRP加筋混凝土梁,测试了梁中CFRP筋电阻随载荷增加的变化曲线。研究表明,CFRP筋具有很好的力学性能和感知性能,对于混凝土结构加强筋和结构受力状态监测具有广泛的应用前景。     

Numerical homogenization of hardened cement paste

Based upon a three-dimensional computer-tomography of hardened cement paste, a finite-element mesh at micrometer length scale is introduced. Effective material properties are obtained through numerical homogenization techniques using representative volume elements. Statistical tests, two- and three-dimensional computations and a comparison with experimental data are shown. For the hydration products of hardened cement paste a visco-plastic constitutive equation of Perzyna type including isotropic damage is introduced. The inelastic material parameters are identified solving an optimization problem through a combination of a stochastic genetic algorithm and the deterministic Levenberg-Marquardt method. The time-consuming evaluations of the corresponding objective function are distributed within a network environment automatically.     

Cryogenic concrete

Available information on the low temperature behaviour of concrete is reviewed. The temperature dependence of strength, elasticity, thermal and other properties is discussed and possible relationships between compressive strength, tensile strength and elasticity are demonstrated.Changes in properties with low temperature depend on the formation of ice in the pores of the material and this is considered in the light of porosity and moisture content and how they are affected by curing, water/cement ratio and age. Their consequent effects on low temperature behaviour are examined. Finally the result of thermal cycling is discussed.