Effects of Micro Silica on Permeability of Plastic Concrete

Water tightness and seepage control are important considerations in the design and construction of dams. Plastic concrete cut-off walls are one of the effective means for seepage control in earth dams constructed on permeable substrate. An important requirement for the plastic concrete in such applications is adequate strength for the design loads and low enough permeability to meet the water tightness requirements of the dam. As permeability of cement-based materials, such as concrete, is mainly dependent on water to cement ratio, the permeability of plastic concretes is much higher than those of normal concretes. It is thus apparent that the achieving to low permeability is an important problem in plastic concrete. So a method, which could reduce the permeability of plastic concretes without unduly increasing their elastic modulus, would provide considerable technical advantages. In the present research, the possibility of lowering the permeability of plastic concrete thorough replacement of a portion of cement by micro silica was investigated. The aim of the experimental research was to investigate the effects of various levels of cement replacement by micro silica, including 0%, 3%, 6%, 9%, 12% and 15% on strength and permeability of plastic concrete. The results illustrate that the use of micro silica for replacement of some of the cement content in plastic concrete results in very substantial decrease in permeability. Obtained results show that through the use of micro silica in plastic concrete, it is possible to keep the strength and elastic modulus at the level of control mix and still achieve to reduction in permeability.     

Effect of Coarse Aggregate Size on Relationship between Stress and Crack Opening in Normal and High Strength Concretes

Fine and coarse aggregates play an important role in the fracture of concrete. However, quantitative information available on the effect of the coarse aggregate size on the fracture properties of concrete is still limited. In the present paper, the effect of coarse aggregate size (single grade of 5～10, 10～16, 16～20 and 20～25 mm) on stress-crack opening (σ-w) relation in normal and high strength concretes (compressive strength of 40 and 80 MPa, respectively)was studied. The investigation was based on three-point bending tests implemented by fictitious crack analysis. The result shows that coarse aggregate size and cement matrix strength significantly influence the shape of σ-w curve.For a given total aggregate content, in normal strength concrete, smaller size of aggregate leads to a high tensile strength and a sharp stress drop after the peak stress. The smaller the coarse aggregate, the steeper the σ-w curve.By contrast, in high strength concrete, the effect of aggregate size on σ-w relation almost vanishes. A similar σ-w relation is obtained for the concrete except for the case of 20～25 mm coarse aggregate size. The stress drop after the peak stress is more significant for high strength concrete than that for normal strength concrete. Meanwhile, the smaller the coarse aggregate size, the higher the flexural strength. Fracture energy and characteristic length increase with increasing coarse aggregate size in both normal and high strength concretes.     

Effect of Superfine Slag Powder on HPC Properties

A superfine slag powder (SP) made from granulated blast furnace slab incorporating activators by using sepecial miling technique,was used as supplementary cementitious material in high performance concrete (HPC),replacing part of the mass of normal Portland cement,The effects of the SP on the workability,mechanical and crack self-healing properties of HPC were studied,The hydration process and microstructure characteristics were investigated by X-ray diffraction(XRD) and scanning electron microscopy (SEM) techniques,resectively,The crack self-healing capacity was evaluated by Brazilian test,The test results indicate that the SP has especially supplementary effect on water reducing and excellent property of better control of slump loss ,The concrete flowability increases remarkably with of HPC containing SP are higher than the corresponding strength of the control concrete at all ages,The crack self-healing ability is highly dependent on SP content of HPLC.     

Effects of Fe2O3 Nanoparticles on Water Permeability and Strength Assessments of High Strength Self-Compacting Concrete

In this work,compressive,flexural and split tensile strength together with coefficient of water absorption of high performance self-compacting concrete containing different amount of Fe2O3 nanoparticles have been investigated.The strength and the water permeability of the specimens have been improved by adding Fe2O3 nanoparticles in the cement paste up to 4.0 wt%.Fe2O3 nanoparticle as a foreign nucleation site could accelerate C-S-H gel formation as a result of increased crystalline Ca(OH)2 amount especially at the early age of hydration and hence increase the strength of the specimens.In addition,Fe2O3 nanoparticles are able to act as nanofillers and recover the pore structure of the specimens by decreasing harmful pores to improve the water permeability.Several empirical relations have been presented to predict the flexural and the split tensile strength of the specimens by means of the corresponding compressive strength at a certain age of curing.Accelerated peak appearance in conduction calorimetry tests,more weight loss in thermogravimetric analysis and more rapid appearance of peaks related to hydrated products in X-ray diffraction results indicate that Fe2O3 nanoparticles up to 4 wt% could improve the mechanical and the physical properties of the specimens.     

Effect of Some Admixtures on the Hydration of Silica Fume and Hydrated Lime

1. IntroductionSuperplasticizers are widely used to produce amore floatable or a very high strength concrete.Whereas, stearic acid is used as a water-repellent admixture to reduce the capillary absorption of fluidsby hardened concrete and mortar. Also, silica fumeis widely used during the last years as an additive oras a partial replacement of Portland cement in concrete. It improves the microstructure of the hardenedcement paste as well as the strength and durability ofconcrete. This is attr…     

Effect of surface pretreatment on adhesive properties of aluminum alloys

The lap-shear strength and durability of adhesive bonded AI alloy joints with different pretreatments were studied by the lap-shear test and wedge test. The results indicate that the maximum lap-shear strength and durability of the bonding joints pretreated by different processes are influenced by the grade of abrasive papers and can be obviously improved by phosphoric acid anodizing. Alkali etching can obviously improve the durability of bonding joints although it slightly influences the maximum lap-shear strength. The process which is composed of grit-finishing, acetone degreasing, alkali etching and phosphoric acid anodizing, provides a better adhesive bonding property of Al alloy.     

Bond Strength of the Sn-Ti-Ⅹ Ternary Active Solders on Sialon Ceramic

The bond strength of the Sn based ternary active solders,Sn-5Ti with various contents of Ag,Cu,In,Ni or AI on sialon ceramic was investigated by the ceramic/ceramic joint joining.The bond strengthof Sn-STi (about 80 MPa) may increase to 90-100 MPa by small addition of Ag or Cu (about5-10%).A little Ni (about 1-3%) in the solder is slightly beneficial,but too more Ni (more than 5%)is harmful.Small addition of In is trivial,but too more In is detrimental.On the other hand,small addi-tion of AI in the active solder decreases greatly the bond strength of the solder,therefore it is veryharmful.In discussion,three suggestions for selection of the third element to increase the bondstrength of the soft solders on ceramic,i.e.with a high surface energy,benefiting to wetting on ce-ramic and strengthening the soft solder itself,have been made.     

Study on the Basic Mechanical Behaviors of High Density Polyethylene Electrofusion Welded Joints at Different Temperature

The basic mechanical behaviors of high density polyethylene electrofusion welded joint at different temperatures were studied by using differently designed specimens in this paper. The results show that the strength of weld bonding plane is higher than that of the pipe and socket materials at room temperature. In order to get the shear strength of electrofusion welded joint,the effective bond lengths were reduced by cutting artificial groove through the socket. The effective bonding length of welded joint to get the shear strength is decreased with decreasing testing temperature. The shear strength and the sensibility to sharp notch of HDPE material increased with decreasing temperature.     

Discussion about the standard system on urban domestic water saving technology in China

Standard is the technical foundation of national economic and social development, and it is the basic rule of establishing social regulation. Researching and constituting the standard system of urban domestic water saving technology is to offer important science basis for revising standard plan and standardize research plan. This paper introduces the present situation of our urban domestic water saving technical standard system, problem and the development direction in the future, as well as project planning of constructing urban domestic water saving technical standard system.     

Analytic Hierarchy Process(AHP)to Optimize the Service in a Water Supply Network

Decision making in drinking water supply networks is increasingly complex due to the large number of variables involved.In order to make better decisions it is necessary to use adequate and robust methodologies.This paper presents the application of the Analytic Hierarchy Process(AHP)related to the operation of the drinking water supply network of the city of Chihuahua,Mexico,where two possible alternatives are delineated with the objective to optimize the service.The application of AHP was carried out in 24 sectors that have substantial differences in their efficiency but with instrumentation and measurement in all the variables contemplated by the operating agency,with a population of 221,722 inhabitants which represent a 30%of the total population of the city,the results indicate that the best alternative is the one with less criteria to be controlled and fewer repercussions on the cost of operation and investment in the rehabilitation and replacement of the network.     

An investigation into the feasibility of formulating ‘self-cure’ concrete

To achieve good cure, excessive evaporation of water from a freshly cast concrete surface should be prevented. Failure to do this will lead to the degree of cement hydration being lowered and the concrete developing unsatisfactory properties. Curing can be performed in a number of ways to ensure that an adequate amount of water is available for cement hydration to occur. However, it is not alway possible to cure concrete satisfactorily. This paper is concerned with achieving optimum cure of concrete without the need for applying external curing methods. The feasibility of curing concrete by adding water-soluble chemicals during mixing that reduce water evaporation in the set concrete, making it ‘self-curing’ is discussed. The chemicals' abilities to reduce evaporation from solution and to improve water retention in ordinary Portand cement was monitored by measuring weight-loss. x-Ray powder diffraction and thermogravimetry measurements were made to assess whether any improvement in water retention was matched by an increase in degree of cement hydration. Initial surface absorption tests and compressive strength measurements were made to determine surface permeability and strength development, respectively. The scanning electron microscope was used to determine the influence of the admixtures on cement paste microstructure. It was found that two of the chemicals studied had a significant ‘self-curing’ effect. One of these chemicals enhanced hydration further than simply by means of water retention. A possible explanation of this behaviour is given.     

Concrete with recycled aggregates: the Portuguese experimental research

The main objective of this study is to define expedient procedures to estimate the properties of structural concrete that contains recycled aggregates. Experimental results from Portuguese research, most of which supervised by the first author, were used to establish a relationship between some properties of hardened concrete (compressive strength, splitting and flexural tensile strength, modulus of elasticity, abrasion resistance, shrinkage, water absorption, carbonation penetration and chloride penetration) and the density and water absorption of the aggregates’ mixture and also the compressive strength of concrete at the age of 7 days. The workability and density were also analysed for fresh concrete. The graphic analysis of each property shows the relationship between those for recycled aggregate concrete (RAC) mixes and a reference mix using natural aggregates only (RC). The density and water absorption of all the aggregates in the mixture, for each substitution rate, were calculated in order to represent the exact proportion of each type of aggregate (natural and recycled). This method proved to be viable to estimate the variation of the properties of concrete with recycled aggregates by obtaining results for the three parameters mentioned above. This innovative procedure can contribute to increasing the use of recycled aggregates in the construction sector and make it a sustainable activity.     

Investigation of the possibility of estimating concrete strength by porosity measurements

The possibility of estimating concrete or mortar strength by measuring the total water porosity by a very simple method is investigated. Cube strengths of mortar and concrete specimens are correlated with water porosity measured from ‘intact’ pieces taken from the failure cones of the crushed cubes. It is shown that it is not possible to acquire a satisfactory correlation of strength and the measured total water porosity of the mortar or concrete mix. However, a very promising correlation (correlation coefficient R>93%) is obtained if the ‘calculated average paste porosity’ P _p is substituted for the measured total water porosity of the mix P _m . P _p can be determined from the measured porosity P _m of the mix, the porosity of the aggregates and the volumetric proportions of the constituents. For the range of strengths investigated, it can be inferred that the strength class of a cement (OPC 35 or OPC 45) does not significantly influence the correlation, but a slightly different correlation seems to exist for pozzolanic cements.     

基于可靠度分析的湿态混凝土强度模型

湿度会改变混凝土的力学参数,当外界水渗入到结构后,混凝土的力学性能会发生较大改变。该文在现有试验结果和理论模型的基础上,认为湿态和干燥混凝土强度的差异主要与表面能和弹性模量的变化有关。利用ANSYS 有限元软件模拟了不同浸没时间下混凝土试件内部的湿度分布,采用概率分布和可靠度数学模型定量计算了不同湿度下混凝土的抗压强度。该文的计算结果与试验结果吻合较好,可为真实水环境中混凝土的设计参数取值提供参考。     

不同加载速率下饱和混凝土的劈拉试验研究及强度变化机理

利用大型液压伺服试验机研究了不同加载速率下饱和混凝土与干燥混凝土的劈拉强度变化情况,试验研究表明:在准静态加载时,与干燥的混凝土相比饱和混凝土的劈拉强度有所降低,随着加载速率的升高,饱和混凝土的的动态劈拉强度有较大的提高。通过对干、湿混凝土强度变化的比较表明:孔隙及裂缝中的自由水对混凝土强度有所影响。根据细观层次上、不同加载速率下裂纹中孔隙水压力的作用形式(即低加载率时孔隙水的“楔入”作用和高加载率时的“Stefan效应”),利用断裂力学来探讨不同加载速率下饱和混凝土劈拉强度的变化机理。结果表明:提出的机理可以较好地解释饱和混凝土强度的变化情况。     

孔隙水对湿态混凝土抗压强度的影响

湿态混凝土受压时会产生孔隙水压力,孔隙水压力的大小与混凝土的变形和裂纹的扩展速度相关。孔隙水压力的产生给混凝土的力学性能带来一些影响,但是目前这方面的理论研究甚微。尝试利用断裂力学的方法,来探讨湿态混凝土在承受单轴压缩荷载时,孔隙水压力对混凝土开裂、扩展和抗压强度的影响。研究结果表明:混凝土中的孔隙水压力减小了阻碍混凝土开裂的摩阻力,相当于楔体的“楔入”作用,加速了混凝土的损伤和微裂纹的扩展,与干燥态的混凝土相比,湿态混凝土的开裂应力和抗压强度都有所降低。     

Study on mechanism of strength distribution development in vacuum-dewatered concrete based on the consolidation theory

The strength and hardness of a concrete slab surface are considered to be significantly affected by concrete bleeding. Vacuum dewatering is reported to be quite effective in imparting high density and strength. However, in Japan, in contrast with concrete work in civil engineering applications, concrete work in the field of building construction has not been successfully treated by this method. In an earlier report, the authors pointed out the strong relationship between strength distribution and density distribution in vacuum-dewatered concrete, both of which gradually decrease from the top surface to a depth of about 15 cm. The main purpose of the present study is to discuss the mechanism of the occurrence of such distribution of strength and density, based on consolidation theory. In an experiment, pore water pressure distribution in concrete is measured by means of an original measuring system. The results of the experiment confirm that the consolidation theory is quite effective in explaining the internal properties of vacuum-dewatered concrete as well as those of press-dewatered concrete. A prediction method for the strength improvement of concrete by vacuum dewatering is also discussed. It was considered likely that pore water pressure distribution generated by vacuum dewatering could be attributable to the influences of capillary tension and viscous resistance. This mechanism was verified by model experiment.     

磁化废水在混凝土中应用的性能研究

预拌混凝土生产企业共同面临的一个问题是混凝土生产中“废水”的回收利用,而回收“废水”的质量对混凝土的强度性能、工作性能、耐久性能影响较大。通过对“废水”的磁化混凝土强度都能达到强度设计要求,磁化“废水”配制的混凝土对凝结时间的影响不大。     

Effect of mix design inputs,curing and compressive strength on the durability of Na2SO4-activated high volume fly ash concretes

This paper aims to advance research on the use in concrete of a high volume of fly ash, with a high loss on ignition value, aiding in sustainable low carbon footprint construction. To this end, the work explores the benefits that may be achieved in terms of long-term concrete performance from the incorporation of fly ash along with a chemical activator. Durability tests are performed on concrete with an activated hybrid cementitious system: Portland cement (PC) and high volume fly ash with sodium sulfate. The chloride diffusion coefficient significantly decreased over time for the activated system (50% PC - 50% fly ash with added sodium sulfate) compared to the control samples (100% PC and 80% PC - 20% fly ash) at the same water to cementitious material ratio. This behavior is particularly evident in samples cured under controlled laboratory conditions (100% RH and 23 °C). However, outdoor curing increases the permeability for all concretes. Long term carbonation is also investigated under natural exposure conditions, and samples that are cured outdoors exhibit a significant carbonation depth. The compressive strength is correlated with the durability parameters: the durability performance improves as the compressive strength increases, indicating that as is the case for Portland cement (but not always for alkali-activated binders), the microstructural factors which yield high strength are also contributing to durability properties.     

Strength and durability of lightweight concrete in hot marine exposure conditions

The paper presents results on strength development and durability of 35 and 50 MPa total lightweight and 50 MPa normal weight concretes exposed to hot marine exposure conditions for a period of two years. An initial water curing of 7 days and subsequent seaside exposure is more beneficial for the strength development of light-weight concrete than it is for normal weight concrete. One day initial curing and subsequent seaside exposure was not very conductive for the strength development of both lightweight and normal weight concretes. The water penetrability of total lightweight concretes was found higher than the normal weight concrete under all the initial curing conditions and on subsequent exposure to the hot-marine environment. In the same vein, the depth of carbonation for the total lightweight concretes was more than that of the normal weght concrete. On the overall, the results suggest that the higher the water penetrability of a given concrete, the more is the penetration of the damaging species like carbon dioxide, sulphate and chloride ions into a concrete.