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、冻融损伤量的关系,运用灰关联熵分析法讨论了BF-SP-FAC气孔结构参数对抗压强度、抗拉强度、冻融损伤量影响的主次关系。结果表明:相同冻融次数下,与其他纤维掺量相比,玄武岩纤维掺量为0.18vol%时,BF-SP-FAC抗冻性能较好,抗压强度和抗拉强度最高;在相同玄武岩纤维掺量下,随含气量、气泡间距系数、气泡平均弦长的增大,BF-SP-FAC抗压强度和抗拉强度减小,而冻融损伤量增大;随气孔比表面积的增加,BF-SP-FAC抗压强度和抗拉强度增大,而冻融损伤量减小。气孔比表面积是影响BF-SP-FAC强度的最主要因素,而气泡平均弦长是影响BF-SP-FAC冻融损伤量的主要因素,最小灰熵关联度分别为0.998和0.993。气孔结构参数与强度、冻融损伤关系的建立,可预估混凝土的强度与冻融损伤。      

Pore structure and chloride ion permeability of mortars containing silica fume

The addition of silica fume in concrete causes a remarkable increase in strength and a drastic reduction in chloride ion permeability. These effects may be due primarily to microstructural changes both in the cement paste phase and in the interfacial zone around aggregates. The standard method of test for rapid determination of the chloride permeability of concrete, AASHTO T 277–831, has increasingly been used to evaluate the permeability of concrete. However, for the concrete containing silica fume, the results of the AASHTO T 277–831 test, which is expressed in terms of electrical charge passed, do not necessarily reflect the real diffusion index of chloride ion through the concrete. There seems to be factors other than the pore structure which govern the results of the AASHTO T 277–831 test in the concrete containing silica fume. In this study, the effects of silica fume to reduce the chloride ion permeability of the mortar were investigated based on the results of pore size distribution measurements, X-ray diffraction analysis, SEM observations and pore solution extraction. The application of the AASHTO T 277–831 test to the evaluation of the chloride ion permeability of the concrete containing silica fume was discussed.     

Rheology and bonding characteristics of self-leveling concrete as a repair material

This paper presents the first part of a study under-taken to measure the potential of self-leveling concrete to perform overhead repairs. The performances of self-leveling concrete in terms of its filling ability and bonding characteristics was compared to dry-mixture shotcrete and to normal cast-in-place concrete. Self-leveling mixture characteristics included the presence of superplasticizer, viscosity-modifying admixture (welan gum), air-entraining admixture (AEA) and silica fume. On self-leveling concrete and ordinary concrete, rheological parameters (yield value and plastic viscosity) were measured with a rheometer, and other fresh properties were determined including air content (ASTM C-231), specific weight, slump (ASTM C-143), and slump-flow. It was observed that self-leveling concrete has very good filling ability and that the absence of segregation and bleeding enhances the characteristics of the interface with the old concrete, even in an inverted position. Microstructure studies have indicated that self-leveling concrete performed as well as dry-mixture shotcrete and is thus a very interesting repair material.     

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.     

Some findings on the usefulness of image analysis for determining the characteristics of the air-void system on hardened concrete

This paper discusses the usefulness of image analysis techniques in order to assess the characteristics of the air-void system in concrete. Test results indicate that such a technique can correctly assess the air-void characteristics as defined in ASTM C 457 Standard test method. However, the accuracy of the test results is not significantly improved as compared with the manual technique and the image analysis method must be very carefully validated before being used as a routine procedure. Test results also indicate that the image analysis technique failed to correctly assess the size-distribution of air voids and, for that reason, this technique cannot be used to provide a better estimate of the real spacing of air voids than the commonly used ASTM C 457 spacing factor.     

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.     

Determining optimum air-void spacing requirement for a given concrete mixture design using poromechanics

The frost resistance of concrete is a function of the concrete constituent properties, entrained air-void system parameters and environmental exposure history. However, only a single maximum value for the void spacing factor is specified for all types of concrete by code writing bodies for successful protection against freezing damage. The advent and utilisation of new materials over the recent years warrant reevaluation of the validity of this single pass/fail criteria established more than 50 years ago. Here, a poromechanical model, capable of incorporating concrete constituent properties, environmental exposure and air-void spacing factor, has been used to determine the role of various concrete constituents and air-void system on the damage propensity of concrete exposed to freezing temperatures. It is found that a maximum threshold of acceptance, for instance a 0.2 mm spacing factor, may not be adequate for all concrete mixture designs subject to various cooling conditions. The model also suggests that concrete with low-porosity, low-permeability mortar matrix, a characteristic property of mortar containing supplementary cementitious materials and/or low water to cement ratio, can perform satisfactorily under freezing temperatures even with a spacing factor greater than the recommended value. If utilised for design, this model will give more freedom to practitioners in ensuring concrete durability by controlling multiple factors including the concrete mixture components and proportions rather than just satisfying a single pass/fail criterion for void spacing factor for all concrete mixtures.     

Probabilistic evaluation of concrete freeze-thaw design guidance

A novel limit-state function using Powers’ models is developed to assess current freeze-thaw exposure categories and design criteria for concrete placements established by American, Canadian, and European standards organizations. Based upon performance assessments by standardized accelerated testing, the current specifications are shown to provide sufficient levels of reliability pending an appropriate mean air-void spacing factor. Sensitivity assessments of the model demonstrate that the spacing factor, saturation state, permeability, and freezing rate significantly influence the response of the air-entrained concrete. The model is validated with a large dataset derived from standard freeze-thaw tests, and an equation is developed to probabilistically design concrete for freeze-thaw resistance.     

Strength development and chloride penetration in rubberized concretes with and without silica fume

The study presented herein has been carried out in order to investigate the strength development and chloride permeability characteristics of plain and rubberized concretes with and without silica fume. For this purpose, two types of tire rubber, namely crumb rubber and tire chips, were used as fine and coarse aggregate, respectively, in the production of rubberized concrete mixtures which were obtained by partially replacing the aggregate with rubber. Two water-cementitious material (w/cm) ratios (0.60 and 0.40), three moist curing periods (3, 7, and 28 days), four designated rubber contents (0, 5, 15, and 25 by total aggregate volume), two silica fume content (0 and 10% by weight of cement), and five different testing ages (3, 7, 28, 56, and 90 days) were considered as experimental parameters. The results indicated that for a given w/cm ratio and moist curing period, the use of rubber significantly aggravated the chloride ion penetration through concrete but the degree of the rate of the increment of the chloride permeability depended on the amount of the rubber used. When the curing period was extended from 3 to 28 days, the reduction in the magnitude of chloride penetration depth was notably higher for the rubberized concretes, even at a rubber content of as high as 25%. It was also observed that silica fume may be considered as a remedy to enhance the chloride penetration resistance of the rubberized concretes.     

Effects of silica fume on the geotechnical properties of fine-grained soils exposed to freeze and thaw

Both the landfill liner and cover systems are the most important parts on a waste disposal landfill site. These systems are generally constructed using compacted fine-grained soils. It is known that the strength and permeability are particularly affected by freezing and thawing cycles in the cold regions. The aim of this study is to reduce the effects of freezing and thawing cycles on the strength and permeability. To modify the fine-grained soils, silica fume generated during silicon metal production as very fine dust of silica from a blast furnace and historically considered a waste product has been used as a stabilizer. The natural fine-grained soils and soil–silica fume mixtures have been compacted at the optimum moisture content and subjected to the laboratory tests. The test results show that the stabilized fine-grained soil samples containing silica fume exhibit high resistance to the freezing and thawing effects as compared to natural fine-grained soil samples. The silica fume decreases the effects of freezing and thawing cycles on the unconfined compressive strength and permeability. We have concluded that silica fume can be successfully used to reduce the effects of freezing and thawing cycles on the strength and permeability in landfill liner and cover systems constructed from compacted fine-grained soils.     

Influence of silica fume content on microstructure development and bond to steel fiber in ultra-high strength cement-based materials (UHSC)

The use of silica fume can significantly enhance mechanical properties of concrete given its beneficial filling and pozzolanic effects. In this study, a simple and effective double-side pullout testing method was adopted to characterize the interfacial bond properties, which include pullout load-slip relationship, bond strength, and pullout energy, of steel fiber-matrix in ultra-high strength cement-based material (UHSC) with 0–25% silica fume by the mass of binder. The effects of silica fume content on flowability, heat of hydration, compressive and flexural strengths, hydration products, and pore structure of matrix at different curing time were evaluated as well. Backscatter scanning electron microscopy (BSEM) and micro-hardness measurement were used to examine the quality of interfacial transition zone (ITZ) around the fiber. In terms of the results, the optimal silica fume content could be in the range of 15%–25%. UHSC mixtures with these dosages of silica fume showed significant improvement in pullout behavior. Its bond strength and pullout energy at 28 d could increase by 170% and 250% compared to the reference samples without any silica fume. The microstructural observation verified the findings on the macro-properties development. Formation of more and higher strength of hydration products and refinement of ITZ around the fiber ensured higher micro-hardness, and thus improved the bond to fiber.     

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.     

Evolution of microstructure and mechanical behavior of concretes utilized in marine environments

Concrete in marine environments is exposed to chemical mechanisms of deterioration, most of them involving chloride and sulfate ions. The principal motivation of this study is to try to minimize the expansive reactions between the aggressive ions and the cement matrix. However, the most effective protection will undoubtedly be the one that prevents the penetration of aggressive substances. The ingress of any aggressive substance in concrete is determined by concrete’s porous structure, especially the accessible connected porosity. This porosity is defined by the composition of the concrete and the chemical characteristics of the cement. The results clearly show that concrete that includes silica fume is significantly less porous and less permeable. In the rest of the mixtures studied, the porosity is higher, and the pore radius is the most decisive factor in defining the permeability.     

Properties of cementitious systems containing silica fume or nonreactive microfillers

The object of this work was to explore the effects of silica fume on the microstructure of hardened paste and of the transition zone between paste and aggregates in concrete. The significance of aggregates as reinforcing fillers and their impact on some properties of the transition zone and bulk paste were resolved. The experimental procedure was based on simultaneous studies of model concretes and paste matrices extracted from fresh model concrete mixes. In addition, continuously graded aggregate concretes were prepared. Three sizes of a nonreactive microfiller (carbon black) and one reactive microfiller (silica fume) were applied. On the basis of microstructural studies and compressive strength tests, it was concluded that the primary effect of silica fume was generated by its physical (microfiller) properties, since the strengthening provided by reactive silica fume was similar to that obtained with nonreactive carbon black of similar size and shape. This effect was more significant from the point of view of the concrete strength enhancement than the chemical (pozzolanic) activity of the silica fume. In concretes containing either silica fume or carbon black, aggregates of high quality could serve as reinforcing filler. This could take place when sufficient densification of the transition zone occurred in the presence of silica fume or carbon black. Significant refinement of pore structure was observed in both types of paste matrices (containing silica fume or carbon black). However, this led to a relatively small influence on the paste strength. Concretes containing reactive silica fume or an inert carbon black microfiller behaved as a composite material, unlike conventional concrete.     

Correlation between compressive strength and certain durability indices of plain and blended cement concretes

In this study, plain, silica fume and fly ash cement concrete specimens prepared with varying water to cementitious materials ratio and cementitious materials content were tested for compressive strength, water permeability, chloride permeability, and coefficient of chloride diffusion after 28 days of water curing. The data so developed were statistically analyzed to develop correlations between the compressive strength and the selected durability indices of concrete. Very good correlations were noted between the compressive strength and the selected durability indices, particularly chloride permeability and coefficient of chloride diffusion, irrespective of the mix design parameters. However, these correlations were observed to be dependent on the type of cement.     

A study of the combined influence of condensed silica fume and a water reducing admixture on water demand and strength of concrete

In recent years condensed silica fume has been acknowledged as one of the most effective pozzolans ever added to concrete. Properly used in adequate amounts and combined with a water reducing admixture, many important qualities of concrete are achieved or secured. The principle of dosage of water reducing admixture adopted in this combination has been to dose a quantity proportional to the amount of silicate fume. The positive effect on concrete strength from this combination of admixtures has been described as an “efficiency factor” for 1 kg silica fume between two to five when the efficiency of 1 kg cement is the unit. This study shows first that the dosing principle used until now is contradicted by experimental evidence and secondly that the water reducing admixture (lignosulfonate) used in the experimental series studied is responsible for 60% or more of the strength gain at 28 days, whereas the silica fume has to be credited for less than 40%. A more correct “efficiency factor” can then be calculated.     

Air entrainment in fresh concrete with PFA

The results of a study into the influence of PFA on air entrainment in fresh concrete are discussed It is shown that the required dosage of AEA to produce an air content of 5.5 ± 0.5% in a PFA modified concrete mix is two-six times that required in the corresponding neat OPC concrete mix. The dosage of a vinsol based air entraining agent (AEA) required appears to be directly related to the PFA content of the mix. Similar direct relationships were obtained with a range of different PFAs. The dosage of an AEA based on the salt of a fatty acid appears to be sensitive to both PFA and OPC contents. For the type of PFA used, the variability of measured air content or the amount of air retained after continued agitation both indicated that vinsol based AEAs show the highest variability whilst fatty acid based AEAs show low variability. The between batch variability of air content was significantly improved by the addition of PFA regardless of the AEA used.     

A model predicting carbonation depth of concrete containing silica fume

Silica fume (SF) has been used since long as a mineral admixture to improve durability and produce high strength and high performance concrete. Due to the pozzolanic reaction between calcium hydroxide and silica fume, compared with ordinary Portland cement, the carbonation of concrete containing silica fume is much more complex. In this paper, based on a multi-component concept, a numerical model is built which can predict the carbonation of concrete containing silica fume. The proposed model starts with the mix proportions of concrete and considers both Portland cement hydration reaction and pozzolanic reaction. The amount of hydration products which are susceptible to carbonate, such as calcium hydroxide (CH) and calcium silicate hydrate (CSH), as well as porosity can be obtained as associated results of the proposed model during the hydration period. The influence of water-binder ratio and silica fume content on carbonation is considered. The predicted results agree well with experimental results.     

Review: Improving cement-based materials by using silica fume

The effects of silica fume as an admixture in cement-based materials are reviewed in terms of the mechanical properties, vibration damping capacity, freeze-thaw durability, abrasion resistance, shrinkage, air void content, density, permeability, steel rebar corrosion resistance, alkali-silica reactivity reduction, chemical attack resistance, bond strength to steel rebar, creep rate, coefficient of thermal expansion, specific heat, thermal conductivity, fiber dispersion, defect dynamics, dielectric constant and workability. The effects of silane treatment of the silica fume and of the use of silane as an additional admixture are also addressed.     

Effect of aggregates on the diffusion properties and microstructure of cement with slurried silica fume based materials

Diffusivity of cement-based materials is an important factor regarding durability and the service life prediction of concrete structures. The present research focuses on investigating the influence of aggregates on tritiated water diffusivity of cement-based materials containing slurried silica fume. Effective diffusion coefficients of mortars with several sand volume fractions varying from 0 to 65% were determined by through-out diffusion tests. Microstructure was examined by scanning electron microscopy associated to energy dispersive spectrometry analysis, thermogravimetric analysis, water and mercury porosimetry, and BET adsorption analysis. It was found that large agglomerated particles of silica fume observed in cement paste and mortar with a low sand content (here 10%), reduce pozzolanic reactivity and thus affect the effectiveness of silica fume on the materials sustainability parameters. The clusters present in these formulations are mainly due to the interaction of silica fume with calcium hydroxide of the mixing solution and not to the initial state of the slurry, which was well stirred and whose particles size were checked before use. However, the presence of high content of aggregates (more than 30% of sand volume fraction) during mortar's mixing improves the dispersion of slurried silica fume particles and helps to ‘shear’ and break up agglomerates of silica fume providing a better homogenization of the material and improving the microstructural and diffusivity parameters. The addition of superplastizer in mortars with more than 50% sand content may also participate in dispersing silica fume.