橡胶混凝土上浮试验研究

通过进行橡胶净浆试验,砂浆试验,橡胶集料混凝土三部分试验,利用Image-pro plus软件得到橡胶颗粒沿高度分布图,研究橡胶在各试件的上浮情况.试验结果表明:三种试验中浆体的流动度以及颗粒粒径是影响橡胶集料上浮的主要因素.净浆试验表明橡胶集料上浮情况与水灰比成正比关系,砂浆试验表明砂等细集料的掺入对橡胶集料上浮情况起抑制作用.混凝土试验表明坍落度不宜大于15 cm,振捣时间宜于5～15 s,当坍落度小于15 cm时,50目橡胶颗粒在混凝土中分布更均匀,当坍落度大于15 cm,20目与50目橡胶颗粒都上浮非常严重,并出现分层离析现象.     

Crack effects on gas and water permeability of concretes

The relationship between load-induced cracking and concrete permeability is studied. Ordinary concrete (OC) and high-performance concrete (HPC), including steel fiber-reinforced concrete (HPFRC), are used. Two discs, 50 mm-thick slices, cut from 110-220 mm cylindrical specimens are diametrically loaded, as for a normal splitting test. The lateral displacement, also called the crack opening displacement (COD) is monitored for each loading cycle. After unloading, gas and finally water permeability tests are both performed, using constant head permeameter, to compare the influence of the percolating fluid and the COD. Due to the wide range of measured gas flow, Klinkenberg's and Dupuit-Forcheimer's laws are applied to compute the intrinsic gas permeability. Results suggest it increases proportionally to the cube of the COD and it matches water permeability, if only the first water percolating time is considered. The roughness parameter of the cracks induced in each concrete, is compared and discussed.     

Crack closure of reinforced concrete by electrodeposition technique

The electrodeposition method for rehabilitation of cracked reinforced concrete, based on the electrochemical technique, is presented here. Investigations on reinforced concrete beams of size 15×15×125 cm were conducted in this study. Specimens cracked by chloride attack for 18 months were immersed in a ZnSO₄ solution, and were applied with a constant current for 8 weeks. The development of crack closure due to the precipitation of electrodeposits (ZnO) was evaluated. Water permeability, chloride ion concentration, polarization curve and half-cell potential were measured in order to assess the effectiveness of this method in the improvement of concrete properties. The results indicate that electrodeposits formed on the concrete surface were able to close the concrete cracks and to decrease the concrete permeability. In addition, the investigation shows that the application of electrodeposition have effects on the desalination of concrete and re-passivity of the reinforcing steel in concrete.     

The permeability of Portland limestone cement concrete

The effect of limestone addition on the air permeability, water permeability, sorptivity, and porosity of limestone cement concrete has been investigated. Six Portland limestone cements (PLCs) with different limestone content (10-35% w/w) were produced by intergrinding clinker, gypsum, and limestone. A water-to-cement ratio (w/c) of 0.70-0.62—depending on the cement strength class—was used to prepare concrete of the compressive strength class C20/25 of EN 206-1. A modified commercial triaxial cell for 100-mm-diameter samples was used for the determination of the gas (N₂) and the water permeability of concretes. In addition, the sorptivity and porosity of the samples were measured, while thin sections of the concrete specimens were examined by means of optical microscopy. It is concluded that the PLC concrete indicates competitive properties with the ordinary Portland cement (OPC) concrete. Furthermore, the limestone addition has a positive effect on the water permeability and the sorptivity of concrete.     

Residual stress-strain relationship for concrete after exposure to high temperatures

An experimental research is performed on the complete compressive stress-strain relationship for concrete after heating to temperatures of 100-800 °C. All concrete specimens are ?15 cm × 30 cm standard cylinders, made with siliceous aggregate. The heated specimens are tested at 1 month after they are cooled to room temperature. From the results of 108 specimens with two original unheated strengths, a single equation for the complete stress-strain curves of heated concrete is developed to consider the shape varying with temperature. Through the regression analysis, the relationships of the mechanical properties with temperature are proposed to fit the test results, including the residual compressive strength, peak strain and elastic modulus. Compared with the experimental curves, the proposed equation is shown to be applicable to unheated and heated concrete for different temperatures. In addition, the split-cylinder tests of 54 specimens are also carried out to study the relationship of splitting tensile strength with temperature.     

Interactions of Cold Radiofrequency Plasma with Solid Wood I. Nitrogen Permeability along the Grain

Exposure of 1.5 cm diameter × 1.2 cm long white fir and Douglas-fir heart-and sapwood samples to the action of cold radiofrequency oxygen, nitrogen, and helium plasmas strongly increased wood permeability to nitrogen flow along the grain. Oxygen was most effective, followed by nitrogen and helium. While an increase in radiofrequency energy level increased the effect, the opposite was true for an increase in reactant gas flow rate. With white fir the rate of permeability increase was higher at the beginning of the plasma treatment. This was attributed to differing ablation rates of cellulose, hemicelluloses, and lignin. No significant time difference in the rate of permeability increase was observed with Douglas-fir. Extractives suppressed the plasmainduced permeability increase; thus extraction with water and ethanol, followed by oxygen plasma treatment increased permeability of Douglas-fir heartwood 32 times, while plasma treatment of the unextracted Douglas-fir increased permeability only 8 times. The permeability of extracted and oxygen plasma-treated wood did not vary much across the grain, but did decrease with distance from the ends of the samples due to incomplete removal of extractives from the center.     

Influence of aggregate size and volume fraction on shrinkage induced micro-cracking of concrete and mortar

In this paper, the influence of aggregate size and volume fraction on shrinkage induced micro-cracking and permeability of concrete and mortar was investigated. Nonlinear finite element analyses of model concrete and mortar specimens with regular and random aggregate arrangements were performed. The aggregate diameter was varied between 2 and 16 mm. Furthermore, a range of volume fractions between 0.1 and 0.5 was studied. The nonlinear analyses were based on a 2D lattice approach in which aggregates were simplified as monosized cylindrical inclusions. The analysis results were interpreted by means of crack length, crack width and change of permeability. The results show that increasing aggregate diameter (at equal volume fraction) and decreasing volume fraction (at equal aggregate diameter) increase crack width and consequently greatly increases permeability.     

Effects and interactions of temperature and stress-level related damage on permeability of concrete

The objective of this study is to investigate damage-temperature-stress level-permeability interactions in structural concrete. The tests are performed on hollow cylindrical concrete specimens, subjected to compressive loading and temperature up to 150 °C. The results emphasize that at stress levels lower than 80% of the peak stress, the variation of permeability is small and it is slightly influenced by the stress. As a matter of fact, the permeability under load is smaller than the permeability measured after unloading. As the load exceeds 80% of the peak stress, micro-cracking increases rapidly, causing an increase of the permeability and a greater sensitivity to the applied load, i.e. a noticeable difference between the permeability measured under load and after unloading, the first becoming greater than the latter. In the post-peak phase the increase of permeability is much larger due to significant crack width growth. The increase of permeability with the applied load seems to be greater with temperature, inducing further alterations of concrete and dilation of the porous structure of the material. Finally, the experimental results seem to agree with the format of coupled evolution of the permeability due to damage and temperature assumed by Gawin et al. [D. Gawin, C.E. Majorana, B.A. Schrefler, Numerical analysis of hygro-thermal behaviour and damage of concrete at high temperature, Mechanics of Cohesive-Frictional Materials 4 (1999) 37-74.].     

Role of fibers in controlling unrestrained expansion and arresting cracking in Portland cement concrete undergoing alkali-silica reaction

An experimental study was undertaken to investigate the role of polypropylene or brass-coated steel fibers in controlling unrestrained expansions and delaying and arresting cracking in Portland cement concrete due to alkali-silica reaction. Portland cement concrete and fiber-reinforced concrete (FRC) mixtures were prepared at a w/c ratio of 0.40 using modified Type I cement, reactive fine particles, and coarse limestone aggregates. Prism (5×5×30 cm) and plate (13.5×13.5×3 cm) specimens were prepared and cured for 7 or 28 days before exposure to a special treatment to accelerate ASR. Expansion, time of cracking, and ultrasonic pulse velocity were determined over a treatment period of 65 days using prism specimens. Ultimate cracking pattern and extent were determined after a treatment period of 85 days using plate specimens. The results showed that while fibers did not contribute significantly to controlling pre-cracking and post-cracking expansions, they played a significant role in delaying cracks formation and limiting their extent. Considering its lower cost and content, the performance of polypropylene fibers was superior to that of brass-coated steel ones. The potential of brass-coated fibers in arresting ASR cracking was significantly affected by age of concrete when subjected to treatment.     

Behaviour of Slurry Infiltrated Fibre Concrete (SIFCON) under triaxial compression

Slurry infiltrated fibre concrete (SIFCON) is one of the recently developed construction material that can be considered as a special type of high performance fibre reinforced concrete (HPFRC) with higher fibre content. In the current research, triaxial compressive behaviours of high strength concrete (HSC), HPFRC and SIFCON were investigated. Purposefully, laboratory tests employed on four types of 75 × 150 mm cylindrical specimens with different steel fibres volumes (0, 2, 5 and 10%). All tests were conducted under four different confining pressure levels (0, 5, 15 and 21.5 MPa) according to triaxial conditions. Consequently, stress-strain curves were obtained and governing failure patterns and failure criterions of HSC, HPFRC and SIFCON samples were discussed. According to the results increasing of fibre volumes increases peak stress, energy absorption, toughness and Poisson's ratio while increasing confining pressures increases peak stress, energy absorption and toughness. Also, this may cause concrete to behave as a plastic material.     

The influence of particle size distribution on the surface appearance of glazed tiles

Matte glaze was milled in for different times (15–50 min) and the glazes obtained were applied over ceramic bodies which were then fired using an industrial cycle and the final surfaces were analyzed using a glossmeter. The same glazes were also uniaxially pressed, forming cylindrical samples; the specimens were placed on engobe surfaces and fired at 1100 °C in a laboratory oven. The contact angles between the melted specimens and the ceramic surfaces were measured to determine their wettability. The results showed a correlation between the glaze particle size distribution and the surface aspect of the final product.     

The effects of deicer salts on the pressure impregnation of bridge deck type concrete with methyl methacrylate

A group of cylindrical concrete specimens, containing various known quantities of deicer salts, were impregnated from the top surface under a pressure of 75 psig (517 KPa) until complete penetration of the concrete (5 in. or 12.7 cm) was achieved. It was found that increasing salt contents, up to 0.75% Cl^? by weight of the mortar fraction of the concrete, had no effect on quantity of monomer inbibed, the volume percent of porosity filled with the monomer, nor the degree of water absorptivity of the polymer impregnated concrete. A second group of specimens was impregnated from the top surface at various pressures ranging from 0 to 75 psig (101 to 517 KPa) to a given level of monomer loading. For a given pressure, monomer loading was found to vary directly with the square root of time. Also, it was shown that higher pressures provide denser polymer loadings, higher rates of impregnation, and for a given time period, increased depth of penetration.     

Effect of strength and age on the stress-strain curves of concrete specimens

Many researchers have rigorously studied the nonlinear behavior of the stress-strain relationship of concrete using mathematical curves. Currently, most empirical expressions for the stress-strain relationship, however, have been focused on hardened concrete and are unable to completely represent the behavior of concrete at an early age. A broad understanding of the behavior of concrete from early age to old age is important in evaluating the durability and service life of concrete structures.In this paper, the effect of five different strength levels and various ages from 12 h to 28 days on the compressive stress-strain curve was observed experimentally and analytically. Tests were carried out on ∅ 100×200 mm cylindrical specimens cured in a standard moist room at a temperature of 20±3 °C. An analytical expression of the stress-strain curve with strength and age was developed using regression analyses on experimental results. For verification of the proposed model equation, the equation was compared to the experimental data and existing model equations.The result shows that the proposed model equation is not only compatible with the experimental data but also describes satisfactorily the effect of strength and age on the stress-strain curve.     

Cement content determination through selective stain in hardened concrete

A cement quantification method has been developed in Ø 15 × 30 cm concrete specimens applying selective stain. A tannic acid–tartaric acid solution was used to stain the cement paste.This paper explains the procedure to determine the quantity of cement in the samples: the reagents utilised for the stain, preparation, cutting and stain of the specimens, the capture and treatment of images of the stained samples, and the preparation of the calibration curves.Next, we assess the accuracy of this method and include the resolution of a practical case in which a comparison is drawn between the values obtained through the application of this method and those obtained using a ‘reference’ method.     

Permeability of silica monoliths containing micro- and nano-pores

For applications as catalyst supports in flow reactors, porous silica monoliths require a combination of connected pores of micron-scale to enable fluid flow plus nm-scale pores to enable high catalyst area and activity. We have synthesised a range porous silica monoliths, characterised their micron and nm-scale pores and measured their permeability coefficients K. K can be controlled over the range 10⁻¹⁰–10⁻¹⁴ m², primarily by adjustment of the polymer/silane concentration ratio, whilst maintaining the specific surface area and nm-scale porosity approximately constant. For the majority of the silica monolith samples, the measured permeability coefficient K is 2–5 times smaller than K for a hypothetical reference system consisting of a monolith with uniform cylindrical pores aligned in the flow direction and with the same average pore diameter and volume fraction.     

Axial strain variations in concrete under uniform uniaxial compressive stress

This paper presents the results of an experimental study to determine the variations of axial strain in concrete under a uniform uniaxial compressive stress. Surface bonded and especially developed embeddable strain gage units are used for measuring axial strains in cylindrical concrete specimens under uniaxial compressive loads. Test results show that the axial strain decrease with the distance from the specimen face becoming uniform at a distance equal to about six times the maximum size aggregate used. These variations, reaching 40 percent in this investigation are confirmed by tests on drilled core cylinders obtained from the test specimens.     

Identification and quantification of cracks in concrete by optical fluorescent microscopy

Cracks in concrete structures can indicate major structural problems and can damage the appearance of monolithic construction. Cracking of concrete is a major factor affecting for the material strength and durability. The development of a crack pattern can contribute to increasing the permeability and the diffusivity of concrete, which is generally connected with a substantial reduction of its durability. This paper describes a method for identification and quantification of crack patterns in concrete by means of optical fluorescent microscopy and image analysis. Results obtained for undamaged and deteriorated specimens are presented. The range of investigation included several concrete mixes made in the laboratory. In order to induce cracks, concrete mixes were exposed to freezing action 0, 1 and 2 h after mixing. The concrete cubes of 100-mm and 150-mm size were frozen for 0 (reference specimens) and 2 days. Investigation of compressive strength, water permeability, chloride migration and analysis of cracks was made after 28 days. The low-temperature deteriorated specimens showed a significant reduction of compressive strength and resistance to water and chloride penetration in concrete. Correlations between density of cracks and compressive strength, depth of water penetration and depth of chloride penetration have been proposed.     

Relation between fibre distribution and post-cracking behaviour in steel fibre reinforced self-compacting concrete panels

In this research, the influence of the fibre distribution and orientation on the post-cracking behaviour of steel fibre reinforced self-compacting concrete (SFRSCC) panels was studied. To perform this evaluation, SFRSCC panels were cast from their centre point. For each SFRSCC panel, cylindrical specimens were extracted and notched either parallel or perpendicular to the concrete flow direction, in order to evaluate the influence of fibre dispersion and orientation on the tensile performance. The post-cracking behaviour was assessed by both splitting tensile tests and uniaxial tensile tests. To assess the fibre density and orientation through the panels, an image analysis technique was employed across cut planes on each tested specimen. It is found that the splitting tensile test overestimates the post-cracking parameters. Specimens with notched plane parallel to the concrete flow direction show considerable higher post-cracking strength than specimens with notched plane perpendicular to the flow direction.     

Determination of the apparent activation energy of one concrete by calorimetric and mechanical means: Influence of a superplasticizer

An instrument has been developed to perform calorimetric tests on concrete in isothermal conditions using 0.11×0.22-m cylindrical samples. The purpose of this article is to determine the apparent activation energy of one concrete as a function of temperature (10, 20 and 40 °C) using this technique and to compare it with that obtained by a mechanical means in order to validate the hypothesis made in maturity measurements, according to which the apparent activation energy values are assumed to be equivalent. The two means give relatively similar results (with differences of the order of 3 kJ/mol). It also seems possible to use a single apparent activation energy value over the entire temperature range (10-40 °C). Lastly, the effect of a superplasticizer on the apparent activation energy is examined, and it appears that its role is relatively small in the case of the concretes studied here.     

Effect of short fibers on residual permeability and mechanical properties of hybrid fibre reinforced high strength concrete after heat exposition

Mechanical and permeability performance of fibre reinforced high strength concrete after heat exposition were evaluated in the experimental study. Cylindrical concrete specimens were exposed to heat with the rate of 10 °C/min of up to 400 °C. In order to study the effect of short fibres on residual performance of heated high strength concrete, polypropylene and steel fibres had been added into the concrete mix. The melting and vaporization of its fibre constituents were found to be responsible for the significant reduction in residual properties of polypropylene fibre reinforced high strength concrete. In terms of non-destructive measurement, UPV test was proposed as a promising initial inspection method for fire damaged concrete structure. Furthermore, the effect of hybrid fibre on the residual properties of heated fibre reinforced high strength concrete was also presented.