Influence of age of loading,water-cement ratio and rate of loading on fracture energy of concrete

The applicability of linear elastic fracture mechanics to a composite material such as concrete has always been questioned. Recently, a new approach to describe failure of concrete has been developed. In this context a material is characterized by its fracture energy and the shape of the descending branch of the strain softening diagram. So far little work has been done to determine experimentally major influences on these material parameters. In this contribution results of test series to study the influence of age of loading, water-cement ratio, and rate of loading are presented. It is shown that a detailed evaluation of the test data necessitates appropriate computer programs. Essentials of these modules are briefly described. It is shown that failure of a beam under three-point bending mode can be predicted in a realistic way if valid material parameters are incorporated in a numerical analysis. Finally, it is pointed out that further studies are needed before a general application of the new approach can be recommended.     

Influence of water-cement ratio on the properties of 2-2 cement based piezoelectric composite

Composites with 2-2 connectivity were fabricated from plates of “PMN” ceramic embedded in a sulphoaluminate cement matrix by a cut-filling process. The influences of the water-cement ratio in the matrix on the properties of the composite were analyzed. The results show that when the water-cement ratio is less than 0.4, the piezoelectric stain factor d₃₃ and piezoelectric voltage factor g₃₃ increase smoothly. When the water-cement ratio is larger than 0.4, d₃₃ and g₃₃ increase obviously with increasing the water-cement ratio. This is attributed to a more effective contact between the active and matrix phases. d₃₃ = 322 pc N^− 1 and g₃₃ = 20.9 mV mN^− 1 at a water-cement ratio of 0.45. The planar electromechanical coupling coefficient K_p of the composite is nearly independent of the water-cement ratio. With increasing the water-cement ratio, the thickness electromechanical coupling coefficient K_t of the composite increases, while the mechanical quality factor Q_m exhibits the trend of decrease.     

Influence of the water/cement ratio on the air permeability of concrete

The durability of concrete structures is mainly affected by the transport of gaseous and liquid substances through its pore system which can potentially cause deterioration of the concrete. Thus, an important indicator of long-term durability is the relative ease with which each aggressive substance is transported through the concrete, in other words, its permeability. Studies were conducted to deepen knowledge of concrete permeability and, in particular, to understand how it is affected by the water-cement ratio, preconditioning temperature and testing pressure. The water-cement ratio is one of the main factors affecting concrete permeability; small changes in this ratio promote large permeability variations. An important increase of air permeability with the water-cement ratio and preconditioning temperature has been noticed. On the other hand, insignificant differences have been observed in the air permeability coefficient at the four testing pressures.     

Predicting the mechanical properties of ordinary concrete and nano-silica concrete using micromechanical methods

By combining several materials with specific mechanical properties, new materials with unknown mechanical properties are obtained. Various experiments are required to determine the mechanical properties of the produced composite materials. Since conducting experiment processes is costly and time-consuming, comprehensive studies have been conducted in recent years to solve the problem. Fortunately, it is possible to easily predict the mechanical properties of composite materials without the need to construct them, by inspecting their constituent’s properties using micromechanical methods. Although various micromechanical methods have been presented so far, few of them yielded precise predictions of the properties of composite materials. Therefore, selecting a method suitable to predict the properties of composite materials is of much importance. In this study, some micromechanical approaches, including Hirsch, Hansen, Bache, Cavento, Mori–Tanaka, Eshelby, self-consistent, effective interface and double-inclusion models, were employed for the estimation of elasticity modulus and Poisson’s ratio of ordinary and nanomaterial concretes. The results obtained from the micromechanical methods were compared to those obtained from experimental tests. The obtained numerical results showed that Bache’s model is the most accurate micromechanics model for predicting the elastic mechanical properties of ordinary and nanomaterial concretes.     

Observations on the nature of micro-cracking in brittle composites

The degree of micro-cracking in BeO-SiC composites due to internal stresses which arise from the mismatch in the coefficients of thermal expansion was monitored by measurements of the thermal diffusivity by the laser-flash technique. The experimental results indicated that micro-cracking was most extensive at approximately 30 and 80 wt% SiC and a minimum at nearly 50 wt% SiC. A theoretical analysis indicated that the magnitude of internal stress increases linearly with SiC content, so that the above observations cannot be attributed to a low internal stress state at ~ 50 wt% SiC. Instead, this effect can be attributed to changes in the statistical variables affecting the brittle fracture as well as the degree of internal stress relaxation. Both these factors are thought to be controlled by the nature of multiaxial stress distribution. At ~ 50 wt% SiC-content, due to anticipated non-hydrostatic triaxial stress distribution, residual stress relaxation is possible in both the components of the composite. However, at low and high fractions of SiC content, such stress relaxation is less likely to occur due to the expected hydrostatic stress distribution in one of the components.     

The investigation of aggregate grain size effect on fracture toughness of ordinary concrete structures

The material under investigation was ordinary conerete of B 20 class, different in the sizes of coarse aggregate and water-cement ratio of the same value w/c=0.55. It was found during the tests that there were two kinds of sample cracking mechanisms. Some samples cracked at the moment of the initiation of slit (when P _Q=P _max); in others the decohesion force P _max exceeded P _Q, which was identified as a minor deflection or relative extremum of the curve. The frequency of the second type of cracking increased with the growing size of coarse aggregate grains. Together with the growth of the grain size, the angle of curve inclination also became smaller. This phenomenon can be explained by the fact that, in the process of decohesion, the aggregate grains are relocated and the system of contact between them changes, so the K _Hc depends on coarse aggregate.     

Influence of Si:Al ratio on the microstructural and mechanical properties of a fine-limestone aggregate alkali-activated slag concrete

Three series of fine limestone aggregate, alkali-activated blast furnace slag (AAS) concretes were fabricated and tested; two through activation with waterglass/NaOH solution, of which one included NaCl as a retarding agent, and one activated by Na₂CO₃. Each of these series was made up of three formulae containing different amounts of Al₂O₃. The compressive strengths of the series activated by waterglass/NaOH after 28 days were ≈65 ± 5.3 MPa, a 22% increase compared to previously reported formulae containing no additional Al₂O₃. Increasing the amount of Al₂O₃ did not further increase strength, however. The Na₂CO₃-activated formulae had strengths of ≈35 ± 3 MPa after 28 days, representing no increase in strength over formulae not containing Al₂O₃ previously reported. X-ray diffraction showed the main binding phase to be calcium silicate hydrate (C–S–H) gel, as is commonly found in ordinary Portland cement (OPC). Fourier transform infrared spectroscopy showed little difference from the previously reported results for formulae not containing Al₂O₃ and strongly resemble the spectra reported elsewhere for C–S–H. Electron microscopy, coupled with energy dispersive spectroscopy, showed the cementing phase to be a single homogenous phase—not a mixed system of geopolymer and C–S–H gel—with a lower volume fraction of unreacted slag than formulae without Al₂O₃. The reason for the increase in strength of Al₂O₃-containing formulae is unclear, but is unlikely to be ascribed to the formation of large amounts of ‘geopolymers’ and may be related to a possible increase in reaction temperature of between 2 and 5°C, depending on amount of additive.     

Development of a confined model for rectangular ordinary reinforced concrete columns

In order to estimate the flexural behaviour of reinforced concrete members, the stress–strain behaviour of the constituent materials must be well established. The behaviour of confined concrete is important to the designer in order to determine the quantity of the confining steel required in reinforced concrete column sections to achieve the ultimate curvatures required in seismic design for ductility. The objective of this study is to develop an analytical model for the confinement mechanism in rectangular reinforced concrete columns. The proposed model was established on the basis of the observations derived from several experimental studies conducted in past years and encompasses some features of already established models. The main variables examined in the model include the volumetric ratio of lateral reinforcement, the characteristics of steel and concrete and the effectiveness confinement coefficient. The comparison with other existing models illustrates the validity of the proposed equations.

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Résumé L’issue fondamentale permettant de définir la capacité flexionnelle des éléments en béton armé est la connaissance des lois contraintes déformations des matériaux constitutifs. Le comportement du béton confiné est donc l’élément capital qui devra permettre au concepteur d’estimer, par utilisation de l’armature transversale, la ductilité de courbure exigée pour une demande de ductilité désirée. L’objectif de cette étude est de développer un modèle analytique tenant compte du mécanisme de confinement dans les poteaux en béton armé. Le modèle est basé sur une analyse rigoureuse de modèles analytiques existants et une exploitation de données expérimentales réalisées durant les deux dernières décennies. L’étude a pris en considération l’influence des différents paramètres, notamment le rapport volumétrique des aciers transversaux, les caractéristiques mécaniques de l’acier et du béton et le coefficient efficace de confinement. La comparaison avec d’autres modèles existants confirme la validité des équations proposées.

     

Influence of the water to cement ratio W/C on the compressive strength of concrete — An application of the cement hydration equation to concrete

The cement hydration equation was used to determine the influence of the water to cement ratio, W/C, on the compressive strength of concrete up to the end of the hydration age. Using the hydration equation, the equations of the compressive strength of concrete were derived for five values of the ratio W/C equal to 0.50, 0.55, 0.60, 0.65, and 0.70. The curves of variation of the compressive strength versus the W/C ratio for the hydration age of 7, 28, 90 and 5475 days were drawn. Based on these curves a thorough study of the influence of the W/C ratio on the compressive strength of concrete was undertaken.     

Effective creep Poisson's ratio for damaged concrete

First, creep data are presented for concrete under high sustained compressive stress which is, over the long-term strength of the concrete. Creep in both axial and lateral directions is reported. Creep Poisson's ratio has remarkable change before failure, and a sharp increase of creep Poisson's ratio can be observed in the region of failure.Secondly, a damage model is developed for the analysis of creep damage in both axial and lateral directions; effective Poisson's ratio of damaged material as a model parameter plays an important role for evaluating lateral damage, which is similar to the effective Young's modulus in evaluating axial damage.     

Mechanical damage of ordinary or prestressed reinforced concrete beams under cyclic bending

The behaviour of metallic materials under repeated loading has been examined since the 19th century, but extended studies are more and more needed especially for reinforced concrete structures such as bridges, where high-cycle fatigue phenomena can be significant. In the present paper, a theoretical model based on fracture mechanics concepts is proposed in order to analyse the mechanical damage of ordinary or prestressed reinforced concrete beams with a rectangular or a T cross-section subjected to cyclic bending. Local phenomena, such as fracturing or crushing of concrete and yielding or slippage of the longitudinal steel reinforcement, are examined. Further, fatigue life is predicted by applying a crack growth law, and the energy dissipated during the plastic shake-down phenomenon is evaluated.     

Influence of size on fracture energy of concrete

Concrete structures frequently exhibit cracks. In order to investigate the influence of cracks on durability and the remaining load bearing capacity of buildings, fracture mechanics models can be used. For materials like concrete non-linear models for example the fictitious crack model introduced by Hillerborg, Modéer and Petersson [1] or the crack band model proposed by Ba?ant and Oh [2] are able to describe crack formation and propagation in a realistic way. But it has been observed, that the parameters of these models depend both, on structural size and geometry. In this paper a simple model to explain the nature of size and geometry dependence of the specific fracture energy is presented. The model is evaluated with experimental data from Trunk and Wittmann [2,4,5].     

等强度下混凝土组分对徐变性能的影响

采用自制的徐变加载装置,研究了聚乙烯醇(PVA)纤维、双掺粉煤灰和矿渣以及减缩剂对7d等强度混凝土徐变性能的影响规律,结合与混凝土同水胶比浆体的化合结合水量分析了其影响机理.结果表明,混凝土徐变系数发展较快,加载100d左右趋于稳定;减缩剂和双掺矿物掺合料均明显降低了混凝土的徐变系数,以掺减缩剂效果更好,450d值仅为...     

Influence of loading rate on concrete cone failure

Three different effects control the influence of the loading rate on structural response: creep of bulk material, rate dependency of growing microcracks and structural inertia. The first effect is important only at extremely slow loading rates whereas the second and third effects dominate at higher loading rates. In the present paper, a rate sensitive model, which is based on the energy activation theory of bond rupture, and its implementation into the microplane model for concrete are discussed. It is first demonstrated that the model realistically predicts the influence of the loading rate on the uniaxial compressive behaviour of concrete. The rate sensitive microplane model is then applied in a 3D finite element analysis of the pull-out of headed stud anchors from a concrete block. In the study, the influence of the loading rate on the pull-out capacity and on the size effect is investigated. To investigate the importance of the rate of the growing microcracks and the influence of structural inertia, static and dynamic analyses were carried out. The results show that with an increase of the loading rate the pull-out resistance increases. For moderate loading rates, the rate of the microcrack growth controls the structural response and the results of static and dynamic analysis are similar. For very higher loading rates, however, the structural inertia dominates. The influence of structural inertia increases with the increase of the embedment depth. It is shown that for moderately high-loading rates the size effect becomes stronger when the loading rate increases. However, for very high-loading rate the size effect on the nominal pull-out strength vanishes and the nominal resistance increases with an increase of the embedment depth. This is due to the effect of structural inertia.     

Influence of corrosion inhibitors on concrete transport properties

The effect on transport properties of admixing corrosion inhibitor in Portland cement based concrete is studied experimentally in this paper. Two commonly available types of corrosion inhibitor have been evaluated: an organic corrosion inhibitor and a nitrite based inhibitor. The gas permeability, water permeability and chloride migration of the resulting concrete were evaluated by means of standard test methods. In some cases, 1% sodium chloride was added to the concrete. It is found that both types of corrosion inhibitor lead to higher transport coefficients. The increment caused by the nitrite based inhibitor is higher than for the amine-ester based inhibitor. The influence of sodium chloride addition during mixing is not prominent. The obtained transport properties are analysed and discussed by means of BSE experiments and image analysis.     

On poisson's ratio and volumetric strain in concrete

A new procedure is proposed for identifying uniaxial stress-strain relationship and Poisson's ratio in compressed plain concrete. The procedure is based on the assumption of an internal core of intact material always present inside a specimen in uniaxial-compression. This involves a modification of the traditionally identified uniaxial stress-strain relationship and Poisson's ratio, which turns out to be almost independent of the loading step. The main finding concerns the volumetric strain, since it appears to be no real increase in the volume of a concrete solid when the solid is placed under pressure.     

Influence of confinement on high strength concrete behavior

An experimental program has been made in order to study the confined concrete behavior when its strength changes from traditional values to high strength values and with confinement levels ranging from 0% to values higher than 4%. The specimen shape and size have also been included as variables. With the data experimentally achieved, the parameters that define the stress-strain curve for concrete were adjusted using a statistical methodology that gives us suitable approximation levels. A stress-strain curve model is proposed, which lets us know precisely the confined concrete behavior up to high strain levels and analyze the material ductility. The achieved improvement, thanks to the confinement, was quantified regarding the parameters that define the concrete behavior, particularly the maximum strength, the strain at the peak and the ductility.

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Résumé Un programme expérimental a été développé pour étudier le comportement du béton confiné avec une teneur en armature transversale de 0% à 4%. L'étude analyse des bétons de résistance entre 25 MPa et 100 MPa. D'autres variables ont été la forme et la dimension des éprouvettes. Avec les résultats expérimentaux obtenus on a réalisé une analyse statistique pour définir les paramètres de la courbe Contrainte-déformation avec une bonne corrélation. On a proposé un modèle de courbe Contrainte-déformation qui montre le comportement du béton à hautes performances confiné jusqu'à des niveaux de déformation très élevés, et qui permet l'analyse de la ductilité du matériau. Les performances obtenues grace au confinement ont été quantifiées d'après les paramètres qui définissent le comportement du béton, en particulier de sa résistance, de la déformation au pic et sa ductilité.