不同冻融介质作用下混凝土力学性能衰减模型

通过快速冻融试验,研究了三种不同冻融介质(水、3.5wt%NaCl、飞机除冰液)对混凝土质量损失、动弹模量以及力学性能的影响,比较了三种冻融介质对混凝土损伤程度的大小,分析了混凝土相对动弹性模量与相对剩余抗压强度和相对剩余抗折强度之间的关系,基于相对动弹性模量建立了相对剩余抗压强度和相对剩余抗折强度衰减方程。结果表明:3.5wt%NaCl溶液对混凝土的损伤度要远大于单纯水冻融循环对混凝土的损伤度,飞机除冰液对混凝土冻融损伤具有抑制作用;混凝土抗压、抗折强度以及相对动弹性模量随着冻融循环次数的增加而降低;三种冻融介质下混凝土抗压、抗折强度损失率大小关系为:3.5wt%NaCl水飞机除冰液;相对动弹性模量与相对剩余抗压强度、相对剩余抗折强度相关性好,可以通过测定混凝土相对动弹性模量来评估混凝土相对剩余强度。     

Damage evaluation in freezing and thawing test of concrete by elastic-wave methods

In the freezing and thawing test of concrete, elastic-wave methods are applied to estimate the frost damage nondestructively. Recently, two test methods are proposed in the RILEM recommendation. The alternative method A is based on the ultrasonic test, which measures the transmission time of the longitudinal wave (P wave), while the alternative method B measures the resonant frequencies of vibrations. With relation to these methods, evaluation of the frost damage by the elastic-wave methods is studied, focusing on the dynamic modulus of elasticity. The freezing and thawing tests were conducted by employing concrete samples after 1 year curing. As a result, the samples were not heavily damaged even after 300 cycles. Concerning the resonant frequencies of concrete samples, a three-dimensional analysis is performed by the boundary element method to identify actual vibration modes. It is clarified that an assumption of the one-dimensional resonant vibration is not applicable, resulting in a false discrepancy between the dynamic modulus and the static modulus of elasticity. It is found that relative modulus obtained from P-wave velocity is comparable to that from the tangential modulus of elasticity in the compression test, and is reasonably recommended to estimate the frost damage nondestructively.     

Considerations on the Italian method for freezing and thawing durability of concrete

The identity of test systems in the different laboratories is essential because the results on the freezing behaviour of concrete should be comparable. In that aim, after many tests, the Working Group “Permeability and Durability” of Unicemento has published a standard on concrete freezing. Series of tests have been specially carried out in order to determine the influence of different parameters on test results (size of specimens, saturation degree, time of single cycle phases, velocity of freezing and thawing, etc...). On the basis of the results obtained, the enclosed standard is proposed.     

Colour and ultrasound propagation speed changes by different ageing of freezing/thawing and cooling/heating in granitic materials

In the present work we determined the chromatic coordinates (L*, a*, b*) and ultrasound propagation speeds on the three spatial planes (V_x, V_y, V_z) of three ornamental granites (Aqueduct of Segovia, Spain) before, during, and after being subjected to 70 cycles of two types of accelerated ageing (typical of cold regions): a) freezing/thawing and cooling/heating (T1), and b) freezing/thawing and cooling/heating + salt crystallization (T2). A multivariate technique (Canonical Biplot) was applied to the data obtained, with the observation of significant variations between the two types of accelerated artificial ageing as compared with those obtained in quarry rock in the three chromatic coordinates (L*, a*, b*). With regard to the ultrasound propagation speed, we only detected differences in the results of the T2 artificial ageing treatment with respect to those of quarry rock. This fact is confirmed by the estimated data of resistance to compression.     

Mould growth on meat at freezing temperatures

Meat is usually spoiled by bacteria, which grow more rapidly than moulds. It is usually suggested that mould spoilage occurs mainly with frozen meat when the temperature permits growth of moulds but not bacteria. Moulds were thought to grow at —10°C or even lower temperatures, while bacteria cannot grow much below —2°C, the freezing point of meat. It has also been suggested that the reduced water activity of frozen media rather than the temperature per se may determine the minimum growth temperatures for moulds. Examination of meat spoilage moulds showed most to be moderately xerotolerant with minimum growth temperatures of —5°C or higher. In only one species was growth limited by reduced water activity rather than temperature. During prolonged storage at —5°C, meat developed a flora dominated by yeasts. Visible mould colonies did not appear until the eighth month of storage. It therefore seems that most cases of mould spoilage arise when the temperature of meat surfaces approaches 0°C with surface drying inhibiting bacterial growth.     

Analysis of damage development in cement paste due to ice nucleation at different temperatures

From the thermodynamic considerations and from a virtual microstructure of cement paste, a quantitative assessment of the internal damage of cement paste due to hydraulic pressure is investigated by 3D lattice fracture model. Two issues are solved in this study: (1) the capillary pores related to the penetration of ice crystals are characterized by a multi-step digitalization algorithm. (2) The hydraulic pressure P_L is achieved through an iterative fracture process simulation where the creation of micro-cracks is taken into account. The iterative fracture process simulation then is verified by a benchmark test. Simulations indicate that randomly distributed micro-cracks are formed at a low ice crystal saturation degree S_C (defined as the molar percentage of water transformed into ice crystals), while local propagation of micro-cracks occurs and transverse cracks can be formed at high S_C. Comparison of S_C and cracks pattern by simulation and by experiments are finally discussed.     

Modeling of concrete nonlinear mechanical behavior at high temperatures with different damage-based approaches

Heating of concrete leads to decrease of mechanical properties due to different damage processes: thermal strain gradients, chemical reactions, increase of vapour pressure (which is not taken and strain incompatibilities (between cement paste and aggregates, which is not taken into account explicitly here). This research project deals with comprehension of mechanical modeling (smeared cracking approach) on concrete’s behavior at high temperatures so as to be able to predict the previous mechanical damage processes and properties decreases. Two types of mechanical models are used: an isotropic elastic damage model (IED) and an orthotropic elasto-plastic damage model (OEPD). Transient thermal creep model (TTC) is also introduced. Different temperature rates are investigated and different virtual mechanical tests are performed (uniaxial tension, uniaxial compression, biaxial tension). The paper concludes, using numerical analysis and comparisons to experimental results, on relevances and performances of these different mechanical models.     

Performance of metakaolin concrete at elevated temperatures

An experimental investigation was conducted to evaluate the performance of metakaolin (MK) concrete at elevated temperatures up to 800 °C. Eight normal and high strength concrete (HSC) mixes incorporating 0%, 5%, 10% and 20% MK were prepared. The residual compressive strength, chloride-ion penetration, porosity and average pore sizes were measured and compared with silica fume (SF), fly ash (FA) and pure ordinary Portland cement (OPC) concretes. It was found that after an increase in compressive strength at 200 °C, the MK concrete suffered a more severe loss of compressive strength and permeability-related durability than the corresponding SF, FA and OPC concretes at higher temperatures. Explosive spalling was observed in both normal and high strength MK concretes and the frequency increased with higher MK contents.     

Dehydration creep of concrete at high temperatures

In this paper, a new approach for modeling the transient component of the load induced thermal deformation is proposed in order to predict the concrete behavior when subjected to high temperatures with a concomitant applied load. This component is conventionally referred to as transient creep strain. In this approach, the transient creep strain is split into a drying creep component and a newly introduced dehydration creep strain. The former is related to the evolution of the hygrometric state of the material, while the latter is related to the material dehydration which results from the heating induced chemical transformations. Therefore, a dehydration variable is defined and then introduced as a driving variable of the transient creep for temperatures exceeding 105°C. This thermo-hydro-damage model is implemented using a finite element code and␣numerical simulations are performed and compared to experimental findings in order to assess the predictive character of the proposed model.

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Résumé Dans cet article, une nouvelle approche pour la modélisation de la composante transitoire de la déformation thermique induite sous charge est proposée afin de prédire le comportement du béton à hautes températures. Cette composante est conventionnellement connue sous le nom du fluage thermique transitoire. Dans cette approche, le fluage thermique transitoire est décomposé en fluage de dessiccation et en une composante, nouvellement introduite, de fluage de déshydratation. La première composante est due à l’évolution hygrométrique du matériau tandis que la deuxième est due à la déshydratation du matériau qui résulte des transformations chimiques induites par l’augmentation de la température. Par conséquent, une variable de déshydratation est définie et est introduite comme une variable régissant le fluage thermique transitoire lorsque la température dépasse 105°C. Ce modèle thermo-hydro-endommageable est implémenté dans un code aux éléments finis. Des simulations numériques sont effectuées et comparées à des résultats expérimentaux pour analyser les capacités prédictives du modèle proposé.

     

Dielectric characterization of concrete at high temperatures

For reliable modelling of microwave heating of concrete its complex permittivity has to be known precisely within the full range of working temperatures. Dielectric characterization of dry concrete cured with different water-to-cement (w/c) ratios and concrete samples from nuclear power plant constructions was performed during heating and cooling cycles from room temperature to 700 °C and back. On average, higher permittivity values are found for concretes cured with smaller w/c ratio (more dens and less porous) as compared to concretes cured with higher w/c ratio (lighter and more porous). Samples from nuclear power plant reveals a permittivity close to the concrete prepared with lowest w/c ratio. Permittivity change along increasing temperature correlates with moisture loss and thermal decomposition reactions. These reactions are irreversible that lead to a permittivity divergence in heating and cooling scenarios. The variations of concrete permittivity because of w/c ratio, water transport and decomposition reactions are discussed.     

Experimental data for freezing and thawing of multi-dimensional objects

Testing the accuracy of freezing and thawing time prediction methods requires accurate experimental data. To complement existing data, 175 experimental measurements, 68 for thawing of rectangular bricks and 107 for both freezing and thawing of 12 different multi-dimensional irregular shapes, were made using Tylose, a food analogue, over a wide range of conditions. Twelve additional experiments were conducted using an actual food material, minched lean beef. Details of all the experimental conditions are reported.     

Internal friction and dynamic modulus transitions in hardened cement paste at low temperatures

Summary Low-temperature dynamic mechanical measurements in the system hardened cement paste and water indicated the existence of an internal friction peak and a modulus transition. The transition occurred in the temperature range −160° to −60°C. The transition was found to be enhanced by increasing the moisture content and the extent of the internal surface. The transition is believed to be associated with a phase change of the adsorbed water. Hence the name “adsorbate transition” seems appropriate. Though the true nature of the low-temperature phase is not known, possible means of determining the characteristics of this phase are referred to and suggested. Stress induced phase transition is regarded as a possible cause for the internal friction peak. The method is regarded as having a great potential value in studying water adsorbing systems. In particular, calibration of the internal friction peak versus some other measure of the internal surface area, yields a new method for determining the extent of internal surface areas on a relative scale.

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Résumé Les measures dynamiques à basse température faites sur le système pate de ciment durcie-eau montrent l'existence d'une valeur maximale du frottement interne et d'un changement de module. Ce changement de module se produit entre −160° et −60°C et on établit qu'il est accentué par l'accroissement de la teneur en eau et de la surface interne. L'auteur pense que ce changement est associé à un changement de phase de l'eau adsorbée. On pourrait donc la désigner comme ?transition d'adsorbat?. Quoique la nature véritable de la phase à basse température soit inconnue, l'auteur suggère des moyens de détermination des caractéristiques de cette phase. Le changement de phase induit par la contrainte est considéré comme une cause possible de la valeur maximale du frottement interne. L'auteur pense que la méthode peut avoir une grande valeur pour l'étude des systèmes à adsorption d'eau. En particulier, de l'étalonnage de la valeur maximale du frottement interne en fonction de quelque autre mesure de la surface interne découle une nouvelle méthode pour déterminer l'extension relative des surfaces internes.

     

High pressure freezing and thawing of foods: a review

The phase diagram of water shows that the melting temperature of water decreases with pressure down to −21°C at 210 MPa while the opposite effect is observed above this pressure. This phenomenon allows the achievement of rapid freezing and thawing of foods that mainly contain water. In addition, pressure-assisted thawing has the advantage of inducing a reduction in drip loss which tends to be a function of process parameters and nature of the product. Concerning pressure shift freezing, this process permits the significant preservation of the microstructure of biological substances. The current status of high pressure freezing and thawing applications in foods is reviewed in this paper. Concepts and principles underlying the application of these technologies are firstly developed. Available literature on the applications of high pressure freezing and thawing is then presented and discussed. Finally, the modelling aspects of theses processes are dealt with.     

Effects of Co content on tensile properties and deformation behaviors of Ni-based disk superalloys at different temperatures

Tensile properties of three Ni-based disk superalloys with 5 wt.%, 15 wt.% and 23 wt.% Co contents were investigated from room temperature (RT) to 725 °C with a constant strain rate of 3 × 10^− 4 s^− 1. It is found that addition of Co enhances the yield strength and the strain hardening capacity of the alloy in the studied temperature regime. It is due to the following two reasons: i.e. the interactions between a large volume fraction of fine secondary and tertiary γ′ precipitates with the dislocations in slip bands at lower temperatures and the formation of deformation microtwins at higher temperatures. However, the formation of deformation microtwins in the high Co-containing content alloy sharply decreases the ductility at higher temperatures.     

A study of concrete hardening at temperatures below zero

The porosity, strength, permeability, and frost-resistance kinetics in concrete hardening at a below-zero temperature have been studied by the methods of heat and mass transfer.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 22, No. 6, pp. 1060–1067, June, 1972.     

Bond shear modulus of reinforced concrete at high temperatures

The effect of fire and high temperature on the behavior and properties of concrete has drawn considerable attention. In this work an experimental program is used to determine the effect of high temperature on the interfacial bond shear modulus between concrete and reinforcement. Steel bars of different diameters were embedded in concrete cylinders for a depth less than that required for total development to assure failure by loss of bond. Specimens were then kept in an oven for different time durations and different temperatures. Specimens were then cooled by either keeping cylinders at room temperature or immersing them in water. The pull-out test was applied, and loads and displacements were recorded. Results from the pull-out test were then used along with an analytical model to calculate the bond shear modulus. The analytical model is based on the physical representation of the pull-out test, assuming linear elastic behavior of both steel and concrete.