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.     

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.     

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.     

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.     

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.

     

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.     

Heat emission accompanying thawing of a vertical ice surface

A dependence is proposed for determining the heat-transfer coefficient at the boundary of an ice massif and a water film running down the ice in the presence of thawing.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 49, No. 6, pp. 1034–1038, December, 1985.     

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.     

Dynamic properties of hardened paste,mortar and concrete

The dynamic moduli and the damping properties of hardened paste, mortar matrix and concrete prisms excited in the fundamental longitudinal, flexural and torsional vibrations are presented. The effects on dynamic properties of curing conditions, water-cement and aggregate-cement ratios, the type of aggregate, mode of vibration and of the presence of rigid inclusions in a relatively soft matrix are investigated. The stress-strain behaviour and the mechanism of damping of cementitious materials are discussed. Relations between dynamic moduli and strength, between damping capacity and compressive strength, and between dynamic moduli and the properties of the constituents are presented.     

Experimental study of creep of hardened Portland cement paste at variable water content

Tests of creep under axial load and torque have been made using tubular specimens of extremely small wall thickness (0.7 mm) in order to achieve sufficiently rapid moisture exchange with the environment. The changes of relative humidity and temperature in a program-controlled environmental chamber have been gradual, so as to minimize the differences in pore humidity throughout the specimen wall and the accompanying residual stresses and microcracking. A number of different humidity and temperature histories, including the drying before and during the creep test, and the humidity changes during the creep test and during the recovery, have been tested. The measurements have revealed a decline of the slope of creep curve in log-time after a sufficiently long drying period; acceleration of creep as well as recovery by both drying and wetting; a smaller and more delayed acceleration at lower humidities; a delay of this acceleration with respect to the weight loss; a similarity of these effects in axial and torsional creep; a higher recovery as well as creep at higher humidities when moisture equilibrium has been approached before loading; a higher creep acceleration by temperature increases or decreases when the humidity is below saturation, but a smaller acceleration at nearly dry state; and other effects.     

Modelling of deformations of high strength concrete at elevated temperatures

A constitutive model for the analysis of deformations of concrete subject to transient temperature and pressures is proposed. In these severe conditions concrete structures experience spalling phenomenon, which is the violent or non-violent breaking off of layers or pieces of concrete from the surface of a structural element when it is exposed to high and rapidly rising temperatures. This process can lead to a loss of load-bearing capacity, trough a loss of section and a loss of protection to steel reinforcement. Many different form of spalling exist, but probably the most dangerous is explosive spalling, because it is sudden and capable to result in a general collapse of the structure.The constitutive model includes thermo-chemical and mechanical damage for taking into account the deterioration of the material due to mechanical loads, high temperatures and chemical changes and it is introduced into a general coupled mathematical model of hygro-thermo-chemomechanical behaviour of concrete structures.In this constitutive model the so called free thermal strains, which are the concrete strains during first heating, are decomposed in three main contributions: thermal dilatation strains (treated in a manner usual in thermomechanics), shrinkage strains (modelled by means of the effective stress principle) and thermo-chemical strains (which take into account for the thermo-chemical decomposition of the concrete and which are related to thermo-chemical damage). Thermo-mechanical strains occurring during first heating of concrete under load, known as LITS (Load Induced Thermal Strains), are also included in the framework of thermodynamics of porous media. The proposed model is applied to an illustrative example that demonstrates its capabilities.     

Behaviour at cryogenic temperatures of tendon anchorages for prestressing concrete

Tendon anchorages are sensitive zones where fractures may be produced and this risk may be increased at low temperatures. No international standards for tendon anchorages at low temperatures have yet been developed. This paper aims at two objectives: to furnish information on the behaviour, at low temperatures, of two commercial tendon anchorage systems and to make some proposals, as well as related testing procedures, for pre-stressing systems intended for cryogenic applications.