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.     

Prediction of spalling in fibre-reinforced high strength concrete at elevated temperatures

This experimental study investigates two spalling test methods, for small scale specimens made of fibre-reinforced high strength concrete exposed to elevated temperatures, namely a blowtorch test and a furnace test. The study also aims at developing a relationship between relative maximum pore pressure and spalling, in order to determine a threshold relative maximum pressure for predicting spalling in heated concrete. The test results showed that the blowtorch test was a more effective and economical test method for carrying out spalling tests in small, unloaded and unrestrained specimens. Since such small specimens almost always do not spall during furnace tests, a blowtorch spalling test can help to provide relevant and useful data for mitigation of spalling in real scale structural elements. Also, with a blowtorch spalling test, a clear relationship between relative maximum pore pressures and spalling in heated concrete was observed, and a threshold relative maximum pore pressure above which spalling is likely to occur was suggested.     

Code provisions for high strength concrete strength-temperature relationship at elevated temperatures

This paper presents results of experiments, conducted at NIST and elsewhere, to measure compressive strength of concrete at elevated temperature. The paper compares the test data with existing design rules and recommendations to assess their applicability to HSC. Based on the compiled data, the paper proposes new strength-temperature relationship for HSC and discusses the need for standardizing the test procedure for testing concrete at high temperature and for a revision of the current design guides to include new data for properties of concrete at high temperature.

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Résumé Cet article présente les résultats expérimentaux, recueillis au NIST et ailleurs, pour mesurer la résistance à la compression du béton soumis à des températures élevées. Cet exposé compare les données disponibles avec les règles et recommandations existantes pour la conception d’un béton afin de déterminer leur applicabilité pour le béton à haute résistance (HSC). Basé sur les données compilées, cet article propose une nouvelle relation entre la résistance et la température pour le béton à haute résistance, On discute de la nécessité de standardiser la procédure de mesure de la résistance du béton à haute température. On discute aussi de la nécessité de revoir les guides actuels pour y inclure les données récemment disponibles concemant les propriétés du béton à haute tempèrature.

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Editorial Note Dr. Long T. Phan is a RILEM Senior Member. He participates in the work of RILEM TC HTC ‘Mechanical concrete properties at high temperature-Modelling and applications’. Dr. Nicholas J. Carino is also a RILEM Senior Member. Both of them work at NIST, a RILEM Titular Member.     

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.     

Photoconductivity of ZnTe thin films at elevated temperatures

Photoconductivity of thermally evaporated ZnTe thin films was studied at different elevated temperatures. A gap type cell configuration with Al electrodes on glass substrates was used. The conductivity was found to obey two distinct conduction mechanisms within the region of applied fields. At low fields the photoconduction is ohmic and at high fields it is of Poole-Frenkel type. With increase of ambient temperatures, the Poole-Frenkel conductivity regions were found to extend to lower fields. The temperature dependence of dark conductivity also was found to be of similar nature. The paper was presented at the 6th Asian Thermophysical Properties Conference (6th ATPC), held at Gauhati University, during 8–11 October 2001.     

Infra-red studies of vitreous silica at elevated temperatures

The purpose of this paper is to present a new approach to the problem of structure of the ordered regions in X-ray amorphous silica glass. The problem is open because of the differences in scientific opinion as to which polymorph of silica is the true crystalline model of vitreous silica. Infra-red absorption spectra of cristobalite and silica glass taken at temperatures ranging from room temperature to 500° C were used to identify a characteristic low-high temperature polymorphic transition relaxed in X-ray amorphous silica after heating at 210° C for at least 10 h. The infra-red spectrum of silica glass from various sources was identified as the spectrum of high-cristobalite, the spectrum after the polymorphic transition in silica glass is identical with that of low-cristobalite. The results give a strong support to the high-cristobalite/cubic/model of normal silica glass, with ordered regions in metastable form at room temperature.     

Transient strain of high strength concrete at elevated temperatures and the impact of polypropylene fibers

This paper presents the results of an experimental study on the transient strain of high strength concrete (HSC) under heating up to 750°C and the impact of polypropylene (PP) fibers. Concerning this topic only few results are available in the literature and systematic investigations are missing. However, basic knowledge is necessary for the understanding of the internal damage processes in the material as well as for heated structures. The transient strain during heating can be separated in two basic components: the free thermal strain and the mechanical strain. They were experimentally determined exemplarily for one HSC. For the determination of the mechanisms of transient strain and particularly the influence of PP fibers different techniques were applied. In this context the monitoring of the microcracking was done for the first time with acoustic emission analysis in combination with ultrasonic measurements. This new approach helps fundamentally to explain the impact of PP fibers on free thermal strain and mechanical strain during heating up. Furthermore weight loss measurements were carried out to characterize the moisture transport. It was shown that the PP fibers cause an acceleration of the moisture transport in the temperature range from 200 to 250°C which leads to drying shrinkage in opposite direction to the free thermal strain. Hence this paper is a contribution to the general understanding of the impact of PP fibers in HSC at high temperatures and emphasizes the important influence of PP fibers on the thermal and mechanical induced strain of HSC.     

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.     

Mechanical properties of amorphous silicon carbonitride thin films at elevated temperatures

The mechanical properties of amorphous silicon carbonitride (a-SiC _x N _y ) films with various nitrogen content (y = 0–40 at.%) were investigated in situ at elevated temperatures up to 650 °C in inert atmosphere. A SiC film was measured also at 700 °C in air. The hardness and elastic modulus were evaluated using instrumented nanoindentation with thermally stable cubic boron nitride Berkovich indenter. Both the sample and the indenter were separately heated during the experiments to temperatures of 300, 500, and 650 °C. Short duration high temperature creep tests (1200 s) of the films were also carried out. The results revealed that the room temperature hardness and elastic modulus deteriorate with the increase of the nitrogen content. Furthermore, the hardness of both the a-SiC and the a-SiCN films with lower nitrogen content at 300 °C drops to approx. 77 % of the corresponding room temperature values, while it reduces to 69 % for the a-SiCN film with 40 at.% of nitrogen. Further increase of temperature is accompanied with minor reduction in hardness except for the a-SiCN film with highest nitrogen content, where the hardness decreases at a much faster rate. Upon heating up to 500 °C, the elastic modulus of the a-SiCN film decreases, while it increases at 650 °C due to the pronounced effect of short-range ordering. The steady-state creep rate increases at elevated temperatures and the a-SiC exhibits slower creep rates compared to the a-SiCN films. The value of the universal constant x = 7 relating the W _p/W _t and H/E ^* was established and its applicability was demonstrated. Analysis of the experimental indentation data suggests a theoretical limit of hardness to elastic modulus ratio of 0.143.     

An anisotropic thermomechanical damage model for concrete at transient elevated temperatures

The behaviour of concrete at elevated temperatures is important for an assessment of integrity (strength and durability) of structures exposed to a high-temperature environment, in applications such as fire exposure, smelting plants and nuclear installations. In modelling terms, a coupled thermomechanical analysis represents a generalization of the computational mechanics of fracture and damage. Here, we develop a fully coupled anisotropic thermomechanical damage model for concrete under high stress and transient temperature, with emphasis on the adherence of the model to the laws of thermodynamics. Specific analytical results are given, deduced from thermodynamics, of a novel interpretation on specific heat, evolution of entropy and the identification of the complete anisotropic, thermomechanical damage surface. The model is also shown to be stable in a computational sense, and to satisfy the laws of thermodynamics.     

The impact-echo response of concrete plates containing delaminations: numerical,experimental and field studies

This paper describes the use of the impact-echo technique— a non-destructive testing technique based on the use of transient stress waves— for detecting delaminations in concrete plate-like structures such as bridge decks, slabs, and walls. Results obtained from numerical (finite element) analysis and controlled-flaw laboratory studies are presented and used to explain the elastic impact response of delaminated plates. Current impact-echo instrumentation is described, and results obtained from concrete bridge decks containing delaminations caused by corrosion of reinforcing steel are presented. These results provide a better understanding of the impact response of delaminated plates.

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Resume On décrit ici l’utilisation de la technique impact-écho-méthode d’essai non destructive basée sur l’utilisation d’ondes de pression acoustique transitoires-pour détecter les clivages dans des plaques de béton, telles que tabliers de pont, poutres et murs. On présente les résultats fournis par les méthodes numériques (élément fini) et l’étude en laboratoire sur éprouvette fissurée témoin, qui sont utilisées pour expliquer la réponse élastique à l’impact des plaques détériorées. On décrit l’appareillage courant et l’on expose les résultats obtenus sur des tabliers de ponts en béton qui présentent des clivages dus à la corrosion de l’armature. Ces résultats contribuent à mieux faire comprendre la réponse à l’impact de plaques détériorées.

     

Oxidation behavior of sputter-deposited Ti–Ni thin films at elevated temperatures

The isothermal oxidation kinetics of sputter-deposited equiatomic Ti–Ni thin films in pure oxygen from 823 to 923 K is studied using thermo-gravimetric analysis. The structure, composition-depth distribution and surface morphology of oxidized Ti–Ni thin films are investigated by X-ray diffraction (XRD), Auger electron spectroscopy (AES) and atomic force microscope (AFM), respectively. The results show that the oxidation kinetics of Ti–Ni thin films obeys a near-parabolic law. TiO₂, TiNi₃ and parent B2 phase are the compositions of oxidized Ti–Ni thin films. A double-layered scale including the outermost layer and the Ni-rich layer is formed outside the B2 matrix of oxidized Ti–Ni thin films. Moreover, thermal oxidation induces a surface smoothening of Ti–Ni thin films and surface roughness of oxidized Ti–Ni films decreases with the increasing oxidation temperature.     

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.     

Modelling basalt fibre reinforced glass concrete slabs at ambient and elevated temperatures

The analysis of tests conducted on small-scale slabs at ambient and elevated temperatures is presented in this paper. The slabs were produced from a new type of concrete containing different levels of glass sand and basalt fibre. Two methods were used for this purpose: a simplified method developed previously and a finite element method, using the software package ABAQUS. For the slabs at ambient temperature, the results showed a good correlation for the load–displacement relationship between the test and the two models up to the failure loads. For the slabs at elevated temperature, the ABAQUS model gave a reasonable prediction for the temperature–displacement relationship while the simplified method gave a conservative prediction for the maximum allowable vertical displacement. As a result, the simplified method underestimated the temperature at which the reinforcement fracture occurs for this type of concrete slab, incorporating glass sand and basalt fibres. Further work is required to remove this conservatism from the simplified design method for this type of concrete.     

Testing techniques to determine the deformation properties of concrete at elevated temperatures

This report describes the development and construction of a universal testing equipment used to determine the uniaxial deformation properties of concrete within the temperature range: +20≤T≤+140°C. Some of the results taken from other sources of the literature have proved to contain systematic testing errors, resulting from the testing techniques used; some partly unconventional solutions had to be devised in order to eliminate these errors, regarding, in particular, the length measurement system, the limitation of the measuring length to half the length of the test specimen, and the sealing. The testing techniques described have been tried for several operating years. The results of further investigation, especially of the current programme testing above all the basic creep of concrete, should all contribute to the objectivation of the presently existing knowledge.

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Résumé Cet article décrit la mise au point et la construction d’un équipement d’essai universel utilisé pour déterminer les caractéristiques de déformation uniaxiale du béton dans une gamme de température de +20≤T≤+140°C. Certains des résultats fournis par d’autres sources bibliographiques révèlent des erreurs systématiques d’essai dues aux techniques utilisées et, afin de les éliminer, on a eu recours à des solutions en partie non conventionnelles. Ceci concerne en particulier le système de mesure des longueurs, la limitation de la longueur de mesure à la moitié de la longueur de l’éprouvette d’essai et le confinement. Les techniques d’essai décrites ont été mises à l’épreuve par plusieurs années d’opération. Les résultats des études ultérieures et spécialement du programme d’essai en cours, surtout pour le fluage du béton, devrait contribuer à donner une forme plus objective à nos connaissances actuelles.

     

Response of high performance concrete plates to impact of non-deforming projectiles

The relatively recent technology, which enables the production of high strength concrete (HSC), makes HSC a prospective material for the construction of impact-resisting barriers. However, current penetration formulae are based on test data of normal strength concrete (NSC) and their extrapolation to higher concrete strengths is unsafe. The response of 80×80 cm high performance concrete (HPC) plate specimens to an impact of non-deforming steel projectiles was examined in an experimental study. The tests were planned with an aim to observe the influence of the concrete mix ingredients and amount and type of reinforcement on the performance of HSC under this type of loading. The variants that were examined were the aggregates (different types and maximum size), addition of micro-silica (MS) and steel fibers, and reinforcement details. The main findings show that design of HPC barriers to withstand impact loads involves several aspects. These are aimed at achieving enhanced properties of the structural element, where only one of which is the concrete's compressive strength.