Residual fracture properties of concrete subjected to elevated temperatures

Investigations on the residual Mode-I fracture behaviors of concrete subjected to elevated temperatures were carried out. Test specimens with the same dimensions, 230?×?200?×?200?mm, were exposed to temperatures varying from 65 to 600°C. The wedge splitting method was employed to obtain the complete load-crack mouth opening displacement curves (P?CCMOD) of the post-fire specimens, from which the initial fracture toughness K _ini, the critical fracture toughness K _ic and the fracture energy G _F were calculated. The results demonstrated that K _ini decreased monotonically with increasing heating temperature T _m; K _ic and G _F sustained a hold-increase?Cdecrease tendency with T _m. Furthermore, the characteristic length l _ch, a brittleness parameter, was calculated and it shared the same tendency as K _ic?CT _m and G _F?CT _m. Thus, any of the three parameters could serve as the index of brittleness for post-fire concrete. The fracture parameters and other material properties of concrete could be closely related to the ultimate weight loss.     

Crack formation and fracture energy of normal and high strength concrete

The crack path through composite materials such as concrete depends on the mechanical interaction of inclusions with the cement-based matrix. Fracture energy depends on the deviations of a real crack from an idealized crack plane. FRACTURE energy and strain softening of normal, high strength, and self-compacting concrete have been determined by means of the wedge splitting test. In applying the numerical model called “numerical concrete” crack formation in normal and high strength concrete is simulated. Characteristic differences of the fracture process can be outlined. Finally results obtained are applied to predict shrinkage cracking under different boundary conditions. Crack formation of high strength concrete has to be seriously controlled in order to achieve the necessary durability of concrete structures.     

The effect of exposure to elevated temperatures on the fracture energy of plain concrete

Results are given for the fracture energy of concrete preheated to temperatures in the range of 20°C–600°C, then tested at room temperature in three-point bending, for both slowly-cooled specimens and those delivered a thermal shock. Fracture energy measurement techniques are discussed, and comparisons made across previous studies. The results suggest a significant difference between residual and ‘hot’ capacities, along with a temperature dependence on the ductile-brittle transition.     

Comparison between high strength concrete and normal strength concrete subjected to high temperature

Two normal strength concretes and three high strength concretes, with 28-day compressive strengths of 28, 47, 76, 79 and 94 MPa respectively, were used to compare the effect of high temperatures on high strength concrete and normal strength concrete. After being heated to a series of maximum temperatures at 400, 600, 800, 1000 and 1200°C, and maintained for 1 hour, their compressive strengths and tensile splitting strengths were determined. The pore size distribution of hardened cement paste in high strength concrete and normal strength concrete was also investigated. Results show that high strength concrete lost its mechanical strength in a manner similar to or slightly better than that of NSC. The range between 400 and 800°C was critical to the strength loss of concrete with a large percentage of loss of strength. Microstructural study carried out revealed that high temperatures have a coarsening effect on the microstructure of both of high strength concrete and normal strength concrete.     

Size effect on fracture resistance and fracture energy of concrete

A recent asymptotic approach dealing with the size effect on the fracture properties of a large plate is further developed to consider the influence of both the front and back free surfaces of small sized specimens. The new extension is applied to experimental results found in the literature, and good agreements have been found between the predictions and the fracture resistance and energy measured using geometrically similar specimens and specimens with identical size but different initial crack or notch lengths. The physics behind the size effect are discussed based on the modified asymptotic approach. It is found that both the specimen geometry and crack length contribute to the size effect on fracture properties besides its physical size. In particular, the ratio of a fracture process zone, size over its distance to a free surface plays a very important role.     

Residual compressive strength and microstructure of high performance concrete after exposure to high temperature

Experimental programs were carried out to study compressive strength and microstructure of high performance concrete (HPC) subjected to high temperature compared with normal strength concrete (NSC). After the concrete specimens were exposed to a peak temperature of 800°C, the compressive strength was tested. Changes of porosity and pore size distribution of the concrete were measured by using mercury intrusion porosimetry (MIP). Test results show that high performance concrete had higher residual strength although the strength of high performance concrete degenerated much more than the normal strength concrete after high temperature exposed. Variations in pore structure of high performance concrete after high temperature indicated the degradation of the mechanical properties. A model by optimizing the parameters in Ryshkewitch model was developed to predict the relationship between porosity and compressive strength of the high performance concrete.

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Résumé Une série de programmes expérimentaux a été réalisée afin d'étudier la résistance à la compression et la microstructure des bétons à haute performance (BHP) soumis à de fortes températures par comparaison aux bétons ordinaires. Les bétons ont été soumis à une température extrême de 800°C, puis la résistance à la compression a été testée. Les changements de porosité et la répartition de la taille des pores dans le béton ont été mesurés par la technique de porosimétrie au mercure. Les résultats des essais ont montré que les bétons à haute performance présentaient des niveaux de résistance résiduelle mais que la résistance des bétons à haute performance se dégradait beaucoup plus que celle des bétons ordinaires après exposition à haute température. Des variations dans la structure des cavités des bétons à haute performance après exposition à de hautes températures ont indiqué la dégradation des propriétés mécaniques. L'étude a donné lieu au développement d'un modèle par optimisation des paramètres du modèle Ryshkewitch afin de prévoir les relations entre la porosité et la résistance à la compression du béton à haute performance.

     

Flow behaviour of high strength high-performance concrete

The workability of flowable High-Performance Concrete (HPC) is nowadays mainly measured using conventional test methods such as the slump test or the slump-flow test. These single-point tests do not seem sensitive enough to characterize the high-workability of HPC. Due to the fluid consistency and uniformity of fresh HPC, it is possible to describe its flow properties by using a rheological test method. To evaluate the flowability based on rheology, fresh HPC is regarded as a two-phase material composed of a matrix phase and a particle phase. In the study, the effects of materials and proportioning on the rheological properties were investigated experimentally. A new rheometer was established by conducting a two-point test to investigate the flow behaviour of high strength HPC. Test results show that the high strength HPC with good uniformity and without tendency of segregation can possess the properties of rheology according to Bingham’s equation. An increase of the fraction of mortar in HPC can lead to a more distinct the rheological behaviour. Moreover, it is found that the application of a rheological method can provide more stable results than any other test method in describing the flowability of high strength HPC.     

Mechanical properties of photocatalytic white concrete subjected to high temperatures

The paper describes the consequences of progressive damage in architectural high performance concrete when exposed to different heating treatments. Specimens were tested for uniaxial compressive, direct, and indirect tensile strengths at ambient conditions approximately one day after the exposure to the high temperature. Modifications in the microstructure, porosity, and pore size distribution of the fire deteriorated specimens were identified using scanning electron microscopy and mercury intrusion porosimetry techniques. Test results revealed no significant variations in the mechanical strengths for specimens exposed to temperatures up to 250 °C. Per contra, significant damage was observed for higher temperature, 500 °C and 750 °C, treatments, similar to that of ordinary concrete made with similar aggregates. Based on X-ray diffraction analysis, photocatalytic properties of the concrete were lost at 750 °C.     

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.

     

Boundary effect on concrete fracture and non-constant fracture energy distribution

This paper extends the local fracture energy concept of Hu and Wittmann [29] and [30], and proposes a bilinear model for boundary or size effect on the fracture properties of cementitious materials. The bilinear function used to approximate the non-constant local fracture energy distribution along a ligament is based on the assumption of the proportionality of the local fracture energy to the fracture process zone (FPZ) height and characterises the FPZ height reduction when approaching a specimen back boundary. The bilinear function consists of a horizontal straight line of the intrinsic fracture energy G_F and a declining straight line that reduces to zero at the back boundary. It is demonstrated that using the bilinear model, the size-independent fracture energy G_F can be estimated from the fracture energy data measured on laboratory-size specimens, and the intersection of these two linear functions, defined as the transition ligament, represents the influence of the back boundary on the fracture properties. It is also demonstrated that the specimen size alone is not sufficient to characterise the size effect in the fracture properties observed on laboratory-size specimens.     

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.     

Residual bond strength of polymer adhesive anchored reinforcement subjected to high temperatures

The results are presented of an experimental project into the residual strength characteristics of polymer adhesives used in anchoring steel reinforcement bars, following exposure to elevated temperatures. The adhesives tested were polyester resin and epoxy resin grouts. Two types of experiment were undertaken designed to investigate the residual bond strength as well as the compressive strength of specimens. For the bond strength determination, pull-out tests were carried out on 150 mm cubes. Compressive tests were undertaken on small cylindrical specimens. All specimens were exposed to different temperatures in pre-heated ovens and then allowed to cool down to normal laboratory temperatures prior to testing. In the bond tests, two types of failure were observed: splitting and slipping. Splitting of the cubes occurred in specimens exposed to temperatures below about 200° C, and the adhesive characteristics were in general enhanced. Slipping failures, by pulling through the adhesive, were obtained in specimens exposed to higher temperatures than about 200°C, and both the residual bond and compressive strengths were observed to diminish with increasing temperature. Additionally, the texture of the adhesive had changed, losing its cohesiveness. Based on this observation, an approach for assessing thein situ bond capacity of anchors is proposed.     

Probabilistic aspects of fracture energy of concrete

A test method to determine fracture energy and strain-softening in direct tension is described. Experimental results on cylinders of equal diameter and varying length are reported. It is found that the tensile strength decreases with increasing volume while the fracture energy remains constant within the observed volume range. By means of numerical simulation, it is shown that in a direct tension test several fracture process zones appear in the initial states of cracking and that final rupture is induced by the development of only one of these fracture zones. This phenomenon has been observed experimentally by other authors. A comparatively large number (44) of identical samples were tested by using the wedge-splitting test. Half the specimens were grooved. The fracture energy of the grooved and ungrooved specimens turned out to be the same within the given range of accuracy. It was observed experimentally and simulated numerically that in grooved specimens the crack is forced to follow a ragged fracture surface which is statistically not the weakest one. In an ungrooved specimen the crack path generally diverts from the centre line and advances through weaker zones. For the formation of these skew cracks, however, more energy is consumed due to aggregate interlock. In addition, the fracture process zone observed in ungrooved specimens is generally wider.     

Residual strain energy in elastoplastic adhesive and cohesive fracture

The problem of peeling an elastoplastic strip from a rigid substrate is re-examined. The mechanics of combined flow and fracture are expressed algebraically, and in terms of work areas both under load and unloaded. The importance of residual elastic strain energy becomes apparent in the analysis and it is shown that determinations of work of fracture from partitioned areas may be significantly in error if residual strain energy is neglected. The extension of the treatment to cohesive fracture is discussed, and it is shown that Turner's I term relating to unloaded work areas should include a residual strain energy component. As an algebraic solution to the problem is available over the whole range of elastoplastic fracture, it is possible to construct an R6 failure assessment diagram over the whole range of deformation.

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Résumé On réexamine le problème du pelage d'une bande élastoplastique d'un substrat rigide. On exprime la mécanique combinée d'écoulement et de rupture par voie algébrique, et en termes de surface de travail, selon qu'il y ait charge ou non. L'importance de l'énergie de déformation élastique résiduelle apparaît dans l'analyse, et l'on montre que le travail de rupture relatif aux surfaces séparées risque d'être déterminé avec une erreur significative si on néglige l'énergie résiduelle de déformation. On discute de l'extension du traitement à la rupture par décohésion, et on montre que le terme I de Turner raltif aux surfaces de travail non sollicitées devrait inclure une composante d'énergie de déformation résiduelle.Comme une solution algébrique est disponsible pour le problème sur toute la gamme du régime élastoplastique de rupture, il est possible d'élaborer un diagramme RG d'établissement des conditions de rupture sur toute la gamme des déformations possibles.

     

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].     

Tensile characteristics and fracture energy of fiber reinforced and non-reinforced ultra high performance concrete (UHPC)

In the framework of this study, various mixtures of fiber reinforced and non-reinforced ultra high performance concrete (UHPFRC and UHPC) were produced and tested with focus on the determination of the fracture energy and its comparison to standard mechanical material parameters. For some mixtures a compressive strength of more than 300 MPa was reached still retaining good fresh characteristics of the UHPC. These mixtures were examined for properties of fresh and hardened concrete, focusing on tensile strength properties and fracture energy. The fracture energy was determined to describe the work capacity, i.e. the potential energy intake until the failure of the material. Thereby, a significant increase of the work capacity could be achieved by the addition of steel fibers. Furthermore, the impact of a vacuum treatment of the freshly mixed concrete in regard to fresh and hardened concrete characteristics as well as the influence of aftertreatment (heat treatment and water storage) on compressive and tensile properties of the UHPC was investigated.     

Multifractal nature of concrete fracture surfaces and size effects on nominal fracture energy

Experimental evidence of the fractality of fracture surfaces has been widely recognized in the case of concrete, ceramics and other disordered materials. An investigationpost mortem on concrete fracture surfaces of specimens broken in direct tension has been carried out, yielding non-integer (fractal) dimensions of profiles, which are then related to the ‘renormalized fracture energy’ of the material. No unique value for the fractal dimension can be defined: the assumption of multifractality for the damaged, material microstructure produces a dimensional increment of the dissipation space with respect to the number 2, and represents the basis for the so-called multifractal scaling law. A transition from extreme Brownian disorder (slope 1/2) to extreme order (zero slope) may be evidenced in the bilogarithmic diagram: the nominal fracture energyG _F increases with specimen size by following a nonlinear trend. Two extreme scaling regimes can be identified, namely the fractal (disordered) regime, corresponding to the smallest sizes, and the homogeneous (ordered) regime, corresponding to the largest sizes, for which an asymptotic constant value ofG _F is reached.

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Resume On a largement établi la preuve expérimentale du caractère fractal des surfaces de rupture dans le cas du béton, des céramiques et d'autres matériaux ‘désordonnés’. Une étudepost mortem menée sur des surfaces de rupture d'échantillons cassés par traction directe révèle des dimensions non intégrales (fractales) des profils dont on a établi la relation avec l'énergie de rupture ‘renormalisée’ du matériau. Il n'est pas possible d'établir une valeur unique de la dimension fractale: en présumant la multifractalité de la microstructure du matériau endommagé, on obtient une augmentation dimensionnelle par rapport au numéro 2 et on établit la base de la loi dite d'échelle multifractale. Dans le diagramme à deux logarithmes on peut voir, une transition du désordre de Brown extrême (inclinaison 1/2) à l'ordre extrême (inclinaison zéro); l'énergie de fracture nominaleG _F augmente avec les dimensions de l'échantillon suivant une tendance non linéaire. On peut voir deux régimes extrêmes d'échelle, c'est-à-dire le régime fractal désordonné) . qui correspond aux dimensions minimales, et le régime homogène (ordonné), qui correspond aux dimensions maximales pour lesquelles on atteint une valeur constante asymptotique deG _F.

     

Low-cycle deformation and fracture of structural graphites at high temperatures

Chelyabinsk Polytechnic Institute. Translated from Problemy Prochnosti, No. 1, pp. 60–65, January, 1988.     

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.