Explosive spalling of normal strength concrete slabs subjected to severe fire

The first part of this paper presents an experimental investigation on explosive spalling of six full-scale normal strength reinforced concrete slabs subjected to conventional fire curve ISO834 and severe hydrocarbon fire curve, performed at the Fire Research Centre, University of Ulster, UK focusing on concrete thermal behaviour and the explosive spalling phenomenon. Each slab was loaded with 65% of its BS8110 design load and was heated from the bottom side only. Temperatures profile was recorded at three depths within the slabs and the moisture content was also measured before and after the tests. The deflection of the slabs was recorded at the middle of the 3 m span. The second part of the paper presents a numerical study on the normal concrete slabs using the finite element method. The concrete slabs were modelled including the embedded reinforcement to conduct a non-linear transient structural analysis taking into account cracks appearance and creep. A comparison between the experimental and the FEM results is presented in the paper.     

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.

     

Strength of concrete subjected to high temperature and biaxial stress: Experiments and modelling

The mechanical behaviour of normal weight concrete subjected to high temperature and biaxial stress is not well understood. The investigation reported is twofold. The strength is studied of unsealed concrete and mortar incorporating quarzitic aggregate and bound by Portland cement and subjected to high temperature and biaxial stresses. This involves short-term tests and includes the influence of the composition. Additionally a mechanical model is developed for the failure of concrete subjected to these stresses which incorporates not only the main variable temperature, but also compositional variables which have a significant influence on the strength of concrete at high temperatures.

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Resume Le but de cette recherche était d'améliorer nos connaissances sur la résistance du béton soumis à de hautes températures ainsi qu'à des contraintes biaxiales. De nombreux essais ont été menés sur des éprouvettes non scellées de béton et de mortier à base de CPA. On a aussi étudié, outre le type de sollicitations, l'influence des paramètres de composition les plus importants. Les essais ont révélé que, dans tout le domaine des sollicitations biaxiales en compression, la composition influence directement la diminution de la résistance du béton en fonction de l'augmentation de la température. Le diamètre maximal des granulats s'avère être un facteur déterminant alors que la nature de ces derniers et le rapport eau/ciment n'ont en général que des effects secondaires. L'amplitude de ces influences diffère. Comme le montrent les essais à température normale, les valeurs de la résistance du béton sont affectées par sa composition avant la désintégration de la portlandite entre 450°C et 550°C. Afin de généraliser le comportement observé, on a modélisé la résistance du béton soumis à des contraintes biaxiales à haute température; pour ce faire, le critère de defaillance de Podgorski a été modifié afin de prendre en compte les paramètres suivants: haute température, rapports eau/ciment et granulats/ciment. Avec quelques simples essais de résistance à température normale et élevée, le modèle peut être facilement ajusté pour décrire le comportement d'un béton possédant une composition spécifique. La théorie ainsi développée est bien vérifiée par les résultats des essais.

     

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.     

Effect of transversal reinforcement in normal and high strength concrete columns

This paper deals with an experimental investigation and numerical simulation of reinforced concrete columns. Behavior of normal and high strength columns is studied with special attention paid to the confinement effects of transversal reinforcement in columns with square cross section. Character of a failure, strengths, ductility and post-peak behavior of columns are observed in experiments and also in numerical solution. Three-dimensional computational model based on the microplane model for concrete was constructed and compared with experimental data. Results of numerical model showed good agreement in many aspects and proved capabilities of the used material model.     

Tensile basic creep versus compressive basic creep at early ages: comparison between normal strength concrete and a very high strength fibre reinforced concrete

The paper presents experimental results concerning the comparison of tensile and compressive basic creep behaviours at early ages of two different concretes: a normal strength concrete (NSC) and a very high strength fibre reinforced concrete (HPFRC). This research project has been done in the context of a bilateral collaboration between Polytechnique Montreal and IFSTTAR. Observations on the HPFRC showed specific compressive creep similar to the specific tensile creep. Moreover, the specific creep curves obtained under compressive and tensile loading had always positive values, i.e. they were in same direction of the applied load on specimens. Measurements made on the NSC revealed specific compressive creep with positive values (in the loading direction). However, specific tensile creep presented negative values (opposite direction of loading) for a long period. A physical explanation based on the existence of two mechanisms with opposite effect is proposed to describe these basic creep results. The first mechanism is a coupling between the microcracking process and the water transfers that lead to additional self-drying shrinkage; the second mechanism is the self-healing of concrete induced by the microcracking.     

Two-layer pre-stressed beams consisting of normal and steel fibered high strength concrete

The paper is focused on analysis of two-layer bending pre-stressed beams consisting of steel fibered (SF) high strength concrete (HSC) in compressed zone and normal strength concrete (NSC) in tensile zone. Investigation of such beams is important for RC structural design, because calculation of fibers volume ratio is significant, like that of reinforcing steel bars for usual RC elements. In other words, such elements are made of high performance concrete (HPC). There is a growing tendency that more effective HPC structures replace NSC ones, first of all in pre-stressed elements. Definition of the HSC class lower limit, to be used in the compressed zone of a two-layer pre-stressed beam, is given. It was demonstrated that SF have little effect on the beam elastic deflections. However, the ultimate deflections of the section increase because additional potential for plastic energy dissipation (PED) in the bending element. NSC, used in the section tensile zone, contributes additionally about 20% to the section’s PED potential compared to one-layer HSC beams. In order to guarantee sufficient section’s ductility of the pre-stressed beams, required to withstand dynamic loadings, a minimum SF ratio is proposed to be considered. The fibers take the tensile stresses, yielding cracks in the concrete matrix. A design method for calculation of the SF volume ratio, as a function of required ductility, is proposed. A numerical example, illustrating the efficiency of this method is presented.     

Variations in the mechanical properties and temperature of concrete subjected to cyclic loads,including high loads

Concrete structures are subjected to cylic loading throughout their service life. This article examines the changes taking place in the mechanical properties of concrete specimens under a limited number of compressive loading cycles involving high loads.     

Effect of loading rate on the strength of concrete subjected to uniaxial tension

In the context of an international co-operation project between the University of Delft (The Netherlands) and the LCPC, an experimental study was made of rate effects in the behaviour of concrete under tensile stress. Very high speed tests ( between 1 and 80 GPa s⁻¹) were carried out in Delft on a Hopkinson bar, and quasi-static tests ( between 5×10⁻⁵ and 5×10⁻³ GPa s⁻¹) were carried out by the LCPC on a hydraulic press. This investigation had two objectives. 1. To verify on a mini-concrete (diameter of the largest particles 10 mm) a result obtained with a micro-concrete (diameter of the largest particles 2 mm) in the course of a previous study. Rate effects are produced by the presence of pore water in the material. 2. To investigate the influence of the water/cement ratio (i.e., the compressive strength of the concrete) on these rate effects. The three main conclusions that can be drawn from this study are (i) it is indeed the presence of pore water in the concrete which is at the origin of rate effects where this materials is concerned, (ii) the effect of speed on the tensile strengthf ₁ increases with the water/cement ratio, and (iii) in absolute value, the increase in strength (f _tdyn-f _tstat ) seems to be independent of the water/cement ratio.

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Resume Dans le cadre d'une coopération internationale entre l'Université de Delft (Pays-Bas), et le Laboratoire Central des Ponts et Chaussées (LCPC), une étude expérimentale a été menée sur les effects de vitesses dans le comportement en traction du béton. Les essais très rapides ( compris entre 1 et 80 GPa s⁻¹) ont été réalisés à Delft sur une barre d'Hopkinson, les essais quasi-statiques ( compris entre 5×10⁻³ et 5×10⁻⁵ GPa s⁻¹) étant réalisés au LCPC sur une presse hydraulique. Deux objectifs sont visés dans cette étude: (i) vérifier sur un mini-béton (diamètre du plus gros grain égal à 10 mm) un résultant obtenu sur micro-béton (plus gros grain égal à 2 mm) lors d'une étude précédente: c'est la présence d'eau libre au sein du matériau qui induit les effects de vitesse; et (ii) étudier l'influence du rapport eau/ciment (c'est-à-dire de la résistance en compression du béton) sur ces effets de vitesse. Les conclusions principales que l'on peut tirer de cette étude sont les trois suivantes: c'est bien la présence d'eau libre dans les bétons qui est l'origine des effects de vitesse relatifs à ce matériau; l'effet de vitesse sur la résistance en traction f₁ augmente avec le rapport eau/ciment; et en valeur absolue, l'augmentation de la résistance (f _tdyn-f _tstat ) semble indépendante de ce rapport eau/ciment.

     

Design shear strength formula for high strength concrete beams

After a brief review on the concrete shear strength mechanisms, two very reliable expressions for predicting the shear strength of beams without transverse reinforcement are reported: the one proposed by Ba?ant and Kim [7], which is valid for Normal strength Concrete (NSC) beams, and the other recently proposed by the authors, which is valid for High Strength Concrete (HSC) beams. Hence a summary of a shear strength model for beams with stirrups is provided, which was derived [27] on the basis of the Ba?ant and Kim expression and therefore is adequate only for NSC beams.On the basis of the expression obtained for HSC without stirrups and of the model already proposed for NSC with stirrups, a shear strength expression for HSC beams with stirrups is derived. The obtained expression is applied to an experimental program of 116 HSC beams with stirrups, and is found to predict the test results better than ACI Code [1], Eurocode [12] and CEB/FIP Model Code [9].A design formula is hence proposed, which is adequately conservative and accurate.A design example of a HSC beam with stirrups is carried out, and the various design expressions previously considered are compared.     

Relation between abrasion resistance and flexural strength of high volume fly ash concrete

In this paper, abrasion of high volume fly ash (HVFA) concretes made with 50% and 70% of cement replacement with fly ash was assessed in terms of its relation to flexural tensile strength. Comparisons were made between normal Portland cement (NPC) concrete and fly ash concrete. Comparisons were also made between fly ash concretes. Investigation results have shown that the abrasion resistance increased as flexural tensile strength increased. Analysis of the results showed that, for concrete with tensile strength of greater than 4–5 MPa, the abrasion resistance of HVFA concrete with 70% replacement with cement was found to be higher than that of counterpart control NPC concrete and concrete made with 50% fly ash. The comparison between the relation of abrasion to compressive strength and abrasion to flexural tensile strength made in terms of R² of the linear regression showed that a stronger relation existed between abrasion and flexural tensile strength than that of abrasion to compressive strength of the concrete studied.

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Résumé L'étude a pour but d'estimer la relation entre la résistance à l'usure avec la résistance à la traction en flexion des bétons contenant de 50% et 70% de cendres volantes. On a comparé le béton pur au béton contenant des cendres volantes. Des comparaisons ont été faites également entre les différents bétons contenant des cendres volantes. Les resultats de la recherche ont montré que la résistance à l'usure augmente à mesure que la résistance à la traction en flexion de 4∼5 MPa ont une résistance à l'usure plus grande s'ils contienneint 70% de cendres volantes que s'ils étaient purs ou contenaient 50% de cendres volantes. La comparaison entre les relation de la résistance à l'usure en compression et de la résistance à l'usure en traction a été établie en termes de R² de la régression linéaire. On a prouvé qu'une relation plus forte a été obtenue entre la résistance à l'usure et la résistance à la traction en flexion par rapport à la résistance à l'usure en compression du béton étudié.

     

Investigation of the fatigue strength of steel DI-22 at normal and high temperature

Strength of Materials -     

Creep of medium and high strength aluminum alloys at normal temperature in moist atmospheres

Direct data have been obtained for the creep of high- and medium-strength aluminum alloys in the stress range of O.6–1.2 of the nominal yield strength _0.2 in laboratory air and in aqueous NaCl solution at room temperature. On this basis using known theories approximating functions have been determined for the creep curves. Stress _0.2 serves as a natural boundary for the macroelastic and macroplastic regions in the first of which creep is only transient, and in the second there are transient, quasisteady-state, and accelerated stages. Extrapolated estimates of creep strain in the macroelastic region from data measured in the macroplastic region are not physically competent. However, a tendency towards an increase in ductility with an increase in time to failure at stresses greater than _o.2 makes it possible to estimate by extrapolation the time for onset of the accelerated creep stage with low test stresses from measured values at greater stresses in the macroplastic region. Fractographic and strain indices revealed the harmful effect of moist atmospheres on the deformation and failure resistance of alloys with prolonged loading.Translated from Problemy Prochnosti, No. 1, pp. 50–58, January, 1990.     

The strength and deformation characteristics of high early strength structural concrete

There are many possible structural applications of concrete with a compressive strength of about 100 N/mm². In practice, however, the development of early strength is far more important, but the combination of both can bring considerable economic benefits to the construction industry. Tests are reported on the strength and deformation characteristics of high early strength structural concrete. Tests using an ultra fine cement with expanded slate lightweight aggregate and granite produced concretes with a strength of 30–40 N/mm² and 60–70 N/mm² respectively in 24 hours. Tests with aluminous cement produced better aggregate-matrix bond and developed strengths of about 95 N/mm² in the same time. Equations are presented to predict the tensile strength and elasticity of the high early strength concrete. It is shown that the rapid hydration results in a high rate of shrinkage and creep initially but the long-term deformation characteristics are comparable to normal concrete. It is suggested that it is worth exploring methods to minimise the effects of conversion.     

The application of pull-out test to high strength concrete estimation

The paper compares the results of non-destructive (pull-out, pulse velocity, hardness) tests conducted to determine the compressive strength of concrete specimen with strength from40 to80 MPa. The pull-out method is seen to exhibit greater sensitivity and precision in the results. It is particularly efficient for high strength concrete (60÷80 MPa). In this case, it may be convenient to combine the ultrasonic/pull-out methods, or else, the ultrasonic/hardness/pull-out methods.

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Résumé On présente les résultats d'essais non-destructifs effectués pour déterminer la résistance en compression des bétons de haute et moyenne résistance (40÷80 MPa). On examine la méthode de l'essai d'adhérence par traction, de la vitesse d'impulsion des ultrasons, du scléromètre, la méthode combinée roumaine (scléromètre, ultrasons). On étudie la sensibilité, la précision de chaque méthode avec une analyse statistique des données expérimentales, en notant les avantages et les désavantages de leur application. La méthode de l'essai d'adhérence en traction est sans doute la méthode la plus précise pour la résistance en compression de40÷80 MPa. Toutefois son emploi est limité; car on ne peut déterminer la résistance du matériau que dans les points de la structure où l'on a placé les tiges d'acier. Les ultrasons, le scléromètre et surtout la méthode combinée, en dépit d'une certaine dispersion des résultats (cette dispersion devient remarquable dans l'intervalle de résistance60÷80 MPa) permettant d'essayer, en peu de temps, une quantité considérable de béton. On conclut que la combinaison de ces méthodes (essai d'adhérence par traction, méthode combinée roumaine ou essai d'adhérence par traction-ultrasons) se prête le mieux à la détermination de la résistance.

     

Numerical investigation of concrete subjected to high rates of uniaxial tensile loading

The present article is concerned with the response of structural concrete prisms to high rates of uniaxial tensile loading. The numerical investigation carried out is based on a finite-element (FE) program capable of carrying out three-dimensional (3D) nonlinear static and dynamic analyses. This program is known to yield realistic predictions to the response of a wide range of plain- and reinforced-concrete structural forms subjected to arbitrary static and earthquake actions. Furthermore, its application has recently been successfully extended in predicting the response of plain-concrete prism elements under high rates of uniaxial compressive loading. The main feature of the FE program is that it incorporates a 3D material model which is characterized by both its simplicity and its attention to the actual physical behaviour of concrete in a structure. Its analytical formulation is based on the assumption that the material properties of concrete are independent of the applied loading rate (strain rate) thus attributing the effect of the applied loading rate on the prism's response to inertia. The validation of this assumption is based on a comparative study between numerical and experimental data which reveals good agreement. This constitutes a major departure from current thinking as regards material modelling of concrete under high-rate loading. In addition, the available data (numerical and experimental) show that the response of the concrete prism elements depends on a number of parameters linked to geometry and material properties of the structural forms under investigation as well as the testing method adopted. This dependence explains, to a significant extent, the scatter that characterizes the available experimental data, and it also suggests that both experimental and numerical results describe structural rather than material behaviour thus raising questions regarding the validity of the use of such data in the constitutive modelling of concrete-material behaviour under high-rate loading conditions.