Strength prediction and failure modes of concrete specimens subjected to the split test

This paper deals with modelling and test of concrete specimens subjected to the Brazilian split test. Based on the fictitious crack concept, a simple model for the crack propagation process in the splitting plane is developed. From the model, it is possible to determine the distribution of residual tensile strength as crack propagation take place. The residual tensile strength is thereafter used in a rigid plastic analysis of the splitting failure. Based on this combined approach, the ultimate load may either be governed by crack propagation or by a plastic failure, which then terminates the crack growth process. It is shown that the model is able to replicate a number of experimental observations. This includes size effect and influence of loading width.     

Strength and complete stress-strain relationships for concrete tested in uniaxial compression under different test conditions

This report discusses the variations in strength and complete stress-strain behaviour which can result from varying some basic experimental conditions. The significance of varying the specimen slenderness ratio, ratio of specimen diameter to maximum size of aggregate, diameter of test platen used to transmit load from the testing machine to the test specimen, and the methods employed for strain measurement, have been evaluated.

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Résumé On examine les variations de résistance et des relations contrainte-déformation qu'on peut obtenir en faisant varier certaines des conditions expérimentales de base. On a étudié les incidences des variations du degré d'élancement de l'éprouvette, du rapport du diamètre de l'éprouvette à la dimension maximale du granulat, du diamètre du plateau d'essai utilisé pour transmettre la charge à l'éprouvette ainsi que de la méthode de mesure des déformations. Tous les essais ont été effectués sur des éprouvettes cylindriques carottées et sciées. Avec ce type d'éprouvette, on a obtenu une rupture uniforme par la suppression de la zone faible au voisinage de la surface de coulée. Les rapports d'élancement et les diamètres des carottes étaient respectivement compris entre 1–3 et 25–100 mm. On a maintenu une vitesse de déformation de 2,5×10⁻⁶/sec. Les principaux résultats de cette étude sont les suivants: a) la variation du comportement contraintedéformation en fonction du degré d'élancement montre différentes tendances selon que les déformations unitaires sont déterminées d'après la mesure des déformations globales ou d'après celle de la zone centrale des éprouvettes. Quoique ces différences diminuent avec l'augmentation du degré d'élancement, elles restent notables. b) Un degré d'élancement de 2,5 convient très bien à l'étude des propriétés du béton non armé. Pour des degrés d'élancement inférieurs, des ruptures se produisent aux extrémités fortement encastrées, tandis qu'à des degrés d'élancement plus élevés on constate un transfert indésirable d'énergie de déformation élastique depuis les régions de rupture aux extrémités vers la zone centrale de rupture. Le taux de variation de la résistance atteint aussi une valeur minimale. c) L'emploi de plateaux d'acier dont le diamètre excède celui de l'éprouvette détermine un accroissement de 3% de la déformation de rupture observée (à partir de la déformation globale); après application de la contrainte maximale le comportement est plus sérieusement affecté. d) Bien que la contrainte correspondant au début de la dilatation augmente avec le degré d'élancement, la contrainte latérale correspondant à ces contraintes de dilatation restait constante à 0,043% près. e) Avec une dimension maximale de granulat (D₀) de 9,52 mm, et pour des éprouvettes de diamètre (D) différent, on constate que la résistance atteint un maximum pour D/D₀=8. Le diamètre optimal d'une éprouvette, correspondant à sa résistance la plus élevée, pour la dimension maximale de granulat, peut être déterminé approximativement par: D/D₀=14,55−6,68 log₁₀D₀.

     

Strength of concrete under combined tensile-compressive stresses

Summary Strength of plain concrete under combined tensile-compressive stresses has been studied by conducting splitting tests on cubes, cylinders, prisms and ellipses. Using these results the influence of lateral compressive stress on the longitudinal tensile strength of concrete has been determined. The results have been critically examined and a failure criterion in the form of an interaction diagram has been given.

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Résumé On étudie la résistance du béton armé sous des efforts combinés en traction-compression au moyen de l'essai brésilien sur des cubes, cylindres, prismes, et des éprouvettes à section elliptique. A l'aide de ces résultats, on détermine l'influence de l'effort de compression latérale sur la résistance en traction longitudinale. Après analyse des résultats, on donne un critère de rupture sous la forme d'un diagramme d'interaction.

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Nonlinear fracture mechanics and plasticity of the split cylinder test

The split cylinder test is subjected to an analysis combining nonlinear fracture mechanics and plasticity. The fictitious crack model is applied for the analysis of splitting tensile fracture, and the Mohr-Coulomb yield criterion is adopted for modelling the compressive crushing/sliding failure. Two models are presented, a simple semi-analytical model based on analytical solutions for the crack propagation in a rectangular prismatic body, and a finite element model including plasticity in bulk material as well as crack propagation in interface elements. A numerical study applying these models demonstrates the influence of varying geometry or constitutive properties. For a split cylinder test in load control it is shown how the ultimate load is either plasticity dominated or fracture mechanics dominated. The transition between the two modes is related to changes in geometry or constitutive properties. This implies that the linear elastic interpretation of the ultimate splitting force in term of the uniaxial tensile strength of the material is only valid for special situations, e.g. for very large cylinders. Furthermore, the numerical analysis suggests that the split cylinder test is not well suited for determining the tensile strength of early age or fibre reinforced concrete.     

Constitutive modeling of creep-fatigue interaction for normal strength concrete under compression

Conventional approaches to model fatigue failure are based on a characterization of the lifetime as a function of the loading amplitude. The Wöhler diagram in combination with a linear damage accumulation assumption predicts the lifetime for different loading regimes. Using this phenomenological approach, the evolution of damage and inelastic strains and a redistribution of stresses cannot be modeled. The gradual degration of the material is assumed to not alter the stress state. Using the Palmgren–Miner rule for damage accumulation, order effects resulting from the non-linear response are generally neglected.In this work, a constitutive model for concrete using continuum damage mechanics is developed. The model includes rate-dependent effects and realistically reproduces gradual performance degradation of normal strength concrete under compressive static, creep and cyclic loading in a unified framework. The damage evolution is driven by inelastic deformations and captures strain rate effects observed experimentally. Implementation details are discussed. Finally, the model is validated by comparing simulation and experimental data for creep, fatigue and triaxial compression.     

The concrete cylinder: stress analysis and failure modes

The rationale for using the circular cylindrical specimen for determining the tensile strength of concrete is reviewed, and the stress fields and fracture modes associated with the familiar splitting test and a pressurized cylinder test are discussed. Special attention is paid to contradictory reports and unresolved issues in the literature as to exactly how the fracture of a concrete cylinder develops and progresses under increasing load.The effect of a macrocrack on the stress field within a cylinder is introduced as a means of understanding the progressive fracture of a cylindrical specimen. In particular, it is argued that, while the idealized stress field in an unflawed cylinder may explain how and where the first macrocrack develops in a cylindrical specimen, it is the stress field modified by the presence of the macrocrack that must be considered to understand subsequent behavior. This point of view enables us to take a unified view of a variety of different observations about the indirect tensile mode of failure.The axial tensile failure of a concrete cylinder loaded by radial pressure is also considered in the context of classical elastic stress analysis. Whereas this failure mode has been described as paradoxical in the literature, we demonstrate that the induced tensile stress field is indeed of sufficient magnitude to explain the axial failure under radial pressure by an elementary strength of materials argument.

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Résumé On examine les motivations d'utiliser une éprouvette à section cylindrique pour déterminer la résistance à la traction du béton, et on discute des champs de contraintes et des modes de rupture correspondant à l'essai classique de séparation et à l'essai sur cylindres soumis à pression. Les rapports de caractère contradictoire, et les données non résolues figurant dans la littérature, sont examinés en particulier en ce qui regarde la maniére exacte selon laquelle se développe et progresse sous charges croissantes une rupture dans un cylindre en béton.Un moyen de comprendre la rupture progressive d'une éprouvette cylindrique est d'introduire l'effet d'une microfissure sur le champ de contraintes dans un cylindre. En particulier, on avance que si un champ de contraintes idéal dans un cylindre sans défauts peut expliquer comment et où la première microfissure se développe dans une éprouvette cylindrique, c'est le champ de contraintes modifié par la présence des microfissures qui est à prendre en considération pour comprendre le comportement subséquent.Cette opinion permet aux auteurs d'adopter une vision globale d'une série d'observations diverses sur un mode de rupture par contraintes indirectes.On considère également le cas de la rupture par contraintes axiales d'un cylindre de béton sollicité par une pression radiale, dans le cadre d'une analyse classique des contraintes élastiques.Bien que ce mode de rupture ait été décrit dans la littérature comme paradoxal, on démontre que l'amplitude du champ de contraintes de traction induites est bien suffisante pour expliquer la rupture axiale sous pression radiale, en recourant à une explication basée sur la résistance élémentaire des matériaux.

     

Numerical simulation of the Split Hopkinson Pressure Bar test technique for concrete under compression

The Split Hopkinson Pressure Bar [SHPB] technique has been commonly used to investigate concrete compressive response under high strain rate. However, there appears to be a lack of agreement in the literature about a number of critical issues pertaining to this test method. In this paper, computational simulation models are employed to critique the technique and obtain a better understanding of it. Influential parameters are identified and attempts are made to shed light on some controversial issues surrounding the interpretation of high strain rate test data. The results show that significantly different strain rates can be obtained from the same SHPB test depending on the method used to estimate the strain rate value. Furthermore, comparing the results of simulations with pressure-independent and pressure-dependent constitutive material models show that strength increases associated with strain rate are strongly, but not totally, reliant upon the confinement introduced by lateral inertial effects and the frictional condition at the interface between the pressure bars and the specimen. Based on these observations, it is argued that the so-called ‘rate-enhanced’ models that explicitly account for strength increases as a function of strain rate should not be used in numerical simulations that already account for the effects of lateral confinement, since such models would tend to double-count the strain rate effect.     

Strength and strain capacities of concrete compression members reinforced with FRP

The analytical compressive behavior of concrete members reinforced with fiber-reinforced polymer (FRP) was examined. The variation in the shape of the transverse cross-section was analyzed. The bearing capacity and the increase in the maximum strain for members having a cross-section which was circular, square or square with round corners reinforced with FRP were determined. The proposed analytical model allows one to evaluate the confining pressure in ultimate conditions considering the effective confined cross-section and also allows one to determine the ultimate strain corresponding to FRP failure through a simplified energetic approach. Analytical results are then compared to experimental values available in the literature, showing good agreement.     

Stresses and failure in the diametral compression test

The validity of the diametral disc test with a small flat ground has been established. The stress distribution was determined by a finite element method for a range of loading conditions. The results show that for the case of a point load, the failure is due to shear and compressive stresses at the loading point. Application of the diametral disc test for a ground flat was proposed and tested. The width of the flattened area must be less than 0.2 times the diameter of the disc to obtain accurate tensile strength.     

Strength failure and crack coalescence behavior of brittle sandstone samples containing a single fissure under uniaxial compression

Uniaxial compression experiments were performed for brittle sandstone samples containing a single fissure by a rock mechanics servo-controlled testing system. Based on the experimental results of axial stress-axial strain curves, the influence of single fissure geometry on the strength and deformation behavior of sandstone samples is analyzed in detail. Compared with the intact sandstone sample, the sandstone samples containing a single fissure show the localization deformation failure. The uniaxial compressive strength, Young’s modulus and peak axial strain of sandstone samples with pre-existing single fissure are all lower than that of intact sandstone sample, which is closely related to the fissure length and fissure angle. The crack coalescence was observed and characterized from tips of pre-existing single fissure in brittle sandstone sample. Nine different crack types are identified based on their geometry and crack propagation mechanism (tensile, shear, lateral crack, far-field crack and surface spalling) for single fissure, which can be used to analyze the failure mode and cracking process of sandstone sample containing a single fissure under uniaxial compression. To confirm the subsequence of crack coalescence in sandstone sample, the photographic monitoring and acoustic emission (AE) technique were adopted for uniaxial compression test. The real-time crack coalescence process of sandstone containing a single fissure was recorded during the whole loading. In the end, the influence of the crack coalescence on the strength and deformation failure behavior of brittle sandstone sample containing a single fissure is analyzed under uniaxial compression. The present research is helpful to understand the failure behavior and fracture mechanism of engineering rock mass (such as slope instability and underground rock burst).     

Microcracking and chloride permeability of concrete under uniaxial compression

In the previous studies on microcracks and rapid chloride permeability tests, microcracks were quantified in terms of total crack length. This was carried out by examining concrete slices after compression tests. No attempts have been made to characterise the microcracks during the compression test prior to the chloride permeability test. In the present study, concrete cylinders were loaded under uniaxial compression between 30% and 95% of the ultimate strength. A non-destructive method of microcrack evaluation was used to study the progressive microcracking in concrete cylinders during compression tests. After the compression test, a rapid chloride permeability test (RCPT) (ASTM C1202) was carried out on a specimen cut from the same cylinder. The total crack length was also determined from the same specimen to compare with the observed microcracking behaviour, assessed by the non-destructive testing. The characteristics of the microcracks in terms of the specific crack area are different when a concrete is under a load and when it is completely unloaded. The chloride permeability of a concrete (after it was unloaded) appears to be influenced by the occurrence of a certain stress level known as the critical stress. When the critical stress is exceeded in a concrete specimen, a comparatively large chloride permeability was measured. Where the critical stress in a specimen is not exceeded, the increase in the permeability is marginal in spite of the large increase in microcracks.     

The interaction of test methods and failure criteria

A specific example is given in which test methods influence the ability of a component to meet a failure criterion. The example is for self-pressurized products such as aerosol containers. The U.S. Department of Transportation (DOT) regulates aerosol containers with respect to acceptable temperatures and pressures of their contents as well as the minimum burst pressures of the containers. Experiments have shown that the burst pressures of the containers are a function of the test methods used to measure the burst pressures. The paper also presents a method to determine the temperature at which a two-piece aerosol container burst, provided that the bottom of the container can be found and it is not severely deformed by impact. While focusing on the specific examples of aerosols, the broader issue is the relationship between test methods and the results achieved, with the ultimate goal of safer engineering outcomes. The nature of various test methods and their relationships to how a mechanical system is likely to stop functioning properly and safely are also discussed. Aerosols present an interesting case study because they involve several disciplines and concepts and are very familiar to most people.     

Buckling length determination of concrete filled steel tubular column under axial compression in standard fire test

The structural behavior of the column under fire becomes essential since it concerns the safety of buildings and people inside in case of fire event occurs. The standard fire tests were usually used to evaluate the resistance of the members under fire. This paper presents a method to determine the buckling length that was used to predict the resistance of the concrete filled steel tubular (CFST) column under axial compression in standard fire test. In this method, the fourth-order differential equation was used to develop the analytical model. Various boundary conditions of the column at working condition, e.g. fixed–fixed, fixed–pinned, and pinned–pinned, were considered in this analytical model. Design formulae and charts were developed to determine the buckling lengths that considered varying flexural stiffness along the column height in fire. Temperature-independent buckling lengths were also proposed to further simplify the fire resistance design procedure. To establish the validity, the determined fire resistance of CFST columns based on the analytical and proposed buckling lengths were compared with test results in literature and predictions based on empirical buckling lengths.     

Strength and fracture of lithium hydride crystals under uniaxial compression

We determine the conditions of fracture of lithium hydride crystals under uniaxial compression and establish the character of deformation and fracture of tested specimens and the ultimate compressive loads that can be withstood by the specimens. We also discovered and statistically analyzed a specific size effect on the fracture processes in crystals that is connected with the fact that the dimension of the crystal in the direction of applied compressive stresses is the principal factor that determines its compression strength. Russian Federal Nuclear Center, All-Russia Scientific Research Institute for Experimental Physics, Sarov, Russia. Translated from Problemy Prochnosti, No. 5, pp. 64–69, September–October, 1999.     

混凝土圆柱体试件在低速冲击下动力效应的研究

为研究混凝土圆柱体试件在冲击荷载作用下的动力效应,利用落锤冲击试验机,对混凝土圆柱体试件在应变率100~101/s范围内进行轴向冲击试验,并采用混凝土连续面盖帽模型（CSCM）对试验过程进行数值模拟。试验中测量不同冲击速度及冲击边界下的锤头冲击力、试件轴向应变时程曲线,获取了试件破坏形态及破坏过程的高速影像,比较分析了不同冲击速度及边界条件下,试件应力、应变峰值,应变率及动力增强系数（DIF）的变化规律。结果表明：随冲击速度的增加,试件的应力、应变峰值,应变率及动力增强系数都呈增加的趋势,冲击力作用时间则减小。混凝土平均强度与冲击速度呈抛物线关系,应变率则与冲击速度呈线性关系。模拟结果表明,CSCM混凝土本构模型在低速冲击范围内,有很好的计算精度,模拟破坏形态与试验结果吻合良好。     

Strength and elasticity of brick masonry prisms and wallettes under compression

The characteristics of brick masonry are influenced by the properties of bricks and mortar. This paper attempts at studying the properties of brick masonry using table moulded bricks and wire-cut bricks of India with various types of mortars. The strength and elastic modulus of brick masonry under compression have been evaluated for strong-brick soft-mortar and soft-brick strong-mortar combinations. Various sizes of prisms and wallettes have been tested during these experiments to study the size effect and different bonding arrangements. The failure mechanisms of such specimens have been studied. Attempts are also made to derive empirical relationships for masonry strength as a function of brick and mortar strength in the Indian context.     

The split tensile test applied to rectangular concrete blocks—Numerical and analytical approach

It has become common use to replace the diametrical tensile split test on cores taken out of concrete blocks by a split test carried out on the block itself, generally at a distance of one of the ends equal to a half of the height of the block. The split tensile strength is expressed using the classical relation established for disks: σ _br =2P/πHl, whereP is the fracture load,H the height of the block andl its thickness. The paper presents an analytical model, based on the theory of elasticity, verified by a finite element model, allowing to compute de horizontal tensile stress in a block of finite length submitted to two uniformly distributed and opposite vertical loads. Computations show that the maximum tensile stress in the block varies between 60 and 90% of the stress value obtained by the relation for the disk. Hence the tensile strength could be overestimated by 10 to 40% as far as the modulus of rupture is equal to the maximum stress, which should be verified in practice. One further shows the importance of the width and the nature of the load support on the results. Concluding, it seems desirable that testing methods and conditions should be refined and better specified before publishing the European standard in this matter.