Numerical investigation of fatigue characteristics of concrete pavement

In concrete pavements, fatigue is one of the major causes of distress. Repeated loads result in the formation of cracks. The propagation of these cracks cause internal progressive damage within the structure, which ultimately leads to failure of the pavement due to fatigue. This paper presents a theoretical investigation of crack propagation within concrete pavement and its fatigue characteristics under cyclic loading. A numerical fatigue performance model has been developed for this purpose. The model is based on fictitious crack approach for the propagation of cracks and stress degradation approach for estimating the bridging stress under cyclic loading. Using the numerical model, a parametric study has been performed for a typical concrete pavement to evaluate its fatigue characteristics for different foundation strengths. The method can be used for prediction of crack propagation in concrete pavement under cyclic loading and gives an estimate of the incremental damage or the entire crack history of the pavement.     

Numerical computation in the viscoelastoplastic continuum damage of hot mix asphalt concrete

The objective of this paper is to develop an accurate and advanced material characterization of hot mix asphalt concrete using an existing viscoelastoplastic constitutive model that accounts for rate of loading, temperature and stress state with growing damage. The modelling strategy of viscoelastoplastic continuum damage is based on modelling strain components separately and then combining the resulting models to obtain a final integrated viscoelastoplastic model. According to this model, the initial-boundary value problem is numerically solved using the constitutive relationship expressed in the convolution integral form. The model is successful in predicting responses up to localization when microcracks start to coalesce.     

Numerical investigation of elastic mechanical properties of graphene structures

The computation of the elastic mechanical properties of graphene sheets, nanoribbons and graphite flakes using spring based finite element models is the aim of this paper. Interatomic bonded interactions as well as van der Waals forces between carbon atoms are simulated via the use of appropriate spring elements expressing corresponding potential energies provided by molecular theory. Each layer is idealized as a spring-like structure with carbon atoms represented by nodes while interatomic forces are simulated by translational and torsional springs with linear behavior. The non-bonded van der Waals interactions among atoms which are responsible for keeping the graphene layers together are simulated with the Lennard-Jones potential using appropriate spring elements. Numerical results concerning the Young’s modulus, shear modulus and Poisson’s ratio for graphene structures are derived in terms of their chilarity, width, length and number of layers. The numerical results from finite element simulations show good agreement with existing numerical values in the open literature.     

Chemomechanics of concrete at finer scales

Concrete, like many other materials (whether man-made, geological or biological), is a highly heterogeneous material with heterogeneities that manifest themselves at multiple scales. As new experimental techniques such as nanoindentation have provided unprecedented access to micro-mechanical properties of materials, it becomes possible to identify the mechanical effects of chemical reactions at the micro-scale, where the reactions occur, and trace these micro-chemo-mechanical effects through upscaling techniques to the macro-scale. The focus of this paper is to review recent developments of a microchemomechanics theory which ultimately shall make it possible to capture chemomechanical deterioration processes at the scale where physical chemistry meets mechanics. This is illustrated through application of the theory to early-age concrete and calcium leaching, and an outlook to biologically mediated deterioration processes in solid materials is given.

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Résumé Le béton comme beaucoup d'autres matériaux, soit artificiels, géologiques ou biologiques, est un matériau très hétérogène, dont les hétérogénéités se manifestent à de multiples échelles. Comme des techniques expérimentales nouvelles, telle la nano-indentation, ont donné un accès non-précédent aux propriétés micromécaniques des matériaux, il est possible d'identifier les effets mécaniques des réactions chimiques à l'échelle microscopique, où les réactions ont lieu, et tracer ces effets au travers des méthodes de changement d'échelle vers l'échelle macroscopique. Cet article fait le point sur le développement d'une modélisation micro-chimicomécanique qui a comme but de modéliser la détérioration chimico-mécanique à partir de l'échelle physico-chimique. Ces développements sont illustrés au travers des applications au béton au jeune age et à la lixiviation des bétons. Enfin, l'extension de cette modélisation aux processus de détérioration bio-chimique des matériaux est mise en perspective.

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Editorial note Prof. Franz-Josef Ulm presented a lecture of this paper at the 2002 RILEM Annual Meeting in Madrid, Spain, when he was awarded the 2002 Robert L'Hermite Medal in recognition of his work in the field of durability mechanics. He is a RILEM Senior Member and Associate Editor forConcrete Science and Engineering.     

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.     

Numerical and analytical investigation of the size-dependency of the FPZ length in concrete

The main objective of this paper is to study the size effect on the fracture characteristics in concrete structures. The numerical investigation is based on a mesoscale modeling approach. Analytically, two size effect laws are investigated: the classical Ba?ant SEL and a new size effect law based on the enrichment of the stress field on the crack tip. The mesoscopic approach is used to study the evolution of the tangential stress along the crack path in order to investigate the fracture process zone variation during the cracking process. In addition, different analytical governing equations are used to evaluate the size-dependency of the FPZ length.     

A laboratory investigation on bonding properties of dowels in concrete roads

Inclined dowel bars and unmovable bars may cause pavement cracking in the vicinity to the bar ends. The aim of the investigation is to study if there are any differences in bonding properties due to dowel material, coating or diameter of the dowel. Steel dowels with different coatings and dowels made of composite material are tested. The maximum draw-out force for a draw-out travel of 1.5 mm is measured. The test is repeated four times and ends with a final cycle to establish the constant force needed for a draw-out travel of 5 mm. Steel dowels with bituminous coating show the lowest initial draw-out force. The draw-out force increased 2 to 3 times with a diameter increase of 50% for steel dowels with plastic coatings. For composite dowels the comparing results showed an increase of draw-out force 2 to 5 times with an increase in diameter with one third. The results from the repeating test for several cycles showed that the draw-out and push-back force were almost the same for all dowels. However, for the dowels with bituminous coating a higher push-back force was needed compared to the draw-out force. It should be noted that the testing speed could affect the results, especially for dowels with bitumen.

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Résumé Des goujons d’assemblage inclinés ou inamovibles peuvent engendrer des fissures au voisinage de leur extrémité. L’objectif de cette étude est de déterminer si les propriétés d’adhésion dépendent du matériau utilisé pour le goujon, de son revêtement ou de son diamètre. Des goujons en acier avec différents revêtements ainsi que des goujons en matériau composite sont testés. La force maximale correspondant à une extraction de 1.5 mm est mesurée. Le test est répété quatre fois et est suivi d’un cycle final pour déterminer la force constante nécessaire à une extraction de 5 mm. La plus petite force initiale d’extraction est obtenue pour les goujons en acier avec un revêtement bitumineux. Pour les goujons en acier avec revêtement plastique, la force d’extraction augmente de deux à trois fois lorsque le diamètre augmente de 50%. Pour les goujons en composite, la force d’extraction augmente de deux à cinq fois lorsque le diamètre augmente d’un tiers. Les tests comportant plusieurs cycles ont montré que les forces d’extraction et de rétraction étaient pratiquement similaires pour tous les goujons. Cependant, pour les goujons avec revêtement bitumineux, la force de rétraction était supérieure à celle d’extraction. Il doit être noté que la vitesse avec laquelle les essais étaient réalisés pouvait affecter les résultats, particulièrement pour les goujons avec revêtement bitumineux.

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Editorial note The Swedish Cement and Concrete Research Institute (CBI) is a RILEM Titular Member.     

Electrochemical method for the investigation of transport properties of alumina scales formed by oxidation

An electrochemical method is applied to the study of the transport properties of -alumina scales developed by oxidation of alumina-forming alloys. This technique consists in plotting the characteristic curvesV versusI of the scale. In this paper, relationships between the transport properties ( _i, _e,t _i, ...) of the scale and the variations ofV withI are analysed and discussed. Information about the conductivity variation in the oxygen chemical potential gradient of the scale can be obtained from the slopes ofV-I curves and from the evolution of such curves for different outerP _O2 imposed to the oxide. This method will be applied to -alumina developed by oxidation of a-NiAl alloy in part II.     

Energy-based equivalence between damage and fracture in concrete under fatigue

Damage in concrete members, occur in a distributed manner due to the formation and coalescence of micro-cracks, and this can easily be described through a local damage approach. During subsequent loading cycles, this distributed zone of micro-cracks get transformed into a major crack, introducing a discrete discontinuity in the member. At this stage, concepts of fracture mechanics could be used to describe the behavior of the structural member. In this work, an approach is developed to correlate fracture and damage mechanics through energy equivalence concepts and to predict the damage scenario in concrete under fatigue loading. The objective is to smoothly move from fracture mechanics theory to damage mechanics theory or vice versa in order to characterize damage. The analytical methods developed here have been exemplified with some already available data in the literature. The strength and stiffness reduction due to progressive cracking or increase in damage distribution, has been characterized using the available indices such as the strength reduction and stiffness reduction factors. It is seen through numerical examples, that the strength and stiffness drop indices using fracture and damage mechanics theory agree well with each other. Hence, it is concluded, that through the energy approach a discrete crack may be modeled as an equivalent damage zone, wherein both correspond to the same energy loss. Finally, it is shown that by knowing the critical damage zone dimension, the critical fracture property such as the fracture energy can be obtained.     

疲劳荷载对橡胶混凝土损伤和断裂性能的影响

虽然橡胶混凝土塑性和疲劳性能较好,但由于掺入橡胶,其在疲劳荷载下离散性增大,损伤过程及最终的断裂机制均不明确.为研究橡胶混凝土在疲劳荷载下的损伤和断裂性能,基于声发射开展了不同橡胶掺量的混凝土在疲劳荷载下的三点弯曲疲劳断裂试验.计算有效裂缝长度,分析疲劳荷载下不同橡胶掺量的混凝土裂缝长度a的变化规律,并利用裂缝长度a和...     

Numerical relationship between creep deformation coefficients of prestressed concrete beams

It is important to precisely predict and control the long-term deformation of a prestressed concrete beam in engineering practice, where creep defection is the primary component. The key to precisely predicting the long-term deflection is clarifying the numerical relationship between the creep coefficient and creep deflection coefficient. In this paper, four simply supported prestressed beams (7.5 m in length) were loaded for 700 days. According to the creep strains at different heights in the mid-span cross section during the loading period, the plane-section assumption was verified for the prestressed beams under long-term loading. Meanwhile, geometry models of the creep strain were established for both fully prestressed and partially prestressed beams. By studying the models, the numerical relationships between the creep coefficient and creep deflection coefficient were derived; for the fully prestressed beams, the creep deflection coefficient is larger than the creep coefficient, while the opposite is true for partially prestressed beams. Moreover, an expression for the creep deflection coefficient was proposed; the coefficient is determined by the creep coefficient, prestress degree, prestress effect, and geometric properties of the cross section. A new method is thus proposed for the accurate calculation of the creep deflection of a prestressed concrete beam.     

Influence of damage on the fractal properties of concrete subjected to pure tension

In a previous work [1] the authors showed how to acquire the meso-structural characteristics of undamaged concrete-like materials by a peculiar laser equipment [2]. In order to extend the analysis to damaged disordered materials, a new direct tension test equipment has been developed, that minimizes flexural effects by freely rotating boundary conditions. Increasing levels of damage are obtained, after reaching the peak load, by proceeding along the descending strain-softening curve. After the desired damage level is reached, the load is removed and the specimen is cut to permit the laser acquisition of the most damaged zone. The progressive rarefaction of the effective stress-carrying cross section is described by means of fractal concepts. It is worth noting that both the fractal dimension and the measure of the stress carrying cross section decrease after the peak load, and vanish when the specimen is broken apart.

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Résumé Lors d'une précédente étude [1], les auteurs ont démontré la possibilité d'apprécier les caractéristiques méso-structurelles des mattériaux désordonnés, comme le béton, au moyen d'un dispositif laser [2]. Afin de pouvoir étendre l'analyse à des matériaux désordonnés endommagés, il a fallu mettre au point un équipement d'essa à traction directe, pour minimiser les effects de flexion tout en permenttant la rotation des sections de extrémité de l'échantillon. Des niveaux d'endommagement croissants sont obtenus par l'application d'une charge supérieure à la valeur de pic, suivant la courbe décroissante de radoucissement (strain-softening). Une fois le niveau d'endommagement souhaité atteint, la charge est enlevée et l'échantillon est coupé. Le dispositif laser est ainsi capable d'analyser la section la plus endommagée. La raréfaction progressive de la section effective est décrite en utilisant des concepts appartenant au domaine de la géométrie fractale. Il est à la fois possible d'observer la réduction de la mesure de la section effective et la diminution de la dimension fractale après que la charge appliquée a franchi le pic, jusqu'à l'annulation lorsque les deux moitiés de l'échantillon sont complètement séparées.

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Editorial Note Prof. Alberto Carpinteri is a RILEM Senior Member.     

Laboratory investigation of rheological properties and scaling resistance of air entrained self-consolidating concrete

An experimental investigation was undertaken to analyze the influence of various admixtures on the rheological properties and scaling resistance of self-consolidating concrete. Such concrete is intended for use as a repair material for filling highly restricted areas, such as forms with closely spaced reinforcing steel bars. Several self-consolidating concrete mixtures having slump flow of 550+50 mm were prepared with water-to-cement ratios varying between 0.35 and 0.41. The mixtures were cast with 0 and 3 percent silica fume, with and without air-entraining admixture. All concretes incorporated superplasticizer and viscosity-modifying admixture to enhance deformability and stability. Rheological parameters (yield value and plastic viscosity) were measured using a concrete viscometer. The air content, unit weight, and consistency were also determined. The consistency was assessed using the slump flow and L-Flow methods. Tests performed on hardened concrete included compressive strength at 28 days (ASTM C 39), scaling resistance (ASTM C 672), durability to freezing and thawing (ASTM C 666) and measurement of the air-void parameters (ASTM C 457). Relationship between the simple slump flow and yield value and plastic viscosity measurements determined using a concrete viscometer are also discussed. In general, the laboratory test results indicate that it is possible to produce a frost durable, self-consolidating concrete with low yield value and high plastic viscosity (for such fluid concrete) which can be use as a repair material to fill highly restricted areas.     

Relationship between pore structure and compressive strength of concrete: Experiments and statistical modeling

Properties of concrete are strongly dependent on its pore structure features, porosity being an important one among them. This study deals with developing an understanding of the pore structure-compressive strength relationship in concrete. Several concrete mixtures with different pore structures are proportioned and subjected to static compressive tests. The pore structure features such as porosity, pore size distribution are extracted using mercury intrusion porosimetry technique. A statistical model is developed to relate the compressive strength to relevant pore structure features.     

The influence of corrosion on bond properties between concrete and reinforcement in concrete structures

The rebar corrosion in reinforced concrete is one of the most extensive pathologies affecting the performance of concrete structures. Chloride-induced rebar corrosion damage results mainly from the use of de-icing salts in cold climates and/or exposure to marine environments. Carbonation damage is a further important degradation mechanism. The internal consequences of corrosion are the modification of the steel behavior and degradation of the steel–concrete bond. This work is devoted to the influence of the controlled corrosion on the adherence between steel and concrete. A new geometry of specimen has been designed to: (i) avoid the lateral confining stresses that appear during the classical pullout tests and (ii) permit to impose a known confinement for the study of its influence on the behavior of the interface. Five specimens with different levels of corrosion have been tested in this contribution. Reinforcing bars embedded in concrete were submitted to accelerated corrosion using an external current source. The magnitude of corrosion was measured using both Faraday’s law and the weight loss method. The level of corrosion varied from 0% to 0.76%. The geometry of the specimens allowed us to take series of digital pictures during the tests, which were analyzed using a digital image correlation, the procedure named CORRELI^LMT. The results of pullout tests proposed in this contribution indicate that: (i) levels of corrosion that are less than 0.4% of weight loss improves the bond stress and (ii) levels of corrosion resulting in more than 0.4% of weight loss lead to a reduction of the bond stress value.     

The statistical evaluation of epoxy concrete heterogeneity

The subject of this paper is the statistical estimation of heterogeneity of epoxy concrete. The study is involved with the potential of its random failures. The main categories and sources of possible uncertainty have been shown. The research concept has been presented, showing the parameters that influence the polymer composite heterogeneity, e.g. composition, or the method of maturing. Experimental results have been analyzed using statistical methods and presented numerically and graphically in the form of histograms and distribution curves. Three types of theoretical distribution — normal, log-normal and Weibull's — have been considered. Conclusions on the variability of the various types of epoxy concrete have been formulated as well as conclusions on the influence of particular factors on this variability.     

单丝复合体系渐进损伤过程模拟及组分性能对损伤过程的影响

为研究单丝复合体系在单向加载过程中的应力传递及损伤演化规律,基于剪滞模型建立了渐进损伤过程的三维数值分析模型。单丝复合体系的渐进损伤过程曲线和临界状态下的纤维段数、应变载荷及纤维轴向应力分布均与文献试验结果非常吻合,表明本文所提出的单丝复合体系渐进损伤模型能够有效模拟单丝断裂过程中的损伤起始、损伤演化和断裂临界状态。研究了模型中组分材料的模量和强度对损伤过程的影响。结果表明：保持组分材料强度不变,增加纤维的模量能够加快损伤过程,基体模量和界面模量的增加对单丝复合体系渐进损伤过程影响不大;在组分材料模量及界面强度不变的情况下,随着纤维强度的增加,单丝复合体系渐进损伤过程的起始应变载荷和临界应变载荷均增加,临界状态下的纤维断点数减少。     

Predictions of comulative damage for concrete and reinforced concrete

Nonlinear damage evolution equation is suggested to predict fatigue life of plain concrete subjected to sequential, constant amplitude loading. This approach seems to eliminate the nonconservative aspect of the linear damage law (the Miner hypothesis) and is based on the observed shape of the damage growth relationship. For serviceability based design of reinforced concrete structures, a simple nonlinear relationship is developed to relate the increases in deflection and crack width with the cycle-ratio. The results compare favorably with experimental data.     

Is concrete a poromechanics materials?—A multiscale investigation of poroelastic properties

Materials and Structures - There is an ongoing debate, in Concrete Science and Engineering, whether cementitious materials can be viewed as poromechanics materials in the sense of the porous media...