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Abstract
A R T I C L E I N F O Keywords: Post-fatigue characteristics High-strength concrete Coupled 3D fatigue-static loading Physico-mechanical properties Fatigue damage model Empirical prediction model A B S T R A C T Concrete structures suffer some damage yet not failed under fatigue load, and then continue to bear 3D redistributed stress. For this, the 3D fatigue loading considering various fatigue factors (e.g. confining stress, axial static stress (ASS) and force amplitude (FA), frequency, and cycle number) is first carried out, and then the 3D static loading is performed. The post-fatigue characteristics of high-strength concrete (e.g. P-wave velocity, S-wave velocity, porosity, gas permeability, triaxial compression strength, and elastic modulus) are gained. The results indicate that 3D fatigue loading weakens mechanical properties, delays wave propagation, and increases seepage paths. An obvious stress threshold is exhibited with increasing axial static stress and force amplitude, that is 80% triaxial compressive strength, where the physico-mechanical characteristics are rapidly weakened due to the energy dissipation caused by crack growth rises increasingly. Compared with 1D fatigue loading, the frequency turning point of weakening effect from decreasing to increasing is advanced under 3D fatigue loading due to the application of 3D stress exacerbates the heat accumulation and creep damage generation. Interestingly , the fatigue damage is likely to be more sensitive to axial load compared to confining stress during 3D fatigue loading. In other words, the promotion effect of axial fatigue load on damage is larger than the restriction of confining stress. Furthermore, the fatigue damage models considering various fatigue factors are proposed. Then, the empirical prediction models of this damage variable to mechanical parameters (strength and elastic modulus) and permeability are established to predict the capacity loss caused by fatigue loading in the design of concrete construction. The testing results in this context could facilitate our understanding of post-fatigue characteristics of high-strength concrete subjected to 3D fatigue loading and guide the safe design of concrete construction.
... To evaluate the behavior of concrete submitted to coupled 3D fatigue-static loading, Yang et al. [137] have performed fatigue tests using the triaxial compression dynamic-static tool. They characterize the relevant post-fatigue parameters of High-Strength Concrete (HSC), starting with those related to (1) wave propagation (longitudinal and transversal waves velocities), for which a delay has been noted, up to (2) the mechanical parameters (triaxial compressive strength, elastic modulus), showing a decreasing in their performances, and finally (3) the physical characteristics (porosity, permeability), for which an increase has been observed. ...
... They characterize the relevant post-fatigue parameters of High-Strength Concrete (HSC), starting with those related to (1) wave propagation (longitudinal and transversal waves velocities), for which a delay has been noted, up to (2) the mechanical parameters (triaxial compressive strength, elastic modulus), showing a decreasing in their performances, and finally (3) the physical characteristics (porosity, permeability), for which an increase has been observed. Yang et al. [137] have also established an empirical prediction model based on the parameters affecting the fatigue (some of them are notably described in our paper) such as axial static stress, loading frequency, force amplitude, cycle number and confining stress, in order to establish a relation between these all parameters. From their results, they showed that the damage is more influenced by the axial loads compared to confining stress. ...
... Deutscher et al. [137] evaluated the effect of temperature increase due to the dynamic testing on the behavior of ultra-high-performance concrete (UHPC). They investigated the loading frequency (3, 10, and 20 Hz), the maximum grain size, and the maximum stress level. ...
This paper contributes by summarizing knowledge on the fatigue behavior of concrete, discussing current standardization, and organizing previous contributions and prospecting future developments in experimental research. It describes classical laboratory fatigue tests and analysis (S-N curves, damage effects on modulus etc.), emphasizing differences expected for real loading (with variation in frequency and temperature, among other effects). Not only current standards diverge in results, but more fundamental effects were observed. Temperature has relevant impact on fatigue life, and during tests it may change due to self-heating. Specimen dimensions are relevant for this phenomenon. Loading frequency seems to influence fatigue, and since tests in laboratory need to be accelerated (high frequency) compared to the field (low frequency), this is a major concern. Other investigated effects include loading waveforms and moisture.
... As a nondestructive testing technology, AE is increasingly used to study the mechanical properties of rock mass materials [37][38][39]. Under the condition of normal temperature curing, the AE characteristics of the rock mass in the process of load failure show a certain law [40]. Stress relaxation will occur when fractures are formed because of the failure of the rock mass structure. ...
Gangue materials have been used to solve mine disasters with a support tunnel along the goaf and filling mining. Mastering the properties and damage characteristics of filling materials is an important basis for effective implementation. Based on the conventional uniaxial compression acoustic emission (AE) test, the effects of cementitious materials, ratio between water and cementitious material, gangue particle size, and grading parameters on the mechanical properties of gangue-cement samples were analyzed. The stage characteristics of compression deformation were studied. The fracture propagation characteristics and rock mass failure types induced by different graded gangues were revealed. The fracture forming mechanism from clustered damage and failure was interpreted. The results show that the compressive strength of the backfill increases with the increase of cementitious material; however, it decreases with the increase of water binder ratio. Controlling the proportion and dosage of materials was the key factor to realizing pumpability and stability. Combined with the deformation and AE characteristics, the failure stage of the backfill body is divided into three stages: linear deformation-low energy changing, block compression-high energy changing, and gentle stability-stable energy changing. Affected by the gangue distribution, the load in each stage will induce fracture to produce five distribution modes of single, turning, breakthrough, bifurcated, and collapsed surrounding gangue. In the process of loading failure, different gradation and particle sizes will also change its stress concentration characteristics, resulting in the transformation of rock failure types. The surface structure and roughness of gangue play an important role in the compressive performance of cement paste. The research results try to provide some guidance for efficient filling mining.
A novel high-strength straight-hole recycled pervious concrete (HSRPC) for the secondary highway pavement was prepared in this paper. This study aimed to investigate the effect of porosity (0.126%, 0.502%, and 1.13%), vehicle loading stress level (0.5 and 0.8) and service life on the resistance to rainstorm-based waterlogging of HSRPC under fatigue loading. The mechanical properties of HSRPC in terms of flexural strength and dynamic elastic modulus were studied. The waterlogging resistance of HSRPC was described by surface water depth and drainage time. The microstructure of HSRPC were observed with scanning electron microscopy (SEM). Results showed that although the dynamic elastic modulus and flexural strength of HSRPC decreased with the increasing number of fatigue loading, the flexural strength of HSRPC was still greater than 5 MPa after design service life of 20 years. After 2.5×105 times of fatigue loading, the permeability coefficient of HSRPC with a porosity of 0.502% and 1.13% increased by 18.4% and 22.9%, respectively; while the permeability coefficient of HSRPC with 0.126% porosity dropped to 0.35 mm/s. The maximum surface water depth of HSRPC with a porosity of 0.126%, 0.502%, and 1.13% were 8, 5 and 4 mm, respectively. SEM results showed that fatigue loading expanded the number and width of cracks around the tiny pores in HSRPC.
The dynamic mechanical behavior of high-strength concrete is sensitive to aggregate properties, thus an accurate description of the effect of aggregate strength on the dynamic tensile mechanical properties of high-strength concrete is essential to evaluate the structural stability of concrete. In this paper, a concrete meso-scale modeling framework was proposed based on discrete element method (DEM). The meso-scale model considered the three-dimensional meso-structure of concrete (coarse aggregate, mortar and interfacial transition zone), and the crushable aggregate model with realistic morphological characteristics was generated by the “clump-cluster” method. Based on this, the effects of aggregate strength on dynamic tensile properties and damage behavior of high strength concrete were quantitatively studied by splitting tensile test. The simulation results reveal the influence of aggregate strength and strain rate on the tensile properties of concrete at both macroscopic and mesoscopic levels, including dynamic deformation behavior, microcracks propagation mode and the proportion of different kinds of microcracks at different loading stages. The results show that the aggregate strength plays an important role in the dynamic tensile properties of concrete. The failure behavior of concrete and the damage degree of different components under dynamic tensile loading are influenced by the ratio of aggregate strength to mortar strength.
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The stress distribution around constructions in deep rock can be modified by dynamic disturbances, such as earthquakes. For this, we proposed a new loading method applied to granite sample under coupled static and dynamic cyclic loading (CSDCL) condition. The variations in P-wave velocity, gas permeability, and mechanical properties of granite before and after CSDCL are revealed. It shows that with increasing axial static stress, the dynamic cyclic loading amplitude and cycle number, P-wave velocity, uniaxial compressive strength (UCS) and elastic modulus decrease, whilst the permeability and Poisson’s ratio increase. It seems that variations in those parameters have a close relation with the exciting frequency. In this case, CSDCL is applied for crack development along the axial direction in the rock samples, and subsequently results in degradation of mechanical properties, delay of P-wave propagation, and increase of the transport paths. The results show that the permeability and Poisson’s ratio are likely to be more sensitive to the CSDCL, in particular under the axial static stress. The empirical relationships of the P-wave velocity with permeability, UCS, and elastic modulus are established for a pragmatic monitoring purpose. From design point of view, we have established the relationships of the disturbance factors to damage variable. Then the correlations of the damage variable to permeability, UCS, and elastic modulus are analyzed. The testing results in this context could facilitate our understanding of rock stability upon excavation subjected to dynamic disturbances.
In this paper, a refined engineering rule for the assessment of remaining fatigue life of concrete under compressive cyclic loading with varying amplitudes is proposed. The rule has been derived based on a combined numerical and experimental investigation of the loading sequence effect. The applied modeling approach is based on a damage model using the equivalent tensile strain rate to govern the fatigue damage evolution upon loading and reloading at subcritical load levels. A systematic calibration and validation procedure of the numerical model was performed based on the available experimental results. The prediction of the numerical model was compared with existing damage accumulation rules for the assessment of the concrete fatigue life exposed to varying loading ranges. Based on these studies, an enhancement of the Palmgren-Miner rule is proposed and validated for several loading sequence scenarios.
This study examines the fatigue performance of plain concrete specimens under uniaxial compression. The experimental program was developed for investigating the fracture evolution in concrete cubic specimens subjected to cyclic compression using the advanced X-ray micro-computed tomography system SkyScan 1173. As compared to other experiments, the 3D micro-CT damage images were shown for a various number of loading cycles. The quantitative evolution of the cracking volume with increasing fatigue damage revealed a strongly non-linear shape. The increase of the total crack volume was higher by 30% as compared to the monotonic fatigue test.
As a solid pollutant, the recycled aggregate can be reused to replace the natural aggregate to cast pervious concrete, promoting resource recycling and reducing environmental pollution. Pervious concrete is usually applied to transportation engineering as pavements and decks, which are often subjected to fatigue loads in service. Therefore, it is essential to investigate the fatigue mechanical properties of pervious concrete. In this study, four-point cyclic bending loading test of natural aggregate pervious concrete and recycled aggregate pervious concrete were conducted under four different stress levels. By analyzing the experimental results, the mechanical performances, including hysteresis curve characteristics, damping ratio, dynamic elastic modulus and cyclic strain, of two kinds of pervious concrete under cyclic loading were revealed. Based on the improved EPF model, the relationship between fracture parameters, plastic strain and unloading strain were obtained. Besides, the relationship between the loading cycles and the ratio of plastic strain to unloading strain was received according to fatigue testing data under different stress levels. Further, the simplified fatigue model of pervious concrete was proposed and the experimental data was fitted with the model results. The fitting result reached a good agreement.
Most of the rock masses after dynamic disturbance return to the original state of stress or have the stress redistributed within a certain time period. One dimensional coupled dynamic and static loading tests were carried out under a new type of static and dynamic-static loading combinations to compare the characteristics of mechanical properties,permeability,wave velocity and acoustic emission of granite. The damage variable defined according to the ultrasonic velocity increases gradually with the increasing of the static axial pressure(ASP),decreases first and then increases with the increasing of frequency. The maximum damage degree reaches 16%. The permeability and Poisson's ratio of samples are larger than the results of undisturbed samples,increase with the increasing of ASP,and increase at first and then decrease with the increasing of frequency. Both show high sensitivity to the high ASP. The strength and elastic modulus are less than the results of undisturbed specimens,increase with the increasing of ASP,and vary differently at the different levels of ASP with the increasing of frequency. Compared with the energy release of undisturbed rock samples,the specimen after the dynamic loading in the uniaxial compression process have no initial and latent stages of energy release and enter directly into the active stage of energy release with the duration prolonged and a lag phenomenon exists in the relative stress at the end and the energy release peak.
This paper reports fatigue deterioration of engineered cementitious composite (ECC), a unique high-performance fiber-reinforced concrete featuring high ductility. Sources of fatigue dependency in ECC microscopic constituent properties were discovered experimentally. These fatigue-dependent fiber and fiber/matrix interface properties were incorporated into a novel multi-scale mechanics-based analytical model to reveal the influences of fatigue dependency on the fiber-bridging and fatigue crack propagation in ECC. The flexural stress-fatigue life (S-N) of ECC were predicted and compared with experimental results. It was found that several fatigue-induced changes of microscopic constituent properties contribute to the fatigue deterioration of fiber bridging of ECC. As a result, saturation of multiple cracks (and thus strain capacity) of ECC was much reduced under fatigue. The flexural fatigue model incorporating the fatigue-dependent fiber-bridging constitutive model developed in this study could be used to predict the flexural stress-fatigue life of ECC and the resulting S-N curve agreed well with experimental results.
The fiber reinforced cementitious material with high ductility has potential use in particular environments and structures that undergo repeated or fatigue loads. In this study, a series of monotonic and fatigue tests were performed to investigate the compressive fatigue behavior of this material. It is found that the fatigue life of this material is higher than that of plain concrete and steel fiber reinforced concrete under the same stress level. In addition, the failure deformation of fiber reinforced cementitious material with high ductility under fatigue load was larger than the monotonic envelope, while the envelope coincides with the monotonic loading curve for concrete or fiber reinforced concrete. The failure surface and damage process were investigated and a new failure mode of polyvinyl alcohol fiber with crushed end was discovered. The fatigue failure surface could be divided into three regions, including fatigue source region, transition region and crack extension region.
This paper presents an experimental study on the gas permeability evolution with deformation and cracking process in a white marble under compressive stresses. Uniaxial and triaxial compression tests with different confining pressures are firstly carried out to generate different states of deformation and induced cracks. The spatial distribution and geometrical form of induced cracks are investigated by the X-ray micro-tomographic imaging technique. Localized splitting-type macroscopic cracks are generated in the uniaxial test, while diffuse-oriented micro-cracks are induced in triaxial compression tests. Then, each of the cracked marble samples is subjected to a hydrostatic compression cycle, with the measurement of permeability and axial and lateral strains. It is found that the initial permeability of the sample with localized cracks is higher than that with diffuse cracks. The permeability of cracked samples exhibits an irreversible evolution between the hydrostatic loading and unloading, and the evolution trend is influenced by the initial crack state induced by the previous mechanical tests. The deformation of cracked samples under hydrostatic compression is clearly anisotropic due to oriented crack distributions and irreversible due to the unilateral closure—opening property of cracks. Finally, we propose to use the nonlinear regression method to formulate best-fitting models to capture the relationships between the permeability evolutions and effective hydrostatic stress or axial and lateral strains of cracked samples.
Based on the continuum damage mechanics theory of concrete materials under fatigue loading; a phenomen-ological fatigue damage model due to nuclei-propagation of microdefects, microvoids and fractal of rendered concrete in conjunction with more compliant are developed by utilizing internal variable theory of thermo-dynamics. Experimental data from fatigue test were employed to verify the model and results shows that the model can describe the damage evolution of concrete materials under different fatigue loading by verifying the predicted fatigue life of concrete materials in uniaxial compression loading. Degradation of Young's Modulus, alteration in the mechanical behaviour, damage in elastic stiffness tensor, evolution in strain, damage development corresponding to the Normalized life, and its accumulation subjected to constant amplitude fatigue loading including its verifications are well discussed.
A numerical method is developed to simulate effectively meso-scopic damage evolution of concrete subjected to cyclic fatigue loading based on considering its heterogeneous material constituents. In the developed method, a fatigue damage evolution gradient controlled procedure is developed to speed up fatigue damage simulation in a reasonable way by using the developed damage model. In the procedure, the block cycle jump strategy is used and the maximal fatigue damage accumulation for each block cyclic loading is controlled to be less than the pre-set value based on the condition of efficiency and precision. In addition, a heterogeneous concrete modeling strategy is developed and used to consider heterogeneous material constituents with lower computational cost, in which material parameters are assumed to obey some kinds of probability distribution such as Gaussian distribution. A representative numerical example supports the effectiveness of the developed numerical method, which illustrates its ability to simulate well fatigue damage evolution of heterogeneous concrete. From simulation results obtained from different finite element models with coarse and fine mesh. We can also find that fatigue analysis results are insensitive to gird mesh by using the developed numerical method.
Fatigue failure is one of the most common failure modes for concrete structures. When subjected to fatigue loading, which may be induced either by vehicle load or by wind load, the damage of concrete would develop gradually due to the evolution of internal micro-cracks. Once the damage value exceeds a critical threshold, the concrete will lose its loading capacity. Therefore, knowledge about the fatigue damage evolution of concrete is crucial for evaluating its time-dependent deterioration as well as establishing the fatigue life prediction model of reinforced concrete structures. This paper dealt with an experimental research of stress level on fatigue performance of plain concrete based on the energy dissipation method. The developments of strain, stiffness degradation and energy dissipation were discussed in accordance with the obtained experimental data. Moreover, the parameter of effective area ratio, e.g. the ratio between the area of matrix and the area of concrete at the same cross section, was proposed to describe the fatigue damage as well as its evolution during the cyclic loading process. The results show that the developments of strain, stiffness degradation and fatigue damage can all be divided into three stages: (i) an instable and relatively brief stage due to the initiation of material original micro-cracks; (ii) a linear developing stage because of the propagation of internal micro-cracks; (iii) a rapid and unstable stage as a result of the growth and coalescence of cracks. In the linear developing stage, the value of strain rate remains constant approximately, irrespective of the stress level. In addition, the fatigue damage and stiffness degradation of concrete are closely linked to the stress level. With the increase of stress level, the fatigue damage enhances. But on the contrary, the stiffness degradation reduces when the stress level rises. The average value of single-cycle energy dissipation per unit volume in the second stage Ed,V increases exponentially due to the increasing of the stress level, and a fatigue life prediction model was developed by means of energy dissipation method. It is demonstrated that the fatigue life decreases with the absorbed energy of concrete and there is a linear relationship between the logarithmic fatigue life and Ed,V.
Concrete structures are increasingly affected by fatigue damage, while new slender building structures with greater mechanical requirements make it necessary to deepen knowledge of their behaviour under fatigue loading. In this research, high-strength plain concrete was studied under two different frequencies of cyclic compressive stress. Wöhler curves were obtained in two different ways, on the one hand, applying a uniform frequency of 10 Hz and, on the other hand, applying the specimen resonance frequency. The results show two different types of behaviour. A reduction in fatigue life was observed in those concretes tested at very high frequency. When loads were applied at resonance frequency, they did not seem to modify the breaking mechanisms of the concrete subjected to fatigue.
High-cycle fatigue loading combined with chloride exposure was studied with a close investigation of reinforcement ratio (1.06%, 1.59%, 2.91%) and fatigue load level (0.2–0.5). This study conducts both, experimentally and service life modeling based on the Monte-Carlo simulation. Results show that: (1) high-value fatigue loads expedite the chloride ingress of compressive concrete; (2) at the load level of ≥ 0.4, chloride ingress of tensile concrete significantly increases and service life decreases ≥ 93%; (3) although reinforcement ratio of 1.59% decreases the chloride ingress of tensile concrete, varying the reinforcement ratio rarely affects the service life.
Precast segmental ultra-high performance concrete (UHPC) columns with post-tensioned (PT) tendons can effectively protect the compression toe from damage and enhance energy dissipation (ED) capacity of column compared to conventional segmental concrete columns. Extensive experiments and numerical studies have been carried out to explore the cyclic response of segmental UHPC columns in the existing literature; however, the effect of such novel columns on the seismic safety of bridge structures has not been thoroughly understood. This study aims to numerically assess the seismic fragility and seismic life-cycle loss of bridge structures supported by the precast segmental UHPC columns. For comparison, the seismic performance of the same bridge with monolithic reinforced concrete (RC) piers are also evaluated. Here, three-dimensional (3D) numerical models are generated to simulate these two bridge types. A performance-based earthquake engineering (PBEE) framework is used to evaluate and compare their seismic performance. The segmental UHPC column model is validated with the previous experimental studies. The validated model is combined into the whole bridge. Numerical results showed that the precast segmental UHPC bridge experiences similar peak acceleration and larger peak displacement in comparison to the monolithic RC bridge. The segmental UHPC bridge pier experiencing lower residual deformation can effectively reduce the damage probability and life-cycle loss of a bridge compared to the conventional monolithic RC pier. Under the design event (2475-year return period) and maximum considered event (5000-year return period), the expected losses of the segmental bridge in the lifetime were approximate 92% and 84% of that of the monolithic bridge, respectively. The segmental UHPC bridge has a noticeable economic benefit when such a bridge is constructed in regions of high seismicity.
Designing or analysing the influence of fatigue on concrete structures is becoming increasingly important for a number of structural elements. For this reason, it is necessary to define a method capable of determining the concrete's fatigue limit in the most economical, fast and efficient way. The aim of this study is to compare and correlate different methods found in the literature in order to reduce the number and duration of the tests required to determine the fatigue limit. This reduction will consequently reduce the economic costs of determining the fatigue limit. For these proposes three different types of self-compacting recycled concrete was used. A linear correlation was found between the analysed methods, which means that the methods are capable of providing similar results and that the results obtained by the economical procedure must be selected. This work opens the door to define an optimal procedure in the process of concrete fatigue characterization, which is capable of yielding results up to five times faster and more economical than with other methodologies, so that resources are not wasted.
Geopolymeric recycled aggregate concrete (GRAC) can greatly facilitate sustainability in the construction industry by the simultaneous utilization of solid waste-based recycled aggregate and eco-friendly binder–geopolymer. This study presents an experimental investigation on the mechanical behaviors of GRAC confined by carbon fiber-reinforced polymer (CFRP) jackets under both monotonic and cyclic compressive loading. A total of 24 CFRP-confined GRAC specimens were fabricated and tested, in which four aggregate replacement ratios (i.e., 0%, 25%, 50%, and 100%) and two thicknesses of CFRP jackets (i.e., 1 and 2 layers) were considered. The failure models, compressive stress-strain behavior, and axial-lateral strain responses of CFRP-confined GRAC were investigated and compared. The characteristics of stress-strain relationships were also discussed in terms of the peak stress, ultimate strain, residual modulus, plastic strain, reloading modulus, and stress deterioration ratio. Moreover, the related results were analyzed by comparing to the predictions ones of the existing models for FRP-confined concrete, to evaluate their applicability and accuracies for CFRP-confined GRAC in this study. The outcomes will enrich the experimental database of CFRP-confined concrete and provide insights into the practical application of CFRP-confined GRAC.
This paper presents the experimental and analytical results of the fatigue behavior of prestressed concrete (PC) beam used in a heavy-haul railway under constant-amplitude and variable-amplitude fatigue loading. The fatigue behaviors including mid-span displacement, bending stiffness and strains of classical materials were investigated comprehensively. Moreover, the residual strain of concrete in compression zone was mainly analyzed and fitted, and the S-N relationships of bottom tensile steel bar and the test beam related to prestressing force were given. The experimental result shows that the fatigue failure mode of PC beam is the fatigue fracture of bottom tensile steel bar in the pure bending section. The main crack width, stiffness degradation and the maximum displacement all show development law of three stages under the constant-amplitude fatigue loading. The fatigue characteristic parameters (stiffness, displacement and strain of concrete in compression zone) of variable-amplitude fatigue test beams exhibit the multi-stage development law due to the addition of multi-level fatigue load. Further, on the basis of the fatigue characterization model of classical materials and the stress analysis of fatigue cracking cross-section, a nonlinear numerical analysis method for the whole fatigue process is proposed. The numerical simulation results are in good agreement with the test data, which shows that the proposed numerical analysis method can simulate the fatigue behavior of PC beams under constant-amplitude and multi-level variable-amplitude fatigue loading. The results of this research provide a reference for fatigue design and fatigue life evaluation of PC beams in heavy-haul railway.
In this research, the fracture processes in concrete subjected to monotonic and fatigue loadings are characterized and the differences in failure mechanisms are studied using acoustic emission (AE) and digital image correlation (DIC) techniques. Experiments are performed on notched plain concrete beams under three-point bending. The micro and macro structural activities are classified based on acoustic energy levels to differentiate between the formation of a fracture process zone (FPZ) under monotonic and fatigue loadings. It is observed that a FPZ within its conventional definition is formed under monotonic loading. On the contrary, the AE results under fatigue loading indicate isolated and dispersed micro cracks up to 95% of fatigue life. A damage index based on AE energy is proposed for concrete subjected to fatigue loading which could be used in health monitoring of structures.
This study experimentally examined variations in the permeability of mechanically damaged concrete under the application and relaxation of confining pressure and investigated the mechanism of the variation from the perspective of pore structure. The results showed that mechanical damage significantly altered the pore structure of concrete specimens, and as a result of micro-cracking the porosity increases above 0.1 micro metre in size. During the loading phase, the permeability gradually decreased with increasing confining pressure, the plot of which could be divided into three segments—sensitive, transition, and insensitive segments. In the unloading phase, the permeability was gradually recovered with the relaxation of the confining pressure. Permeability values during the relaxation were lower than those during pressure application. This is related to the partial opening of microcracking and pore, while the deformed microcracking and pore cannot be fully recovered. The confining pressure changes the pore structure dominated by mesopores into micropores.
With the development of material technology, high-strength concrete has been widely used in immersed, underwater and deep-buried tunnels, which exhibit elastic, brittle and plastic states under various stress conditions. However, the research on constitutive models of high-strength concrete is insufficient, especially regarding the elastoplastic coupling properties. Therefore, in this paper, an elastoplastic coupling analysis of high-strength concrete is conducted via conventional triaxial compression testing and cyclic loading testing. The failure process and the associated micro-cracking behaviour of the concrete during loading are monitored via the acoustic emission (AE) technique. By defining a confining stress function and plastic internal variables, the variations in the deformation parameters and the strength parameters with the plastic internal variables are analysed. With the increase in the plastic internal variables, the cohesion decreases, and the internal friction angle gradually increases. Based on the Mohr-Coulomb yield criterion, an improved cohesion weakening - friction strengthening (CWFS) model is proposed. The dilation angle of the concrete is smaller than the internal friction angle, and the non-associated flow rule should be adopted in the elastoplastic coupling model of the high-strength concrete. Finally, an elastoplastic coupling model is established based on the test results to provide an experimental and theoretical basis for numerical calculations and engineering applications.
This paper presents an experimental study of the permeability evolution of thermally damaged basalt fiber-reinforced concrete (BFRC) under effective stress. Initial permeability, temperature damage coefficient, stress sensitivity, and maximum permeability damage rate of BFRC increased gradually with temperature, while they decreased with the increased cycles of effective stress loading and unloading. The permeability of BFRC shows nonlinear attenuation in the effective stress loading stage, and the permeability recovery degree of BFRC decreases with the decrease of effective stress in the unloading stage. The mechanism of permeability evolution is revealed based on the changes in pore characteristics.
As structural design and analysis become more refined there is an increased demand for fundamental information on the performance of concrete under loads other than static. Apart from the application of different types of loading, the material behaviour of concrete is highly heterogeneous and complex in nature due to the occurrence of extensive micro-cracking and toughening mechanisms. In this work, an attempt has been made to estimate the crack growth in concrete members under the action of fatigue loading considering complex material behaviour. In addition to the dependency of size and shape of the member on fatigue behaviour, the effect of loading history under the action of varying amplitude load cycles with sudden overload is important and has been considered in this work. Further, the effect of loading frequency on crack growth and fatigue life has been demonstrated through thermal diffusion coefficient. The mathematical formulation for the prediction of crack growth rate has been derived on the basis of theoretical arguments utilizing the concepts of self-similarity and intermediate asymptotic. Available experimental data on concrete fatigue has been taken up from the literature and are used to determine the unknown coefficients in the formulation. The predicted results based on the proposed formulation are found to capture the effects of loading history, sudden overload, and loading frequency in addition to the effect of structural size. The proposed model is then solved using higher-order numerical integration techniques enabled by surrogate modelling followed by a sensitivity analysis. Loading frequency and energy release rate due to overload cycle are found to be less sensitive on the fatigue life prediction.
Fatigue failure is usually modelled by estimating fatigue lifetime with corresponding S-N curve and fatigue damage accumulation law such as Miner's law. This approach can neither track the stress redistribution induced by fatigue damage nor explain the mechanism of fatigue damage evolution properly. In this paper, a constitutive model for concrete fatigue is developed within the framework of continuum damage mechanics, where fatigue damage can capture the gradual degradation of mechanical performance in concrete. The conventional phenomenologically modelled fatigue damage evolution is built with an explanation on its physical mechanism. Then, in order to increase computational efficiency instead of taking cycle-by-cycle approach to track fatigue damage evolution in FEM simulation, an accelerating numerical scheme based on two temporal scales homogenization is employed to make computational cost acceptable. The simulation results are compared with fatigue experimental results to validate the proposed constitutive model and accelerating numerical scheme.
In order to study the fracture behavior of three-graded concrete under cyclic loading after initial static loading, the three-point bending test of three-graded concrete beam is carried out in this paper. In the course of loading, acoustic emission (AE) and 3D digital image correlation (3D DIC) are used to record the AE signal and surface crack of concrete beam under dynamic load. And the damage evolution behavior of three-graded concrete beams is described quantitatively based on the results of AE and DIC. The results show that the damage indexes of AE hits and crack opening area are in good agreement with that of Secant Modulus of P-CMOD curve of beams under three-point bending, which can be used to quantitatively characterize the damage degree of concrete. In addition, by comparing the crack morphology and fracture surface characteristics of three-graded concrete, the crack propagation mechanism of three-graded concrete with different loading frequencies is analyzed.
This paper presents a discussion on the subject of fracture energy of concrete and effective parameters. Therefore, it serves as an overview of the available approaches in determining the fracture energy. It also includes experimental methods which form the basis for measuring the fracture energies obtained from work of fracture (GF) and size effect models (Gf) and thus provides a comparison between different models. Furthermore, the behavior of the load-displacement curve, factors affecting the fracture energy and the crack path in the specimen are discussed. In order to build a background for the current understanding of fracture energy, references from both former and present authors are included to tie the subject together within the course of time.
A unified damage model applicable to both monotonic and fatigue loading is developed for concrete in the framework of mesoscopic stochastic fracture models. Besides traditional strain-based failure criteria, a new kind of energy-based failure criteria for meso-elements is established relying on nanoscale mechanics. Verifications show that the proposed model can not only reasonably reproduce the randomness in fatigue lifetime but also well describe the nonlinearity in fatigue damage evolution of concrete. In addition, the physical mechanisms lying behind these randomness and nonlinearity are clearly demonstrated for the first time.
Concrete structures often serves under discontinuous fatigue loading. In this work, discontinuous fatigue tests were designed for the first time to investigate the fatigue performance of ordinary concrete under discontinuous cyclic loading. During discontinuous fatigue tests, repeated stress cycles were interrupted by no-load time intervals with different durations. Results showed that time interval could promote the development of plastic deformation produced by cyclic loading. The residual strain in a spaced cycle (S cycle), which follows one time interval, was significantly larger than that in a normal cycle (N cycle), which is not preceded by any interval. The fatigue life, which was estimated by an energy dissipative model, of the specimen used in discontinuous fatigue tests was smaller than that from conventional fatigue tests. However, the dilatancy angle stayed constant in both S and N cycles. The threshold of the time interval was identified at 120 s, with the maximum catalytic rate of 117% for the tested concrete.
A fatigue model for plain concrete under variable amplitude loading is proposed by unifying the concepts of damage mechanics and fracture mechanics through an energy equivalence, in conjunction with the principles of dimensional analysis and self-similarity. The effects of stress ratio and overloads that accelerate the crack growth rate is included in the model, in order to capture the realistic behaviour under variable amplitude loading. Experiments are performed under variable amplitude fatigue loading in order to calibrate and verify the validity of the model. The model proposed in this work encapsulates the complex behaviour of concrete under fatigue and provides a more rational method for computing fatigue life of concrete structures.
The fatigue test is conducted, and from which the evaluation law of the energy dissipation is revealed and analyzed. In the second stage of the fatigue test with invariable cyclic, the energy dissipation within each cycle stay constant, and has an exponential relationship with the stress level; the critical dissipated energy is concerned only with the mechanical property of the material, but immune to the testing load. The process of energy dissipation is discussed, and a novel fatigue life prediction method of concrete based on the energy dissipation is proposed. The life prediction of concrete is compared with experimental results, and the proposed prediction method is validated.
The safe use of strain-hardening cement-based composites (SHCC) in structural and non-structural applications often requires a solid knowledge of the mechanical performance of this novel material under cyclic loading. The article at hand presents the findings of a comprehensive experimental investigation focusing on fatigue behaviour and failure mechanisms of SHCC made with polyvinyl-alcohol fibre and subject to various loading regimes. Uniaxial tests were performed both as tension-swelling tests and alternating tension-compression cyclic tests. While the upper reversal point was controlled – depending on the chosen regime – either by a given deformation increment or load level, the lower reversal point was always controlled by a given load value. The experiments revealed a pronounced decrease in the number of load cycles to failure with increasing upper stress level, smaller applied strain increments and with transition from purely tensile loading to alternating tension-compression regime. Furthermore, the effects of these tests parameters on strain capacity and other material properties of SHCC were investigated. On the basis of the test results, evaluation of the crack patterns and microscopic analysis of the conditions of the fracture surfaces, four various failure modes were identified, each of them typical for particular cyclic loading scenarios.
This paper presents an analytical study on the fatigue response of reinforced concrete beams strengthened with fiber reinforced polymers (FRP). The study gathers the experimental results from the international literature and from authors' experiments and investigates the critical parameters that affect fatigue performance of reinforced concrete (RC) beams. A new analytical model is proposed for predicting the fatigue life of FRP strengthened RC beams. The parameters used are the maximum stress of tensile steel (σmax) to the yielding strength (fy) ratio, as well as the axial rigidity of longitudinal steel (ks) and FRP (kf) reinforcement. The predictions of the proposed model are compared against the experimental results as well as against the predictions of fatigue models in the literature.
An equation is proposed for the determination of the fatigue strength of plain, ordinary, and lightweight concrete when subjected to compressive stresses. The equation expresses both the Woehler and the Smith diagram, and it is shown that the Woehler diagram should be constructed for constant values of the ratio between the lowest and the highest compressive stress under pulsating load, and not for constant stress amplitudes or for constant lower stresses. The equation is verified by laboratory experiments which are presented and by test data taken from the literature.
Crack growth caused by load repetitions in geometrically similar notched concrete specimens of various sizes is measured by means of the compliance method. It is found that the Paris law, which states that the crack length increment per cycle is a power function of the stress intensity factor amplitude, is valid only for one specimen size (the law parameters being adjusted for that size) or asymptotically, for very large specimens. To obtain a general law, the Paris law is combined with the size-effect law for fracture under monotonic loading, proposed previously by Bazant. This leads to a size-adjusted Paris law, which gives the crack length increment per cycle as a power function of the amplitude of a size-adjusted stress intensity factor. The size adjustment is based on the brittleness number of the structure, representing the ratio of the structure size d to the transistional size d0, which separates the responses governed by nominal stress and stress intensity factor. Experiments show that d0 for cyclic loading is much larger than d0 for monotonic loading, which means that the brittleness number for cyclic loading is much less than that for monotonic loading. The crack growth is alternatively also characterized in terms of the nominal stress amplitude.
A nonlinear cumulative damage theory that can model the effects of the magnitude and sequence of variable-amplitude fatigue loadings is proposed. Concrete beam specimens are prepared and tested in fourpoint flexural loading conditions. Variable-amplitude fatigue loadings in two and three stages are considered. The present experimental study indicates that the fatigue failure of concrete is greatly influenced by the magnitude and sequence of applied variable-amplitude fatigue loadings. It is seen that the linear damage theory proposed by Palmgren and Miner is not directly applicable to concrete under such loading cases. The sum of the cumulative damage is found to be greater than 1 when the magnitude of fatigue loading is gradually increased and less than 1 when the magnitude of fatigue loading is gradually decreased. The proposed nonlinear damage theory, which includes the effects of the magnitude and sequence of applied fatigue loadings, allows more realistic fatigue analysis of concrete structures.
This paper presents a general four-variable relationship for predicting the fatigue strength of concrete. Based on the relationship, two equations are proposed for the prediction of fatigue strength of concrete, one for the high-cycle fatigue and one for low-cycle fatigue. These equations have been substantiated by compressive and flexural tests reported in literature.
This paper explores the strain response and damage behavior of concrete under uniaxial compression cyclic loading. Through present experimental investigation on irreversible strain accumulation, strain range variation and the analysis of fatigue modulus of concrete with microcracks, the fatigue failure process has been found to be related to the strain response behavior. It has also been found that irreversible strain accumulation, strain range variation, and fatigue modules degradation of concrete under fatigue loading are associated with and can be used to describe fatigue damage evolution of concrete. On the basis of the experimental results and continuum damage mechanics, a fatigue model is proposed in this paper, which can be used to account for the strain response properties, fatigue damage evolution and S-N relation of concrete under uniaxial compression cyclic loading.
Results of an experimental study of fatigue fracture og geometrically similar high-strength concrete specimens of very different sizes are reported and analyszed. Three-point bend notched beams 1.5, 4.24, and 12 in deep were subjected to cyclic loading with a lower load limit of 0.07Pu and an upper limit between 0. 73 and 0.84 Pu. It is found that the Paris law for the crack length increment per cycle as a function of the stress intensity factor, which was previously verified for normal concrete, is also appicable to high-strength concrete. However, for specimens of different sizes, an adjustment for the size effect needs to be introduced, of a similar type as previously introduced for normal concrete.
The behavior of 11 columns was studied experimentally and analytically. A numerical method for computing deformation of columns under fatigue loading is proposed.
The fracture response of concrete under low-cycle variable amplitude loading at frequencies up to 10 Hz was investigated. The applied loading history, selected to reflect earthquake loads on concrete dams, consisted of a basic sinusoidal oscillation interrupted by occasional spikes. Test results of specimens with different sizes and loading histories are reported. It was determined that the induced damage is both size and loading history-dependent; further, it was found that spikes in the loading history are likely to accelerate crack growth. On the basis of the experimental results, a fracture mechanics-based empirical law for crack propagation under variable amplitude cyclic loading is proposed.
In this paper, an improved lattice model of heterogeneous cohesive frictional solids is developed to simulate the fracture process of recycled concrete under tensile and compressive loadings. In this numerical simulation, the recycled concrete is considered as a composite material of five phases, including natural aggregates, old hardened mortar (HM), new HM, new interfacial transition zone (ITZ) and old ITZ. In addition to the numerical simulation, a series of laboratory tests are designed to investigate the failure process of the recycled concrete. The fracture process of the numerical simulation is in a satisfactory agreement with the experimental observations. It is demonstrated that the cracks always appear around ITZs at first, extend to old HM secondly and then to new HM in the recycled concrete. Under the condition of this investigation, the simulated stress–strain relationship of recycled concrete is fit well with the laboratory test results under tensile loadings.