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This work studies the creep characteristics of concrete sustaining initial damage by considering both the initial damage to concrete and the damage caused by crack growth in the creep process. The damage state of concrete is characterized by the characteristics of the amplitude of ultrasonic waves. Creep tests on concrete prisms with different degrees of initial damage were carried out at loads of 40%, 50%, and 60% compressive strength. The results show that the initial damage to concrete significantly impacts its creep behavior. The modified creep curve of concrete, including the initial damage variable, is consistent with the creep curve of undamaged concrete at different stress levels, proving the applicability of the damage variable. [ABSTRACT FROM AUTHOR]

Creep affects the long-term deformation of concrete structures. Nonlinear creep further overestimates the safety factor of structures and affects the safety service performance. The coupling of creep and a damage model considering the rate effect is conducive to accurate prediction of nonlinear creep, but the iterative process of strain makes the calculation method more complex. The purpose of this study is to propose a nonlinear creep explicit method that considers the damage evolution of concrete under compression. Two groups of axial compression members with compressive stresses of 0.2 fc and 0.4 fc were made. Considering the law of concrete damage evolution under uniaxial compression, coupled with elastic creep and damage incremental strain, the lower limit of the medium stress level that gives rise to nonlinear creep is analyzed. The concrete nonlinear creep amplification coefficient with a loading age of 28 days and loading duration of 360 days is studied with consideration for the uncertainty of relative humidity and the theoretical thickness of the component. On this basis, the explicit calculation formula of the nonlinear creep amplification coefficient related to the concrete axial compressive strength and stress level is given. The results indicate that the nonlinear creep amplification coefficient increases nonlinearly with an increase in the stress level, and, when the compressive stress level ratio is higher than 0.6, the nonlinear creep amplification coefficient increases significantly; when the stress level is determined, the creep amplification coefficient decreases gradually with an increase in the compressive strength of the concrete. It is suggested that a stress level range of 0.35~0.75 should be used for the study of a nonlinear creep amplification factor under the medium stress state. [ABSTRACT FROM AUTHOR]

The available models for estimating the creep strains of concrete generally assume concrete as a homogeneous material. Since concrete is a composite, such models cannot take into account incompatible strains between cement paste and aggregates during creep loading. The main objective of this paper is to demonstrate that initial defects could increase the creep strain level and justify the application of a coupling between creep and damage at the macroscopic scale. To do so, different contents of air-entraining agent were used to simulate the internal defects of concrete and high-stress continuous load was applied to concrete samples with different initial defects. By analyzing the stress-strain curves of concretes containing different initial defects after compression, it was concluded that the existence of internal defects had a significant effect on the basic mechanical properties of concrete samples. Acoustic emission technology was used to obtain the internal acoustic emission response of concrete samples under high stress. Creep development rate is nonlinear to load holding level based on the analysis of experimental data and internal structure of concrete samples. [ABSTRACT FROM AUTHOR]

A damage-plasticity model is extended to account for the effects of simultaneously occurring stiffness degradation, residual deformation, and creep. Assuming the additivity of small strains, the model combines damage mechanics, plasticity theory, and an improved version of the ACI model to characterize creep at low stress levels. The coupling between damage and creep produced by medium and high stress levels is accounted for by introducing a damage-dependent influence function. An explicit numerical algorithm is developed to implement the proposed model in the simulations of structural response. The proposed model is systematically validated by comparing its results with experimental data, suggesting that it offers promise for capturing the long-term mechanical behavior of reinforced concrete structures. [ABSTRACT FROM AUTHOR]

Many prestressed concrete bridges are reported to suffer from excessive vertical deflections and cracking during their service life. Creep softens the structure significantly, and therefore an accurate prediction of creep is necessary to determine long-term deflections in elements under eccentric axial compression such as prestressed concrete girders. This study proposes a modification to the creep damage model of Model Code 2010 to account for the effect of load eccentricity. The modified creep model considers damage due to differential drying shrinkage. Initially, the creep behaviour of small scale concrete specimens under eccentric compression load is investigated experimentally. Twelve small-scale concrete prisms were subjected to eccentric axial loading to assess their shrinkage and creep behaviour. The main parameters investigated include the load eccentricity and exposure conditions. Based on the experimental results, an inverse analysis is conducted to determine the main parameters of the modified creep model. Subsequently, a numerical hygro-mechanical simulation is carried out to examine the effect of load eccentricity on the development of shrinkage and creep, and on the interaction between drying, damage and creep. The results indicate that eccentric loading leads to different tensile and compressive creep through the cross section, which contradicts the current design approach that assumes that tensile and compressive creep are identical. The proposed model also predicts accurately the long-term behaviour of tests on reinforced concrete elements available in the literature. This study contributes towards further understanding of the long-term behaviour of concrete structures, and towards the development of advanced creep models for the design/assessment of concrete structures. [ABSTRACT FROM AUTHOR]

The detrimental influence of high levels of sustained load on the compressive strength of concrete has been acknowledged and investigated since the 1950s. Despite the potential significance of this phenomenon in many situations, current design codes still provide limited guidance on how to account for this effect at ultimate limit state. In addition, the practical case of a significant permanent stress followed by stress increases due to variable loads is not addressed in most codes of practice. The aim of this paper is to present a general but simple design approach to account for the influence of sustained loading on the uniaxial behavior of concrete in compression, both in terms of its strength and deformation capacity. This approach is based on a previously developed mechanical model accounting for nonlinear creep and material damage development on the long‐term response of concrete. On that basis, a design approach is formulated in a simple code‐like manner and used to investigate several practical design situations involving both sustained and variable actions. This approach is shown to be consistent and applicable to any general loading pattern, accurately reproducing the results of the general mechanical model and of available experimental results. [ABSTRACT FROM AUTHOR]

In 1967-1985, a research campaign comprising a unique set of long-term experimental data on concrete beams was conducted in joint collaboration with four Belgian research institutes to determine the influence of creep and shrinkage on the long-term behavior of reinforced concrete members. The main aim of the research campaign was the determination of the long-term behavior of cracking and deformations subjected to permanent loads considering the influence of the magnitude of the loads and various reinforcement ratios. The objective of this article is twofold: to provide an overview of the measured data of the reinforced beams of the research campaign, which has never been published before, and to propose a simplified calculation method based on available models in literature that can predict the available measurement data. A simplified calculation model is proposed, which accounts for nonlinear creep strains due to high stresses, shrinkage, aging, and cracking in reinforced concrete beams. This numerical method is based on a cross-sectional analysis formulated using the layered Euler-Bernoulli beam theory, allowing fast and accurate predictions of strains, stresses, and deflections as a function of time based on fib Model Code 2010 and EN1992-1-1. The measurements of the beams subjected to high permanent loads during a time period of 4 years are compared to the results evaluated with the proposed simplified calculation model. The results show that the proposed simplified calculation method based on the current models of EN1992-1-1 and fib Model Code 2010 can predict the long-term behavior of reinforced concrete beams subjected to high loads in good agreement with the measu [ABSTRACT FROM AUTHOR]

The material behavior of concrete is highly nonlinear and time-dependent. In particular, the evolution of deformation due to creep is crucially dependent on the acting stress and the environmental conditions. Numerous experimental campaigns for determining the complex material behavior of concrete in various loading conditions and environmental conditions can be found in the literature. However, the experimental investigation of the time-dependent deformation due to high sustained loading, i.e., nonlinear creep, has received less attention. Especially, the evaluation of the impact of different environmental conditions on nonlinear creep is still pending. In this contribution an experimental study on basic and drying creep under moderate and high uniaxial compression is presented. The experimental results indicate that the nonlinearity of the deformation due to creep in high uniaxial compression is dependent on the environmental conditions. Additionally, the capability of selected constitutive models for representing nonlinear creep is assessed in a numerical study based on the novel experimental results. • Experimental study on nonlinear creep considering both sealed and drying conditions. • Distinct influence of the humidity conditions on the nonlinear creep behavior. • Extension of the well-known B4 model to nonlinear creep. • Comparison of the test results with the prediction of modern creep models. • Calibration of the extended B4 model using the experimental results. [ABSTRACT FROM AUTHOR]

For high stress levels, the dependence of the creep strain rate of concrete on the acting stress is nonlinear. Furthermore, very high stress levels may lead to a critical growth of microcracks resulting in material failure. For the lifetime assessment of creep sensitive concrete structures by means of numerical simulations, the realistic description of nonlinear creep is crucial. However, many concrete models are formulated for either nonlinear time-independent material behavior, or time-dependent material behavior, restricted to linear creep. This is the motivation for extending a damage-plasticity model, aiming at a unified and computationally efficient approach for representing the highly nonlinear time-dependent material behavior of concrete. Well established approaches for modeling the evolution of material properties, inelastic deformation, damage, creep and shrinkage serve as basis for considering nonlinear creep and material failure due to high sustained acting stress. The extended material model for concrete is validated by comprehensive experimental results from the literature. • Extension of a damage-plasticity model for concrete to nonlinear creep. • A novel approach for modeling nonlinear creep and failure due to creep. • Application of a gradient-enhanced formulation for mesh-insensitive results. • Extensive validation of the proposed model by uniaxial and multiaxial test data. • Application to 3D FE simulations of a beam characterized by failure due to creep. [ABSTRACT FROM AUTHOR]

• Uniaxial nonlinear creep test of concrete under different ages and stress-strength ratios. • Quantitative relationship between creep characteristics and control variables. • Establish a concrete three-phase numerical simulation model. • Simulate the development process of concrete nonlinear creep, damage and crack. • Verify the mesoscopic mechanism of concrete nonlinear creep. In order to study the nonlinear creep mechanism of concrete materials, predict and evaluate the safety of the structure, uniaxial nonlinear creep tests of concrete specimens under different ages and stress-strength ratios (σ / f c) were carried out. Based on the discrete element theory (DEM), a concrete three-phase numerical simulation model is established, which considers the real aggregate shape and the strength difference of each phase material. The study found that the nonlinear creep duration of the early-age concrete specimens increases exponentially with age, and decreases exponentially with the increase of stress-strength ratio. Age has the greatest influence on total deformation of concrete nonlinear creep. The quantitative relationship between creep characteristics (duration, total deformation) and control variables (age, stress-strength ratio) is established. The numerical simulation model simulates the uniaxial nonlinear creep process of the concrete specimen. Compared with the measured data, it is found that the creep curve is in good agreement. At the same time, the overall trend of the measured damage value defined by the wave velocity and the damage simulation value defined by the number of cracks is in good agreement, which objectively reflects the general law of damage and crack development in the nonlinear creep process. Through the analysis of the failure mode and fabric diagram of the concrete specimen, the microcracks first occurred on the interface. At the same time, the cement gel in the cement mortar undergoes viscous flow, the stress is redistributed, and the microcracks continue to produce, expand, and connect, which ultimately leads to the complete failure of the specimen. The quantitative relationship and numerical simulation model proposed in this paper can be applied to the creep prediction and safety calculation of components or buildings under the condition of high stress-strength ratio, and has a good engineering application prospect. [ABSTRACT FROM AUTHOR]

• Nonlinear creep test on circular RACFST stub columns. • Influence of the RCA on the nonlinear creep of circular CFST. • Variation of confinement effects with time for RACFST. Encasing recycled aggregate concrete (RAC) in circular steel tube can improve the mechanical behaviours of the RAC without considerably increasing the cost. The confinement effects, which significantly increase the strength and the ductility of the RAC, also influence the nonlinear creep behaviour of RAC, which has not been well investigated. This paper presented nonlinear creep tests on recycled concrete filled steel tubes (RACFST) for up to 500 days. The load level varied between 0.41 and 0.79. Two concrete strengths were included in the tests (i.e. 30 MPa and 50 MPa). All these members used 100% of the recycled coarse aggregates (RCA) and have the steel ratio (the ratio of the steel area over the concrete area) of 0.08. The tested time-dependent deformations were compared against those reported for the circular CFST members to investigate how the full use of RCA would influence the nonlinear creep behaviour of the composite sections. The confinement effects were also carefully monitored during the long-term tests. It was found that the nonlinear creep can increase the creep coefficient of the RACFST members by 52%–115%, which needs to be carefully considered in the design. Confined by steel tubes, the encased RAC could stand for up to 500 days without creep failure under the stress level of 0.78–0.98 f cm , where f cm is the uniaxial strength of the RAC. The use of recycled aggregates can considerably increase the nonlinear creep effects by up to 22%, and such influence increased with the sustained loading level to the maximum value at the stress level of approximately 0.68 and then started to decrease. [ABSTRACT FROM AUTHOR]