Creep Behavior Of Concrete Research Articles

Laboratory Evaluation of Tensile Creep Behavior of Concrete at Early Ages

Short-term uniaxial tensile creep tests were conducted for early-age concrete at different ages in an effort to characterize early-age concrete as an aging viscoelastic material. Based on the test results, the viscoelastic material properties of the test concrete were characterized in terms of the Dirichlet series creep compliance and relaxation modulus functions. Applications of the model to the numerical analysis of the test samples showed a good agreement between the tests and computational results for the samples tested at different ages. This paper presents the test procedures and data analysis with a brief introduction of theoretical background.

Influence of low-grade metakaolin on creep behavior of concrete

Clay is an abundant material worldwide, but due to the high demand for pure kaolinite clay, research into the application of low-grade metakaolin (MK) is required. However, research on the effect of low-grade MK on the volume stability of concrete is lacking. In this work, concrete composed of low-grade MK, replacing 20%–60 % of Portland cement, is studied using nanoindentation, thermogravimetric, creep and strength measurement in order to demonstrate the impact of high-volume metakaolin on the long-term volume stability of concrete materials. Results show that although the incorporation of high-volume MK degrades the early-age mechanical properties of concrete (7–28 d), the addition of an appropriate amount of MK (20 %) improves the long-term compressive strength of concrete (60–180 d). Meanwhile, concrete mixed with MK (～60 %) shows lower creep stress than plain specimen, and the specimen mixed with 20 % MK has the lowest creep stress. From a nanoscale perspective, incorporating an appropriate amount of MK increases the content of high-density calcium silicate hydrate (C–S–H), which provides a positive effect on reducing the creep stress of concrete. For the 20 % MK-modified specimen, the volume fraction of C–S–H increases by 4.23 % compared to the reference sample, and the relative volume fraction of high-density C–S–H increases by 146.77 %. Meanwhile, the contents of pore and Ca(OH)2 phase, as well as the widths of interfacial transition zone (ITZ), are decreased in the 20 % MK-modified samples compared to plain concrete. The results of thermogravimetric analysis also show that the content of Ca(OH)2 decreases with the increase in MK substitution level, and the non-evaporable water content of the 20 % MK-modified paste is similar to that of plain paste. This study provides theoretical guidance for the application of low-grade clay minerals in concrete.

Prediction of high-temperature creep in concrete using supervised machine learning algorithms

In this paper, supervised machine learning techniques were employed to develop a prediction model for concrete creep at elevated temperatures. Several algorithms were considered including artificial neural networks (ANN), decision tree regression (DTR), random forest regression, linear regression and Gaussian process regression (GPR) models. A dataset of short-term, basic creep was compiled from experimental data reported in the literature for algorithm training. The selected input variables were time, temperature, 28-day compressive strength, compressive stress, fine and coarse aggregate mass, and steel and PP fiber mass. The data was split into training and verification sets, with 70% used for model training and the remaining 30% for model verification. The results demonstrate that ANN, DTR, RFR and GPR models can accurately reflect concrete creep behavior at elevated temperatures based on statistical indicators. However, further analysis showed that the GPR was the best-performing model, providing highly accurate representations of creep behaviour and demonstrating superior performance to existing empirical equations. Conversely, the DTR was found to not accurately reflect experimental data. Additionally, a Shapley additive explanation analysis was conducted that assessed the significance of each input parameter on model prediction. This research highlights the potential of machine learning techniques to accurately model high -temperature concrete creep behavior and thus represents a powerful tool for engineers and researchers.

A numerical model for basic creep of concrete with aging and damage on beams

This work presents a methodology to model the basic creep of concrete, considering aging and damage. Continuum elements with viscoelastic material represent the basic creep with aging using the solidification theory; interface elements with a cohesive zone model describe damage behavior through a linear softening law. Given the challenges of obtaining a unique set of parameters from experimental tests, a calibration process based on decoupling the physical phenomena is adopted. In addition, an inverse analysis is used when characterization tests are not available, allowing an indirect model calibration. Afterward, numerical results are compared with experimental data from bending creep tests. These results show that the proposed methodology can accurately represent concrete creep behavior at different load levels and specimen sizes besides predicting the beams’ residual capacity observed in tests with high load levels.

Flexural creep in plain concrete: State of the Art

Concrete, being a viscoelastic material, creeps with time causing additional strains and deflections in flexural members. Most of the studies in creep of concrete are based on long-term tests on plain concrete standard cylinders under axial compression, very limited studies have been reported on creep of concrete in flexure and direct tension. Whether the mechanism of creep in tension is different from creep in compression is still being debated. Numerous studies have been reported in the literature showing varied results with regard to the effects of creep in direct compression, direct tension and flexure. This paper provides a brief review of various creep tests on plain concrete reported and attempts to understand the effects of various parameters on the creep behaviour of concrete. Some studies reported that the creep in tension is higher than that in compression, whereas a few others obtained contradictory results from creep tests. This may be attributed to the fact that the relative creep behaviour in tension and compression is highly sensitive to the material composition, exposure conditions and loading level. The studies by various researchers on flexural creep in plain concrete are also discussed.

Creep Behavior of Early-Age Concrete Made with Different Cement Compositions under the Controlled Temperature History

A temperature stress testing machine (TSTM) was used to investigate effect of cement composition on cracking sensitivity and creep behavior of concrete. Results show that the cracking sensitivity of concrete varied with the ratio of C3S/C2S and it is observed that the temperature difference of concrete linearly increased with smaller ratio of C3S/C2S. Furthermore, it is also found that concrete with higher ratio of C3S/C2S had weaker creep behavior based on the specific creep deduced from results of TSTM.

Predicting the creep behavior of concrete at high temperature using the variable-order fractional model

PurposeCreep behavior of concrete at high temperature has become a major concern in building structures, such as factories, bridges, tunnels, airports and nuclear buildings. Therefore, a simple and accurate prediction model for the high-temperature creep behavior of concrete is crucial in engineering applications.Design/methodology/approachIn this paper, the variable-order fractional operator is introduced to capture the high-temperature creep behavior of concrete. By assuming that the variable-order function is a linear function with time, the proposed model benefits from the advantages of both formal simplicity and the physical significance for macroscopic intermediate materials. The effectiveness of the model is demonstrated by data fitting with existing experimental results of high-temperature creep of two representative concretes.FindingsThe results show that the proposed model fits well with the experimental data, and the value of order is increasing with the increase of the applied stress levels, which meets the fact that higher stress can accelerate the rate of creep. Furthermore, the relationship between the model parameters and loading conditions is deeply analyzed. It is found that the material coefficients are constant at a constant temperature, while the order function parameters are determined by the applied stress levels. Finally, the variable-order fractional model can be further written into a general equation of time and applied stress.Originality/valueThis paper provides a simple and practical variable-order fractional model for predicting the creep behavior of concrete at high temperature.

A coupled creep and damage model of concrete considering rate effect

From experimental perspective, it is well-known that relationship between concrete creep and loading level is nonlinear at middle and high loading levels. This nonlinear characteristic is considered to be closely related to micro-crack propagation and damage accumulation in concrete during creep. It has also been revealed in experiments that the strength of concrete under long-term sustained loading is significantly lower than that under static loading test, which can also be ascribed to the nonlinear characteristic (micro-crack propagation and damage accumulation) under long-term sustained loading. Focused on the nonlinear creep of concrete at middle and high loading levels, a coupled creep and damage model considering rate effect is constructed at meso-scale in this study. In the coupled creep and damage model, the creep behavior of concrete is characterized using B3 model and rate effect is taken into account by modifying Mazars damage model via intensity amplification factors DIFs. The presented model is validated through comparisons with experiment results from literatures. The dynamic damage mechanism and nonlinear creep mechanism are comprehensively analyzed and discussed using the model. It is proven that the presented model is capable of characterizing the nonlinear creep and creep failure of concrete at medium and high loading levels. The consideration of rate effect in calculation shows better results and the characterization of its role on the creep evolution is correct and essential.

Interpretable Ensemble-Machine-Learning models for predicting creep behavior of concrete

This study aims to provide an efficient and accurate machine learning (ML) approach for predicting the creep behavior of concrete. Three ensemble machine learning (EML) models are selected in this study: Random Forest (RF), Extreme Gradient Boosting Machine (XGBoost) and Light Gradient Boosting Machine (LGBM). Firstly, the creep data in Northwestern University (NU) database is preprocessed by a prebuilt XGBoost model and then split into a training set and a testing set. Then, by Bayesian Optimization and 5-fold cross validation, the 3 EML models are tuned to achieve high accuracy (R2 = 0.953, 0.947 and 0.946 for LGBM, XGBoost and RF, respectively). In the testing set, the EML models show significantly higher accuracy than the equation proposed by the fib Model Code 2010 (R2 = 0.377). Finally, the SHapley Additive exPlanations (SHAP), based on the cooperative game theories, are calculated to interpretate the predictions of the EML model. Five most influential parameters for concrete creep compliance are identified by the SHAP values of EML models as follows: time since loading, compressive strength, age when loads are applied, relative humidity during the test and temperature during the test. The patterns captured by the three EML models are consistent with theoretical understanding of factors that influence concrete creep, which proves that the proposed EML models show reasonable predictions.

Structural behaviour of CFRP reinforced concrete members under monotonic and cyclic long-term loading

A large percentage of the damages to reinforced concrete structures is caused by corrosion of the reinforcement steel, which often leads to expensive repairs or new construction of existing structures. Due to their high strength and resistance to corrosion, reinforcements made of carbon fibre-reinforced polymers (CFRP) are becoming more and more important in structural engineering. It is expected, that the service life of CFRP reinforced concrete (RC) members can be significantly increased as the strength-reduction due to corrosion is negligible compared to conventional RC members. Therefore, precise knowledge of the long-term behaviour of CFRP RC members is required in order to ensure safe and economic design. This paper presents experimental investigations on the long-term behaviour of CFRP RC members as well as steel-reinforced RC members under monotonic and cyclic long-term loading with varying load levels, different cross-sectional shapes and shear slendernesses. Accompanying experiments on the concrete creep behaviour that were conducted within the investigations are shown. Within the scope of the experiments, the deflections as well as the strains on the top and bottom side of the RC members were measured using displacement sensors and strain gauges. The experimental data is evaluated, especially with regard to the time-dependent deflections. The data is compared to existing mechanical and empirical models, which are usually derived for steel-reinforced RC members. Based on the experimental data, the time-dependent reduction of stiffness and conclusions for the calculation of deflections are shown.

Microstructure homogenization of concrete used in nuclear power plants

Nearly all nuclear power plants in the United States are operating past their intended lifetimes or are requesting lifetime extensions. Therefore, understanding changes to the concrete containment structure over time is crucial to evaluate the structure’s continued viability. Concrete materials are heterogeneous particulate composites that exhibit viscoelastic material properties, which can lead to slow deformation over time, causing stress redistribution and the potential for creep cracking. A code to generate random, three dimensional (3D) concrete microstructures has been developed and paired with finite element analysis to predict the long-term viscoelastic properties of concrete. Data from these simulations are used to develop constitutive equations for the viscoelastic behavior of the homogenized concrete. The codes in this work are used to virtualize laboratory experiments, to obtain long-term creep data in a faster, cheaper manner. To validate this work, the simulated creep behavior of concrete is compared to 800 d of experimental data that has been extended to 27 y of data using the Time-Temperature superposition (TTS) principal. Excellent agreement between the simulation results and experimental data is seen.

Numerical simulation of concrete creep behaviour using integral creep algorithm with alternating stresses

To investigate the accuracy of commonly used algorithms for prediction of creep and shrinkage effects in concrete structures, a comparative study is carried out using the finite element (FE) models with various user-defined time-dependent material properties. Numerical results are compared with available benchmark tests to justify the FE models. Results show that the FE model following the concrete integral creep (CIC) algorithm is close to the test result. The secondarily developed subroutine on the universal commercial FE platform is able to represent the time-dependent behavior of the concrete cylinders. CIC shows higher accuracy once alternating load history is considered, though the rate of creep method (RCM) and age-adjusted effective modulus method (AEMM) show the same accuracy as CIC under constant load. CIC shows outstanding compatibility with the material model specified in contemporary design codes. It seems that the CIC model can be used for a detailed evaluation of concrete structures, while RCM and AEMM might be used for preliminary and rough analysis.

Analysis and Selection of Creep Models for Concrete Structures in Tropical Regions

There are a number of models for predicting concrete creep behaviour such as models proposed in RILEM - B3, EN 19921-1-:2004, SABS 0100-1; MC -10… The existence of these different models for concrete creep analysis is resulted from the dependence of creep characteristics on many factors, including internal factors and external factors. Each model is therefore suitable for a specific condition. This study aims to analyse the creep models of concrete given in some concrete standards. Characteristics and scope of application of each model were clarified. These models were then applied to calculate concrete creep based experimental results on long-term deformation of reinforced concrete column sample in Viet Nam. A method of determining concrete creep coefficients was also established in this study. Compare the concrete creep coefficient calculated from the above methods and the results of the experiment to select the expected model. The results show that models ACI 209-2008 and EN 1992-2004 were the most suitable ones for applying in Vietnamese conditions. We recommend using these models for creep analysis of concrete in Viet Nam and in tropical climate.

Effect of aggregate shapes on local fields in 3D mesoscale simulations of the concrete creep behavior

Describing accurately the creep behavior of concrete is of significant importance for the evaluation of the long-term performance of structures. In this regard, a finer characterization of mesostructure effects and material non-linearity provides very insightful information. In this work, the effects of aggregate shapes on the creep response are studied using numerical simulations on 3D mesoscopic samples. The main focus is put on the assessment of the representativeness of generated samples versus real specimens obtained by tomography. Several mesostructures are generated by randomly distributing aggregates with different geometries, from simple spheres to realistic ones extracted from tomography. Creep simulations with finite element (FE) and Fast Fourier Transform (FFT) methods are then performed on different spatial refinements. Moreover, a classical linear viscoelastic (VE) and a viscoelastic-viscoplastic (VE-VP) behavior able to reproduce non-recoverable strains are adopted for describing the matrix behavior, to assess the relevance of a more accurate model. It is shown that numerical samples generated with tomographic aggregates may be regarded as a good approximation of the real specimen, while more ‘isotropic’ shapes, especially spherical, lead to significant differences at both local and macroscopic levels. Results obtained with FE and FFT methods are very close, indicating that while FFT is well adapted, FE remains attractive in this context. Finally, notable differences are observed between VE and VP response due to the development of residual strains in the matrix and correspondingly more limited strain redistribution, which indicates that VP-like models should be preferred to capture accurately the creep features at mesoscale.

Structural effects of FRC creep

Research studies in the last 20 years allowed to obtain reliable rules for designing structures made of fiber reinforced concrete (FRC). However, design aspects like the long-term behavior of FRC, especially when synthetic fibers are adopted, require further research. Long-term behavior includes aging and creep. Aging represent the change of fiber properties into the concrete environment, which may reduce the structural bearing capacity; when present, it is an important issue for the structural safety, especially when fibers are the only reinforcement. Aging of fibers must be proven by experimental tests. Creep is a complex phenomenon, roughly considered by building codes even for traditional reinforced concrete (RC) structures. The introduction of fibers do not change anything in concrete matrix and, before cracking, in the material concrete creep behavior is not expected any change. After cracking, the structural effect of FRC creep depends on the degree of structural redundancy and on the presence of rebars since creep produces a stress redistribution in the structure or from FRC to the rebars. When FRC post-cracking resistance is necessary for equilibrium requirements, in structures with cracked sections in service conditions the structural deferred response has to be analyzed by considering the FRC creep behavior. When FRC is used for resisting secondary actions and rebars are present for equilibrium requirements, the response of a FRC element (with rebars and fibers) will be identical to a conventional RC; FRC contributes by controlling the crack development under both short and long term loading.

Creep behavior of concrete containing glass powder

Glass powder (GP) is a solid waste with increasing reserves. GP can be used as a supplementary cementitious material (SCM) to produce concrete in order to effectively save resources and solve environmental pollution problems. The effect of GP to replace cement partially by weight on the compressive strength, elastic modulus and creep of concrete has experimentally been studied, and the internal microstructure is also determined by mercury intrusion porosimetry, scanning electron microscope and nanoindentation techniques. The results show that the use of GP reduces the compressive strengths and elastic modulus at the early ages, but the use of GP content less than 20% increases the compressive strengths and elastic modulus at the later ages. The use of GP content less than 20% can obviously reduce the creep and the use of 20% GP content seems to be the best in terms of the reduction of the creep. The use of GP with the appropriate content can effectively improve the internal microstructure of concrete and increase the content of high density calcium silicate hydrate at the later ages which is helpful in reducing the creep. This may be attributed to the pozzolanic reaction and microfiller effect of GP.

EFFECT OF CONCRETE CREEP ON THE SHORTENING OF VERTICAL ELEMENTS IN HIGH-RISE BUILDINGS

Vertical structural elements are commonly subjected to the time-dependent behavior effects caused by creep which causes shortening in these elements in the direction of the load being applied. Significant amount of creep takes place instantaneously with loading and carries on for the long-term dimensions. In high-rise buildings, the axial shortening in columns is inevitable, so it cannot be ignored. The shortening is differential between vertical structural members due to the difference in axial stiffness and load distribution areas on these elements. Conventional structural analysis assumes that all structural loads are instantaneously applied to the entire completed structure. The construction sequence and loading sequence may be different depending on the construction plan. Therefore, the actual structural behavior can be significantly different from the conventional analytical behavior based on the above assumption. The objective of this study is to highlight the effect of concrete creep behavior on the shortening of vertical elements specifically in high-rise buildings. In this study, we will be considering the ACI 209R-92 model. Based on experimental data, a procedure is developed to compute the elastic shortening due to creep. A 250 meters high-rise is studied taking into consideration the stress and modulus of elasticity depending its construction time and height.

A New Multicoefficients Creep Model for Concrete

Abstract The existing creep models for concrete are usually accompanied by complex calculations or a limited application scope because the factors that influence the creep behavior of concrete are numerous. A new, multicoefficients compressive creep model for concrete was proposed. This model is suitable for humidity from 3 % to 100 %, temperature from −20°C to 80°C, effective size from 20 mm to 500 mm, loading age from 1 day to 1,000 days, compressive strength from 15 MPa to 180 MPa, and stress level η ≤ 0.76. First, creep experiments were conducted on 188 concrete prisms under controlled room conditions and humidity conditions, which included different axial compressive strengths, different theoretical stress levels, different loading ages, and different load durations. Moreover, the linear creep coefficient model was obtained by regression analysis of the linear creep experimental data. Then the correction coefficients of concrete strength, loading age, temperature, humidity, and effective size of specimen on the creep behavior of concrete were obtained by fitting the 1,148 experimental data from references. Finally, the amplification coefficient formula of nonlinear creep was obtained from the relation between nonlinear convergences and linear creep experimental data, and the proposed creep model was verified by experimental data.

Effects of Variable Humidity on the Creep Behavior of Concrete and the Long‐Term Deflection of RC Beams

The influences of variable humidity on the creep behaviors of concrete and the long‐term deflections of RC beams are analyzed in this paper. A total of 6 prismatic specimens and 5 RC beams were subjected to sustained loads and 3 types of variable humidity conditions for 331 days. The creep strains of the prismatic specimens and the deflections of the RC beams were recorded to investigate the long‐term deformation characteristics. The test results reveal that both the creep strain and the deflection of specimens under cyclic humidity conditions exhibit approximately linear tendencies that are obviously less than those of specimens exposed only to natural air. During certain wetting cycles, the deformation rate became slower and the creep strain even recovered with an increase in the humidity, especially for shorter wetting‐drying cycles. Long‐term deflections predicted using the CEB‐FIP 90 and ACI 209R models were compared with the test results to evaluate the versatility of these current specifications. The findings included rather large errors between the predicted results and experimental data when the average relative humidity was adopted in the analytical models.

Strength and Deformation Properties of Alkali-Silica Reaction (ASR) Damaged Concrete Exposed to External Alkalis

In the present paper the results of strength and creep of a specific pavement concrete after ASR performance test are reported and discussed. The FIB cyclic climate storage test was applied which was developed in Germany. The used specific pavement concrete was tested exposed to water and external alkalis such as NaCl solution as deicer. Concrete prisms (100 x 100 x 400 mm) were tested on length change. After 252 days of storage, strength properties, creep and shrinkage were determined. The results of the experimental study indicate that ASR affects not only expansion but also static modulus of elasticity and creep behavior of concrete. Furthermore the tested concrete exposed to NaCl solution shows higher level of creep strain than exposed to water.