Creep Prediction Models Research Articles

Dynamic Analysis of Train-Rail-Bridge Interaction considering Concrete Creep of a Multi-Span Simply Supported Bridge

A moving wheel-rail interaction element incorporating rail irregularity is derived, and the finite element equations of motion for the train-rail-bridge interaction system are established. Then, the long-term creep deformation increment of an actual pre-stressed concrete girder is predicted based on the modified creep-prediction model, and the real rail creep irregularity of a multi-span simply supported bridge is analyzed in consideration of the rail smoothing effect. Finally, the influence of the rail smoothing effect on the dynamic response is discussed by a numerical example. Also, the influences of the creep irregularities at different periods are investigated, and the effects of a control method of delaying the track-laying time for about half year for mitigating the creep irregularity are studied. The results indicate that the periodic creep irregularity can excite significantly both the natural vibration and the forced vibration for the vehicle, and leads to the sharp impact at the beam end, thereby increasing dramatically the car body acceleration and the contact force. The continuous rail structure can smooth obviously the turning angle of the creep deformation and the impact at the beam end, and should be taken into account for a more accurate analysis. Moreover, the control method has distinct control effects, especially for the car body acceleration as well as the bridge acceleration.

Creep Behavior of High-Performance Concrete

Realistic prediction of concrete creep is of crucial importance for the safety, durability and long term serviceability of concrete structures. High performance concrete (HPC) contains combinations of various components, such as aggregate, cement, water-reducing agent and other ingredients which affect the properties of the HPC including creep. This paper reviews the accuracy of the conventional concrete (CC) creep prediction models, including B3,GL2000 and CEB-FIP(2010). Further, a new creep prediction model based on the comprehensive analysis is proposed. The improved model was calibrated through a joint optimization of laboratory creep tests. Comparisons of the results of the proposed methold with 76 data points of creep coefficient showes good agreement.

A Revised the B3 Creep Prediction Model Based on Short-Time Tests

The calculation of structure creep effect depends on the computational accuracy of concrete prediction model. Eight groups of test data were collected and used to evaluate six different prediction models. The differences between the calculated and the test data were analyzed. Based on the analysis, the method to improve the B3 prediction model was discussed, and the examples of revised B3 prediction model by the use of short-time tests were also presented. The results show that updating the prediction model based on short-time tests can improve the accuracy of B3 creep prediction model.

Concrete Creep and Shrinkage Effect Analysis Program Development

In order to analysis of concrete based on ABAQUS, A creep and shrinkage of concrete analysis program TDPA was developed. Firstly, the theory of creep and shrinkage effect of concrete was introduced. with the help of a Dirichlet series properties, creep stepwise recursive method was introduced to avoid stress history of large capacity storage; Secondly, using three user subroutine interface USDFLD, GETVRM, UEXPAN provided by ABAQUS, analysis program TDPA was developed by Fortran program language; Finally, typical long-term load tests of pre-stressed concrete and steel concrete composite members long term tests were selected to prove the reliability of TDPA. Research results show that: the TDAP has high applicability and reliability. It is suitable for a variety of creep and shrinkage prediction model, and provides an effective method for creep and shrinkage of concrete analysis on ABAQUS platform.

Analysis on Creep Property and Model of Bridge Girder Concrete with Various Mix Proportions

Combined with the construction of Jinghang Canal Bridge of Jiangliu highway, shrinkage and creep property of the cast-in-situ high performance concrete of bridge girder was researched. The results showed early shrinkage and creep development of the high performance concrete was relatively rapid, and creep tended to be stable 120 days later. With the admixture increasing, shrinkage and creep of the concrete were reduced. Meanwhile, early creep rate deceased. Similar results applied to the reduction of sand rate and water-cement ratio. Based on creep prediction models both at home and abroad, hyperbolic power function and exponential function model were put forward, taking the influence of material parameter and environmental factors on shrinkage and creep into consideration. In accordance with the model, concrete creep formula was given out and would provide a relatively scientific basis for practical prediction of concrete creep.

Dynamic versus static artificial neural network model for masonry creep deformation

One of the inherent modelling problems in structural engineering is creep of quasi-brittle materials such as concrete and masonry. The creep strain represents the non-instantaneous strain that occurs with time when stress is sustained. It has long been known that creep in brickwork results in deformations that increase gradually over time, and dependable and accurate prediction models for the long-term, time-dependent creep deformation of brickwork structures are, therefore, required. Several models, with limited accuracy, have been developed over recent decades to predict creep in concrete and masonry structures. The stochastic nature of creep deformation and its reliance on a large number of uncontrolled parameters (e.g. relative humidity, time of load application, stress level) make the process of prediction difficult and the development of accurate mathematical models almost impossible. Artificial neural networks have been introduced as an efficient modelling technique for applications incorporating a large number of variables and have proven successful in many cases, especially in problems for which the characteristics of the process are difficult to describe using mathematical models. This study introduces a creep prediction model, based on non-linear autoregression with exogenous inputs (Narx), which is able to detect and consider time dependency (which is the major factor in creep deformation of brickwork structures) within its architecture. The performance of the proposed Narx model was verified with experimental creep data from brickwork assemblages collected over the last 15 years. The results show that the accuracy of the Narx model outperforms existing artificial neural network models and is able to achieve a prediction error of less than 12%, strongly suggesting that the Narx model is more suitable for modelling a time-varying process such as creep prediction.

Analysis of Creep Effects in High-performance Concrete Bridge Based on AEMM

Analysis of creep behavior of High Performance Concrete (HPC) is significant due to the increase of the amount of HPC used in bridges. The common creep prediction models are mostly established on a large numbers of testing data of creep of normal-strength concrete whose simplified strength adjustment coefficient could not satisfy the accuracy in the analysis of HPC bridges, this makes the discussion on creep behavior of HPC used in bridges of paramount importance. Based on the revised creep prediction model on HPC, a creep strain incremental recurrence formulas for HPC was proposed according to the age adjusted effective modulus method and CEB-FIP model code 2010. Then, the stress relaxation function was deduced. With the secondary development function of the finite element software ANSYS, the subroutine USERCR.F and USERMAT.F have been developed. Finally, according to the incremental recurrence formulas, an ANSYS model of continuous rigid-frame bridge has been modeled analyzed, and some meaningful conclusions were obtained, which is helpful to provide a reference for the creep behavior analysis of HPC bridges.

Simplified Procedure for Evaluating the Effects of Creep and Shrinkage on Prestressed Concrete Girder Bridges and the Application of European and North American Prediction Models

The effects of time-dependent phenomena on concrete prestressed girder bridges are investigated. The study concerns the case of bridges built directly in their final configuration and that of bridges built by a sequence of stages in which geometry, restraints, and loads vary until the final configuration is achieved. An analytical approach based on the principles of aging linear viscoelasticity and the age-adjusted effective modulus method is followed. The paper has two aims: the first is to provide an efficient and simplified tool for the evaluation of the structural response in the early stages of design; the second is to compare the results of the analyses on actual cases of bridges when different shrinkage and creep prediction models are used. Numerical applications show that the influence of stress redistribution on bending moments, attributable to the change in static schemes, is reduced by the load-balancing effect of the dead load and prestressing and that bending moment diagrams for bridges in service do not change significantly with the different prediction models. Opposite results are found on deflections, which differ significantly in reference to the model used. Differences between predictions models are underlined, suggesting that designers should manage them with prudence to predict the long-term performance of concrete. Finally, Eurocode predictions always underestimate final deflections and stress redistribution because of the static scheme variation.

Sensitivity Study on Parameters for Fatigue-Creep Modeling of Stainless Steel Materials

In present paper, sensitivity study on parameters for fatigue-creep model of stainless steel materials has been carried out. An analytical model has been established to predict fatigue-creep life of AISI347 stainless steel. Manson’s Universal Slopes equation has been used as an empirical correlation which relates fatigue endurance to tensile properties. Different creep prediction models have been studied in order to correlate the results of shout-time elevated temperature tests with long-term service performance at more moderate temperatures. Comparison between the different creep prediction models and experimental results were carried out for a range of stresses and temperatures. A linear damage summation method has been used to combined fatigue and creep. The results of sensitivity analysis indicate that strain range and temperature have very significant influence on creep-fatigue life estimation.

Re-evaluation of CEB-FIP 90 prediction models for creep and shrinkage with experimental database

This paper aims to evaluate the CEB-FIP 90 model, which is commonly utilised to predict the creep and shrinkage effects of concrete structures, by comparing it with an extensive compiled database which combines the available data in literature and newly collected data from China. This database considers only concrete specimens with an average 28-day compressive strength between 30MPa and 80MPa, and restricts the relative humidity of the experimental environment to a maximum value of 95%. Three statistical methods are applied to evaluate the CEB-FIP 90 model: the residual method, the B3 coefficient of variation method, and the CEB coefficient of variation method. Based on the statistical regression analysis of the shrinkage and creep test data, the CEB-FIP 90 model is revised by modifying the influencing coefficients of the compressive strength of concrete and the time development functions of creep and shrinkage. The modified model is then subjected to evaluation and verification using the residual method, B3 coefficient of variation method and CEB coefficient of variation method. Based on verification with experimental data and corroboration with statistical analysis, the modified model performs better than CEB-FIP 90 model, especially with regards to high strength concrete.

Excessive Long-Time Deflections of Prestressed Box Girders. I: Record-Span Bridge in Palau and Other Paradigms

The segmental prestressed concrete box girder of Koror-Babeldaob (KB) Bridge in Palau, which had a record span of 241 m (791 ft), presents a striking paradigm of serviceability loss because of excessive multidecade deflections. The data required for analysis have recently been released and are here exploited to show how the analysis and design could be improved. Erected segmentally in 1977, this girder developed a midspan deflection of 1.61 m (5.3 ft) compared with the design camber after 18 years, and it collapsed in 1996 as a consequence of remedial prestressing, after a 3-month delay. Compared with three-dimensional analysis, the traditional beam-type analysis of box girder deflections is found to have errors up to 20%, although greater errors are likely for bridges with higher box-width-to-span ratios than the KB Bridge. However, even three-dimensional finite-element analysis with step-by-step time integration cannot explain the observed deflections when the current American Concrete Institute, Japan Society of Civil Engineers, Comite Euro-International du Beton (or Comite Euro- International du Beton—Federation internationale de la precontrainte), and Gardner and Lockman prediction models for creep and shrinkage are used. These models give 18-year deflection estimates that are 50-77% lower than measured and yield unrealistic shapes of the deflection history. They also predict the 18-year prestress loss to be 46-56% lower than the measured mean prestress loss, which was 50%. Model B3, which is the only theoretically based model, underestimates the 18-year deflection by 42% and gives a prestress loss of 40% when the default parameter values are used. However, in Model B3, several input parameters are adjustable and if they are adjusted according to the long-time laboratory tests of Brooks, a close fit of all the measurements is obtained. For early deflections and their extrapolation, it is important that Model B3 can capture realistically the differences in the rates of shrinkage and drying creep caused by the differences in the thickness of the walls of the cross section. The differences in temperature and possible cracking of the top slab also need to be taken into account. Other paradigms on which data have recently been released are four bridges in Japan and one in the Czech Republic. Their excessive deflections can also be explained. The detailed method of analysis and the lessons learned are presented in Part II. DOI: 10.1061/(ASCE)ST.1943-541X .0000487. © 2012 American Society of Civil Engineers. CE Database subject headings: Creep; Shrinkage; Box girders; Serviceability; Deflection; Span bridges. Author keywords: Prestressed box girder; Bridges; Segmental erection; Shear lag; Design standards; Concrete; Relaxation.

Creep Prediction Model for Concrete Made of Crushed Clay Bricks as Coarse Aggregate

A simple design office oriented empirical model containing only two parameters has been developed to predict creep behavior of concrete made of crushed clay bricks as coarse aggregate. For this, concrete samples having three different normal compressive strengths in the range of 18.9 up to 24.0 N/mm2 are first prepared and then tested for their creep deformation. For each of the samples, a hyperbolic equation is developed from their creep-time behavior. These equations are then combined and modified according to statistical norms to finally obtain a generalized equation. Comparison of creep strain obtained from this equation with that of experimental values show that the proposed model can closely predict creep in brick aggregate made concrete.

Creep Property of Concretes with Different Types of Coarse Aggregates

The rheological phenomenon of concrete materials, also termed as creep, is one of very important properties of concrete. Excessive deformation caused by creep does more detrimental effects on prestressed concrete structures than otherwise. Even though some investigations had been conducted on the normal concrete mixtures by the researchers and engineers all over the world, and the conclusions and the creep prediction models based on those investigations were developed, their effectiveness to be extended to all the concrete mixtures is very limited due primarily to the susceptibility of the creep property of concrete to the variation of the properties of aggregate, cement, water to cementitious materials ratio and their proportions. Especially for prestress concrete, creep induced prestress loss puts severe threat on the safety and durability of prestressed structures. Thus, in order to thoroughly understand the creep characteristics of the typical concrete mixtures used in Florida, this project was carried out by University of Florida and Florida Department of Transportation. The investigation found out that the creep strains of the concretes with different types of coarse aggregates at 91 days are very close, while the differences of creep coefficients of the concretes with different aggregates are fairly significant.

Selecting creep models using Bayesian methods

The development of creep prediction models has been a field of extensive research and many different models have already been proposed. This paper presents an evaluation method of the prediction quality of creep models for specific experimental data. Within the scope of this paper, the model according to Bockhold and the model according to Heidolf are examined. First, the parameters of the models are identified with respect to existing experimental data. This is done using a sampling based approach of Bayesian updating developed by Bažant and Chern. In extension to the method by Bažant and Chern, the uncertainty coming from inaccurate measurement data is taken into account in the definition of the likelihood function within the updating algorithm. The more inaccurate the measurements are, the more uncertain the estimated model parameters and model prognoses become. The identification is performed for different short- and long-term creep tests. The intension is not to validate these models intensively, but to evaluate their prognoses for the individually tested creep behavior. The results show that the identifiability of the models’ parameters is different for both models and consequently the models prognoses differ in their uncertainties. Second, the models are evaluated using two different strategies: the stochastic model selection according to MacKay, Beck and Yuen based on the Ockham factor, and a comparison of the uncertainties taking into account parameter and model uncertainties. The results of the evaluation of the creep models differ for various experimental tests. Model Heidolf is more flexible and gives a better fit to the data, however, it fails to predict reliable long-term creep deformations using only short-term measurements compared to model Bockhold. Comparing the evaluation methods, the analysis of uncertainties of the creep prognosis proofs to be more stable than the evaluation using the stochastic model selection.

Improved Algorithm for Efficient and Realistic Creep Analysis of Large Creep-Sensitive Concrete Structures

Recent compilation of data on numerous large-span prestressed segmentally erected box girder bridges revealed gross underestimation of their multi-decade deflections. The main cause has been identified as incorrect and obsolete creep prediction models in various existing standard recommendations and is being addressed in a separate study. However, previous analyses of the excessive deflections of the Koror-Babeldaob (KB) Bridge in Palau and of four Japanese bridges have shown that a more accurate method of multi-decade creep analysis is required. The objective of this paper is to provide a systematic and comprehensive presentation, appropriate not only for bridges but also for any large creep-sensitive structure. For each time step, the solution is reduced to an elastic structural analysis with generally orthotropic elastic moduli and eigenstrains. This analysis should normally be three-dimensional (3-D). It can be accomplished with a commercial finite element code such as ABAQUS. Based on the Kelvin chain model, the integral-type creep law is converted to a rate-type form with internal variables, which account for the previous history. For time steps short enough to render aging during each step to be negligible, a unique continuous retardation spectrum for each step is obtained by Laplace transform inversion using simple Widder's formula. Discretization of the spectrum then yields the current Kelvin chain moduli. The rate-type creep analysis is computationally more efficient than the classical integral-type analysis. More importantly, though, it makes it possible to take into account the evolution of various inelastic and nonlinear phenomena such as tensile cracking, cyclic creep, and stress relaxation in prestressing tendons at variable strain, as well as the effects of humidity and temperature variations, and the effect of wall thickness variation on drying creep and shrinkage. Finally, the advantages compared to the existing commercial programs, based on step-by-step integration of memory integrals, are pointed out and illustrated by a simple example.

English

The main goal of this study is to investigate the creep fatigue behavior of 316 stainless steel materials. In the low cycle thermal fatigue life model, Manson’s universal slopes equation was used as an empirical correlation which relates fatigue endurance to tensile properties. Fatigue test data were used in conjunction with different modes to establish the relationship between temperature and other parameters. Statistical creep models were created for 316 stainless steel materials. In order to correlate the results of shout-time elevated temperature tests with long-term service performance at more moderate temperatures, different creep prediction models, namely Basquin model, Sherby-Dorn model and Manson-Haferd model, were studied. Comparison between the different creep prediction models were carried out for a range of stresses and temperatures. A linear damage summation method was used to establish life prediction model of 316 stainless steels materials under fatigue creep interaction. Key words: Stainless steel materials, low cycle thermal fatigue, statistical creep model.

Rheological concrete creep prediction model for prestressed concrete bridges constructed by the free cantilever method

The primary objective of this study is to construct a simpler and more reasonable rheological model than existing models for predicting the creep phenomenon of concrete. Deformations of concrete under sustained stress can be expressed by the sum of immediate elastic deformation, time-dependent and time-independent short-term creep, and long-term creep, according to the mechanism and time-dependency. To simulate these deformations, a rheological model with six parameters was constructed. In composing each parameter, the microprestress-solidification theory and existing design codes were used. Moreover, the numerical approach was also used for the specific parameters that cannot be deduced from existing theory and design specifications. Finally, actual test data from prestressed concrete bridges built using the free cantilever method were analysed for verification of the proposed model and the suitability of the model was confirmed by comparisons with existing prediction models and design codes.

Evaluation of prestress losses in nuclear reactor containments

The most critical safety barrier in a nuclear power plant, the concrete containment, is prestressed by hundreds of tendons, both horizontally and vertically. The main purpose of the containment is to prevent radioactive discharge to the environment in the case of a serious internal accident. Due to creep and shrinkage of concrete and relaxation of the prestressing steel, tendon forces decrease with time. These forces are thus measured in Swedish containments with unbonded tendons at regular in-service inspections. In this paper, the prestress losses obtained from these in-service inspections are compared to losses estimated using several prediction models for creep, shrinkage and relaxation. In an attempt to increase the accuracy of these models, existing expressions for the development of shrinkage were modified using previous findings on the humidity and temperature inside two Swedish containments. The models which were used and modified for predicting creep and shrinkage were CEB-FIP Model Codes 1990 and 1999, ACI 209, Model B3 and GL2000. Eurocode 2 was used for the prediction of relaxation. The results show that the most accurate of the models were CEB/FIP MC 99 and ACI 209. Depending on the model, the accuracy of the prediction models was increased by 0.5–1.2 percentage points of prestress losses when using the modified development of shrinkage. Furthermore, it was found that the differences between the different models depend mainly on the prediction of creep. Possible explanations for the deviation between the calculated and measured models can be the influence of reinforcement on creep and shrinkage of concrete and the influence of friction on horizontal tendons.

State of the art of creep of concrete filled steel tubular arches

The state of the art of creep of Concrete Filled Steel Tubular (CFST) arch bridges including CFST members is presented in this paper. It is delineated that researches on creep of CFST is not mature and need much study compare to creep of plain concrete and reinforced concrete. In comparison with the plain concrete members, creep magnitude in CFST is relatively low but, the duration is longer. Creep has much influence on the deformation, stress redistribution between steel tube and core concrete and to reduce the buckling strength of CFST arches; how to implement the research results on actual arch bridge design is necessary part of research in pending. A good creep prediction model is the urgent essence of today’s design works with CFST element.

Uncertainty and sensitivity analysis of creep models for uncorrelated and correlated input parameters

The intention of this paper is to evaluate the uncertainties and sensitivities of creep prediction models of standard concrete. The development of creep prediction models has been a field of extensive research and many different models have already been proposed. The four major models are: model GL2000 by Gardner and Lockman, model MC90 according to CEB-FIP Model Code 1990, model ACI209 according to the American Concrete Institute and model B3 by Bažant and Bajewa. First, a sensitivity study is performed in order to determine the parameters which mostly contribute to the uncertainties of the model prediction. This is done for uncorrelated and correlated input parameters and the differences are pointed out. Due to high parameter correlation, most standard sensitivity methods are not applicable and, therefore, a new method developed by Xu and Gertner is applied. Second, the uncertainties of the creep prediction for all of the models are compared and reveal significant differences. Due to the consideration of parameter and model uncertainties, a measure for the total variation of the model response is achieved. A special FEM code is developed to include the existing creep models in structural analysis. Utilising the FEM application, arbitrarily distributed creep strains, non-linear creep and single reinforcement bars can be taken into account. Finally, this FEM code is used and the prescribed creep models and uncertainty method are applied to a pre-stressed concrete bridge. The uncertainties of the loss of pretensioning force and axial shortening are calculated and show a reduction in comparison to uncertainties of pure creep strain. Together, the presented method proves its ability to determine uncertainties and sensitivities of models of time-dependent behaviour for uncorrelated and correlated parameters in an efficient way.