Development and validation of the model B4 for concrete creep and shrinkage

An analysis of 69 recorded bridge deflection histories reveals that the existing formulations for the creep and shrinkage of concrete structures greatly underestimate the multi-decade creep deflections of structures, often designed for lifetimes >100 years. Despite having the correct form, the existing model B3 (a 1995 RILEM Recommendation) markedly underestimates the multi-decade deflections. So does the new fib Model Code 2010. Presented is a new model, labeled B4, which overcomes two main shortcomings of the ACI, CEB, JSCE and GL models. First, the new model is calibrated using the collected bridge deflection data to ensure the proper terminal slope of the compliance function in logarithmic time. Second, model B4 extends the scope of prediction to modern concretes with various types of admixtures and high strengths

Long-term shrinkage prediction from theoretical considerations and data analysis

Upon the release of the data from the tragic collapse in 1996 of the record-span segmental box-girder bridge in Palau, it was found that the 18-year deflection was 200 - 400% larger than the predictions based on the American, European and Japanese design codes or recommendations. This finding triggered further studies that led to a collection of deflection histories of 69 large-span segmental bridges, most of which suffered excessive, logarithmically growing, deflections with no sign of an asymptotic bound. It thus became clear that major improvements in design codes and practices are required. Data collection efforts led to a new database of laboratory concrete creep and shrinkage data. With over 3000 test curves, this database more than doubles the size of the previous RILEM database. Unfortunately, the duration of about 94% of the available lab tests is <6 years, 97% ≤12 years, and only 3% attains 30 years, while 100-year lifetimes are generally desired. So it became evident that the only way to develop a realistic multi-decade prediction model was by joint statistical optimization of the fit of the laboratory data and the multi-decade bridge data. Regrettably, most of the bridge data are insufficient for inverse FE analysis. The relative increases of multi-decade deflection after about 1,500 days could be used for calibration. The combination of incomplete multi-decade bridge data with the short-time laboratory database posed a challenge for statistical optimization of the model parameters. Nonlinear least-square regression was used to inform the information of obtained from the database with the bridge deflection measurements. The database of laboratory tests has further been extended to include high-strength concretes (up to 167 MPa strength at 28 days), as well as modern concretes with various admixtures, classified into six classes, some of which decrease and others increase the creep and shrinkage. Through correlation analyses and the incorporation of previously studied trends, new formulas for estimating the model parameters from concrete strength and composition (with admixtures) have been identified

Development of a Testing and Analysis Framework for Validation of Rehabilitating Pipe-in-Pipe Technologies

Aging natural gas pipeline infrastructure needs rehabilitation, and trenchless, pipe-in-pipe (PIP) technologies offer a versatile solution. For example, legacy cast/wrought iron pipes have been subject to elevated incident rates for decades (www.phmsa.dot.gov). In an effort to accelerate innovation, the United States (U.S.) Department of Energy (DOE) has invested in a recently initiated, 3-year research program focused on pipeline “REPAIR”. To establish industry adoption of new technologies, a robust framework to evaluate and validate systems under in-service loading conditions is required. This paper introduces the approach taken by the Testing and Analysis team to develop a framework that confirms a 50-year design life for the PIP technologies. Testing protocols involve a comprehensive literature review, performance criteria, and relevant load cases and failure modes of PIP technologies. We use numerical and analytical modelling to investigate failure modes and severe conditions, thus informing testing protocols. In this paper, we discuss analytical frameworks and proposed model validation methods. We further discuss plans for test geometries (e.g., circumferentially cracked host pipe) and protocols (e.g., cyclic/dynamic traffic loading) to apply relevant load cases and probe failure modes in service. Modifications and enhancements are investigated in light of the insights gained from review and modelling. The testing and analysis framework for validating service life performance of trenchless PIP repair methods is intended to accelerate the development and adoption of new and safe repair technologies in the gas industry, as well as other critical lifeline systems.Patrick G. Dixon, Brad P. Wham, Jacob Klingaman, Allan Manalo, T. Tafsirojjaman, Khalid Farrag, Thomas D. O, Rourke, Mija H. Hubler, Shideh Dasht

Recalibration and uncertainty quantification of the B3 creep model for long term estimates using Bayesian methods

Arecalibration of the B3 model for creep and shrinkage has become possible with the availability of a new expanded statistical database on creep and shrinkage at Northwestern University that now also covers high performance concretes with admixtures. The recently collected evidence on excessive multi-decade deflections of numerous pre-stressed long-span box-girder bridges shows that this recalibration is indeed necessary. Data containing precise information of concrete materials as well as curing and testing conditions are needed in order to update the model parameters.To accurately capture the long-termbehavior of a structure, the model must also be calibrated with long-term data.Yet, precise laboratory tests pertain mostly to durations <6 years and are not available on the time scale of many decades that engineers need to predict the behavior with desired lifetimes >50 years and often>100 years.To performthe required recalibration, two databases are used simultaneously— one for laboratory test data with accurate material information but unknown long-term behavior and another for long-term bridge deflections with unknown or uncertain material information and structural characteristics. A Bayesian approach to this problem is presented. First the parameters of the B3 model are recalibrated with the enlarged laboratory test database, which is regarded as the Bayesian prior information, see Figure 1. Then the model is updated in the Bayesian sense using the incomplete bridge deflection data to obtain the posterior information. The resulting model predicts the long-term structural behavior more accurately and provides the associated measures of uncertainty based on both sets of data

Multi-decade creep and shrinkage of concrete : extended laboratory-bridge database and improved prediction model

The pursuit of new material and structural analysis models for creep and shrinkage of concrete has largely been driven by poor practical experience with long-term predictions. A key insight was obtained from the study of long-time deflections of the ill-fated record-setting KB Bridge in Palau [1] and from the subsequent finding that similar excessive deflections plagued many similar large-span prestressed segmental bridges around the world [2]. In particular, it transpired that the asymptotic final slope in material based prediction models can be significantly improved by joint statistical calibration of the materials model by laboratory data and bridge deflection data. In collaboration with the TC-MDC, research at Northwestern achieved major progress in three aspects: (1) the size of the world-wide database of laboratory tests was nearly doubled (>4000 test curves), (2) a new database of long-term deflections of 69 bridges was compiled, and (3) a new model M4, which is an improvement of the existing model B3 (1995 RILEM Recommendations), has been developed. This newly developed model is able to capture the correct asymptotic evolution of creep, agreeing with multi-decade bridge observations. It covers both normal and high-strength concretes, and takes into account the effects of environmental humidity and temperature, cross section size and concrete strength and composition, including the effects of aggregate type and of the admixtures used in modern concretes

The B4 model for multi-decade creep and shrinkage prediction

Presented is a new model, labeled B4, which can overcome some of the shortcomings of the CEB-fib, ACI, JSCE and GL prediction models for concrete creep and shrinkage. The B4 model represents an extension and systematic recalibration of the theoretically founded model B3, a 1995 RILEM recommendation. In addition to introducing the so far missing separation of autogenous and drying shrinkage, model B4 takes into account the cement-type and admixture parameters, as well as the effects of various types of aggregate. The new predictors for the creep compliance function more accurately capture the composition information and are recalibrated to match the multi-decade behavior. This behavior has recently been documented by observed deflection records of many bridges, which is evidence that has so far been systematically underestimated. The improved model was calibrated through a joint optimization of a new significantly enlarged database of laboratory creep and shrinkage tests and a new database of bridge deflection records

Model B3.1 for multi-decade concrete creep and shrinkage : calibration by combined laboratory and bridge data

Recent comparisons with numerous data on excessive multi-decade deflections of segmentally erected prestressed box girder bridges revealed the need for recalibration of RILEM B3 model for creep and shrinkage of concrete. With this aim, data from 69 large bridge spans have already been collected at the time of writing but a joint effort of Northwestern University and RILEM TC-MDC. The NU-ITI Laboratory Database has been updated by new data from JSCE, Japan, and a large collection of datasets for modern concretes containing admixtures. Extensive sensitivity analysis of the dependence of model B3 parameters on the material composition and strength is presented. The laboratory data are weighted, with emphasis on long-time tests, particularly the 30-year laboratory tests of Brooks. Combined optimization of both the bridge and laboratory data leads to better formulae of model B3.1 for predicting creep and shrinkage parameters from material strength and composition

Creep and shrinkage prediction models for concrete : an uncertainty quantification

The accuracy of prediction models significantly influences the lifetime performance of engineering structures. For aging concrete structures, these are mainly deterioration models (chloride ingress, reinforcement corrosion, carbonation), as well as models for changes in material properties and long term deformations, such as creep and shrinkage (CS). Prediction models not only govern the inherent safety level in design but also represent key elements of any performance assessment. Although the numerical framework for the latter is well established, suitable stochastic models for many input parameters are still missing. A recently expanded database of laboratory CS tests as well as multi-decade bridge deflection data has now become available, making it possible to quantify the uncertainty of CS models embodied in the current design codes and standard recommendations. In this study, the stochastic models for the major input parameters are presented, the formulations of the new model B4, which represents a significant update and expansion of model B3 (1995 RILEM Recommendation), are introduced and applied to a case study in order to obtain sensitivity factors and prediction bounds

Pervasive lifetime inadequacy of long-span box girder bridges and lessons for multi-decade creep prediction

The present study was stimulated by the paradigm of KB Bridge in Palau, a record span segmentally erected concrete box girder, which deflected excessively within 18 years, and collapsed 3 months after remedial prestressing. Computations at Northwestern showed that obsolete creep and shrinkage models in standard recommendations are largely to blame. A literature search found that 68 further spans with similar excessive deflections and service lives shortened to 20-40 years were identified. Outlined is a method of 3D FE computation of multi-decade creep effects. It is shown that the computations match the multi-decade bridge observations of the KB and several other bridges provided one uses a recalibrated B3 model, while the obsolete creep and shrinkage models lead to severe underestimation of the multi-decade deflections and prestress losses. A recalibration of the B3 model is achievable through statistical coupling of partial data on bridge deflections with a world-wide laboratory database

Model B4 : a multidecade prediction model for creep and shrinkage

The multi-decade creep and shrinkage prediction for modern high–performance concretes poses a significant challenge. Here we present Model B4, the improved and extended successor of the 1995 RILEM recommendation Model B3. We demonstrate its strength by comparing it statistically to a wide set of previous models, such as Model B3, the fib Model Code Models 90/99 and 2010, the 1992 ACI 209 Model, and the Gardner–Lockman Model 2000. The calibration of Model B4 is based on a new laboratory database, developed at Northwestern University, and additionally on multi-decade deflections of 69 large-span prestressed bridges. The multi–decade creep behavior is updated using a joint optimization of the laboratory and bridge deflection databases. Two sets of equations to predict the parameters of the creep and shrinkage model, one utilizing solely strength, and one based on composition information, have been calibrated. They include the effects of admixtures, aggregates, and cement type