Theoretical Modeling on Chemical Composition and Mechanical Properties of Well Cement under Carbonation Reactions

This paper presents theoretical models of the phase transformation and the change of mechanical properties of oil well cement due to underground carbonation reactions. Application of oil well cement has become common practice in CO2 underground storage formations. However, the CO2 stored underground may leak through damaged well cement, and it can cause carbonation reactions which may significantly decrease the mechanical properties of well cement. Extensive research has been conducted by using experimental methods in laboratories to characterize the carbonation reactions of cement underground. However, no theoretical formulations have been developed considering the mechanical properties change due to the carbonation effect. In this paper, the CO2 profile was predicted first by the error function solution of a 1D CO2 diffusion equation and further the carbonation reaction rate was calculated. The phase transformation of well cement due to carbonation were then analyzed by using a stoichiometric model. After that, the changes of mechanical properties of well cement were modeled using a two-phase generalized self-consistent model at different scales. Specifically, the decrease of the elastic modulus of well cement during the carbonation process was used as an example application of the model. Finally, the modeling result was validated with published experimental results. &nbsp;</p

Engineered Ureolytic Microorganisms Can Tailor the Morphology and Nanomechanical Properties of Microbial-Precipitated Calcium Carbonate

We demonstrate for the first time that the morphology and nanomechanical properties of calcium carbonate (CaCO ) can be tailored by modulating the precipitation kinetics of ureolytic microorganisms through genetic engineering. Many engineering applications employ microorganisms to produce CaCO . However, control over bacterial calcite morphology and material properties has not been demonstrated. We hypothesized that microorganisms genetically engineered for low urease activity would achieve larger calcite crystals with higher moduli. We compared precipitation kinetics, morphology, and nanomechanical properties for biogenic CaCO produced by two Escherichia coli (E. coli) strains that were engineered to display either high or low urease activity and the native producer Sporosarcina pasteurii. While all three microorganisms produced calcite, lower urease activity was associated with both slower initial calcium depletion rate and increased average calcite crystal size. Both calcite crystal size and nanoindentation moduli were also significantly higher for the low-urease activity E. coli compared with the high-urease activity E. coli. The relative resistance to inelastic deformation, measured via the ratio of nanoindentation hardness to modulus, was similar across microorganisms. These findings may enable design of novel advanced engineering materials where modulus is tailored to the application while resistance to irreversible deformation is not compromised.</p

Experimental and theoretical investigation of the fracture behavior of glass beads/epoxy composites using microscratching

Abstract: Concrete, ceramic, and other quasi-brittle materials contain pre-existing cracks and complex structure on various internal length-scales. Quantifying the fracture behavior of these heterogeneous materials using different testing methods is a challenge and is an argumentative subjec

Optimization method, choice of form and uncertainty quantification of Model B4 using laboratory and multi-decade bridge databases

The preceding article describes a new multi-decade creep and shrinkage prediction model, labeled B4, which extends and improves the previous RILEM recommendation B3, and a separate article presents a new large worldwide database of laboratory data on creep, drying shrinkage and autogenous shrinkage. This article presents the general optimization concepts used to verify and calibrate Model B4. The main objective is multi-decade, even 100-year, prediction, which is what is needed for sustainable design of long-span bridges, tall buildings and other large concrete structures. Since the existing worldwide database is insufficient by far for purely experimental verification and calibration of multi-decade creep, the importance of choosing a model form that is theoretically and physically justified is emphasized. So is the choice of a model form that is able to fit closely individual broad-range creep and shrinkage curves for one and the same concrete, which are free of the huge obfuscating scatter due to differences in concrete composition. The development and calibration of the formulae for predicting the parameters of the creep and shrinkage equations from the composition and strength of concrete is described. An effective method for statistical optimization of the fits of a new database comprising thousands of laboratory test curves is presented. Various types of bias in the database are counteracted by data weighting. To predict and calibrate multi-decade creep, a method to combine the laboratory database with a database of excessive multi-decade deflections of large span bridges is outlined. This leads to a significant increase of the slope of the terminal trend of predicted creep in a logarithmic plot. Finally, the statistical approaches for using the hybrid laboratory-bridge database for multi-objective fit optimization and for Bayesian updating are explained and discussed

Comprehensive database for concrete creep and shrinkage : analysis and recommendations for testing and recording

The first large worldwide database of creep and shrinkage tests was assembled at Northwestern University (NU) in 1978. It was expanded as the RILEM database in 1992 and further in 2008. A major expansion, completely restructured and verified, named the NU Database, is now presented. The number of the test curves of creep and drying shrinkage is more than doubled and over 400 test curves of autogenous shrinkage are added. The database covers longer measurement periods and encompasses the effects of admixtures in modern concrete mixtures. The database contains roughly 1400 creep and 1800 shrinkage curves, of which approximately 800 creep and 1050 shrinkage curves contain admixtures. Their analysis shows significant influence of admixtures on the creep and shrinkage behavior The mixture proportions, testing conditions, and specimen geometries are documented in greater detail, and information on the admixture contents and aggregate types is included. The new database makes it possible to calibrate and verify improved creep and shrinkage prediction models. Additionally, the statistics of the mixture parameters, strength distributions, and scatter of the compliance curves have been extracted for applications in reliability engineering and probabilistic performance assessment. Data analysis brings to light various recommendations for testing and recording, and suggests corrections of various oversights distorting the reported data. These recommendations would make future test data more useful, consistent, complete, and reliable. The NU database is now available for free download at www.civil.northwestern.edu/people/bazant/ as well as at www.baunat.boku.ac.at/creep.html

Statistical justification of Model B4 for drying and autogenous shrinkage of concrete and comparisons to other models

The shrinkage prediction part of Model B4 presented in the preceding paper is here statistically justified by optimal fitting of the new NU database containing 1050 test curves and by statistical comparisons with the existing shrinkage prediction models. Rather than attempting a point-wise constitutive model for free shrinkage, Model B4 predicts the average shrinkage of cross sections of long members, which are affected by nonuniform residual stresses relaxing due to creep and microcracking. The main improvement in Model B4, which extends the 1995 Model B3 (a RILEM recommendation), is that separate formulae are given for: (1) the drying shrinkage, which represents most of the shrinkage observed in normal concretes of high water-cement ratios, and (2) for the autogenous shrinkage, which has a different physical mechanism and is important for modern high-performance concretes with admixtures, additives and low water-cement ratios. The effect of elevated temperature on the shrinkage rate is captured through an equivalent accelerated time based on activation energy. Model B4 is statistically calibrated by the new NU database of laboratory shrinkage tests through a sequential optimization procedure which isolates different physical behaviors. The new shrinkage equations are shown to match the time curves of individual shrinkage tests well, and fit the database with minimum error. Statistics of extensive comparisons with Model B3 and with the models of various engineering societies, including those of ACI and fib, document a superior fit of the new model to the database

Statistical justification of model B4 for multi-decade concrete creep using laboratory and bridge databases and comparisons to other models

This paper presents: (1) statistical justification and calibration of model B4 using laboratory creep data and long-term bridge deflection data, and (2) statistical comparisons of various types with the existing creep prediction models of engineering societies. The comparisons include the 1995 RILEM Recommendation (Model B3), fib Model Code 1999, Model Code 2010, ACI Committee-209 Model, and the 2000 Canadian Model by Gardner and Lockman. The statistics and comparisons rely on a separately presented combined database of laboratory tests and multi-decade bridge deflection measurements, which has been developed at Northwestern University (NU). The laboratory data assembled in the NU database more than double the size of the previous RILEM database. The collected bridge data include multi-decade deflections of 69 large-span prestressed bridge spans, most of them excessive. The multi-decade bridge data are the only available and a significant source for long-term calibration because only 5 % of laboratory creep tests in the database had durations > 6 years, and only 3 % are > 12 years. Joint optimization of the laboratory and bridge data is conducted. Improved equations are obtained to predict the basic parameters of the compliance function for creep from the environmental conditions and concrete composition parameters, including the water-cement and aggregate-cement ratios, cement content and type, and admixture content. Comparisons with measured individual compliance curves are included as an essential check to validate the form of the compliance function

Comparison of main models for size effect on shear strength of reinforced and prestressed concrete beams

This paper presents a critical comparison of the existing code provisions for the shear strength of concrete beams. The comparison is based on the computerized filtering-out of the in-evitable statistical bias from the available multivariate database on shear strength, on an examination of the predicted size effects on shear strength and their underlying hypotheses and on the results of recent high-fidelity numerical simulations of shear failure. In addition to examining the existing models, the present comparison also provides several key considerations for testing the scientific soundness of any model of shear failure in concrete beams, which is necessary for future revisions to the design code provisions