Avoiding Overfitting in Inverse Modeling of Chloride Migration in Concrete

Inverse analysis for reactive transport of chlorides through concrete in the presence of electric field is presented. The model is solved using MATLAB’s built-in solvers “pdepe.m” and “ode15s.m”. The results from the model are compared with experimental measurements from accelerated migration test and a function representing the lack of fit is formed. This function is optimised with respect to varying amount of key parameters defining the model. Levenberg-Marquardt trust-region optimisation approach is employed. The paper presents a method by which the degree of inter-dependency between parameters and sensitivity (significance) of each parameter towards model predictions can be studied on models with or without clearly defined governing equations. Eigen value analysis of the Hessian matrix was employed to investigate and avoid over-parametrisation in inverse analysis. We investigated simultaneous fitting of parameters for diffusivity, chloride binding as defined by Freundlich isotherm (thermodynamic) and binding rate (kinetic parameter). Fitting of more than 2 parameters, simultaneously, demonstrates a high degree of parameter inter-dependency. This finding is significant as mathematical models for representing chloride transport rely on several parameters for each mode of transport (i.e., diffusivity, binding, etc.), which combined may lead to unreliable simultaneous estimation of parameters.

Frequency and distribution of group I intron genotypes of Candida albicans colonising critically ill patients

A study is performed to examine the distribution and frequency of 25S rRNA intron genotypes of Candida albicans isolated from different anatomical sites of patients in an intensive care unit (ICU) setting. Germ-tube positive Candida isolates (n=65) from 65 patients are included and isolates are characterised by 25S intron genotyping, whereby all can be subdivided into four genotypes (A-D). Results demonstrated that there were no significant differences between the frequency and genotype distribution of the Candida isolates and the anatomical site of colonisation. Furthermore, analysis of the transposable intron region in the 25S rRNA gene demonstrated equal distribution, regardless of age and anatomical site of isolation (groin, throat, etc.). Therefore, there does not appear to be any selective pressure associated with any anatomical site, resulting in an ecological shift in the frequency of genotypes present. This suggests that C. albicans intron genotypes equally colonise those sites of the body examined in this study. Although such an ecological finding as this is interesting, it perpetuates the continued need to find a genotypic typing scheme that helps to identify the source (nosocomial or endogenous) and mode of entry of C. albicans into patients in the ICU setting, resulting in C. albicans bloodstream infection.