Development of service life model CHLODIF++

Abstract

The concept of reinforced concrete durability started to develop only 25 years ago, when it became obvious that reinforced concrete could have serious durability problems, especially when exposed to the actions of aggressive environment. In marine environment chlorides are always the most decisive environmental action for the corrosion initiation. Most engineering models for the calculation of service life are based on the mechanism of chloride ingress into concrete due to diffusion process. Fifteen years ago started a development of the numerical model CHLODIF for the calculation of service life of reinforced concrete structures exposed to chloride action, based on Fick's second law and deterministic approach. Following experience collected during 15 years and numerous experimental researches are recently incorporated into the new version of the model CHLODIF++, taking into account comparative analysis of experimental (laboratory and field) results and computational analysis. Numerical model computes the required initiation period through the analysis of chloride ions diffusion in a semi-infinite medium. The influences of different surface chloride concentration and concrete properties on the service life are obtained. The existing model is expanded to three levels of design, from deterministic (deem-to satisfy rules), through performance based, to a probabilistic analysis, taking into account the scatter of materials and structural properties. Using the well-known analytical solutions of the chloride ion diffusion problem in a semi-infinite medium, and a computer experiment, an expression which best fits continuous diffusion process is created. Time dependent diffusion coefficient and initial concentration of chloride ions are considered. The factors influencing the diffusion coefficient are analyzed in detail, especially those which contribute to the binding capacity of chloride ions (C3A, fly ash, sulfates...), those which improve density of concrete (slag, silica fume, superpalsticiers...), as well as temperature and humidity changes and cover cracking contributions during the initiation time of corrosion. The increase of the initial chloride concentration is defined on the basis of the wind influence (airborne chlorides) and capillary pore volume and the quantity of salt crystals that can fill the pores in different environmental conditions. Calibration of the model was performed by the comparison of measured profiles on the existing bridges at the Adriatic coast (Krk bridge, Maslenica bridge and Dubrovnik bridge) and predicted profiles by the model. In the model are formed several databases with currently available data regarding materials properties and environmental conditions. The aim is to improve permanently these databases and to improve the existing numerical model to become fully operational for the structural and materials' engineers.