Corrosion-induced Cracking in Reinforced Concrete Structures -a numerical study

Buildings and infrastructure constructions represent a major part of the investments made today and thus, the owners make severe demands on the economy when building new structures. Durability and service life of structures are two key parameters strongly related to the economy. Increased durability and longer service life decreases the total amount of materials needed when considering construction and maintenance of structures decreasing the total costs. Further optimisation of material usages is beneficial when considering sustainability as extraction of raw materials, production of structural members, repair and disposal constitute a large part of the total amount of carbon dioxide emission during the life cycle of a structure. One of the most decisive factors controlling the durability and service life of a structure - designed and constructed without flaws and deficiencies - is deterioration of the structure due to environmental effects. Reinforced concrete structures comprise a great part of the structures taking form today and in structures, such as bridges, tunnels, parking garages etc. Corrosion of the reinforcing steel is the most significant deterioration mechanism in reinforced concrete structures. Reinforcement corrosion is therefore a major concern during design and service life.
The aim of the present work was to further develop the knowledge within the field of service life of reinforced concrete structures. To be exact further development of service life modelling based on reinforcement corrosion.
First an existing finite element model simulating corrosion-induced cracking was taken a step further. The existing deterministic model, which was developed at DTU, is divided into five distinct domains; concrete, reinforcement, a corrosion layer, cracking, and debonding domain (crack opening and sliding at reinforcement surface). Applying a fictitious thermal load to the elements in the corrosion layer expansion of the corrosion products was simulated applying a discrete crack modelling approach. Due to expansion of the corrosion products the stresses in the reinforcement/concrete interface increases reaching at some point the tensile strength of the concrete initiating cracking. Penetration of corrosion products into the surrounding concrete was included in the modelling postponing the initiation of the crack following experimental observations of the corrosion process. With further expansion of the corrosion products the crack continues to develop in the concrete cover layer towards the concrete surface. As several studies have shown that corrosion products precipitate non-uniformly along the circumference of the reinforcement the model was further developed changing the precipitation of corrosion products along the circumference of reinforcement from uniform to non-uniform varying the corrosion current density along the circumference of the reinforcement.
Secondly a new corrosion-induced damage model was developed. Applying a smeared crack modelling approach the new model is cable of modelling the initiation and propagation of multiple micro- and macrocracks. The model is based on finite element theory and is divided into three domains: a concrete domain surrounding a corroding and a non-corroding steel domain. To simulate the expansive nature of solid corrosion products a thermal expansion coefficient was applied to the corroding steel elements. Similar to the discrete crack model cracking was initiated when stresses in the reinforcement/concrete interface exceeded the tensile strength of the surrounding concrete. Besides non-uniform precipitation of corrosion products along the circumference of the reinforcement the modelling also included effects such as penetration of corrosion products into the surrounding concrete and creep.
Parallel to the modelling the simulated results were compared to experimental data. The applied experimental data was results of experiments produced at DTU and observations described in the literature. The comparison showed good estimates of both deformations in the reinforcement/concrete interface and crack width. Comparing with experimental data the discrete and the smeared crack modelling approach was also compared indicating that a smeared crack modelling approach show more realistic cracking pattern however this modelling approach is strongly depend on the number of steps during simulations.
Finally both model approaches was used to numerically study the influence of different mechanisms and geometrical parameters on corrosion-induced cracking. Among others the influence of non-uniform precipitation of corrosion products was investigated. The investigation showed that the crack pattern and the development of a surface crack width strongly depend on the formation of the precipitated corrosion products and that the degree of reinforcement corrosion varies at time-to surface crack initiation.