Stefan Ullmann

In a classic concrete repair, the damaged concrete is removed and re-profiled with a repair mortar or concrete. After such a repair, a structural element is usually considered to be as good as new. A detailed statement on the remaining service life of a repaired reinforced concrete component was not possible until recently. However, this is of great interest to the owner or user of a reinforced concrete structure from an economic point of view.

As a result of two research projects at the Technical University of Munich, it has now been possible to transfer the approach to probabilistic lifetime design according to Gehlen into a semi-probabilistic, and thus strongly practice-oriented approach. It is now possible to calculate the remaining service life of a reinforced concrete component with regard to carbonation-induced and chloride-induced corrosion, depending, among other things, on the weather conditions, the applied mortar layer thicknesses and the material-specific resistance to chloride penetration and carbonation.

The basis of the calculation approaches is Fick's 2nd law, which describes the diffusion mechanisms and allows a material-specific estimation of the penetration behaviour of liquids and gases into the concrete on a chemical level. The ageing of a component and its material resistance to chemical attack is taken into account in the case of carbonation using the root-time approach and in the case of chloride penetration by adjusting the chloride diffusion coefficient using an experimentally determined age exponent.

The influence of mechanics on the description of aging processes in a repair layer has not been integrated in the current models for lifetime design so far. The need for research in this area becomes clear when considering the controllability of stress distributions between repair material and old concrete via the variation of the Young's modulus in the repair layer. A soft system avoids the load, while a stiffer system relieves the old concrete and takes over the load transfer itself. As a result of deformations, micro-cracking of the hardened cement paste matrix occurs long before the formation of cracks visible to the eye. This is accompanied by a change in the material resistances relevant to durability.

The aim of the work is therefore to determine ideal combinations of mechanical parameters between old concrete and repair material, initially on the basis of laboratory tests and depending on numerous variables (e.g. class of old concrete, geometry of concrete repair, load scenario, modulus of elasticity, strength) in order to achieve the longest possible service life. The stress distribution in the cross section of the component and also on the composite joint is to be considered. In the following, these considerations will be linked to investigations on the influence of mechanical ageing on the chloride diffusion coefficient.