Band 6, Heft 3

Cement based layers are frequently used during the repair of reinforced concrete structures nowadays. They restore the protection of the reinforcement against the penetration of aggressive substances. Protective coatings can also be applied to new structures in order to improve their durability. This paper discusses a concept for the improvement of durability of existing and new reinforced concrete structures using protective layers. To offer protection and to prevent the transport of harmful substances inside the reinforced concrete structure these layers have to be free from cracks during the planned service life. Under usual conditions such layers are submitted to different phenomena like thermal, hygral and autogenous shrinkage and swelling. These processes create a complex state of eigenstresses inside the system layer-subbase. The tensile stresses can lead to the formation of cracks and delamination of the protective layer after a few years. This paper suggests an approach for correct design of systems of protective as repair layers. For this purpose a numerical model is used in order to determine the time dependent stress distribution and the crack formation due to different loads and boundary conditions. On this basis the requirements for a layer can be formulated ad hoc as a function of the probabilistic properties of each damaged building and the risk of failure can be minimized.

The paper deals with the corrosion resistance of different stainless steels in chloride contaminated and carbonated concrete. Stainless steel reinforcement has a higher corrosion resistance as compared to the normal carbon steel reinforcement; stainless steels can, however, be subject to localized corrosion if the chloride content in the concrete exceeds a certain critical value. This critical value depends on the pH value of the concrete (i.e. carbonated or alkaline concrete), the temperature, the corrosion potential (function of environmental conditions), and the composition and microstructure of the stainless steel. Furthermore, in the rehabilitation of corroding reinforced concrete structures, stainless steel is often used in structures reinforced with normal carbon steel and galvanic coupling can occur. The results of measurements of free corrosion potential, corrosion rate and macrocouple current in reinforced concrete specimens are reported as a function of chloride concentration and humidity. The consequence of coupling with carbon steel reinforcement is also considered.

Profound knowledge of the moisture transport in porous building materials is essential to understand and to improve their durability. Water flow in these materials is mainly driven by capillary forces and the capillary suction of a porous building material depends on its pore structure. We have designed a statistical model to simulate the capillary water absorption of porous building materials based on invasion percolation rules. A 3D-network with 30x30x100 sites was used and the trapping effect is included. The input for the simulated invasion percolation is a pore size distribution obtained by a stereological analysis of the results of automated image analysis measurements on SEM-BSE-images of polished sections. Pore size distributions of nine different materials have been determined experimentally by SEM and by MIP. The simulated absorption curves agree reasonably well with the experimental absorption curves.

The degradation of monumental stones is the result of the interaction between the material and environmental factors, such as mechanical loads, moisture, freezing and thawing, heat, atmospheric pollutants and living organisms. This interaction starts at the stone surface and progresses inwards reaching several millimetres in depth. The common decay or degradation processes result in significant loss of the strength properties of the outermost layers of stone material and quite often these exhibit erosion features, a cohesionless appearance and typical disintegration forms. At present there is no method which can measure with sensitivity, reliability and quasi-non-destructively the cohesive properties of stones both at the exposed surface and at larger depths (few centimetres), which can be used both in laboratory and in situ. The objective of this work is to verify the capability of a phenomenological model to characterize the cohesional properties of stones and the influence of wear on drill-bit performance based on appropriate microdrilling measurements performed by using a new portable Drilling Force Measurement System (DFMS).