Overview
In contrast to reinforcement corrosion due to carbonation of the concrete no consensus about the long-term and most economical measures for chloride-induced corrosion is available (new building or repair). The application of stainless steel reinforcement is one possibility to improve the durability of reinforced concrete structures. However, in many cases the limitations of use of such steels are not clarified yet and standardised test procedures are needed to determine and compare the corrosion behaviour of different steel qualities in chloride contaminated concrete rapidly, economically and reliably. The standard SIA 262/1 "Betonbau – Ergänzende Festlegungen" contains no requirements to the corrosion resistance of reinforcing steel. This gap is to be filled with a SIA guideline for the application of stainless steels in concrete structures. The guideline shall not only contain corrosion-technical notes for the use of stainless steel, but also illustrate possible characteristics regarding design and design-engineering. Additionally the guideline shall contain also a validated corrosion test procedure including requirements to test the manufacturer products at their own expense. Such a test procedure does not exist until today and therefore has to be developed by laboratory investigations.
Goal of the project is the development of a standardised corrosion test procedure to investigate the different stainless reinforcing steel qualities regarding certain requirements and specify the limitations of use at expense of the manufacturer. This test procedure shall be part of the new guideline SIA 2029 "supplementary specifications for stainless reinforcing steels". A further goal of the project is the clarification of fundamental requirements of stainless steel reinforcement e.g. mechanical characteristics (strength, ductility, fatigue, behaviour in case of fire), weldability, application in mixed reinforcements, processing/handling on site, registration etc..
The corrosion-technical investigations are conceptualised and carried out in the laboratories of the SGK. The laboratory has modern equipment for electrochemical and corrosion-technical studies. The TFB has an accredited laboratory for various tests on concrete, the example can be used for the production of test specimens
Funding
Results
The results of the present investigations show, that stainless steel reinforcements extend the initiation phase for uncracked as well as for cracked concrete. However, this prolongation does, for given exposure conditions, not only depend on the steel quality but is also highly influenced by the surrounding concrete. Thus, it is a distinctive system behavior of material properties, exposure conditions and geometrical conditions. Hence additional factors like concrete composition, concrete quality, exposure conditions of a structure, including the influence of humidity and pollutants, as well as structural conditions must be considered for the selection of the appropriate stainless steel reinforcement. The electrochemical solution tests with different chloride concentrations and pH-values
allowed the determination of pitting potential and, thus, a differentiation of the different steel qualities. The solution test showed that molybdenum-free stainless steels are more
sensitive to pH-drops. The main advantage of solution tests is the simple and quick test procedure. However, the test results are, compared to test setups in concrete, too optimistic due to a virtually ideal, crevice free setup, which means that the corrosion resistance tends to be overestimated.
The suction test with concrete blocks and integrated chloride sensors allowed the determination of the chloride threshold value for non carbonated and carbonated concrete.
However, the tests and their evaluations are very time-consuming. Furthermore, in concrete with CEM III/B or carbonated concrete the chloride sensors are of limited suitability because their potential is strongly influenced by sulfides and/or pH-drops. The chloride threshold value is not only influenced by the amount of alloying elements but also by the type of cement. It decreases by lowering the clinker content (CEM I → CEM II/A-LL → CEM III/B). The lower corrosion protection effectiveness of a concrete with CEM III/B is partly compensated by its lower chloride diffusion coefficient and, if corrosion is activated, by a high electrical resistivity and a slower corrosion rate, respectively. However, carbonation of concrete with CEM III/B causes a steep rise of the chloride diffusion coefficient and a distinctive reduction of the chloride threshold value, which is caused by the pHreduction within the hardened cement paste.
In cracked concrete the corrosion risk increases due to the faster ingress of pollutants, esp
Technical Implications
Although the application of stainless steel reinforcement aims for a preferably corrosion free design working life, in some situations corrosion attacks can occur. The mass loss of corroding stainless steel rebars in concrete is lower than for unalloyed reinforcing steel. However the corrosion depth (loss in cross section) over time is comparable because the attack is more localized. Due to the lower volume of rust the corrosion on stainless steel rebars might be detected by visual inspections at a later time compared to normal reinforcing steel. Although large efforts for the investigation of the chloride threshold value were undertaken worldwide optimal or generally accepted test methods do not exist, yet. The suction tests used in this report is not ideal too, mainly due to the unsuitability and instability of the chloride sensors (new cement types, a long test period) and the long duration of the tests. Further investigations are necessary. Thereby the test procedure given by new RILEM recommendations is only partly appropriate.