Influence of alternative binder systems on the corrosion behavior of the steel reinforcement - Part 2

• Applicants: Professor Dr.-Ing. Horst-Michael Ludwig; Professor Dr.-Ing. Michael Raupach
• Subject Area: Construction Material Sciences, Chemistry, Building Physics
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 416337623

Due to the climate debate, alternative sustainable binders are being developed. These alternative binders are expected to meet the same durability requirements as Portland cement. However, only a few investigations have been carried out so far to systematically evaluate the corrosion protection for steel reinforcement. The aim of the first project phase was to be able to determine the time-dependent passivity (without external exposure under natural conditions), the corrosion initiation processes, as well as corrosion processes and rate-determining substeps in systems with alternative binders. This was done by means of extensive investigations on the system of reinforcing steel - alternative binders with regard to passivity at a young age, corrosion initiation processes, and the corrosion process itself. In order to gain an understanding of these processes, the hydration of the alternative binders was also recorded in a time-dependent and detailed manner. Relevant results of the first project phase are:1. Hydration behavior of the binders (phase, microstructure and strength formation, composition of the pore solution), are in agreement with the literature2. No clear correlation between carbonation and pore structure or Ca(OH)2 content can be determined 3. Common test methods for determining the chloride migration coefficient are applicable to all binders investigated here4. Depending on the electrochemical potential of embedded steel after 28 days and the pH values, passivation of the steel is expected for each binder system.In the proposed second phase of the project, the focus will be on up-scaling to concretes under real-world exposures. The main differences between the previously investigated model systems and the use of the binders in reinforced concrete components are, on the one hand, the use of coarse aggregate and, on the other hand, the different crack formation. The coarser aggregate leads to distinct interfaces between aggregate and binder matrix (interfacial transition zone; ITZ), which have an influence on transport processes and the microstructural properties. Exposure, e.g. in frost or under mechanical stress, leads to microcracks, which also have a decisive influence on transport processes and corrosion-relevant properties. The quality of the steel-concrete interface (SCI) is important for the initiation of corrosion, as well as the transport processes directly at the steel surface and thus also for the corrosion rate. This results in the following aims of this project phase:1. Quantitative description of the migration of chloride and carbonation on undisturbed concrete specimens2. Description of the influence of ITZ and SCI on transport processes and corrosion3. Description of the influence of microcracks on the electrolyte resistance, as well as carbonation and chloride migration4. Application possibilities and limits of alternative binders in reinforced concrete components under critical exposures

• DFG Programme: Research Grants