Performance and durability of concrete under effect of multi-deterioration mechanisms

As a result of the environmental loading concrete gradually deteriorates with time. Very rarely is concrete subject to a single deterioration mechanism, yet, performance-based durability design is typically based on modelling the effect of such single deterioration mechanisms. Due to the possible synergetic effect of two or more deterioration mechanisms acting simultaneously or intermittently, this deterioration rate might differ remarkably from the predicting rate. In Finland, concrete has to perform in rather difficult environmental conditions due to the extremely harsh winters. This results in unique combinations of deterioration mechanisms. The most common deterioration mechanisms affecting concrete are freeze-thaw and reinforcement corrosion due to both concrete carbonation and the ingress of chlorides (typically due to use of de-icing salts). This paper provides an overview on the effect of combined deterioration mechanisms on concrete durability performance, based on research performed at VTT Technical Research Centre of Finland Ltd over the last few years. The research project built on 30+ years of concrete durability research, including ca. 15 years of field studies. The durability performance was assesses with both accelerated laboratory testing and in-situ exposure results. The scope of the research has been broad, i.e., for example evaluating how cracks resulting from freeze-thaw deterioration influence chloride ingress; how freeze-thaw cycles without cracking assist chloride transport in the concrete; or how carbonation changes the surface properties affecting freeze-thaw scaling. Results are based on both accelerated laboratory testing and in-situ exposure testing at the three Finnish testing sites, and also at the parallel Swedish testing site (from Baltic coast to Arctic exposure sites). These results presented here give some examples of the wide range of laboratory and field investigations that have been a part of the Finnish studies on the effect of multi-deterioration mechanisms. The research has shown that single attacks tested in laboratory conditions yield different results than what may actually be experienced in field applications or in-situ concrete structures, where multiple deterioration types are occurring simultaneously. Field testing will go on to obtain long term results for calibration and verification of durability and service life models. Another goal is to continue also with more comprehensive studies on the multi-deterioration mechanisms. Research into modelling multi-deterioration mechanisms has also led to the development of service life design/assessment tools, with the objective to assist engineers in more realistically estimating long-term concrete infrastructure performance. The research results show that a holistic approach to concrete durability performance should be taken into account when estimating the service life of concrete structures.