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CSLA - Concrete Service Life Assessment - Modelling frost attack degradation in the presence of chlorides 2012 - 2015

CSLA - Concrete Service Life Assessment - Modelling frost attack degradation in the presence of chlorides

The durability design of reinforced concrete structures (RCS) lags behind structural design especially in terms of design methodology. From a normative perspective, the traditional approach to durability design has been based on deem-to-satisfy rules, while structural design has adopted a probabilistic approach based on the definition of ultimate and serviceability limit states with verification based on the use of reliability indexes. This traditional approach to durability design, despite being based on experience and expert opinion, fails to find adequate solutions for concrete performance of all type of structure whatever the exposure conditions are. In addition, no actual quantifiable knowledge is obtained concerning the performance of the chosen concrete and, therefore, also no exact knowledge is obtained on the actual service life of the structure.

A decade ago the international scientific community started to address the problems associated with the deem-to-satisfy approach by directing its attention to the performance based approach of durability design. Several European Projects spearheaded this research, some of which VTT was closely involved in. As a result of this move towards performance based durability design, the construction industry gradually changed the manner in which RCS where designed including, as an intrinsic part of the project, the durability design of concrete and the service life design of RCS. Within a few years, many countries included new codes for durability design and assessment of RCS using such performance based approaches. As an example, in recent years, the BY50 Finnish concrete code included a procedure for service life assessment of RCS.

Since then research into the durability of concrete has developed significantly and knowledge concerning certain degradation mechanisms has reach quite a high standard. Recently, the international research community has recognized that the use of a performance based approach in its current format considers individually the effect of a degradation mechanism. While often this is considered to be acceptable due to a dominant form of degradation, in some cases coupled deterioration cannot be over looked. In reality, RCS are subject to a multitude of degradation mechanism acting either simultaneously or cyclically, and most often with synergetic effect on the degradation of concrete. A sign of the importance of this topic is the number of international technical bodies that are endorsing research in this field. An example is the recently created RILEM Technical Committee – TDC created in 2011, dedicated to studying new test methods to determine durability of concrete under combined environmental actions and mechanical loads.

In Finland, RCS have to perform in rather difficult conditions due to the extremely harsh winters. This results in unique combinations of degradations mechanism. Traditionally frost attack and carbonation have been considered the predominant degradation mechanism, with the influence of chlorides normally being relegated to secondary importance. However, in light of recent research at VTT (DURAINT project 2008-2011) and internationally, more attention is now being drawn to other degradations mechanism such as chloride penetration and alkali-aggregate reactions and the relationship of these to frost attack.

As a consequence, this research project was set up to investigate the influence of frost attack of concrete on the transport characteristics of chloride ion in concrete. The results of this project will give Finnish industry good opportunity to address this issue systematically and to incorporate the findings into Finnish practice.