An experiential petrographic classification scheme for the condition assessment of concrete in façade panels and balconies

In Finland during the past decade, petrographic examination of concrete has become an important part of concrete structure surveys. In particular, the unexpectedly high deterioration rate in the prefabricated external wall panels and balconies of buildings has heightened the need to utilize more elaborate examination methods than simple strength tests. Deficiencies in frost resistance, corrosion of reinforcement and filling of air voids are the main factors causing concern. Alkali reactivity of aggregate is an uncommon phenomenon in Finnish mixes. Thin-section analysis is the method found to yield the most comprehensive data for condition assessment purposes. However, the lack of a sufficiently consistent procedure to describe and categorize the observed microstructural phenomena has occasionally hampered the usefulness of the results. A petrographic classification scheme based on the experiences gained from more than 1200 thin-section analyses and 160 building renovation projects is presented. Emphasis is put on the properties and damages related to frost weathering. A division into four classes for each parameter is considered practical enough to improve mutual understanding between condition survey engineers, petrographers and repair design engineers.