Forensic engineering of structures with durability problems

Forensic assessment of structural durability can be efficiently based on the probabilistic approach provided in the newly developing international standard ISO 'General principles on the design of structures for durability'. Although this document provides a significant basis of durability assessment, its operational use in forensic practice requires additional information on the theoretical models of basic variables and durability criteria. It appears that the durability criteria essential for the forensic structural assessment should be differentiated taking into account the character of the considered limit state, consequences of its infringement, costs of structural improvements, time of the expected intervention and the discount rate. It is shown that the methods of probabilistic optimisation provide valuable data that are useful for specifying the target reliability levels and adequate durability criteria. An example of a reinforced concrete member exposed to carbonation illustrates the general principles. It is shown that the predicted service life depends significantly on the specified reliability level. The optimum reliability level and a corresponding concrete cover, assuring the minimum total cost, increases with increasing costs owing to the durability failure, and with decreasing intervention time and discount rate.