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Concrete In Australia : December 2014
50 Concrete in Australia Vol 40 No 4 FEATURE: DURABILITY Durability and service life of important concrete infrastructure Odd E. Gjørv Norwegian University of Science and Technology, NTNU, Trondheim, Norway Upon completion of new concrete structures, the achieved construction quality typically shows a high scatter and variability, and in severe environments, any weaknesses and deficiencies will soon be revealed whatever durability specifications and materials have been applied. To a certain extent, a probability approach to the durability design can accommodate the high scatter and variability. However, a numerical approach alone is insufficient for ensuring the durability: greater control and improvements in durability also require the specification of performance-based durability requirements that can be verified and controlled during concrete construction in order to achieve quality assurance. The documentation of achieved construction quality and compliance with the specified durability is essential to any rational approach to controlling and increasing the durability. For a more controlled service life of the structure, the production of a service manual for future condition assessment and preventive maintenance is also an essential part of the durability design. In the present paper, current experience with probability-based durability design and performance-based concrete quality assurance is briefly outlined and discussed. 1.0 INTRODUCTION In recent years, deterioration of major concrete infrastructure has emerged as a most severe and demanding challenge facing the construction industry. Both public agencies and private owners are spending significant and rapidly increasing proportions of their limited construction budgets for repairs and maintenance of existing concrete infrastructure. Enhanced durability and service life of new concrete infrastructure is not only important from a cost point of view; it also directly affects the sustainability of our society. Although a number of different deteriorating processes may affect the durability and service life of concrete structures in severe environments, extensive experience demonstrates that it is not the disintegration of the concrete itself but rather chloride-induced corrosion of embedded steel which represents the greatest threat to the structures and challenge to the durability design. If the concrete is produced dense enough to resist an uncontrolled ingress of chlorides, current experience demonstrates that the concrete will also be good enough to resist most other potential deteriorating processes in a severe environment. In many countries, the increasing amount of de-icing salt has created a special challenge, but chloride-induced corrosion has shown to be an even more extensive and costly problem for concrete structures in the marine environment. After a comprehensive surveying of concrete structures in U.S. waters, the problem of corrosion of embedded steel was already pointed out by Wig and Ferguson in 1917 (1). In Norwegian waters, chloride-induced corrosion of embedded steel has been the major problem to the operation and safety of a large number of important concrete structures for a long period of time (2-5), and still this represents a challenge to the durability design and construction of such structures. In order to take the high variability of achieved construction quality into account, there has been a rapid international development in recent years on probability-based durability design (6-8). However, numerical solutions alone are insufficient for ensuring the durability of concrete structures in severe environments; it is essential also to specify some durability requirements which can be verified and controlled during concrete construction in order to achieve quality assurance. In 2004, new recommendations and guidelines for production of more durable concrete infrastructure in Norwegian harbours were introduced (9). In order to obtain the best possible potential durability of the given concrete structure for the given period of service in the given environment, a probability-based durability design is carried out. As a result, performance-based durability requirements are established, which provide the basis for concrete quality control and quality assurance during construction with documentation of achieved construction quality and compliance with the specified durability. Since the service life of the structure also very much depends on regular condition assessment and preventive maintenance during operation of the structure, a service manual for the future control of chloride ingress is also an essential part of the durability requirements. In the following, current experience with the above recommendations and guidelines for durability design and concrete quality assurance are briefly outlined and discussed. 2.0 DURABILITY DESIGN 2.1 General For several years, probability-based durability design has been applied to a number of new major concrete structures in many countries (10-13). In Norway, such design was initially based on the guidelines from the European research project 50-56 - Gjorv.indd 50 50-56 - Gjorv.indd 50 21/10/14 2:24 PM 21/10/14 2:24 PM