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Concrete In Australia : December 2014
Concrete in Australia Vol 40 No 4 55 To show compliance with such a durability requirement, a new durability analysis is carried out based on the achieved average values and standard deviations of both the 28-day chloride diffusivity and the concrete cover from the quality control as new input parameters. For this durability analysis, all the other previously assumed input parameters from the original durability analyses, which may have been somewhat difficult to select, are kept the same. Hence, this documentation primarily reflects the results obtained from the regular quality control during concrete construction, including the scatter and variability involved. 4.3 In situ quality The achieved chloride diffusivity on the construction site during the construction period is primarily based on the testing of a number of concrete cores removed from the given structure under construction. In order not to weaken the structure too much by coring, however, one or more representative dummy elements are also normally produced on the construction site, from which a number of additional cores are removed and tested during the construction period. Based on the achieved chloride diffusivity on the construction site after one year combined with the site data on cover thicknesses as new input parameters, a new durability analysis is carried out. Also here, all the other previously assumed input parameters from the original durability analyses are held constant. Hence, this analysis provides documentation of the achieved in situ quality during the construction period. 4.4 Potential quality For most binder systems, the development of chloride diffusivity tends to plateau after about one year of water curing at 20 °C in the laboratory. Therefore, to estimate the potential construction quality and durability of the structure, the chloride diffusivity is also determined on separately produced, water cured specimens in the laboratory for a period of up to one year. This chloride diffusivity in combination with the achieved site data on concrete cover as new input parameters provides the basis for documenting the potential construction quality of the structure. Also here, all the other originally assumed input parameters to the durability analysis are held constant. 5.0 CONDITION ASSESSMENT AND PREVENTIVE MAINTENANCE Even if the strongest requirements to both concrete quality and concrete cover have been specified and achieved during concrete construction, extensive experience demonstrates that for all concrete structures in chloride containing environments, a certain rate of chloride ingress will always take place during operation of the structures. Upon completion of the given structure, therefore, the owner must also be provided with a service manual for regular control of the real chloride ingress taking place. It is such a service manual which helps provide the ultimate basis for obtaining a more controlled and increased service life of the given concrete structure. For each new condition assessment of the structure, new estimates for the probability of corrosion are developed using input parameters based on data from the observed chloride ingress (5). Before this probability of corrosion becomes too high, appropriate protective measures should be implemented. 6.0 CONCLUDING REMARKS The durability of concrete structures in severe environments is related not only to design and materials but also to construction. Extensive field experience demonstrates that much of the durability problems that develop after some time can be attributed to an absence of proper quality assurance and special problems during concrete construction. Therefore, construction quality and variability must be firmly grasped before a more controlled durability can be reached. For the service life of the structures, regular condition assessment and preventive maintenance are also very important. For a number of commercial projects where the above procedures for probability-based durability design and performance-based concrete quality assurance have been applied in recent years, the specified durability has been achieved with a proper margin. As a result of the performance-based concrete quality assurance, it has been possible to reveal and correct any unacceptable deviations during concrete construction. The required documentation of achieved construction quality and compliance with specified durability has also clarified the responsibility of the contractors for the quality of the construction process. As a result, reduced scatter and variability of achieved construction quality have been observed. For the owners of the structures it has been very important to receive documentation of achieved construction quality and compliance with the specified durability before the structures have been formally handed over from the contractors. Upon completion of the structures, it has also been very important for the owners to receive a service manual for regular condition assessment and preventive maintenance of the structures. It is such a service manual which helps provide the ultimate basis for achieving a more controlled service life of the given concrete structure in the given environment. 7.0 REFERENCES 1. Wig, R.J . and Furguson, L.R. “ What is the Trouble with Concrete in Sea Water?” Engineering News Record, V. 79, 532, 641, 689, 737 and 794, 1917. 2. PANE, “Concrete in Seawater“, Publication No. 3, The Concrete Committee, The Professional Association for Norwegian Engineers, Oslo, 1936. (In Norwegian) 3. Gjørv, O.E., “Durability of Reinforced Concrete Wharves in Norwegian Harbours“, Ingeniørforlaget AS, Oslo, 1968. 4. Gjørv, O.E., “Steel Corrosion in Concrete Structures Exposed to Norwegian Marine Environment”, Concrete International, V. 16, No. 4, 1994, pp. 35 -39. 5. Gjørv, O.E., “Durability Design of Concrete Structures in Severe Environments“, Second Edition, Taylor & Francis, CRC Press, London and New York, 2014. 6. Siemes, T., Vrouwenvelder, T. and Beukel, A. Van den, “Durability of Buildings: A Reliability Analysis”, HERON, V. 30, No. 3, 1985, pp. 2-48. 50-56 - Gjorv.indd 55 50-56 - Gjorv.indd 55 21/10/14 2:24 PM 21/10/14 2:24 PM