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Concrete In Australia : December 2014
36 Concrete in Australia Vol 40 No 4 FEATURE: DURABILITY and unrelated to the specific durability requirements of the structure. For cracks caused by the applied loads and resulting from bending and direct tension, AS3600-2009 requires that the stress in the reinforcement crossing the crack due to service loads does not exceed a specified maximum value. The maximum value depends only on the bar diameter or the bar spacing. In addition, certain detailing requirements are imposed involving maximum limits on concrete cover and bar spacing. These requirements are independent of the exposure classification, the type and importance of the structure, the mix proportions, the concrete material properties (including the shrinkage strain), the heat of hydration, etc. – all of which affect crack widths and, therefore, should be considered in the design for crack control. With regard to restrained shrinkage and temperature cracking, minimum quantities of reinforcements are specified depending on whether a “strong”, “moderate” or “minor” degree of crack control is required. Again, many of the factors that affect crack widths are not considered, including the level of shrinkage or the exposure classification. 8.0 Z7/07 DURABILITY – TESTING Warren Green was the chairman of Z7/07 but authorship of the document was widespread, not surprisingly given the broad nature of the testing methods. Z7/07 provides guidance on performance tests for durability design and implementation. Often several test methods supply similar information. The limitations and advantages of test methods are reviewed, and recommendations provided on which test is the most suitable for project specifications. Test methods are available to assess various aspects of durability performance through a concrete structure’s life cycle including: • Mix Acceptance Tests (including tests to validate values used in modelling). • Tests For Quality Assurance. • Tests Where Placed Concrete is Suspect. • Tests For Condition Monitoring. A wide range of tests designed to demonstrate the potential durability performance of concrete have been introduced over the years. This has caused some uncertainty for: • Asset Owners: To understand what methods are available, the appropriateness of those methods to the structures’ exposure, environment and life cycle, and the most cost effective testing regimes to achieve the required outcomes and level of certainty that they are looking to achieve. • Designers: To know which tests are the most appropriate to specify and how much test data is required to ensure that the level of statistical confidence from the test results underpinning the design is appropriate. • Contractors and Material Suppliers: To understand and have confidence in the consistency, repeatability and validity of trial data and quality control performance testing they are required to undertake for compliance with the project specification. • Suppliers of Laboratory Testing Services: To maintain and calibrate equipment, train staff, maintain third party accreditation for the tests (e.g. perform the tests to sufficient frequency, provide regular proficiency training of staff and keep detailed records) and competitively price test methods despite some being not often specified. Z7/07 aims to reduce the confusion and uncertainty. It provides guidance on performance tests for durability design and implementation. Often several test methods supply similar information. The limitations and advantages of these methods are reviewed, and recommendations provided on which test is the most suitable for project specifications. Design phase durability testing requirements are recommended to be clearly specified for four stages: • Mix trials to confirm the mix is suitable. • Quality assurance tests as construction proceeds. • Tests at the end of the defects liability period to create a list of items for repair. • Tests during the design and service life including monitoring. Construction phase materials testing and selection requirements recommended are: • Materials testing and selection must be completed in accordance with the project specifications prior to use in the works. Additional testing is required prior to a change in supply of materials or a new source of materials. • Verification of concrete mix designs to meet project specification durability requirements can take considerable time, and unscheduled changes in concrete supply during construction may result in program delays. Durability testing of concrete such as chloride diffusion, water permeability, drying shrinkage, etc. may have a long test period (e.g . up to 3 months). • Variability of durability tests must be taken into account by the durability consultant, with specification test criteria allowing for alternative solutions to achieve the required durability if the test results do not achieve the specified values. This can be achieved by conservative durability design and/or provision for use of additional measures such as protective coatings or special additives or other measures. Operations and maintenance phase monitoring and testing recommended are: • Practical Completion Inspection – Prior to a structure going into service it’s important to determine if any defects need to be contractor repaired and to document the initial structure characteristics and condition for future reference and comparison. • Periodic In-Service Visual Inspection – A reactive approach to on going maintenance be limited to visual inspections only and these may be performed on a regular basis or ad- hoc. This may be adequate provided no major defects are found and may be sufficient to prevent minor defects from becoming major ones. If appropriate, follow up repairs are performed as required. This approach may be suitable for minor structures and/or structures with a short design life. • In-Service Condition Monitoring and Testing – Proactive maintenance will involve early intervention to prevent or delay the onset of corrosion initiation. This will require regular inspections in conjunction with additional activities such as structural monitoring and non-destructive testing, as required. 31-37 - Papworth.indd 36 31-37 - Papworth.indd 36 21/10/14 12:29 PM 21/10/14 12:29 PM