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Concrete In Australia : June 2008
maintenance) for principal structural elements of 100 years or more. The Authors understand that the Gateway Motorway Upgrade project in Queensland specifi es a minimum of 300 years resistance to carbonation induced corrosion. In 1992 the AustRoads Bridge Design Code was released, specifying a nominal design life of 100 years for bridge structures. This value was recognised as being somewhat arbitrary, being based on the value of 120 years in BS 5400, and on the historical perspective that bridges tended to be one of the more “permanent” structures used by our society. Even so, as shown in Table 3, the majority of Australian Standards relevant to reinforced concrete are still based on nominal 40-60 years or 50 years design life. As a result, the old favourite standby of “design in accordance with the code and she’ll be right” is no longer acceptable for many infrastructure commissions. For example, there are obvious discrepancies between applying provisions based on 40-60 year design life in the concrete code (AS 3600) and the piling code (AS 2159) to the 100 year design life of piled bridge foundations in accordance with the current bridge design code (AS 5100). Design life modelling In concert with the implementation of durability planning, there has been an increase in use of mathematical modelling of deterioration processes to validate the likely achievement of design life, and to provide numerical performance parameters for the generation of materials specifi cations. In the case of reinforced concrete, modelling may include any or all of: chloride ingress and chloride induced corrosion damage; carbonation ingress and carbonation induced corrosion damage; cracking due to restraint, thermal dimension change, and shrinkage effects; aggressiveness of surface or ground waters and soils, including potential and actual acid sulfate soils; moisture/water movement; and alkali aggregate reactivity. Specifi cation and compliance The basic standard for concrete supply in Australia is AS 1379. Projects involving construction in concrete will have some form of specifi cation written for supply, placement, fi nishing and curing of that concrete. The complexity and detail of the specifi cation will be dictated by the size of the project, and the specifi c requirements for the concrete. It should, however, be noted that premix concrete suppliers have their plant, delivery system, recording, and QA/QC systems set up in accordance with the requirements of AS 1379. In the opinion and experience of the authors, it is best to refer as much of the works specifi cation as is feasible to the provisions of AS 1379 in order to simplify compliance issues and to only enforce deviations where there is a specifi c need to do so. Of principal interest to this paper are the following provisions of AS 1379: • Specifi cation of “class” and “grade”: Clause 1.6 of AS 1379 describes specifi cation of “normal-class” and “special-class” concretes. Normal-class concretes are specifi ed by strength grade only, designated by the prefi x “N” followed by the characteristic strength in MPa selected from a Table of standard values, and must possess a range of attributes as specifi ed in the standard. Where any of the attributes Test methods for concrete Available methods for assessment of potential durability Where properties other than those routinely referred to in AS 1379 are specifi ed, the specifi er must nominate both the criterion to be applied in each case, and the test method to be used. It is essential that the chosen test method provides the necessary information, is suited to the type of concrete being considered and possesses the necessary repeatability and reproducibility to discriminate between concrete samples that are within or out of specifi cation. Table 4 lists a number of test methods in use in Australian concrete specifications for demonstration of potential durability related performance. It should be noted that Table 4 does not include tests such as the water absorption tests contained in AS 4058, AS 4198, or AS/NZS 4676, as these are tests specifi ed for particular precast concrete products and are not generally used outside the relevant manufacturing standard. Nor are matters such as shrinkage or AAR considered. Due to space limitations, not all of the test methods presented in Table 4 will be discussed further in this paper. Only those methods in relatively common use will be examined in detail. One issue that is obvious from perusal of Table 4 is that the majority of durability related testing methods are either “proprietary”, or where they are national standards, are foreign rather than Australian. Development of Australian standards, or adoption of either international standards or privately developed methods as Australian standards has been slow. Although this is not necessarily a problem per se, it should be borne in mind that the majority of concrete test samples in Australia are routinely manufactured as cylinders in accordance with AS 1012.8 (currently AS 1012.8.1-2000), which is a relatively old standard with many of the provisions for making cylinders essentially unchanged from at least as long ago as the 1973 version. Foreign standards often do not include sample Concrete in Australia Vol 34 No 2 51 described in the standard are varied from the values given, or where additional attributes are specifi ed, or where the characteristic strength varies from the stipulated values, the concrete has to be specifi ed as special-class, denoted by the prefi x “S” followed by the required characteristic strength. • Production assessment or project assessment: Clause 6 of AS 1379 describes two systems of quality control at the concrete batch plant. In brief, production assessment is used for routine concrete supply, where a given grade of concrete is supplied over an extended period, possibly to several projects, and even from more than one batch plant. Where a higher level of scrutiny is desirable, and in particular for special-class concretes, the concrete supplied to a particular project is treated uniquely in a process known as “project assessment”. Where durability related parameters such as a maximum chloride ingress coeffi cient, water permeability, or water absorption are required, the resulting concrete will always have to be specifi ed as special-class. If project assessment is required, it must be nominated in the concrete supply specifi cation for the works.