Z7-02 Durability Exposure Classification

Failure to correctly specify exposure is one of the most common durability design faults. It typically has three causes:

1. Confusion – about what exposure class to specify, due to the various Australian Standards dealing with concrete structures classifying the same exposure in different ways. Confusion also arises where inadequate explanation is given in Australian Standards on where an exposure class applies.
2. Unpredicted exposures – Australian Standard exposure classes may seem to relate to a location on a drawing when considered in isolation. However, in a broader analysis of the structure much more severe macro and micro climates may be identified.
3. Limited range of exposure classes – designers may force an exposure class from the limited number available in Australian Standards to fit an exposure that is more complex and severe in practice.

Confusion sometimes occurs in the design of elements in acid ground conditions where AS 2159), AS 3600 and AS 3735 all have different classifications for concrete in ground with the same pH.  A classic case of unpredicted yet predictable severe exposure in Europe was the design of car parks as internal dry environments, whereas cars bringing in de-icing salts and water created a severe exposure that the car park design could not cope with. Significant premature reinforcement corrosion damage has occurred and cathodic protection has been applied to many such structures.

An example of limited range of exposure classes relates to elements immersed in seawater. The AS 3600 (2) exposure classification described as “permanently submerged” may be appropriate only where this condition applies to all the surfaces of an element. There is no exposure class in Australian Standards for elements where a wall is immersed on one side and has air on the other, e.g. brine tanks.

In Z7-02, exposure classes from the various Australian Standards dealing with concrete durability have been considered and combined into one set of exposure classes, applicable to any and all structures. This should help reduce confusion.

Z7-02 introduces exposure classes not included in Australian Standards which are commonly dealt with by durability consultants. It is anticipated that the wider range of exposure classes will assist designers in correctly identifying the appropriate exposure.

The increased range of exposure classes in Z7-02 will help designers identify mechanisms that are not immediately apparent, and reduce cases similar to the European car parks. For most structures a detailed exposure assessment is required, and this can often only be obtained from a site visit, previous experience of the structure and exposure types, and exposure tests.

Many of the exposure designations used in Z7-02 will be new to most Australians. However they are essentially an amalgam of all the exposure classes from all Australian structures codes, split into different exposure types.

The three principal reasons for not using the exposure class terminology in Australian Standards throughout were:

1. Z7-02 use of exposure class specific to each type of exposure will permit better targeting of materials suited to each exposure.
2. Lack of consistency between exposure classes within Australian Standards.
3. Australian Standard exposure classes are more limited than Eurocode 2, that defines more macro and micro climates, which are generally included in this Z7-02.