Environmental exposure classifications for concrete construction A relook

摘要

Durability of concrete structures is dependent on the design mix used (I.e. ratio and quality of ingredients namely water, cement, fine and coarse aggregates, additives etc.), the concreting practices adopted (I.e. transportation, placing, compaction, curing, formwork used, cover adopted etc.) and on the exposure conditions to which it is subjected to during its life. The Indian Code IS 456:2000 for plain and reinforced cement concrete specifies five exposure classifications namely, mild, moderate, severe, very severe and extreme, which seem to be arbitrary and prescriptive in nature regarding durability requirements. Further, the classifications do not necessarily address the relevant mechanisms of concrete deterioration adequately. Given the importance of environmental effects on concrete service life and performance, it is necessary to have detailed classifications catering to all exposure conditions, which address the appropriate mechanisms of deterioration. This paper describes an attempt to propose a new environmental classification system for concrete construction in India. The shortcomings of the present system are described first, followed by a summarisation of the international developments in exposure classifications. Concrete mix designs from several construction sites across India executed by Central Public Works Department are then presented and analyzed in the light of the prescriptions made by codal provisions from a number of countries across the world. The results of the analyses, along with the relevant features of international developments, are used to finally propose a rational system for classification of concrete exposure conditions.