Use of grade 25 concrete in design of water retaining structures based on BS 8007 recommendations

Abstract

The grade of concrete is very important for strength and durability of reinforced concrete structures. Although strength and durability of concrete are inter-related, it is not always true that high strength concrete has high durability. This is because, the durability depends on the porosity of concrete rather than strength. As far as the reinforced concrete water retaining structures are concerned, the durability requirements are more demanding because concrete is subjected to more severe environmental conditions in those structures. Because of this reason, higher grade of concrete is normally specified for reinforced concrete water retaining structures. In the recent past, Grade 25 concrete was used for water retaining structures based on BS 5337 recommendation. Even before BS 5337 came into practice, the nominal mixes such as 1: 11/2:3 (equivalent to grade 25) were used for water retaining structures. At present, recommendations given in BS 8007 are used for design and construction of water retaining structures in Sri Lanka in which minimum recommended grade of concrete is grade 35. Furthermore, the maximum cement content is limited to 400 kg/m3 in the present code. Because of these recommendations, there is difficulty in adopting BS 8007 due to inability to produce grade 35 concrete in Sri Lanka, especially in volume batched concrete. In this context, this study was carried out to explore the possibility of using Grade 25 concrete with the design recommendations given in BS 8007. A literature survey was carried out to find out the design practices in other countries such as India, U.S.A., and Australia. A field survey was carried out to collect information on durability and the performance of the water retaining structure built using Grade 25. In addition, information was collected on production of Grade 25 and 35 concrete for construction of water retaining structures in Sri Lanka. It was found that the Indian standard recommends grade 25 concrete for water retaining structures and all other codes recommend higher grades of concrete for water retaining structures. Furthermore, it was observed that concrete in almost all the structures, which were design and built based on BS 5337 and CP 2007 are in good conditions. The collected concrete strength data shows that the production of Grade 35A concrete by volume batching was not that successful. But it is possible to obtain Grade 25 concrete without any difficulty even with volume batching. Furthermore, design calculations showed that the reinforcement requirement to control thermal and shrinkage cracks could be reduced with the use of lower grade of concrete such as Grade 25 concrete. Finally, amendments are proposed to design recommendation given in BS 8007 to accommodate Grade 25 concrete. Basest on this study it can be concluded that Grade 25 concrete can be used with design recommendation given in BS 8007 to design water retaining structures more economically.