WORKABILITY OF CONCRETE, DURABILITY AND THE CONCRETE MIX PROPORTIONING.

The durability of concrete depends on its resistance to deterioration and the environment in which it is placed. The resistance of concrete to weathering, chemical attack, abrasion, frost and fire depends largely upon its quality and constituent materials. Susceptibility to corrosion of the steel is governed by the cover provided and the permeability of concrete. The cube crushing strength alone is not a reliable guide to the quality and durability of concrete;it must also have an adequate cement content and a low water cement ratio. One of the main characteristics influencing the durability of any concrete is its permeability. With strong, dense aggregates, a suitably low permeability is achieved by having a sufficiently low water cement ratio by ensuring as a thorough compaction of the concrete as possible and by ensuring sufficient hydration of cement through proper curing methods. As such, for given aggregates, the cement content should be sufficient to provide adequate workability with a low water cement ratio so that concrete can be completely compacted with the means available. The mix proportions shall be selected to ensure that the workability of the fresh concrete is suitable for the conditions of handling and placing, so that after compaction it surrounds all reinforcements and completely fills the form work. When concrete is hardened, it shall have the required strength, durability and surface finish. The determination of the proportions of cement, aggregates and water to attain the required strength shall be made by designing the concrete mix, such concrete is called Design concrete mix and also by adopting nominal concrete mix, such concrete shall be called Nominal mix concrete. Design mix concrete is preferred to nominal mix. If design mix concrete can't be used for any reason on the work for grades of M-20 or lower, nominal mixes may be used with the permission of engineer -in-charge, which however, is likely to involve a higher cement content. In specifying a particular grade of concrete, the following information shall be included (a) Type of mix, that is, design mix concrete, or nominal mix concrete. (b) Grade designation (c) Type of cement (d) Maximum nominal size of aggregate (e) Minimum cement content (for design mix concrete) (f) Maximum water cement ratio. (g) Workability and (h) Mix proportion (for nominal mix concrete) . In appropriate circumstances, the following additional information may be specified (a) Type of aggregate. (b) Maximum cement content, and (c) Whether an admixture shall or shall not be used and type of admixture and the conditions of use.