Preliminary Data Necessary For Mix Design
Concrete mix design stands for a method that is used for choosing the proper materials of concrete and their relevant proportions with the purpose of arranging concrete with specific minimum strength, required workability and stability in a cost-effective manner.
While going to choose a concrete mix design, the following data should be obtained in advance since few design requirements are freezed based on these data.
Elementary Data Necessary For Mix Design:
Exposure Condition: Exposure Conditions of the structure: The normal atmosphere, under which the concrete is vulnerable throughout its service life, is segregated into five class to severity, with reference to IS 456.
Given below, the details about several exposure conditions :-
Mild: The concrete surfaces to be defended against weather or imperious conditions exclusive of those located in costal area.
Moderate: Concrete surfaces protected from extreme rain or freezing whereas wet concrete vulnerable to condensation & rain.
Concrete constantly remains under water.
Concrete touching or buried under non-aggressive soil/ground water.
Concrete surfaces protected from saturated salt air in costal area.
Severe: Concrete surfaces vulnerable to extreme rain, alternate wetting and drying or infrequent freezing whereas wet or severe condensation.
Concrete fully absorbed in sea water.
Concrete vulnerable to coastal atmosphere.
Very severe: Concrete surfaces vulnerable to sea water spray, corrosive fumes or extreme freezing condition when wet.
Concrete that touches or buried under aggressive sub-soil or ground water.
Extreme: Surfaces of members in tidal area.
Members which directly touches with liquid/solid aggressive chemicals.
The exposure condition restricts the least cement content, maximum water-cement ratio and minimum grade of concrete.
For exposure condition, the above data is suitable for dealing with the first trial and attaining its mix ratio.
If lower cement content can bring desired result, then it is required to provide extra with adherence to IS 456
Minimum thickness of member – The size of aggregate should be in excess of one fourth of the minimum thickness of member, specifically 20 mm nominal size aggregate is extensively used for most works. It is recommended to select for the maximum nominal size of aggregate to save an quantity of cement per unit of concrete.
Cement Grade – Locally accessible cement type/grade should be used during the construction period.
Workability – The workability of concrete is based on the placing condition of concrete, low – slump of 25-75 mm (lightly reinforced sections in slab, beam, column) to high – slump of 100-150 mm (slip form, pumped concrete) .
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