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SELECTION OF WATER CONTENT

From Table 2, maximum water content = 186 litre (for 25 to 50 mm slump range) for 20 mm aggregate

CALCULATION OF CEMENT CONTENT

Water-cement ratio = 0.55
Cement content = 186 / 0.55 = 338.18 kg/m3
From Table 5 of IS 456, minimum cement content for ‘mild’ exposure condition = 300 kg/m3 338 kg/m3 > 300 kg/m3 , hence O.K.

PROPORTION OF VOLUME OF COARSE AGGREGATE AND FINE AGGREGATE CONTENT

From Table 3, volume of coarse aggregate corresponding to 20 mm size aggregate and fine aggregate (Zone I) for water-cement ratio of 0.50 = 0.60.
In the present case water-cement ratio is 0.45. Therefore, volume of coarse aggregate is required to be increased to decrease the fine aggregate content. As the water-cement ratio is lower by 0.05, the proportion of volume of coarse aggregate is increased by 0.01 (at the rate of -/+ 0.01 for every + 0.05 change in water-cement ratio). Therefore, corrected proportion of volume of coarse aggregate for the water-cement ratio of 0.40 = 0.61.

Note – In case the coarse aggregate is not angular one, then also volume of coarse aggregate may be required to be increased suitably, based on experience For pumpable concrete these values should be reduced by 10 percent Therefore, volume of coarse aggregate = 0.61 x 0.9 = 0.55 Volume of fine aggregate content = 1 – 0.55 = 0.45

MIX CALCULATIONS

MIX PROPORTIONS FOR TRIAL NUMBER 1

Cement = 350 kg/m3 Water = 157.6 kg/m3
Fine aggregate = 873 kg/m3 Coarse aggregate = 1079 kg/m3
Chemical admixture = 3.5 kg/m3
Water Cement ratio = 0.45

Adjustments for moisture in aggregates and water absorption of aggregates:
Correction for aggregates:

Free (surface) moisture is nil in both fine and coarse aggregates.
Thus, the surface water contributed by fine aggregates = 3-2= 1 percent.
Corrected water content = 186 + 873 (0.01) + 1079 (0.005) = 171.7 kg.

The estimated batch masses (after corrections) are as given below:
Cement = 350.0 kg/m3
Water = 171.7 kg/m3
Fine aggregate = 873.0 kg/m3
Coarse aggregate = 1079 kg/m3
Superplasticiser = 3.5 kg/m3

The slump shall be measured and the water content and dosage of admixture shall be adjusted for achieving the required slump based on trial, if required. The mix proportion shall be reworked for the actual water content and checked for durability requirements
Two more trials having variation of + 10 percent of water-cement ratio in A-10 shall be carried out and a graph between three water-cement ratios and their corresponding strengths shall be plotted to work out the mix proportions for the given target strength for field trials. However, durability requirement shall be met.

Will be Continued in Issue 3..


References:
REFERENCE MANUAL FOR FIELD ENGINEERS ON BUILDING CONSTRUCTION


Published by:
Task Force for Quality Assurance in Public Constructions
GOVERNMENT OF KARNATAKA ISBN


Important Note:
This reference manual is intended for the use of Government Engineers.


Disclaimer:
Every effort has been made to avoid errors or omissions in this publication. In spite of this, some errors might have crept in. Any mistake, error or discrepancy noted may be brought to our notice which shall be taken care of in the next edition. It is notified that neither the publisher nor the authors will be responsible for any damage or loss of action to any one, or any kind, in any manner, therefrom.