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 0.55. Therefore, volume of coarse aggregate is required to be decreased to increase the fine aggregate content. As the water-cement ratio is higher by 0.05, the proportion of volume of coarse aggregate is decreased 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 0.55 = 0.59.
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 it is desirable to reduce these values up to 10 percent
Considering a reduction of 5% in the volume of coarse aggregate, we have
volume of coarse aggregate = 0.59 x 0.95 = 0.56
Volume of fine aggregate content = 1 – 0.56 = 0.44
MIX CALCULATIONS
MIX PROPORTIONS FOR TRIAL NUMBER 1
Cement = 300 kg/m3 Water = 165 kg/m3
Fine aggregate = 843 kg/m3
Coarse aggregate = 1102 kg/m3
Chemical admixture = 3 kg/m3
Water Cement ratio = 0.55
NOTE : Aggregates should be used in saturated surface dry condition. If otherwise, when computing the requirement of mixing water, allowance shall be made for the free (surface) moisture contributed by the fine and coarse aggregates. On the other hand, if the aggregates are dry, the amount of mixing water should be increased by an amount equal to the moisture likely to be absorbed by the aggregates. Necessary adjustments are also required to be made in mass of aggregates. The surface water and percent water absorption of aggregates shall be determined according to IS 23867 .
Adjustments for moisture in aggregates and water absorption of aggregates: Correction for aggregates: Fine aggregate (for 3% surface moisture) = 843 x 1.03 = 868.3 kg (wet) It may be noted that absorbed water does not become the part of the mixing water and must be excluded from the adjustment in added water. Thus, the surface water contributed by fine aggregates = 3-2= 1 percent.
Corrected water content = 165 - 843 (0.01) = 156.57 kg.
The estimated batch masses (after corrections) are as given below:
Cement = 300 kg/m3
Water = 156.7 kg/m3
Fine aggregate = 868.3 kg/m3
Coarse aggregate = 1102 kg/m3
Chemical admixture = 3 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. Mix Proportioning for manual placing of Concrete for M 20 Grade without the use of Admixture (as is the practice in normal house constructions) For manual placing concrete (non pumpable) workability in terms of slump required will be in the range of 25 mm to 50 mm. Considering the same data as in illustrative example for pumpable method of placing, mix proportioning steps from 1 to 4 remain same.
Will be Continued in Issue 13..
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.