An example illustrating the mix proportioning for a concrete of M 20 grade is given through Steps 1 to 11.
STIPULATIONS FOR PROPORTIONING
- Grade of designation : M 20
- Type of cement : OPC 43 grade conforming to IS 8112-19896 (Reaff. 2005)
- Maximum nominal size of aggregate : 20 mm
- Minimum cement content : 300 kg/m3
- Maximum water-cement ratio : 0.55
- Workability in terms of Slump : 100 mm
- Exposure condition : Mild (for reinforced concrete)
- Method of concrete placing : Pumping
- Degree of supervision : Good
- Type of aggregate : Crushed angular aggregate
- Maximum cement content : 450 kg/m3
- Chemical admixture type : Superplasticiser
TEST DATA OF MATERIALS
- Cement : OPC 43 grade conforming to IS 8112
- Specific gravity of cement : 3.15
- Chemical admixture : Superplasticizer conforming to IS 9103-1999
- Specific gravity of admixture : 1.145
- Specific gravity of : 1) Coarse aggregate : 2.67 2) Fine aggregate : 2.60
- Water absorption (IS 2386-19637)
- Coarse aggregate : 0.5 percent
- Fine aggregate : 2.0 percent 55 CEMENT, CONCRETE AND MIX PROPORTIONING
- Free (surface) moisture:
- Coarse aggregate : Nil (absorbed moisture also nil)
- Fine aggregate : 3%
- Sieve analysis (IS 2386 Part I 8)
- Coarse aggregate
TARGET STRENGTH FOR MIX PROPORTIONING
f’ck = fck + 1.65 s
where
f’ck = target average compressive strength at 28 days
f ck = characteristic compressive strength at 28 days, and
s = standard deviation
From Table 1, standard deviation, s = 4 N/mm2
Therefore, target strength = 20 + 1.65 x 4 = 26.6 N/mm2
SELECTION OF WATER – CEMENT RATIO
From Table 5 of IS 456, maximum water cement ratio = 0.55 for Mild exposure.
Let us adopt water – cement ratio as 0.55 only
SELECTION OF WATER CONTENT
From Table 2, maximum water content = 186 kg.
(for 25 to 50 mm slump range) for 20 mm aggregate
Estimated water content for 100mm slump = 186 + 6 x 186 100 = 197 kg
As superplasticizer is used, the water content can be reduced up to 20 percent and above
Based on trials with superplasticizer, water content reduction of 20 percent has been achieved.Hence, the arrived water content = 197 x 0.80 = 157.6 kg.
CALCULATION OF CEMENT CONTENT
Water-cement ratio = 0.55
Cement content = 157.6 0.55 = 286.5 kg/m3
From Table 5 of IS 456, minimum cement content for ‘mild’ exposure condition = 300 kg/m3
286.5 kg/m3 <300 kg/m3 , hence, adopt Minimum Cement content = 300 kg/m3
For this cement content total water content (for SSD condition) becomes 165 kg/cu.m of concrete
Will be Continued in Issue 11..
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.