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Mix Design
Introduction Of Text
A number of publications put forward methods of mix design, and it cannot
be said that there is a best method; each has its merits, depending on the
materials to be used and the intended purpose of the concrete. It must be
stressed, too, that all mix design procedures are no more than means of
estimating proportions for the initial mix. This estimate needs to be checked by
making at least one trial mix, and it will frequently be found that the proportions
have to be adjusted according to experience gained from the trial.
All the methods are based on the strength and durability requirements;
there are usually fixed by the specifier. For concrete not exposed to the weather,
for example concrete used in the interior of a building, the required strength is
likely to be the main factor influencing the choice of the appropriate mix
proportions.Provided concrete can be fully compacted, its strength will be
inversely related to its water/cement ratio; typical curves, showing how strength
rises as water/cement ratio is reduced, may be found in many references.
When designing a mix it is also necessary to allow for the fact the
measured strength of concrete will vary as the result such factors fluctuations in
the properties of the materials, inconsistencies in batching, differences in
compaction, and variations in sampling and testing.
Objective
To achieve set of physical properties in finished mix either in its strength,
water-cement ratio, workability and cohesiveness.
Apparatus
British Standard Method – used for calculation and reference
BRE (building research establishment) – as reference
Procedures
Actually there is no field work required. Just require the information of the specimen for the concrete mix design to calculate.
Here we include an example of how to use B.S method to find concrete mix design.
• Design Mix– Interior columns of a building
– Characteristic compressive strength 25MPa
– The materials to be used are:
• Type GP Portland cement 3150 kg/m3
• Coarse aggregate – 20mm and 10 mm crushed quartzite
density 2680 kg/m3
• Fine aggregate – washed river sand density 2610 kg/m3
fineness modulus 2.74, 43% passing 600mm sieve
British Method – 1 water cement ratio
Use 0.63 as max w/c ratio
Type of cement
Type of coarse
aggregate
Compressive strengths (MPa)
Age (days) 3 7 28
uncrushed 17 24 38 Type GP orSR crushed 22 31 45
uncrushed 24 36 47 HE crushed 31 39 52
Max value
British method – 2 water content
Slump Max. size of
aggregate (mm)
Type of aggregate 0-10 10-30 30-60 60-180
uncrushed 150 180 205 225 10 crushed 180 205 230 250
uncrushed 135 160 180 195 20 crushed 170 190 210 225
uncrushed 115 140 160 175 40 crushed 155 175 190 205
Free-watercontent (kg/m3)
cf WW31
32
Fine uncrushedCoarse crushed 242225
31250
32
Kg/m3
British method – 3 cement content
• Divide free-water contentby free-water cement ratio
C = 242 0.63 = 384
British method – 4 aggregate
Estimate density of compacted concrete (2370 kg/m3)Total aggregates = D – C – W = 2370 – 384 – 242= 1744 kg/m3
British Method – 5 fine /coarse aggregate
British Method – 5 fine /coarse aggregate
Mix of coarse aggregates10mm:20mm ratio 1:210mm = 1/3 x 784 = 261 kg/m3
20mm = 2/3 x 784 = 523 kg/m3 sand = 960 kg/m3
cement = 384 kg/m3
water = 242 kg/m3
Design and Results
From calculation of concrete mix design, we designed a concrete mix based on
the following characteristics:
Target strength - 25N/mm² at 28 days.
5% proportion defective with no previous result available
Cement strength class 42.5MPa
Coarse agg – 20mm crushed quartzite. Fine agg – washed river sand
uncrushed and 50% passed the 600µm sieve
Workability - 30-60 mm slump
Since there was previous result available, 8N/mm² had been chosen as a
standard deviation while K = 1.64 was used in accordance to 5% proportion
defective.
The calculation for mixed designed can be seen in mix design form the appendix.
Hence from the result and the calculation we get the quantities for cement,
water, fine aggregate and coarse aggregate. (See table below)
Quantities Cement (kg) Water(kg or litres)
Fine aggregate (kg)
Coarse aggregate (kg) – 20 mm size
Per m³ (to nearest 5 kg)
355 210 697 1138
Per trial mix of 0.0135 m³
4.8 2.8 9.4 15.4
Conclusion
From the result of the test we obtained for designed mix, it can be
calculate that designed mix for any specific characteristics require certain
proportion of water, cement and aggregates. Any changes in the concrete
proportion may results different in the strength obtained. A small quantity of trial
mix can be done to check and prove the calculated design.
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