Mix Design

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



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.

Documents

Mix Design