# Concrete Mix Design – M20 Grade of Concrete

Let’s say we have to design a Reinforced Concrete Cement(RCC) with a slump of 100 using Cement (Specific Gravity 3.14), 20mm Angular Coarse aggregate (Specific Gravity 2.45) and Zone 2 Sand (Specific Gravity 2.50) for a Mild exposure condition. These are the steps you will take:

STEP 1 : TARGET MEAN STRENGTH

For MILD exposure RCC using 20mm coarse aggregate we need to atLeat use M20 Grade of concrete (As per Table 5 of IS 456 2000)

So lets design M20

Target Mean Strength Fckt = Fck + 1.65xS

Fck = 20MPa (for M20)

Standard deviation for M20-M25 = 4.0 (Table 1 IS 10262 2009)

Fckt = Fck + 1.65×4

Target Mean Strength= 20+ 1.65*4.0=26.60

STEP 2 : CALCULATION FOR WATER CONTENT

For 20mm Aggregates MAX Water Content =186.0 (Table 2 10262 2009)

This value is for Slump within the range of 25-50

For Every 25mm increase in slump we will increase

water by 3%

Current Slump is :100 Which is :50 mm above 50mm

Increase Water Content By :6.00%

186.0+(6.00*186.00)/100 =197.16 Kg/m3

STEP 3 : SELECTION OF WATER CEMENT RATIO

For • REINFORCED Concrete and MILD exposure

condition MAX W:C=0 55 (As per Table 5 of IS 456 2000)

STEP 4 : CALCULATION OF CEMENT CONTENT

Cement=WaterContent/w:C

Cement Content=197.16/0.55 = 358.47 Kg/m3

(As per Table 5 of IS 456 2000)Minimum Cement Content is 300

Since

358.47>300 OK

STEP 5 : CALCULATION OF RATIO OF AGGREGATES

For 20mm Coarse and Zone 2 Fine,Volume of Coarse to Total Aggregates=0.62(Table 3 10262 2009)

This value is for w:C =0.50 our w:C=0.55

Decrease Coarse Ratio by 0.01 for every increase of

0.05 in w:C

For 0.55 there is an increase of :0.05

So Decrease Factor=0.01

New Coarse Ag Ratio=0.62-0.01=0.61

Fine Agg Ratio=1-Coarse Agg Ratio=0.39

DESIGN MIX CALCULATIONS

Vol of Cement= WT OF CEMETN/(SPGR of

CEMENT*1000)=358 47/(3.14*1000)=0.11

Vol of Water=Wt of Water/1000

=197.16/1000

=0 .20

TOTAL AGGREGATE Vol(e) = 1-(Cement+Water)=0.69

Wt of Coarse Aggregate=e*CoarseRatio*SPGR*1000

= 0.69*0.61*2.45*1000=1029.23 Kg/m3

Wt of Fine Aggregate=e*FineRatio*SPGR*1000

=0.69*0.39*2 50*1000=671 46 Kg/m3

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In a future article we are going to design the M30 concrete mix