Mix Design as per IS:IS10262-2009- M20 concrete.
Objective: To find out the combination/constituents of material and its quantity to get a desired property of fresh and hardened concrete as per requirement of design.
2. Information required before start of Mix Design:
a. Coarse Aggregate: Rounded/Crushed aggregate:
b. Maximum size of the coarse aggregate, 10mm, 20mm, or 40mm.
c. Grading zone of Fine aggregate.Zone-1, II,III or IV.
d. Sp. Gr. of Aggregates or density of aggregate.
e. Type of Cement and cement strength(may or may not be depending upon the method of Mix Design).
f. Type of Admixture (plasticizer, accelerator etc)
g. Absorption property of aggregate and moisture content .
(It is necessary to find out the total water to be added to the concrete to keep the W.C ratio as per design. All aggregates shall be Saturated Surface Dry condition)
3. Some Important properties one should know, not mandatory to mention in the Mix Design.
a. Fresh concrete Properties depend on.
water content, Grading of aggregate and maximum size of Aggregate.
Shape of aggregate.
b. Hardened concrete Properties depend on W/C ratio.
c. All other mechanical properties i.e tensile strength, shear strength, Modulas of elasticity of concrete etc depends on compressive strength of concrete.
d. Durability of concrete i.e long term strength is controlled by w/c ratio, cement content, Grade of concrete etc.
e. There may be large numbers of methodology of Mix Design.
f. Cube strength is a function of (W/(C+F), F stands for fly ash added, as designed, replacing cement i.e function of water- cement ratio.
g. Workability is a function of (W=water, P=Plasticizer, Aggregate packing)
h. Durability is a function of (W/(C+F), curing).
i. Total volume of all ingredients is conserved.
4. Assumption of Mix Design:
a. M.S.A( Maximum size of aggregate): 20mm
b. Degree of workability: 0.8
c. Degree of QC: good
d. Type of Exposure: Mild
e. Water Cement Ratio: 0.47
f. Cement: OPC-53
g. Sp. Gr. Of Cement: 3.15
h. Sp. Gr. of coarse aggregate:2.7
j. Water absorption of C/aggregate in SSD condition: 0.5%
k. Water absorption of F/aggregate in SSD condition: 1%
l. Free moisture in C/Aggregate: 1%
m. Free moisture in F/Aggregate: 2%
n. Additional moisture content in coarse aggregate= 1- 0.5= 0.5%
o. Additional moisture content in fine aggregate= 2- 1= 1%
p. Bulk density of Cement: 1440 kg/m3
q. Bulk density of Coarse aggr: 1650 kg/m3
r. Bulk density of Fine aggr: 1800 Kg/m3
5. Step-1- Target Mean Strength:
a. Fm= fck+1.65Ϭ, Fm= target mean strength, Ϭ=Standard Deviation.
Standard deviation shall be determined at site from Min. 30 Samples. However in absence of site data, then IS-456, Table-4 shall be followed.
for M20-M25, standard deviation, Ϭ= 4 Mpa.
Fm= 20 + 1.65*4=26.6 Mpa.
6. Step-2: selection of w/c ratio from curves
For mild exposure and m.s.a 20mm from Table-5 of IS 456.
W/C ratio =0.55, Table-5 of IS 457.
Adopted w/c ratio=0.48 i.e. less than 0.55.
7. Step-3: selection of water content.
From Table-2 of IS10262-2009 up to M35 concrete
8. Step-4 calculation of cement content:
Cement content =Water content/W.C ratio(SL no 20)=186kg / 0.48= 387 Kg.
Minm. Cement content for moderate exposure = 300 kg. As per T/5 of IS 456-2000
9. Step-5 proportion of volume of Course Aggregate and Fine Aggregate.
From Table-3 of IS10262-2009.
For 20mm aggregates and Zone-II sand, against total volume of Aggregates taking-1, Volume of Course Aggregate= 0.62, Volume of Fine Aggregate= 1-0.62=0.38.
10. Step-6 Mix Calculation.
a. Volume of concrete=1cum.------(A)
b. Volume of cement=Mass of Cement/Mass density of Cement=387kg/3150Kg/m3=0.123 cum.--------(B)
c. Volume of water(SL no. 20)=186kg=0.186cum.----------(C)
d. Volume of all in aggregate= Total volume of concrete-(Volume of Water+ Volume Cement)=A-(B+C)=1-(0.123+0.186)=0.691cum.
e. Coarse Aggregate- actual Volume of C/aggre in 01 m3 concrete(SL no-31 &41)= (0.62/1)*0.691=0.4284 cum.
Density of Coarse Aggregates= 2.7*1000 Kg/m3=2700Kg/m3.
Mass of Coarse aggregate=Volume x Density=0.4284 x 2700=1157Kg.
f. Fine Aggregate- actual Volume of Fine Aggregate(SL no-31 &41)= 0.38 x 0.691=0.2626cum.
Density of fine Aggregate=2650 Kg/m3
Mass of fine Aggregate=0.2626 x 2650=696 Kg.
11. Step-7 Mix Proportion at SSD condition.
Cement=387 kg
Fine Aggregates=696 Kg
Coarse Aggregates=1157 Kg.
Cement : Fine aggregates : Coarse aggregates=1: 1.8 : 3
12. Corrected Mass of Ingredients after moisture
a. Cement=387 Kg.
b. Fine Aggregate= 696kg + 1% of 696(para 4(o)) = 696 + 6.96kg= 703 kg
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c. Coarse Aggregate= 1157 kg + 0.5% of 1157(para 4(n)) = 1157 + 5.78 kg= 1163 kg
Remarks: Sl no. 4 (l) and 4(m) above may vary with atmosphere , accordingly water adjustment shall be made on daily basis or more frequently.
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