Concrete Mix Design
Data Required for Concrete Mix Design
(i) Concrete Mix Design Stipulation
(a) Characteristic compressive strength required in the field at 28 days grade designation — M 25 (b) Nominal maximum size of aggregate — 20 mm (c) Shape of CA — Angular (d) Degree of workability required at site — 50-75 mm (slump) (e) Degree of quality control available at site — As per IS:456 (f) Type of exposure the structure will be subjected to (as defined in IS: 456) — Mild (g) Type of cement: PSC conforming IS:455 (h) Method of concrete placing: pump able concrete(ii) Test data of material (to be determined in the laboratory)
(a) Specific gravity of cement — 3.15 (b) Specific gravity of FA — 2.64 (c) Specific gravity of CA — 2.84 (d) Aggregate are assumed to be in saturated surface dry condition. (e) Fine aggregates confirm to Zone II of IS - 383Procedure for Concrete Mix Design of M25 Concrete
Step 1 — Determination Of Target Strength
Himsworth constant for 5% risk factor is 1.65. In this case standard deviation is taken from IS:456 against M 20 is 4.0.ftarget = fck + 1.65 x S
= 25 + 1.65 x 4.0 = 31.6 N/mm2
Where, S = standard deviation in N/mm2 = 4 (as per table -1 of IS 10262- 2009)Step 2 — Selection of water / cement ratio:-
From Table 5 of IS 456, (page no 20) Maximum water-cement ratio for Mild exposure condition = 0.55 Based on experience, adopt water-cement ratio as 0.5. 0.5<0.55, hence OK.Step 3 — Selection of Water Content
From Table 2 of IS 10262- 2009, Maximum water content = 186 Kg (for Nominal maximum size of aggregate — 20 mm)Table for Correction in water content
Parameters | Values as per Standard reference condition | Values as per Present Problem | Departure | Correction in Water Content |
Slump | 25-50 mm | 50-75 | 25 | (+3/25) x 25 = +3 |
Shape of Aggregate | Angular | Angular | Nil | - |
Total | +3 |
Step 4 — Selection of Cement Content
Water-cement ratio = 0.5 Corrected water content = 191.6 kg /m3 Cement content = From Table 5 of IS 456, Minimum cement Content for mild exposure condition = 300 kg/m3 383.2 kg/m3 > 300 kg/m3, hence, OK. This value is to be checked for durability requirement from IS: 456. In the present example against mild exposure and for the case of reinforced concrete the minimum cement content is 300 kg/m3 which is less than 383.2 kg/m3. Hence cement content adopted = 383.2 kg/m3. As per clause 8.2.4.2 of IS: 456 Maximum cement content = 450 kg/m3.Step 5: Estimation of Coarse Aggregate proportion:-
From Table 3 of IS 10262- 2009, For Nominal maximum size of aggregate = 20 mm, Zone of fine aggregate = Zone II And For w/c = 0.5 Volume of coarse aggregate per unit volume of total aggregate = 0.62Table for correction in estimation of coarse aggregate proportion
Parameter | Values as per Standard reference condition | Values as per present problem | Departure | Correction in Coarse Aggregate proportion | Remarks |
W/c | 0.5 | 0.5 | Nil | - | See Note 1 |
Workability | - | pump able concrete | - | -10% | See Note 2 |
Total | -10% |
Step 6: Estimation of the mix ingredients
a) Volume of concrete = 1 m3 b) Volume of cement = (Mass of cement / Specific gravity of cement) x (1/100) = (383.2/3.15) x (1/1000) = 0.122 m3 c) Volume of water = (Mass of water / Specific gravity of water) x (1/1000) = (191.6/1) x (1/1000) = 0.1916 m3 d) Volume of total aggregates = a – (b + c ) = 1 – (0.122 + 0.1916) = 0.6864 m3 e) Mass of coarse aggregates = 0.6864 x 0.558 x 2.84 x 1000 = 1087.75 kg/m3 f) Mass of fine aggregates = 0.6864 x 0.442 x 2.64 x 1000 = 800.94 kg/m3Concrete Mix proportions for Trial Mix 1
Cement = 383.2 kg/m3 Water = 191.6 kg/m3 Fine aggregates = 800.94 kg/m3 Coarse aggregate = 1087.75 kg/m3 W/c = 0.5 For trial -1 casting of concrete in lab, to check its properties. It will satisfy durability & economy. For casting trial -1, mass of ingredients required will be calculated for 4 no's cube assuming 25% wastage. Volume of concrete required for 4 cubes = 4 x (0.153 x1.25) = 0.016878 m3 Cement = (383.2 x 0.016878) kg/m3 = 6.47 kg Water = (191.6 x 0.016878) kg/m3 =3.23 kg Coarse aggregate = (1087.75 x 0.016878) kg/m3 =18.36 kg Fine aggregates = (800.94 x 0.016878) kg/m3 = 13.52 kgStep 7: Correction due to absorbing / moist aggregate:-
Since the aggregate is saturated surface dry condition hence no correction is required.Step 8: Concrete Trial Mixes:-
Concrete Trial Mix 1:
The mix proportion as calculated in Step 6 forms trial mix1. With this proportion, concrete is manufactured and tested for fresh concrete properties requirement i.e. workability, bleeding and finishing qualities. In this case, Slump value = 25 mm Compaction Factor = 0.844 So, from slump test we can say, Mix is cohesive, workable and had a true slump of about 25 mm and it is free from segregation and bleeding. Desired slump = 50-75 mm So modifications are needed in trial mix 1 to arrive at the desired workability.Concrete Trial Mix 2:
To increase the workability from 25 mm to 50-75 mm an increase in water content by +3% is to be made. The corrected water content = 191.6 x 1.03 = 197.4 kg. As mentioned earlier to adjust fresh concrete properties the water cement ratio will not be changed. Hence Cement Content = (197.4/0.5) = 394.8 kg/m3 Which also satisfies durability requirement. Volume of all in aggregate = 1 – [{394.8/(3.15x1000)} + {197.4/(1 x 1000)}] = 0.6773 m3 Mass of coarse aggregate = 0.6773 x 0.558 x 2.84 x 1000 = 1073.33 kg/m3 Mass of fine aggregate = 0.6773 x 0.442 x 2.64 x 1000 = 790.3 kg/m3Concrete Mix Proportions for Trial Mix 2
Cement = 384.8 kg/m3 Water = 197.4 kg/m3 Fine aggregate =790.3 kg/m3 Coarse aggregate = 1073.33 kg/m3 For casting trial -2, mass of ingredients required will be calculated for 4 no's cube assuming 25% wastage. Volume of concrete required for 4 cubes = 4 x (0.153 x1.25) = 0.016878 m3 Cement = (384.8 x 0.016878) kg/m3 = 6.66 kg Water = (197.4 x 0.016878) kg/m3 =3.33 kg Coarse aggregate = (1073.33 x 0.016878) kg/m3 =18.11 kg Fine aggregates = (790.3 x 0.016878) kg/m3 = 13.34 kg In this case, Slump value = 60 mm Compaction Factor = 0.852 So, from slump test we can say, Mix is very cohesive, workable and had a true slump of about 60 mm. It virtually flowed during vibration but did not exhibit any segregation and bleeding. Desired slump = 50-75 mm So , it has achieved desired workability by satisfying the requirement of 50-75 mm slump value . Now , we need to go for trial mix-3 .Concrete Trial Mix 3:
In case of trial mix 3 water cement ratio is varied by +10% keeping water content constant. In the present example water cement ratio is raised to 0.55 from 0.5. An increase of 0.05 in the w/c will entail a reduction in the coarse aggregate fraction by 0.01. Hence the coarse aggregate as percentage of total aggregate = 0.558 – 0.01 = 0.548 W/c = 0.55 Water content will be kept constant. Cement content = (197.4/0.55) = 358.9 kg/m3 Hence, volume of all in aggregate= 1 – [{(358.9/(3.15 x 1000)} + (197.4/1000)] =0.688 m3
Mass of coarse aggregate = 0.688 x 0.548 x 2.84 x 1000 = 1070.75 kg/m3 Mass of fine aggregate = 0.688 x 0.452 x 2.64 x 1000 = 821 kg/m3