ACI Mix design Procedure
(a) The W/C ratios selected from curve for the target mean strength.
(b) The water content is selected from table 4 for the desired workability and maximum size of aggregate
(c) The cement content is calculated from the water content and W/C ratio
(d) The coarse aggregate content is estimated from the table 5 for the maximum size of aggregate and fineness modulus of sand.
(e) The fine aggregate content is determined by subtracting the sum of the absolute volume of coarse aggregate, cement, water and air content from the unit volume of concrete.
Determine the slump depending on the degree of workability and placing condition. A concrete of the stiffest consistency (lowest slump) that can be placed efficiently should be used.
Determine the maximum size of coarse aggregate that is economically available and consistent with dimensions of the structure.
Determine the amount of mixing water for the given slump and maximum size of coarse aggregate from Table 4. This table also indicates approximate amount of entrapped air.
Determine the minimum water-cement ratio from the curve.
Determine the amount of cement per unit volume of concrete from steps3 and 4. This cement content should not be less than the cement content required based on durability or some other criterion.
Determine the amount of coarse aggregate required for a unit volume of concrete from Table 5. The value thus obtained is multiplied by the dry rodded unit weight if the aggregate to get the required dry weight.
Determine the amount of fine aggregate. At completion of step 6, all ingredients of the concrete have been estimated except the fine aggregate. If the weight of concrete per unit volume is assumed, the required weight of fine aggregate is simply the difference between the weight of fresh concrete and the total weight of all other ingredients. An estimate of weight of fresh concrete can be made either by using Eq. 1 or Table 6
Wm = 10ρA(lOO-A)+γc(1- ρA/ ρC) -γw(ρA – 1) (1)
Wm = weight of fresh concrete, kg/n.
ρA = weighted average specific gravity of combined
fine and coarse aggregate
ρC = specific gravity of cement (= 3:15)
γc = cement requirement, kg/m3
γw = mixing water requirement, kg/m3
A = air content, percent
Adjust the mixing water quantity based on the moisture content in the aggregate.
Check the calculated mix proportions by means of trial batches prepared and tested in accordance with the relevant IS specifications and make another trial, if necessary.
CALCULATIONS AS PER ACI MIX DESIGN METHOD
Specific Gravity of Cement = 3.10
Specific Gravity of Fine Aggregate = 2.65
Specific Gravity of Coarse Aggregate = 2.70
Dry Density of Coarse Aggregate = 1560 kg/m3
Water Cement Ratio = 0.403
Volume of dry-rodded Coarse Aggregate = 0.64
per unit volume of concrete
Fineness Modulus of Fine Aggregate = 2.40
Fineness Modulus of Coarse Aggregate = 7.20
Trail mix was prepared for,
Workability = Medium
Compaction Factor = 0.85 to 0.92
Slump = 50mm to 75mm
Approximate amount of entrained air = 2%
Mean Target Compressive ft = fck + k . s
Strength of concrete , = 35 + 1.65 x 5
= 43.25 N/mm2
Water required for 50-75mm slump using = 200 l
53 grade cement as per sp23-1982,
Water Cement Ratio = 0.403
Quantity of cement per m3 of concrete = 200 /.403
= 496.3 kg
Quantity of coarse aggregate required = 0.6264 x1560
for 1m3 of concrete = 977.2 kg
By: Dr. B.N. KRISHNASWAMI
DEPT OF CIVIL ENGG
NIT, TRICHY ORGANISED
CIVIL ENGINEERS & ARCHITECTS ASSOCIATION,