Question

Find the weight (in kg) of cement, water, flyash, fine and coarse aggregate to produce a cement/flyash mix for a column, using the British Method that has a characteristic 28-day compressive strength of 60 MPa. One hundred laboratory test results on the controlling mix show a standard deviation of 6.0 MPa (k=1.65). The exposure classification for durability purposes is A1 and the conditions require a fly ash/ high early strength cement blend (with a 40% proportion of fly ash). The specific gravity of the crushed coarse 20 mm aggregate to be used is 2850 kg/m3 and its dry rodded density is 1600 kg/m3. The specific gravity of fine aggregate is 2750 kg/m3 and its fineness modulus is 2.7. Assume the high early strength cement density is 3150 kg/m3 and fly ash density to be 2500 kg/m3

Answer 1

Ans) Let the total volume of trial mix be 1 m3 ..According to British method slump suitable for construction of column is 75 mm.For 75 mm slump and nominal aggregate size of 20 mm , amount of water required per cubic meter for high strength concrete is 175 kg

=> Amount of water = 175 kg per m3 concrete

For mild exposure condition, amount of air = 2%

Now, required compressive strength is larger of

f'cr = f'c + 1.34 s

or

f'cr = 0.9f'c + 2.33S

=> fcr = 60 + 1.34(6) = 68 MPa

or

=> f'cr = 0.9(60) + 2.33(6) = 68 MPa

Hece, required strength = 68 MPa

Now, for compressive strength of 68 MPa, water cement ratio is 0.25

Hence, amount of cement + fly ash = Amount of water / w-c ratio = 175/ 0.25 = 700 kg

Hence, amount of cement = 0.60 x 700 = 480 kg

Amount of fly ash = 700 - 480 = 220 kg  

  Now, for nominal aggregate size of 20 mm and fineness modulus of 2.7 , volume of coarse aggregate is 0.72 m3

=> Amount of coarse aggregate = dry rodded density x volume = 1600 x 0.72 = 1152 kg

Volume of fine aggregate = Total volume of concrete - Volume of all water, cement, fly ash, coarse aggregate and air

=> Fine aggregate volume = 1 - [(175/1000) + (480 / 3150) + (220 / 2500) + (1152/ 2850) + 0.02]

=> Fine aggregate volume = 1 - 0.84 = 0.16 m3

=> Amount of fine aggregate = volume x specific gravity = 0.16 x 2750 = 440 kg

Hence, required batch weight for trial mix of 1 cubic meter is as follows :

Ingredient	Amount (kg)
Cement	480
Fly ash	220
Water	175
Fine aggregate	440
Coarse aggregate	1152