Question

Find the weight (in kg) of cement, water, flyash, fine and coarse aggregate to produce a cement/flyash mix for pumped concrete, using the British Method that has a characteristic 28-day compressive strength of 45 MPa. One hundred laboratory test results on the controlling mix show a standard deviation of 5.3 MPa (k=1.65). The exposure classification for durability purposes is A2 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 2.5. Assume the high early strength cement density is 3150 kg/m³ and fly ash density to be 2000 kg/m³

Answer 1

1. Target mean strength & standard deviation.

f_ck' = f_ck + kS ...... k = constant = 1.65 , Standard deviation = 5.3 MPa

f_ck' = 45 + 5.3*1.65 = 53.745 MPa. = 54 MPa.

2. water-cement ratio

exposure condition is A2 as per British standard, max. w/c ratio = 0.5

3. selection off water content

for crushed 20mm aggregate & concrete of slump 25-50mm water content = 186kg/m³

For pumped concrete slump of 75-100 mm

therefore,

add extra 3% water for evvery 25mm increse in slump, 2 * 3 % = 6 %

water content = 1.06 * 186 = 197.16 kg/m^3.

4. cement conternt = 197.16 / 0.5 = 394.32 kg/m3 of concrete ( less than minimum required of 400kg/m3 for M55) therefore 400kg/m3

flyash = 400 * 0.4 = 160 kg/m3

cement = 400-160 = 240 kg/m3

w/c = 197.15/400 = 0.49....ok

5. CA & FA

for w/c ratio of 0.5, percentage of CA of 20mm is 0.64 of total aggregaates.

VCA = 0.64 * total aggregates

VFA = 0.36 * total aggregates

The volume of total aggregate = 1 - { [ ( C/Sc) + (F ash/ Sflyash) + (W/1) ] / 1000 }

= 1 - { [ ( 240/3.15) + (160/ 2) + (197.16/1) ] / 1000 }

= 0.6466 m³

V_CA = 0.64* 0.65 = 0.415 m3

V_FA = 0.36 *0.65 = 0.2335 m3

M_CA = 0.415 m3 * 2850 kg/m3 = 1,182.75 kg

M_FA = 0.2335 * 2750 = 642.125‬ kg

Therefore,

water = 197.16 kg/m³

cement = 240 kg/m³

flyash = 160 kg/m³

CA = 1182.75 kg/m³

FA = 642.125 kg/m³