Compressive Strength of Concrete cubes is the determination of Strength of Concrete in 28 days. Compressive Strength of Concrete is an important element in designing R.C.C Structure. Quality of the concrete manufacture is often measure by its Compressive Strength.
Compressive Strength of Concrete Cubes
Characteristic Strength = Grade of Concrete. Cubes to test at 7 and 28 days for Compressive Strength. The cubes are examined with calibrated compression machine.
Concrete Cube Properties
Size of cube 15 cm X 15 cm X 15 cm
Density=Mass/volume
Mass=Density X volume
Volume of cube=150 mm X 150 mm X 150 mm
Mass=2400 X (0.15*0.15*0.15) = 8.1
Result
Weight of cube in kg = 8.1
Concrete Density in kg/cum  Volume of 150 mm size cube  Corresponding Weight of cube in kg 
2400  0.003375  8.1 
2425  0.003375  8.184 
2450  0.003375  8.269 
2475  0.003375  8.353 
2500  0.003375  8.438 
Compressive Strength of Concrete at 7 Days & 28 Days
Grade of Concrete 
Minimum compressive strength N/mm^{2} at 7 days  Specified characteristic compressive strength (N/mm^{2}) at 28 days 
M 15 
10 
15 
M 20 
13.5 
20 
M 25 
17 
25 
M 30 
20 
30 
M 35 
23.5 
35 
M 40 
28 
40 
M 45 
30 
45 
Calculation
Compressive Strength of concrete = Maximum compressive load / Cross Sectional Area
Cross sectional Area = 150 mm X 150 mm = 22500 mm^{2} or 225 cm^{2}
Assume the compression load is 710 KN,
Details 
Samples 

1 
2 
3 

Compressive Load (KN) 
710 KN 
705 KN 
700 KN 
Compressive Strength (N/mm^{2}) 
710000/22500 = 31.000 N/mm^{2}  705000/22500 = 31.333 N/mm^{2}  700000/22500 = 31.111 N/mm^{2} 
Average Compressive Strength 
= (31.000+31.333+31.111)/3 = 31.148 N/mm^{2} 
Frequency of Sampling
Quantity of Concrete in Work (M^{3}) 
Number of Samples of concrete cubes for 7 days and 28 days (Three for 7 days and Three for 28 days)

15 
6 
615 
12 
1630 
18 
3150 
24 
51 and above 
4 plus one additional sample for each additional 50 m^{3} 