Overall Strength of a structure such as flexural resistance and abrasion directly depends upon the compressive strength of concrete.
According to Wikipedia, Compressive Strength of concrete is defined as the Characteristic strength of 150mm size concrete cubes tested at 28 days.
Why do we test at 7, 14 & 28 days?
Concrete is a macro content with Sand, Cement, & Coarse aggregate as its microingredient (Mix Ratio) and gains its 100% strength over time at the hardened state.
Take a look at the below table.
Concrete Strength Overtime
Days after Casting  Strength Gain 
Day 1  16% 
Day 3  40% 
Day 7  65% 
Day 14  90% 
Day 28  99% 
As you can see the concrete gains its strength rapidly til 7^{th} & 14^{th} Days. Then gradually increases from there. So we can’t predict the strength until the concrete comes to that stable state.
Once it attains certain strength at 7 days, then we know (according to the table) only 9% of strength going to increase. So at sites, we do normally test concrete at this interval. If the concrete fails at 14 days, then we will reject that batching.
Compressive Strength Table of Concrete at 7 & 28 Days
Grade of Concrete  Minimum compressive strength N/mm2 at 7 days  Specified characteristic compressive strength (N/mm2) at 28 days 
M15  10  15 
M20  13.5  20 
M25  17  25 
M30  20  30 
M35  23.5  35 
M40  27  40 
M45  30  45 
Compressive Strength of Concrete Lab Test
Objective
To find compressive strength value of concrete cubes.
Required Equipment & Apparatus
 150 mm Cube Moulds (with IS Mark)
 Electronic Weighing Balance
 G.I Sheet (For Making Concrete)
 Vibrating Needle & other tools
 Compressions Testing Machine

Procedure
Cube Casting
 Measure the dry proportion of ingredients (Cement, Sand & Coarse Aggregate) as per the design requirements. The Ingredients should be sufficient enough to cast test cubes
 Thoroughly mix the dry ingredients to obtain the uniform mixture
 Add design quantity of water to the dry proportion (watercement ratio) and mix well to obtain uniform texture
 Fill the concrete to the mould with the help of vibrator for thorough compaction
 Finish the top of the concrete by trowel & tapped well till the cement slurry comes to the top of the cubes.
Curing
 After some time the mould should be covered with red gunny bag and put undisturbed for 24 hours at a temperature of 27 ° Celsius ± 2
 After 24 hours remove the specimen from the mould.
 Keep the specimen submerged under fresh water at 27 ° Celsius. The specimen should be kept for 7 or 28 days. Every 7 days the water should be renewed.
 The specimen should be removed from the water 30 minutes prior to the testing.
 The specimen should be in dry condition before conducting the testing.
 The Cube weight should not be less than 8.1 Kgs
Testing
 Now place the concrete cubes into the testing machine. (centrally)
 The cubes should be placed correctly on the machine plate (check the circle marks on the machine). Carefully align the specimen with the spherically seated plate.
 The load will be applied to the specimen axially.
 Now slowly apply the load at the rate of 140kg/cm^{2} per minute till the cube collapse.
 The maximum load at which the specimen breaks is taken as a compressive load.
Calculation
Compressive Strength of concrete = Maximum compressive load / Cross Sectional Area
Cross sectional Area = 150mm X 150mm = 22500 mm2 or 225 cm^{2}
Assume the compression load is 450 KN,
Compressive Strength = (450000 N / 225)/9.81 = 204 kg/cm^{2}
Observation Result (Lab Report)
Details  Samples  
Specimen 1  Specimen 2  Specimen 3  
Compressive Load (KN) 
375 KN  425 KN  435 KN 
Compressive Strength (Kg/Cm2) 
(375000/225) / 9.81 = 170 kg/cm^{2} 
(425000/225) / 9.81 = 192.5 kg/cm^{2} 
(435000/225) / 9.81 = 197.0 kg/cm^{2} 
Average Compressive Strength  = (170+192.5+197)/3
= 186.5 Kg/cm^{2} 
Notes
 The above experiment should be conducted at the temperature of 27° Celsius ± 2°.
 As per IS 516, the individual variation in compressive load should not be more than plus minus 15% of the average value.
Frequency of Sampling
According to IS 456:2000, the Minimum frequency of Concrete Sampling
Quantity of Concrete in Work (M3)  Number of Samples 
15  1 
615  2 
1630  3 
3150  4 
51 and above  4 plus one additional sample for each additional 50 m3 
Experiment Video
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