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Concrete is one of the most widely used building materials in the world, yet people still find it difficult to take samples correctly and manage the testing & certification  process.

Compressive Strength Test

This test give us an idea about all the characteristics of concrete. With the help of this  test we can check that whether Concreting has been done properly or not. and compressive strength is the ability of material or structure to carry the loads on its surface without any crack or deflection. A material under compression tends to reduce the size, while in tension, size elongates

Compressive strength of concrete depends on many factors such as water-cement ratio, cement strength, quality of concrete material, quality control during production of concrete etc.

The cubes are generally tested at 7  &  28 days unless specific early tests are required and Strength of concrete increase with age showen in Below Table :

Age	Strength percent
1 day	16%
3 days	40%
7 days	65%
14 days	90%
28 days	99%

Procedure of Test

For this test mainly 150mm * 150 mm * 150 mm cubes are used

• Clean the mounds properly and apply oil inside the cube frame
• Fill the concrete in the molds in layers approximately 50mm thick
• Compact each layer with not less than 35 strokes per layer using a tamping rod (steel bar 16mm diameter and 600 mm long, )
• Level the top surface and smoothen it with a trowel
• The concrete cubes are removed from the moulds between 16 to 72 hours, usually this done after 24 hours.  Remove the specimen from water after specified curing time and wipe out excess water from the surface.Take the dimension of the specimen to the nearest 0.2mm And then place the specimen in the machine in such a manner that the load shall be applied to the opposite sides of the cube cast.Align the specimen centrally on the base plate of the machine.Rotate the movable portion gently by hand so that it touches the top surface of the specimen.
• Apply the load gradually without shock and continuously at the rate of 140 kg/cm²/min. till the specimen fails
• Record the maximum load and note it
•  

The compressive strength of concrete is given in terms of the characteristic compressive strength of 150 mm size cubes tested at 28 days  The characteristic strength is defined as the strength of the concrete below which not more than 5% of the test results are expected to fall.”

Number of samples for testing Compressive Strength

Quantity of Concrete (in m3)	Number of samples for testing Compressive Strength
1-5	1
6-15	2
16-30	3
31-50	4
51 +	4 + 1 cube for each additional 50m

Note :-

1. The concrete cubes are removed from the moulds between 16 to 72 hours, usually this done after 24 hours.   . 
2. The top surface of these specimen should be made even and smooth
3. concrete is poured in the mould and tempered properly so or use vibrator that no void remain in the cube
4. These specimens are tested by compression testing machine after 7 days curing or 28 days curing.
5. The cubes should be covered with a damp cloth and a plastic sheet and stored in dry environment at a temperature range of 20 ± 5 degrees
6. The curing tank needs to operate at a temperature between 20 ± 2 degrees and provides a moist environment that allows the cubes to hydrate properly. 
7.  Load should be applied gradually at the rate of 140 kg/cm2 per minute till the Specimens fails. Load at the failure divided by area of specimen gives the compressive strength of concrete.
8. The water for curing should be tested every 7 days and the temperature of water must be at 27+-2oC.
9. Minimum three specimens should be tested at each selected age. If strength of any specimen varies by more than 15 percent of average strength, results of such specimen should be rejected. Average of three specimens gives the crushing strength of concrete. The strength requirements of concrete.
10. Standard tests for determining the strength are Cube Test and Cylinder Test
11.  

Calculations Format

Size of the cube =150mm x 150mm x 150mm

Area of the specimen =22500 m²

Noted load from machine =……….tones = ………….N

Compressive strength = (Load in N/ Area in mm²⁾=…………N/mm²

Compressive Strength of Different Grades of Concrete at 7 and 28 Days

Grade	Min. strength 7 days	Min. Strength at 28 days
M15	10  N/mm2	15 N/mm2
M20	13.5  N/mm2	20  N/mm2
M25	17  N/mm2	25  N/mm2
M30	20  N/mm2	30 N/mm2
M35	23.5 N/mm2	35 N/mm2
M40	27 N/mm2	40 N/mm2
M45	30 N/mm2	45 N/mm2

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