Compressive Strength of Concrete | Cube Test, Procedure, calculation & facts

 What Is The Compressive Strength?

Compressive strength is the ability to carry loads of material or structure on its surface without any cracking or deformation. An object under compression will reduce in size and, under tension, the size will continue to lengthen.

Compressive Strength = Load / Cross-sectional Area

IMPORTANCE OF COMPRESSIVE STRENGTH IN CONCRETE

Concrete is a mixture of sand, cement and aggregate. The concrete strength depends on the individual compressive strength of its components (cement, sand, aggregates), the quality of the materials used, curing methods, water-cement ratio, the ratio of air entrainment mixture and many other factors. Effects of temperature.

The Compressive strength test helps us to know the overall strength and the above factors. By conducting this test, one can easily determine the strength psi of the concrete and the quality of the concrete being produced.

The Concrete Cube Test will give compressive strength of concrete which gives an idea of all the properties of concrete. By this unique test, we can decide whether the concreting was done correctly or not.

The compressive strength of concrete ranges from 15 MPa (2200 psi) to 30 MPa (4400 psi) for residential concrete and is high in commercial structures. Some applications use forces greater than 10,000 psi (70 MPa).

Must read: What Is Slump Cone Test | Procedure | Slump patterns | Slump Value | Observations | FAQ 

 

PROCEDURE FOR TESTING THE COMPRESSIVE STRENGTH  OF CONCRETE CUBES

 

Purpose of the test to To find compressive strength value of the concrete cube

Equipment & Apparatus Required For The Test

 

PREPARATION OF CONCRETE CUBE SPECIMEN

All products should be stored at around 27 C +- 3 C degree Celsius. Ensure there are no lumps in the cement by mixing it uniformly with a trowel

 

Mixing of concrete

Mix the concrete either by hand or in a laboratory batch mixer

Hand mixing

The process must be done on the G.I Sheet (For Making Concrete) until a homogenous mix is obtained.

·       First mix fine aggregates and cement in dry state

·       Second Add coarse aggregates with equal distribution

·       Finally add the amount of water to the package until stability is reached.

Machine mixing

The component should not be rotated for more than 2 minutes and the following method should be followed

·       Amount of water

·       coarse aggregates 50%

·       fine aggregates

·       cement

·       coarse aggregates 50%

Sample casting (specimen)

The casting molds are rubbed with grease on the inner side for easy removal. This sample should be poured in 3 layers (5 cm each) and mixed properly to avoid honeycomb formation.

Compaction

Using tamping rod Compact each layer with not less than 35 strokes per layer. This tamping bar has the dimension of steel bar diameter 16mm and a length of 0.6m.

Age of test

Compressive strength Cube testing can be done in 1, 3, 7, 14 and 28 days. Sometimes, the strength of the older ones is required, which is carried out at 14 to 54 weeks.

Number of samples

We should have at least 3 samples for testing from different batches as mandatory. The mean of compressive strength achieved by this samples is used to determine the actual strength of the batch.

Must read: Laboratory Tests On Cement | 8 Different Types Of Cement Test | LCETED

 

PROCEDURE FOR CONCRETE CUBE TEST

1. Remove the samples from the water after curing time specified and excess water from the surface should be wiped.

2. The dimension of the samples to the nearest 0.2m taken

3. The bearing surface of the testing machine should be cleaned

4. Place the sample in the machine so that the load is applied to the opposite sides of the cube cast.

5. Align the centre of the sample with the machine's base plate.

6. Rotate the movable portion gently by hand so that it touches the top surface of the specimen.

7. Gently rotate the movable part by hand so that it touches the upper surface of the sample.

8. Apply load gradually at a rate of 140 kg / cm2 per minute until the samples fail.

9. The maximum load recorded and any unusual features in the type of failure noted.

Must read: Curing of Concrete | Reasons | Duration | Types | Weather

CALCULATION

Compressive Strength of concrete = Maximum compressive load carried by specimen / Cross Sectional Area surface of specimen

Take it as,

Cross sectional Area = 150mm X 1500mm = 22500 mm² or 225 cm²

Assume the compression load is 400 KN,

Compressive Strength = (400000 /22500) = 17.77 /9.81 = 181.22 kg/cm2

Note – 1 kg is equal to 9.81 N

 

The same calculation is done for the sample at different ages as stated below:

before that know Q/A 408: Compressive Strength of Concrete at Various Ages

NO OF SAMPLING FOR AREA OF CONCRETE

Minimum frequency of concrete sampling according to IS456: 2000

TIPS 408: COMPRESSIVE STRENGTH OF DIFFERENT GRADES OF CONCRETE AT 7, 14, 21 AND 28 DAYS

Note:

At least three samples should be tested at each selected age as per IS: 516-1959 Which means three samples in 7 days, three samples at 14 days and 28 days If the strength of one sample exceeds 15% of the average force, such a sample should be rejected.

click to know about Facts On Concrete Strength Test

Must read: Compaction factor test | Workability test for concrete | Fresh concrete test

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