How to Check Compressive Strength of Concrete:

The Compressive strength of concrete cube test is very important to test that provides an idea about all the characteristics of concrete. By performing this test we can predict about manufactured Concrete is done properly or not. The Concrete strength for normal construction work varies from 15 MPa (2200 psi) to 30 MPa (4400 psi) and more in commercial and industrial structures.

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Test for compressive strength is can be dobe by either on cube or cylinder. There is various standard codes recommends concrete cylinder or concrete cube as the standard specimen for the test. The American Society for Testing Materials of construction ASTM C39/C39M provides Standard Test Method for Compressive Strength of cube and Cylindrical Concrete Specimens.

**Compressive Strength Definition:**

Compressive strength can be defined as the ability of material or structure to carry the loads on it without any crack or deflection. A material under compressive load tends to reduce the size, while in tension, size elongates.

**Compressive Strength Formula:**

Check Compressive Strength of Concrete of any element can be calculated by dividing load applied at the point of failure to the cross-section area of the face on which load was applied.

Compressive Strength = Load at failure / Cross-sectional Area of element

**Procedure:**

Compressive Strength Test of Concrete Cubes

For cube test of concrete two types of specimens either cubes of 15cm X 15cm X 15cm or 10cm X 10cm x 10cm depending upon the size of aggregate are used for concrete making. For most of the concrete works cubical moulds of size 15cm x 15cm x 15cm are commonly used.

This concrete is filled in the mould and tempered properly so as to minimise any air voids available in concrete and then After 24 hours these moulds are opened and test specimens are soaked in water for curing. The upper surface of these specimen should be made even and smooth. This is done by spreading cement paste on whole area of specimen.

These cubes are tested by compression testing machine after 7 days curing or 28 days curing. The Load on cube should be applied gradually at the rate of 140 kg/cm2 per minute till the Specimens fails. Load at the failure of cube divided by area of specimen gives the compressive strength of concrete.

Following is the method for testing Compressive strength of Concrete Cubes

**Apparatus for Concrete Cube Test:**

Standard Compression testing machine

**Preparation of Concrete Cube Specimen:**

The proportion and material for making these test specimens are should be taken from the same concrete used in the field.

**Specimen:**

Minimum 9 cubes of 15 cm size Max. M15 or above

**Mixing of Concrete for Cube Test:**

Mix the concrete can be done by hand or in a laboratory batch mixer

**Hand Mixing:**

Mix the coarse aggregate, cement and fine aggregate on a water platform until the mixture is thoroughly blended and is of uniform color.

After that add water and mix it until the concrete appears to be homogeneous and of the desired consistency.

**Sampling of Cubes for Test:**

Clean the mounds of concrete and apply oil. Fill the concrete in the molds in 3 layers. Compact each layer of mix with not less than 35 strokes per layer using a tamping rod (Steel rod 16mm diameter and 60cm long, bullet pointed at lower end) The top level of concrete cube and smoothen it with a trowel.

**Curing of Cubes:**

**Curing of Concrete Cube:**

The test specimens are stored in moist air atmosphere for 24 hours and after this period the specimens are given marking and removed from the molds and kept submerged in fresh water until taken out for testing.

**Precautions for Tests:**

The water used for curing should be tested every 7 days and the temperature of water must be at 27+-20 C.

**Procedure for Concrete Cube Test:**

- Remove the concrete cube from water after specified curing time and clear out excess water from the surface.
- Measure the dimension of the specimen to the nearest 2mm.
- Clean the test surface of the testing machine.
- Place the cube sample in the machine in such a manner that the load shall be applied to the opposite sides of the cube cast.
- Place the specimen centrally on the base plate of the machine.
- Rotate the machine movable portion gently by hand so that it touches the top surface of the specimen.
- Apply the load gradually to cube without shock and continuously at the rate of 140 kg/cm2/minute till the specimen fails.
- Record the breaking load and note any unusual features in the type of failure.

**Calculations of Compressive Strength**:

Check Compressive Strength of Concrete

Size of the concrete cube = 15 cm x 15 cm x 15 cm

Area of the cube sample =225 cm2

Characteristic compressive strength(f ck)at 7 days =…………. (20 N/mm2)

Expected maximum load =fck x area x f.s

Range to be selected is …………………..

Same calculation should be done for 28 day compressive strength

Maximum load applied =………….KN(400KN)

Compressive strength = (Load in N/ Area in mm2)=……………N/mm2(400/22.5)

=……………………….N/mm2(17.77 N/mm2)

**Reports of Cube Test:**

Identification mark : M-20, date

Date of test:

Age of specimen: 3, 7,28 days

Results of Concrete Cube Test:

Avg. compressive strength of the concrete cube = ………….N/ mm2 (at 7 days)

Avg. compressive strength of the concrete cube =………. N/mm2 (at 28 days)

Compressive Strength of Concrete days:

The strength of concrete is increases with age. Following table shows the strength of concrete at different ages in comparison with the strength at 28 days after casting.

**Percent of strength increases in days**

1 day – 16 %

3 day – 30 %

7 days – 65 %

14 days – 90 %

28 days – 99 %

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