Compressive strength of Concrete can be defined as the ability of a material or structure to carry the loads on it without any crack or deflection. material under compressive load tends to reduce the size, while in tension, size elongates. The compressive strength of concrete can be calculated by dividing the load applied to the concrete cube at the point of failure by the cross-section area of the cube (15x15x15 cm) on which the load was applied.

The concrete compressive strength for normal construction work varies from 15 MPa (2200 psi) to 30 MPa (4400 psi) and more in commercial and industrial structures. The strength of concrete depends on factors such as the water-cement ratio, the strength of cement use, the quality of concrete materials, quality control during the production of concrete,** **etc.

## Importance of Compressive Strength

The compressive strength test of concrete plays a vital role in ensuring the quality and durability of structures. It helps in determining the ability of concrete to withstand the loads and forces without experiencing any cracks or failures. By conducting this test, engineers can assess the structural integrity of concrete elements and make informed decisions regarding construction techniques and materials.

The results of the test provide valuable information for structural design and allow engineers to accurately calculate the load-bearing capacity of a concrete structure. Moreover, the compressive strength test also assists in quality control during the production of concrete, ensuring that the mix proportions and curing processes are optimized for achieving the desired strength.

Overall, the importance of the compressive strength test cannot be overstated, as it ensures the safety and reliability of concrete structures.

## Compressive Strength Formula

The following is the compressive strength of concrete formula,

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

**IS Code For Concrete Cube Test:**

Cube Test of Concrete as per IS Code **IS 516 (1959):** Method of Tests for Strength of Concrete and **IS 456 2000:** Concrete Strength Acceptance Criteria

## Compressive Strength of Concrete

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

This** **concrete is filled in the mould and tempered properly so as to minimize any air voids available in concrete. After 24 hours these moulds are opened and test specimens are soaked in water for curing.

The upper surface of these specimens should be made even and smooth. This is done by spreading cement paste on the whole area of the specimen. These cubes are tested by a compression testing machine after 7 days of curing or 28 days of curing.

The Load on the cube should be applied gradually at the rate of 140 kg/cm2 per minute till the specimen fails. Load at the failure of a cube divided by the area of the specimen gives the compressive strength of concrete.

**Apparatus for Concrete Cube Test**

- Standard Compression testing machine
- Cube mould of standard size 15 cm x 15 cm x 15 cm

**Preparation of Concrete Cube Specimen**

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

**1. Specimen**

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

**2.** **Mixing of Concrete for Cube Test**

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

**3.** **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.

**4. Pouring Concrete In Cubes**

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 the lower end) The top level of the concrete cube and smoothen it with a trowel.

**5. Curing of Cubes**

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

**6.** **Precautions for Tests**

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

**Concrete Cube Test Procedure**

- Remove the concrete cube from the water after the specified curing time and clear out excess water from the surface.
- Measure the dimension of the specimen to the nearest 2mm.
- Clean the
- 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’s movable portion gently by hand so that it touches the top surface of the specimen.
- Apply the load gradually to the 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 the Compressive Strength of Concrete

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

Area of the cube sample =225 cm^{2} (22500 mm^{2})

The same calculation should be done for 28 days of compressive strength

The maximum load applied or load at failure** **of cube = 400 KN (400×1000 N)

Compressive strength = (Load in N/ Area in mm2)=400×1000/22500 N/mm^{2}

= 17.77 N/mm^{2}

**Percentage of Strength Gain by Cubes In Days**

The** **following table shows the percentage gain of compressive strength of concrete at 7 days, 14 days, and 28 days

Days | % Gain Strength |

1 Day | 16 % |

3 Day | 30 % |

7 Day | 65 % |

14 Day | 90 % |

28 Day | 99 % |

**Concrete compressive strength gain after 3, 7, 14, and 28 days**

## Compressive Strength of Different Grades of Concrete

The following table shows the compressive strength of various concrete grades at different ages.

Grade of Concrete | 1 Day (16%) | 3 Day (30%) | 7 Day (65%) | 14 Day (90%) | 28 Day (28%) |

M 15 | 2.4 | 4.50 | 9.75 | 13.50 | 14.85 |

M 20 | 3.20 | 6.00 | 13.00 | 18.00 | 19.80 |

M 25 | 4.00 | 7.50 | 16.25 | 22.50 | 24.75 |

M 30 | 4.80 | 9.00 | 19.50 | 27.00 | 29.70 |

M 35 | 5.60 | 10.50 | 22.75 | 31.50 | 34.65 |

M 40 | 6.40 | 12.00 | 26 | 36.00 | 39.60 |

M 45 | 7.20 | 13.50 | 29.25 | 40.50 | 44.50 |

**Alternative Testing Methods:**

**Non-Destructive Testing (NDT):**

**Ultrasonic Testing (UT):** Utilizes high-frequency sound waves to assess the internal structure and integrity of concrete. Changes in wave velocity can indicate variations in compressive strength.

**Rebound Hammer Test:** Measures the surface hardness of concrete by rebounding a hammer off the material. The rebound distance is correlated with compressive strength.

**Penetration Resistance Testing:** Involves driving a probe into the concrete and measuring the penetration depth. The resistance encountered provides an estimate of compressive strength.

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