**What is the Compressive Strength of Concrete?**

Compressive strength of cement concrete is the strength of concrete against crushing due to direct compressive load. It is considered to be the most important property of concrete and is often taken as the index of the overall quality of the concrete.

**Factors Affecting Compressive Strength of Concrete**

- Type and fineness of cement
- Strength, size, shape, surface texture and grading of aggregate
- Water to cement ratio
- Aggregate to cement ratio
- Curing time and temperature
- Age of concrete

**Compressive Strength Test**

The compressive strength test is performed on 15 cm × 15 cm × 15 cm cube, Metalic mould is used for casting concrete cubes.

Fresh concrete is filled in three layers in a mould and each layer is compacted 25 times by tamping road, sometimes vibration table is used for compaction of the cube.

After that cube is kept in a wet room whose temperature is about 27-degree Celsius. After that crushing is done in the usual way.

The cube can be tested after 3, 7, 14 and 28 days. Cube is placed in compression testing machine and axial load is applied at the rate of 140 kg/ cm2.

In actual concrete is required sufficient time to gain strength. Usually 28 days strength is considered as the full strength of concrete. Sometimes the strength of concrete may be required to be tested earlier, generally 7 days.

**Apparatus for Compressive Strength Test**

- Cube Mould (150 mm × 150 mm × 150 mm or 100 mm ×100 mm × 100 mm)
- Tamping Rod (Dia. 16mm, Height 600mm)
- Compression Testing Machine

**Example:**

Grade of concrete = M25

Cube size = 150mm×15mm×150mm

Age of cube = 28 days

S.No. | Date of Casting | Date of Testing | Load (KN) | Area of Cube(150×150) mm^{2} | Compressive Strength(N/mm^{2}) = Load/Area | Remark |

1. | 11/10/18 | 08/11/18 | 610 | 22500 | (610×1000)/22500= 27.11 | |

2. | 11/10/18 | 08/11/18 | 580 | 22500 | (580×1000)/22500= 25.77 | Avg. =26.96N/mm2 |

3. | 11/10/18 | 08/11/18 | 630 | 22500 | (630 ×1000)/22500= 28 |

**Notes:**

- The strength of any individual cube should not vary by more than 15% of average % the set of three cubes. If it is more than 15% then it should be considered fail.
- Three specimens shall be made for each sample.

**Frequency of Sampling**

S.No. | Quantity of concrete in the work (m3) | Number of samples |

1 | 1-5 | 1 |

2 | 6-15 | 2 |

3 | 16-30 | 3 |

4 | 31-50 | 4 |

5 | 51 and above | 4 + one additional sample for each additional 50 m3 |

**Compressive Strength of Concrete at Various Ages**

Concrete Age | Strength % |

1 Day | 16% |

3 Days | 40% |

7 Days | 65% |

14 Days | 90% |

28 Days | 99% |

**Compressive Strength of Different Grade of Concrete at 7 days and 28 days**

Concrete Grade | Minimum compressive strength at 7 days (N/mm2) | Characteristic compressive strength at 28 days(N/mm2) |

M-15 | 10 | 15 |

M-20 | 13.5 | 20 |

M-25 | 17 | 25 |

M-30 | 20 | 30 |

M-35 | 23.5 | 35 |

M-40 | 27 | 40 |

M-45 | 30 | 45 |

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