The compressive strength test of concrete is one of the destructive tests widely done at the site to find out the compression strength of a particular concrete batch.

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

The ability of hardened concrete cube to resist the compression loads applied on the surface is known as the compressive strength of concrete.

Otherwise, the compressive strength of concrete is defined as the maximum crushing stress endured by the concrete.

**Purpose of this Test**

Assume a slab at our site is designed to cast M25 grade of concrete, but we could not define its strength in the semi-solid state.

For that matter, A certain quantity of concrete shall be cast as a cube while concrete pouring time to find the compressive strength of that concrete batch. The cube will be preserved and curated well for almost 28 days to be tested by the Compression Testing Machine.

**Compressive Strength of Concrete Test**

**Apparatus Required**

- Steel Mould (150mm x 150mm x 150mm)
- Tamping rod
- CTM Machine
- Trowel
- Weighing machine

**Procedure**

**Cube Casting**

- Estimate the adequate ingredients to prepare the concrete with a proper water-cement ratio.
- Ensure that the cube mould should be free from dust and rust.
- Now, pour the concrete into the mould by proper compaction with the help of a tamping rod.
- Finish the top surface as smooth by the trowel.
- The mould should be covered with a gunny bag and placed undisturbed for 24 hours at a temperature of 27 ° Celsius ± 2.
- After 24 hours, the cube specimen shall be removed from the mould, and it should be immersed into the water for 7 or 28 days based on the test.

**Method of Testing**

- The cube should be removed from the water 30 minutes before the test, and it should be in dry condition.
- The specimen should be weighed before the test.
- The specimen should be placed between the plate with proper alignment.
- Now, apply the load (kilonewton) gradually to the specimen.
- The specimen will break at maximum load (1 Division = 5 KN), which is noted down as the crushing value of the cube.
- The average (Minimum Three Sample) crushing value should be note down as the compression value of the selected concrete batch.

Video Courtesy – NCTEL

**How To Calculate Compressive Strength Of Concrete Value? – Formula**

Compressive Strength of concrete cube = Maximum load/Area of the cube

**Example Calculation**

- Assume that the compression load is 375 KN (1 Kg = 9.81 N)
- Cross Sectional Area – 15 x 15 = 225 Sqcm.
- Compressive Strength = (375 x 1000/225) = 1666/9.81 = 169.82 Kg/Sqcm.

**Lab Report**

Grade |
Age of Test |
Specimen |
Load (KN) |
Tensile Strength (Kg/Sq.cm) |
Average Strength |

M10 | 28 Days | Sample 1 | 300 | 136 | 138 |

28 Days | Sample 2 | 290 | 131 | ||

28 Days | Sample 3 | 325 | 147 |

**Frequently Asked Doubts **

DAYS | PERCENTAGE OF STRENGTH |
---|---|

Day 1 | 16% |

Day 3 | 40% |

Day 7 | 65% |

Day 14 | 90% |

Day 28 | 100% |

**Gradual Difference of Compressive Strength of Concrete**

GRADE | CRUSHING STRENGTH @ 3 DAYS | CRUSHING STRENGTH @ 7 DAYS | CRUSHING STRENGTH @ 28 DAYS |
---|---|---|---|

M10 | 4 | 6.5 | 10 |

M15 | 6 | 9.7 | 15 |

M20 | 8 | 13 | 20 |

M25 | 10 | 16.25 | 25 |

M30 | 12 | 19.5 | 30 |

QUANTITY OF CONCRETE (CUM) | NUMBER OF SAMPLES |
---|---|

1-5 | 1 |

6-15 | 2 |

16-30 | 3 |

31-50 | 4 |

51 & above | 4+1 additional sample for each additional 50 Cum concrete. |

**Example -**If the concrete grade is M20, then the individual crushing value should not be less than 15 N/Sqmm. The individual crushing value difference between the cube should not be greater than 15%.

Hope you enjoyed the topic. Happy Learning 🙂