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**The compressive strength of the concrete cube test provides **an idea about all the characteristics of concrete. By this single test one judge that whether Concreting has been done properly or not. Concrete compressive strength for general construction varies from 15 MPa (2200 psi) to 30 MPa (4400 psi) and higher in commercial and industrial structures.

Compressive strength of concrete depends on many factors such as water-cement ratio, cement strength, quality of concrete material, quality control during the production of concrete, etc.

Test for compressive strength is carried out either on a cube or cylinder. Various standard codes recommend a concrete cylinder or concrete cube as the standard specimen for the test. American Society for Testing Materials ASTM C39/C39M provides Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens.

**Compressive Strength Definition**

Compressive strength is the ability of material or structure to carry the loads on its surface without any crack or deflection. A material under compression tends to reduce the size, while in tension, size elongates.

**Compressive Strength Formula**

Compressive strength formula for any material is the load applied at the point of failure to the cross-section area of the face on which load was applied.

**Compressive Strength = Load / Cross-sectional Area**

**Procedure: Compressive Strength Test of Concrete Cubes**

For cube test 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 most of the works cubical molds of size 15cm x 15cm x 15cm are commonly used.

This concrete is poured in the mold and appropriately tempered so as not to have any voids. After 24 hours, molds are removed, and test specimens are put in water for curing. The top surface of these specimen should be made even and smooth. This is done by placing cement paste and spreading smoothly on the whole area of the specimen.

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

**Following are the procedure for testing the Compressive strength of Concrete Cubes**

**Apparatus for Concrete Cube Test**

Compression testing machine

**Preparation of Concrete Cube Specimen**

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

**Specimen**

6 cubes of 15 cm size Mix. M15 or above

**Mixing of Concrete for Cube Test**

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

**Hand Mixing**

- Mix the cement and fine aggregate on a watertight none-absorbent platform until the mixture is thoroughly blended and is of uniform color.
- Add the coarse aggregate and mix with cement and fine aggregate until the coarse aggregate is uniformly distributed throughout the batch.
- 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 and apply oil.
- Fill the concrete in the molds in layers approximately 5 cm thick.
- Compact each layer with not less than 35 strokes per layer using a tamping rod (steel bar 16mm diameter and 60cm long, bullet-pointed at lower end).
- Level the top surface and smoothen it with a trowel.

**Curing of Cubes**

The test specimens are stored in moist air for 24 hours and after this period the specimens are marked and removed from the molds and kept submerged in clear freshwater until taken out prior to the test.

**Precautions for Tests**

The water for curing should be tested every 7 days and the temperature of the water must be at 27+-2oC.

**Procedure for Concrete Cube Test**

- Remove the specimen from the water after specified curing time and wipe out excess water from the surface.
- Take the dimension of the specimen to the nearest 0.2m
- Clean the bearing surface of the testing machine
- Place the specimen in the machine in such a manner that the load shall be applied to the opposite sides of the cube cast.
- Align the specimen centrally on the base plate of the machine.
- Rotate the movable portion gently by hand so that it touches the top surface of the specimen.
- Apply the load gradually without shock and continuously at the rate of 140 kg/cm
^{2}/minute till the specimen fails - Record the maximum load and note any unusual features in the type of failure.

**Note:**

Minimum three specimens should be tested at each selected age. If the strength of any specimen varies by more than 15 percent of average strength, the results of such specimens should be rejected. The average of three specimens gives the crushing strength of concrete. The strength requirements of concrete.

**Calculations of Compressive Strength**

Size of the cube =15cmx15cmx15cm

Area of the specimen (calculated from the mean size of the specimen )=225 cm^{2}

Characteristic compressive strength(f ck)at 7 days =

Expected maximum load =fck x area x f.s

Range to be selected is .......................

Similar calculation should be done for 28 day compressive strength

Maximum load applied =..........tones = .............N

Compressive strength = (Load in N/ Area in mm^{2)}=...............N/mm^{2}

=............................N/mm^{2}

**Reports of Cube Test**

- Identification mark
- Date of test
- Age of specimen
- Curing conditions, including date of manufacture of specimen
- Appearance of fractured faces of concrete and the type of fracture if they are unusual

**Results of Concrete Cube Test**

Average compressive strength of the concrete cube = .............N/ mm^{2} (at 7 days)

Average compressive strength of the concrete cube =.......... N/mm^{2} (at 28 days)

**Compressive Strength of Concrete at Various Ages**

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

Age | Strength percent |

1 day | 16% |

3 days | 40% |

7 days | 65% |

14 days | 90% |

28 days | 99% |

**Compressive Strength of Different Grades of Concrete at 7 and 28 Days**

Grade of Concrete | Minimum compressive strength N/mm^{2} at 7 days | Specified characteristic compressive strength (N/mm^{2}) at 28 days |

M15 | 10 | 15 |

M20 | 13.5 | 20 |

M25 | 17 | 25 |

M30 | 20 | 30 |

M35 | 23.5 | 35 |

M40 | 27 | 40 |

M45 | 30 | 45 |

**Some Facts on Concrete Strength Test**

**Why Compressive Strength Test of Concrete is Important?**

The compressive strength of the concrete cube test provides an idea about all the characteristics of concrete. By this single test one judge that whether Concreting has been done properly or not.

**What is compressive strength of commonly used concrete?**

Concrete compressive strength for general construction varies from 15 MPa (2200 psi) to 30 MPa (4400 psi) and higher in commercial and industrial structures.

**What is compressive strength after 7 days and 14 days?**

Compressive strength achieved by concrete at 7 days is about 65% and at 14 days is about 90% of the target strength.

**Which test is most suitable for concrete strength?**

A concrete cube test or concrete cylinder test is generally carried out to assess the strength of concrete after 7 days, 14 days or 28 days of casting.

**What is the size of concrete cubes used for testing?**

For cube test 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 most of the works cubical molds of size 15cm x 15cm x 15cm are commonly used.

**Which machine is used for concrete strength test?**

The compression testing machine is used for testing the compressive strength of concrete.

**What is the rate of loading on compression testing machine?**

Load should be applied gradually at the rate of 140 kg/cm2 per minute till the Specimens fails.

**Which ACI Code is use for Concrete Strength Test?**

American Society for Testing Materials ASTM C39/C39M provides Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens.

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