Determining the compressive strength is an effective way to evaluate how much load material or surface can withstand.

**Concrete compressive strength for the general construction ranges from 22 N/mm2 to 25 N/mm2 or 15 MPa to 30 MPa or 2200 psi to 4400 psi, where it goes higher in industrial and commercial structures.**

The compressive strength of concrete is defined as the resistance to failure under the decided force. It is the maximum load applied at any cross-sectional area. You can calculate the compressive strength of concrete by the formula,

F = P / A

Where,

F is the compressive strength

P is the maximum load applied in Newton

A is the cross-sectional area

Generally, the concrete mix may be designed to get the required durability and engineering properties. Therefore, testing the compressive strength after 28 days of pouring concrete is pretty necessary. Some of the other engineering properties of hardened concrete involve density, tensile strength, creep coefficients, elastic modulus, coefficients of thermal expansion, etc.

**What Is The Reason Of Testing Concrete Compressive Strength After 28 Days?**

Concrete gains strength with the passing time after pouring and casting. Concrete takes too much time to gain 100% strength, and nobody can estimate time. That is why it often remains unknown.

The rate to gain concrete compressive strength is higher during the first 28 days of casting, which gradually slows down. A table shows concrete compressive strength gain after one day, two days, seven days, 14 days, and 28 days considering the concrete grade you use.

Days After Casting | Compressive Strength Gained |
---|---|

One day | 16% |

Three Days | 40% |

Seven Days | 65% |

14 Days | 90% |

28 Days | 99% |

**Compressive Strength Gained After Casting**

As per the table, you can observe that concrete gains around 16 percent strength in one day of pouring, 40 percent in three days, 65 percent in seven days, 90 percent in 14 days, and about 99 percent in 28 days.

Hence it is clear that concrete strength gains faster in the initial days after pouring. Even if its strength reaches up to 99% in 28 days, concrete keeps gaining strength afterwards. But the rate of gain in compressive strength is lesser than the initial 28 days.

That is why the test of concrete compressive strength is done after 28 days to check whether it has achieved the desired strength. There is no clear evidence of when concrete gains 100% strength, but the assumption based on empirical data says that it takes around one year.

**Calculation Of Concrete Compressive Strength**

The basic formula used to evaluate the compressive strength of concrete is:

Compressive strength F = load at failure P / Cross-sectional area A

To test the concrete, two types of specimens such as 15 cm * 15 cm * 15 cm or 10 cm * 10 cm * 10 cm cubes are used. But generally, cubical moulds of size 15 cm * 15 cm * 15 cm are used. Load on the cube is applied at the rate of 140 kg / square cm per minute till the specimen fails.

Here, load at the failure of a concrete cube divided by the area gives the result of compressive strength of concrete. Let us understand briefly with the example below –

Size of the cube = 15 cm * 15 cm * 15 cm

Area of the cube = 225 square cm or 22500 square mm

The same calculation must be done for 28 days strength as below –

Load at failure or Maximum load applied = 400 KN or 4,00,000 N (400 * 1000)

Thus,

Compressive strength = load in N / Area in square mm = 4,00,000 / 22500 N/Sq. mm

Which is equal to 17.77 N / square mm.

In this way, experts calculate the compressive strength of concrete during the test done after casting. The strength varies with the grade of concrete and the load applied. Besides, many more factors affect the rate of strength gain.

**What Are The Factors Affecting Compressive Strength Gain?**

Below is a list of some critical factors that impact the rate of compressive strength gain –

**Size Of Aggregates**

Concrete is made by combining cement, aggregates, water, sand, and other ingredients. But even with the appropriate mixing, you can observe some micro cracks. They develop because of the thermal and mechanical properties of cement and aggregates mix, leading to concrete failure.

Technologists of concrete suggested theoretical implications as per the size of aggregates. So if the aggregate size is increased, it will lead to increased compressive strength. But this theory was disapproved later because experiments proved that greater aggregate size showed increased stability in the initial days but saw reduction gradually.

The big reason for this strength drop was the decreased surface area for associated bonding strength between aggregates and cement matrix.

**Air Entrainment**

Concrete air entrainment was one of the concepts developed by cold nations to prevent damages because of thawing and freezing. Next, experiments show multidimensional benefits of air entrainment with the improved workability of concrete at a low ratio of water to cement.

The needed workability at low water contents helped acquire high strength of concrete, which leads to light concrete with great compressive strength.

**Water To Cement Ratio**

Everybody might know about the danger of excess water to concrete strength. Cement plays a significant role as a binding material in concrete that requires water for the hydration procedure but is limited to around 0.20% to 0.25% of cement content.

Excess water turns out to be beneficial in contributing to the workability and finishing of concrete. Excess water is considered dangerous because the drying of water in a concrete matrix leaves large spaces in between cement and aggregate. This interstitial space becomes primary cracks during the test of concrete strength.

**You can check compressive strength of concrete as per their grades in 3, 7, 14, and 28 days respectively in the table below –**

Concrete Grades | 3 days strength | 7 days strength | 14 days strength | 28 days strength |
---|---|---|---|---|

M 10 | 4 | 6.5 | 9 | 10 |

M 15 | 6 | 9.75 | 13.5 | 15 |

M 20 | 8 | 13 | 18 | 20 |

M 25 | 10 | 16.25 | 22.5 | 25 |

M 30 | 12 | 19.5 | 27 | 30 |

M 35 | 14 | 22.75 | 31.5 | 35 |

M 40 | 16 | 26 | 36 | 40 |

M 45 | 18 | 29.25 | 40.5 | 45 |

M 50 | 20 | 32.5 | 45 | 50 |