The **compressive strength test** is the most common destructive test conducted on hardened concrete because it is easy to perform and most of the properties of the concrete are related to its compressive strength.

**What is the compressive strength of concrete?**

The compressive strength of concrete is defined as the maximum crushing stress that the concrete can withstand without undergoing any failure.

Various factors can affect the compressive strength of concrete such as water-cement ratio, grade of cement, quality of water used, usage of admixtures, etc.

It is a very important parameter while designing the structural elements of a building and also can be taken as an index to indicate the **overall quality of concrete**.

**Purpose of this test**

The compressive strength test is carried out to confirm that the batch of concrete used at the site has developed the required strength.

We know that the concrete is in a semi-solid state while casting the structural elements, so there is no way to test the strength of the concrete at that time.

In order to test the strength of concrete at a later stage, test samples are made by casting **cubes or cylinders** from the actual batch of the concrete used.

Usually, these test samples are tested **after 7 and 28 days from casting**. Even though, we know the test results late, it helps us to understand the quality of the concrete used.

The concrete will usually give the required strength if we have followed proper quality control measures during its production.

However, one has to remember that this standard compressive strength gives a measure of the potential strength of concrete, **not** the actual strength of concrete in the structure.

**Compressive strength of concrete test**

**Aim**

Our main aim is to determine the compressive strength of concrete cube specimens by using a Compression Testing Machine.

**Apparatus required**

- Raw materials for concrete
- Weighing machine
- Tamping rod
- Trowel
- A concrete cube mould of 150 mm x 150 mm x 150 mm dimensions
- A Compression Testing Machine (CTM)

**Sample preparation**

The concrete samples have to be taken from the concrete batch at the site (highly recommended), if not possible, prepare the concrete mix at the lab using the same proportion.

The following steps are carried out if the concrete mix is prepared in the lab.

1) Properly **weigh the raw materials** required for preparing concrete of the desired grade according to the mix design.

The raw materials may include cement, fine aggregate (sand), coarse aggregate (gravel), and water. (Add admixtures, if any)

2) The concrete raw materials shall be **mixed** by hand, or preferably by laboratory batch mixer in such a manner that the whole mix will become homogeneous.

3) Clean the concrete mould of any dust. Either cubical or cylindrical mould can be used. If **cubical mould** is used, then the dimension of the mould shall be **150 x 150 x 150mm.**

If the largest nominal size of the aggregate used shall not exceed **20mm**, then the cube mould of 100 x 100 x 100 mm may be used as an alternative. However, for determining the compressive strength of concrete mostly the previous one shall be used.

4) Concrete is poured into the mould and compacted layer by layer to reduce the number of air voids.

5) After 24 hours, the moulds are removed and the concrete specimens are placed in water for curing.

6) These specimens are subjected to curing until they are tested on respective days like 3,7, and 28 days.

These days are calculated from the time of the addition of water to the dry ingredients of concrete.

**How many samples do you need?**

The number of samples to be prepared depends upon the quantity of concrete produced in each batch.

According to IS 456, the minimum number of samples for various quantities of concrete is listed below.

Quantity of concrete (m^{3}) | Number of samples |

1-5 | 1 |

6-15 | 2 |

16-30 | 3 |

31-50 | 4 |

51 and above | 4 + 1 sample for each additional 50 m^{3} |

However, according to IS 456 clause 15.4, we need at least 3 specimens to establish the test results.

So whatever the quantity, prepare a **minimum of 3 sample specimens** to determine the strength at respective days and additional samples according to the quantity of concrete prepared.

**Testing procedure**

1) Get the specimen out of the water and wipe out the excessive moisture on the surface of the specimen.

2) Measure the cross-sectional area of each surface

3) Take the weight of the concrete cube specimen using a weighing machine

4) Clean the bearing surface of the CTM machine

5) Place the cube in between the bearing surfaces of the CTM machine. The specimen should be placed in such a way that the load shall be applied to the face perpendicular to the casting direction.

6) The axis of the specimen shall be carefully aligned with the center line of the base plate.

7) Apply the load **gradually **without shock and increase the load continuously at the **rate of 140 kg/cm ^{2}/minute** until the specimen fails.

8) Record the maximum load applied to the specimen.

Repeat the same procedure for all the 3 samples and note it in the report.

**Calculations**

Cross-sectional area of cube = 150 mm x 150 mm = 22,500 mm^{2}

Compressive strength = Load/Area

Average compressive strength = (Compressive strength of cube 1 + Comp strength of cube 2 + Comp strength of cube 3)/3

Cube No. | Age of cube (days) | Weight of cube (kg) | Cross-sectional area (mm^{2}) | Load (N) | Compressive strength (N/mm2) | Average Compressive strength (N/mm2) |

1 | 28 | 8.102 | 22500 | 605000 | 26.88 | 26.67 |

2 | 28 | 8.053 | 22500 | 597000 | 26.53 | |

3 | 28 | 8.201 | 22500 | 599000 | 26.62 |

We have to check our result with the criteria provided in **IS 456 clause 15.4**.

**It states that the individual variation of these three specimens should not be more than or less than 15% of the average.**

**Let’s check our results:**

Average Compressive strength = 26.67 N/mm^{2}

15% of average compressive strength = 4.00 N/mm^{2}

So, the compressive strength of our 3 cubes must be in the range of

- 26.67 – 4 = 22.67 N/mm2 and
- 26.67 + 4 = 30.67 N/mm2.

Obviously, the compressive strength of our 3 cubes is in the given range. **Hence our test result is valid.**

**Factors affecting test results of the specimen**

Now as you know the test procedure, you should be aware of the factors that can potentially affect the test results. Some of the important factors are listed below.

**1) State of moisture – **The concrete specimen is subjected to curing until the day of testing. On the day of testing, the specimen should be taken out of water and the excessive water on the surface should be wiped out.

If the specimen is subjected to testing while it is **too wet**, the cohesion between the particle will be less and the test result show **lesser strength** than the original.

If the specimen is subjected to testing while it is **too dry**, the cohesion between the particle will be higher and the test result **show more strength** than the original.

**2) Position of loading – **The loading on the cube specimen **should be axial** i.e. it should pass through the center line of the specimen.

If loading becomes eccentric, then additional bending develops in the specimen. This extra load will soon crush the specimen and result in a false load.

**3) Rate of loading – **The specimen should be **gradually** loaded at the rate of 140 kg/cm^{2}/minute until the specimen fails.

If the rate of loading changes, the test result will also vary drastically due to the creep effect.

**4) Size of specimen –** When the size of the specimen increases the possibility of defects in the internal region of the specimen also increases, which results in reduced strength of concrete. Hence specimen with the correct dimension should be used.

**Frequently Asked Questions (FAQ)**

**1) What are the codes I can refer to for the compressive strength test of concrete?**

- IS 516 (1959): Method of tests for strength of concrete
- IS 1199 (1959): Method of sampling and analysis of concrete
- SP 23 – 1982: Handbook on concrete mixes
- IS 10086 (1982): Specification for moulds for use in tests of cement and concrete
- IS 456- 2000: Code of practice for plain and reinforced concrete

**2) Why should the concrete specimen be immersed in water before testing?**

The specimen should be immersed in water for **curing** purposes.

From the moment water is added to the cement, a chemical reaction called hydration takes place. This hydration process converts the powdered cement into concrete. Water is needed for the hydration process.

The water we use to mix the concrete eventually depletes due to the heat of hydration and environmental conditions.

So we maintain optimum moisture content and desirable temperature for a certain time after concrete placement, so that the hydration process may continue until the concrete can develop sufficient strength. This process is called** curing**.

**Read**: Curing of concrete – Everything you need to Know

**3) What is the purpose of weighing the cube specimen?**

You may think that we didn’t use that value anywhere in the calculation then why do I need to measure the weight?

The main purpose is to identify the **unit weight of concrete**. We know that **the unit weight of concrete is 2400 kg per cubic meter**. By measuring the weight of the cube, we can cross-check this value.

How?

Simple. Take the weight of one cube as 8 kg.

Volume of concrete specimen = 0.15m * 0.15m * 0.15m = 0.0033 m3.

If a 0.0033 m3 volume cube weighs about 8kg, then we can easily find how much the weight of concrete will be if the volume is 1m3 by cross multiplication.

0.0033 m3 | 8 kg |

1m3 | x |

By cross multiplying, we get the unit weight of concrete = 8 kg/0.0033m3 = 2424 kg/m3.

**4) What is the concrete strength at 7 days? / What is the concrete strength at 14 days?**

By conducting extensive research, scientists published a relationship between the concrete age and its strength.

Concrete Age | % of strength gained |

1 day | 16% of fck |

3 days | 50% of fck |

7 days | 65% of fck |

14 days | 90% of fck |

28 days | 99% of fck |

Where, fck = characteristic compressive strength of concrete.

In simpler words,

If you are using an M25 grade of concrete, then at the end of 28 days, you should expect a compressive strength of 25 N/mm2. Here fck = 25N/mm2.

In that case, if you are casting an M25 grade concrete, then the following strength can be attained on respective days.

M25 Grade of concrete | |

Concrete Age | % of strength gained |

1 day | 16% of 25 = 4 N/mm2 |

3 days | 50% of 25 = 12.5 N/mm2 |

7 days | 65% of 25 = 16.25 N/mm2 |

14 days | 90% of 25 = 22.5 N/mm2 |

28 days | 99% of 25 = 24.75 N/mm2 |

You can follow the same method for any grade of concrete.

Hope you understand everything you need to know about the **compressive strength test of concrete**. If you find this article helpful, let us know in the comment section.

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