Contents

## What is Compressive Strength?

## What is Compressive Strength of Concrete?

Compressive strength is an important property of concrete. By conducting the cube test, we can get an idea about the concrete mix has done properly or not. We can check the strength of concrete at 7th, 14th& 28th day, without damaging the structure.

### Material required for compressive strength test

**Cube mould (150mm X 150 X 150mm)****Tamping rod 60 cm long of 16mm dia****Trowel****Compression testing machine****Spoon****Vibrating machine****Trey****Weighing machine****Water****Cement****Aggregate****Sand****Record sheet****Dung spade****Oil**

## Procedure to conduct a compressive test of Concrete

- For casting of concrete cubes, first measure the dry proportions of ingredients as per given mix design. Some standard design mix-

**M5 – 1:5:10****M7.5- 1:4:8****M10- 1:3:6****M15- 1: 2:4****M20- 1:1.5:3****M25- 1:1:2**

If we talk about M25, by weight 1kg is cement, 1kg is Sand, 2kg is aggregate.

**How to Find out water amount by using water cement ratio. **

**Example, if water cement ratio is 0.50& cement is 50kg**

**Then,**

**Water/Cement=0.50**

**Water= 0.50 X 50**

**Water = 25 liters**

### Procedure to Conduct Test

- Mix the dry ingredient properly to obtain a uniform mixture; the mixture should be sufficient to cast three moulds.
- Take water as per design mix of concrete and add water in to dry mixture of ingredient.
- Now mix concrete mixture properly by using a spade to obtain a proper mixture of concrete.
- Before filling concrete into moulds, apply oil on all surface of mould to get a smooth finish of cubes.
- By using spoon or trowel fill concrete in to mould, concrete mould should be filled into 3 layers.
- Compact concrete by using the tamping road by giving 25 blows after each layer. Compaction also can be done by using table vibrator or needle vibrator.
- After filling the final layer of concrete, finish the concrete surface to the top level of the mould by using a trowel. The concrete top surface should be smooth.
- Details should be written on the concrete cube- Grade of concrete, casting date, Site name & very important is cube number.
- After some time of cube casting, the cube should be covered with wet gunny bags for 24 hrs.
- After 24 hours from casting remove concrete cubes from moulds and keep cubes into the curing tank for a period of 7, 14 or 28 days on which cube is to be tested. The temperature should be 27°C ± 2°C. Water shall be renewed after every seven days into which specimens are submerged.
- After curing, remove cube specimens from curing tank just half an hour before the test. For cube compression test specimens should be in surface dry condition.
- Place a cube in compression testing machine to conduct compression test. The axis of the specimen shall be properly aligned with the center of the spherical plates of the machine.
- Now start to apply load gradually and observe dial gauge of compression testing machine. Note the final reading at the time of cube specimen breaks. That reading is the maximum load carried by the cube which is in KN.
- Carried out the same test on all specimens and note the readings carefully on the record sheet.
- A Compressive strength of cube specimen we can find out by dividing maximum compressive load taken by cube specimen by its cross-sectional area.

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## The Compressive Strength of Concrete Formula

A Compressive Strength of Concrete cube can be calculated by dividing Maximum Load Carried by Concrete cube by Cross sectional area of Concrete Cube

### Compressive Strength of Concrete Calculation

### Result of Cube Test

- Average of Compressive Strength at 7 days = _________N/mm²
- Average of Compressive Strength at 28 days = _________N/mm²

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