What is Compressive Strength?

The compressive strength of concrete is a fundamental property that plays a crucial role in determining the structural integrity and durability of concrete structures. 

It is a measure of the concrete's ability to withstand axial loads or forces that tend to squeeze or crush the material. 

Compressive strength is one of the key factors considered in the design and assessment of concrete structures, and it is vital in ensuring that concrete can support the applied loads without failing.

This piece of writing will cover all about the Compressive Strength of concrete. This topic is important for all the upcoming civil engineering examinations, including SSC JE CE and RRB JE Civil.

What is Compressive Strength?

Compressive strength is the ability of concrete to resist crushing or compression under a load. 

It is one of the most important mechanical properties of concrete, as it helps determine the load-carrying capacity of structures such as columns, beams, slabs, and foundations.

 The strength is measured in terms of the maximum stress the concrete specimen can withstand before failure.

 Concrete with higher compressive strength is more durable and suitable for heavy structures like bridges, dams, and multi-storey buildings.

How is Compressive Strength Measured?

The compressive strength of concrete is determined using a cube or cylinder that is tested in a compression testing machine (CTM). 

A standard-sized specimen is cast, cured under specific conditions, and then crushed in the CTM after a set period (usually 7 or 28 days). 

The load at failure is recorded and divided by the cross-sectional area to calculate the compressive strength.

 The standard IS code for this test is IS 516:1959.

Specimens Used for Compressive Strength Testing

To accurately test the compressive strength of concrete, standardized specimens are required.

 The shape and size of these specimens affect the result, as different geometries distribute stress differently during testing.

 In India, cubes are widely used, whereas cylinders are more common in countries like the USA. 

The choice depends on national standards and testing facilities. The table below shows the commonly used specimen types along with their standard dimensions and application areas:

Types of Concrete Specimens

Type of Specimen Standard Size Commonly Used In

Cube -  150 mm × 150 mm × 150 mm Indian standards (IS code)

Cylinder - 150 mm diameter × 300 mm height American standards (ASTM)

Compressive Strength Test Procedure

Conducting a compressive strength test requires careful attention to the preparation and handling of the concrete specimen. Even minor errors like air voids, improper curing, or misalignment in the testing machine can significantly affect the results. That's why following the proper procedure as per IS 516:1959 is crucial.

Here’s a step-by-step explanation of how the test is conducted:

1. Sample Preparation: The fresh concrete is poured into moulds in layers and compacted properly to eliminate air pockets.

2. Curing: After 24 hours, the specimen is demoulded and submerged in water at 27°C ± 2°C to allow proper strength development.

3. Surface Cleaning: Before testing, the cube is removed from the curing tank and dried to remove excess surface moisture.

4. Testing: The cube is placed centrally in the compression testing machine. Load is applied at a controlled rate until the specimen fails.

5. Calculation: The maximum load at failure is divided by the cross-sectional area of the cube to get the compressive strength in MPa.