Understanding Concrete Cube Sampling as per IS 456

Concrete undergoes various tests to ensure its quality, and one crucial examination is the compressive strength test. This test provides insights into the concrete’s strength, especially when it comes to on-site pouring. A common concern for site engineers during this test is determining the appropriate number of concrete cubes to cast. In this article, we will explore the process of Concrete Cube Sampling in line with the guidelines set by IS 456, offering practical examples to enhance understanding.

Concrete Cube Sampling Guidelines

To maintain the quality of concrete, IS 456 provides a set of guidelines for the minimum frequency of sampling based on the grade of concrete. The table below outlines the recommended number of samples for a 28-day compressive strength compliance requirement.

Concrete Cube Sampling Table

Grade of Concrete	Number of Samples (for 28-day testing)
M10 to M15	2 samples (6 cubes)
M20 to M25	2 samples (6 cubes)
M30 to M50	4 samples (12 cubes)
M60 and above	6 samples (18 cubes)

Note: Additional samples may be required for 7-day testing or when striking the formwork, as instructed by the engineer-in-charge.

Examples of Concrete Cube Sampling

Example 1: Quantity of Concrete – 14 m3

Let’s consider a scenario where the quantity of concrete is 14 m3. According to the table, the prescribed number of samples is 2 samples (6 cubes).

Hence, for each batch of 7 m3, 2 samples (6 cubes) should be cast for 28-day testing. Additionally, 1 sample (3 cubes) should be cast for 7-day testing.

Example 2: Quantity of Concrete – 220 m3

Now, let’s explore a case with a larger quantity of concrete, say 220 m3. Referring to the table, the prescribed number of samples for 220 m3 is 8 samples.

Breaking it down:

• 4 samples (16 cubes) for the first 50 m3,
• 1 sample (3 cubes) for 50 – 100 m3,
• 1 sample (3 cubes) for 100 – 150 m3,
• 1 sample (3 cubes) for 150 – 200 m3,
• 1 sample (3 cubes) for 200 – 220 m3.

For 28-day testing, 8 samples (32 cubes) should be cast for each batch, and an additional 2 samples (6 cubes) for 7-day testing, as directed by the engineer in charge.

Conclusion

Understanding and implementing Concrete Cube Sampling as per IS 456 is vital for ensuring the structural integrity of concrete. By following these guidelines and examples, site engineers can make informed decisions on the number of samples to cast, contributing to the overall quality and reliability of the concrete used in construction projects.