What is cube test? When is it done? | Q & A

What Is a Cube Test?

A cube test is a quality control test conducted on concrete to determine its compressive strength and to verify whether the concrete has achieved the specified design grade (M20, M25, etc.).

It is one of the most important tests in RCC construction.

Purpose of Cube Test

• To check compressive strength of concrete

• To confirm concrete quality

• To ensure compliance with design specifications

• To decide acceptance or rejection of concrete work

When Is the Cube Test Done? 1. During Concreting (Casting Stage)

• Cubes are cast at site from fresh concrete

• Standard cube size:

  • 150 mm × 150 mm × 150 mm

• Usually:

  • 3 cubes per sample

2. Testing Ages

Cubes are tested in a compression testing machine (CTM) at:

Age              Purpose

7 days	Early strength check
28 days	Final acceptance strength

(Sometimes 3-day test for early assessment)

Cube Test Procedure (Brief)

1. Concrete poured into cube mould in 3 layers

2. Each layer compacted (tamping rod or vibrator)

3. Cubes demoulded after 24 hours

4. Cured in water until testing day

5. Tested under CTM for compressive strength

Acceptance Criteria (Interview Level)

• Average of 3 cubes ≥ characteristic strength

• Individual cube ≥ 0.8 × characteristic strength
  (As per IS 456)

One-Line Interview Answer (Perfect)

Cube test is conducted to determine the compressive strength of concrete; cubes are cast during concreting and tested at 7 and 28 days to verify the concrete grade.

Common Interview Follow-Up Questions

Q: Why are 3 cubes cast?
A: To take average value and reduce testing error.

Q: What if cube strength fails?
A: Conduct further tests like core test or NDT as directed by consultant.