Concrete Cube Size vs Aggregate – approval for 100 mm test cubes As per standard practice, 100 mm (10 cm) cubes may be used for work-test compressive strength only if the engineer-in-charge approves and the maximum nominal size of coarse aggregate does not exceed which limit?

Introduction / Context:
Concrete compressive strength is commonly determined using standard cube or cylinder specimens. In Indian practice, 150 mm cubes are standard. However, smaller 100 mm cubes are sometimes permitted for work tests to save materials and curing space, provided certain conditions regarding aggregate size and authority approval are satisfied. This question checks the correct aggregate-size limit that allows the use of 100 mm cubes.

Given Data / Assumptions:

• Work test cubes for compressive strength under normal laboratory procedure.
• Standard 100 mm and 150 mm cube moulds are available.
• Engineer-in-charge may approve 100 mm cubes under codal limitations.

Concept / Approach:

The specimen must be sufficiently large compared with the maximum aggregate size to ensure representative sampling, minimal wall effects, and proper compaction without segregation. Codes and handbooks cap the use of 100 mm cubes to mixes where the maximum nominal size of coarse aggregate is not more than 20 mm. For larger aggregate, a 150 mm cube is required so that the specimen contains an adequate number of particles across the section and provides reliable strength values.

Step-by-Step Solution:

Verification / Alternative check:

Laboratory practice aligns with the general rule of keeping specimen dimension ≥ 4 to 5 times the maximum aggregate size. For a 100 mm cube, 100/20 = 5 satisfies this guidance; larger aggregate fractions would violate it for 100 mm cubes but comply with 150 mm cubes (150/25 = 6).

Why Other Options Are Wrong:

10 or 15 mm are overly restrictive and not required by practice; 25 or 30 mm exceed the limit for 100 mm cubes and may yield non-representative results due to boundary effects and poor compaction around large particles.

Common Pitfalls:

Using 100 mm cubes without checking aggregate size; not adjusting compaction energy; comparing strengths from different cube sizes without correlation.

Final Answer:

20 mm