In concrete quality control, the ‘‘test strength’’ of a sample is defined as the average compressive strength of how many standard specimens (e.g., 150 mm cubes) cast from that sample?

Introduction / Context:
Acceptance of concrete strength on site relies on statistical sampling. A ‘‘sample’’ comprises multiple test specimens so that random variability is averaged out. The test strength is then compared with specified characteristic strength for compliance checks and quality adjustments.

Given Data / Assumptions:

• Standard compressive strength specimens are 150 mm cubes (or cylinders as specified).
• Multiple specimens are cast from a single sample of concrete.
• The test strength equals the arithmetic mean of the specimen strengths.

Concept / Approach:

Using three specimens per sample balances practicality and statistical reliability. Averaging three values reduces the influence of outliers due to minor casting or curing variations and provides a robust estimate of the sample strength.

Step-by-Step Solution:

Verification / Alternative check:

Specifications also impose limits on individual specimen results to avoid masking poor units by averaging; nevertheless, the mean of three remains the core test strength metric.

Why Other Options Are Wrong:

• 2 specimens: Too few for reliable averaging.
• 4, 5, or 6 specimens: Provide more data but are beyond the standard definition of a single sample’s test strength.

Common Pitfalls:

• Mixing specimens from different batches into one sample.
• Not recording curing or age precisely, which skews results.

Final Answer:

3 specimens