Quality control in compressive strength testing: the preliminary test set is repeated if the range between the highest and lowest strengths among three specimens exceeds approximately how much?

Introduction / Context:
Consistency in compressive strength testing of concrete or cement mortars is vital for trustworthy acceptance decisions. Most standards prescribe limits on the spread (range) within a set of nominally identical specimens; exceeding the limit indicates test or specimen anomalies.

Given Data / Assumptions:

• Three nominally identical compressive specimens are tested.
• Preliminary testing context for mix qualification.
• Decision criterion is based on the difference between maximum and minimum strength.

Concept / Approach:
A practical threshold often used in preliminary checks is a maximum allowable range of about 15 kg/cm² among three results. If the spread exceeds this value, the batch is considered inconsistent and the test is repeated to rule out outliers caused by casting, curing, or testing errors.

Step-by-Step Solution:
Compute range = max - min strength among the three cubes/prisms.Compare with threshold ≈ 15 kg/cm².If range > 15 kg/cm² → repeat preliminary test set.

Verification / Alternative check:
Laboratory practice frequently uses similar maxima for small specimen sets to ensure reliability before proceeding to full acceptance lots.

Why Other Options Are Wrong:

• 5, 8, 10, 12 kg/cm²: too stringent for typical field variability; would trigger excessive retesting.

Common Pitfalls:
Ignoring curing uniformity; mixing end effects; capping/grinding errors; and loading rate deviations, all of which can inflate variability.

Final Answer:
15 kg/cm²