The preliminary test is repeated if the difference in compressive strength of three test specimens, exceeds:
📊 The Need for Consistency in Testing
When testing the strength of a material like cement or concrete, it's never enough to test just one sample. Due to minor, unavoidable variations in mixing, compaction, and curing, no two specimens will be perfectly identical. To get a reliable result, multiple specimens (typically three) are tested.
The results from these specimens are then averaged. However, if one specimen gives a result that is wildly different from the others, it might indicate a flaw in that specific sample or the testing procedure. To ensure the average is representative, standards set a limit on the acceptable variation between the individual test results.
📝 Detailed Analysis of the Options
The question refers to the acceptance criteria for the compressive strength test of cement mortar cubes.
(a) 5 kg/cm², (b) 8 kg/cm², (c) 10 kg/cm²
These values represent very tight tolerances. While desirable, achieving such low variation consistently in a standard lab environment is difficult. Setting the rejection limit this low would lead to an excessive number of re-tests.
(d) 15 kg/cm²
This is the correct answer. According to standard procedures for testing cement, if the range (the difference between the highest and lowest strength value) of the three tested cubes exceeds 15 kg/cm², the test results are considered unreliable and must be discarded. A new set of cubes must then be made and the test repeated.
📋 Compressive Strength Test: Acceptance Criteria
| Step | Procedure |
|---|---|
| 1. Testing | Test three mortar cube specimens for compressive strength. Let the results be S₁, S₂, and S₃. |
| 2. Calculation | Calculate the average strength: S_avg = (S₁ + S₂ + S₃) / 3. |
| 3. Variation Check | Find the range: Range = (Highest Strength - Lowest Strength). |
| 4. Decision | If Range ≤ 15 kg/cm², the test is valid. The average strength (S_avg) is reported. If Range > 15 kg/cm², the results are discarded and the test is repeated. |
💡 Study Tips
- Memorize the Limit: The value "15 kg/cm²" is a key number to remember for the cement compressive strength test's validity.
- Understand "Range": The rule applies to the difference between the maximum and minimum values, not the difference from the average.
- Why Repeat?: The goal is to ensure the reported average strength is a true representation of the cement's quality, free from outlier results caused by testing errors.