Measuring workability of low w/c concrete — suitable field test For concrete mixes with a low water–cement ratio (stiff mixes) where slump may be near zero, which test is more appropriate to assess workability?
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ADirect tensile strength test
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BSlump test
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CCompaction factor test
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DFlexural strength test
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ENone of these
Answer
Correct Answer: Compaction factor test
Explanation
Introduction / Context:Workability tests must match the consistency range of the concrete. Slump is insensitive for very stiff mixes because even workable low w/c concretes can show little to no slump, giving misleading results.
Given Data / Assumptions:
- Mixes have low water–cement ratios (stiff, harsh consistency) but are still compactable with vibration.
- Field-friendly testing is desired.
Concept / Approach:The compaction factor test measures the degree of compaction achieved under standard conditions and is more sensitive in the low-workability range. The slump test is better for medium consistencies. Tensile or flexural strength tests are hardened concrete tests and do not measure fresh workability.
Step-by-Step Solution:
Match test method to consistency: compaction factor for low-workability mixes.Exclude slump for near-zero slumps; it lacks resolution in this range.Exclude strength tests; they are not fresh-concrete workability measures.Verification / Alternative check:Standards and textbooks recommend compaction factor or Vebe time tests for stiff concretes.
Why Other Options Are Wrong:
- Slump: insensitive at low slump values for stiff mixes.
- Tensile/Flexural: hardened concrete properties, not workability tests.
Common Pitfalls:Relying on slump alone across all consistencies; not considering vibration effort available at site.
Final Answer:Compaction factor test