Concrete Practice – choose the incorrect statement (workability, controlled concrete, and sand bulking) Identify the single incorrect statement among the following site-practice observations.
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AWorkability of the concrete mix decreases when moisture content increases
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BConcrete for which preliminary mix tests are conducted is termed controlled concrete
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CBulking of sand depends on the fineness (particle size) of the sand
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DA 1 : 6 : 12 mix is used as a lean mass concrete (e.g., in piers and foundations)
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EAll of the above
Answer
Correct Answer: Workability of the concrete mix decreases when moisture content increases
Explanation
Introduction / Context:Many site decisions hinge on correct understanding of fresh concrete behavior and aggregate characteristics. This item asks you to separate one false statement from a group of otherwise acceptable descriptions regarding workability, controlled concrete, and bulking of sand.
Given Data / Assumptions:
- No special water-reducers unless stated; observations refer to ordinary concretes.
- “Controlled concrete” implies that trial mixes and preliminary tests govern proportions.
- Bulking is the apparent volume increase of moist fine aggregate due to a water film.
- Lean mixes such as 1 : 6 : 12 are associated with mass concrete where high strength is not the primary goal.
Concept / Approach:
Workability generally increases with additional moisture (to a point), because more paste fluidity allows easier placement and compaction. Thus, claiming that workability decreases with an increase in moisture is incorrect. The other statements reflect accepted practice: controlled concrete is based on trials and tests; bulking depends on fineness with finer sands showing greater bulking; and very lean mixes like 1 : 6 : 12 are used for mass concreting purposes rather than structural RCC elements.
Step-by-Step Solution:
Assess (a): False—added water typically increases workability though it may harm strength if w/c rises excessively.Assess (b): True—trial mixes determine proportions; hence “controlled concrete”.Assess (c): True—finer sand exhibits higher bulking for the same moisture.Assess (d): True—1 : 6 : 12 is a lean mass-concrete proportion used in non-RCC works like foundations/piers.Verification / Alternative check:
Slump typically rises with water content; bulking curves show peak bulking for fine sands around 4–6% moisture; construction handbooks list lean mixes for mass works.
Why Other Options Are Wrong:
(b), (c), and (d) align with standard guidance; “All of the above” cannot be right because (a) is false.
Common Pitfalls:
Confusing “workability increases” with “strength increases”—they are not the same; ignoring that excessive water harms durability even though it may improve slump.
Final Answer:
Workability of the concrete mix decreases when moisture content increases