MCQs on Concrete Operations — Page 3
Questions 35–50 — Tap an option to check your answer.
Q35: Non-uniform compaction may cause the concrete to become:
Difficulty: Medium
Correct Answer: D. All options are correctSolution:Non-uniform compaction leaves some areas with high air voids (porous), others with uneven aggregate distribution (non-homogeneous), and the presence of voids in any zone reduces the overall structural strength. All three effects occur simultaneously.
Q36: The process of hardening the concrete by keeping its surface moist is known as:
Difficulty: Easy
Correct Answer: C. CuringSolution:Curing is the specific process of maintaining sufficient moisture in concrete during the early hardening period to enable continued hydration and full strength development. It is distinct from ‘wetting’ (general water application) and ‘placing’ (depositing concrete).
Q37: Of keeping concrete wet to enable it to attain full strength is known as:
Difficulty: Easy
Correct Answer: A. CuringSolution:Keeping concrete continuously wet (or moisture-controlled) to allow it to reach its full design strength is the definition of curing. ‘Wetting’, ‘drenching’, and ‘quenching’ are general water-application terms not used in this technical context.
Q38: Curing a concrete for long period ensures better:
Difficulty: Easy
Correct Answer: D. All options are correctSolution:Extended curing maximises hydration, which simultaneously improves strength (more C-S-H gel formed), water resistance (fewer capillary pores), and volume stability (reduced drying shrinkage as pores fill with hydration products).
Q39: Curing:
Difficulty: Easy
Correct Answer: D. All options are correctSolution:Curing simultaneously reduces shrinkage (by maintaining moisture that prevents rapid drying), preserves concrete properties (by enabling complete hydration), and prevents moisture loss by evaporation (which is the mechanical means by which it achieves the first two).
Q40: Curing of concrete can be done by:
Difficulty: Easy
Correct Answer: D. Any of theseSolution:Spraying, ponding, and covering with moist cloth are all valid and commonly used wet curing methods. Each provides moisture to the concrete surface to enable continued hydration. The choice depends on the surface geometry, climate, and project conditions.
Q41: Steam curing is not used with:
Difficulty: Medium
Correct Answer: C. High alumina cementSolution:Steam curing accelerates hydration through elevated temperature and humidity. However, with High Alumina Cement (HAC), applying external heat triggers a ‘conversion’ reaction where the metastable hydration products transform to more stable but weaker phases, causing a drastic and irreversible loss of strength. Steam curing is compatible with OPC, RHC, PPC, and PSC.
Q42: While compacting the concrete by a mechanical vibrator, the slump should not exceed ________ cm:
Difficulty: Medium
Correct Answer: B. 5.0 cmSolution:IS 2505 and standard practice specify that mechanical vibrators should be used for concrete mixes with a slump not exceeding 50 mm (5.0 cm). Beyond this, the mix is too fluid and vibration will cause segregation — heavy aggregates sink, cement paste rises.
Q43: The object of curing is NOT to:
Difficulty: Easy
Correct Answer: B. Reduce the strength of concreteSolution:Curing aims to INCREASE strength by maintaining moisture for hydration. Reducing strength is the exact opposite of curing’s purpose. Options A, C, and D are all genuine objectives of curing.
Q44: In concrete, while hand mixing is adopted, excess cement to be added is:
Difficulty: Easy
Correct Answer: B. 10%Solution:Hand mixing is less efficient than machine mixing, resulting in some cement loss and non-uniform mixing. To compensate for this inefficiency and ensure the concrete achieves its design strength, IS 456 and standard practice recommend adding 10% extra cement when mixing by hand.
Q45: Ready mix plant and central mix plant differ in:
Difficulty: Medium
Correct Answer: A. TransportationSolution:In a Ready Mix Plant (RMC), materials are batched at the plant and mixing occurs (partially or fully) in the transit mixer during transportation to the site. In a Central Mix Plant, complete mixing (including water addition) is done at the plant before discharge into transport vehicles. The key difference is the transportation stage — RMC mixes in transit; central mix completes mixing at the plant.
Q46: Which of the following methods is NOT used for measuring air content in fresh concrete?
Difficulty: Medium
Correct Answer: A. Blaine air permeability methodSolution:The Blaine air permeability apparatus measures the fineness (specific surface area) of dry cement — not air content of fresh concrete. The three standard methods for measuring air content in fresh concrete are: Gravimetric method (IS 1199), Pressure method (IS 1199), and Volumetric method (IS 1199).
Q47: Separation of coarse aggregates from mortar during transportation is known as:
Difficulty: Easy
Correct Answer: C. SegregationSolution:Segregation is the separation of the constituent materials of concrete — specifically the coarse aggregate from the cement-sand mortar. During transportation over rough terrain, dropping from height, or due to excessive vibration, the heavier aggregates tend to settle and separate from the lighter mortar.
Q48: Bleeding is good to an extent if it occurs when concrete is:
Difficulty: Medium
Correct Answer: C. PlasticSolution:Minor bleeding while concrete is still in the plastic (freshly placed, not yet set) state can be tolerable — it helps lubricate the surface for finishing and is a natural result of settlement. However, excessive bleeding is detrimental as it creates a weak laitance layer at the top and increases the effective w/c ratio locally. Bleeding during transportation or after placing creates more serious problems.
Q49: Excess vibration during compacting of concrete can lead to:
Difficulty: Medium
Correct Answer: B. Segregation (Note: Commission invalidated this question in the revised answer key)Solution:Note: This question was marked as invalidated by the examining commission. However, in most standard texts, the primary consequence of over-vibration is segregation — heavy aggregates sink, cement paste and water rise. Bleeding can also occur as a secondary effect. The commission likely invalidated it due to ambiguity between options A and B.
Q50: The risk of segregation is more for:
Difficulty: Medium
Correct Answer: D. All options are correctSolution:All three factors increase segregation risk: (A) Wetter mixes have less cohesion, making it easier for heavy aggregates to settle; (B) Larger maximum aggregate size means heavier particles that settle faster in the paste; (C) Coarser cement grinding produces a less cohesive paste that cannot hold aggregates in suspension as effectively.