MCQs on Physical Quality of Concrete – Part 1

Answer: A. Blaine air permeability method

The Blaine air permeability apparatus measures the fineness (specific surface area) of dry cement — not air content. The three standard methods for measuring air content in fresh concrete are: Gravimetric method, Pressure method, and Volumetric method (all per IS 1199).

Answer: C. Segregation

Segregation is the separation of the constituent materials of concrete — specifically the coarse aggregate from the cement-sand mortar. It commonly occurs during transportation when concrete is jolted or dropped, causing heavy aggregates to settle and separate. IS 456 limits the drop height to 1.5 m to prevent segregation.

Answer: C. Plastic

Minor bleeding while concrete is still in the plastic state (freshly placed and workable) is tolerable. It helps lubricate the surface for finishing operations. However, excessive bleeding at any stage reduces impermeability, weakens the top surface layer (laitance), and creates capillary channels that make the concrete porous.

Answer: B. Segregation

Note: The commission's revised answer key considered this question incorrect/invalidated. In standard texts, the primary result of over-vibration is segregation — heavy aggregates sink while cement paste and water rise. Bleeding is a closely related secondary effect. The standard accepted answer for this question is (b) Segregation.

Answer: D. All options are correct

All three factors increase segregation risk: (A) Wetter mixes have lower cohesion; (B) Larger maximum aggregate size means heavier particles that settle faster; (C) Coarser cement grinding produces a less cohesive paste that cannot suspend aggregates as effectively. Additionally, segregation increases voids — 2% voids reduces strength by 10%, 5% voids by over 30%.

Answer: B. Only B (Impermeability is NOT a property of fresh concrete)

Impermeability is a property of hardened concrete, not fresh concrete. Fresh concrete is plastic and fluid — impermeability is irrelevant at this stage. Once concrete hardens, its resistance to water permeation (impermeability) becomes a critical durability property.

Answer: B. Bleeding

Bleeding is the phenomenon where free mix water rises to the surface of freshly placed concrete due to the settlement of heavier solid particles (cement, aggregate). The water on the surface also carries fine particles of cement and sand. Bleeding makes concrete porous by creating upward capillary channels.

Answer: C. 150 cm

IS 456:2000 specifies that to prevent segregation, concrete should not be dropped/placed from a height greater than 150 cm (1.5 m). When dropped from greater heights, the kinetic energy of impact causes coarse aggregates to bounce and roll away from the cement-sand mortar.

Answer: D. Amount of water added during mixing

The amount of mixing water is the most important factor affecting concrete shrinkage. Water that is not consumed by cement hydration must eventually evaporate, and this loss of volume causes drying shrinkage. A higher water-cement ratio means more free water, more evaporation, and more shrinkage. Aggregate (fine and coarse) actually restrains shrinkage.

Answer: D. Cement-aggregate ratio

Durability of concrete is directly proportional to the cement-aggregate ratio. More cement relative to aggregate means a richer mix with better binder content, denser paste, and lower permeability — all of which extend the service life. However, durability is also heavily influenced by permeability, which is inversely related to w/c ratio.

Answer: D. Both cider and vinegar

Cider (apple-based acidic drink) and vinegar (acetic acid solution) are both acidic substances that react with calcium hydroxide (Ca(OH)₂) in hardened concrete, degrading the cement matrix and reducing durability. Alcohol (option C) does not significantly attack concrete under normal conditions.