Special Concretes MCQ with answer and solution

Concept

Properties and mix proportioning principles of High Strength Concrete (HSC).

Step-by-step solution

1. Analyze the characteristics of High Strength Concrete (HSC). 2. HSC uses a low water-cement ratio (often < 0.35) and high cement/binder content along with mineral and chemical admixtures. 3. Smaller maximum aggregate size is often preferred in HSC to reduce the stress concentration at the aggregate-paste interface. 4. Due to dense microstructure and low permeability, HSC is more prone to explosive spalling under high temperatures (fire). 5. HSC exhibits high resistance to chemical attacks, including the alkali-silica reaction (ASR). 6. Therefore, statement (B) is false.

Explanation

High strength concrete (HSC) requires a very low water-cement ratio to achieve high density and strength, which is typically achieved using high cementitious material content combined with superplasticizers, not high water content. Option (B) states a 'low cement content and high water content', which is characteristic of weak, low-grade concrete and would drastically reduce strength.

Why the other options are wrong

A: Smaller maximum size of aggregate improves bond strength and homogeneity in HSC. C: Dense matrix traps moisture, causing high susceptibility to fire-induced spalling. D: Low permeability limits the ingress of moisture and ions, providing high resistance to alkali-silica reaction.

Shortcut

Remember: High strength means low water-cement ratio, hence low water and high cement/binder content.

Common mistake

Confusing high-strength concrete mix design with normal concrete mixes, assuming high strength requires more water for workability (which is solved using superplasticizers instead).