Special Concretes MCQ with answer and solution

Concept

Flowability and segregation resistance requirements in SCC

Formula

NONE

Step-by-step solution

1. Self-compacting concrete (SCC) must flow into formwork without mechanical consolidation while maintaining a cohesive matrix. 2. High dosages of superplasticizers (polycarboxylates) disperse cement grains, providing high deformability and flowability. 3. High powder content (materials < 0.125 mm) increases plastic viscosity and cohesive strength, preventing aggregate segregation. 4. Therefore, an optimal superplasticizer dosage combined with high powder content is the critical combination. Answer: C

Explanation

Self-compacting concrete (SCC) requires high deformability to flow under its own weight and high cohesion to prevent segregation and bleeding during flow. An optimal dosage of polycarboxylate superplasticizer achieves high deformability, while a high powder content (cement, fly ash, silica fume, or limestone powder) provides the necessary plastic viscosity and stability.

Why the other options are wrong

A) Low w/c ratio without superplasticizer produces an unworkable stiff mix that cannot self-compact. B) High superplasticizer with low powder content causes severe aggregate segregation and bleeding. D) High coarse aggregate increases particle interlocking and causes blockage across reinforcement.

Shortcut

SCC requires high flowability (superplasticizer) + high segregation resistance (powder content).

Common mistake

Assuming superplasticizer alone creates SCC, forgetting that high powder content is vital to prevent segregation and bleeding.