Durability of Concrete MCQ with answer and solution

Concept

Cement paste density and concrete permeability

Formula

NONE

Step-by-step solution

1. Concrete permeability is governed by the volume and continuity of capillary pores in the hardened paste. 2. Rich mixes contain a high proportion of cementitious paste relative to aggregates at an appropriately controlled water-cement ratio. 3. The excess hydrated cement paste completely coats aggregate surfaces and densely fills interfacial transition zones and capillary pores. 4. This dense paste structure creates discontinuous pores, drastically lowering fluid permeability. Answer: B

Explanation

Rich concrete mixes contain a higher proportion of cementitious paste relative to aggregates at an appropriate water-cement ratio. This extra volume of hydration paste thoroughly coats every aggregate particle and fills interstitial capillary voids, producing a dense, cohesive microstructure that significantly restricts fluid ingress and reduces permeability.

Why the other options are wrong

A) Permeability depends on paste density and porosity, not simply low absolute water content. C) Aggregate size selection does not automatically define rich mix matrix permeability. D) Curing is critical for rich mixes to achieve complete hydration and prevent thermal microcracking.

Shortcut

Rich mix = Abundant cement paste filling voids and interfacial transition zones = Low permeability.

Common mistake

Thinking rich mixes simply contain less water rather than understanding void filling by hydrated paste.