Admixtures MCQ with answer and solution

Concept

Mechanism of air-entraining agents in freeze-thaw durability

Formula

NONE

Step-by-step solution

1. Capillary water expands roughly 9% in volume upon freezing, generating high bursting pressures within concrete pores. 2. Air-entraining agents incorporate microscopic, disconnected air voids uniformly spaced throughout the cement paste. 3. These voids act as miniature relief chambers for freezing water to expand into without cracking the paste. 4. Consequently, air-entraining agents primarily improve resistance to freeze-thaw cycles. Answer: C

Explanation

Air-entraining agents generate discrete microscopic air bubbles uniformly distributed throughout the concrete paste. These entrained air voids act as expansion reservoirs that relieve hydraulic and osmotic pressures created when water freezes within capillary pores, thereby significantly improving resistance to freeze-thaw cycles and scaling.

Why the other options are wrong

A) Air entrainment introduces voids, which slightly reduces density rather than increasing it. B) Entrained air does not systematically reduce permeability compared to well-cured dense concrete. D) Air-entraining admixtures do not increase the chemical heat of hydration.

Shortcut

Air entrainment = Microscopic expansion voids -> Superior freeze-thaw durability.

Common mistake

Believing entrained air increases concrete density, whereas adding air voids decreases unit weight and density.