Admixtures MCQ with answer and solution

Concept

Role of air-entraining admixtures in improving concrete freeze-thaw resistance.

Step-by-step solution

1. Understand the primary purpose of air-entraining admixtures: to incorporate millions of microscopic, non-coalescing air bubbles into concrete. 2. Analyze the effect on freeze-thaw resistance: these bubbles provide space for water to expand when it freezes, reducing internal stresses. 3. Evaluate options: (A) is correct as it directly reduces stress due to freeze-thaw; (B) is false because workability actually increases; (C) is false because excessive air voids generally reduce compressive strength.

Explanation

Air-entraining admixtures introduce microscopic air bubbles into the concrete matrix. These bubbles act as expansion chambers that relieve hydrostatic pressure caused by freezing and thawing of water, thereby greatly enhancing durability under freeze-thaw cycles.

Why the other options are wrong

B: Air-entraining admixtures actually improve/increase workability. C: Air voids generally decrease compressive strength due to increased porosity. D: Incorrect since option (A) is correct.

Shortcut

Air entrainment improves workability and freeze-thaw resistance, but decreases compressive strength.

Common mistake

Assuming air voids increase strength; air voids actually decrease the compressive strength of concrete.