Admixtures MCQ with answer and solution

Concept

Role and classification of concrete admixtures in modifying fresh and hardened properties of concrete.

Step-by-step solution

1. Understand the definition of admixtures as per IS 9103 and standard concrete technology. 2. Evaluate Option A: Admixtures are widely used to modify fresh concrete properties like workability and setting time (True). 3. Evaluate Option B: Admixtures significantly alter and improve hardened concrete properties such as strength, durability, and permeability (False). 4. Evaluate Option C: Admixtures help ensure overall concrete quality under varying site conditions (True). 5. Evaluate Option D: Accelerators or retarders help overcome emergencies like sudden weather changes during operations (True).

Explanation

Admixtures are chemical or mineral substances added to concrete batches during mixing to modify its fresh and/or hardened properties. Therefore, stating that they have no use in modifying properties of hardened concrete is incorrect, as many admixtures (like pozzolans, shrinkage reducers, and corrosion inhibitors) specifically improve hardened concrete characteristics.

Why the other options are wrong

A: Admixtures like plasticizers are commonly used to modify fresh concrete properties. C: They help maintain consistency and ensure quality. D: Accelerators help overcome emergency low-temperature conditions.

Shortcut

Remember that admixtures modify BOTH fresh concrete (workability, setting) and hardened concrete (strength, durability).

Common mistake

Assuming admixtures are only meant for controlling setting time or workability of fresh concrete, completely ignoring their major role in enhancing durability and strength in hardened concrete.