IS sand grading zones — what does Zone I indicate? According to IS grading of fine aggregates for concrete, sands classified in Zone I are generally considered to be of which coarseness category?

Introduction / Context:
Indian Standards categorize fine aggregates into grading Zones I to IV. These zones reflect particle size distributions that influence water demand, workability, pumpability, and finish. Designers choose sand zones to balance cohesiveness with economy and performance of the mix.

Given Data / Assumptions:

• IS-based grading zones I–IV.
• Zone I lies toward coarser sands; Zone IV toward finer sands.
• Normal concrete applications are considered.

Concept / Approach:
Zone I sands have higher percentages retained on coarser sieves, thus they are designated as coarse. Coarser sand reduces paste demand and can improve economy by lowering water and cement requirements, but may compromise finishability if excessively coarse or poorly graded. Finer sands (Zones III–IV) increase water demand at a given slump and can improve finish, but may elevate shrinkage and require more cement or admixture to maintain workability and strength targets.

Step-by-Step Solution:

Verification / Alternative check:
Gradation tables and fineness modulus values corroborate that Zone I corresponds to higher fineness modulus (coarser distribution).

Why Other Options Are Wrong:

• (b), (c), (d) misstate the coarseness implied by Zone I, which is not medium or fine.

Common Pitfalls:
Assuming Zone I is universally better; optimal performance depends on overall grading, paste content, and finishing requirements.

Final Answer:
Coarse sand