Aggregate grading for quality concrete: to ensure durable, workable concrete with reduced voids and economy of paste, which type of aggregate should be preferred?

Introduction / Context:
Aggregates comprise the majority of concrete volume. Their grading controls void content, paste demand, workability, pumpability, and strength. Selecting the right grading is a core decision in mix design.

Given Data / Assumptions:

• Objective: produce workable, economical concrete with minimal segregation and shrinkage.
• Paste (cement + water + admixtures) is costly and shrinks; reducing paste volume by lowering aggregate voids is desirable.
• Practical site control of grading via combined sieve analysis is assumed.

Concept / Approach:
A well-graded aggregate (continuous distribution of sizes) minimizes voids by allowing smaller particles to fill the gaps between larger ones, thus reducing the paste required to fill remaining voids. This improves economy and often improves durability. Single-sized or two-sized blends leave more voids, increasing paste demand and shrinkage risk.

Step-by-Step Solution:
Compare grading options in terms of void content.Well-graded → lowest voids → least paste → improved economy and performance.Therefore select “Graded (well-graded) aggregates.”

Verification / Alternative check:
Mix design methods (e.g., DOE/ACI) encourage target combined grading envelopes for optimum packing and workability.

Why Other Options Are Wrong:

• Single- or two-sized: high voids, high paste demand.
• Only coarse/fine: imbalanced grading, poor workability and cohesion.

Common Pitfalls:
Assuming strength comes only from cement content; ignoring packing and particle shape effects.

Final Answer:
Graded (well-graded) aggregates