Fresh concrete rheology: internal friction among concrete ingredients is reduced most effectively by which adjustment to the mix?

Introduction / Context:
“Internal friction” in fresh concrete refers to resistance to flow caused by particle interlock and paste viscosity. Site crews often adjust water content or aggregate grading to achieve target workability; however, these changes affect segregation and strength. Understanding which change actually reduces interparticle friction is critical.

Given Data / Assumptions:

• Workability depends on water content, paste volume, and aggregate grading/shape.
• Well-graded aggregates (proper fines to coarse balance) minimize voids and friction.
• Excess water increases workability but risks bleeding and strength loss.

Concept / Approach:
Rheology improves when the paste sufficiently lubricates the aggregate skeleton and when aggregate grading reduces particle interlock. Increasing water (within limits) lowers paste viscosity and, if paired with a sound grading that includes appropriate coarse fractions, reduces internal friction. Merely adding fines increases surface area and water demand, often increasing friction at a fixed water content.

Step-by-Step Solution:
Identify the friction sources: angularity, poor grading, insufficient paste.Consider adjustments: more water + proper grading decreases friction and eases compaction.Reject alternatives: less water increases friction; more fines raise water demand; “rich mix” without water change can still be sticky.Therefore, option (d) best reduces internal friction.

Verification / Alternative check:
Use of water-reducing admixtures can also lower friction without increasing water; the principle remains that lubrication and grading reduce interparticle resistance.

Why Other Options Are Wrong:

• Less water: stiffer, higher internal friction.
• More fines: higher surface area and paste demand, often stickier.
• Rich mix only: can become cohesive and less workable if w/c is unchanged.

Common Pitfalls:
Over-watering to chase slump, leading to segregation and strength loss; ignoring aggregate shape (rounded vs crushed).

Final Answer:
Using more water along with well-graded (including coarser) aggregates