Surface finish issues in fresh concrete — identifying the cause A concrete mix that makes it difficult to obtain a smooth, closed surface finish is most likely exhibiting which characteristic?

Introduction / Context:
Finishability of concrete depends on mix proportioning, aggregate characteristics, and fresh properties such as cohesiveness and water demand. When the mortar fraction is inadequate or aggregate texture causes excessive drag, finishing becomes laborious and produces rough, open textures often described as “harsh.”

Given Data / Assumptions:

• Fresh concrete experiencing difficulty in trowelling/closing the surface.
• No special finishing aids or surface hardeners considered.
• Standard aggregates and cement used.

Concept / Approach:
A harsh mix has high internal friction among particles, commonly due to angular or flaky aggregates, insufficient fines, or low paste content. Such mixes resist movement under the trowel and do not readily consolidate at the surface, hampering the development of a smooth finish. By contrast, bleeding pushes water to the surface, sometimes falsely easing trowel movement but damaging durability; segregation separates coarse aggregate, creating variable texture but not necessarily universal finishing difficulty. “Hardness” relates to the hardened state, not fresh finishing.

Step-by-Step Solution:

Verification / Alternative check:
Improving sand grading, increasing fines, using rounded aggregates, or deploying water-reducing admixtures usually improves finishability, confirming the friction-based explanation.

Why Other Options Are Wrong:

• (a) Segregation causes uneven stone-rich zones, not a consistent inability to smooth the surface.
• (c) Hardness applies after setting/hardening.
• (d) Bleeding yields excess surface water; it can temporarily lubricate, yet harms surface quality later.

Common Pitfalls:
Adding extra water to “fix” finishability, which raises water–cement ratio and reduces durability; instead, adjust grading, paste content, and admixtures.

Final Answer:
High internal friction (harshness)