Aggregates for concrete: Bulking of sand occurs primarily due to which factor?

Introduction / Context:
Bulking is the apparent increase in volume of sand when it contains a small percentage of moisture. This affects batching by volume; if not corrected, concrete can become harsh and under-sand, compromising workability and finish.

Given Data / Assumptions:

• We consider fine aggregates (sand) used in concrete and mortar.
• Moisture content is in the range that causes maximum bulking (typically 3–6% for many sands).
• Measurement is by loose volume.

Concept / Approach:
Thin films of water around sand particles create surface tension forces that increase inter-particle repulsion and trap small amounts of air, causing the sand mass to occupy more volume. This is most pronounced at low to moderate moisture contents and reduces as sand becomes saturated.

Step-by-Step Solution:
Identify mechanism: surface moisture → water films → increased separation between grains.Observe behavior: volume expands at a few percent moisture; then contracts as saturation eliminates suction and the sand consolidates.Conclude cause: surface moisture is the primary reason; air and clay can influence but are not the principal mechanism.

Verification / Alternative check:
Bulking test (field jar test) visually confirms rise in sand volume with small moisture additions.

Why Other Options Are Wrong:
Air voids: present but secondary to the water film effect.Viscosity: not the controlling factor for bulking behavior.Clay content: can change water demand but is not the primary bulking mechanism.

Common Pitfalls:
Ignoring bulking corrections during volumetric batching; always determine bulking percentage before mix proportioning by volume.

Final Answer:
Surface moisture coating the sand grains