A concrete design mix with a low water/cement ratio and also using larger aggregates results in:
Correct Answer: D. Gains in concrete compressive strength
📚 Detailed Explanation: Low w/c + Larger Aggregates = Higher Strength
Why D (Gains in compressive strength) is correct:
Effect 1 — Low w/c ratio: Per Abrams' Law, compressive strength is inversely proportional to w/c ratio. Reducing w/c reduces capillary porosity after hardening, directly increasing strength.
Effect 2 — Larger aggregates: Larger particles have less specific surface area (surface area per unit mass). Less surface area = less water needed to coat all particles = for the same mix water content, effective w/c ratio is lower = higher strength. Additionally, larger aggregates provide better mechanical interlock in the hardened paste matrix.
Effect 1 — Low w/c ratio: Per Abrams' Law, compressive strength is inversely proportional to w/c ratio. Reducing w/c reduces capillary porosity after hardening, directly increasing strength.
Effect 2 — Larger aggregates: Larger particles have less specific surface area (surface area per unit mass). Less surface area = less water needed to coat all particles = for the same mix water content, effective w/c ratio is lower = higher strength. Additionally, larger aggregates provide better mechanical interlock in the hardened paste matrix.
How Low w/c and Large Aggregates Both Improve Strength
| Factor | Mechanism | Strength Effect |
|---|---|---|
| Low w/c ratio | Fewer capillary pores after hydration | Direct strength increase |
| Larger aggregate | Less surface area → less water demand | Allows lower effective w/c |
| Combined effect | Denser paste + less porous matrix | Significant strength gain |
- Low w/c = key driver of strength (Abrams' Law: fc ∝ 1/w/c ratio).
- Larger NMSA: less surface area → less water demand → supports lower w/c → higher strength.
- Note: very large aggregates (>40mm) can introduce stress concentrations at paste-aggregate interface, limiting gains.