For a satisfactory workable concrete with a constant W.C. ratio increase in aggregate cement ratio ……………………..
Correct Answer: B. Decreases the strength of concrete
📚 Detailed Explanation: A/C Ratio at Constant W/C
Abrams' Law specifies that w/c governs strength at a given aggregate-cement ratio. When A/C changes while w/c is held constant, the absolute cement content changes, which affects the total amount of binding paste per unit volume — and therefore, strength.
Why B (Decreases strength) is correct: Consider two mixes both at w/c = 0.50:
Mix 1: A/C = 4.0 (per m³: ~350 kg cement, 1400 kg aggregate)
Mix 2: A/C = 6.0 (per m³: ~250 kg cement, 1500 kg aggregate)
Mix 2 has less cement paste per unit volume. The paste must coat more aggregate surface area per unit of paste, leading to thinner, weaker paste bonds. The compressive strength of Mix 2 is lower despite the same w/c. While w/c sets the intrinsic paste strength, the A/C ratio determines how much paste is available to bind the aggregate matrix — both matter.
Mix 1: A/C = 4.0 (per m³: ~350 kg cement, 1400 kg aggregate)
Mix 2: A/C = 6.0 (per m³: ~250 kg cement, 1500 kg aggregate)
Mix 2 has less cement paste per unit volume. The paste must coat more aggregate surface area per unit of paste, leading to thinner, weaker paste bonds. The compressive strength of Mix 2 is lower despite the same w/c. While w/c sets the intrinsic paste strength, the A/C ratio determines how much paste is available to bind the aggregate matrix — both matter.
Effect of Changing A/C at Constant W/C
| A/C Ratio | Cement Content (kg/m³) | Paste Volume | Strength |
|---|---|---|---|
| 4.0 (lower) | Higher (~360) | More | Higher |
| 5.0 (medium) | Medium (~290) | Medium | Medium |
| 6.0 (higher) | Lower (~240) | Less | Lower |
Key Concepts for Students
- Abrams' Law holds strictly only when aggregate type, grading, and A/C ratio are all fixed — w/c is the variable.
- Increasing A/C at constant w/c reduces cement content, reduces paste volume, and reduces strength.
- A/C ratio is a secondary parameter after w/c in mix design — it is used to adjust workability and economy.