The strength of concrete made with angular aggregate and rounded aggregate is practically same at the water/cement ratio is
Correct Answer: D. 0.65
📚 Detailed Explanation: Crossover Point for Angular vs Rounded Aggregate
Aggregate shape affects the concrete strength primarily through the quality of the aggregate-paste interface. Angular aggregates provide better mechanical interlock and a rougher surface for cement paste to grip. However, this advantage diminishes as water content increases.
Why D (0.65) is correct: At low w/c (<0.65): Angular aggregates outperform rounded ones because the mechanical interlock at the aggregate-paste interface governs failure — angular particles with rough surfaces bond better. At w/c = 0.65: The crossover point where both aggregate types give equal strength. At w/c > 0.65: The paste quality (low strength due to high porosity) governs failure — aggregate shape no longer matters since the paste itself is the weak link. Option C (55) is clearly a typo for 0.55, but the accepted answer is 0.65.
Effect of Aggregate Shape on Strength vs W/C
| W/C Range | Stronger Aggregate | Governing Factor |
|---|---|---|
| Low (<0.50) | Angular | Aggregate-paste bond |
| Medium (0.50–0.65) | Angular (slightly) | Mixed — bond + paste |
| ~0.65 (crossover) | Equal strength | Transition point |
| High (>0.65) | Equal / Rounded | Paste porosity governs |
Key Concepts for Students
- At w/c = 0.65, angular and rounded aggregates give equal concrete strength — the specific crossover value to memorise.
- For high-strength concrete (w/c <0.40), angular crushed aggregate is mandatory — the bond quality critically governs strength.
- Rounded river gravel works well in mass concrete (w/c ~0.55–0.65) where workability is more important than maximum strength.