Mass concrete for dams — maximum nominal size of coarse aggregate In constructing cement-concrete dams (large mass concrete placements), what is the typical maximum permissible nominal size of coarse aggregate used under standard practice and handling?

Introduction / Context:
Mass concrete elements such as dams benefit from larger nominal maximum aggregate sizes to reduce cement and water demand, limit heat of hydration, and control shrinkage and thermal cracking. However, aggregate size is constrained by placement equipment, layer thickness, and reinforcement/confinement details.

Given Data / Assumptions:

• Conventional gravity dam placements with minimal congested reinforcement.
• Adequate control of gradation and cleanliness of aggregates.
• Formwork and lift thickness compatible with larger aggregate sizes.

Concept / Approach:
For mass concrete, nominal maximum sizes higher than those in ordinary RCC work are adopted. Many specifications permit up to about 75–80 mm aggregates where placing methods and section thickness allow. Within the options provided, 80 mm represents the common upper bound used for dam concretes prepared and placed using standard equipment.

Step-by-Step Solution:

Verification / Alternative check:
Numerous dam projects specify nominal maximum sizes in the 75–80 mm range, with larger sizes possible in special circumstances subject to testing.

Why Other Options Are Wrong:

• 40–70 mm: Common in RCC, but dam mass concrete often allows larger sizes to reduce paste content.

Common Pitfalls:
Using very large aggregates without adjusting lift thickness or ensuring proper consolidation; ignoring the effect on finishing near surfaces.

Final Answer:
80 mm