Mass concrete for gravity dams — expected 28-day cube strength range For mass concrete in gravity dams using aggregates with maximum size about 5 cm, what is the typical 28-day cube strength range adopted in practice?

Introduction / Context:
Mass concrete in dams prioritizes thermal control, volume stability, and long-term durability over very high early strength. Large aggregate sizes and low heat binders are often used to minimize temperature rise and thermal cracking, which leads to moderate 28-day strengths compared with heavily reinforced structural concrete.

Given Data / Assumptions:

• Gravity dam construction with maximum aggregate size about 50 mm.
• Low heat cement or blended cements may be used.
• Strength values at 28 days (standard cube test).

Concept / Approach:
Specifications for mass concrete typically target moderate strengths (order of 15–30 MPa, i.e., about 150–300 kg/cm²). Higher strengths increase cement content and heat of hydration, contradicting the primary objectives of mass concrete. Early age temperature management outweighs the need for high early strength in these placements.

Step-by-Step Solution:

Verification / Alternative check:
Historic and contemporary dam projects show specified strengths commonly within 15–25 MPa at 28 days, with long-term gains expected under continued curing.

Why Other Options Are Wrong:

• (b) Excessively high for mass concrete, would raise heat of hydration.
• (c) Overly broad and includes very high strengths not typical for dams.
• (d) Below 200 kg/cm² can be acceptable in some elements, but typical ranges extend up to 300 kg/cm² and beyond, making (a) the best representative answer.
• (e) Not applicable, as (a) correctly reflects practice.

Common Pitfalls:
Assuming higher strength is always better; in mass concrete, thermal and volumetric stability dominate design decisions.

Final Answer:
Between 150 and 300 kg/cm²