Concrete strength of standard 15 cm cubes at 28 days: Under favorable materials, mix, and curing, what maximum crushing strength is commonly cited in elementary building practice texts?

Introduction / Context:
Compressive strength of concrete is measured using standard 15 cm cubes at 28 days. While design uses specified characteristic strengths, elementary building construction texts often quote indicative ranges in traditional units (kg/cm²) for common mixes and curing. This question asks for the upper bound referenced for “favorable circumstances.”

Given Data / Assumptions:

• Standard cube size = 150 mm.
• Strength assessed at 28 days under controlled curing.
• Traditional unit system maintained to match the option set.

Concept / Approach:

Historically, ordinary mixes yielded 100–300 kg/cm², with good quality materials, proper proportioning, compaction, and curing enabling substantially higher strengths. Textbook tables frequently remark that up to about 500 kg/cm² can be achieved with quality control, though modern high-strength concretes may far exceed this in SI units.

Step-by-Step Solution:

Verification / Alternative check:

Cross-reference with typical design grades: 200–300 kg/cm² (≈20–30 MPa) for ordinary structural concrete; higher strengths are achievable with better cement, low w/c ratio, admixtures, and curing.

Why Other Options Are Wrong:

100–400 kg/cm² represent lower or mid-range strengths and do not reflect the “maximum in favorable circumstances.”

Common Pitfalls:

Confusing characteristic design strength with occasional maximum test strengths; mixing units (MPa vs kg/cm²) without conversion (1 MPa ≈ 10.197 kg/cm²).

Final Answer:

500 kg/cm²