Stone selection for rubble masonry: which property should stones used in rubble masonry primarily possess? (Choose the characteristic that ensures long-term performance under load and weather.)

Introduction / Context:
Rubble masonry, comprising irregular or roughly dressed stones, is often used for foundations, retaining walls, and rural structures where economy and durability matter. The principal requirement is that the stones resist crushing and weathering. Hence, hardness and soundness are key selection criteria.

Given Data / Assumptions:

• Rubble units are irregular and rely on interlock and mortar for stability.
• Exposed faces may encounter cycles of wetting and drying, salts, and frost.
• Load-bearing performance depends on the stone's compressive strength and abrasion resistance.

Concept / Approach:
A hard, durable stone minimizes spalling and attrition under service. While density and weight influence handling and dead load, hardness directly addresses strength and abrasion resistance, ensuring longevity even with uneven contact surfaces typical of rubble work.

Step-by-Step Solution:

Verification / Alternative check:
Construction handbooks recommend hard, tough stones (e.g., basalt, granite, quartzite) for rubble masonry, particularly in foundations and water-retaining structures.

Why Other Options Are Wrong:

• Soft: prone to crushing and erosion.
• Light or heavy: mass alone does not define durability; hard stones may be dense, but density is not the primary criterion here.

Common Pitfalls:
Selecting stones by appearance or ease of dressing rather than mechanical durability; ignoring bedding orientation in stratified stones.

Final Answer:
Hard