Property index under combined temperature and load: which test parameter measures the strength of a refractory when it is subjected simultaneously to high temperature and mechanical load?

Introduction / Context:
During service, refractories rarely experience temperature alone; they carry the weight of superstructure, burden, or gas pressure while being heated. A key quality metric is the ability to sustain shape and strength under this combined stress. The standard figure of merit for this behavior is refractoriness under load (RUL).

Given Data / Assumptions:

• We consider a compressive load applied while the sample is heated.
• RUL temperature is typically defined at a specified deformation (e.g., 0.5% or 1%).
• Other properties like porosity or thermal conductivity do not directly capture load-bearing at temperature.

Concept / Approach:
RUL characterizes the temperature at which a refractory begins to soften and deform under a given load. It integrates chemical composition, bonding, and microstructure into a single functional performance metric. High RUL indicates that the refractory can support mechanical loads at elevated temperatures without unacceptable creep or collapse.

Step-by-Step Solution:
Define need: measure of hot strength under load.Identify available indices: porosity, specific gravity, thermal conductivity, RUL.Only RUL directly tests strength at temperature under load.Select RUL accordingly.

Verification / Alternative check:
Data sheets routinely state both PCE (pyrometric cone equivalent) and RUL; RUL is invariably lower than PCE and is used for designing load-bearing zones such as arches, crowns, and stove checker supports.

Why Other Options Are Wrong:
Porosity: affects properties but is not a strength-at-temperature test.Specific gravity: density measure, not a high-temperature strength metric.Thermal conductivity: heat-transfer property, unrelated to mechanical load capacity.Permanent linear change: measures dimensional stability after heating, not load-bearing strength during heating.

Common Pitfalls:
Confusing PCE (softening without load) with RUL (softening with load).Over-relying on room-temperature strength to predict hot performance.

Final Answer:
RUL (Refractoriness Under Load)