Fire and Concrete – Residual strength at about 1000°C A reinforced concrete member is subjected to a fully developed fire and concrete temperature reaches approximately 1000°C. What is the approximate residual compressive strength of the concrete compared to its original (ambient) strength?

Introduction / Context:
Concrete performance in fire is critical for structural safety. With rising temperature, free and chemically bound water are driven off, microcracking develops, and strength declines. Understanding approximate residual strengths helps in post-fire assessment and conservative design checks when detailed testing data are unavailable.

Given Data / Assumptions:

• Peak concrete temperature around 1000°C (not just gas temperature).
• Ordinary siliceous aggregate concrete without special heat-resistant constituents.
• Assessment refers to residual compressive strength after cooling.

Concept / Approach:

Residual strength decreases sharply beyond 600–800°C. Typical data indicate roughly 50–60% remaining at 600°C, 30–40% at 800°C, and near 10–20% at around 1000°C for siliceous aggregate concrete. The exact value depends on mix, moisture, heating rate, and aggregate type; however, a representative conservative estimate near 15% is widely used for rapid appraisal.

Step-by-Step Solution:

Verification / Alternative check:

Fire engineering guides and experimental studies (siliceous aggregates) support residual strengths in the low-teens at ~1000°C, with carbonate aggregates sometimes retaining slightly more.

Why Other Options Are Wrong:

10% is possible but often represents severe damage; 20–25–35% are optimistic for siliceous concrete at ~1000°C; choosing 15% balances conservatism with typical data.

Common Pitfalls:

Confusing steel temperature with concrete temperature; assuming gas temperature equals concrete core temperature; neglecting spalling which can further reduce capacity.

Final Answer:

15%