TALK by Prof. Mitsuo Ozawa: Fundamental study on concrete damage in high-temperature environments.

Fundamental study on concrete damage in high-temperature environments.

Guest lecture by
Professor Mitsuo Ozawa
Cluster of Materials and Environment
Program of Civil and Environmental Engineering
Gunma University, Japan

September 25, 2026 at 3 p.m.
SF1009 – Sandford Fleming Building

Abstract
Following the accident at the Tokyo Electric Power Company Fukushima Daiichi Nuclear Power Station (1F) that occurred in March 2011, an visual investigation in the reactor containment vessel for Unit 1 was conducted in 2023. As the result, it was confirmed that the reinforced concrete inside the pedestal had largely disappeared, leaving only the exposed reinforcement. The conditions and mechanisms responsible for this concrete loss remain uncertain. Among the potential causes, concrete spalling induced by the intense heat of high‑temperature molten materials has been identified as a plausible factor. Spalling mechanisms generally include thermal stress, vapor pressure, and the influence of restraint conditions.

This study aims to clarify the fundamental characteristics of concrete spalling under high‑temperature conditions with mechanical restraint. Heating experiments were conducted in accordance with the JCI standard ring‑restrained heating test method. Disk‑shaped specimens were cast within steel rings (φ300 mm × h50 mm × t8 mm), and the effects of restraint conditions and heating temperature on spalling behavior were examined. During testing, spalling occurrence, restraining stress, and vapour pressure were continuously monitored to evaluate their interrelationship. A design compressive strength of 100 MPa was selected to promote spalling susceptibility.

The main findings are as follows. In the heating tests, specimens restrained by the steel ring exhibited explosive spalling following increases in vapour pressure and restraining stress, whereas unrestrained specimens did not experience explosive failure.

Biography
Mitsuo Ozawa is a Professor in the Cluster of Materials and Environment, Program of Civil and Environmental Engineering at Gunma University, Japan. He received his Doctor of Engineering degree from Gifu University in 2003. He has served as Vice Chair of the Japan Concrete Institute (JCI) Technical Committee on the Potential Performance of Concrete under High-Temperature Conditions (JCI-TC154A). He was also a member of RILEM TC SPF, Spalling of Concrete due to Fire: Testing and Modelling.

In addition, he chaired the Japan Society of Civil Engineers (JSCE) Technical Committee 365, Damage and Performance Evaluation of Reinforced Concrete Structures after Fire. He currently serves as Chair of the JSCE Subcommittee on Guidelines for Fire Damage Assessment of Civil Concrete Structures (Committee 250).

His research interests include the mechanisms of fire-induced spalling of concrete, the development of ring‑restrained testing methods, modelling of concrete behaviour at high temperatures, and durability assessment of concrete after exposure to elevated temperatures.