Researchers have demonstrated that applying plaster to reinforced concrete (RC) beams significantly improves their structural performance and moment capacity after exposure to uncontrolled fire. The findings, published in Scientific Reports, show how a simple protective layer can help RC structures withstand extreme heat and retain strength after fire events.
Study: Flexural performance of reinforced concrete beams exposed to uncontrolled fire with and without plaster protection. Image Credit: Nature Peaceful/Shutterstock.com
Background
Fire-induced damage in buildings can range from superficial discoloration to total structural collapse, with serious consequences for safety and resilience. Concrete itself is often described as fire-resistant because of its low thermal conductivity, but steel reinforcement has very different thermal properties. When exposed to high temperatures, steel loses strength quickly, undermining the performance of reinforced concrete.
This dual behavior makes RC structures—particularly beams and columns—a key focus of fire safety research.
Full-scale fire testing has been used to study these effects, typically involving furnaces or controlled heating and cooling systems. However, such testing is costly, logistically demanding, and often hazardous. Numerical modeling has emerged as a practical alternative, allowing researchers to predict post-fire performance with reduced experimental burden.
Within this context, the study in question set out to investigate whether plaster could function as an effective insulating material for RC beams under uncontrolled fire exposure.
Methods
A total of 36 RC beams were prepared for the investigation, divided into two reinforcement configurations:
- M4D6: four main bars with six stirrups
- M5D4: five main bars with four stirrups
All beams were cast using Ordinary Portland Cement along with locally available coarse and fine aggregates. To test the role of plaster as an insulating layer, a 1:6 plaster mix was applied to some beams at a thickness of 20 mm, while others were left unplastered.
The fire exposure was designed to simulate realistic uncontrolled conditions. Beams were subjected to open-air fires lasting either 3 hours or 6 hours, during which temperatures reached between 600 °C and 800 °C. After exposure, the specimens were cooled in open air for 24 hours, washed, and painted white to make surface cracking more visible.
To assess structural performance, the beams underwent a three-point loading test. This began with an initial load of 5 kN, applied using a universal testing machine with a maximum capacity of 1800 kN. Researchers then analyzed cracking patterns, failure modes, load-bearing behavior, and residual moment capacity.
Results and Discussion
The experiments revealed clear differences in how plastered and unplastered beams responded to fire.
- Peak Load Capacity: Fire exposure reduced peak loads in all beams, but plastered beams consistently retained higher capacities. Plastered M5D4 beams carried greater loads than plastered M4D6, while among unplastered beams, M4D6 suffered more severe reductions than M5D4.
- Moment Capacity: Post-fire moment capacity dropped significantly across all specimens. However, plastered beams preserved more capacity than their unplastered counterparts. Interestingly, M4D6 beams retained more moment capacity when plastered than M5D4, showing that plaster’s effectiveness also depends on reinforcement configuration.
- Predictability: Analytical and experimental results aligned more closely in plastered beams, suggesting that their fire performance was not only stronger but also more consistent and predictable.
Additional observations reinforced these findings. Cracking patterns, failure progression, and load-displacement curves showed little variation between beams with and without stirrups. This indicated that shear failure was not dominant, likely because the shear span-to-depth ratio (a/d) was less than 4—an expected condition where flexural failure governs instead.
The researchers also developed a simplified design methodology to estimate moment capacity of RC beams after fire exposure. Their graphical analysis emphasized that plaster provided measurable resistance to deflection and enhanced residual strength, marking it as a practical addition to fire-resilient design.
Conclusion
In simple terms, the study shows that plaster really does make a difference when RC beams are exposed to fire. By testing different reinforcement layouts and fire durations, the researchers found that a plaster layer slows down heat damage, helps beams hold on to more of their strength, and makes their performance easier to predict.
The work was carried out under realistic open-fire conditions, but it didn’t cover every factor. For example, the effects of uneven heating or beams already carrying load during a fire weren’t part of the experiment. The team notes that these gaps are important for future research.
Still, the takeaway is clear: plaster is a straightforward, low-cost way to improve the fire resistance of RC beams and boost their safety after a fire.
Journal Reference
Hisbani, N., Shams, M. A., Stanikzai, M. A., Almujibah, H., & Benjeddou, O. (2025). Flexural performance of reinforced concrete beams exposed to uncontrolled fire with and without plaster protection. Scientific Reports, 15(1). DOI: 10.1038/s41598-025-00146-8. https://www.nature.com/articles/s41598-025-00146-8
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