Researchers have developed a new semi-analytical multiscale model that accurately predicts how concrete's stiffness and thermal-expansion behavior change at high temperatures, offering engineers a clearer view of fire-related damage mechanisms.
Study: A multiscale model for predicting the Young’s modulus and the thermal-expansion coefficient of concrete at high temperatures. Image Credit: Bannafarsai_Stock/Shutterstock.com
Concrete structures exposed to fire can suffer sudden and severe degradation, but understanding exactly how and why this happens—especially at the microscale—has remained challenging.
Traditional models tend to operate at a broad scale and often overlook critical details like load-induced thermal strains (LITS), which appear almost instantly when heated concrete is under compression. These overlooked mechanisms are key to understanding how thermal loads compromise structural integrity.
This new study, published in Construction and Building Materials, addresses that gap by introducing a model that captures the full picture—from micro-level chemical reactions to macro-scale structural effects. By doing so, it helps explain how stiffness loss and expansion occur under heat, and what role microcracking, material composition, and strain behavior play in the process.
Bridging the Microscale and Macroscale
The model works across three levels of observation: cement paste, mortar, and full concrete. In the first stage, it calculates how the Young’s modulus of cement paste evolves as it’s heated, based on input parameters like cement composition and heating rate. In the second stage, it uses those results to predict the behavior of mortar and concrete, employing an iterative homogenization method—specifically, the Mori–Tanaka scheme—to combine the effects of cement paste, fine aggregates, and coarse aggregates.
This layered approach allows the model to simulate how different components of concrete respond to heat, not in isolation, but as part of a composite system. It also incorporates a fracture mechanics method to estimate how cracks form and spread due to mismatches in thermal expansion between the paste and the aggregates.
What the Model Reveals
When compared to experimental data, the model showed strong agreement in predicting how both stiffness and thermal-expansion coefficients change with temperature. One key insight was that chemical changes in the cement paste—like dehydration—had a relatively minor effect on the concrete’s overall stiffness. Instead, the aggregate phase, which makes up most of the volume, played a larger role.
At temperatures above 200 °C, the breakdown of aggregates began to dominate, especially in concretes with less heat-resistant materials. Those using stable aggregates like basalt maintained stiffness longer, up until thermal incompatibility between components triggered microcracking.
The model also showed that the amount of pre-existing microcracks in hardened cement paste had a big influence on how the material behaved under heat. Interestingly, at moderate temperatures, much of the apparent stiffness loss wasn’t caused by cracking at all—it was due to LITS, a type of strain that doesn’t reverse after cooling. This underscores the importance of including LITS in any realistic prediction of concrete performance under fire conditions.
Why it Matters
This semi-analytical multiscale model offers more than just accurate predictions. By connecting specific damage mechanisms to design variables—like cement type, aggregate choice, and water-to-cement ratio—it provides a useful tool for engineers looking to improve fire resistance through material selection and mix design.
It also opens the door to predicting other important properties under high temperatures, such as thermal conductivity, permeability, and diffusivity. And because it’s based on closed-form equations, the model can be readily integrated into existing design software, making it both practical and precise.
Journal Reference
Peters, S., Vu, G., & Meschke, G. (2025). A multiscale model for predicting the Young’s modulus and the thermal-expansion coefficient of concrete at high temperatures. Construction and Building Materials, 479, 141259. DOI: 10.1016/j.conbuildmat.2025.141259, https://www.sciencedirect.com/science/article/pii/S0950061825014072
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