Conference Agenda

The durability of concrete under extreme temperatures is primarily influenced by the behavior of calcium silicate hydrates (C-S-H), the primary binding phase in the material. During heating, the release of chemically bound water within C-S-H leads to degradation of material properties, ultimately contributing to spalling. To accurately predict concrete’s performance in fire scenarios, comprehensive thermo-hydro-chemical (THC) models are indispensable. This research presents a novel THC model that simulates the entire lifecycle of concrete, from its initial casting to potential fire accidents. The model comprehensively accounts for the interplay between temperature, humidity, and the chemical reactions taking place within the concrete microstructure. By incorporating the effects of C-S-H hydration and dehydration, the model provides a more accurate representation of concrete’s behavior under various conditions. A key innovation in this work is the development of a novel adsorption-desorption model that captures the irreversible nature of chemical damage within the porous structure. This model simplifies the calibration process, requiring fewer experimental inputs. The model is validated against experimental data, demonstrating its ability to accurately predict concrete’s response in both early-age behavior and high-temperature conditions. The results underscore the significant impact of the initial hygral state on the concrete’s fire resistance, highlighting the importance of considering this factor in structural design and analysis, especially in real-world conditions, where the heterogeneity is high both in the water distribution and, consequentially, in the resulting microstructure. Finally, another novelty of the current work is the availability of the developed thermo-hydro-chemical code made publicly available on GitHub, facilitating its application and further research in the field of concrete fire safety.

This study addresses the challenges of understanding the complex mechanisms of concrete behavior under fire, particularly damage and spalling. Using a standardized mortar, extensive thermal, mechanical, transport and microstructural characterizations were conducted to provide input data for a thermo-hygro-chemo-mechanical model. Preliminary results highlight the impact of elevated temperatures on the integrity of mortar, providing insight into the degradation mechanisms that affect its structural performance under thermal stress.

During a fire, concrete structures experience rapid heating leading to thermal instability (spalling), compromising load-bearing capacity by reducing the concrete’s cross-section or exposing steel reinforcement to flames. Polypropylene fibers are recognized for their effectiveness in mitigating instability risks. The study aims to enhance understanding of fiber’s behavior during heating to optimize their dosage and limit adverse effects on fresh concrete. Four concretes have been studied with calcareous gravels and mortar aggregates, with and without polypropylene fibers. Two samples of each concrete type were tested: C10 (calcareous aggregate), C1-18/32(0.5) (calcareous aggregate with fibers), C1MA (mortar aggregate), and C1MA-18/32(0.5) (mortar aggregate with fibers). The use of mortar aggregates allows us to investigate the influence of thermal mismatch and to better understand the interaction between several phenomena: cracking due to thermal mismatch and porosity and cracking due to fiber melting. Controlled heating from 80°C to 450°C at a rate of 2°/min was conducted, followed by a 3-hour stabilization before radial permeability testing. Results indicate a temperature-dependent increase in intrinsic permeability across all concrete types, with mortar aggregates showing lower permeability values. Mortar aggregate induces minimal thermal mismatch compared to calcareous aggregate, owing to its similar expansion and shrinkage characteristics to cement paste. This reduced thermal mismatch results in fewer cracks when exposed to elevated temperatures, further contributing to the lower permeability of concretes containing mortar aggregate. In contrast, fiber-reinforced concretes exhibited higher permeability, primarily due to the melting and expansion of polypropylene fibers around 170°C. Overall, these findings suggest that the mechanism that increases permeability and reduces spalling is primarily linked to the addition of fibers. while polypropylene fibers are effective in enhancing concrete's permeability and minimizing thermal instability, the thermal expansion of aggregates also has a high influence in this risk.. Further research would explore the effects of mechanical loading on the intrinsic permeability of these concrete types and investigate polypropylene fibers with varying geometries and dosages to compare results with the current findings.

The aim of this study was to experimentally investigate the dehydration phenomenon of cement-based products in high-performance concrete exposed to high temperatures, using neutron imaging. Graywacke and limestone were employed as coarse aggregates in the high-strength concrete. The results revealed that when one side of a concrete block was heated, dehydration initiated at temperatures exceeding 100°C during the early stages of heating. With prolonged heating, dehydration was observed to begin at temperatures below 100°C. In limestone concrete, the region experiencing dehydration above 100°C extended deeper into the sample compared to that in Graywacke concrete. These findings suggest that the dehydration of cement hydrates under high temperature conditions is influenced by the state of the interface between the coarse aggregate and the cement matrix.

Concrete's vulnerability to fire-induced spalling poses a critical challenge in structural applications, especially under extreme conditions. This phenomenon is highly material-dependent, with an elevated risk for modern concrete mixes, such as high-performance concrete (HPC), due to their dense microstructure. Understanding and mitigating this risk is crucial for enhancing structural safety in fire scenarios.

This study investigates the two-stage mechanism underlying spalling: crack initiation and crack instability, both driven by thermal and hygral interactions and strongly influenced by the fracture behaviour of concrete. In the first stage, spalling begins when driving forces—such as pore pressure and thermal stress—surpass the tensile strength of concrete. Biot's coefficient, the shape and dimension of the structure are critical in determining this threshold. The second stage involves rapid crack propagation, driven by thermal energy conversion through vaporization. This vaporisation occurs adjacent to the cracked region, pressurising the crack and accelerating it for instability.

To explore these mechanisms, innovative testing methods were employed, including a frameless direct-tension test setup to evaluate fracture behaviour and a small-scale spalling test to analyze instability thresholds. The results reveal the complex interplay of pore pressure, thermal stresses, and moisture content in spalling dynamics.