The mainstream use of intumescent coatings for the design of fire-safe steel structures has fostered extensive research into the development of effective intumescent coatings and the assessment of ...their fire performance as thermal barrier to structural elements and materials. The research community has highlighted numerous challenges related to the current design framework based on standard fire resistance furnace testing and the current methodologies have been questioned due to its over-simplifications, limitations and uncertainties. In order to overcome these challenges, numerous research studies have proposed comprehensive approaches and methodologies to investigate the effectiveness of intumescent coatings exposed to a range of heating and fire conditions. This review covers the more recent developments in the field of fire testing and analysis of the fire performance of steel elements protected with intumescent coatings. Apart from the chemical formulation, the substrate boundary conditions, the applied coating thickness, the heating conditions and the fire test method appeared to be the key governing factors. All of them should be considered for a rigorous and systematic testing and design environment that allows the explicit quantification of the effectiveness of intumescent coatings.
•Recent extensive research into the development of effective and reliable intumescent coatings and the assessment of fire performance.•Numerous challenges related to the current design framework and engineering design methodologies based on standard fire resistance furnace tests.•Several research studies have proposed comprehensive approaches and methodologies to investigate the effectiveness of intumescent coatings.•The effectiveness of intumescent coatings have been investigated according to different methodologies and using different experimental setups.•The substrate boundary conditions, the applied coating thickness, the heating conditions and the fire test method appeared to be the governing factors.
In the past decades, the steel industry has experienced a rapid growth in the use of intumescent coatings for protecting load-bearing steel structures in the event of a fire. The onset of swelling is ...key for assuring the effectiveness of thin intumescent coatings for providing thermal insulation to the steel substrate during fire. The study presented herein investigates the onset of swelling for a commercially available thin intumescent coating applied on steel plates and exposed to a wide range of heating conditions. Experiments were performed using an array of radiant panels for controlling incident radiant heat flux at the exposed surface of coated steel samples. Within the scope of this research study, the onset of swelling was defined based on two conditions: (1) visual observation of swelling during heating or (2) time-history of the steel temperature. Research outcomes derived from this work defined a threshold for the onset of swelling in terms of steel and coating temperatures and it concluded that the onset of swelling is directly influenced by the heating conditions at the exposed surface and the original applied dry film thickness.
Purpose
Intumescent coatings are nowadays a dominant passive system used to protect structural materials in case of fire. Due to their reactive swelling behaviour, intumescent coatings are ...particularly complex materials to be modelled and predicted, which can be extremely useful especially for performance-based fire safety designs. In addition, many parameters influence their performance, and this challenges the definition and quantification of their material properties. Several approaches and models of various complexities are proposed in the literature, and they are reviewed and analysed in a critical literature review.
Design/methodology/approach
Analytical, finite-difference and finite-element methods for modelling intumescent coatings are compared, followed by the definition and quantification of the main physical, thermal, and optical properties of intumescent coatings: swelled thickness, thermal conductivity and resistance, density, specific heat capacity, and emissivity/absorptivity.
Findings
The study highlights the scarce consideration of key influencing factors on the material properties, and the tendency to simplify the problem into effective thermo-physical properties, such as effective thermal conductivity. As a conclusion, the literature review underlines the lack of homogenisation of modelling approaches and material properties, as well as the need for a universal modelling method that can generally simulate the performance of intumescent coatings, combine the large amount of published experimental data, and reliably produce fire-safe performance-based designs.
Research limitations/implications
Due to their limited applicability, high complexity and little comparability, the presented literature review does not focus on analysing and comparing different multi-component models, constituted of many model-specific input parameters. On the contrary, the presented literature review compares various approaches, models and thermo-physical properties which primarily focusses on solving the heat transfer problem through swelling intumescent systems.
Originality/value
The presented literature review analyses and discusses the various modelling approaches to describe and predict the behaviour of swelling intumescent coatings as fire protection for structural materials. Due to the vast variety of available commercial products and potential testing conditions, these data are rarely compared and combined to achieve an overall understanding on the response of intumescent coatings as fire protection measure. The study highlights the lack of information and homogenisation of various modelling approaches, and it underlines the research needs about several aspects related to the intumescent coating behaviour modelling, also providing some useful suggestions for future studies.
Three different experimental setups corresponding to three different fire scenarios were used to investigate how different heating conditions and heating rates affect the behaviour of two different ...thin intumescent coatings (a solvent-based and a water-based paint). Coated steel samples were exposed to different standard and non-standard fire conditions in an electric oven, in a gas furnace and in a cone heater. A common trend was observed in the thermal resistance development of the tested coatings and three phases (inert phase, transient phase and steady phase) were identified according to four critical points: activation, end of reaction, binder exhaustion and steel austenitization point. The results also showed that the water-based paint performed better at low heating rates, while the tested solvent-based paint performed better at high heating rates and did not activate or provide proper insulation at very low heating rates. In summary, the study confirms that the current procedure for the design of intumescent coatings has shortcomings, as different paints have different performances according to the heating conditions and, in particular, according to the fire heating rate.
•Common trend in the thermal resistance development for the tested intumescent coatings based on three general phases and four critical points.•Clear difference between the two tested products and importance of the heating conditions on the performance of intumescent coatings.•The tested water-based paint had better performance for low heating rates, while the tested solvent-based paint for high heating rates.•For very low heating rates, the solvent-based paint did not activate or provide proper insulation to the steel substrate.•The current design procedure has shortcomings, as different products have different performances according to the heating conditions.
The study presented herein describes an exploratory investigation on the fire performance of intumescent coatings used on timber elements. Timber samples, uncoated or coated with three different ...thicknesses of a commercially available thin intumescent coating, were tested using high-performance radiant panels according to the H-TRIS fire test method. Test samples were heated for 60 min at a constant incident radiant heat flux of 50 kW/m2. Uncoated samples quickly ignited, while coated samples showed good adherence between the intumescent coating and the timber substrate and limited flaming. At the start of the heating exposure, the intumescent coating rapidly swelled up to a quasi-steady thickness. The presence of the intumescent coating at the exposed surface of timber samples seemed to delay the onset of timber charring and also to reduce the average charring rate after initiation of charring. The delay is proportional to the DFT of the intumescent coating, up to 40 min from the start of heating for a DFT of 2.1 mm (based on 300 °C isotherm). The experimental results described herein showed that thin intumescent coatings may be effectively used on timber for delaying the onset of charring and assuming a reduced timber charring rate during heating.
This research study presents a heat transfer model aimed at estimating the thermal and physical response of intumescent coatings. The numerical model is inspired by the outcomes of an experimental ...study focused on analysing the insulating effectiveness of a commercial intumescent coating for a range of heating conditions and initial coating thickness. The model solves the one-dimensional heat conduction problem using the finite-difference Crank-Nicolson method, and it assumes that the effectiveness of intumescent coatings is mainly dependent on their ability to develop swelled porous char. The coating swelling is implemented in the model by adopting an approach based on expanding the mesh representing the physical domain in proximity to the substrate-coating interface. The model described herein offers researchers and engineers a tool to estimate the heat transfer of swelling intumescent coatings (i.e. in-depth thermal gradient). Outcomes of the analysis shown herein demonstrate that the heat conduction within intumescent coatings is governed by the physical coating swelling and the thermal conditions at the coating-substrate interface. The numerical model shows that its accuracy is highly influenced by the coating thickness ahead of the reaction zone. Consequently, the coating swelling rate plays a key role, while the thermo-physical properties of the intumescent coating have a secondary effect. According to its assumptions, the model defines a quasi-steady-state thermal problem: it is more accurate for conditions close to steady-state (e.g. high heat fluxes), but it loses accuracy for cases characterised by transient phenomena (e.g. phases prior to the onset of swelling and low heat fluxes).
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•Transient heat transfer model for swelling intumescent coatings.•Effectiveness of intumescent coatings based on ability to develop swelled char.•Swelling by expanding physical domain at the substrate-coating interface.•Correct prediction of the thermal gradient within swelling intumescent coatings.•Based on assumptions, quasi-steady-state thermal problem.
A commonly used approach to represent the thermal load in a compartment fire is the Eurocode Parametric Fire Curve (EPFC), which specifies gas temperatures (or rather adiabatic surface temperatures). ...Recognizing the significant deviations between real fires and the EPFC framework, the concept of model uncertainty is explored. This study does not aim to assess or improve the EPFC, but introduces a model uncertainty, allowing for reliability-based structural fire engineering (SFE). It presents a stochastic correction factor for the fire load density, based on the maximum temperature in steel sections. The focus is on the fire load density, but in general other parameters can be jointly taken into account as well. This correction factor considers protected and unprotected sections, incorporating variations in section factor and protection thickness. The findings reveal that the fire load density within the EPFC framework can be modified to better represent the severity of fire experiments. This approach ensures physical consistency of the obtained compartment gas temperatures, as opposed to alternative approaches for addressing the EPFC model uncertainty. While promising results are evident in this proof of concept, exploration for other types of structural elements and evaluation for structural systems is necessary before integration into design practices.
•The fire severity model uncertainty is quantified by fire load correction factor.•The declared thermal inertia of the linings strongly affects the correction factor.•Application of the correction factor allows for reliability-based design.
Summary
The experimental study presented herein investigates the influence of the substrate thermal conditions on the behaviour of thin intumescent coatings. Steel plates coated with a commercially ...available solvent‐based thin intumescent coating were exposed to a constant incident radiant heat flux of 50 kW/m2 in accordance with the heat‐transfer rate inducing system (H‐TRIS) test method. The influence of different substrate thermal conditions was investigated using sample holders capable of controlling the thermal boundary conditions at the unexposed surface of tested steel plates and comparing them to coated timber samples. Experimental results evidence that the substrate thermal conditions govern the swelling of intumescent coatings, thus their effectiveness in protecting load‐bearing structural elements. The substrate temperature controls the swelling of intumescent coatings because it defines the temperature experienced by the reacting virgin coating located close to the coating‐substrate interface. The physical and thermal properties of the substrate control the capacity of the system to concentrate/dissipate heat in proximity of the coating‐substrate interface. In this way, the substrate thermal conditions govern the temperature evolution of the reacting intumescent coating, consequently the swelling process. Accordingly, high swelling rates were recorded for highly insulating conditions (timber substrate), while low swelling rates for poorly insulating conditions (water‐cooled heat sink).
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