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Bench-scale fire stability testing - Protective layers in carbon fibre reinforced polymer laminates
(2023)
Fire resistance testing of components made of carbon fibre reinforced polymers (CFRP) composites usually demands intermediate-scale or full-scale testing. In this study, a bench-scale test is presented as a practicable and efficient method to assess the improvement in structural integrity of CFRP with different protective interlayers during fire.
Fire resistance testing of components made of carbon fibre reinforced polymers (CFRP) composites usually demands intermediate-scale or full-scale testing. In this study, a bench-scale test is presented as a practicable and efficient method to assess the improvement in structural integrity of CFRP with different protective interlayers during fire.
Fire stability is one of the biggest issues of carbon fibre reinforced polymer (CFRP) composites, particularly when they are using in load bearing applications. As soon as the glass transition temperature of polymer matrix (100-200°C) is achieved, the composite loses its structural integrity, what leads to the distortion and failure. The principal fire stability test is based on simultaneous application of fire and mechanical load. Since the carbon fibres can transfer the tensile loads quite well, the compression load is chosen as a required mechanical load for a test. The fire tests were preceded by a static load test at room temperature to determine ultimate failure load. The specimen was loaded with a compression force until the failure load was reached, which was observed as a buckling. For the fire tests, 10% of compression failure load and direct flame of a fully developed fire (heat flux ≈ 180 kW m-2) were applied simultaneously to the specimen, while the time to failure was measured.
The possibilities of bench-scale fire stability testing were presented by investigating new types of laminate structures, which exhibit promising flame retardancy and fireproof properties. The new approach of CFRP laminate differs with a protective concept. Every system consists of two different interlayers (titanium foil, thermoplastic foil PEI, ceramic layer WHIPOX, rubber tape Pyrostat, basalt fibres and kenaf fibres), where the one layer constitutes the fireproof protection, that delays the rise of temperature in the rest of laminate, and the second layer provides very good structural connection with carbon fibre layers, thus improves the mechanical integrity of CFRP composite in fire.
The time to failure of CFRP composite was 17s. Protective systems significantly enhance the fire stability and increased time to failure by 3 to 10 times. Thicker specimens showed longer resistance time, however CFRP laminate with ceramic layer and titanium foil presented outstanding results and the best performance. The protection with Kenaf and basalt fibres offered a natural fibre solution with also good fire resistance performance
Although bench-scale tests are limited with respect to assessing the performance of components and structures, they are valuable in the assessment of different materials concepts. Furthermore, the reduced effort of conducting bench-scale test (lower costs, time, personnel resources) makes it more practical and effective.
Intumescent coatings are commonly used in civil engineering to improve the fire resistance of steel constructions. Especially in the case of tension bars, where mostly circular or rectangular solid sections are used, intumescent coatings offer an efficient measure to improve the fire resistance taking advantage of profile-following application and low coating thickness requirements. Thus, the architectural appearance of slender profiles can be preserved. The paper describes real-scale mechanically loaded and unloaded fire tests of circular and rectangular solid steel tension bars with intumescent coating. The aim of these tests is to investigate the influence of the different profile types as well as different bar orientations on the performance of intumescent coatings. The results are used to specify a normative test and assessment procedure to be implemented in a new European standard for determining the contribution of intumescent coatings to the fire resistance of circular or rectangular bars used as tension members.
Textile-reinforced concrete (TRC) is a new composite material comprising high-performance concrete and textile reinforcement from textile yarns with a matrix, usually consisting of epoxy resins (ER). The most significant advantage of ER is the homogenization of all filaments in the yarn and full utilization of its tensile potential. Nevertheless, ER matrix is a critical part of TRC design from the perspective of the fire resistance due to its relatively low resistance at temperatures of approximately 120 C. This work expands the previously performed mechanical tests at normal temperatures with cement suspension (CS) as a non-combustible material for the yarn matrix. Here, the mechanical properties of CS matrix at elevated temperatures were verified. It was found that the addition of polypropylene fibers into HPC negatively affected the mechanical results of CS matrix specimens. Simultaneously, thermal insulation effect of the covering layers with different thicknesses did not significantly influence the residual bending strength of specimens with CS matrix and achieved similar results as reference specimens. Furthermore, all specimens with ER matrix progressively collapsed. Finally, CS as a textile reinforcement of yarn matrix appears to be a suitable solution for increasing the temperature resistance of TRC structures and for substituting synthetic resins.
Textile-reinforced concrete is currently most frequently used for non-load–bearing structures, but there is a vision for also using it in load–bearing construction elements. In recent years, this construction material has been subjected to detailed examination. Different combinations of materials for potential use in textile-reinforced concrete have been described. These differ in the type of concrete mix and the composition of the textile reinforcement. The aim of this work is to test the application of a specific textile-reinforced concrete, consisting of high-performance concrete, textile reinforcement from carbon fibers and its epoxy resin matrix, at an elevated temperature. The combination of these materials makes it possible to produce subtle load–bearing structures with excellent mechanical properties. The critical issue is the behavior of these structures when exposed to fire. A series of medium-scale fire condition experiments were carried out with a temperature load based on the ISO 834 curve, followed up by mechanical tests. The aim of these experiments was to describe critical areas of textile-reinforced concrete in fire and to propose possible solutions. In an indicative fire experiment, experimental samples displayed massive spall of concrete layers, and interaction between materials was lost due to the low temperature resistance of the epoxy resin. Concurrently, the optimal quantity of polypropylene fibers was experimentally determined. This paper presents an experimental demonstration of the problematic aspects of textile-reinforced concrete and subsequent recommendations for future work with practical application in the design of load–bearing structures.
The Burning of Plastics
(2021)
The burning of a polymer is a physico–chemical process strongly influenced by the coupling of a chemical reaction – oxidation of fuel – in the gas phase with a chemical decomposition reaction – pyrolysis – in the condensed phase via heat and mass transfer. The heat and mass flux control the intensity of fire and the ablation of fuel. Indeed, the temperature profile as a function of time may be one of the most important responses of a specimen to understand its burning behavior. Further, several physical phenomena, such as the heat absorption of the materials, thermal conductivity, and also melt flow and dripping, play a major role in determining ignition, flammability, and fire behavior. The burning of a polymer is very complex. The various phenomena interact with each other, e. g., pyrolysis also influences the viscosity of the melt, and, thus, whether dripping or charring results in a protective layer, increasing the shielding effect of the residual protective layer. Only a detailed and comprehensive description opens the door to a well-founded understanding of the burning behavior of polymeric materials.
A systematic approach was used to investigate the weathering-induced degradation of a common water–based intumescent coating. In this study, the coatings are intended for humid indoor applications on steel substrates. The coating contains ammonium polyphosphate, pentaerythritol, melamine, and polyvinyl acetate. By replacing each ingredient with a less water-soluble substance, the most vulnerable substances, polyvinyl acetate and pentaerythritol, were identified. Furthermore, the weathering resistance of the system was improved by exchanging the ingredients. The coatings were stressed by artificial weathering tests and evaluated by fire tests. Thermogravimetry and Fourier-transform infrared spectroscopy were used to study the thermal decomposition. This study lays the foundation for the development of a new generation of water-based intumescent coatings.
Although concrete itself is not a combustible material, concrete mixtures with high density, such has high-performance concretes (HPCs), are susceptible to significant damage during fires due to explosive spalling. Past research has shown that the inclusion ofpolymer fibres in high density concrete can significantly mitigate this fire damage. The exact mechanisms causing this increased spalling resistance are not yet fully understood, but it is thought that the fibres facilitate moisture transport during fire exposure, which in turn contributes to relief of internal stresses in the spalling-susceptible region. In this study, X-ray Computed Tomography (CT) was applied to observe the interaction between polymer fibres and cracking during thermal exposure. For this purpose, two concrete samples containing different polymer fibre types were subjected to incremental application of a defined thermal exposure. CT images were acquired before and after each thermal exposure and powerful image processing tools were used to segment the various material components. This enabled a detailed analysis of crack formation and propagation as well as the visualization and quantification of polymer fibre characteristics within the concrete. The results demonstrated that the orientation of both fibres and cracks in polymerfibre reinforced concrete tend to be anisotropic. The results also indicated that crack geometry characteristics may be correlated with fibre orientation, with cracks tending to run parallel to fibre beds. Clear quantitative relationships were also observed between heating and increasing cracking levels, expressed in terms of both crack surface area and crack volume.
Flame retarded polymeric materials are used in various applications in which a certain fire behavior is demanded. Protection goals are defined, such as limited flammability in terms of hindered sustained ignition or limited contribution to a fire, and these protection levels are tested with defined specimens or components in defined fire scenarios, that is to say, different fire tests. Passing a specific fire test by meeting whatever its demands is often the most important development goal, so the parameters of the different fire tests vary widely to emphasize different fire properties. Some fire tests are used to screen or provide a general assessment of flame retardant polymers during development, while other fire tests and tailored experiments are performed to address special phenomena or understand the flame retardancy modes of action. For all fire testing, the devil is in the details – demanding know-how and crucial efforts to manage the quality of investigations and advanced interpretation. This chapter aims to offer a structured overview of all these aspects.
For the purpose of increasing payload and reduce freight cost, lightweight composite tank containers used for Transportation have been progressively developed during the last years. Compared to conventionally produced cylindrical steel tanks, the fiber-reinforced solutions allow greater flexibility in the tank design. Despite a number of further material-related benefits of fiber-reinforced composites as non-conductive and non-magnetic behavior as well as corrosion resistance and high strength, the optimization of their thermal degradation properties during combustion is still a challenge. To improve the fire performance of lightweight composite containers, special intumescent fire protection coatings can be applied onto the outside tank surface. This paper presents fire tests on glass-fiber-reinforced plastic transport tanks with complex geometries sheltered with different surface-applied fire protection systems. To evaluate the fire resistance of the tank structures, a fiber optic monitoring system was developed. This system is based on distributed temperature measurements using high-Resolution optical backscatter reflectometry and pointwise reference measurements using fiber Bragg gratings. Thereby, all the fiber optic sensors were directly integrated in the composite layer structure of the tanks. The focus of the presented work is on the demonstration of capability of fiber optic monitoring system in such high-temperature application. Moreover, the fiber optic measurements provide new insights into the efficiency of intumescent coating applied for fire protection of fiber-reinforced plastic transport tanks.