Weerakkody, Neythra Geetanjelee (2022) Evaluation of exterior wall cavity fires using an intermediate scale test method. Research Master thesis, Victoria University.

Abstract

External wall (EW) cavities are an integral part of an external wall system (EWS), providing space for structures, building services, insulation, waterproofing membranes and drainage. Over the past three past decades, the number of external wall fire incidents have gradually risen worldwide, with the most notable of these being the Grenfell Tower fire in London. The 2017 Grenfell Tower fire started when a non-flashover kitchen fire spread into the external wall cavity which included combustible insulations and combustible external cladding. Within wall cavities, combustible insulation can provide fuel load and vertical air gaps provide an arrangement that helps shield and insulate fires during the incipient stage, while at the same time supplying enough ventilation to support fire spread. This study aims to investigate fire spread within an EW cavity containing combustible materials within an otherwise, ‘deemed’ non-combustible EWS. The cavity test rig design was based on FM Global’s Cavity Fire Test method referenced within the FM4411-2020 standard. The experimental component of this research has selected different test parameters (such as ignition type, ignition size, cavity widths and chosen test specimens) to that examined under the FM Global Cavity Fire Test study, to gain further understanding of fire behaviours within an EW cavity. These test parameters included the following: • Liquid fuel based ignition sources of either methylated spirits or heptane. • Three fire ignition sizes created by using either: o One tray, ~6-8 kW methylated spirits fire (fuel surface area of 0.0125m2), o One tray, ~80kW heptane fire (fuel surface area of 0.0125m2), o Two trays, ~200kW heptane fire (fuel surface area of 0.025m2). The Heat Release Rate (HRR) of these ignition sources were influenced by the surrounding boundary conditions created within the cavity. • Main cavity width of 65mm air gap plus the thickness of the installed insulation (a cavity air gap width of ~130mm was used to conduct Cavity Rig characterisation tests only). • A range of cavity materials of varying fire performance that included two thermoplastic insulations (polystyrene board and polyester batts), two thermosetting insulations (polyisocyanurate foam and phenolic foam) and one type of foil faced polypropylene sarking. The thermosetting materials were supplied with a protective facing adhered to both sides. These materials were tested with and without the protective facing. Three ignition sizes were chosen to represent a range of possible cavity fire scenarios. The size of the one tray methylated spirits fire (reduced scale ignition) represented a small, localised fire, developing on materials, created from an electrical fault. The exposure conditions and ignition size of the methylated spirits fire also, most closely represents the ignition source devised for the FM Global Cavity Fire Test under FM4411-2020. The size of the heptane tray fire (base scale ignition) represented a pre-flashover (or post flashover) compartmental fire breach into the cavity (such as was evident for the Grenfell Tower fire). An additional two tray heptane fire (sensitivity scale ignition) was adopted to examine if an increased fire size would reveal further discriminatory data and/or fire behaviour between the chosen materials. The one tray heptane fire was concluded to be sufficient in establishing ignition and providing discriminatory results (in terms of HRR, temperature, radiant heat and fire progression and post-test damage data). It also revealed a range of reaction-to-fire behaviours between the tested materials. Such behaviours included the swelling of char layer formation on the polyisocyanurate material, extended smouldering combustion of both polyisocyanurate and phenolic foams after the ignition source burnout, and differing formation of molten flow and pool fire spread between polyester batts and polystyrene board. Except for the single type of foil faced polypropylene sarking, all materials experienced fire spread to the top of the cavity under one and two tray heptane fires. The methylated spirits fire ignition source did not promote fire spread for most materials (it did involve the aluminium paper facing on phenolic board and resulted in some limited spread on exposed PIR). Fire hazards associated with thermoplastic materials such as the production of pool fires that have the potential to flow and spread laterally within a cavity floor, are not fully captured by small-scale test methods like AS1530.2 and AS1530.3. The fire size and exposure conditions of these tests are too small and do not represent cavity fire scenarios. Currently, large-scale tests provide the most reliable information in predicting real-scale fire risk posed by a particular EWS design, however large-scale test methods do not address potential fire hazards resulting from fires originating within a cavity. The intermediate scale sized test rig addresses the shortcomings of both small- and large-scale test methods in representing cavity fire scenarios. Overall, the design of this cavity fire test successfully enabled investigation of ignition and reaction to fire behaviour of a range of different types of cavity insulation and sarking within an end-use arrangement.

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