ASSESSMENT OF TIMBER PARTITION MATERIALS WITH FIRE RETARDANTS WITH A ROOM CALORIMETER

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1 , Volume 6, Number 3, p , 2004 ASSESSMENT OF TIMBER PARTITION MATERIALS WITH FIRE RETARDANTS WITH A ROOM CALORIMETER C.W. Leung, W.K. Chow Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China G.W. Zou, H. Dong and Y. Gao Department of Building Engineering, Harbin Engineering University, Harbin, Heilongjiang, China (Received 16 February 2004; Accepted 12 April 2004) ABSTRACT Treating combustibles with suitable fire retardants can improve their fire behaviour, say limiting the rate of flame spread. There are requirements on treating materials such as polyurethane foam with fire retardants in the local legislations. To understand behaviours of burning those fire retarded materials under real fires, more indepth studies should be carried out with fire scenarios due to accidental or arson. Three different types of fire retardants commonly used were assessed by full-scale burning tests. The fire retardants were applied to the surface of chipboard partitions. Wallpaper was also tested over chipboard to investigate its effects on flame spread. Those products were tested under an accidental fire and a flashover fire in a room calorimeter. Ten full-scale burning tests were conducted and the results on the heat release rate, floor heat flux, upper layer gas temperature, surface temperature over materials and temperature profiles inside the room and at the doorway of the room will be reported in this paper. 1. INTRODUCTION Timber boards are widely used for internal partitioning for buildings in the Far East. Offices, karaokes and rooms are partitioned by those combustibles. There are concerns on their fire behaviour as flame spread over the timber partitions might give big fires. At the moment, local legislative requirements on flame spread in the fire codes [e.g. 1] include specifications for lining materials used in ductings, concealed locations and protected means of escape only, though the fire codes are under review. Those materials are required to be tested according to the bench-scale test BS 476: Part 7 [2] and satisfy Class 1 or 2 of the testing criteria. This gives some limits on flame spread. Materials should be brought up to that standard by the use of a fire retardant product approved by the Fire Services Department (FSD). Some requirements on the application of fire retardants in karaoke establishments and registered premises are also specified in the licensing requirements issued by the Home Affairs Department [3] such that all combustible false ceilings, partitions and wall furnishings are required to be coated with approved fire retardant products. Applying appropriate fire retardants would delay the time to ignition, reduce the heat release and thus reduce the fire hazard. However, the effectiveness of those fire retardants in slowing down the flame spread rate over materials depends on the application method, the substrate, geometry and the fire size. Fourteen full-scale burning tests on bare timber products and timber products with paint, wallpaper, fiberglass and fire retardants were carried out in a room calorimeter in Harbin in China [4]. Materials were tested based on the ISO 9705 testing scenario [5]. The products were covered on the ceiling and the walls of the room calorimeter of 3.6 m long, 2.4 m wide and 2.4 m high with a doorway 0.8 m wide and 2.0 m high. A square gasoline fire of side length 0.26 m and a heat output of about 45 kw was used as the ignition source. The materials were evaluated for 20 minutes or until flashover, whichever occurred first. Comparing the heat release rate from burning bare chipboard and chipboard with fire retardants, it was found that the specific fire retardant coating was very useful in reducing the flame spread over the tested wood surfaces. The time to ignition was delayed and the burning area was limited to the vicinity of the ignition source. Only limited amount of heat was released. But the fire retardant is effective only in the particular applications with the fire scenario being only an accidental fire starting from a small source comparable to a paper basket fire. The protection provided by such fire retardant in flashover conditions is unknown. Further tests should be carried out. 122

2 To compare different fire retardants and to investigate their performances under different fire conditions, eight full-scale tests were carried out on chipboard with and without fire retardants in the Chinese Assembly Calorimeter. Fire retardants passing the BS 476: Part 7 test were selected to compare their performances under accidental fire and flashover conditions. The materials were assessed for 20 minutes in both scenarios or until no burning over the materials was observed. The data on time to ignition, time to flashover, heat release rate, floor heat flux, surface temperature of the materials, upper layer gas temperature and temperature profiles at the doorway, at the front wall corner of the room and at the center of the room were collected. Two tests were also carried out on chipboard with and without wallpaper to compare the effects of finishing materials under the two fire scenarios. 2. THE TESTS The ISO 9705 room-corner fire test is considered to be appropriate for assessing the burning behaviour and flame spread over materials [6]. However, this test requires relatively high cost and is quite timeconsuming to be carried out. Further, a large amount of materials, i.e. 32 m 2 is required per test. This might not be good for material quality control and the development of new products. There, many tests should be carried out to provide representative results with acceptable repeatability. A new testing approach was used in this paper for reducing the use of excess materials; and allowing more focused and precise measurements on a smaller area without spending too much resources. The materials were still tested in the room calorimeter. Instead of covering materials on all the walls and ceiling surfaces as in ISO 9705, materials were only attached on the rear wall of the room, 2.4 m wide and 2.4 m high (about 6 m 2 ) as shown in Fig. 1a. Two wood panels of 1.2 m wide and 2.4 m high were used in each test. Two testing conditions were designed: Accidental fire A square gasoline fire of side length 0.35 m and 0.25 m high was used to simulate an accidental fire. About 4.5 litres gasoline was burnt to give an average heat output of 100 kw. The heat release rate was over 100 kw for 730 s and then over 80 kw for 470 s as in Fig. 2. The fire source was put at the left rear wall corner as shown in Fig. 1a. The results are useful for studying the fire growth, development to flashover, flame spread from the fire source over the materials, and the rate and extent of burning. Fig. 1: Experimental setup 123

3 Heat release rate / kw Flashover fire source measuring the upper layer gas temperature (T1 to T6). Temperature readings were recorded every 1 s. 3. MATERIALS TESTED An 8 mm thick chipboard sample widely used in Hong Kong was selected. Three common types of fire retardants for wood substrates were applied for comparing their effectiveness in fire control: Time / s Fig. 2: Heat release rate measured Flashover Accidental fire source Using a small fire source might only involve a limited part of the materials in burning. The heat released in such limited burning might not be very high. This is not good enough to show the contribution of materials in a post-flashover fire or an arson fire. The materials might burn much more vigorously and give out much more heat under a strong heat flux higher than 20 kwm -2. To study that, flashover was set off by burning 12 litres gasoline in a bigger pool of diameter 1.0 m at the center of the room as shown in Fig. 1a. Tests were repeated by trial and error to ensure that the amount of gasoline used was sufficient to give flashover and then burnt off with little contribution to the fire with the chipboard partitions ignited. Such testing condition is similar to the cone calorimeter where the materials were exposed to an external radiant heat flux at or above flashover values. The heat release rate curve is shown in Fig. 2. The time to ignition was observed visually. All smoke and combustible products were collected by a hood outside the doorway, extracted through the duct section for determining the heat release rate by the oxygen consumption method. Heat release rate data were recorded every 5 s. A heat flux meter was located on the floor. Forty-two 1 mm type K inconel sheathed thermocouples were used for temperature measurement as shown in Figs. 1b and 1c. Twenty points (R1 to R20) were attached on the materials for recording the surface temperature. Three thermocouple trees were used to measure the temperature profiles. There were six points at the center of the room (M1 to M6), six points at the front corner of the room (C1 to C6) and four points at the doorway (D1 to D4). Six thermocouples were fixed at 30 mm below the ceiling for FR1: A synthetic binder and fire retardant material; FR2: A fire retardant paste composed of potassium silicate, mica powder and quartz sand; FR3: An intumescent water-based formulation combining flame retardancy with decorative finish. FR1 and FR3 are approved fire retardants by the FSD. Products treated with FR1 can be classified as Class 2 and those with FR3 as Class 1 under BS476: Part 7. Only limited flame spread would be observed for both samples. FR2 is a common fire retardant paste for wood in China. Another set of tests was carried out by covering the chipboard with a common brand of wallpaper to investigate its effects on flame spread under the two testing scenarios. 4. RESULTS ON SURFACE TEMPERATURE The materials were attached on the rear wall of the test room. The flame spread progress could only be observed through the doorway. Drawing gridlines on the surface might help to trace the flame front. For safety reasons, operators cannot stay too close to the test room due to the high heat output generated during the test. But, on the other hand, viewing from a far distance might not provide accurate results. In the flashover scenario, the flame generated from the large pool fire at the center of the room would totally block the view. Visual observation of the flame spread over the rear wall surface before the pool fire dies down is impossible. To solve this problem, high temperature rated thermocouples were attached to the material surface for tracing the flame front and flame spread progress. Twenty thermocouples were located in an array of four rows and five columns as shown in Fig. 1b. According to previous full-scale roomcorner fire tests on wood products [4], it was found that the flame most likely spread upward due to the fire source located at the corner. When the flame 124

4 tip reached the ceiling, the ceiling materials would be ignited and the flame spread radially, followed by lateral spread along the ceiling-wall interface. When a flame layer was formed over the ceiling, flashover occurred and the flame spread downward from the ceiling-wall interface. Little lateral spread from the upward spread zone was observed. Without ceiling materials in these new sets of tests, it was expected that flashover might not occur and the materials far away from the fire source might not contribute to burning. So, more thermocouples were located at the region close to the source, i.e. the left panel. The points on the right panel were for base measurement of the temperature. In the accidental fire scenario, the side length of the gasoline container was 0.35 m. It was assumed that the flame width would be equal to the side length of the fire source. The first column of thermocouples was located at 0.3 m from the left edge, which was in the burner flame region and where the materials would be first ignited. The second and third columns were located 0.3 m apart. Since the major flame spread region was expected to be kept within the vicinity of the fire source, the fourth and fifth columns were located with 0.6 m separations. The first row of thermocouples was located at 0.15 m below the ceiling, which was the zone in which lateral flame spread was expected. For the flashover scenario, the materials were exposed to a high radiant heat flux at the same time. A regular array would be suitable. However, the same array used in the accidental fire scenario was used to allow comparisons. The peak surface temperature are identified and shown in Fig. 3 and Table 1. This provides a picture of the critical temperature on the materials during the tests and for investigating the extent of heat transfer. Accidental fire From the profiles obtained in the accidental fire tests as in Figs. 3b to 3f, it is observed that the highest temperature was recorded in the first column above the burner due to the direct flame impingement from the fire source and vigorous upward flame spread. High temperature values were also recorded in the first row below the ceiling, indicating that lateral flame spread was resulted when the upward flame tip reached the ceiling. Comparing the temperatures in Column 2, bare chipboard reached 370 o C. For chipboard with FR1, the temperature was up to over 200 o C. With FR2, FR3 and wallpaper, there were only a little temperature rise. The temperatures were up to around 160 o C only. This indicated that it is easier to have lateral spread from the upward spreading region for bare chipboard and chipboard with FR1. Little downward spread was shown and the region far away from the vicinity of the fire source remained at a low temperature below 100 o C. Flashover Under flashover, no significant temperature contour can be observed as shown in Figs. 3g to 3k. The surface temperature recorded for bare chipboard, chipboards with FR1 and FR2 were higher, with values up to over 600 o C. 14 out of 20 points in the chipboard with FR1 were over 200 o C. For the chipboards with FR3 and wallpaper, the surface temperature of the whole surface was below 260 o C. There was relatively little temperature rise when compared to the other three cases under the same flashover condition. It appears that adding wallpaper could provide some protection to the chipboard. The surface temperature-time curves are shown in Figs. 4 and 5. Assuming that the materials are ignited when the surface temperature reaches the ignition temperature, the time to ignition and thus the propagation of the flame front can be monitored. In the accidental fire scenario, the time to ignition of the materials can be obtained through visual observation. Taking bare chipboard as an example, the surface behind the source, R16 was ignited at 110 s. Comparing the surface temperature at 110 s at R16 in Fig. 5a(vi), it can be assumed that the ignition temperature for bare chipboard is 200 o C. By identifying the time for the other points to reach such apparent ignition temperature, the flame spread progress and rate can be determined. The same apparent ignition temperature can be used in the flashover scenario for the same material. 5. RESULTS ON UPPER LAYER GAS TEMPERATURE The upper layer gas temperature was measured. All points were fixed at 30 mm below the ceiling as shown in Fig. 1c. T1 was located directly above the accidental fire source. Flashover The upper layer gas temperatures in the flashover tests were over 600 o C due to the onset of flashover inside the room as shown in Fig. 6. There was only one peak in the temperature-time curve, indicating that flashover occurred only once in the compartment, which was onset by the pool fire source. No flashover was caused by further burning of the materials. 125

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18 Accidental fire The highest temperature value was recorded at T1 as shown in Fig. 7, which was up to 800 o C due to the direct flame attack from the upward flame spread caused by the accidental fire source. For bare chipboard, it took 270 s for the surface of the material, R1 to reach the ignition temperature, say 200 o C as shown in Fig. 5a. By visual observation, it took about 300 s for the flame tip to reach the ceiling. Comparing with 400 o C at T1 at 300 s, a reference can be provided for determining the time for the upward flame spread to the ceiling. For the other points fixed at the center of the ceiling, i.e. T2 to T6, the results were between 250 o C to 400 o C for all cases, indicating that there was no flashover inside the room when only the rear wall was covered with the materials. 6. RESULTS ON TEMPERATURE PROFILES Three thermocouple trees were located at the center of the room, at the left front corner of the room and at the doorway as shown in Figs. 1b and 1c. There were six points at the corner to provide the gas temperature profile in the room and for determining the descent of the hot gas layer. The thermocouple tree at the center was located above the flashover pool fire in the flashover scenario. This can provide the flame temperature of the pool fire and the gas temperature profile in the accidental fire scenario. The thermocouple tree at the doorway provided an indication of the hot gases and smoke flowing out of the room. This can allow the determination of the approximate transient smoke layer interface height. Flashover The temperature profile obtained in the flashover scenario was dominated by the pool fire. The effects of the burning materials were small and there was no significant difference between the results of different materials as shown in Figs. 8 to 10. The temperatures collected above the pool fire were all over 600 o C due to the direct attack of the pool fire as shown in Fig. 9. From the doorway temperature profiles, D1 to D4 were all above 500 o C as shown in Fig. 10. This indicated that the hot smoke layer descended below D4, i.e. 1.1 m above the floor. This agreed with the visual observation where the smoke layer descended to 0.8 m at the doorway. Accidental fire The corner temperature profiles are shown in Fig. 11. The peak temperature recorded for bare chipboard was the highest, up to 290 o C. It is still far below the upper layer gas temperature, say 500 o C at flashover. The profiles for chipboards with FR1, FR2, FR3 and wallpaper were similar. The temperatures recorded at points below 1.42 m (C4) were maintained at below 100 o C throughout the tests for all cases. The temperature profiles at the center as in Fig. 12 were similar to those at the corner. At the doorway, distinct temperature readings were obtained for the upper two points, D1 and D2, and the lower points, D3 and D4 as shown in Fig. 13. This showed that the smoke layer interface height lay between D2 and D3, i.e. 1.7 m and 1.4 m from the floor, with hot gases flowing out the door above D3 and cool air supply into the room below D3. Comparing the temperature profiles among different materials, the profiles for chipboard with FR1 stayed at the peak values for a longer period. 7. RESULTS ON HEAT RELEASE RATE The heat release rates in both scenarios are compared in Figs. 14 and 15. The peak heat release rate and total heat released are tabulated in Table 2. Flashover The heat release rate was measured until the pool fire died down and no burning over the materials was observed. The heat release rate curves obtained in the flashover tests were very different from those obtained under an accidental fire. There was no significant shifting of the curves. It is mainly due to the high heat output, up to 2.4 MW, from the flashover pool fire. The contributions from the materials were comparatively small. However, a reduction in the heat release rate by applying those fire retardants can still be observed. The heat release rate curves are shown in Fig. 14a. The peak values are shown in Table 2. The highest peak heat release rate was recorded in the chipboard with wallpaper, which was about 300 kw higher than bare chipboard but the total heat output was 2 MJ less. 139

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27 Adding fire retardants helped to reduce the peak heat release rate. There were reductions of about 300 kw for FR1 and FR2, and 400 kw for FR3. However, with FR1, the high heat release rate was kept for a longer time to give a flatter curve. The total heat released was even 20 MJ higher than bare chipboard as shown in Table 2 and Fig. 15a. Applying FR3 gave the lowest peak heat release rate and the total heat release was reduced by 28 MJ. This is the best fire retardant among the three fire retardants for improving the fire behaviour. Accidental fire In the accidental fire tests, the highest peak heat release rate was recorded in burning bare chipboard as shown in Fig. 14b. With wallpaper covering, the peak heat release rate was slightly less than bare chipboard. However, more rapid burning and flame spread was observed when wallpaper was added, causing the heat release rate curve to shift to the left. The total heat released was 16 MJ more than bare chipboard, which is much larger than the difference under flashover. For the three fire retardant coatings, FR2 was not too satisfactory. The total heat release was 10 MJ less than bare chipboard but there was only a little reduction in the peak heat release rate and the time to reach the peak was shortened. With FR3, there was a much longer time delay in reaching the peak heat release rate and the heat release rate curve was shifted to the right. The flame size was observed to be the smallest and the peak heat release rate was 400 kw lower than bare chipboard. The total heat release was the lowest and 24 MJ less than bare chipboard. By adding FR1, the peak heat release rate was reduced by 170 kw but the total heat release was 1 MJ higher than bare chipboard. Similar to the flashover testing results, a flatter heat release rate curve was obtained. The heat release rate was kept at about 300 kw, which is the peak value, for over 3 minutes. When comparing with bare chipboard, it appears like removing the top part of the heat release rate curve of bare chipboard. So, eventhough the peak heat release rate was kept for a relatively longer period, still some protection was provided and there was some reduction in the heat release when FR1 was applied. For all the materials, the heat release rate curves return to around 100 kw after about 600 s, with heat given out from the fire source only. There was no further flame spread over the materials. In comparing the heat release rate results of both scenarios, applying FR3 provided a good protection and reduction in both the peak heat release rate and total heat release. FR1 performed well under accidental fires but poor in flashover. Similar protection was achieved by FR2 in both scenarios. 148

28 Adding wallpaper gave different results in the tests. Under flashover, the total heat release was slightly less than bare chipboard but the peak heat release rate was higher. Under an accidental fire, there was a slight reduction in the peak heat release rate but the total heat released was 16 MJ higher than bare chipboard. Such distinct results showed the necessity of testing composite materials under different fire scenarios. 8. RESULTS ON HEAT FLUX In the flashover tests, the peak heat flux at floor level was about 15 kwm -2, which is in the order of 10 times the values recorded in the accidental fire tests, i.e. about 1 to 2 kwm -2 only as shown in Figs. 16 and 17. The high heat flux in the flashover scenario only lasted for about 180 s and there was only surface charring over the materials under such exposure. The materials were not contributed to complete burning. It appears that the materials are quite safe under such flashover scenario. However, the total heat released from the materials only, excluding the contribution from the fire source, as tabulated in Table 2 under flashover within 6 minutes was similar to that released under an accidental fire for 20 minutes. The values are comparative. For a higher heat flux, say 50 kwm -2 or a longer exposure time, the materials might ignite and burn vigorously. Conducting tests with a cone calorimeter might help to find out a correlation with the existing results and for further studies on using higher heat flux values. 9. OBSERVATIONS The conditions of the materials were observed after the tests. The burnt zones under the same scenario appeared to be similar as shown in Figs. 18 and 19. Flashover There was only surface burning over the materials. The back of the materials remained unburnt. Only the upper edge of chipboard with FR1 was burnt after the test and other materials remained in their full sizes. The lower part, about 0.6 m from ground level, remained unaffected for all cases. There was no significant burning and only little parts of the surfaces were darkened as in Fig. 18. Applying fire retardants FR1 showed the least protection on the material structure. Thick layers of char were formed over the surface, with large pieces of charred surfaces fell off. The upper edge of the panels was burnt out. There were lots of cracking. With FR2, small parts of the charred surfaces fell off. Nearly half of the materials at the lower part remained unburnt. With FR3, many bubbles were formed due to the foaming up characteristics of intumescent coatings. The char remained adhered to the material surface and there was no dripping. Note that the fallen flaming surfaces or smoldering charring pieces in the other tests might cause further fire spread, especially when the floor is covered with easily ignitable materials like carpet. The wallpaper was involved in burning and peeled off from the chipboard surface. Some darkened bubbles were formed. The exposed wood surface under the wallpaper was charred. Accidental fire The region above the fire source and the upper edge of the materials were burnt out in all the cases. The burnt areas were similar in all the tests, with a larger charring zone for bare chipboard. There was upward flame spread due to the fire source which caused the zone above it to burn out. When the flame tip reached the ceiling, flame spread along the ceiling-wall interface laterally. There was no significant lateral spread caused by the fire source or the burning zone above it and no further downward spread was induced by the burning zone along the interface of the ceiling and the upper wall edge. This agreed with the results on surface temperature. Char was formed at the burnt edges which helped to stop the flame from spreading out. Note that in the room-corner fire tests, when the flame reaches the ceiling and with lateral spread along the ceiling-wall interface, the ceiling would be ignited. A flame layer would be formed and it would shortly go to flashover in the compartment. It is believed that flashover would be occurred in this set of accidental fire tests if all the wall and ceiling surfaces are covered with the materials. 10. CONCLUSION Local fire codes and licensing requirements [1,3] allow the use of fire retardants to give some safety protection. Perhaps, the flame spreading rate over combustible construction materials might be slowed down, to be confirmed by standard test BS476: Part 7. However, BS 476: Part 7 is only a bench-scale test. Fire retardants are required to be applied to those materials with high flame spread rate [1,3]. Those fire retardants would appear on an approved product list if they pass this test. However, restricting flame spread over the materials in such a test might not provide adequate protection in other scenarios such as under flashover fire. 149

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33 Ten full-scale tests were carried out in the Chinese Assembly Calorimeter on bare chipboard and chipboards with three types of fire retardants, with two appeared on the FSD approved list, and wallpaper. The products were assessed under an accidental fire and a flashover fire. The materials were covered on the rear wall of the room calorimeter in both tests. No flashover was caused by the heat given out from the materials in both testing scenarios. A larger area of materials were burnt out in the accidental fires. Only surface burning and charring occurred in the flashover tests. The extent of protection provided by the fire retardants and the effects of wallpaper on the heat release rate are different under the two fire scenarios. Treating chipboards with fire retardant might not necessarily provide good protection. As tested, higher total heat released was found under flashover with FR1 added. More rapid burning was resulted in the accidental fire tests by adding FR2. A fire retardant performing well in an accidental fire might not necessarily provide the same protection under flashover. Among the three fire retardants tested, the intumescent coating FR3 performed better than the other two. The peak heat release rate and total heat release were reduced in both scenarios. Under an accidental fire, adding wallpaper gave out 207 MJ, which was 16 MJ higher than bare chipboard. The peak heat release rate was 456 kw, only 52 kw lower than 508 kw for bare chipboard. Under flashover, the peak heat release rate with wallpaper was 330 MW, about 300 kw higher than bare chipboard but the total heat released was similar, about 480 MJ in both tests. REFERENCES 1. Codes of Practice for Minimum Fire Service Installations and Equipment and Inspection, Testing and Maintenance of Installations and Equipment, Fire Services Department, Hong Kong Special Administrative Region (1998). 2. BS476: Part 7: 1997, Fire tests on building materials and structure, Part 7, Method of test to determine the classification of the surface spread of flame of products, British Standards Institution, London, UK (1997). 3. Licensing requirements for registered premises (hotel, guesthouse, holiday flat, holiday camp, club, bedspace apartments), Office of the Licensing Authority, Home Affairs Department, Hong Kong Special Administrative Region (1999). 4. C.W. Leung, W.K. Chow, G. Zou, H. Dong and Y. Gao, Preliminary experiment results on fire behaviour of timber partition materials with a room calorimeter, International Journal on Engineering Performance-Based Fire Codes - Submitted for consideration to publish, January (2004). 5. ISO 9705: 1993(E), Fire tests Full-scale room test for surface products, International Standards Organization, Geneva, Switzerland (1996). 6. C.W. Leung and W.K. Chow, Assessing flame spreading of materials with smaller-scale tests - Submitted for consideration to publish, November (2003). The measurements of the surface temperature of materials allow the estimations of the time to ignition, flame spread progress and further derivation of the flame spread rate. The transient smoke layer interface height can be estimated from the doorway temperature profiles. In this paper, it is shown that fire retardants passing a bench-scale test might not provide significant protection in larger scale tests or even postflashover fires. Materials covered with finishing materials might behave differently under different fire scenarios. The Authority might consider specifying other more appropriate assessment tools for testing fire retardants and finishing materials. ACKNOWLEDGEMENT This project is funded by a PolyU research grants under account number G-W