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Experimental Study on Fire Characteristics of Building External Thermal Insulation Composite Systems under Corner Wall

Cheng X. D.*, Xu L., Yao Y. Z., Yang H., Zhang R. F., Zhang H. P.

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui, China

*Corresponding author email: chengxd@ustc.edu.cn

ABSTRACT The research objective was to gain the fire characteristics of the External Thermal Insulation Composite Systems (ETICS), which is widely used in China. A series of large-scale experiments were conducted on the ETICS with the core of rigid polyurethane foam (PUF) under corner wall in relatively open space. Both flame-retardant and flammable rigid PUF were used in the experiments, and the various thicknesses (3 mm and 5 mm) of the outer layer were also considered. Under the fire source impact, a cavity inside the ETICS was formed during the flame evolution period, which was obviously different from the pure thermal insulation panel. For better assessing how to reduce the fire risk of ETICS, a detailed analysis was performed to quantify some major parameters, such as heat release rate (HRR) and temperature distribution. The results indicated that the peak of HRR could be reduced 54% by the thicker outer layer (5 mm), and reduced 40% by the flame-retardant treated rigid PUF. Furthermore, the fire growth rates of PUO-5mm and PUZ-3mm were 73% and 58%, respectively, lower than that of PUO-3mm. A special variation of the temperature on the ETICS was also observed and analyzed. The results also indicated that the outer layer and the flame-retardant treatment were the two important factors for the fire safety of the ETICS.

KEYWORDS: ETICS, HRR, temperature distribution, rigid PUF foam, fire risk.

INTRODUCTION

The External Thermal Insulation Composite Systems (ETICS) is widely popularized by the energy conservation policy in China. A remarkable success has been achieved on the building energy conservation, which is meaningful to Energy Conservation and Emission Reduction. However, lots of flammable materials, such as rigid polyurethane foam (PUF), are used as the thermal insulation core material on ETICS, which have greatly increased the fire risk of the buildings, especially the high-rise buildings. Therefore, more attention should be paid to the fire risk of the ETICS with rigid PUF core.

Some researchers [1-8] have studied the upward flame spread process for organic insulation materials, in which the finite width of the sample [2], heat-feedback of flame [3], external radiation [4] and melting behavior [5] were discussed. The rigid PUF has also been studied fully as the materials have been widely used during the past decades. Singh et al. summarized that the rigid PUF are easily ignitable, support combustion, and burn quite rapidly [9]. Muller et al. investigated the parameters influencing the mechanical resistance of intumescent chars [10]. Pau et al. used thermogravimetry at different heating rates of 1, 5, 20 and 60 °C/min to obtain the foams decomposition behavior [11]. Levchik et al. thought that the density of the foam and air flow are the other two important factors for the flame spread [12]. Xue et al. performed a series of cone calorimeter experiments to investigate the degradation properties of PUF [13]. However, the fire characteristics of ETICS have not been adequately investigated in both modeling and experimental studies before, which is a new subject in distribution inside the ETICS and the effect of outer layer are still unknown. The ETICS is similar to the Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8), pp. 60-67 Edited by Chao J., Liu N. A., Molkov V., Sunderland P., Tamanini F. and Torero J. Published by USTC Press ISBN:978-7-312-04104-4 DOI:10.20285/c.sklfs.8thISFEH.007

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polymer sandwich composites with multilayer structures. In the previous experimental research, the polymer sandwich composites were always an interesting subject in fire research [12, 14-17]. The impact response of fire-damaged polymer sandwich composites had been studied [15, 17, 18], and the fire decomposition effect had also been researched [14]. Griffin et al. conducted room fire tests according to the ISO 9705 protocols, on steel-skinned sandwich panels with expanded polystyrene (EPS) cores [17]. Chow performed a cone calorimeter experiment to composites polyurethane sandwich panels for temporary accommodation units [19]. In the numerical simulation study, fire structural modeling of polymer composites had been reviewed [20], but not suitable for the complex structure of ETICS. Therefore, the fire risk of the ETICS with rigid PUF core should be studied by experimental research, especially the rapid upward flame spread on ETICS.

Due to the limitation of the small-scale experiment, the large-scale experiment is more effective and meaningful for assessing the fire risk of ETICS, since it closely simulates the conditions in practical applications. A series of large-scale experiments were performed in relatively open spcae under corner fire, which is a typical fire phenomenon in residential building façade. The effects of the core material property and the outer layer thickness were considered. The temperature distribution and heat release rate (HRR) were measured and analyzed.

MATERIAL AND EXPERIMENT

Material and sample

Rigid PUF is considered to be an excellent material for the ETICS. It is resistant to temperatures up to 120 °C and degradation by the process of deploy condensation commences at 220 °C. Two different types of Rigid PU foam were used in the experiments. One is marked as PUO, which is flammable, and the other is marked as PUZ, which is treated by the flame-retardant with the addition of fire-retardant compounds-tris (2, 3-dichlorpropyl) phosphate (TCPP).

According to the ETICS standard, JG 149-2003 [21], the sample structure was simplified as the rear wall, the core material identifying as rigid PUF and the outer layer, as shown in Fig. 1. The outer layer consisted of plastering mortar, which had a variable thickness of 3 mm and 5 mm. The core of the ETICS used in this study was 30 mm thick rigid PUF, which is a common organic thermal insulation material in China. The rear wall, which was made by 20 mm thick ceramic fiberboard, produced an insulation environment in the back of the rigid PUF. The dimension of the sample panel was 1200 mm high, 600 mm wide and 30 mm thick, respectively.

Figure 1. Schematic of the experimental system.

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The large-scale experiment

The experimental apparatus consists of sample frame, fire source, thermocouples, video camera, and data acquisition system. As shown in Fig. 1 and Fig. 2, in the larger-scale experiment, two sample panels were vertically installed on a steel frame; three groups of K-type thermocouples were fixed on one sample panel defining as the main wall; a propane-gas burner was placed at the corner formed by the two sample panels. The whole experimental setup was positioned in a large open space, which is similar to outdoor condition. The gas species released from the burning, such as CO, CO2 and O2, were collected by the above hood and measured by a gas analyser. The measurement of HRR was taking place simultaneously based on oxygen consumption.

A 170 mm × 170 mm propane-gas burner with the power of 50 kW was used as the fire source. A CCD camera was located in mid line of the sample frame. Three groups of thermocouples were installed on the main wall, which were used to obtain the temperature information of the samples, as shown in Fig. 2. The locations of the thermocouple groups were at different heights of the sample. The two groups installed in the axis between the two samples were defined as Group-1 and Group-3, and the one installed in the middle height of the sample was called Group-2.

Figure 2. The group of thermocouples identified and installed on the sample.

According to one group of thermocouples, each thermocouple was identified by the symbol “-” and another letter. Taking Group-1 for example as shown in Fig. 3, the four thermocouples were installed with different distances from the front surface. The two thermocouples installed on and behind the outer layer surface were marked as 1-s and 1-b, while 1-m and 1-r were used to mark the thermocouples installed in the middle and rear of the rigid PUF, respectively.

Figure 3. The thermocouple identified and installed of one group.

300 mm

150 mm 150 mm

1-r 1-m

Polymer material

Rear wall

1-s

1-b

Outer layer

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RESULTS AND DISCUSSION

In the initial stage of the experiment, the ECITS was firstly exposed to the fire source. While the propane-gas burner was turned on, the surface temperature of ETICS started to increase. Due to the heated shrinkage of rigid PUF, a cavity between the outer layer and the rear wall was formed, which could be observed after the sample burnt off, as shown in Fig. 4. Once the temperature reached to a certain value, the rigid PUF would start to decompose, generating a mixture of volatile gas and char as products. While the outer layer had been cracked under the repetitive flame impingement, the flame pyrolysis volatile gas was flashed. At the end of the combustion process, the propane-gas burner was shut off.

Figure 4. The cavity of the ETICS formed after the experiment.

Evolution of flame

The development of the ETICS fire could be approximately divided into four stages, which was marked as initial stage, ignition stage, accelerate stage and decay stage. Taking PUZ-5mm for example, a typical evolution of flame was observed in the experiment as shown in Fig. 5.

At the initial stage, the propane-gas burner with a relatively stable power of 50 kW was started in the corner of the sample panels, as shown in Fig. 5(a). With the sustained heating, lots of pyrolysis gases were volatilized from the crack of sample plate and ignited, which was marked as the ignition stage.

The accelerate stage was quite different from that of general wall fire, which had a typical feature due to the influence of outer layer. Along with the heated shrinkage and pyrolysis of the rigid PUF, a cavity was formed between the outer layer and the rear wall and lots of flammable gases were accumulated in the cavity. Under the repetitive flame impingement the outer layer had been cracked and the pyrolysis gas was flashed. And then the flame was overflowed from the cavity through the edge and cracked area of the sample. During the decay stage, the rigid PUF burned out and the flame morphology was similar to the initial stage. Then the burner was turned off and the experiment had been finished.

Cavity

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(a) Initial (b) Ignition (c) Accelerate (d) Decay

Figure 5. The flame progress of PUZ-5mm sample.

Heat release rate

Under large-scale corner wall fire tests, the heat release rate (HRR) were measured for the fire risk of different ETICS. As shown in Fig. 6, the HRR of experimental samples were displayed, excluding the effect of propane-gas burner. The peak HRR of PUZ-3mm arrived 60 seconds later than that of PUO-3mm. The peak HRR of PUZ-3mm was 15.06 kW, which is approximately 40% lower than that of PUO-3mm. It shows that the flame-retardant of the rigid PUF plays an important role in the ETICS according to the heat release. Comparing with the PUO-3mm, the peak HRR of PUO-5mm was approximately 54% lower and arrived 90 seconds later, as shown in Fig. 6(b). Here the ETICS fire can be considered as traditional time square fire model, and the parameter of fire growth rate can be used to directly describe the fire risk. According to the experimental results and fire model, the fire growth rates of PUZ-3mm and PUO-5mm were 58% and 73%, respectively, lower than that of PUO-3mm. The comparative results indicate an interesting phenomenon that the outer layer of ETICS has the similar effect with the flame-retardant core material for restraining combustion.

According to the data of the experiments, the thicker outer layer even had a better effect. However, the thicker outer layer is used restrictedly in the building, which means that the outer layer cannot be extremely thick. In other words, the thicker outer layer has better effect on the fire safety of the building to some degree. In order to further study the use of outer layer, the temperature distribution of the ETICS was obtained and analyzed in the following section.

(a) PUO-3mm and PUZ-3mm (b) PUO-3mm and PUO-5mm

Figure 6. Heat release rate of different ETICS samples.

100 s 300 s 800 s 240 s

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Vertical temperature distribution

As shown in Fig. 2, there are three groups of thermocouples to measure the temperature distribution of the ETICS. Each group contained four thermocouples, which was marked as Ts, Tb, Tm and Tr, as shown in Fig. 3. In order to eliminate the burner influence, Group-2, which was placed in the middle of the sample, was selected to investigate the vertical temperature distribution through-thickness direction. The curves in Fig. 7(a) present the temperature distribution trends with the effect of the outer layer, which is different from that of general wall fire. After the temperature of Ts exceeding the ignition point of the rigid PUF, there were two obvious temperature jumps of Tb or Tm at the time of approximately 450 s, which indicated that the cavity fire inside the ETICS appeared under the propane-gas burner fire condition. Moreover, the time-interval of temperature jumps between Tb and Tm was shorter in PUZ-5mm than that in PUO-5mm. The main reason for this phenomenon is that the combustion condition in the cavity is quite different from that in the open space. As we known, the action of flame-retardant was restraining the chemical reaction of the material, such as pyrolysis. At first, the charred layer of PUZ-5mm was thicker which could restrain the pyrolysis of the rigid PUF, and then it prevented the overflow of pyrolysis volatilization of PUZ-5mm while the temperature increased. After the outer layer being destroyed under the fire impingement, the pyrolysis gas was ignited in the cavity, and then the charred layer and the virgin material could also be ignited in the cavity. Therefore the fire was enhanced sharply by the cavity influence. In the same period for PUO-5mm sample, there were less charred layer and more overflow of pyrolysis gas. The flammable rigid RPUF in the PUO-5mm sample was easier to reach the thermal decomposition status, and most of the core materials have already been changed to pyrolysis gas before the appearance of the cavity fire. While the cavity fire appeared, the combustion of the sample occurred on the top edge of the sample, so that the fire in the position of Group-2 was relatively small.

The cavity is also the reason for the strange phenomenon in Fig. 5(b). Since the crack of thinner outer layer was earlier, the curve of Tb for PUZ-3mm was higher than that for PUZ-5mm. The flame of PUZ-3mm has been appeared in the surface not in the cavity, while the thicker outer layer of PUZ-5mm burn mainly in the cavity, so that the curve of T1 for PUZ-5mm was higher. Above all, the vertical pyrolysis through-thickness direction of the samples was enhanced by the thicker outer layer and the flame-retardant due to the cavity flames influence. On the one hand, the flame was fastened in the cavity by the thicker outer layer. On the other hand, the pyrolysis gas could not overflow from the cavity and had mainly been burning in the cavity, as the flame-retardant treated on the inflammable rigid PUF.

(a) PUO-5mm and PUZ-5mm (b) PUZ-3mm and PUZ-5mm

Figure 7. The vertical temperature distribution toward to the sample surface.

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Upward temperature distribution

The value of Ts, influenced by the pulsating propane-gas burner fire, could not exactly show the surface temperature of the ETICS. The thermocouples assembled behind the outer layer, were chosen to investigate the upward flame character of the ETICS. The value of Tb was regarded as the representation of the ETICS, so that the value of Tb at different heights was analyzed.

All the temperatures increased suddenly between 400 s and 600 s, which occurred at the same time as the appearance of cavity fire, as shown in the above vertical temperature distribution results. The time-interval of temperature jump between Group-1 and Group-3 was shorter for PUO-5mm than that for PUZ-5mm, which indicated a quicker upward flame spread for PUO-5mm (Fig. 8(a)). The time-interval of temperature jump for PUZ-5mm was approximately 3 times longer than that for PUZ-3mm, as shown in Fig. 8(b). Therefore, the velocity of upward flame spread could be decelerated by the flame-retardant and the outer layer. Although the cavity can affect the spread of vertical temperature of the flame, the upward temperature plays a more important effect in the ETICS fire. Therefore, the PUZ with thicker thickness can decrease the fire risk of the ETICS.

(a) PUO-5mm and PUZ-5mm (b) PUZ-3mm and PUZ-5mm

Figure 8. The temperature distribution upward the sample.

CONCLUSIONS

Based on a series of large-scale ETICS fire experiments conducted under corner wall in relatively open space, the fire characteristics of the ETICS were investigated. Two typical factors of ETICS, the thickness of outer layer and the flame-retardant, were taken into account in the experiments. Due to the fire source impact, a cavity could be formed between the outer layer and the rear wall of the ETICS. The cavity could enhance the flame spread on the ETICS, once the outer layer was destroyed. Under the influence of the thicker outer layer (5 mm), the peak of HRR was reduced by 54% and the moment arrived at the peak of HRR was lagged 90 seconds. Similarly, the peak of HRR was reduced by 40% and lagged 60 seconds by the flame-retardant treatment. The fire growth rates of PUZ-3mm and PUO-5mm were 58% and 73%, respectively, lower than that of PUO-3mm. The measured temperature data upward the sample also indicated that the flame-retardant material with thicker outer layer could obviously decrease the fire risk of ETICS.

ACKNOWLEDGEMENTS

This work was supported by National Natural Science Foundation of China (51206156) and National Basic Research Program of China (973 Program) (2012CB719701). The authors deeply appreciate the support.

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