Assessing Fire Hazard of Cladding Materials in Existing Buildings

Assessing Fire Hazard of Cladding Materials in Existing Buildings

The University of Queensland, in collaboration with the Non-Conforming Building Products (NCBP) Audit Taskforce in Queensland, Australia, have proposed a framework to provide a robust methodology to assess the fire hazard of cladding materials in existing buildings based on a thorough understanding of the relevant fire phenomena. Thermogravimetric Analysis enhances the robustness of the framework. Read here how the method is applied to evaluate the risk of external fire spread on buildings.

The Cladding Materials Library comprises an extensive database of cladding materials based on their composition and flammability as individual components. It can further be used to perform hazard analyses. The database is a tool for qualified engineers to enable an adequate fire hazard identification and quantification of the potential fire spread of cladding materials.

The University of Queensland, in collaboration with the Non-Conforming Building Products (NCBP) Audit Taskforce in Queensland, Australia, have proposed a framework to provide a robust methodology to assess the fire hazard of cladding materials in existing buildings based on a thorough understanding of the relevant fire phenomena.

The flammability of cladding materials (aluminum composite panels, insulation, etc.) is defined based on well-established testing frameworks widely accepted in the fire safety engineering community. These frameworks have been applied and peer reviewed within the scope of fire research studies on the fire performance of aluminum composite panels and insulation materials at The University of Queensland and the University of Edinburgh.

Detailed information on the framework can be accessed here!

The methodology includes the identification of pyrolysis and oxidation with the NETZSCH STA 449 F3 Jupiter®.

How does Thermogravimetry Analysis Support the Framework?

Thermogravimetric analysis (ASTM E1131) is used to analyze the thermal decomposition of materials as a function of temperature. Reactions where mass loss occurs, such as pyrolysis and oxidation, can be identified using this technique. TGA is included in this testing protocol to enhance the robustness of the framework. The other material identification and quantification techniques are in theory sufficient but the addition of an extra technique to verify the results ensures that potential error is reduced.

Sample Preparation

TGA samples were taken from the side of a given specimen so that data was averaged across the depth. The outer most encapsulation layer was first removed in order to eliminate the effect of the sampling tool. Specimens took the form of small flakes (0.5 – 3 mm in length) to minimize the thermal gradients through the sample. These were lightly pressed into the bottom of the crucible to ensure good thermal contact with the thermocouple on the load cell.

How to Perform the Analysis with Thermogravimetric Analysis

  • Constant heating rate of 20°C min-1 from 50 to 800°C. A ten-minute isothermal heating regime in air was added at the end of each test to prevent crucibles from becoming stuck to the load cell;
  • One test in air atmosphere and one test in nitrogen atmosphere, for a total of two tests per sample;
  • A flow rate of 150 ml min-1 for the gas, with an additional 20 ml min-1 nitrogen purge flow in all cases;
  • Alumina crucibles (Al2O3) with a volume of 85 μl, a diameter of 8 mm, and without lids were used;
  • Sample mass in general cases was 10.0 mg with a maximum deviation of 2.5 mg. Nonetheless, for some samples, this was not possible for the crucible volume, and so the target mass was reduced.
Fire Cladding Database_TGA measurements
Figure 1: TGA measurement of a cladding material in nitrogen (right) and air (left) atmosphere

The TGA plots shown are for a type of insulation foam, with one test in air and one in nitrogen. The results show that the material undergoes pyrolysis (245 – 383°C) and generates a char in nitrogen. In oxygen, this char then has an exothermic oxidation reaction (peak 581°C). The char yield is quite low and similar to some of the poor performing foams in fire, and so the performance of this foam would also be expected to be poor.

Conclusion

Despite a high oxidation temperature, the thermal inertia of this material is extremely low and so the material will ignite rapidly. Alternatives to the Cladding Materials Library simply note that it is an insulation, and would not provide any indication of this possible fire behavior.

“The aim of the Cladding Materials Library has been to provide evidence of fire performance to practicing engineers so that they can evaluate the risk of external fire spread on buildings. The database aids the engineer in their decision-making, and is both cheap and easy to cross-reference. The TGA results provide extra robustness in material identification, which is used to cross-reference the material fire performance without the engineer having to run the fire testing themselves.”

–   Dr Martyn S. McLaggan, Research Fellow and Co-ordinator of “Cladding Materials Library”

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