Effects of building concave structure on flame spread over extruded polystyrene thermal insulation material

Extruded polystyrene (XPS) thermal insulation material is widely installed on building exterior wall for energy saving, and the concave structure is a typical structure of building facade in architectural design. In this paper, both experimental methods and theoretical analysis are employed to investigate effects of concave structure on flame spread behaviors of XPS foam. Firstly, the upward flame spread tests are conducted. Significant fluctuation of the XPS flame front is observed while the movement of the pyrolysis front is relatively stable. The concave structure promotes the upward flame spread of XPS. The flame spread rate increases by 0.37 folds with the rising of concave structure factor (П) from 0 to 1.6. The growth rate of flame spread rate is 27.76% for П from 0 to 0.8, while it is merely 7.33% with П rising from 0.8 to 1.6. Reduced influence of the increase in the concave structure factor on the flame spread rate is deduced. The mechanism of concave structure effect is revealed. Moreover, a prediction model that evaluates the influence of the concave structure on flame spread characteristics of XPS is established. The predicted results are consistent with the experimental ones. The results of this work contribute to the fire hazard assessment of XPS and fire safety design of concave building facade.