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At about 8.00 am of 20^th December 2015, a fire started in a shop in Plasco building, located in center of Tehran, which resulted in tragic collapse of the building and claimed the lives of 22 fire fighters and other people. According to the findings of the investigations, the main reason of ignition was non-permitted extension of cablings in the shop of origin of fire; for use of electrical heating devices. This incident took the attention of Iranian society, Engineers and authorities to the need of improvement of fire safety of buildings. Plasco was collapsed after about three and a half hours of the ignition. Therefore the question was arisen how the scenario of the fire growth and temperature rise in the building was. The place of the building and depot of debris were visited and examined by special teams. Steel, concrete and other materials, which some of them were molten due to high temperature, was taken from the debris for further experimental works. Different tests were carried out on the samples. Heat release rate of textiles was discussed in another article. In this paper; the results of physical, mechanical and XRD/XRF tests and mineralogy/petrography examinations have been presented. The most tests carried out on the concrete samples taken from the ceiling system of the building. For comparison purposes, samples were also taken from the apparently non exposed ceilings. A number of these un-exposed samples were tested in a fire resistance furnace and their changes were utilized as a reference for comparison with samples of burnt sections of the building. The results revealed that the concrete cores taken from the fire exposed ceilings had experienced temperatures between 500-700 ᵒC. Taking the thermal and physical properties of concrete into account; this revealed that the gas temperature of fire in Plasco was much higher than these amounts. This was in agreement with findings of FDS modeling, which was presented in another paper and showed temperatures of about 1000-1100 ᵒC occurred in some parts of the building in fire. On the other hand, molten blends of different materials were seen in the debris, comprising molten metal, glass and cementitious materials. It showed that temperatures about 1400 ᵒC could be occurred in the incident. But these temperatures are not usually seen in enclosure fires; rather it must be happened under the debris. Fifteen stories were collapsed at the end of tragedy. In that time, the massive amounts of textiles and other combustibles were burning in five upper stories, while there were still large quantities of textiles and clothes in the lower stories. So, after the collapse of the building, all combustible materials and even gasoil fuel stored in tanks in the basement could be burnt under the debris, approved by observations in next days during the operation of extinguishment and removal of debris. The failed construction systems comprised large panels and steel elements; therefore air flow was possible under the debris for supporting the combustion. On the other hand, large amounts of debris made a relative thermally insulated condition, especially in lower parts, which could produce a temperature rise considerably higher than recorded in enclosure fires. The observation of molten blends of cementitious materials supported this assumption and it was also in agreement with possible maximum temperatures in adiabatic combustions of hydrocarbons, which are far higher than these figures.

Keywords: Fire, Plasco, building materials, debris, XRD/XRF

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