Combustion Behavior Of Bituminous Pavement Under Tunnel Fire And Synergistic Inhibition Mechanism Of Composite Flame Retardant

Bituminous pavement is widely used in tunnel engineering due to its advantages of high skid resistance,low noise,comfortable driving,short construction period,convenient maintenance,etc.However,the thermal decomposition of bituminous pavement under tunnel fire produces a lot of heat and toxic smoke,which brings great difficulties to escape and fire rescue.There are few studies on the combustion kinetics of bitumen and its components and the dynamic release process of volatiles during tunnel fires.The composite flame retardant used in previous studies lacks an efficient and synergistic inhibitory effect on the multi-stage combustion behavior of bitumen.In view of this,this paper simulates the tunnel fire under the various temperature field of bituminous pavement,studies the gradient distribution characteristics of different temperature range of bituminous components and the release of volatiles at different heating rates,and determines the environmentally friendly nano flame-retardant that matches the combustion characteristics of each component and its optimal dosage.According to the content of bitumen components,a composite formulation of the nano flame-retardant is proposed.Moreover,the synergistic flame retardant and smoke suppression mechanism of the composite flame retardant on the thermal decomposition of bituminous pavement is studied,which is of great significance to improve the fire safety of tunnel bituminous pavement.Firstly,the thermogravimetry-differential scanning calorimetry-mass spectrometry-Fourier transform infrared spectroscopy technique was used to analyze the gradient distribution characteristics of combustion temperature range of four component at different heating rates,and the combustion kinetics of each component of bitumen was studied.The results showed that with the increase of the heating rate,the combustion process of four components lags,and the combustion of resins and asphaltenes was less affected by the heating rate.Kissinger differential and FWO integral methods were used to calculate and calibrate the combustion kinetic parameters of bituminous components,the activation energy of the saturates and aromatics in the stage 1 is less than the stage 2.The activation energy of resins and asphaltenes is higher,but no more energy is needed to maintain the combustion after ignition.The combustion of carbonized layer mainly occurs in stage 2.Then,by studying the combustion volatile constituents of bituminous components,combined with scanning electron microscopy-energy spectroscopy analysis,the microstructure morphology and elemental composition of the combustion residues were determined.It is found that the release rate and the release amount of volatiles increase with the increasing of the heating rate.Except H₂O and CO₂,acetaldehyde and propane were the main harmful volatile released by four components.In stage 1 of combustion,the release amount of volatiles exceeded that in stage2.The heating rate had a more obvious effect on the release behavior of saturates and aromatics.Combustion residues of saturates,aromatics,resins,asphaltenes tended to be increasingly complete,mainly containing C,O,S and other elements.Secondly,four environmentally friendly nano flame-retardants,such as Ca-Al hydrotalcite,ammonium polyphosphate,microencapsulated red phosphorus and expanded vermiculite,which match the combustion temperature range of the four components of bitumen,were selected respectively.The composite formulation was proposed,and the composite nano flame-retardants were developed to explore their synergistic flame retardancy and smoke suppression effects on the multi-stage combustion process of bitumen.The results show that with the increase of the heating rate,the incomplete combustion of bitumen and composite flame retardant bitumen increases.However,the composite flame retardant decomposes and absorbs heat during the bitumen combustion,resulting in a thicker expansion layer.Due to the synergistic effects of heat absorption,blocking,dilution and neutralization,the formation of molten droplets of bitumen,the types and release of macromolecular volatiles are significantly reduced.The synergistic flame retardant and smoke suppression effect of composite nano flame retardant on bitumen combustion is revealed.In addition,the flame retardant mechanism of composite flame retardant to bitumen mixture was studied by cone calorimeter test.It was found that the composite flame retardant significantly increased the char yield of bitumen mixture,the contents of combustion residues increased significantly,the residues were completer and denser,and played a synergistic flame retardant effect in solid phase.This significantly prolonged the ignition time of the bitumen mixture,reduced the heat release rate,effective combustion heat,total heat release amount,average specific extinction area,toxic smoke release amount,CO release rate and maximum CO₂yield.The composite flame retardant exerted a good synergistic flame retardant effect in the gas phase,revealing the synergistic suppression mechanism of the components of the composite flame retardant on the combustion behavior of the bitumen mixture.Meanwhile,the smoke suppression mechanism of composite flame retardants on the combustion of bitumen mixture was studied based on solid-phase microextraction-gas chromatography-mass spectrometry.The results showed that the composite flame retardant reduced the types of alkanes in the soot of bitumen mixture,prevented the release of unsaturated alkenes and alkynes,and significantly reduced the species of combustion residues.More soot is found in bitumen mixture,but the molecular weight of combustion residues is generally smaller.The composite flame retardant bitumen pavement is more difficult to burn with less soot,and the molecular weight of residuals is generally larger.Finally,the effect of composite flame retardant on the road and combustion performance of bitumen and its mixture was studied.It was found that the composite flame retardant slightly increased the ductility and softening point,but reduced the penetration,and improved the resistance to high temperature deformation and low temperature cracking.The composite flame retardant reduced the combustion grade of bitumen from V-2 to V-1,and the limit oxygen index increased from 19.6%to 25.1%.Composite nano flame retardants slightly improved the high temperature stability and water stability of bitumen mixture,but reduced the low temperature crack resistance,which still met the specifications.In sum,the multi-stage combustion dynamics of bitumen and the dynamic release process of volatiles from the level of bituminous components are explored and a composite formulation of nano flame retardant was proposed.The composite flame retardant has a good flame retardant and smoke suppression effect on bitumen,which reveals the synergistic suppression mechanism of the composite flame retardant on the multi-stage combustion behavior and smoke release of bitumen mixture.And the composite flame retardant does not affect the road performance of bitumen and its mixture,which promotes the rapid development of environmentally friendly tunnel bituminous pavement.