| Bitumen pavement has been widely used in road tunnels because of its excellent road performance and driving comfort. However, once the tunnel caught fire, it would not only exacerbate the spread of fires but also cause serious damage to tunnel pavements. Traffic would be interrupted for a long time and lead to a lot of casualties. In order to improve fire retardant property of tunnel bitumen pavement, preparation and application of flame retardant bitumen and concrete have been attracted great attentions at home and abroad. Adding flame retardant is an important method for improving fire retardant properties of bitumen. Currently, bitumen flame retardant agents are mainly halogen and inorganic hydroxides fire retardant agents. Flame retardant effect of former one is better than that of later one. However, large amount of toxic and corrosion gas and smoke would be produced in the construction of bitumen mixture and combustion of bitumen pavement. These would seriously harm constructors’health and even lead to suffocation death of trapped people. Larger amount of inorganic hydroxides flame retardant agent would increase the cost of flame retardant bitumen and influence performance of bitumen mixture gravely. Therefore, developing non-toxic, environmental, low cost, good overall performance flame-retardant bitumen and its mixture has become a critical project for flame-retardant bitumen pavement of the tunnel.This paper first takes different origins and types of matrix bitumen and SBS modified bitumens as research objects. Combustion and thermal performance of bitumen has been evaluated by using oxygen index, ignition and flash point and thermal property. Relationship between bitumen chemical components and combustion performance has been discussed. Then the impact of inorganic hydroxides, layered silicates (expanded vermiculite, organic vermiculite, montmorillonite, organic montmorillonite and rectorite) and inorganic hydroxide/layered silicate on oxygen index, ignition and flash point and thermal property of SBS modified bitumen, as well as basic bitumen was studied. X-ray diffraction (XRD) and thermal-mass technology (TG-DSC-MS) were also used to analyze the synergistic flame retardant mechanism of inorganic hydroxide/layered silicate. Finally, the effect of fire retardant of inorganic hydroxide/layered silicate on volume performance, road performance and fire-retardant property was researched. Main conclusions of the study are as follows:(1) Chemical composition of bitumen has little effect on oxygen index but more on ignition and flash point. Bitumen with high saturates has lower ignition and flash point, but it with high resins has higher ignition and flash point. Chemical composition of bitumen has noticeable effect on its thermal properties. Bitumen with high saturates has lower initial temperature of thermal weight loss.(2) SBS modifiers have great impact on combustion characteristics and thermal properties of bitumen. With the addition of SBS, the oxygen index of bitumen becomes lower, but the ignition and flash point and thermal stability are increased. SBS is more vulnerable for combustion because they only has C, H elements, which leads to the decrease of oxygen index. The increase of ignition and flash point and thermal stability attributes to the formation of cross-linked network structure of SBS in bitumen, and that structure increases the viscosity of bitumen and obstructs the loss of volatile components in bitumen.(3) Aluminum hydroxide has better inflaming retarding effects on bitumen than magnesium hydroxide, which due to the lower dehydration temperature of crystal water in aluminum hydroxide, and its supreme endothermic peak temperature is close to initial thermal weight loss temperature of bitumen. Absorbing heat of dehydration can inhibit increasing temperature of bitumen and formed steam can dilute volatile component of bitumen. But the dehydration temperature and supreme endothermic peak temperature of magnesium hydroxide are both high, which result in little effects on restraining increasing temperature of bitumen and dilution of volatile component of bitumen, and inflaming retarding effect is comparatively poor.(4) Single vermiculite, organic vermiculite, montmorillonite, organic montmorillonite and rectorite have limited improvement on oxygen index of bitumen. Vermiculite has better ability in improving flame retardant property of bitumen than montmorillonite and rectorite, and organic vermiculite is superior to expanded vermiculite.(5) Aluminum hydroxide composed with expanded vermiculite, organic vermiculite, montmorillonite, organic montmorillonite and rectorite separately. And then they can be added into bitumen for fire retardant. Increment of bitumen oxygen index for composed fire retardants is comparatively higher than that of single flame retardants, which shows very obvious synergistic flame retardant effect, especially for composite of aluminum hydroxide and expanded vermiculite. Aluminum hydroxide used in tandem with expanded vermiculite not only has the advantages of smoke suppression, non-toxic, as well as environmental protection, but also could decrease the amount of aluminum hydroxide, lowering the cost of flame retardant bitumen.(6) XRD analysis indicates that layered structure of expanded vermiculite and organic vermiculite would be damaged and vermiculite layers would be exfoliated after burning, but the layered structure of phlogopite in vermiculite would not be damaged. The layered structure of expanded vermiculite in the composite system mixed with bitumen, expanded vermiculite and aluminum hydroxide could not be destroyed, while that in the composite system mixed with bitumen, organic vermiculite and aluminum hydroxide has been damaged. After high temperature calcination, the layered structures of vermiculite and phlogopite in these two composite systems were exfoliated, and layered structure of montmorillonite in the composite system mixed with bitumen, montmorillonite and aluminum hydroxide has been damaged, montmorillonite layers have been exfoliated and dispersed in bitumen.(7) Analysis of TG-DSC-MS indicates that water steam ion flow intensity in volatile component of SBS modified bitumen/aluminum hydroxide system is higher than that of SBS modified bitumen before400℃, while the situation is contrary after400℃. Carbon dioxide ion flow intensities of volatile components in SBS modified bitumen and SBS modified bitumen/aluminum hydroxide are similar before350℃, but it reverses after350℃. Water vapor and carbon dioxide ion flow intensity of volatile components in SBS modified bitumen/aluminum hydroxide/expanded vermiculite are obviously less than that in SBS modified bitumen/aluminum hydroxide.(8) The analysis results combined XRD and TG-DSC-MS reveal the synergistic flame retardant mechanism of SBS modified bitumen/aluminum hydroxide/layer silicate: when bitumen is heated up to around260℃, aluminum hydroxide begins to dehydrate and absorbs part of heat, thus the system temperature can be reduced. The layered silicate dispersed in bitumen uniformly would barrier the volatilization of water steam released from aluminum hydroxide, light component of bitumen as well as thermal decomposed products, and the permeation of oxygen in air, then the combustion of bitumen is hindered. As temperature rises further, water released from aluminum hydroxide and decomposition of bitumen would become more and more. The interlayer of layered silicate would be filled with increasing steam, and then layered silicate is exfoliated. Meanwhile, silicate barrier layer would be formed gradually. Alumina particles formed from the decomposition of aluminum hydroxide would be filled into the interlayer, forming compact barrier layers. It is more effective to inhibit the volatilization of decomposition products of flame retardant bitumen and the permeation of oxygen to achieve the goal of synergistic flame retardant. In addition, synergistic usage of layer silicates and aluminum hydroxide can promote the formation of charcoal during combustion.(9) The differences of synergistic flame retardant effect between SBS modified bitumen/aluminum hydroxide/expanded vermiculite and SBS modified bitumen/aluminum hydroxide/organic vermiculite would be:the layered structure of organic vermiculite has been damaged during the preparation process of flame retardant bitumen, the vermiculite layers disperse in bitumen randomly, while the layered structure of expanded vermiculite has not been spoiled, thus compact solid barrier layer would be easily formed, which leads to better solid flame retardant effect. The main reason that flame retardant property for montmorillonite is poorer than that for expanded vermiculite is:montmorillonite can be more easily intercalated by bitumen molecules compared to expanded vermiculite, montmorillonite layers are exfoliated in the preparation, then it is hard to form solid barrier layer. Furthermore, the dilatability and heat insulation properties of montmorillonite are weaker than that of vermiculite. (10) Compared with SBS modified bitumen mixtures, the water adsorption, voidage and voids in the mineral aggregate of SBS modified bitumen mixtures with the addition of aluminum hydroxide and expanded vermiculite are decreased slightly, while the bulk density and voids filled with bitumen are increased a little, the Marshall stability, soaking residues stability degrees, tensile strength ratio and dynamic stability are all improved, water stability and high temperature stability performance of bitumen mixture would be improved to a certain extent.(11) After burning tests, residual Marshall strength ratio of bitumen mixtures prepared with aluminum hydroxide and expanded vermiculite were apparently higher than that common SBS modified bitumen, and only gasoline on surfaces was burning, and the level of destruction of samples was comparatively low, which indicates that flame retardant agents have improved the flame retardant property of bitumen mixture effectively. |
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