| High-strength concrete has the advantages of small floor area,high strength,good corrosion resistance and so on.It is a hot topic in the application and research of building materials.However,high-strength concrete will burst under high temperature,many scholars have carried out a lot of research work on this phenomenon,but up to now,there is no consensus on the cause and mechanism of high-strength concrete bursting.The research shows that adding fiber into high-strength concrete is one of the effective methods to restrain the burst,the addition of fiber can delay the occurrence of bursting and improve the residual mechanical properties of concrete after high temperature.At present,the research on fiber reinforced concrete mainly focuses on the improvement of concrete performance after the incorporation of steel fiber and polypropylene(PP)fiber,in fact,carbon fiber and PVA have good mechanical properties and smaller diameter,which may significantly inhibit the high-temperature bursting of high-strength concrete.In this paper,the modification of 100 MPa high-strength concrete by adding PVA fiber and carbon fiber is studied,and the mechanical properties of eight hundred and thirty concrete specimans at room temperature,the bursting behavior at high temperature and the residual properties after high temperature are studied.The optimal fiber content of two materials to improve the high-temperature explosion resistance of concrete is explored.Finally,the high-temperature bursting problem of concrete is analyzed by means of theoretical analysis and numerical simulation.The mechanical model of high-temperature bursting under the combined action of steam pressure and thermal stress is given,and the change rule of high-temperature steam pressure and critical steam pressure of high-strength concrete are obtained.The following work has been carried out:1.A series of mechanical properties tests of fiber reinforced concrete at room temperature were carried out,and the variation laws of compressive strength,splitting strength,flexural strength and axial compressive strength with the amount and type of fiber are obtained.Thetest results show that the effect of PVA and carbon fiber is significantly different,PVA fiber can reduce the compressive strength of concrete,but it can enhance the splitting strength and flexural strength of concrete,so that the axial compressive strength increases first and then decreases,and the optimal content of comprehensive performance is about 0.1%-0.2%.The effect of carbon fiber on the compressive strength,splitting strength,flexural strength and axial compressive strength of high strength concrete is first increased and then decreased,and the optimal content is 0.2%.2.The whole process of high-temperature bursting of high-strength concrete with carbon fiber/PVA fiber was studied.The process images of high-temperature bursting of high-strength concrete with different fiber content,temperature rise system,moisture content and different sizes were obtained.The influence of the above factors on the bursting behavior of high-strength concrete was compared and analyzed.In the experiment,the optimum content of suppressing burst fiber was tested,and the gradation of slag after high-strength concrete burst was analyzed.The results show that PVA fiber has a good explosion suppression effect,when the content of PVA fiber is more than 0.1%,it can inhibit the occurrence of explosion;when the content of carbon fiber is around 0.2%,the bursting strength can be reduced.When the content of carbon fiber is more than 0.2%,the burst burst of the specimen will become a periodic and sustained burst.With the increase of heating rate,the time of first explosion will be advanced and the duration of bursting will be shortened;with the low moisture content,the time of first explosion of concrete specimen will be delayed,but the bursting will become more sudden and more violent.In the aspect of slag grading,when the specimens without carbon fiber and the content of carbon fiber are 0.2%-0.4%,there are more large blocks formed by the cracking of high strength concrete,and there are more fragments with the other fiber content between2.36mm-26.5mm.3.Considering the characteristics of autoclave in concrete,the hollow sphere model was used to analyze the coupling effect of air pressure change and temperature stress in concrete under high temperature,and the calculation formula of circumferential and radialstress of concrete around the cavity under the action of steam pressure and temperature was obtained.Referring to the temperature and ultimate tensile stress strength of concrete recorded in the burst test of high strength concrete,the required burst under the test condition was obtained cracking critical vapor pressure.The influence of temperature difference between inside and outside and porosity on the stress of high strength concrete was also analyzed.Conclusion shows: the control effect of circumferential tensile stress on failure.When the vapor pressure is constant,the risk of bursting increases with the increase of porosity(wall thickness becomes thinner),and decreases with the decrease of temperature difference.4.According to the European code,considering the pores brought by fiber and the increased pores after melting,the formula for calculating the thermal conductivity of fiber reinforced concrete was given by using Maxwell model,and the temperature field of the specimen with fiber action was calculated.The relevant equations of vapor pressure in high strength concrete were given.The vapor pressure was calculated by the finite element software,and the influence of heating mechanism,size and other factors on the temperature field and vapor pressure change was discussed.Conclusion: with the increase of the fire surface,the temperature rise speed of the concrete specimen is accelerated,the peak time of the steam pressure is advanced,and the peak value is increased;with the increase of the specimen size,the temperature difference between the internal and external temperature of the concrete specimen is increased.After the peak value of vapor pressure at each point of the concrete specimen is pushed,the peak value of vapor pressure at the internal point increases,and the peak value of vapor pressure at the surface point decreases.5.The changes of properties of coarse aggregate,fine aggregate,clean slurry and mortar at different temperatures were studied,and the morphological changes of fiber and cement slurry were observed by electron microscope.The appearance change,quality loss and residual strength of fiber reinforced high strength concrete after high temperature are recorded.The results show that the crushing value of coarse aggregateand fine aggregate increase slowly at 25?-600?,and increases rapidly at 600?-800?.The compressive strength of paste and mortar increase first and then decreases,due to the difference of thermal expansion coefficient between fine aggregate and cementitious material,the peak value of mortar strength decreases obviously.For fiber reinforced concrete,the compressive strength increases first and then decreases after high temperature,and the splitting strength,flexural strength and axial compressive strength basically decrease with the increase of temperature.However,it is found that the mechanism of PVA fiber and carbon fiber is not the same.High temperature gasification of PVA fiber reduces the damage degree of high-temperature bursting of concrete,while carbon fiber plays a role in pulling and jointing concrete,which is beneficial to the improvement of residual strength.In conclusion,the residual strength of concrete under high temperature is related to the change characteristics of the strength of gel system,coarse aggregate,fine aggregate and high temperature performance of fiber.At the same time,the influence of mismatch of thermal expansion coefficient of fine aggregate and cementitious material is also very important. |
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