| Modern basic engineering is constantly developing in the direction of ultra-long span,ultra-long distance,ultra-high-rise,and underground ocean,and it also puts forward higher requirements for traditional cement-based composite materials in terms of mechanical properties and durability.Polyvinyl alcohol(PVA)fiber-reinforced cement-based composites(PVA-FRCC)have attracted the attention of many scholars because of their excellent mechanical properties,tensile properties and crack control capabilities at room temperature.At the same time,with the unique"nano effect" of small size effect and surface effect,nanomaterials have shown broad application prospects in many fields,and the addition of nanoparticles to modify traditional cement-based composite materials has also become a research hotspot at home and abroad.In recent years,building fires have occurred frequently,and the high temperature generated by fires has great destructive power,which is easy to cause concrete cracking and loss of structural bearing capacity,which seriously affects the service life of the structure.Therefore,studying the high temperature resistance of new cement-based composite materials is of great significance for assessing the degree of damage after fire and the ability to formulate treatment plans in time.In view of this,in this study,cement,fly ash,quartz sand are used as raw materials,and different amounts of nano-SiO2 and PVA fibers are added at the same time to prepare nano-SiO2 and PVA fiber-reinforced cement-based composite materials.And a large number of experimental studies have been conducted on the mass loss,mechanical properties,and microstructure of PVA fiber and nano-SiO2 reinforced cement-based composites after high temperature.At the same time,the relationship between relative mechanical strength and temperature has been established.The main research contents and results include:(1)Through repeated trials and adjustments,the water-binder ratio and the appropriate dosage of cement,fly ash,quartz sand and water reducing agent have been determined.Refer to the previous research of the research group to determine the dosage of PVA fiber and nano-SiO2,and finally the 10 mix ratios of PVA fiber and nano-SiO2 reinforced cement-based composite materials were determined,and a suitable mixing process that can ensure good workability of the cement-based composite materials was determined after trial mixing.(2)The high temperature test at different temperatures was carried out on all the mechanical properties of the specimens,the heating rate was set to 5℃/min,the constant temperature duration was 2 hours,and the two cooling methods of natural cooling and water spray cooling were adopted to prevent the high temperature during heating.The experimental phenomena in the furnace and in the process of water spray cooling were analyzed.(3)Through the high-temperature heating test,the mass loss of PVA fiber and nano-SiO2 reinforced cement-based composites after being subjected to different temperatures and high temperatures was measured,and the changes in the appearance characteristics of the specimens were observed.The PVA fibers and nanometers were analyzed after being subjected to high temperature.The relationship between the quality loss and appearance of SiO2 reinforced cement-based composites and temperature,and the corresponding influence rules have been obtained.The results show that the apparent color of each group of specimens gradually becomes lighter with the continuous increase of temperature,and it has become gray-white at 800℃,and the degree of cracks,peeling and looseness gradually increased.At the same time,the mass loss of specimens increased with the continuous temperature.It rises and gradually increases.It drops sharply at 400℃,and stabilizes when it is higher than 400℃.At 800℃,the remaining mass of the sample is only about 78%of that at room temperature.(4)Through the basic mechanical performance test of 585 PVA-FRCC specimens in 5 groups of mix ratio and benchmark group after high temperature,the cubic compressive strength,axial compressive strength and resistance of PVA-FRCC after being subjected to different temperatures and high temperatures are measured.Folding strength and splitting tensile strength.After being subjected to high temperature,the influence of PVA fiber on the cubic compressive strength,axial compressive strength,flexural strength and splitting tensile strength of cement-based composite materials is analyzed,and the corresponding results are obtained.The influence law of,reveals the mechanism of the influence of factors such as heating temperature,PVA fiber content,specimen cooling method on the cubic compressive strength,axial compressive strength,flexural strength and splitting tensile strength of cement-based composite materials.The results show that the PVA-FRCC specimen exhibits ductile failure when the temperature is lower than 400℃,and brittle failure when the temperature is higher than 400℃;the mechanical strength of PVA-FRCC gradually decreases with the increase of temperature,but the rate of decrease The difference is that the compressive strength of the cube rises at 400℃.At the same time,when the temperature is lower than 400℃,the mechanical strength of each PVA fiber increases first and then decreases with the increase of the PVA fiber content.The best PVA fiber content is 1.2%.(5)Through the mechanical performance test of 312 nano-SiO2-reinforced cement-based composite material specimens after high temperature in the 4 groups of mix ratios and the benchmark group,the cubic compressive strength and axial resistance of the cement-based composite material after being subjected to different temperatures and high temperatures are measured.Compressive strength and split tensile strength,analyzed the influence of nano SiO2 on the compressive strength,axial compressive strength and split tensile strength of cement-based composites after being subjected to high temperature,and obtained the corresponding influence rules.The mechanism of the influence of factors such as heating temperature,nano-SiO2 content,and cooling method of specimens on the compressive strength,axial compressive strength and split tensile strength of cement-based composite materials is revealed.The results show that the compressive strength generally shows a downward trend with the increase of temperature,but the rate is different.The strength increases at 400℃,while the splitting tensile strength changes monotonously with increasing temperature.At different temperatures,the mechanical strength increases first and then decreases with the increase of nano-SiO2 content,and the best content is 1.0%.(6)Using scanning electron microscope to test and analyze the microstructure of the specimens subjected to high temperature,explore the micro morphology of nano-SiO2 reinforced cement-based composite materials,and analyze the reinforcing effect of nano-SiO2 on cement-based composite materials,revealing The high-temperature damage mechanism of nano-SiO2 reinforced cement-based composites is discussed.The results show that the microstructure inside the matrix gradually deteriorates with the increase of temperature.At 800℃,a large amount of hydration products inside the matrix decompose and become brittle,resulting in a further decrease in the strength of nano-SiO2 reinforced cement-based composite specimens. |
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