Research On Mechanics Performance Of Hybrid Fibers Reinforced Recycled Aggregate Concrete After High Temperature

Recycled aggregate concrete（RAC）is a new kind of environmental concrete material.The use of RAC has significant environmental benefits such as achieving the recycling of waste concrete and saving landfill spaces,leading to an increasing amount of research and attention.The performance of RAC after being exposed to high temperature must be considered as an important issue,when it is applyed to the building structure.In order to study the effects of fibers on RAC properties under high temperature,polypropylene fibers with low elastic modulus and basalt fibers with high elastic modulus are selected and added into RAC in a single incorporation and mixed one.The influences of variable parameters such as the heating temperature,fiber content and cooling method on the mechanical properties of RAC were studied.It mainly includes the following elements:（1）A series of polypropylene fiber reinforced RAC and polypropylene-basalt hybrid fiber reinforced RAC specimens with different fiber content were designed,and the specimens were subjected to high temperature tests at different heating temperatures（20℃、200℃、400℃、600℃、800℃）.The changes of the appearance morphology and mass loss of fiber reinforced RAC specimens after different temperatures were studied.It was found that with the increase of heating temperature,the appearance characteristics and mass loss of polypropylene fiber reinforced RAC and hybrid fiber reinforced RAC have the same vary trend.The specimens appear to change from shallow to deep and then shallow in color,and the degree of surface deterioration gradually increases.The mass loss increases gradually with the increasing temperature,and the mass loss is most serious in 200℃⁴00℃.Under the same temperature condition,the mass loss of polypropylene fiber reinforced RAC was found to increase with the increase of fiber contents,and the mass loss of hybrid fiber reinforced RAC also gradually increased except for 200℃.（2）The mechanical property tests of polypropylene fiber reinforced RAC after exposure to elevated temperatures were carried out.The cubic compressive strength,splitting tensile strength and flexural strength of polypropylene fiber reinforced RAC were obtained.The effects of heating temperature,fiber content and cooling method on polypropylene fiber reinforced RAC mechanical strength were investigated.The results show that the mechanical strength of polypropylene fiber reinforced RAC decreases with the increase of heating temperature.For the same exposure temperature of specimens,the mechanical strengths increases first and then decreases with the increasing polypropylene fiber content.Compared with natural cooling method,the water cooling method aggravates the strength damage of polypropylene fibers reinforced RAC at 200℃and 400℃,however,it is conducive to the recovery of strength at 600℃and 800℃.（3）The mechanical property tests were carried out on polypropylene-basalt hybrid fiber reinforced RAC after exposure to elevated temperatures.The cubic compressive strength,splitting tensile strength and flexural strength of hybrid fiber reinforced RAC were obtained.The effects of heating temperature,fiber content and cooling method on hybrid fiber reinforced RAC mechanical strength were investigated.The results show that the compressive strength and splitting tensile strength of hybrid fiber reinforced RAC increases first and then decreases with the heating temperature increases.An increase in strength can be found at200℃.With the heating increase of temperatures,the flexural strength decreases.The compressive strength,splitting tensile strength and flexural strength of hybrid fiber reinforced RAC are higher than that of plain RAC under the same high temperature conditions.The strength of hybrid fiber reinforced RAC specimens is the highest when the content of basalt fiber is 2.6kg/m³ under the same high temperature conditions.Compared with the natural cooling method,water cooling decreases the high temperature performance of the hybrid fiber reinforced RAC at 200℃and 400℃,While the high temperature performance increases when the temperature reaches up to 600℃and 800℃.（4）By the experiment of scanning electron microscope,the microstructural changes of hybrid fiber reinforced RAC after different heating temperatures were studied.The results show that after exposure to the temperature not higher than 200℃,cement slurry structure is complete,dense and tidy.After exposure to 200℃⁸00℃,C-S-H gel and Ca（OH）₂ crystals are gradually dehydrated and decomposed and become loose and broken.With the increase of temperature,the bonding between aggregate and cement paste gradually becomes loose and the cracks gradually develop.At room temperature,polypropylene fiber and cement paste bonds tightly,and the interface transition zone is complete and dense.When the temperature is higher than the melting point of polypropylene fiber,the polypropylene fiber melts,escapes and leaves channels within the RAC.With the temperature increases,the cracks inside and around the melting channels of polypropylene fiber are constantly formed and expanded,meanwhile the melting channels have always existed.At 200℃,the bonding interface between basalt fiber and cement paste is more compact than that at 20℃.After that,the density of interfacial transition zone gradually decreases and the bonding property decreases with increasing temperature.