High Temperature Performance Test And Pore Structure Analysis Of Hybrid Fiber Fly Ash Concrete

The damage caused by building fire is serious,and the deterioration degree of concrete structure after fire has been widely concerned.The research shows that adding fly ash and fiber into concrete can improve the high temperature resistance of concrete.The mixed fiber fly ash concrete is made by using the quality of fly ash with the content of 0%,10%,20%,30% to replace the cement content,mixing 0.1% basalt and0.1% polypropylene fiber into the concrete mixture with the volume ratio respectively.The mechanical properties and pore structure of hybrid fiber fly ash concrete at different temperatures(20℃,200℃,400℃,600℃,800℃)are studied,and the microstructure of the concrete before and after the high temperature is analyzed in combination with the microstructure.The main contents are as follows:(1)The mechanical properties(compressive strength and tensile strength)of hybrid fiber concrete specimens with different fly ash content in different curing ages(3d,7d,28d)at room temperature were studied.The results showed that the compressive and tensile strength of concrete specimens increased with the increase of curing days,and reached the maximum after 28 days.Compared with other fly ash content,the concrete specimens with 10% fly ash show better compressive and tensile properties.(2)The mechanical properties(compressive strength and tensile strength)of hybrid fiber concrete specimens with different fly ash content after experiencing different temperatures were studied.The appearance of concrete after different temperatures was observed,and its mass loss and ultrasonic velocity were measured.The results indicate that with the increase of temperature,the compressive and tensile strength increase slightly at 200℃ compared with 20℃,and then decrease to varying degrees.The surface of the concrete test specimen changes from blue to white,and the quality and wave velocity continue to decline,which is due to the loss of free water and gel water in the concrete caused by high temperature and the decomposition of hydration products.(3)The pore structure of concrete specimens after high temperature is studied,and the air content and pore frequency distribution of concrete specimens at different temperatures are explored.The relationship between pore structure parameters(air content,spacing factor,average chord length,specific surface area)and macro mechanical properties is further analyzed.The results show that after 200℃,the number and frequency of pores in concrete specimens with different fly ash content at the chord length of 0-0.01 mm increase compared with 20℃,while the number and frequency of pores gradually decrease when the temperature is higher than 200℃;When the chord length is 0.5-1.0mm,the air content and pore frequency of concrete with different fly ash content are reduced to the lowest after 200℃,and the concrete specimens with 10% fly ash content is the minimum,which are 30.01% and 1.41%respectively.This shows that adding 10% fly ash can significantly reduce the number of pores in the long chord and reduce the air content in the concrete.In addition,the fitting curve between pore structure parameters and strength shows that the fitting coefficient is good,and the compressive strength and tensile strength decrease with the increase of air content,spacing factor and average chord length,but increase with the increase of specific surface area.(4)The grey entropy correlation method is used to determine the main pore structure parameters that affect the compressive strength and tensile strength of concrete,and the changes of internal microstructure of concrete before and after high temperature are studied by analyzing SEM image.The results indicate that the main factor affecting the compressive and tensile strength is the specific surface area of pores,whose gray entropy is greater than 0.998.Air content has the least effect on compressive and tensile strength.In addition,it is found that the hydration products at20℃ are complete,and the internal pores of the concrete are less.At 200℃,a part of the hydration products generated fill the pores,making the interior of the concrete relatively dense.However,with the increase of temperature,the internal hydration products continue to decompose.After the temperature of 800℃,the hydration products decompose completely,and the overall structure becomes loose and porous.Compared with the concrete specimens without fly ash,the microstructure deterioration of the concrete specimens with 10% fly ash is lower.Figure [34] Table [16] References [93]...