Mechanical Properties And Pore Structure Of Hybrid Fiber Fly Ash Concrete Under Freeze-thaw Cycles

Fly ash is often added to concrete to optimize its performance,reduce engineering costs,and also reduce fly ash pollution to the environment.However,with the application of fly ash concrete in engineering,its durability problem is becoming more and more serious,especially in severe cold regions and cold regions.The fly ash concrete with the content of 10%was selected as the reference concrete.On this basis,the hybrid fiber fly ash concrete was composed of polypropylene fiber and polyacrylonitrile fiber with different volume contents(0.05%,0.10%,0.15%and 0.06%,0.12%,0.18%),different lengths(6 mm,12 mm,19 mm and 6 mm,12 mm,19 mm).The optimal ratio of hybrid fiber fly ash concrete was obtained based on the orthogonal test.The basic mechanical properties and pore structure of hybrid fiber fly ash concrete under different cycles(0,15,30,45,60 times)were tested in water and 3%NaCl solution.(1)An orthogonal test with hybrid fiber fly ash concrete as the index of compressive and tensile strength was conducted,and it was found that the four factors influenced the compressive strength in descending order of polyacrylonitrile fiber volume rate,polyacrylonitrile fiber volume rate,polyacrylonitrile fiber length and polyacrylonitrile fiber length;the splitting tensile strength was influenced in descending order of polyacrylonitrile fiber length,polyacrylonitrile fiber length,polyacrylonitrile fiber volume rate and polyacrylonitrile fiber volume rate.The optimum level parameters for the four factors were 0.10%volume rate of polypropylene fiber,0.12%volume rate of polyacrylonitrile fiber,12 mm length of polypropylene fiber,and 12 mm length of polyacrylonitrile fiber,respectively.(2)The specimens were subjected to water and salt freezing cycles based on the orthogonal test results,and the specimens were analyzed for appearance,basic mechanical properties,relative dynamic elastic modulus and mass loss rate under different number of freeze-thaw cycles.The results showed that the surface damage of the test specimens was more serious under the salt freezing cycle than under the water freezing cycle,and the hybrid fibers could effectively reduce the surface spalling of the test specimens and delay the development of cracks to the internal damage of the test specimens.Under the same freeze-thaw cycles,the tensile strength and relative dynamic elastic modulus of the test specimens after water freeze-thaw cycles were better than those of salt freeze-thaw cycles,and the mass loss rate was less than that of salt freeze-thaw cycles,indicating that the deterioration rate of the test specimens was faster under salt freeze-thaw cycles,and the incorporation of hybrid fibers could effectively improve the frost resistance of fly ash concrete.The compressive strength and splitting tensile strength of hybrid fiber fly ash concrete are 8.5%and 9.5%,46.8%and 12.1%higher than those of reference fly ash concrete after 60 cycles of water freezing and salt freezing.(3)The pore structure tests were conducted on fly ash concrete and hybrid fiber fly ash concrete under water freezing and salt freezing cycles,and the pore distribution characteristics,pore structure characteristic parameters and SEM micrographs inside the test specimens under different numbers of freeze-thaw cycles were analyzed.The results show that with the increase of the number of freeze-thaw cycles,the pore gas content and the frequency of large pores increase and the frequency of small pores decrease;from the pore distribution of concrete,the distribution of pore structure is more reasonable in water freezing cycle compared with salt freezing cycle,and the distribution of pore structure is more reasonable in hybrid fiber fl y ash concrete compared with the base fly ash concrete.All four pore structure characteristic parameters have good correlation with the number of freeze-thaw cycles;the air content,average chord length and spacing coefficient of pores are smaller,the specific surface area is larger and the test specimens are more severely damaged under salt freeze cycle;mixing with hybrid fibers can effectively improve the pore structure characteristic parameters.The deterioration rate of test specimens under salt freezing cycle is faster,and blended fibers can improve the integrity and crack resistance of fly ash concrete test specimens,thus improving the frost resistance of concrete.Figure[41]Table[24]References[70]...