Study On Size Effect Of Fly Ash Geopolymer Concrete Compressive Strength At High Temperature

The size effect is prevalent in all kinds of concrete materials and is significantly affected by temperature.Compared with ordinary concrete,fly ash geopolymer concrete has better high-temperature resistance and is a good building material in the context of frequent fires.Therefore,this article selects fly ash geopolymer concrete as a research object and investigates its size effect at high temperatures.In this paper,a combination of experimental,theoretical and numerical analysis was adopted to investigate the size effect on the compressive strength of fly ash ground aggregate concrete cubic specimens under different proportioning,size and temperature conditions(up to 600℃).The main contents and conclusions of study are as follows:(1)High temperature loading and compressive strength tests were conducted on 108 cube specimens of fly ash geopolymer concrete with three ratios and three sizes.The test results showed that:after loading at different temperatures,the surface color of the specimens changed from dark to light as the temperature increased,and the fine cracks on the surface increased;the compressive strength of fly ash geopolymer concrete increased to a certain extent when the temperature reached 200℃,and the compressive strength decreased when the temperature exceeded 200℃.The compressive strength has a tendency to decrease;Microscopic electron microscopy analysis revealed that fly ash improves the microstructure of concrete at high temperatures,resulting in improved toughness and homogeneity of fly ash geopolymer concrete.(2)The fitting results are analyzed by combining the cube compressive strength of fly ash geopolymer concrete with Bazant’s size effect law formulation based on energy release.The results show that Bazant’s size effect theory can be applied to the analysis of the size effect of the cube compressive strength of fly ash geopolymer,and based on the framework of the size effect formulation proposed by Bazant,a modified theoretical formulation applicable to the high temperature case is proposed.The internal damage mechanism of fly ash geopolymer concrete was also analyzed,and it was found that the microstructure of fly ash geopolymer concrete under high temperature conditions was improved and the toughness was enhanced,and the combined effect of both was beneficial to reduce the internal defects and the size effect had a tendency to weaken.(3)A random aggregate generation script was developed using Python language and a micro finite element model was established.The damage characteristics of fly ash geopolymer concrete were analyzed by numerical simulation,and its size effect behavior at high temperatures was studied.The results indicate that the random aggregate model can effectively describe the failure characteristics and size effects of fly ash geopolymer concrete.A linear fit of the compressive strength of the cubes of different sizes derived from the numerical simulation yields that the absolute value of the slope k decreases with increasing temperature and the size effect diminishes.Meanwhile,the results of numerical simulations are more consistent with the modified Bazant size effect law proposed in this paper.