| In the cold regions of Northwest China,especially for the area with low temperature and the winter snow and ice covers the ground over a long period of time,the mechanical properties and salt frost resistance of concrete play a key role in the evaluation of concrete performance.Salt freeze-thaw damage is exposed to deicing salt,snow melting agent,saline alkali land,sea water and other salt environment with the freeze-thaw cycle,which is also the focus and difficult research in the field of concrete durability.It combines with the erosion of environmental media in Xinjiang,the orthogonal experiment is designed to simulate the salt-freezing environment,and the effects of salt swelling and frost-thaw cycle with different admixtures is studied,under the action of erosion of compound salt solution and freeze-thaw cycle.The internal microstructure of concrete with different admixtures and different days is analyzed.The admixture adopts single fly ash with 15%,20%,25%,single silica fume with 4%,6%,8%and both in proportion of 1:1,1:3,3:1(accounting for 20%of the total cement)to design the concrete test plan.The test results show that:(1)The addition of fly ash and silica fume has little effect on improving the working performance of concrete.The addition of fly ash will increase the slump and reduce the gas content.Both single fly ash and fly ash-silica fume concrete have the effect of lowering slump,among which fly ash+silica fume(5%+15%)concrete decreases the most which is 4.4%lower than that of foundation concrete.The single silica fume has the greatest influence on the gas content of concrete.The gas content drops quickly with the increase of the silica fume content,and the gas content of 8%silica fume concrete drops 22.6%.(2)The use of fly ash and silica fume can effectively improve the mechanical properties of concrete.The single fly ash concrete has a significant effect on the strength improvement of concrete in the later period,with speed increase by 7%faster than that of foundation concrete in the same period.Both the single silica fume and fly ash-silica fume concrete have obvious effects on the improvement of concrete strength,with a faster growth rate in the early stage,but the single silica fume is better.The growth rate of the single silica fume in the early stage is 44%more than foundation coagulant soil in the same period.(3)In the single freeze-thaw factors(water)environment,combine with the multiple regression analysis to choose the best antifreeze effect of four kinds admixture concrete,the fly ash-silica fu me concrete has good frost resistance effect,fly ash+silica fume(15%+5%)is the best content,after 200 frozen water cycle,the relative loss modulus is more than 86%,mass loss is less than 0.6%.(4)By means of range and variance analysis of orthogonal test results,the strongest corrosion combination of compound salt solution concentration is selected.Under the combined action of salt solution and freeze-thaw,the single fly ash concrete has the worst anti-salt-freezing effect.After 200 times of salt-freezing cycle,the relative modulus drops to 42.1%.The single silica fume has the best anti-salt-freezing effect.After 0,50,100,150 and 200 cycles of salt freezing,the compressive strength loss rate of the single fly ash concrete is the highest,and the compressive strength loss rate of the single silica fume concrete is the lowest.The compressive strength loss rate of single addition of silica fume is 7.5%.(5)The microstructure analysis of four kinds of admix tures concrete showed that the internal structure of fly ash and foundation concrete was loose,especially the concrete generates many cracks and salt erosion after he salt freezing cycle.The inner structure of the two kinds of concrete mixed with silica ash is relatively dense with few microcracks and many calcium silicate hydrate crystals with high compactness are generated.(6)The concrete with 15%fly ash+5%silica fume has good mechanical properties,anti-salt-frost resistance and excellent economic benefits. |
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