Experimental Study On The Performance Of Early Frost-damaged Basalt Fiber Concrete Under The Compound Effect Of Salt And Freezing

In the cold and arid areas of northwest China,hydraulic concrete buildings suffer from early freezing on the basis of the coupling effect of compound salt erosion and freeze-thaw cycles during service,which is a complex engineering problem with initial damage and multiple coupling effects.Fiber concrete by adding fibers to the concrete can make its mechanical properties and durability can be improved to varying degrees,and is recognized by the engineering community as a kind of high-quality concrete suitable for the compound disaster environment,in which basalt fiber as a new fiber material,with its own green,efficient,low energy consumption and other characteristics are gradually applied to fiber concrete,the study on its performance test has gradually become a hot spot.Therefore,it is necessary to carry out experimental research on the performance of basalt fiber concrete under the action of early damage coupled with salt freezing for the special conditions of winter water conservancy project construction in the cold and arid areas of northwest China,and combined with the unique salt freezing medium action of the sedimentary phase foundation in northwest China.In this paper,the experimental study on the performance deterioration of basalt fiber concrete with early frost damage coupled with salt freezing was carried out by simulating the site environment of early frost and salt freezing cycle of hydraulic buildings in Gansu Jingdian irrigation area.The performance indexes analyzed in macroscopic tests include: mass loss rate,relative modulus of elasticity,compressive strength,etc.;meanwhile,the fine-scale performance analysis was carried out by applying SEM scanning electron microscopy,defect detection by ultrasonic method,and pore size detection by nuclear magnetic resonance.Based on the experimental results,the correlation between the relative elastic modulus,compressive strength and pore structure parameters was qualitatively analyzed by using gray correlation theory,and on this basis,reasonable pore structure representative parameters were selected to establish a model of material pore structure and compressive strength under the influence of different factors.The main conclusions are as follows:(1)The effect of incorporating basalt fibres on the 28 d compressive strength of early frozen concrete is not very significant,but the effect on strength at later coupling is more significant.The rate of increase of compressive strength tends to increase and then decrease with the increase of fibre admixture,and the greatest increase of compressive strength is achieved at 0.15% of fibre admixture.The analysis of the measured data values shows that the formulae for the contribution of basalt fibre admixture to the compressive strength of concrete specimens with different freeze-thaw times are polynomial and well correlated,with correlation coefficients above 0.85.(2)The performance parameters such as mass loss rate,relative modulus of elasti city and compressive strength of the injured basalt fiber concrete with different volum e admixtures(0%,0.05%,0.10%,0.15% and 0.20%)in the compound salt freezing en vironment all gradually decreased with the increase of the number of salt freezing,but showed a variability of the integrated macroscopic deterioration effect.The test results show that,under the coupling effect of freezing and salt damage at the same early a ge,the incorporation of basalt fiber can effectively slow down the macroscopic perfor mance degradation of concrete in the process of salt freezing and thawing,and the eff ect is enhanced first and then reduced with the increase of basalt fiber incorporation,and the best effect is achieved when the fiber incorporation amount is 0.15%.(3)Microanalysis methods such as SEM scanning electron microscopy,ultrasonic method defect detection and NMR pore size detection were applied to evaluate the microstructural characteristics of basaltic concrete during the tests from different perspectives,respectively.Comparative analysis using SEM micromorphology shows that the internal damage of the early frozen concrete specimens without basalt fibers is serious,and the incorporation of fibers can effectively inhibit the structural microscopic deterioration under the effect of coupled salt freezing with injury;Application of ultrasonic nondestructive testing of defect clouds shows that the incorporation of basalt fibers can improve the internal pore space of concrete and enhance the transmission of wave velocity to some extent;Using nuclear magnetic resonance(NMR)technique,the pore structure characteristics of concrete with damage were measured for different number of freeze-thaw cycles and different basalt fiber admixtures,and the results showed that the total porosity of basalt fiber concrete specimens with different volume admixtures was positively correlated with the number of freeze-thaw cycles.The pore structure damage shows that the proportion of harmless pores is reduced,and the proportion of less harmful pores,harmful pores and multi-harmful pores is increased.The addition of basalt fiber can increase the proportion of harmless pores,reduce the content of multi-harmful pores,optimize the pore structure of concrete,and improve the durability of concrete against freezing,among which the specimens with 0.15% fiber admixture showed the most superior performance,which is consistent with the results of the material macroscopic performance test.(4)The application of gray correlation analysis based on experimental data showed that there was a strong correlation between harmless and multi-harmful pores and the relative dynamic elastic modulus of concrete,and a strong correlation between harmful pores and the compressive strength of concrete.And on this basis,the selected harmful pore as a parameter,the established model of the pore structure and compressive strength under the influence of different factors,through the test measured data to verify that the fitting accuracy is high,can be quantitative analysis of the state of compressive strength of basalt fiber concrete materials under the action of coupled salt freezing with injury through the corresponding pore structure changes.The related research results can lay the theoretical and technical foundation for the engineering application of basalt fiber concrete in specific environments and the application of fiber concrete under similar environmental effects.