Research On The Frost-Resisting Durability Of Graphene Oxide Concrete In Glacial Ice-Rich Permafrost Zone

Sichuan-Tibet Railway as China’s "One Belt,One Road" interconnection of key projects is currently accelerating its construction.As the construction continues,more and more infrastructures are bound to serve along the railroad line.Sichuan-Tibet Railway passes through the glacial ice-rich permafrost zone of the Qinghai-Tibet Plateau.The freeze-thaw cycle and continuous negative temperature environmental effects caused by high altitude,large temperature difference and ice-rich frozen soil in the region are easy to cause the deterioration of the performance of concrete materials in the maintenance and service stages,leading to a significant reduction in service life.In view of this,it is necessary to effectively evaluate the frost-resisting durability of concrete in the glacial ice-rich permafrost zone,and to propose effective anti-freeze measures to improve the service performance of concrete in such environments.Graphene oxide(GO),as an emerging nanomaterial,is rich in oxygen-containing functional groups on its surface,which can provide nucleation sites for the growth of cement hydration products and promote the hydration reaction.It can be used as one of the potential external admixtures to improve the frost resistance of concrete.In this dissertation,graphene oxide is incorporated into concrete at 0.01%,0.03% and 0.05% to explore its suitability for frost resistance of concrete in glacier-rich permafrost areas.Relying on the Sichuan-Tibet Railway,this dissertation establishes two test systems of freeze-thaw cycle and continuous negative temperature curing by investigating the actual engineering climate and combining with relevant test specifications,and the experimental study on the frost-resisting durability of graphene oxide concrete under two kinds of test systems is carried out.The macro-microscopic property evolution law of graphene oxide concrete is analyzed by compressive strength,dynamic elastic modulus,nuclear magnetic resonance(NMR),scanning electron microscopy(SEM),X-ray diffraction(XRD)and other testing methods.Based on the experimental data and the existing theory,the frost damage model of concrete under freeze-thaw cycles is established and the life prediction analysis is carried out.In addition,the prediction models of compressive strength of concrete under two kinds of environmental systems are developed and validated.The effect law of graphene oxide dosing on the freezing point of concrete is analyzed based on the temperature test method.Finally,in order to save material cost and improve economy,the relationship models between compressive strength of concrete and ultrasonic velocity under two types of environmental regimes are established with a view to applying the nondestructive testing method of ultrasonic testing to strength testing in practical engineering.The main research conclusions are as follows:(1)After the freeze-thaw cycles,the apparent damage to the concrete was obvious,and damage such as desiccation,corner dropping and cracking occurred.Under the action of freeze-thaw cycles,the compressive strength,mass and dynamic elastic modulus of concrete increase first and then decrease rapidly.With the increase of freeze-thaw cycles,the porosity of concrete increases,the number and proportion of harmful macropores gradually increase,and the pore size distribution tends to deteriorate.The incorporation of graphene oxide not only effectively improves the strength and dynamic elastic modulus of concrete at the initial and end stages of freeze-thaw,and delayed the apparent damage process of concrete,but also refines the pore size distribution of concrete,increases the number and proportion of harmless pores.SEM tests show that graphene oxide induce the hydration products to grow into petal-like and polyhedral crystalline clusters,which effectively improve the microstructure of concrete,inhibit the formation and expansion of microcracks under the action of freeze-thaw cycles,and have a positive effect on the frost resistance of concrete.(2)Based on the Weibull distribution function,the frost damage model of concrete is established,which can better reflect the damage degree of concrete in different freeze-thaw cycle stages.By introducing the strength attenuation coefficient k and the graphene oxide correction coefficient λ,the strength attenuation model of concrete is further established.The model can better predict the strength of graphene oxide concrete in different freeze-thaw stages.By solving the Weibull freezing damage model and strength decay model in conjunction,the conversion between concrete strength,relative dynamic elastic modulus and damage degree can be realized,which is conducive to further understanding the change relationship between concrete freeze-thaw damage indicators.Finally,based on the established frost damage model and the existing life prediction model,the service life of concrete is analyzed.The life prediction results show that 0.01%,0.03% and 0.05% graphene oxide can extend the service life of concrete in the worst freeze-thaw areas of the Qinghai-Tibet Plateau by about 1 year,6 years and 4 years,respectively;and extend the service life of concrete in most of the freeze-thaw areas by about 1.5 years,8 years and 4years.Among them,0.03% dose of graphene oxide is most beneficial for concrete frost durability improvement.(3)The strength development of concrete under continuous negative temperature curing is seriously hindered.The strength at 3 d is less than 20% of that under standard curing,and the strength at 28 d is less than 30% of that under standard curing.The results of pore structure and XRD test show that the hydration of cement is inhibited under negative temperature environment,and the amount of hydration products is greatly reduced,which leads to the loose pore structure of concrete,the increase of porosity and the increase of the number of macropores,which seriously damages the frost resistance of concrete.Graphene oxide promotes the development of concrete strength under negative temperature curing,accelerates the hydration of cement clinker and the formation of hydration products,and effectively improves the compactness and frost resistance of concrete.However,excessive graphene oxide content will weaken the frost resistance of concrete,and its optimal dosing is 0.03%.(4)Based on the maturity theory,the prediction model of concrete strength development under negative temperature curing system is established.By analyzing the model parameters and prediction errors,it is concluded that the model can better simulate the development process of concrete strength in the negative temperature curing stage.The logarithmic relationship model of concrete strength under standard curing and negative temperature curing is further established.The strength of negative temperature curing concrete can be reasonably speculated according to the strength of standard curing concrete at the same age,which can provide reference for the strength prediction of deep buried concrete such as pile foundation in glacial ice-rich permafrost zone.(5)The effect law of graphene oxide admixture on the freezing point of concrete is tested by thermometric method.The test results show that graphene oxide incorporation effectively reduce the freezing point of concrete,delay the freezing process of concrete,and have a positive effect on the anti-freezing performance of concrete in low negative temperature environment.(6)To investigate the feasibility of ultrasonic nondestructive method for testing concrete strength,the relationship models between concrete strength and ultrasonic velocity under two test systems are established respectively.The fitting results show that the relationship between concrete strength and wave velocity is most consistent with the parabolic function.Based on this,a unified parabolic function expression between concrete strength and wave velocity is finally established,which can provide a reference for evaluating the strength of concrete by non-destructive testing in glacial ice-rich permafrost zone.