| With the continuous expansion of the scale of infrastructure construction,the demand of sand for building is increasing day by day.However,the high-quality sand resources are facing depletion,which leads to the rising of construction costs.Therefore,it is of great importance to explore new sand sources for the field of concrete engineering.The aeolian sand,which is widely distributed around the world,can become a green resource after the processes of reasonable utilization due to its convenient mining and low cost.In this paper,the aeolian sand on the surface of Mu Us sand land was used as raw material to product concrete,and the influence of aeolian sand content on the mechanical properties of concrete was studied,as well as its influence mechanism was revealed combining with the physical characterization techniques such as SEM,NMR and X-CT,and the chemical ones of X-ray single crystal diffraction(XRD)and in situ infrared spectrometer(FT-IR)from multi-scale.Meanwhile,the durability deterioration tests of aeolian sand concrete were carried out to simulate different service conditions,the deterioration laws of macroscopic properties such as mass,dynamic elastic modulus and compressive strength were analyzed;the evolution of pore structure,interfacial transition zone(ITZ),morphology and phase composition of hydration products were characterized by the SEM,NMR,XRD and other equipment.At the same time,the water transport model and frost heave failure mechanical model were established based on the one-dimensional capillary water absorption theory and porous elastic media theory,the water absorption characteristics and freezing stress evolution of aeolian sand concrete under freeze-thaw action were analyzed,and the degradation mechanism of freeze-thaw and salt erosion was revealed.In addition,a life prediction model was established to predict the service life of aeolian sand concrete based on the indoor and outdoor tests results to provide services for the promotion and use of aeolian sand concrete in infrastructure construction in desert areas,and the main research contents and conclusions are as follows:(1)The influence of aeolian sand content on the slump and compressive strength of concrete was studied,and its mechanism was revealed from multi-scale.The results show that using a proper amount of aeolian sand to take the place of river sand can configure aeolian sand concrete that has reliable workability and mechanical properties,and the best content is 20-30%.The physical mechanism of aeolian sand affecting the mechanical properties of concrete lies in its small particle size and low surface friction.It can perfect the particle gradation of fine aggregates,reduce both the effects of "micro-zone bleeding" and "boundary"and strengthen the ITZ structure.The chemical mechanism is that it is weakly alkaline,and the small particles have active and heterogeneous nucleation effects,which can have secondary hydration reactions with the cement hydration products,thereby changing the chemical structure(functional groups)and microscopic morphology of hydration products and reducing the initial defects inside the concrete.Then,the harmful pore rate was defined,the linear relationship between it and concrete strength was found,and the concrete strength prediction model was established by considering the influence of the chemical activity of aeolian sand or the fineness modulus of sand,which can better predict the early strength development law of aeolian sand concrete.(2)The durability degradation test of aeolian sand concrete with the environments of freeze-thaw in water and air,as well as freeze-thaw and salt corrosion(3%NaCl solution)were carried out to analyze its durability degradation law,and the damage mechanism was revealed from the physical,chemical points of view and multi-scale.The results indicate that the damage of concrete is the result of the coupling of "force-heat-flow",and the concrete mainly exhibits physical damage and weak chemical damage when subjected to freeze-thaw cycles in water.The continuous moisture transmission from the outside is the fundamental reason of concrete freeze-thaw deterioration.The concrete with aeolian sand content of 100%has the worst mechanical properties but best frost resistance.The damage of concrete caused by the freeze-thaw and salt corrosion is a physical-chemical process.On the one hand,part of NaCl in concrete has a complex chemical reaction with the hydration products to form the corrosive product Friedel salt.On the other hand,part precipitates in the form of crystals at the low temperatures,thus aggravated the freeze-thaw damage of concrete.The freeze-thaw damage degree of concrete in various media is 3%NaCl>water>air when the number of freeze-thaw cycles is fixed.The mass and dynamic modulus of aeolian sand concrete are relatively stable,and the small-size pores in concrete tend to decrease after immersion in 3%NaCl solution for the long-term.(3)The evolution law of water absorption characteristics of aeolian sand concrete with freeze-thaw damage was analyzed,and the water transport model was established based on the one-dimensional capillary water absorption theory.Based on the theory of poroelastic medium and pore relative saturation,the frost heave failure mechanics model was established and the evolution law of freezing stress was analyzed.The results show that the water absorption coefficient,water absorption depth and pore saturation velocity of damaged concrete all increase with the increase of the number of freeze-thaw cycles,and the capillary water absorption curve presents the typical "bilinear" characteristic.The freeze-thaw damage rate of concrete is positively correlated with the pore saturation rate and relative saturation,and negatively correlated with aeolian sand content.The freezing stress is the result of the combined effect of the crystallization pressure and hydrostatic pressure caused by the volume expansion caused by the water-ice phase transition and the water migration caused by the supercooling phenomenon,and its value is closely related to the pore structure,relative saturation,liquid saturation and the degree of supercooling.The larger the pore size and pore relative saturation,the smaller the liquid saturation,the greater the freezing stress,the faster the freeze-thaw damage rate of concrete,and the more serious the damage.(4)The natural exposure test was carried out to simulate the deterioration law of concrete structure in the semi-arid climate of seasonal freezing zone,and the natural service life prediction model was established based on the meteorological conditions of the coldest month(January)and the equivalent effects of indoor and outdoor freeze-thaw.The results show that aeolian sand concrete experienced 118 natural freeze-thaw cycles in a frozen age in northern Shaanxi,which is equivalent to 12.66 cycles of damage caused by the rapid freeze-thaw cycles in water environment.The deterioration behavior of durability evaluation indexes such as mass loss rate and dynamic elastic modulus loss rate is closely related to the external environmental humidity(which affects the direction of water transport at the air-concrete interface).After a frozen age,the mass,dynamic elastic modulus and compressive strength of aeolian sand concrete suffered little damage.The service life of concrete can be predicted based on both the mass loss rate and the dynamic modulus loss rate.It is predicted the natural freeze-thaw life of aeolian sand concrete in water environment in northern Shaanxi is about 21 years,and that of superstructure is about 54.96-61.56 years. |
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